Method for carrying out deep decarburization on ultra-pure ferritic stainless steel in AOD (Argon Oxygen Decarburization) furnace

Abstract

The invention discloses a method for performing deep decarburization on ultra-pure ferritic stainless steel in an AOD (argon oxygen decarburization) furnace. The invention belongs to the field of steelmaking, and relates to a method for deeply decarburizing ultra-pure ferritic stainless steel in the refining process of an AOD furnace and the ultra-pure ferritic stainless steel, that is, iron ore (Fe2O3 is more than or equal to 60%) is creatively added after oxygen blowing is finished and before reduction, so that iron oxide in the iron ore is molten into molten steel and further reacts with carbon element in the molten steel, and the decarburization of the ultra-pure ferritic stainless steel is realized. And the purpose of deep decarburization is achieved. Through the practice of the invention, the adding amount, adding time and inert gas stirring strength of the iron ore are determined, and a set of technological parameters for deep decarburization after the iron ore is added are designed. After the method is implemented, the content of the carbon element is greatly reduced, direct production of the ultra-pure ferritic stainless steel without VOD furnace refining and AOD furnace refining is realized, and the smelting process cost of the ultra-pure ferritic stainless steel is reduced.

Description

Technical Field

[0001] The invention belongs to the field of steelmaking, and in particular relates to a method for deep decarburization of ultrapure ferrite stainless steel in an AOD furnace. Background Art

[0002] Carbon will form carbides with chromium in stainless steel. The higher the carbon content, the more chromium carbides will be formed. The lower the chromium content in the solid solution, the lower the electrode potential of the steel, and the lower the corrosion resistance of the steel. Therefore, stainless steel generally requires a lower carbon content. In addition, stainless steel often suffers from corrosion damage along the grain boundaries, which is called intergranular corrosion. It is generally believed that intergranular corrosion is the process of carbon being removed from the saturated crystal structure in the form of Cr. 23 The precipitation of C6 morphology makes the structure at the grain boundary chromium-poor. Therefore, avoiding chromium-poor grain boundaries is an effective way to prevent intergranular corrosion. Titanium and niobium have a greater affinity with carbon than chromium. After adding them to steel, carbon preferentially combines with them to form titanium carbide and niobium carbide, thus avoiding the precipitation of chromium carbide and causing chromium-poor grain boundaries, thereby effectively preventing intergranular corrosion.

[0003] Ultrapure ferritic stainless steel is a ferritic stainless steel with extremely low carbon and nitrogen content. Titanium alloy and niobium alloy are added for alloying during the smelting process. It has excellent corrosion resistance and processing performance and is widely used in automobile exhaust systems, home appliance decoration industries, etc. Its carbon content is generally less than or equal to 0.02% or less than or equal to 0.03%. In the actual production process, the carbon content needs to be controlled within 100ppm.

[0004] Under conventional processes, the AOD refining furnace cannot reduce the carbon content of ultra-pure ferritic stainless steel to below 100ppm, and secondary refining in a VOD furnace is required. Taking ultra-pure ferritic stainless steel SUH409L as an example, after refining in the AOD furnace, it is necessary to refining in the VOD furnace. The carbon content of the molten steel can be reduced to 50-80ppm, with an average of 60ppm. The molten steel is then subjected to the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel in the finished steel reaches 60-100ppm, with an average of 70ppm. Taking ultra-pure ferritic stainless steel TTS443 as an example, after refining in the AOD furnace, it is necessary to refining in the VOD furnace. The carbon content of the molten steel can be reduced to 60-100ppm, with an average of 70ppm. The molten steel is then subjected to the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel in the finished steel reaches 70-120ppm, with an average of 80ppm. Therefore, the smelting process of ultra-pure ferritic stainless steel is expensive, and it is necessary to develop a method for directly producing ultra-pure ferritic stainless steel by refining in the AOD furnace without refining in the VOD furnace.

[0005] In the refining process of ultra-pure ferritic stainless steel in the AOD furnace, a method for deep decarburization of ultra-pure ferritic stainless steel is involved, that is, after the oxygen blowing is completed and before reduction, iron ore (Fe2O3 ≥ 60%) is innovatively added to make the iron oxide in the iron ore melt into the molten steel and further react with the carbon element in the molten steel to achieve the purpose of deep decarburization. Through the practice of the invention, the amount of iron ore added, the addition time, and the stirring intensity of the inert gas are determined, and a set of process parameters for deep decarburization after adding iron ore are designed. After the implementation of the invention, after the ultra-pure ferritic stainless steel SUH409L is refined in the AOD furnace, the carbon content of the molten steel is reduced to 50-80ppm, with an average of 60ppm. The molten steel is then passed through the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel of the finished steel reaches 60-100ppm, with an average of 70ppm. After the AOD furnace refining of ultra-pure ferritic stainless steel TTS443, the carbon content of molten steel is reduced to 60-100ppm, with an average of 70ppm. The molten steel is then subjected to LF furnace, continuous casting or die casting, and the carbon content of the finished steel reaches 70-120ppm, with an average of 80ppm. The carbon content is greatly reduced, and ultra-pure ferritic stainless steel is directly produced by AOD furnace refining without VOD furnace refining, which reduces the cost of ultra-pure ferritic stainless steel smelting process. Summary of the invention

[0006] The object of the present invention is to provide a method for deep decarburization of ultrapure ferritic stainless steel in an AOD furnace in view of the above problems.

[0007] The object of the present invention is achieved as follows: A method for deep decarburization of ultrapure ferritic stainless steel in an AOD furnace, comprising the following steps: Step 1: Blow oxygen in the AOD furnace until the carbon content is within 200ppm, after the oxygen blowing is completed, before reduction, add 300-500kg of iron ore, after adding the iron ore, blow inert gas at the bottom of the AOD furnace with an intensity of 0.3-0.5Nm 3 / (min*t), continue stirring for 3-5min; Step 2: Blow inert gas at the bottom of AOD furnace with an intensity of 0.3-0.5Nm 3 / (min*t), after continuous stirring for 3-5 minutes, the intensity of the inert gas blowing at the bottom of the AOD furnace is increased to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8min; Step 3: Increase the intensity of inert gas blowing at the bottom of the AOD furnace to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8 minutes, and then carry out the reduction operation.

[0008] The method relates to an ultra-pure ferritic stainless steel of SUH409L steel grade, whose steel grade components are C≤0.030%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.030%, Cr: 10.5-11.75%, Ti: 6*C~0.85%, Ni≤0.60%, N≤0.030%; and another ultra-pure ferritic stainless steel of TTS443 steel grade, whose steel grade components are C≤0.020%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.015%, Cr: 20.5-23.0%, Cu: 0.30-0.80%, Ti, Nb or a combination thereof: 10*(C+N)~0.80%, N≤0.020%, and the rest are Fe and unavoidable impurities.

[0009] The beneficial effects of the present invention are as follows: after the implementation of the invention, the carbon content of the molten steel of the ultra-pure ferritic stainless steel SUH409L is reduced to 50-80ppm, 60ppm on average, after the AOD furnace refining is completed, the molten steel is then subjected to the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel of the finished steel reaches 60-100ppm, 70ppm on average. After the AOD furnace refining of the ultra-pure ferritic stainless steel TTS443 is completed, the carbon content of the molten steel is reduced to 60-100ppm, 70ppm on average, after the molten steel is then subjected to the LF furnace, continuous casting or mold casting, the carbon content of the molten steel of the finished steel reaches 70-120ppm, 80ppm on average. The carbon content is greatly reduced, and the ultra-pure ferritic stainless steel is directly produced by AOD furnace refining without VOD furnace refining, thereby reducing the cost of the ultra-pure ferritic stainless steel smelting process. DETAILED DESCRIPTION

[0010] Under conventional processes, the AOD refining furnace cannot reduce the carbon content of ultra-pure ferritic stainless steel to below 100ppm, and secondary refining in a VOD furnace is required. Taking ultra-pure ferritic stainless steel SUH409L as an example, after refining in the AOD furnace, it is necessary to refining in the VOD furnace. The carbon content of the molten steel can be reduced to 50-80ppm, with an average of 60ppm. The molten steel is then subjected to the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel in the finished steel reaches 60-100ppm, with an average of 70ppm. Taking ultra-pure ferritic stainless steel TTS443 as an example, after refining in the AOD furnace, it is necessary to refining in the VOD furnace. The carbon content of the molten steel can be reduced to 60-100ppm, with an average of 70ppm. The molten steel is then subjected to the LF furnace, continuous casting or mold casting, and the carbon content of the molten steel in the finished steel reaches 70-120ppm, with an average of 80ppm. Therefore, the smelting process of ultra-pure ferritic stainless steel is expensive, and it is necessary to develop a method for directly producing ultra-pure ferritic stainless steel by refining in the AOD furnace without refining in the VOD furnace.

[0011] The purpose of the present invention is to design a method for deep decarburization of ultrapure ferritic stainless steel in an AOD furnace. In the AOD furnace, that is, after the oxygen blowing is completed and before reduction, iron ore (Fe2O3 ≥ 60%) is innovatively added to melt the iron oxide in the iron ore into the molten steel and further react with the carbon element in the molten steel to achieve the purpose of deep decarburization. Through the practice of the invention, the amount of iron ore added, the time of addition, and the intensity of inert gas stirring are determined, and a set of process parameters for deep decarburization after adding iron ore are designed. After the implementation of the invention, the carbon content is greatly reduced, and ultrapure ferritic stainless steel is not refined in a VOD furnace, but directly produced by AOD furnace refining, which reduces the cost of the ultrapure ferritic stainless steel smelting process.

[0012] The invention relates to an ultrapure ferritic stainless steel of SUH409L, wherein the steel composition is C≤0.030%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.030%, Cr: 10.5-11.75%, Ti: 6*C~0.85%, Ni≤0.60%, and N≤0.030%; and another ultrapure ferritic stainless steel of TTS443, wherein the steel composition is C≤0.020%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.015%, Cr: 20.5-23.0%, Cu: 0.30-0.80%, Ti, Nb or a combination thereof: 10*(C+N)~0.80%, and N≤0.020%.

[0013] The steps of the embodiment are as follows: The present invention designs ultra-pure ferritic stainless steel, blows oxygen in the AOD furnace until the carbon content is within 200ppm, and before the oxygen blowing is completed and the reduction is performed, 300-500kg of iron ore (Fe2O3≥60%) is added. After the iron ore is added, the iron oxide in the iron ore reacts violently with the carbon element in the molten steel, and the inert gas intensity of the AOD furnace bottom is 0.3-0.5Nm 3 / (min*t), stirring continuously for 3-5min. Inert gas blowing intensity at the bottom of AOD furnace is 0.3-0.5Nm 3 / (min*t), after continuous stirring for 3-5 minutes, the intensity of the inert gas blowing at the bottom of the AOD furnace is increased to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8min. The intensity of inert gas blowing at the bottom of AOD furnace is increased to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8 minutes, and then carry out the reduction operation. Example

[0014] The steel grade of this example is ultra-pure ferritic stainless steel of SUH409L, and its steel grade composition is C≤0.030%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.030%, Cr: 10.5-11.75%, Ti: 6*C~0.85%, Ni≤0.60%, N≤0.030%. The steps of the embodiment are as follows: (1) Blow oxygen in the AOD furnace until the carbon content is 152ppm. After the oxygen blowing is completed and before reduction, 412kg iron ore (Fe2O3: 81%) is added. After the iron ore is added, the inert gas intensity of the AOD furnace is blown to 0.38Nm3 / (min*t), and stirring is continued for 4.1min.

[0015] (2) The inert gas blowing intensity at the bottom of the AOD furnace was 0.38 Nm3 / (min*t). After continuous stirring for 4.1 min, the inert gas blowing intensity at the bottom of the AOD furnace was increased to 0.72 Nm3 / (min*t), and continuous stirring was continued for 6.5 min.

[0016] (3) The intensity of inert gas blowing at the bottom of the AOD furnace was increased to 0.72 Nm3 / (min*t), and after continuous stirring for 6.5 min, the reduction operation was carried out.

[0017] After AOD reduction, the carbon content of the molten steel is 61ppm. After the molten steel passes through the LF furnace and continuous casting, the carbon content of the finished steel is 73ppm.

[0018] Another example is ultra-pure ferritic stainless steel SUH409L, whose steel composition is C≤0.030%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.030%, Cr: 10.5-11.75%, Ti: 6*C~0.85%, Ni≤0.60%, N≤0.030%. The steps of the embodiment are as follows: (1) Blow oxygen in the AOD furnace until the carbon content is 152ppm. After the oxygen blowing is completed and before reduction, 386kg iron ore (Fe2O3: 76%) is added. After the iron ore is added, the AOD furnace bottom is blown with an inert gas intensity of 0.35Nm3 / (min*t), and stirring is continued for 3.7min.

[0019] (2) The inert gas blowing intensity at the bottom of the AOD furnace was 0.35 Nm3 / (min*t), and stirring was continued for 3.7 min. Then, the inert gas blowing intensity at the bottom of the AOD furnace was increased to 0.68 Nm3 / (min*t), and stirring was continued for 7.2 min.

[0020] (3) The intensity of inert gas blowing at the bottom of the AOD furnace was increased to 0.68 Nm3 / (min*t), and after continuous stirring for 7.2 min, the reduction operation was carried out.

[0021] After AOD reduction, the carbon content of the molten steel is 66ppm. After the molten steel passes through the LF furnace and continuous casting, the carbon content of the finished steel is 79ppm.

[0022] Another example is an ultra-pure ferritic stainless steel continuous casting slab of TTS443 steel, whose steel composition is C≤0.020%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.015%, Cr: 20.5-23.0%, Cu: 0.30-0.80%, Ti, Nb or their combination: 10*(C+N)~0.80%, N≤0.020%. The steps of the embodiment are as follows: (1) Blow oxygen in the AOD furnace until the carbon content is 168ppm. After the oxygen blowing is completed and before reduction, 456kg iron ore (Fe2O3: 85%) is added. After the iron ore is added, the AOD furnace bottom is blown with an inert gas intensity of 0.46Nm3 / (min*t), and stirring is continued for 4.5min.

[0023] (2) The inert gas blowing intensity at the bottom of the AOD furnace was 0.46 Nm3 / (min*t). After continuous stirring for 4.5 min, the inert gas blowing intensity at the bottom of the AOD furnace was increased to 0.76 Nm3 / (min*t), and continuous stirring was continued for 7.2 min.

[0024] (3) The intensity of inert gas blowing at the bottom of the AOD furnace was increased to 0.76 Nm3 / (min*t), and after continuous stirring for 7.2 min, the reduction operation was carried out.

[0025] After AOD reduction, the carbon content of the molten steel is 76ppm. After the molten steel passes through the LF furnace and continuous casting, the carbon content of the finished steel is 88ppm.

[0026] Another example is an ultra-pure ferritic stainless steel continuous casting slab of TTS443 steel, whose steel composition is C≤0.020%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.015%, Cr: 20.5-23.0%, Cu: 0.30-0.80%, Ti, Nb or their combination: 10*(C+N)~0.80%, N≤0.020%. The steps of the embodiment are as follows: (1) Blow oxygen in the AOD furnace until the carbon content is 176ppm. After the oxygen blowing is completed and before reduction, 450kg iron ore (Fe2O3: 83%) is added. After the iron ore is added, the AOD furnace bottom is blown with an inert gas intensity of 0.48Nm3 / (min*t), and stirring is continued for 4.6min.

[0027] (2) The inert gas blowing intensity at the bottom of the AOD furnace was 0.48 Nm3 / (min*t), and stirring was continued for 4.6 min. Then, the inert gas blowing intensity at the bottom of the AOD furnace was increased to 0.70 Nm3 / (min*t), and stirring was continued for 6.9 min.

[0028] (3) The intensity of inert gas blowing at the bottom of the AOD furnace was increased to 0.70 Nm3 / (min*t), and after continuous stirring for 6.9 min, the reduction operation was carried out.

[0029] After AOD reduction, the carbon content of the molten steel is 81ppm. After the molten steel passes through the LF furnace and continuous casting, the carbon content of the finished steel is 88ppm.

[0030] 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 deep decarburization of ultrapure ferritic stainless steel in an AOD furnace, characterized in that: The following steps are involved: Step 1: Blow oxygen into the AOD furnace until the carbon content is within 200ppm. After the oxygen blowing is completed, add 300-500kg of iron ore before reduction. After adding the iron ore, blow inert gas at the bottom of the AOD furnace with an intensity of 0.3-0.5Nm 3 / (min*t), continue stirring for 3-5 minutes; Step 2: Blow inert gas at the bottom of AOD furnace with an intensity of 0.3-0.5Nm 3 / (min*t), after continuous stirring for 3-5 minutes, the intensity of the inert gas blowing at the bottom of the AOD furnace is increased to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8min; Step 3: Increase the inert gas blowing intensity at the bottom of the AOD furnace to 0.5-0.8Nm 3 / (min*t), continue stirring for 5-8 minutes, and then carry out the reduction operation.

2. The method for deep decarburization of ultrapure ferritic stainless steel in an AOD furnace according to claim 1, characterized in that: The method relates to an ultra-pure ferritic stainless steel of SUH409L steel grade, wherein the steel grade composition is C≤0.030%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.030%, Cr: 10.5-11.75%, Ti: 6*C~0.85%, Ni≤0.60%, N≤0.030%; Another type of ultra-pure ferritic stainless steel involved is TTS443, whose steel composition is C≤0.020%, Si≤1.0%, Mn≤1.0%, P≤0.035%, S≤0.015%, Cr: 20.5-23.0%, Cu: 0.30-0.80%, Ti, Nb or their combination: 10*(C+N)~0.80%, N≤0.020%, and the rest is Fe and unavoidable impurities.