Alternating-current electric arc furnace auxiliary system and protective gas top-blowing method thereof

Abstract

The invention discloses an alternating-current electric arc furnace auxiliary system and a protective gas top-blowing method thereof, and belongs to the technical field of alternating-current electric arc furnace metallurgy. The technical problem that in the melting period of scrap steel smelting of an existing alternating-current electric arc furnace, the nitrogen content of molten steel is increased due to air ionization of an electrode is solved. The alternating-current electric arc furnace comprises a refractory small furnace cover and a top-blowing protective gas device. A plurality of graphite electrodes are arranged on the refractory small furnace cover; the small refractory furnace cover is arranged in the center of the water-cooled furnace cover; the top-blowing protective gas device is arranged on the refractory small furnace cover and is fixedly connected with the refractory small furnace cover; and the protective gas top-blowing device can blow protective gas into the furnace. The supersonic jet spray gun is mounted on the refractory small furnace cover, so that protective gas can be blown to exhaust air in the furnace when the furnace is electrified, and the content of nitrogen fused in molten steel due to ionization of nitrogen in air by an electric arc after the three-phase electrode of the alternating-current electric arc furnace is electrified is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of AC electric arc furnace metallurgy, and in particular to an AC electric arc furnace auxiliary system and a method for top blowing protective gas thereof. Background Art

[0002] At present, the two main steelmaking processes in the world are the long converter process and the short electric furnace process. Generally, the nitrogen content of molten steel produced by converter is about 20-30ppm, while the nitrogen content of molten steel produced by electric furnace (mainly equipped with AC arc furnace, and a small number of DC arc furnaces) is 70-120ppm, or even higher.

[0003] The main reason for the above differences in nitrogen content is that the current electric furnace smelting process is generally divided into a melting period and an oxidation period. In the melting period, high-energy arc heat is mainly used to melt scrap steel, and the arc temperature is as high as 4000°C. The high-temperature arc will ionize nitrogen in the air, and the ionized nitrogen atoms enter the molten steel.

[0004] With the international and domestic market's demand for low-carbon smelting and low-carbon steel products, the converter process uses molten iron from blast furnaces as raw materials, resulting in high carbon emissions per ton of steel, which is contrary to the market's low-carbon demand. The charge structure of electric furnaces has also changed from the original "more molten iron + less scrap steel" to high scrap steel ratio and all-scrap steel smelting. At the same time, due to cyclical fluctuations in domestic and foreign markets, the economic benefits of smelting low-end products (with no requirements for nitrogen content) in electric furnaces are getting lower and lower. The AC arc furnace production of this type of enterprise has gradually changed from low-end building steel to high-end special steel (with certain requirements for nitrogen content).

[0005] With the change of charge structure to high scrap steel ratio / all scrap steel caused by low-carbon smelting, coupled with the high-end special steel production task, the nitrogen content of molten steel obtained by using existing smelting equipment is high and the prepared products cannot meet the requirements. Summary of the invention

[0006] In view of the above analysis, the present invention aims to provide an AC electric arc furnace auxiliary system and a method for top blowing protective gas thereof, so as to solve the technical problem that when the existing AC electric arc furnace is smelted due to high scrap steel ratio / full scrap steel, during the scrap steel melting period, the molten steel pool and foamy slag have not yet been formed in the furnace, and the electrode arc is exposed to the air to ionize the air, resulting in an increase in the nitrogen content of the molten steel.

[0007] The purpose of the present invention is mainly achieved through the following technical solutions:

[0008] On the one hand, the present invention provides an AC electric arc furnace auxiliary system, including a small refractory furnace cover and a top-blown protective gas device; the small refractory furnace cover is provided with a plurality of graphite electrodes; the small refractory furnace cover is arranged at the center of the water-cooled furnace cover of the AC electric arc furnace; the top-blown protective gas device is arranged on the small refractory furnace cover and the two are fixedly connected; the top-blown protective gas device can spray protective gas into the furnace.

[0009] In one possible design, the top-blowing shielding gas device includes a supersonic spray gun and a gas source system;

[0010] The gas source system is connected to the supersonic spray gun, and the gas source system can provide protective gas for the supersonic spray gun.

[0011] In a possible design, a spray gun mounting hole is provided on the small refractory furnace cover, and the supersonic spray gun is fixed in the spray gun mounting hole, and the lowermost end of the supersonic spray gun is flush with the lowermost end of the small refractory furnace cover.

[0012] In a possible design, a plurality of embedded fixings are provided along the circumferential direction of the spray gun mounting hole of the refractory small furnace cover, and the embedded fixings and the refractory small furnace cover are integrally formed by casting;

[0013] A plurality of spray gun fixings are arranged along the circumferential direction on the outer edge of the gun body; the number of the embedded fixings and the spray gun fixings are equal and their positions correspond; the supersonic spray gun is fixed in the spray gun mounting hole through the embedded fixings and the spray gun fixings.

[0014] In a possible design, threaded holes are provided on both the embedded fixing part and the spray gun fixing part, and after the two are aligned, they are fixedly connected by bolts.

[0015] In a possible design, the supersonic spray gun includes a gun head and a gun body, and the gun head and the gun body are welded.

[0016] On the other hand, the present invention also provides a method for top blowing protective gas of an AC arc furnace auxiliary system, using the above-mentioned AC arc furnace auxiliary system; the method for top blowing protective gas comprises the following steps:

[0017] Step 1: Open the water-cooled furnace cover and load the scrap steel. The AC arc furnace starts smelting. During this stage, the top-blowing protective gas device continuously blows air.

[0018] When the AC arc furnace is filled with scrap steel, the water-cooled furnace cover is rotated, and the protective gas is sprayed after the water-cooled furnace cover is rotated into place; at the same time, when the water-cooled furnace cover of the AC arc furnace is rotated into place, the graphite electrode begins to descend and is energized, the graphite electrode arcs and begins to melt the scrap steel.

[0019] Step 3: After the melting of scrap steel is completed, the AC arc furnace molten pool forms a molten steel pool, and then enters the AC arc furnace oxidation period to produce foam slag. When the foam slag completely covers the graphite electrode arc, the top blowing shielding gas device stops blowing shielding gas and changes to blowing air;

[0020] Step 4: After the AC arc furnace smelting is completed, the steel is tapped and steps 1 to 3 are repeated to start the next furnace smelting operation.

[0021] Furthermore, in step 1, the air blowing flow rate is constant at 100-200m 3 / h.

[0022] In a possible design, in step 2, the blowing flow rate q of the top-blown shielding gas is ≥ 15* rated capacity Q of the AC electric arc furnace, where the unit of the shielding gas blowing flow rate q is m 3 / h, the unit of rated capacity Q of AC arc furnace is t.

[0023] Further, in step 2, the protective gas is argon gas or carbon dioxide gas.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] (1) The present invention provides a top-blown protective gas device on the refractory cover of the AC electric arc furnace, so that the three-dimensional space of the arc region of the three graphite electrodes of the AC electric arc furnace is filled with protective gas, thereby reducing the air content, that is, reducing the nitrogen content. By reducing the ionization of nitrogen in the air, the nitrogen content in the molten steel produced in an electric furnace with a high scrap steel ratio or all-scrap steel is reduced, and the nitrogen content in the molten steel is reduced by ≥30%, thereby reducing the oxidation of the graphite electrodes, and the reduction in the oxidation amount of the graphite electrodes is ≥0.005kg / t of steel.

[0026] (2) The present invention arranges multiple groups of embedded fixings on the refractory furnace cover and multiple groups of spray gun fixings on the gun body. The three groups of fixings all play the role of assembling the supersonic spray gun, so that the supersonic spray gun will not be relatively displaced with the refractory furnace cover when spraying the supersonic jet.

[0027] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the embodiments of the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0029] Figure 1 This is a schematic diagram of the installation of a supersonic spray gun in a refractory furnace cover;

[0030] Figure 2 It is a cross-sectional view of the jet and graphite electrode of a three-hole supersonic spray gun;

[0031] Figure 3A top view of the small refractory cover of the electric furnace with reserved spray gun installation holes and embedded fixings;

[0032] Figure 4 A cross-sectional view of the small refractory cover of the electric furnace with reserved spray gun installation holes and embedded fixings;

[0033] Figure 5 It is a top view of the external welding spray gun fixing part of the supersonic spray gun;

[0034] Figure 6 Schematic diagram of the basic internal structure of a supersonic spray gun.

[0035] Reference numerals:

[0036] 1-supersonic spray gun; 2-graphite electrode; 3-AC electric arc furnace auxiliary system; 4-supersonic spray gun for small refractory furnace cover; 5-spray gun mounting hole; 6-embedded fixings; 7-first threaded hole; 8-spray gun fixings; 9-gun body; 10-second threaded hole; 11-water inlet; 12-water return port; 13-air inlet; 14-Laval hole. DETAILED DESCRIPTION

[0037] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0038] The present invention provides an AC arc furnace auxiliary system 3, such as Figures 1 to 6 As shown, the AC electric arc furnace includes a small refractory furnace cover 4 and a top-blown protective gas device; the small refractory furnace cover 4 is provided with a plurality of graphite electrodes 2 that can extend into the furnace; the small refractory furnace cover 4 is arranged at the center of the water-cooled furnace cover; the top-blown protective gas device is arranged on the small refractory furnace cover 4 and the two are fixedly connected; the top-blown protective gas device can spray protective gas into the furnace.

[0039] Specifically, the AC electric arc furnace is an open-cover AC electric arc furnace, which is provided with a water-cooled furnace cover on the top, on which are installed three graphite electrodes 2 which are all parallel to the central axis of the water-cooled furnace cover; in addition, the water-cooled furnace cover is a circular furnace cover, and a circular refractory furnace cover 4 is also provided at the center of the water-cooled furnace cover, and a top-blowing protective gas device is installed at the center of the refractory furnace cover 4.

[0040] In the prior art, there is no molten steel pool in the AC arc furnace during the melting period, so the foam slag has not been completed, resulting in the electrode arc being unable to be covered by the foam slag and being exposed to the furnace atmosphere. The high-temperature arc will ionize the air (the air contains 78.08% nitrogen, 20.95% oxygen, and other small amounts of gases). In particular, after the nitrogen is ionized, it is very easy to merge into the newly formed molten steel and the molten steel pool at the bottom of the AC arc furnace, resulting in an increase in nitrogen content, which affects the subsequent cleanliness of the molten steel, the surface quality of continuous casting and the quality of the final product. Compared with the prior art, the present invention can control the furnace atmosphere during the melting period of the AC arc furnace by arranging a top-blowing protective gas device on the refractory small furnace cover 4, reduce the increase in the nitrogen content of the molten steel caused by the high-temperature arc ionization of the air, improve the quality control of the molten steel and the continuous casting billet in the AC arc furnace and the subsequent refining outside the furnace and continuous casting, and ultimately improve the quality of steel products. In addition, the nitrogen content of molten steel is reduced by ≥30%, which can also reduce the oxidation of the graphite electrode 2, and the reduction in the oxidation amount of the graphite electrode 2 is ≥0.005kg / t steel.

[0041] It should be emphasized that the AC arc furnace auxiliary system 3 of the present invention can be used to smelt special steels with strict requirements on nitrogen content under high scrap steel ratio (scrap steel ratio = scrap steel / (scrap steel + molten iron) ≥ 30%) or full scrap steel (scrap steel ratio = 100%), and can also be used to smelt steel products with requirements on nitride inclusion content, and can also be used to smelt products that require rapid / ultra-rapid solidification casting or continuous casting and are sensitive to gas precipitation.

[0042] It should be noted that if Figure 1 As shown, three graphite electrodes 2 are fixed on the water-cooled furnace cover and are evenly distributed around the central axis of the water-cooled furnace cover; the small refractory furnace cover 4 is located at the center of the area surrounded by the three.

[0043] In order to top blow protective gas for an AC arc furnace, the top blowing protective gas device of the present invention includes a supersonic spray gun 1 and a gas source system; the gas source system is connected to the supersonic spray gun 1, and the gas source system is used to provide protective gas to the supersonic spray gun 1.

[0044] Specifically, the shielding gas is provided by the gas pipeline of the AC arc furnace plant, and the gas source pressure is greater than or equal to 1.0 MPa. During the melting period of the AC arc furnace, the shielding gas includes argon and CO2 gas; the shielding gas is preferably argon.

[0045] In order to install the supersonic spray gun 1, a spray gun mounting hole 5 is provided on the refractory furnace cover 4 of the AC arc furnace of the present invention, and the supersonic spray gun 1 is fixed in the spray gun mounting hole 5, and the lower end of the supersonic spray gun 1 is flush with the lower end of the refractory furnace cover 4.

[0046] Specifically, the spray gun mounting hole 5 is vertically downward, and the axis of the spray gun mounting hole 5 coincides with the central axis of the water-cooled furnace cover and the small refractory furnace cover 4; the purpose of making the bottom end of the supersonic spray gun 1 flush with the bottom end of the small refractory furnace cover 4 is to prevent the spray gun from sticking to the slag, and to ensure that the protective gas jet can be blown into the furnace to the maximum extent.

[0047] Compared with the prior art, the present invention provides a spray gun mounting hole 5 at the center of the refractory small furnace cover 4 and the mounting hole is arranged vertically downward, so that when the supersonic spray gun 1 adopts a single-hole Laval hole 14, the gas blowing direction is parallel to the gun body 9 of the supersonic spray gun 1 and the three graphite electrodes 2, ensuring that the protective gas around the three graphite electrodes 2 is evenly distributed; when a three-hole Laval hole 14 is adopted, it is ensured that the angle between the three-hole jet direction and the central axis of the water-cooled furnace cover is 2°-10° (such as Figure 2 shown).

[0048] In order to enhance the firmness of the installation of the supersonic spray gun 1, Figure 3 As shown, a plurality of embedded fixings 6 are provided along the circumferential direction of the spray gun mounting hole 5 of the refractory small furnace cover 4, and the embedded fixings 6 and the refractory small furnace cover 4 are integrally formed by casting; a plurality of spray gun fixings 8 are provided along the circumferential direction of the outer edge of the gun body 9; the number of embedded fixings 6 and the spray gun fixings 8 are equal and the positions correspond; the supersonic spray gun 1 is fixed in the spray gun mounting hole 5 by the embedded fixings 6 and the spray gun fixings 8.

[0049] Specifically, Figure 3 and Figure 5 As shown, when prefabricating the small refractory furnace cover 4, first reserve a spray gun mounting hole 5 at the center of the small refractory furnace cover 4, and embed a pre-embedded fixing part 6 every 120° on the 360° circumference of the spray gun mounting hole 5, for a total of three pre-embedded fixing parts 6. The height of the pre-embedded fixing part 6 is greater than or equal to 15 cm, and the pre-embedded fixing part 6 and the small refractory furnace cover 4 are cast as one piece. Correspondingly, spray gun fixings 8 are welded on the outer edge of the gun body 9 of the supersonic spray gun 1 at an average interval of 120° on the 360° circumference, and the number of spray gun fixings 8 is three, which is equal to the number of embedded fixings 6; when installing the supersonic spray gun 1, first place the supersonic spray gun 1 into the spray gun mounting hole 5 accordingly, with the gun head of the supersonic spray gun 1 facing downward. At this time, the three spray gun fixings 8 correspond to the positions of the three embedded fixings 6 respectively, and by fixing the three groups of spray gun fixings 8 and the three groups of embedded fixings 6 together, each fixing plays the role of assembling the supersonic spray gun 1, so that the supersonic spray gun 1 will not be relatively displaced with the refractory furnace cover 4 when spraying the supersonic jet.

[0050] In order to facilitate installation and disassembly of the supersonic spray gun 1, threaded holes are provided on the embedded fixing member 6 and the spray gun fixing member 8 of the present invention, and after the two are aligned, they are fixed by bolts.

[0051] Specifically, Figure 4 As shown, the embedded fixing member 6 of the present invention is a semi-I-shaped embedded fixing member 6, which includes an upper transverse portion, a vertical portion and a lower transverse portion. The upper transverse portion is arranged in parallel with the lower transverse portion, and the ends of the upper transverse portion and the lower transverse portion are respectively welded and fixed to the two ends of the vertical portion. The space surrounded by the upper transverse portion, the vertical portion and the lower transverse portion constitutes a bolt accommodating space. The upper transverse portion is provided with a first threaded hole 7, and a bolt is arranged in the first threaded hole 7. The spray gun fixing member 8 of the present invention is a rectangular iron block, and the rectangular iron block is provided with a second threaded hole 10. The second threaded hole 10 has the same shape and size as the first threaded hole 7. When installing the supersonic spray gun 1, align the three groups of embedded fixing members 6 with the three groups of spray gun fixing members 8, and then insert the bolts into the corresponding second threaded holes 10 and the first threaded holes 7 in sequence for fixing.

[0052] Compared with the prior art, the present invention provides a first threaded hole 7 on the embedded fixing part 6 and a second threaded hole 10 on the spray gun fixing part 8. After the two are fixed by bolts, it can ensure that the supersonic spray gun 1 will not shake when spraying protective gas and will not be misaligned with the refractory furnace cover 4, thereby ensuring that the supersonic spray gun 1 can work stably when spraying protective gas.

[0053] In order to smoothly blow the protective gas to the top of the AC arc furnace, the supersonic spray gun 1 of the present invention comprises a gun head and a gun body 9, and the gun head and the gun body 9 are welded together.

[0054] Specifically, the gun head is made of water-cooled copper, and the gun body 9 is made of non-magnetic stainless steel. The water-cooled copper gun head and the non-magnetic stainless steel gun body 9 are welded and fixed; the water-cooled copper gun head is made of red copper; and the non-magnetic stainless steel is made of 304 stainless steel.

[0055] It should be noted that the water-cooled copper gun head of the present invention is provided with a single-hole or three-hole Laval hole 14, which can realize supersonic gas blowing. When the gun head of the supersonic spray gun 1 adopts a single-hole Laval hole 14, the single-hole jet direction is parallel to the gun body 9 of the supersonic spray gun 1. When the gun head of the supersonic spray gun 1 adopts a three-hole Laval hole 14, the angle between the three-hole jet direction and the central axis of the water-cooled furnace cover is 2°-10°, and the three-hole Laval hole 14 respectively sprays downward obliquely towards the axes of the three graphite electrodes 2. When spraying protective gas, the protective gas enters from the air inlet 13 of the supersonic spray gun 1 and is ejected from the Laval hole 14 of the supersonic spray gun 1.

[0056] The Mach number of the supersonic spray gun 1 is selected according to the tonnage of the AC arc furnace. The Mach number of the supersonic spray gun 1 of the present invention is 1.0-3.0, and the jet length of the supersonic jet of the supersonic spray gun 1 is greater than or equal to 1.2m; it should be pointed out that the design formula of the Mach number of the supersonic spray gun 1 is: Mach number = 0.000622*furnace shell inner diameter-0.00178*electrode center circle diameter, wherein the Mach number is a dimensionless number, taking two significant digits after the decimal point; the units of the furnace shell inner diameter and the electrode center circle diameter are mm.

[0057] For AC arc furnaces of different tonnages, the recommended Mach number values ​​of the supersonic spray gun 1 are shown in Table 1.

[0058] Table 1 Mach number of supersonic spray gun 1 of AC arc furnace of different tonnage

[0059] AC arc furnace rated capacity / t 50 80 100 150 Furnace shell inner diameter / mm 5000 5800 6200 7000 Graphite electrode 2 diameter / mm 450 500 600 610 Electrode center diameter / mm 1150 1200 1250 1350 Electrode lifting stroke / mm 2800 5000 5500 6000 Mach number 1.06 1.47 1.63 1.95 Jet length / m 1.2 1.5 1.7 2

[0060] The top blowing protective gas device of the present invention also includes a cooling water system, such as Figure 6 As shown, the cooling water system includes an inlet pipe and a return pipe, wherein the inlet pipe is connected to the water inlet 11 on the gun body 9, and the outlet pipe is connected to the return water port 12 on the gun body 9. The inlet pipe and the return pipe of the cooling water system are both installed at the support arm of the AC electric arc furnace cover without affecting the lifting and rotation of the graphite electrode 2 and the water-cooled furnace cover. The cooling water system is a separate water supply waterway, which is not connected in series with the cooling water of the furnace body and the water-cooled furnace cover. The inlet pipe and the return pipe of the cooling water system are both provided with thermometers. The water temperature of the inlet pipe should be lower than 25°C, and the water temperature of the return pipe should be higher than 40°C. The thermometer signal is used to monitor and prompt danger, and ensure that the supersonic spray gun 1 can operate safely in the high-temperature smelting environment of the AC electric arc furnace under circulating water cooling.

[0061] The present invention also provides a method for top blowing protective gas of an AC arc furnace auxiliary system 3, characterized in that the AC arc furnace auxiliary system 3 is used; the method for top blowing protective gas comprises the following steps:

[0062] Step 1: Open the water-cooled furnace cover and load the scrap steel. The AC arc furnace starts smelting. During this stage, the top-blowing protective gas device continuously blows air.

[0063] In the above step 1, the purpose of continuously blowing air during the charging process is to prevent slag from hanging on the furnace cover. In addition, the air blowing flow rate is constant at 100-200m 3 / h.

[0064] Step 2, after the AC arc furnace is filled with scrap steel, the water-cooled furnace cover is rotated, and the protective gas is sprayed after the water-cooled furnace cover is rotated into place; at the same time, after the water-cooled furnace cover of the AC arc furnace is rotated into place, the graphite electrode 2 begins to descend and is energized, the graphite electrode 2 starts arcing and begins to melt the scrap steel, and the scrap steel smelting enters the melting period, and the arc temperature in the melting period is 4000°C;

[0065] In the above step 2, as shown in Table 2, the blowing flow rate of the protective gas q≥15*rated capacity Q of the AC arc furnace, where the unit of the blowing flow rate of the protective gas q is m 3 / h, the unit of rated capacity of AC arc furnace is t; when adding scrap steel in 1 to 3 times, air needs to be blown continuously during the feeding process. When the feeding is completed and the water-cooled furnace cover is closed, the protective gas needs to be blown continuously because the foam slag has not covered the arc at this time.

[0066] Table 2 Relationship between the injection flow rate q of the protective gas and the rated capacity Q of the AC arc furnace

[0067] AC arc furnace rated capacity / t 50 80 100 150 <![CDATA[Protective gas flow rate / m 3 / h]]> ≥750 ≥1200 ≥1500 ≥2250

[0068] In the above step 2, the protective gas is preferably argon gas, and carbon dioxide gas is less preferred.

[0069] Step 3, after the melting of scrap steel is completed, the molten steel pool is formed in the AC arc furnace molten pool, and then the AC arc furnace enters the oxidation period to produce foamy slag; when the foamy slag completely covers the arc of the graphite electrode 2, the shielding gas is stopped and the air is started.

[0070] It should be explained that after the foam slag is produced during the oxidation period of the AC arc furnace, the foam slag completely covers the graphite electrode 2 arc. At this time, there is no need to spray protective gas. The supersonic spray gun 1 is changed to spray air. The purpose of spraying air is to prevent the foam slag from clogging the spray gun nozzle, and the gas spray flow rate is constant at 100-200m 3 / h, the air injection will be stopped until the furnace cover is rotated into place after the scrap steel is added next time.

[0071] Step 4: After the AC arc furnace smelting is completed, the steel is tapped and steps 1 to 3 are repeated to start the next furnace smelting operation.

[0072] Compared with the prior art, the present invention solves the technical problem that when an AC electric arc furnace is smelting high scrap steel ratio / all scrap steel, during the scrap steel melting period, a molten steel pool and foamy slag have not yet been formed in the furnace, and the electrode arc is exposed to the air, thereby ionizing the air and causing the nitrogen content in the molten steel to increase. The present invention improves the quality control of the molten steel and continuous casting billet in the entire process of the AC arc furnace and subsequent refining outside the furnace and continuous casting, thereby improving the quality of steel products.

[0073] The AC electric arc furnace auxiliary system and the method for top blowing protective gas provided by the present invention can reduce the nitrogen content of molten steel, and are applicable to various types of alloy steel, stainless steel, and high-strength steel products with strict requirements on nitrogen content. The low nitrogen content in molten steel can be controlled during high scrap steel ratio or full scrap steel smelting in the AC electric arc furnace, and the nitrogen content can be controlled to be less than 70ppm. The present invention can also transfer steel grades with nitrogen content ≤70ppm from the converter steelmaking process to the electric furnace steelmaking process, and realize the production of low-carbon steel products.

[0074] Example 1

[0075] This embodiment takes a 100-ton open-top AC electric arc furnace as an example, and its basic equipment parameters are: the inner diameter of the furnace shell is 6200 mm, the electrode diameter is 600 mm, and the electrode center circle diameter is 1250 mm.

[0076] The supersonic spray gun 1 used is welded from a water-cooled copper gun head (red copper) + non-magnetic stainless steel (304 stainless steel). It is a three-hole supersonic spray gun 1. The Mach number of each hole is designed to be 1.63. The angle between the three-hole spray hole direction and the central axis is 5°. When installed on the small furnace cover, the three spray holes are respectively facing the central axis of the three graphite electrodes 2 and spraying obliquely downward.

[0077] The water temperature of the water inlet pipe of the supersonic spray gun 1 is 25°C, the water temperature of the water return pipe is 27°C, and the temperature difference between the return water and the inlet water of the cooling water system is 2°C.

[0078] The method of top blowing protective gas includes the following processes:

[0079] Step 1: When the arc furnace smelting begins, the steel material must be added first. When the water-cooled furnace cover of the AC arc furnace is opened and the scrap steel material is loaded, the supersonic spray gun 1 continuously sprays air, and the gas spray flow rate is constant at 100-200m 3 / h;

[0080] Step 2: After the AC arc furnace is filled with scrap steel, the water-cooled furnace cover is rotated. After the water-cooled furnace cover is rotated into place, the protective gas is sprayed, and the protective gas spray flow rate q ≥ 15 * AC arc furnace rated capacity Q, where the unit of the protective gas spray flow rate q is m 3 / h, the unit of rated capacity of AC arc furnace is t; when the water-cooled furnace cover of AC arc furnace rotates into place, graphite electrode 2 starts to descend, power is turned on, graphite electrode 2 starts arcing, and starts to melt scrap steel;

[0081] During the melting period, the protective gas sprayed was argon, the gas source pressure of argon was 1.1 MPa, and the spray flow rate was 2000m 3 / h, the jet length is greater than 1.8m.

[0082] Step 3, after the melting of scrap steel is completed, the arc furnace molten pool forms a molten steel pool; thereafter, the AC arc furnace enters the oxidation period to form foam slag. When the foam slag completely covers the electrode arc, it is no longer necessary to spray protective gas, and the supersonic spray gun 1 is changed to spray air.

[0083] It should be noted that in order to prevent the supersonic spray gun 1 from being blocked during the oxidation period and the non-smelting period, it is necessary to use the supersonic spray gun 1 to spray air. When spraying air, the air source pressure is 1.0MPa, and the air spray flow rate is constant at 150m 3 / h.

[0084] Step 4: After the arc furnace smelting is completed, the arc furnace taps steel and prepares for the next smelting operation.

[0085] The actual smelting results are as follows: when the AC arc furnace adopts 50% high scrap steel ratio for smelting, only one charging is required, and the average nitrogen content of the samples taken at the end of smelting is 32ppm.

[0086] When the AC electric arc furnace adopts 75% high scrap steel ratio for smelting, the material is added in two times, and the average nitrogen content of the samples taken at the end of smelting is 39ppm.

[0087] When the AC arc furnace uses 100% scrap steel for smelting, the material is added in two batches, and the average nitrogen content of the samples taken at the end of smelting is 46ppm.

[0088] The present invention utilizes a top-blown protective gas device to spray protective gas into the furnace during the scrap steel melting period, which can avoid the three graphite electrodes 2 being exposed to the air and ionizing the air when power is turned on, resulting in an increase in the nitrogen content of the molten steel. That is, the present invention can reduce the nitrogen content of the molten steel.

[0089] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An AC arc furnace auxiliary system, characterized in that: Including a refractory small furnace cover and a top-blown protective gas device; The small refractory furnace cover is provided with a plurality of graphite electrodes; the small refractory furnace cover is arranged at the center of the water-cooled furnace cover of the AC arc furnace; the top-blowing protective gas device is arranged on the small refractory furnace cover and the two are fixedly connected; the top-blowing protective gas device can spray protective gas into the furnace.

2. The AC arc furnace auxiliary system according to claim 1, characterized in that: The top-blowing protective gas device comprises a supersonic spray gun and a gas source system; The gas source system is connected to the supersonic spray gun, and the gas source system can provide protective gas for the supersonic spray gun.

3. The AC arc furnace auxiliary system according to claim 2, characterized in that: The small refractory furnace cover is provided with a spray gun mounting hole, the supersonic spray gun is fixed in the spray gun mounting hole, and the lowermost end of the supersonic spray gun is flush with the lowermost end of the small refractory furnace cover.

4. The AC arc furnace auxiliary system according to claim 3, characterized in that: A plurality of pre-embedded fixing parts are provided along the circumferential direction of the spray gun mounting hole of the small refractory furnace cover, and the pre-embedded fixing parts and the small refractory furnace cover are integrally formed by casting; The outer edge of the gun body is provided with a plurality of spray gun fixings along the circumferential direction; the number of the embedded fixings and the spray gun fixings are equal and their positions correspond; the supersonic spray gun is fixed in the spray gun mounting hole through the embedded fixings and the spray gun fixings.

5. The AC arc furnace auxiliary system according to claim 4, characterized in that: The embedded fixing part and the spray gun fixing part are both provided with threaded holes, and after the two are aligned, they are fixedly connected by bolts.

6. The AC arc furnace auxiliary system according to claim 1, characterized in that: The supersonic spray gun comprises a gun head and a gun body, and the gun head and the gun body are welded together.

7. A method for top blowing protective gas for an AC arc furnace auxiliary system, characterized in that: The AC electric arc furnace according to any one of claims 1 to 6 is used; the method of top blowing protective gas comprises the following steps: Step 1: Open the water-cooled furnace cover and load the scrap steel. The AC arc furnace starts smelting. During this stage, the top-blowing protective gas device continuously blows air. Step 2: After the AC arc furnace is filled with scrap steel, the water-cooled furnace cover is rotated, and the protective gas is sprayed after the water-cooled furnace cover is rotated into place; at the same time, when the water-cooled furnace cover of the AC arc furnace is rotated into place, the graphite electrode begins to descend and is energized, the graphite electrode arcs and begins to melt the scrap steel. Step 3: After the melting of scrap steel is completed, the AC arc furnace molten pool forms a molten steel pool, and then enters the AC arc furnace oxidation period to produce foam slag. When the foam slag completely covers the graphite electrode arc, the top blowing shielding gas device stops blowing shielding gas and changes to blowing air; Step 4: After the AC arc furnace smelting is completed, the steel is tapped and steps 1 to 3 are repeated to start the next furnace smelting operation.

8. The method for top blowing protective gas for an AC arc furnace auxiliary system according to claim 7, characterized in that: In step 1, the air injection flow rate is constant at 100-200m 3 / h.

9. The method for top blowing protective gas for an AC arc furnace auxiliary system according to claim 7, characterized in that: In step 2, the blowing flow rate q of the top-blown protective gas is ≥ 15* rated capacity Q of the AC arc furnace, wherein the unit of the blowing flow rate q of the protective gas is m 3 / h, the unit of rated capacity Q of AC arc furnace is t.

10. The method for top blowing protective gas for an AC arc furnace auxiliary system according to claim 9, characterized in that: In the step 2, the protective gas is argon gas or carbon dioxide gas.

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