Method for reducing nitrogen content in electric furnace full scrap steel smelting

Abstract

The invention relates to the field of steelmaking. More specifically, the invention relates to a process for controlling nitrogen increase in electric furnace smelting. The invention provides a method for producing low-nitrogen steel from electric furnace full waste steel. The method specifically comprises the steps that waste steel is loaded into an electric furnace, CO2 gas is injected into the electric furnace before and after loading, air is exhausted, and micro-positive pressure in the electric furnace is kept. The method comprises the following steps: after an electric furnace forms a molten pool, injecting limestone powder at the top of the furnace with the injection amount of 60-80kg / t and the injection carrier gas of CO2 gas, after the electric furnace forms the molten pool, injecting oxygen and carbon powder by adopting a carbon-oxygen lance to make foamed slag, injecting CO2 gas into the molten pool to decarburize, produce two times of CO gas and intensify denitrification, and during tapping of the electric furnace, protecting steel flow from ending with air by adopting CO2 gas, and feeding into the next process for smelting after tapping of the electric furnace. Application proves that when the method is used for smelting low-nitrogen steel, nitrogen in the electric furnace can be controlled not to be increased, N is smaller than or equal to 0.003% after tapping of the electric furnace, the smelting rate of the low-nitrogen steel grade is increased, and low-carbon emission production of special varieties is achieved.

Description

Technical Field

[0001] The invention relates to the field of steelmaking, and in particular to a method for reducing nitrogen content in electric furnace full scrap steel smelting. Background Art

[0002] The electric furnace smelting process is very easy to increase nitrogen, especially the production smelting of all-scrap steel electric furnace process will increase the nitrogen content in the steel. The presence of nitrogen in the steel greatly affects the performance of the steel, and will reduce the toughness, welding performance and toughness in the thermal stress zone of the steel. In particular, the nitrogen element in high-grade automotive steel directly affects the deep drawing performance of automotive steel plates.

[0003] The sources of nitrogen in steel are generally divided into three situations: 1) brought in by raw materials; 2) the exposed molten steel surface absorbs nitrogen from the atmosphere; 3) nitrogen is added to the ionized air in the arc zone of the electric arc furnace and LF furnace. At present, the main direction of controlling the nitrogen content in steel is to start from the source of nitrogen in steel and the denitrification of molten steel during the smelting process. Most of them are done by reducing the nitrogen content of raw materials and doing a good job of nitrogen addition by ionizing air with foam slag isolation electrodes. However, when the electric furnace is full of scrap steel smelting, there is no large-scale molten metal in the early stage of the electric furnace, and foam slag cannot be made. Nitrogen addition to molten steel mainly occurs in the melting and tapping stages of scrap steel. The conventional nitrogen control method is to use a 30-70% iron-to-water ratio to quickly form a molten pool and make foam slag at the initial stage of melting. However, the carbon content of the molten steel in the molten pool is insufficient, and the denitrification effect and the effect of the foam slag covering the surface of the molten steel are not ideal. The nitrogen content of the molten steel at the end of the electric furnace is 50-100ppm. For this reason, it is urgent to develop a method for producing low-nitrogen steel in an electric furnace to provide technical support for the smelting of all-scrap steel and low-carbon emission steel.

[0004] The purposes of the present invention are as follows: 1. A method for reducing nitrogen content in electric furnace all-scrap steel smelting is invented, wherein CO2 gas is continuously sprayed into the electric furnace to exhaust the air in the electric furnace, and a slight positive pressure is maintained in the furnace to prevent air from entering the electrode and contacting the steel liquid surface. 2. Limestone powder is sprayed during the melting period of the electric furnace to isolate air from contacting the electrode. At the same time, CaO generated by the decomposition of limestone powder on the slag interface can quickly form slag, and the generated CO2 gas can ensure a slight positive pressure in the electric furnace, preventing external air from entering the electric furnace and reducing the ionization and nitrogen increase of the electrode. 3. After the electric furnace forms a molten pool, a carbon oxygen gun sprays carbon powder and O2, and the slag is foamed by the CO reaction. At the same time, CO2 gas is sprayed into the molten pool to react with [C] inside the molten steel, C+CO2=2CO, and 2CO is generated while decarburizing, so that the molten steel is further denitrified. Summary of the invention

[0005] The purpose of the present invention is to provide a method for reducing nitrogen content in electric furnace full scrap steel smelting in view of the above problems.

[0006] The object of the present invention is achieved as follows: A method for reducing nitrogen content in electric furnace full scrap steel smelting comprises the following steps: Step 1: Before charging the electric furnace, a spray gun is used to spray CO2 gas into the electric furnace, and the electric furnace is filled with CO2 gas to exhaust air. When the electric furnace is charged and powered on, CO2 gas is continuously sprayed, and the gas spraying amount is 3 to 8 Nm 3 / min, maintain a slight positive pressure in the furnace to prevent air from entering the furnace and contacting the molten steel surface; Step 2: After the furnace is drilled, limestone powder is sprayed from the top of the furnace cover. The particle size of the limestone powder is 1-10mm, the spraying amount of the limestone powder is 60-80kg / t, and the carrier gas is CO2 gas; Step 3: After the molten pool is formed at the bottom of the furnace, oxygen is blown to assist melting and decarburization. A carbon-oxygen gun is used to spray carbon powder and oxygen. The carbon powder spraying amount is 4-6kg / t, and the oxygen spraying flow rate is 40-60Nm 3 / min, foam slag is produced during the electric furnace smelting process, slag FeO: 20-25%, slag basicity is controlled at 2.0-2.3, and the height of foam slag is controlled at 650-750mm. When the weight of the electric furnace molten clean reaches more than 70% of the charged amount, a spray gun is used to spray CO2 gas into the molten pool to stir the molten steel, and the spray flow rate is 30-40Nm 3 / min; Step 4: After the electric furnace is melted, continue to spray CO2 gas to stir the molten steel for 2-3 minutes. The temperature is 1630-1650℃, and the steel is tapped after C≤0.2% and P≤0.015%. CO2 gas is sprayed at the tapping port for protection to prevent the steel flow from contacting with the air and increasing nitrogen; Step 5: After tapping, send it into LF to complete refining and cast it into billet.

[0007] The injection flow rate of CO2 gas in step 4 is 55-65Nm 3 / min.

[0008] The beneficial effects of the present invention are as follows: 1. By spraying CO2 gas and lime powder in advance in the electric furnace, air is prevented from entering the electric furnace to increase nitrogen; 2. The CaO generated by the decomposition reaction of limestone powder has high activity and can quickly form slag, promoting the desulfurization and dephosphorization reactions; the generated CO2 isolates the air; 3. The CO2 gas is sprayed in the molten steel to react with [C] in the steel to produce twice the amount of CO gas, promoting the denitrification of the molten steel. 4. By adopting this method, the nitrogen content after the electric furnace is tapped can be controlled at 20-30ppm. DETAILED DESCRIPTION

[0009] Before charging the electric furnace, CO2 gas is sprayed into the electric furnace, and the electric furnace is filled with CO2 gas. CO2 gas is continuously sprayed when the electric furnace is powered on in the early stage to maintain a slightly positive pressure in the furnace (the pressure in the electric furnace is greater than the air pressure, that is, the pressure in the electric furnace is >1.01325*105Pa (1 atmosphere)). After the electric furnace is drilled, limestone powder is sprayed from the top of the furnace; after the electric furnace forms a molten pool, a carbon-oxygen gun sprays carbon powder and oxygen to form foamed slag, and a spray gun is used to spray CO2 gas into the molten pool, and the CO2+C=2CO↑ reaction is used to enhance stirring, and CO bubbles are used for denitrification; when the electric furnace is tapping steel, CO2 gas is sprayed around the tapping port to prevent the tapping steel flow from contacting with air.

[0010] 1. Before charging the electric furnace, use a spray gun to spray CO2 gas into the electric furnace, fill the electric furnace with CO2 gas and exhaust the air. When the electric furnace is charged and powered on, continue to spray CO2 gas, with a spray volume of 5Nm 3 / min, maintain a slightly positive pressure in the furnace (the pressure in the electric furnace is greater than the air pressure, that is, the pressure in the electric furnace is greater than 1.01325*105Pa (1 atmosphere)), and prevent air from entering the electric furnace and contacting the steel liquid surface.

[0011] 2. After the furnace is drilled, limestone powder is sprayed from the top of the furnace cover, with a particle size of 1-10 mm, a powder spraying speed of 60-80 kg / t, and a CO2 gas as the carrier gas. Limestone powder will undergo a decomposition reaction at high temperature, CaCO3=CaO+CO2↑, and the generated CaO will become a slag-forming agent to promote rapid slag formation. The generated CO2 prevents external air from entering the furnace, avoiding the electrode ionization of the surrounding air to form nitrogen.

[0012] 3. After the molten pool is formed at the bottom of the electric furnace, oxygen is blown to assist melting and decarburization. At the same time, a carbon-oxygen gun is used to spray carbon powder and oxygen gun. The carbon powder spraying amount is 5kg / t, and the oxygen spraying flow rate is 40-60Nm 3 / min, CO reaction is used to foam the slag, and foam slag is produced during the electric furnace smelting process. The slag FeO: 20-25%, the slag basicity is controlled at 2.0-2.3, and the foam slag height is controlled at 650-750mm. When the weight of the electric furnace molten clean reaches more than 70% of the charged amount, another spray gun is used to spray CO2 gas into the molten pool to stir the molten steel, and the spray flow rate is 30-40Nm 3 / min, CO2 reacts with [C] inside the molten steel, C+CO2=2CO↑, decarburization generates twice as much CO gas at the same time, promoting further denitrification of the molten steel. (Full scrap steel smelting in an electric furnace means that the raw materials used in the electric furnace are all scrap steel, without molten iron, alloy and other raw materials).

[0013] 4. After the furnace is melted, continue to spray CO2 gas to stir the molten steel for 2 to 3 minutes, with a spray flow rate of 60Nm 3 / min, to promote uniform temperature and composition of molten steel. When the temperature and composition are appropriate, the steel is tapped and CO2 gas is sprayed at the tapping port to protect the steel flow from contacting with air and increasing nitrogen.

[0014] 5. After the steel is tapped, it is sent to LF to complete the refining task and cast into qualified steel billets. Example 1 (160-ton electric furnace smelting IF low-nitrogen steel)

[0015] This embodiment uses electric furnace full scrap steel to produce automobile plate IF steel (composition: C≤0.002%, Si≤0.03%, Mn≤0.2%, P≤0.015%, S≤0.005%, N≤0.004%). Process route: electric furnace full scrap steel smelting - LF deep desulfurization - RH deep decarburization - continuous casting.

[0016] (1) After the last batch of steel is tapped, a spray gun is used to spray CO2 gas into the electric furnace, so that the electric furnace is filled with CO2 gas. 170t of scrap steel is loaded into the electric furnace, and electricity is supplied for melting. CO2 gas is continuously sprayed into the electric furnace, with a spray volume of 5Nm 3 / min, maintain a slight positive pressure in the furnace and prevent air from entering the interior of the electric furnace.

[0017] (2) After the furnace is drilled, limestone powder is sprayed from the top of the furnace cover. The particle size is 1-10 mm, the powder spraying speed is 60-80 kg / t, the spraying carrier gas is CO2 gas, and the gas flow rate is 10 Nm 3 / min. Limestone powder will decompose at high temperature to form CaCO3=CaO+CO2↑, and the generated CaO will become a slag-making agent to promote rapid slag formation. The generated CO2 will prevent the outside air from entering the electric furnace, thus avoiding the arc ionization of the surrounding air to form nitrogen.

[0018] (3) After the bottom of the electric furnace forms a molten pool, oxygen is blown to assist melting and decarburization. At the same time, a carbon-oxygen gun is used to spray carbon powder and oxygen. The oxygen spray flow rate is 40Nm 3 / min, CO reaction is used to make the slag foam, lime, light-burned dolomite and other slag-making materials are added to the electric furnace, and foamed slag is produced during the smelting process. The slag FeO: 20-25%, the slag basicity is controlled at 2.0-2.3, and the height of the foamed slag is controlled at 650-750mm. When the amount of electric furnace melting reaches more than 80%, another spray gun is used to spray CO2 gas into the molten pool to stir the molten steel, and the spray flow rate is 40Nm 3 / min, CO2 reacts with [C] inside the molten steel, C+CO2=2CO↑, and decarburization simultaneously generates twice as much CO gas, which promotes further denitrification of the molten steel.

[0019] (4) After the furnace is melted, continue to spray CO2 gas to stir the molten steel for 3 minutes, with a stirring intensity of 60Nm 3 / min promotes uniform temperature and composition of molten steel. When the temperature and composition are appropriate, the steel is tapped and CO2 gas is sprayed at the tapping port to protect it and prevent the steel flow from contacting with air and increasing nitrogen.

[0020] (5) After the steel is tapped, it is sent to LF and RH to complete the refining task and cast into qualified steel billets.

[0021] In this embodiment, the electric furnace uses all scrap steel to produce automobile steel IF steel. The nitrogen content of the steel after being tapped from the electric furnace can reach 0.0028%, and the nitrogen content of the finished product is 0.0030%, which meets the requirement of low nitrogen content of automobile steel. Example 2 (90 tons of electric furnace all scrap steel smelting pure iron YT01)

[0022] This embodiment uses electric furnace full scrap steel to produce pure iron YT01 steel (composition: C≤0.003%, Si: 0.15-0.25%, Mn≤0.05%, P≤0.015%, S≤0.005%, N≤0.004%). Process route: electric furnace full scrap steel smelting - LF deep desulfurization - RH deep decarburization - continuous casting.

[0023] (1) After the last batch of steel is tapped, a spray gun is used to spray CO2 gas into the electric furnace to fill the furnace with CO2 gas. 95 tons of scrap steel is loaded into the electric furnace and the furnace is powered for melting. CO2 gas is continuously sprayed into the electric furnace at a rate of 5 Nm 3 / min, maintain a slight positive pressure in the furnace and prevent air from entering the interior of the electric furnace.

[0024] (2) After the furnace is drilled, limestone powder is sprayed from the top of the furnace cover. The particle size is 1-10 mm, the powder spraying speed is 60-80 kg / t, the spraying carrier gas is CO2 gas, and the gas flow rate is 10 Nm 3 / min. Limestone powder will decompose at high temperature to form CaCO3=CaO+CO2↑, and produce CaO which becomes a slagging agent to promote rapid slagging. The produced CO2 prevents the outside air from entering the electric furnace, thus avoiding the electrode ionization of the surrounding air to form nitrogen.

[0025] (3) After the bottom of the electric furnace forms a molten pool, oxygen is blown to assist melting and decarburization. At the same time, a carbon-oxygen gun is used to spray carbon powder and oxygen. The oxygen spray flow rate is 40Nm 3 / min, CO reaction is used to make the slag foam, lime, light-burned dolomite and other slag-making materials are added to the electric furnace, and foamed slag is produced during the smelting process. The slag FeO: 20-25%, the slag basicity is controlled at 2.0-2.3, and the height of the foamed slag is controlled at 650-750mm. When the amount of electric furnace melting reaches more than 80%, another spray gun is used to spray CO2 gas into the molten pool to stir the molten steel, and the spray flow rate is 40Nm 3 / min, CO2 reacts with [C] inside the molten steel, C+CO2=2CO↑, and decarburization simultaneously generates twice as much CO gas, which promotes further denitrification of the molten steel.

[0026] (4) After the furnace is melted, continue to spray CO2 gas to stir the molten steel for 3 minutes, with a stirring intensity of 60Nm 3 / min promotes uniform temperature and composition of molten steel. When the temperature and composition are appropriate, the steel is tapped and CO2 gas is sprayed at the tapping port to protect it and prevent the steel flow from contacting with air and increasing nitrogen.

[0027] (5) After the steel is tapped, it is sent to LF and RH to complete the refining task and cast into qualified steel billets.

[0028] In this embodiment, the electric furnace uses all scrap steel to produce pure iron YT01 steel. The nitrogen content of the steel after the electric furnace is discharged can reach 0.0026%, and the nitrogen content of the finished product is 0.0028%, which meets the requirement of low nitrogen content of pure iron.

[0029] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are included in the patent scope of the present invention.

Claims

1. A method for reducing nitrogen content in electric furnace full scrap steel smelting, characterized in that: The following steps are involved: Step 1: Before charging the electric furnace, use a spray gun to spray CO2 gas into the electric furnace. Fill the electric furnace with CO2 gas to exhaust the air. Continue to spray CO2 gas when charging the electric furnace and supply power. The gas spraying amount is 3-8Nm 3 / min, maintain a slight positive pressure in the furnace to prevent air from entering the furnace and contacting the steel liquid surface; Step 2: After the electric furnace is drilled, limestone powder is sprayed from the top of the furnace cover. The particle size of the limestone powder is 1 to 10 mm, the spraying amount of the limestone powder is 60 to 80 kg / t, and the spraying carrier gas is CO2 gas; Step 3: After the molten pool is formed at the bottom of the electric furnace, oxygen is blown to assist melting and decarburization. Carbon powder and oxygen are sprayed using a carbon oxygen gun. The carbon powder spraying amount is 4-6 kg / t, and the oxygen spraying flow rate is 40-60 Nm 3 / min, foam slag is produced during the electric furnace smelting process, slag FeO: 20-25%, slag basicity is controlled at 2.0-2.3, and the height of foam slag is controlled at 650-750mm. When the weight of the electric furnace molten metal reaches more than 70% of the charged amount, a spray gun is used to spray CO2 gas into the molten pool to stir the molten steel, and the spray flow rate is 30-40Nm 3 / min; Step 4: After the electric furnace is melted, continue to spray CO2 gas to stir the molten steel for 2-3 minutes. The temperature is 1630-1650℃, C≤0.2%, P≤0.015%, and then the steel is tapped. CO2 gas is sprayed at the tapping port for protection to prevent the steel flow from contacting with the air and increasing nitrogen; Step 5: After the steel is tapped, it is sent to LF for refining and cast into billets.

2. The method for reducing nitrogen content in electric furnace full scrap steel smelting according to claim 1, characterized in that: The injection flow rate of CO2 gas in step 4 is 55-65Nm 3 / min.