Control method for smelting low-nitrogen steel in electric furnace

Abstract

The invention discloses a control method for smelting low-nitrogen steel by an electric furnace, which comprises the following steps of: S1, after the production of steel in the previous furnace is finished, reserving molten steel which accounts for 10-20wt% of the capacity of the electric furnace in the electric furnace, and simultaneously reserving casting powder which accounts for 10-30wt% of the total slag quantity; s2, raw materials including scrap iron or direct reduced iron are added into the electric furnace, and meanwhile coke and silicon iron are added; s3, in the smelting process of the electric furnace, CO2 gas is blown from the bottom of the electric furnace, after the raw materials are melted down to form molten steel, carbon in the molten steel is oxidized, dispersed CO bubbles are formed, and nitrogen dissolved in the molten steel is taken away; and S4, when the electric furnace smelting end point is reached, non-deoxidation tapping is adopted. According to the method, nitrogen absorption of the molten steel is reduced, nitrogen removal of the molten steel is increased, and the nitrogen element in the molten steel is controlled to be 40 ppm or below, so that the purpose of reducing carbon emission by using waste steel or directly reducing iron is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of electric furnace smelting, and in particular to a control method for smelting low-nitrogen steel in an electric furnace. Background Art

[0002] Under the background of global carbon emission reduction, my country has put forward higher requirements for carbon emission majors such as the steel industry, advocating to vigorously reduce carbon dioxide emissions. At present, conventional plates are produced by converters. Electric furnaces need to use scrap steel or direct reduced iron as raw materials. Although it can reduce carbon emissions, one of the biggest problems of electric furnace production is that it leads to nitrogen increase in molten steel.

[0003] Due to the smelting characteristics of the electric arc furnace refining furnace, the smelting process needs to be carried out through ultra-high power electrode discharge to transmit electric energy. Due to the impact power of the arc, the molten steel in the arc action area cannot be effectively covered by slag. The strong arc dissociates the surrounding air, causing the molten steel to combine with the nitrogen in the ionized air, resulting in serious nitrogen increase in the molten steel. This smelting process is an inherent defect of steelmaking in the electric arc furnace system, and it has long been unable to achieve the superiority of converter production of low-nitrogen steel grades. Chinese patent CN211035983U proposes a gas protection device, which uses protective gas to isolate air to solve the defect problem that the electric arc furnace system cannot produce low-nitrogen steel grades. At the same time, due to the cooling effect of the inert gas, the consumption of electric arc furnace electrodes can be effectively improved. Chinese patent publication number CN108715912A proposes a method for smelting low-nitrogen steel in an electric furnace. In the primary smelting process of the EAF furnace, the amount of molten iron added is 35-55%, the molten steel is slightly overoxidized, the oxygen content is 100-500ppm, the tapping temperature is 1620-1680℃, and 3-4kg / t of lime is added to the ladle during tapping. It is said that the molten steel can reach N≤0.0040% in the steel; this technology mainly protects nitrogen in the air from dissolving into the molten steel through highly surface active oxygen elements. Japanese patent JP6485058B2 proposes a method for smelting low-nitrogen steel, focusing on controlling the nitrogen absorption rate of steel tapping. This technology provides a smelting method for low-nitrogen steel that inhibits the absorption of nitrogen from the surrounding air during the tapping process. Before tapping, the periphery of the opening for introducing sealing gas into the ladle is used to isolate the air and reduce nitrogen absorption. The technology of controlling nitrogen absorption proposed by Chinese patent CN112981038B is as follows: the primary smelting of the EAF electric arc furnace adopts arc smelting of a smelting charge containing scrap steel and molten iron to obtain a primary molten steel, wherein the oxygen content of the primary molten steel is 0.02-0.04%, and the carbon content is 0.5-1.0%; the first part of lime is added to the smelting charge at the 8th to 12th minute after the arc smelting begins, and the second part of lime is added to the smelting charge at the 20th to 30th minute after the arc smelting begins to form a slag layer in the EAF electric arc furnace; during the arc smelting process, oxygen is sprayed by an oxygen lance on the furnace wall side of the EAF electric arc furnace as a carrier gas for smokeless carbon powder so that the smokeless carbon powder is blown into the slag layer to form foamy slag; the main technical principle of this technology is to use foamy slag and rapid slag formation to protect the surface of the molten steel and prevent nitrogen absorption.

[0004] In summary of the above technologies, these mainly prevent molten steel from absorbing nitrogen by isolating the air or protecting slag, but these technologies have disadvantages such as high cost, low efficiency, and complex related processes. Moreover, they do not systematically analyze and control the metallurgical physical thermodynamics of the dissolution and exclusion of nitrogen elements, so as to reduce nitrogen in molten steel. Summary of the invention

[0005] In view of the defects existing in the prior art, the purpose of the present invention is to provide a control method for smelting low-nitrogen steel in an electric furnace, which controls the nitrogen element in the molten steel to below 40 ppm by reducing the nitrogen absorption of molten steel and increasing the denitrogenation of molten steel, so as to achieve the purpose of reducing carbon emissions by utilizing scrap steel or direct reduced iron.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] The present invention provides a control method for smelting low-nitrogen steel in an electric furnace, comprising the following steps:

[0008] S1, after the last heat of steel production is completed, 10-20wt% of molten steel of the electric furnace capacity is left in the electric furnace, and 10-30wt% of protective slag of the total slag is left;

[0009] S2, adding raw materials including scrap iron or direct reduced iron into the electric furnace, and adding coke and ferrosilicon at the same time;

[0010] S3, during the electric furnace smelting process, CO2 gas is blown from the bottom of the electric furnace. After the raw materials are melted to form molten steel, the carbon in the molten steel is oxidized to form diffuse CO bubbles, which take away the nitrogen dissolved in the molten steel;

[0011] S4, when the electric furnace smelting ends, the steel is tapped without deoxidation.

[0012] Preferably, in step S2, the coke is added in a ratio of 1.5-2.5wt% of the carbon content in the molten steel.

[0013] Preferably, in step S2, the ferrosilicon is added in a ratio of 0.1 to 1.0 wt% of the silicon content in the molten steel.

[0014] Preferably, in step S3, during the electric furnace smelting process, the flow rate of CO2 is 100-400 Nm 3 / h.

[0015] Preferably, in step S3, after the raw materials are melted to form molten steel, CO2 is blown at the bottom of the electric furnace and oxygen is blown at the top of the electric furnace.

[0016] Preferably, the flow rate of the oxygen is 5000-7000 Nm 3 / h.

[0017] Preferably, in step S3, after the raw materials are melted to form molten steel, the smelting temperature is 1550-1600°C.

[0018] Preferably, in step S4, the steel tapping temperature at the end point of the electric furnace smelting is 1590-1650°C.

[0019] Preferably, in step S4, at the end of electric furnace smelting, the oxygen content in the molten steel is 500-800 ppm, and the nitrogen content is ≤40 ppm.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The control method for smelting low-nitrogen steel in an electric furnace of the present invention controls the nitrogen element in the molten steel within an ideal range by reducing nitrogen absorption by the molten steel and increasing denitrification of the molten steel, so as to achieve the purpose of reducing carbon emissions by using scrap steel or direct reduced iron;

[0022] 2. The control method for smelting low-nitrogen steel in an electric furnace of the present invention can be widely used in the process of smelting plates in an electric furnace, reduce nitrogen content, and improve the purity of molten steel;

[0023] 3. The control method for smelting low-nitrogen steel in an electric furnace of the present invention performs the operation of retaining steel and slag to facilitate arc starting of the electric furnace and ensure that the arc starting process of the electric furnace is conducive to rapid slag formation to form a protective slag layer. At the same time, carbon and ferrosilicon are added to the electric furnace, and bottom blowing of CO2 is used to increase the reaction of carbon and CO2 to form diffuse CO bubbles, which bring the dissolved nitrogen elements in the molten pool out of the molten steel. At the same time, due to the formation of a CO2 protective atmosphere on the slag surface, the nitrogen molecules in the ionized air are prevented from dissolving into the molten steel when the electrode is heated. In short, nitrogen is brought out by the diffuse CO in the molten pool, and dissolved nitrogen is avoided by the protective gas CO2, so that the final nitrogen of the electric furnace is controlled below 40ppm. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0025] Figure 1 It is a schematic flow chart of a control method for smelting low-nitrogen steel in an electric furnace according to the present invention;

[0026] Figure 2 This is a trend diagram of the influence of element composition in molten steel on [C] activity, where (a) is the influence of Si content in molten steel on [C] activity, (b) is the influence of S content in molten steel on [C] activity, and (c) is the influence of Mn content in molten steel on [C] activity. DETAILED DESCRIPTION

[0027] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form.

[0028] Combination Figure 1As shown, in order to reduce the problem of nitrogen increase in molten steel during electric furnace smelting, the present invention provides a control method for smelting low-nitrogen steel in an electric furnace, which works from two aspects: reducing nitrogen absorption in molten steel and increasing denitrification. Specifically, nitrogen in molten steel is controlled from four aspects, including the following steps:

[0029] S1, after the last heat of steel production is completed, 10-20wt% of molten steel of the electric furnace capacity is left in the electric furnace, and 10-30wt% of protective slag of the total slag is left;

[0030] Specifically, the electric furnace adopts the technology of retaining steel and slag. After the production of the previous furnace of steel is completed, 10-20wt% of the molten steel of the electric furnace capacity is left in the furnace to facilitate the arc starting; at the same time, 10-30wt% of the protective slag of the total slag is left in the electric furnace to ensure that the arc starting induction dissolution process of the electric furnace electrode is conducive to rapid slag formation and formation of a protective slag layer;

[0031] S2, adding raw materials including scrap iron or direct reduced iron into the electric furnace, and adding coke and ferrosilicon at the same time;

[0032] Specifically, raw materials are added into the electric furnace while leaving steel and slag, and the raw materials are scrap iron or direct reduced iron. At the same time, coke and ferrosilicon are added into the electric furnace, wherein the coke is added in a ratio of 1.5-2.5wt% of the carbon content in the molten steel, and the ferrosilicon is added in a ratio of 0.1-1.0wt% of the silicon content in the molten steel; in a specific embodiment, the amount of coke added is calculated as 2.0% of the total amount of raw materials (i.e. the total amount of direct reduced iron and scrap steel added); the amount of ferrosilicon added is calculated as 0.5% of the silicon content in the total amount of raw materials (i.e. the total amount of direct reduced iron and scrap steel added).

[0033] The amount of carbon and silicon added in this step is based on the following:

[0034] Combined with the basic components in Table 1, the influence of related elements on the activity of C and Si is explored. On this basis, the composition of other elements is adjusted and the solution is obtained according to the function (1) proposed by Wagner (Huang Xihu. Principles of Iron and Steel Metallurgy [M]. Beijing: Metallurgical Industry Press, 2002. P110); the activity action coefficient can be seen in Table 2.

[0035]

[0036] Table 1 Basic components for activity calculation

[0037] element C(％) Si(%) Mn(%) P(％) S(％) content 0.5 0.1 0.1 0.015 0.004

[0038] Table 2 Interaction coefficient e of elements in iron solution B K

[0039]

[0040] Taking the composition in Table 1 as the base, changing the content gradient of a related element, the influence of C, Si, Mn, P, and S on C and C are calculated respectively. The results of the influence of related elements on C activity are as follows: Figure 2 As shown in the figure, it can be seen that the Si content has a greater influence on the C activity. As the Si content increases, the C activity increases; the Mn content weakens the C activity; the S content has basically no effect on the C activity because the absolute amount does not change much.

[0041] S3, during the electric furnace smelting process, CO2 is blown from the bottom of the electric furnace. After the raw materials are melted to form molten steel, the carbon in the molten steel is oxidized to form diffuse CO bubbles, which take away the nitrogen dissolved in the molten steel;

[0042] Specifically, during the entire electric furnace smelting process, CO2 gas is blown from the bottom of the electric furnace, and the flow rate of CO2 gas is 100-400Nm 3 / h; after the raw materials are melted to form molten steel, the activity of carbon in the molten steel is increased by carbon and silicon addition in step S2, so that the reaction in (2) is carried out, forming a large number of dispersed tiny bubbles (i.e., CO bubbles), and the nitrogen dissolved in the molten steel is taken away by the tiny bubbles. At the same time, due to the CO2 blowing at the bottom of the electric furnace, the gas on the slag surface of the electric furnace is covered by the gas, so that the partial pressure of nitrogen in the air is reduced, thereby avoiding the occurrence of nitrogen absorption reaction of the molten steel.

[0043] C+CO2=2CO (2)

[0044] Among them, before the raw materials are melted, the flow rate of CO2 is 100~400Nm 3 / h; after the raw materials in the molten pool are melted, the flow rate of CO2 is 400Nm 3 / h, while the electric furnace top blows oxygen (with a flow rate of 5000~7000Nm 3 / h), through the boiling at the bottom and the top, the C and Si in the molten steel are oxidized, and the nitrogen dissolved in the molten pool is brought out of the molten steel.

[0045] After the raw materials are melted to form molten steel, the smelting temperature is 1550-1600℃.

[0046] S4, when the electric furnace smelting ends, the steel is tapped without deoxidation.

[0047] Specifically, after the smelting in step S3 is completed and the end point of the electric furnace smelting is reached, the steel is tapped without deoxidation. At this time, the oxygen content in the molten steel is 500-800ppm. Due to the surface activity of oxygen, the nitrogen absorption during the steel tapping process is controlled. Among them, the steel tapping temperature at the end point of the electric furnace smelting is 1590-1650℃; the nitrogen content in the molten steel is ≤40ppm.

[0048] The following is a further introduction to a control method for smelting low-nitrogen steel in an electric furnace with reference to specific examples.

[0049] Example 1

[0050] In this embodiment, the following implementation is carried out according to the control method for smelting low nitrogen steel in an electric furnace.

[0051] One day, in a 100-ton electric furnace, it was necessary to use direct reduced iron to smelt plates, and the operation steps were as follows:

[0052] (1) In a 100-ton electric furnace, 15% of the steel is retained, that is, 15 tons of molten steel is left; at the same time, 20% of the slag is retained, that is, 10 tons of electric furnace slag is left.

[0053] (2) First add 1 ton of coke, 305 kg of ferrosilicon (silicon content is 82%), and 50 tons of scrap steel or direct reduced iron.

[0054] (3) Electrodes are introduced for smelting, and CO2 gas is introduced into the bottom of the electric furnace at a flow rate of 1 / 3 of the maximum flow rate of bottom blowing. After the molten pool is melted, the initial composition of the molten steel is measured, and the carbon content is 1.8%, the silicon content is 0.13%, and the nitrogen content is 50ppm. The electric furnace oxygen gun begins to blow oxygen (with a flow rate of 5000Nm 3 / h), and at the same time, the bottom of the electric furnace starts to blow at the maximum flow rate (i.e. 400Nm 3 / h) to blow CO2 gas. Through the boiling at the bottom and top, the carbon in the molten steel is oxidized, and the silicon is oxidized at the same time, and the dissolved nitrogen in the molten pool is taken out of the molten pool. At the same time, the temperature is increased, and the specific smelting temperature is 1550℃.

[0055] (4) At the end of smelting, the steel is tapped without deoxidation. The tapping temperature is 1620°C. The oxygen content in the molten steel is 500ppm, the carbon content is 0.04%, the silicon content is 0.005%, and the nitrogen content is 35ppm.

[0056] Example 2

[0057] In this embodiment, the following implementation is carried out according to the control method for smelting low nitrogen steel in an electric furnace.

[0058] One day, in a 100-ton electric furnace, it was necessary to use direct reduced iron to smelt plates, and the operation steps were as follows:

[0059] (1) In a 100-ton electric furnace, 20% of the steel is retained, that is, 20 tons of molten steel is left; at the same time, 30% of the slag is retained, that is, 15 tons of electric furnace slag is left.

[0060] (2) First add 1.4 tons of coke, 438 kg of ferrosilicon (silicon content is 82%), and 70 tons of scrap steel or direct reduced iron.

[0061] (3) Electrodes are introduced for smelting, and CO2 gas is introduced into the bottom of the electric furnace at a flow rate of 1 / 2 of the maximum bottom blowing flow rate (i.e. 200 Nm 3 / h). After the molten pool was melted, the initial composition of the molten steel was measured, and the carbon content was 1.7%, the silicon content was 0.12%, and the nitrogen content was 50ppm. The electric furnace oxygen lance began to blow oxygen (with a flow rate of 7000Nm 3 / h), and at the same time, the bottom of the electric furnace starts to blow at the maximum flow rate (i.e. 400Nm 3 / h) to blow CO2 gas. Through the boiling at the bottom and top, the carbon in the molten steel is oxidized, and the silicon is oxidized at the same time, and the dissolved nitrogen in the molten pool is taken out of the molten pool. At the same time, the temperature is increased, and the specific smelting temperature is 1580℃.

[0062] (4) At the end of smelting, the steel is tapped without deoxidation. The tapping temperature is 1590°C. The oxygen content in the molten steel is 600 ppm, the carbon content is 0.03%, the silicon content is 0.006%, and the nitrogen content is 30 ppm.

[0063] Example 3

[0064] In this embodiment, the following implementation is carried out according to the control method for smelting low nitrogen steel in an electric furnace.

[0065] One day, in a 100-ton electric furnace, it was necessary to use direct reduced iron to smelt plates, and the operation steps were as follows:

[0066] (1) In a 100-ton electric furnace, 10% of the steel is retained, that is, 10 tons of molten steel is left; at the same time, 10% of the slag is retained, that is, 5 tons of electric furnace slag is left.

[0067] (2) First add 1.2 tons of coke, 366 kg of ferrosilicon (silicon content is 82%), and 60 tons of scrap steel or direct reduced iron.

[0068] (3) Electrodes are introduced for smelting, and CO2 gas is introduced into the bottom of the electric furnace at a flow rate of 1 / 2 of the maximum bottom blowing flow rate (i.e. 200 Nm 3 / h). After the molten pool was cleared, the initial composition of the molten steel was measured, with a carbon content of 1.9%, a silicon content of 0.15%, and a nitrogen content of 50ppm. The electric furnace oxygen lance began to blow oxygen (with a flow rate of 6000Nm 3 / h), and at the same time, the bottom of the electric furnace starts to blow at the maximum flow rate (i.e. 400Nm 3 / h) to blow CO2 gas. Through the boiling at the bottom and top, the carbon in the molten steel is oxidized, and the silicon is oxidized at the same time, and the dissolved nitrogen in the molten pool is taken out of the molten pool. At the same time, the temperature is increased, and the specific smelting temperature is 1600℃.

[0069] (4) At the end of smelting, the steel is tapped without deoxidation. The tapping temperature is 1650°C. The oxygen content in the molten steel is 800 ppm, the carbon content is 0.05%, the silicon content is 0.004%, and the nitrogen content is 25 ppm.

[0070] In summary, the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A control method for smelting low nitrogen steel in an electric furnace, characterized in that: The following steps are involved: S1, after the last heat of steel production is completed, 10-20wt% of molten steel of the electric furnace capacity is left in the electric furnace, and 10-30wt% of protective slag of the total slag is left; S2, adding raw materials including scrap iron or direct reduced iron into the electric furnace, and adding coke and ferrosilicon at the same time; S3, during the electric furnace smelting process, CO2 gas is blown from the bottom of the electric furnace. After the raw materials are melted to form molten steel, the carbon in the molten steel is oxidized to form diffuse CO bubbles, which take away the nitrogen dissolved in the molten steel; S4, when the electric furnace smelting ends, the steel is tapped without deoxidation.

2. The control method for smelting low nitrogen steel in an electric furnace according to claim 1, characterized in that: In the step S2, the coke is added in a ratio of 1.5-2.5wt% of the carbon content in the molten steel.

3. The control method for smelting low nitrogen steel in an electric furnace according to claim 1, characterized in that: In the step S2, the ferrosilicon is added in a ratio of 0.1 to 1.0 wt% of the silicon content in the molten steel.

4. The control method for smelting low nitrogen steel in an electric furnace according to claim 1, characterized in that: In step S3, during the electric furnace smelting process, the flow rate of CO2 is 100-400 Nm 3 / h.

5. The control method for smelting low nitrogen steel in an electric furnace as claimed in claim 1, characterized in that: In step S3, after the raw materials are melted to form molten steel, CO2 is blown at the bottom of the electric furnace and oxygen is blown at the top of the electric furnace.

6. The control method for smelting low nitrogen steel in an electric furnace as claimed in claim 5, characterized in that: The oxygen flow rate is 5000~7000Nm 3 / h.

7. The control method for smelting low nitrogen steel in an electric furnace as claimed in claim 1, characterized in that: In step S3, after the raw materials are melted to form molten steel, the smelting temperature is 1550-1600°C.

8. The control method for smelting low nitrogen steel in an electric furnace as claimed in claim 1, characterized in that: In the step S4, the tapping temperature at the end point of the electric furnace smelting is 1590-1650°C.

9. The control method for smelting low nitrogen steel in an electric furnace as claimed in claim 1, characterized in that: In the step S4, at the end of the electric furnace smelting, the oxygen content in the molten steel is 500-800 ppm, and the nitrogen content is ≤40 ppm.

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