Method for controlling hydrogen in high-nitrogen stainless steel

Abstract

The invention relates to a method for controlling hydrogen in high-nitrogen stainless steel, which comprises the following steps of: baking, reducing and adjusting materials in stages by using a steel ladle, fully removing moisture in original and auxiliary materials, strictly controlling the source of hydrogen in the smelting process of the high-nitrogen stainless steel, and simultaneously providing that materials which are easy to absorb moisture, such as lime, are not supplemented when the carbon content of molten steel is less than 0.5% in the AOD smelting process. And in the AOD adjustment period, the baked lime and the fluorite are added to adjust the alkalinity of the slag, the good deoxidation capacity of the slag is guaranteed, the deoxidation process is considered while the hydrogen content of the molten steel is controlled, and the bubble defect and the edge cracking defect of the high-nitrogen stainless steel are overcome.

Description

Technical Field

[0001] The invention relates to the fields of high-nitrogen stainless steel smelting and material baking, and in particular to a method for controlling hydrogen in high-nitrogen stainless steel. Background Art

[0002] At present, high nitrogen stainless steel has strict requirements on hydrogen content. When the hydrogen content of high nitrogen stainless steel increases (>8ppm), a large number of bubble defects are generated on the surface of the strip during the rolling process, resulting in low steel yield and a significant increase in scrap rate. However, it is difficult to control the hydrogen content during the smelting process. The main source of hydrogen is the moisture brought in by the raw and auxiliary materials used in the smelting process, which is decomposed into hydrogen atoms and absorbed by the molten steel under the high temperature of the molten steel. The hydrogen content of molten steel is greatly affected by air humidity and raw material moisture during the smelting process. The hydrogen content of molten steel in the smelting furnace increases significantly in rainy and snowy weather. The moisture brought in by the raw materials added during the main decarburization period is removed by the CO produced by the violent carbon-oxygen reaction in the molten pool. When the carbon content of the molten pool is less than 0.5%, the carbon-oxygen reaction rate decreases, and the moisture brought in by the added raw and auxiliary materials is difficult to remove, resulting in an increase in the hydrogen content of the molten steel. The main reason affecting the hydrogen content of the molten steel during the smelting process is the moisture brought in by the lime added in the low-carbon zone, especially in rainy and snowy weather, the air humidity is high and the lime absorbs moisture seriously. If lime and fluorite are not added in the low-carbon zone, reduction and adjustment stages of the AOD process, the basicity in the refining process will be low, the deoxidation effect will be poor, the oxygen content in the molten steel will increase, and serious edge cracking defects will occur during the rolling process.

[0003] The purpose of the present invention is to provide a method for controlling hydrogen in high-nitrogen stainless steel. Summary of the invention

[0004] The purpose of the present invention is to provide a method for controlling hydrogen in high nitrogen stainless steel in view of the above problems.

[0005] The object of the present invention is achieved in this way: a method for controlling hydrogen in high-nitrogen stainless steel, comprising the following steps: step 1: during the smelting process of high-nitrogen stainless steel, when the carbon content of the molten pool is less than 0.5%, no more lime is added to the high-level silo, and the small scrap steel dried in the high-level silo is used to balance the process temperature of 1710-1740°C during the blowing process; step 2: during the reduction and adjustment stage of molten steel, baked lime, fluorite, ferrosilicon, and silicon manganese are added in two batches, and the materials used in the reduction and adjustment stage are controlled to be less than 7ppm while the oxygen content of the molten steel is ensured to be less than 25ppm; step 3: 3-5h before smelting, two ladles for baking raw materials are prepared for baking reduction and adjustment period materials, and 3.0-4.0t lime, 3.5-4.0t fluorite, 5.0-6.0 t ferrosilicon, 3.8-4.5t silicon manganese, 1.5-2.0t lime and 0.8-1.2t fluorite are added to the ladle of the baking adjustment period material. After the materials are added, the ladle is hoisted to the ladle baker for baking. The material baking time is 2-3h; Step 4: 30-60min before reduction, the reduction period and adjustment period materials baked in the ladle are turned into a special material trough. After the AOD process enters the reduction stage, the baked reduction material is added into the furnace using the material trough. The reduction alkalinity is controlled at 1.85-2.0, and the alkalinity in the adjustment period furnace is controlled at 2.3-2.4; Step 5: After AOD steel is discharged, it enters the LTS process for slagging operation, and the slag thickness is 150-200mm. After slagging, it enters the LF process for refining. During the process, no lime is added to adjust the slag alkalinity to avoid adding auxiliary materials to increase H during the refining process.

[0006] In step 2, the baking time of lime, fluorite, ferrosilicon and silicon manganese is 2-3 hours, and the temperature is 550-650°C.

[0007] The beneficial effects of the present invention are: 1. By reducing the addition of auxiliary materials such as lime in the low-carbon zone during the AOD smelting process, hydrogenation at the end of oxidation is avoided.

[0008] 2. Use a ladle to bake the reduced material to fully remove moisture from the material, ensure that the material used for reduction is completely dry, and avoid moisture brought into the material during the reduction stage, which may lead to hydrogenation of the molten steel. Through material baking, the hydrogen content of the duplex steel ingot is stably controlled below 7ppm, and there is no bubble defect after rolling.

[0009] 3. During the adjustment phase, 2000 kg of baked lime and 800 kg of fluorite were added into the furnace to stably control the basicity of steel tapping in the AOD process at 2.3-2.4. The slag had good deoxidation capacity, the oxygen content of the ingot was stably controlled below 25 ppm, and the edge crack defects of the duplex steel were reduced by 30%. DETAILED DESCRIPTION

[0010] A method for controlling hydrogen in high-nitrogen stainless steel is provided by using ladle baking reduction and adjusting period materials to fully remove moisture from the materials, and at the same time, no lime, fluorite and other auxiliary materials are added in the low-carbon zone and refining process during the smelting process, and the source of hydrogen in the smelting process is strictly controlled to achieve the purpose of controlling the hydrogen content of high-nitrogen steel grades, improve defects such as bubbles and cracks in high-nitrogen stainless steels such as duplex steel, and improve the product yield rate.

[0011] 1. During the smelting of high nitrogen stainless steel such as duplex steel, when the carbon content of the molten pool in the AOD process is less than 0.5%, lime is no longer added to the high-level silo during the oxidation period to prevent the addition of lime and other auxiliary materials during the low-carbon period to increase hydrogen. During the blowing process, small dry scrap steel in the high-level silo is used to balance the process temperature to ensure a normal decarburization rate.

[0012] 2. In order to improve the basicity of slag, ensure the good deoxidation ability of slag, and reduce the oxygen and hydrogen content of molten steel, baked adjustment period materials (lime, fluorite, ferrosilicon and other raw materials) are added during the reduction and adjustment stage of molten steel to remove moisture from raw and auxiliary materials and prevent hydrogen from increasing during the reduction stage. While controlling the hydrogen content of molten steel, the oxygen content of molten steel is guaranteed to be less than 25ppm to reduce cracking defects.

[0013] 3. 3 hours before smelting, prepare two ladles for baking raw materials for baking reduction and adjustment period materials. Add 3.0t lime, 3.5t fluorite, 5.0t ferrosilicon and 4.5t silicon manganese into the ladle for baking reduction materials. Add 2.0t lime and 0.8t fluorite into the ladle for baking adjustment period materials. After the materials are added, lift the ladle to the ladle baking machine for baking. The material baking time is more than 2h.

[0014] 4. 30 minutes before reduction, turn the materials in the reduction period and adjustment period baked in the ladle into the special trough. After the AOD process enters the reduction stage, add the baked reduction materials into the furnace using the trough. The reduction alkalinity is controlled at 1.85-2.0 to ensure a good reduction effect. The alkalinity in the furnace during the adjustment period is controlled at 2.3-2.4. 5. After AOD steel is tapped, it enters the LTS process for slag removal, with a slag thickness of 150-200mm. During the refining process, no lime is added to adjust the slag basicity. Example 1

[0015] Steel grade S32205; Smelting process: Use EAF+IF melt to smelt S32205 in AOD.

[0016] 1. The medium frequency furnace melts 75 tons of high chromium, and the electric furnace process melts 110 tons of chromium-nickel pig iron, and completes the dephosphorization operation. After the medium frequency steel is tapped, the pre-melted liquid is transferred to the electric furnace for steel tapping. The mass percentage of the pre-melted liquid after the medium frequency furnace and the electric furnace are mixed is: C: 3.37%, Si: 0.816%, Cr: 23.68%, Ni: 5.50%.

[0017] 2. During the AOD desiliconization period, 5070 kg of lime and 2000 kg of high chromium are added, and 1272 Nm³ of oxygen is blown before slag dumping.

[0018] 3. After slag pouring, the main decarburization period begins. During the main decarburization period, 16.1 tons of lime, 10.1 tons of ferromolybdenum, 8 tons of nickel scrap steel, and 8 tons of chromium scrap steel are added to balance the process temperature, adjust the alkalinity of the slag during the decarburization period, and ensure normal molten pool temperature and decarburization speed.

[0019] 4. When the total oxygen blowing volume reaches 6976Nm³ and the carbon content of the molten steel reaches 0.6%, the addition of lime to the high-level silo is stopped. The total oxygen blowing volume is 7669Nm³. When the carbon content is calculated to be 0.38%, the top gun oxygen supply is stopped. In the subsequent blowing process, the temperature of the molten pool is balanced by adding dry chromium scrap steel to avoid excessively high molten pool temperature, which may lead to deterioration of the slag condition in the furnace and affect the decarbonization rate.

[0020] 5. After the oxygen blowing volume reaches 11161Nm³, it enters the reduction period. Before reduction, the material trough is used to add the baking reduction materials (3.0t lime, 3.5t fluorite, 5.0t ferrosilicon, 4.5t silicon manganese). After 12 minutes of reduction, the slag is poured and samples are taken to confirm the composition of the molten steel in the furnace.

[0021] 6. After the analysis results of the reduction sample come out, use the material trough to add the baked adjustment period materials (2.0t lime, 0.8t fluorite) and make fine adjustments to the chromium, nickel and other components.

[0022] 7. After steel is tapped, LTS performs slag removal, leaving slag of 150-200mm, and transports it to the LF process for refining and adjusting the temperature of the molten steel.

[0023] 8. After continuous casting, the ingot was sampled and analyzed, and the hydrogen content was 7.1ppm, the oxygen content was 22ppm, and there was no porosity defect or fission defect after rolling. Example 2

[0024] Steel grade S32205; Smelting process: Use EAF+IF melt to smelt S32205 in AOD.

[0025] 1. The medium frequency furnace melts 74.8 tons of high chromium, and the electric furnace process melts 109 tons of chromium-nickel pig iron, and completes the dephosphorization operation. After the medium frequency steel is tapped, the pre-melted liquid is transported to the electric furnace for steel tapping. The mass percentage of the pre-melted liquid after the medium frequency furnace and the electric furnace are mixed is: C: 3.32%, Si: 0.796%, Cr: 23.52%, Ni: 5.47%.

[0026] 2. During the AOD desiliconization period, 4895 kg of lime and 2075 kg of high chromium were added, and 1235 Nm³ of oxygen was blown before slag dumping.

[0027] 3. After slag pouring, the main decarburization period begins. During the main decarburization period, 15.7 tons of lime, 10.0 tons of ferromolybdenum, 9.2 tons of nickel scrap steel, and 7.5 tons of chromium scrap steel are added to balance the process temperature, adjust the alkalinity of the slag during the decarburization period, and ensure normal molten pool temperature and decarburization speed.

[0028] 4. When the total oxygen blowing volume reaches 6869Nm³ and the carbon content of the molten steel is calculated to reach 0.6%, the addition of lime to the high-level silo is stopped. The total oxygen blowing volume is 7598Nm³. When the carbon content is calculated to be 0.37%, the top gun oxygen supply is stopped. In the subsequent blowing process, the temperature of the molten pool is balanced by adding dry chromium scrap steel to avoid excessively high molten pool temperature, which may lead to deterioration of the slag condition in the furnace and affect the decarbonization rate.

[0029] 5. After the oxygen blowing volume reaches 11095Nm³, it enters the reduction period. Before reduction, the material trough is used to add the baking reduction materials (2.9t lime, 3.6t fluorite, 4.9.0t ferrosilicon, 4.4t silicon manganese). After 12 minutes of reduction, the slag is poured and samples are taken to confirm the composition of the molten steel in the furnace.

[0030] 6. After the analysis results of the reduction sample come out, use the material trough to add the baked adjustment period materials (1.9t lime, 0.7t fluorite) and make fine adjustments to the chromium, nickel and other components.

[0031] 7. After steel is tapped, LTS performs slag removal, leaving slag of 150-200mm, and transports it to the LF process for refining and adjusting the temperature of the molten steel.

[0032] 8. After continuous casting, the ingot was sampled and analyzed, and the hydrogen content was 6.6ppm, the oxygen content was 21ppm, and there was no porosity defect or fission defect after rolling.

[0033] 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 all covered by the patent scope of the present invention.

Claims

1. A method for controlling hydrogen in high nitrogen stainless steel, characterized in that: The following steps are involved: Step 1: During the smelting process of high nitrogen stainless steel, when the carbon content of the molten pool is less than 0.5%, no more lime is added to the high-level silo, and the dry small scrap steel in the high-level silo is used to balance the process temperature of 1710-1740℃ during the blowing process; Step 2: During the steel reduction and adjustment stage, baked lime, fluorite, ferrosilicon, and silicon manganese are added in two batches. While controlling the hydrogen content of the steel liquid to less than 7ppm, the oxygen content of the steel liquid is ensured to be less than 25ppm. Step 3: 3-5 hours before smelting, prepare two ladles for baking raw materials for baking reduction and adjustment period materials. Add 3.0-4.0t lime, 3.5-4.0t fluorite, 5.0-6.0t ferrosilicon and 3.8-4.5t silicon manganese into the baking reduction material ladle. Add 1.5-2.0t lime and 0.8-1.2t fluorite into the baking adjustment period material ladle. After the materials are added, lift the ladle to the ladle baking machine for baking. The material baking time is 2-3h. Step 4: 30-60 minutes before reduction, turn the reduction period and adjustment period materials baked in the ladle into a special trough. After the AOD process enters the reduction stage, add the baked reduction materials into the furnace using the trough. The reduction alkalinity is controlled at 1.85-2.0, and the alkalinity in the furnace during the adjustment period is controlled at 2.3-2.4; Step 5: After AOD is tapped, it enters the LTS process for slag removal, with a slag thickness of 150-200mm. After slag removal, it enters the LF process for refining. During the process, lime is no longer added to adjust the slag basicity, avoiding the addition of auxiliary materials to increase H during the refining process.

2. A method for controlling hydrogen in high nitrogen stainless steel according to claim 1, characterized in that: The method comprises the following steps: in step 2, the baking time of lime, fluorite, ferrosilicon and silicon manganese is 2-3 hours and the temperature is 550-650°C.