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

Abstract

The invention discloses a method for performing deep decarburization on ultrapure ferritic stainless steel in a VOD (Vacuum Oxygen Decarburization) furnace, which comprises the following steps of: adding 50-300kg of iron ore with the Fe2O3 content of more than or equal to 60 percent into the VOD furnace within 3-8 minutes in the boiling period of the end of oxygen blowing of the VOD furnace under the conditions that the limit vacuum degree is less than or equal to 5mbar and the bottom blowing stirring strength of a steel ladle is 3.0-10.0 NL / (min * t); after the iron ore is added, the limit vacuum degree smaller than or equal to 5 mbar is kept, the steel ladle bottom blowing stirring strength is reduced to 2.0-5.0 NL / (min * t), and steel ladle bottom blowing stirring lasts for 2-5 min under the strength; and after steel ladle bottom blowing stirring is conducted for 2-5 min under the strength, the steel ladle bottom blowing stirring strength is increased to 3.0-10.0 NL / (min * t), and steel ladle bottom blowing stirring lasts for 5-10 min under the strength. According to the method, the iron ore is added at the ultimate vacuum degree in the boiling period after VOD oxygen blowing is finished, ferric oxide in the iron ore further reacts with carbon in molten steel, the content of the carbon in the ultra-pure ferritic stainless steel is greatly reduced, the corrosion resistance of the product is improved, and due to the fact that the adding amount of the titanium alloy and the niobium alloy is reduced, the corrosion resistance of the product is improved. And the production cost of the ultra-pure ferritic stainless steel is obviously reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of ferritic stainless steel production, and in particular relates to a method for deep decarburization of ultrapure ferritic stainless steel in a VOD 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, resulting in a lower chromium content in the solid solution, a lower electrode potential of the steel, and a lower 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, i.e., 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, so 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. The standard for the carbon content of ultrapure ferritic stainless steel is generally less than or equal to 0.02% or less than or equal to 0.03%, that is, w C ≤0.02% or w C ≤0.03%, the titanium content or niobium content or titanium content plus niobium content is generally greater than or equal to six times the carbon content or ten times the carbon content plus nitrogen content, that is, w Ti ≥6w C , w Nb ≥6w C ,(w Ti +w Nb )≥10(w C +w N ).

[0004] Under the conventional process of the prior art, after the ultra-pure ferritic stainless steel SUH409L is refined by the VOD furnace, the carbon content of the molten steel can be reduced to 50-100ppm, with an average of 70ppm. After the molten steel is further refined by the LF furnace, continuous casting or die casting, the carbon content of the molten steel of the finished steel reaches 60-150ppm, with an average of 100ppm; after the ultra-pure ferritic stainless steel TTS443 is refined by the VOD furnace, the carbon content of the molten steel can be reduced to 70-150ppm, with an average of 100ppm. After the molten steel is further refined by the LF furnace, continuous casting or die casting, the carbon content of the molten steel of the finished steel reaches 80-200ppm, with an average of 120ppm. However, since the carbon content is still at a relatively high level, it affects the corrosion resistance of the ultra-pure ferritic stainless steel. Therefore, it is necessary to further reduce the carbon content of the ultra-pure ferritic stainless steel through process innovation, improve the corrosion resistance, and reduce the alloy cost. Summary of the invention

[0005] In order to solve the technical problems existing in the above-mentioned prior art, the present invention provides a method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace, comprising:

[0006] During the boiling period when oxygen blowing of the VOD furnace is finished, 50 to 300 kg of iron ore with a Fe2O3 content of ≥60% is added to the molten steel in the VOD furnace under the condition of a vacuum degree of ≤5 mbar and a stirring intensity of 3.0 to 10.0 NL / (min×t) at the bottom of the ladle, wherein the addition of the iron ore is completed within 3 to 8 minutes;

[0007] After adding the iron ore, the vacuum degree is maintained at an ultimate vacuum degree of ≤5 mbar, and the ladle bottom blowing stirring intensity is reduced to 2.0-5.0 NL / (min×t). The ladle bottom blowing stirring is continued for 2-5 minutes at an intensity of 2.0-5.0 NL / (min×t);

[0008] After the ladle bottom blowing and stirring are carried out at an intensity of 2.0 to 5.0 NL / (min×t) for 2 to 5 minutes, the ladle bottom blowing and stirring intensity is increased to 3.0 to 10.0 NL / (min×t), and the ladle bottom blowing and stirring are continued at an intensity of 3.0 to 10.0 NL / (min×t) for 5 to 10 minutes.

[0009] As a specific implementation manner, the grade of the ultra-pure ferritic stainless steel is SUH409L, and its chemical composition is as follows by mass percentage: 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 the rest is Fe and unavoidable impurities.

[0010] As a specific implementation manner, the grade of the ultra-pure ferritic stainless steel is TTS443, and its chemical composition is as follows by mass percentage: 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%, N≤0.020%, Ti, Nb or a combination thereof: 10×(C+N)~0.80%, and the rest is Fe and unavoidable impurities.

[0011] The method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of the present invention has the following advantages and beneficial effects: by innovatively adding iron ore under extreme vacuum during the boiling period at the end of VOD oxygen blowing, the iron oxide in the iron ore is melted into molten steel and further reacts with the carbon element in the molten steel to achieve the purpose of deep decarburization, and by controlling the amount of iron ore added, the addition time and the stirring intensity of the ladle bottom blowing after the addition of the iron ore, the carbon content in the ultrapure ferritic stainless steel is greatly reduced, the corrosion resistance of the product is improved, and due to the reduction in the amount of titanium alloy and niobium alloy added, the production cost of the ultrapure ferritic stainless steel is significantly reduced, and the economy and competitiveness of the product are effectively improved. DETAILED DESCRIPTION

[0012] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0013] The present invention aims to reduce the carbon content of ultrapure ferritic stainless steel, improve the corrosion resistance of the product, and reduce the addition amount of titanium alloy and niobium alloy at the same time, effectively reducing the production cost by designing an innovative ultrapure ferritic stainless steel VOD deep decarburization method. The main technical measures of the method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace provided by the present invention are: in the boiling period at the end of oxygen blowing, under the extreme vacuum, iron ore (Fe2O3≥60%) is added to the molten steel, so that the iron oxide in the iron ore is melted into the molten steel and further reacts with the carbon element in the molten steel to achieve the purpose of deep decarburization.

[0014] Specifically, the method of deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of the present invention comprises:

[0015] During the boiling period when oxygen blowing of the VOD furnace is finished, 50 to 300 kg of iron ore with a Fe2O3 content of ≥60% is added to the molten steel in the VOD furnace under the condition of a vacuum degree of ≤5 mbar and a stirring intensity of 3.0 to 10.0 NL / (min×t) at the bottom of the ladle, wherein the addition of the iron ore is completed within 3 to 8 minutes;

[0016] After adding the iron ore, the vacuum degree is maintained at an ultimate vacuum degree of ≤5 mbar, and the ladle bottom blowing stirring intensity is reduced to 2.0-5.0 NL / (min×t). The ladle bottom blowing stirring is continued for 2-5 minutes at an intensity of 2.0-5.0 NL / (min×t);

[0017] After the ladle bottom blowing and stirring are carried out at an intensity of 2.0 to 5.0 NL / (min×t) for 2 to 5 minutes, the ladle bottom blowing and stirring intensity is increased to 3.0 to 10.0 NL / (min×t), and the ladle bottom blowing and stirring are continued at an intensity of 3.0 to 10.0 NL / (min×t) for 5 to 10 minutes.

[0018] The present invention is described in detail below in conjunction with specific embodiments. The implementation methods in the following embodiments without specifying specific conditions are carried out according to conventional methods and conditions, and the present invention does not make specific limitations on this.

[0019] Example 1

[0020] Embodiment 1 of the present invention is applied to VOD deep decarburization of ultrapure ferritic stainless steel of SUH409L steel grade, the chemical composition of the steel grade is as follows by mass percentage: 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 the rest is Fe and unavoidable impurities. The method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of Embodiment 1 is specifically implemented as follows:

[0021] (1) During the boiling period when oxygen blowing in the VOD furnace is finished, 70 kg of iron ore with a Fe2O3 content of 86% is added to the molten steel in the VOD furnace within 5 minutes while maintaining the ultimate vacuum degree of 3 mbar and the stirring intensity of the ladle bottom blowing of 5.0 NL / (min×t);

[0022] (2) After adding the iron ore, the vacuum is maintained at an ultimate vacuum of 4 mbar, and the ladle bottom blowing stirring intensity is reduced to 3.0 NL / (min×t), and this process lasts for 3 min;

[0023] (3) After the above process time of 3 minutes, the stirring intensity of the ladle bottom blowing is increased to 5.0NL / (min×t), and this process time lasts for 8 minutes.

[0024] Through the implementation of Example 1 of the present invention, after the VOD furnace refining is completed, the carbon content of the molten steel is 25ppm. After the molten steel is subsequently refined in the LF furnace and continuously cast, the carbon content of the molten steel in the finished steel is only 48ppm.

[0025] Example 2

[0026] Embodiment 2 of the present invention is applied to VOD deep decarburization of ultrapure ferritic stainless steel of SUH409L steel grade, the chemical composition of the steel grade is as follows by mass percentage: 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 rest is Fe and unavoidable impurities, and the method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of Embodiment 2 is specifically implemented as follows:

[0027] (1) During the boiling period when oxygen blowing in the VOD furnace is finished, 120 kg of iron ore with a Fe2O3 content of 82% is added to the molten steel in the VOD furnace within 3 minutes while maintaining the ultimate vacuum degree of 4 mbar and the ladle bottom blowing stirring intensity of 6.0 NL / (min×t);

[0028] (2) After adding the iron ore, the vacuum is maintained at an ultimate vacuum of 3 mbar, and the ladle bottom blowing stirring intensity is reduced to 2.0 NL / (min×t), and this process lasts for 3 min;

[0029] (3) After the above process time of 3 minutes, the stirring intensity of the ladle bottom blowing is increased to 6.0NL / (min×t), and this process time lasts for 9 minutes.

[0030] Through the implementation of Example 2 of the present invention, after the VOD furnace refining is completed, the carbon content of the molten steel is 31ppm. After the molten steel is subsequently refined in the LF furnace and continuously cast, the carbon content of the molten steel in the finished steel is only 51ppm.

[0031] Example 3

[0032] Embodiment 3 of the present invention is applied to VOD deep decarburization of ultrapure ferritic stainless steel of TTS443 steel grade, the chemical composition of the steel grade is as follows by mass percentage: 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%, N≤0.020%, Ti, Nb or a combination thereof: 10×(C+N)~0.80%, the rest is Fe and unavoidable impurities, and the method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of Embodiment 3 is specifically implemented as follows:

[0033] (1) During the boiling period when oxygen blowing in the VOD furnace is finished, 210 kg of iron ore with a Fe2O3 content of 92% is added to the molten steel in the VOD furnace within 8 minutes while maintaining the ultimate vacuum degree of 2 mbar and the ladle bottom blowing stirring intensity of 7.0 NL / (min×t);

[0034] (2) After adding the iron ore, the vacuum was maintained at an ultimate vacuum of 3 mbar, and the ladle bottom blowing stirring intensity was reduced to 3.0 NL / (min×t), and this process lasted for 4 min;

[0035] (3) After the above process time of 4 minutes, the stirring intensity of the ladle bottom blowing is increased to 6.0NL / (min×t), and this process time lasts for 9 minutes.

[0036] Through the implementation of Example 3 of the present invention, after the VOD furnace refining is completed, the carbon content of the molten steel is 46ppm. After the molten steel is subsequently refined in the LF furnace and continuously cast, the carbon content of the molten steel in the finished steel is only 63ppm.

[0037] Example 4

[0038] Embodiment 4 of the present invention is applied to VOD deep decarburization of ultrapure ferritic stainless steel of TTS443 steel grade, the chemical composition of the steel grade is as follows by mass percentage: 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%, N≤0.020%, Ti, Nb or a combination thereof: 10×(C+N)~0.80%, the rest is Fe and unavoidable impurities, and the method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of Embodiment 4 is specifically implemented as follows:

[0039] (1) During the boiling period when oxygen blowing in the VOD furnace is finished, 240 kg of iron ore with a Fe2O3 content of 90% is added to the molten steel in the VOD furnace within 6 minutes while maintaining the ultimate vacuum degree of 3 mbar and the ladle bottom blowing stirring intensity of 5.0 NL / (min×t);

[0040] (2) After adding the iron ore, the vacuum is maintained at an ultimate vacuum of 4 mbar, and the ladle bottom blowing stirring intensity is reduced to 4.0 NL / (min×t), and this process lasts for 5 min;

[0041] (3) After the above process time of 5 minutes, the stirring intensity of the ladle bottom blowing is increased to 8.0NL / (min×t), and this process time lasts for 9 minutes.

[0042] Through the implementation of Example 4 of the present invention, after the VOD furnace refining is completed, the carbon content of the molten steel is 55ppm. After the molten steel is subsequently refined in the LF furnace and continuously cast, the carbon content of the molten steel in the finished steel is only 72ppm.

[0043] In summary, by implementing the method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace of the present invention, after refining in the VOD furnace, the carbon content of the molten steel of the ultrapure ferritic stainless steel SUH409L is reduced to 10-50 ppm, with an average of 30 ppm. After the molten steel is further subjected to an LF furnace, continuous casting or mold casting, the carbon content of the molten steel of the finished steel reaches 20-60 ppm, with an average of 50 ppm. After refining in the VOD furnace, the carbon content of the molten steel of the ultrapure ferritic stainless steel TTS443 is reduced to 30-60 ppm, with an average of 50 ppm. After the molten steel is further subjected to an LF furnace, continuous casting or mold casting, the carbon content of the molten steel of the finished steel reaches 40-90 ppm, with an average of 70 ppm.

[0044] Therefore, the present invention innovatively adds iron ore under extreme vacuum during the boiling period at the end of VOD oxygen blowing, so that the iron oxide in the iron ore is melted into the molten steel and further reacts with the carbon element in the molten steel to achieve the purpose of deep decarburization, and by controlling the amount of iron ore added, the addition time and the stirring intensity of the ladle bottom blowing after the addition of the iron ore, the carbon content in the ultra-pure ferritic stainless steel is greatly reduced, the corrosion resistance of the product is improved, and due to the reduction in the amount of titanium alloy and niobium alloy added, the production cost of the ultra-pure ferritic stainless steel is significantly reduced, effectively improving the economy and competitiveness of the product.

[0045] It should be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such article or device.

[0046] Finally, it should be noted that 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace, characterized in that: include: During the boiling period when oxygen blowing of the VOD furnace is finished, 50 to 300 kg of iron ore with a Fe2O3 content of ≥60% is added to the molten steel in the VOD furnace under the condition of a vacuum degree of ≤5 mbar and a stirring intensity of 3.0 to 10.0 NL / (min×t) at the bottom of the ladle, wherein the addition of the iron ore is completed within 3 to 8 minutes; After adding the iron ore, the vacuum degree is maintained at an ultimate vacuum degree of ≤5 mbar, and the ladle bottom blowing stirring intensity is reduced to 2.0-5.0 NL / (min×t). The ladle bottom blowing stirring is continued for 2-5 minutes at an intensity of 2.0-5.0 NL / (min×t); After the ladle bottom blowing and stirring are carried out at an intensity of 2.0 to 5.0 NL / (min×t) for 2 to 5 minutes, the ladle bottom blowing and stirring intensity is increased to 3.0 to 10.0 NL / (min×t), and the ladle bottom blowing and stirring are continued at an intensity of 3.0 to 10.0 NL / (min×t) for 5 to 10 minutes.

2. The method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace according to claim 1, characterized in that: The ultrapure ferritic stainless steel is SUH409L, and its chemical composition is as follows by mass percentage: 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 the rest is Fe and unavoidable impurities.

3. The method for deep decarburization of ultrapure ferritic stainless steel in a VOD furnace according to claim 1, characterized in that: The ultra-pure ferritic stainless steel is of TTS443 grade, and its chemical composition is as follows by mass percentage: 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%, N≤0.020%, Ti, Nb or a combination thereof: 10×(C+N)~0.80%, and the rest is Fe and unavoidable impurities.