Low-aluminum low-sulfur high-chromium high-nitrogen pressure vessel steel and smelting method

Abstract

The application discloses a kind of low-aluminum low-sulfur high-chromium high-nitrogen series pressure vessel steel and smelting method, belong to steelmaking technical field, its chemical component and weight percentage are as follows: C:0.06-0.15%, Si:0.2-0.5%, Mn:0.3-0.6%, P≤0.025%, S≤0.003%, Alt 0.005-0.015%, Cr:8.0-13.5%, Mo:0.8-1.2%, V:0.15-0.3%, Nb:0.05-0.1%, N:0.03-0.08%, the balance is Fe and other inevitable impurities.Smelting method is converter smelting, LF refining, RH vacuum treatment and conventional continuous casting.The application does not need to increase equipment, ensures deep desulfurization under low Alt condition, optimizes LF refining parameter, smelts high N high Cr steel using LF furnace, stably controls each component, so that the composition of molten steel meets the requirements.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking technology, specifically relating to a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel and its smelting method. Background Technology

[0002] Low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel not only possesses high oxidation resistance and corrosion resistance in high-temperature steam environments, but also exhibits excellent impact toughness, high and stable creep ductility, and high thermal strength. Therefore, it is commonly used in high-temperature pressure vessel equipment. At operating temperatures below 620℃, the allowable stress of low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel is higher than that of austenitic stainless steel. At operating temperatures above 560℃, the designed allowable stress of low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel is approximately twice that of high-grade carbon tool steel. This superior performance allows it to support the operation of high-temperature superheaters and reheaters in subcritical and supercritical boilers (wall temperature ≤ 625℃), as well as high-temperature headers and steam pipes at wall temperatures ≤ 590℃. Furthermore, this steel can also be used in nuclear power plant heat exchangers and furnace tubes in petroleum cracking units.

[0003] The alumina (Al) content in steel should not be too high. When the alumina content exceeds 0.02%, brittle alumina inclusions are easily formed, reducing the steel's impact resistance and fatigue resistance, and promoting steel plate oxidation. During production, high aluminum content leads to nodule formation during casting and surface scaling on the billet, resulting in numerous surface defects in the finished product. Furthermore, during the recrystallization process in the finished product's annealing, high-alumina precipitates Al-N along subgrain boundaries or original grain boundaries. Excessive Al-N inhibition prevents the desired microstructure from being achieved during annealing and recrystallization. Moreover, when the aluminum content is high, it easily combines with dissolved oxygen in the ladle to form Al2O3, etc. These harmful inclusions significantly reduce the mechanical and weldability of the steel plate, resulting in low welding heat input and easy fracture in the weld zone. When Alt < 0.005%, deoxidation of the molten steel is insufficient, resulting in high oxygen content and the formation of numerous pores in the billet during casting. Therefore, the alumina content should be controlled between 0.005% and 0.015%.

[0004] Sulfur (S) is a harmful element in steel. It easily combines with manganese to form MnS inclusions, which reduces the low-temperature impact toughness of steel. Su is also prone to segregation and enrichment in steel, which reduces the corrosion resistance of steel. Therefore, it is necessary to control the S content in molten steel and keep it ≤ 0.003%.

[0005] Cr is the main alloying element in this steel, improving its hardness and wear resistance without making it brittle. It also provides good high-temperature oxidation resistance and resistance to oxidative corrosion, and increases the steel's hot strength. While increasing Cr content is beneficial for improving oxidation resistance, when its content exceeds a certain value, it can generate a highly coarsening Z phase during service and consume V and Nb elements in the steel. Furthermore, increased Cr content leads to the formation of more δ-ferrite phase. Therefore, it is necessary to control the Cr content in the steel, ideally between 8.0% and 13.5%.

[0006] The nitrogen (N) content is controlled between 0.03% and 0.08%. The role of N in steel is mainly reflected in two aspects: firstly, it acts as a solid solution strengthening agent. Because N has very low solubility in steel at room temperature, during the post-weld heat-affected zone and post-weld heat treatment, the solid solution and precipitation of nitrogen (VN) will occur successively in the microstructure, enhancing the stability of the microstructure, making it dense and solid, and improving the creep strength of the heat-affected zone. Secondly, it acts as a dispersion strengthening agent. The nitrides formed by the combination of trace element Al and N in steel can improve the strength, hardness, wear resistance, and corrosion resistance of the steel.

[0007] Patent document CN103643117B discloses an ultra-low aluminum steel and its smelting method. The steel has a tensile strength of 1290-1300 MPa, a yield strength of 945-950 MPa, an elongation of 15-15.5%, and an impact energy (Aku) of 52-54 J. The chemical composition of the ultra-low aluminum steel, by mass percentage, is: 0.12% ≤ C ≤ 0.18%, 0.95% ≤ Si ≤ 1.05%, 1.1... The composition is as follows: 5% ≤ Mn ≤ 1.25%, 0.02% ≤ Ti ≤ 0.03%, 0.10% ≤ Ni ≤ 0.20%, 0.20% ≤ Mo ≤ 0.30%, 0.02% ≤ V ≤ 0.03%, 0 ≤ P ≤ 0.010%, 0 ≤ S ≤ 0.010%, 0 ≤ Al ≤ 0.0015%, 0 ≤ O ≤ 0.0020%, 0 ≤ H ≤ 0.0010%, 0 ≤ N ≤ 0.0010%, with the balance being Fe. This invention employs a production process involving deep desaturation (S and P) in molten iron pretreatment, converter decarburization, silicon-manganese deoxidation and alloying during tapping, argon blowing for slag removal, deep desaturation (O, S, and P) in ladle refining with microalloying of elements such as Ti, Ni, Mo, and V, RH vacuum circulation degassing, soft argon blowing, and fully protected casting. The high sulfur content in this invention increases the brittleness of the steel; the absence of chromium means it cannot meet the requirements for use as a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel; and the low nitrogen content means it does not belong to the type of steel strengthened by nitrogen solution and is classified as a controlled element.

[0008] Patent document CN103060519.B discloses a deoxidation method for high-nitrogen steel, characterized in that the deoxidation process is carried out in a medium-frequency vacuum induction furnace, and the applicable alloy composition range is: Cr: 20-21%, Ni: 6-7%, Mn: 8-10%, Fe balance. Nitrogen addition to the molten steel is achieved through alloy nitriding. Aluminum is selected as the deoxidizer, and a continuous smelting process of 15-25 minutes is required after the addition of aluminum. This invention features high Cr, Ni, and Mn contents; furthermore, the deoxidation of the molten steel is carried out in a medium-frequency vacuum induction furnace.

[0009] Patent document CN108286015.A discloses a steel plate for pressure vessels and its production method. The chemical composition and weight percentage of the steel plate are as follows: C: 0.04-0.15%, Si: 0.13-0.45%, Mn: 0.35-0.73%, Cr: 0.74-1.21%, Mo: 0.40-0.65%, P≤0.007%, S≤0.007%, Ni: 0.15~0.30%, Cu≤0.20%, with the balance being Fe and unavoidable impurities. In this invention, the Cr content is low, the S content is high, and the Alt and N contents are not specifically specified, so it does not belong to the type of solid solution strengthening using N element. The process of this invention adopts converter smelting → LF furnace refining → VD furnace vacuum treatment → continuous casting billet → slab heating → hot rolling → heat treatment → finished product, which is a full-process production method. Its focus is on the control of steel rolling and heat treatment processes, and it belongs to the category of steel rolling.

[0010] Patent document CN111286677.A discloses an ultra-low sulfur, low aluminum, high nitrogen steel and its smelting method. The steel's composition and weight percentage content are as follows: C: 0.17-0.19%, Si: 0.26-0.40%, Mn: 1.6-1.7%, Alt: 0.005-0.020%, P≤0.010%, S≤0.0015%, Ni: 0.28-0.40%, V: 0.15-0.17%, N: 0.0100-0.0150%. The remainder consists of Fe and unavoidable impurities. The N content in this invention is 0.01-0.015%, which is relatively low, and no Cr is added. The smelting method is KR desulfurization → converter smelting → LF refining → RH vacuum treatment → conventional continuous casting billet. The nitrogen content in the steel is controlled only by continuously controlling the RH vacuum degree and vacuum time in three stages. However, the addition of KR desulfurization to the smelting process increases the process cost. In addition, this invention only increases N by controlling the vacuum degree and vacuum time in the RH process, and the nitrogen control cannot meet the product requirements.

[0011] Patent document CN100593578C discloses a method for smelting high-nitrogen steel, the smelting steps of which are: (1) melting molten steel in an electric arc furnace or induction furnace, (2) simultaneously melting a nitrogen-containing alloy in another electric arc furnace or induction furnace, matching the total composition of the molten steel in steps (1) and (2) with the steel grade, (3) adding the molten steel to the nitrogen-containing alloy liquid, and refining under nitrogen protection to obtain high-nitrogen steel. Although this method does not require complex pressurization and remelting equipment, the total smelting time is greatly shortened, only half the time of smelting high-nitrogen steel in the prior art. It is simple to operate, and the chemical composition of the steel is stable, with small fluctuations in the composition of the steel between furnaces. At the same time, it can reduce the amount of deoxidizing reducing agent and reduce smelting costs. However, it requires two electric arc furnaces or induction furnaces to melt molten steel and melt the nitrogen-containing alloy separately, which is complicated and costly. Summary of the Invention

[0012] The purpose of this invention is to provide a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel and a smelting method. While ensuring the content of each alloying element, it solves the problems of difficulty in ensuring deep desulfurization in LF furnaces under low Alt conditions and the inability to increase nitrogen and unstable composition control during the smelting of high-N and high-Cr steel in LF furnaces without adding equipment, so that the N content can be stably controlled at 0.03%-0.08%.

[0013] The objective of this invention can be achieved through the following technical solutions:

[0014] A low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel has the following chemical composition and weight percentage content: C content 0.06-0.15%, Si content 0.2-0.5%, Mn content 0.3-0.6%, P content ≤0.025%, S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-13.5%, Mo content 0.8-1.2%, V content 0.15-0.3%, Nb content 0.05-0.1%, N content 0.03-0.08%, with the balance being Fe and other unavoidable impurities;

[0015] Furthermore, the chemical composition and weight percentage content of the low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel are as follows: C content is 0.06-0.12%, Si content is 0.2-0.4%, Mn content is 0.3-0.6%, P content is ≤0.025%, S content is ≤0.003%, Alt content is 0.005-0.015%, Cr content is 8.0-10.5%, Mo content is 0.8-1.1%, V content is 0.16-0.25%, Nb content is 0.05-0.07%, N content is 0.03-0.06%, and the balance is Fe and other unavoidable impurities.

[0016] A smelting method for low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel includes the following steps:

[0017] S1. Converter smelting:

[0018] Molten iron and scrap steel are fed into a top-and-bottom blown (BOF) converter for smelting, with a molten iron to scrap steel ratio of 16±0.5 tons: 5±0.5 tons; oxygen lance blowing is performed, with an oxygen supply flow rate of 45000 Nm³. 3 / h, after blowing for 14 minutes, the lance is lifted. The final blowing temperature is ≤1650℃, and the final carbon content is ≤0.055% and the oxygen content is ≤800ppm. After slag removal, the molten steel is tapped. During tapping, lime is added sequentially for slag washing, followed by the addition of aluminum blocks for pre-deoxidation, and finally, low-carbon ferrochrome is added for alloying. Nitrogen is bottom-blown from the bottom of the ladle during tapping, with a nitrogen flow rate of 400Nm. 3 / h, molten steel is transferred to the argon station and top slag modifier is added to the top slag of the ladle to modify the top slag and obtain the required raw material molten steel;

[0019] S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, lime and low-silicon refining pre-melted slag are added in batches. After the raw steel is heated to ≥1605℃, aluminum blocks are added. While stirring to desulfurize, ferrochrome alloy is added in batches to obtain steel with S content ≤0.003%, Alt content 0.01-0.015%, Cr content 8.0-13.5%, and N content ≥0.03%.

[0020] S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. The circulating gas during the entire RH treatment process uses nitrogen blowing mode. The RH treatment lasts 25-35 minutes. For the first 10 minutes, a single-stage vacuum pump is used; for the following 15-25 minutes, a four-stage or five-stage pump is used. The circulating gas flow rate is 140 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h-180Nm 3 / h, after 2 minutes adjust the circulation flow rate to 180Nm 3 / h-220Nm 3 After processing for 10 minutes, a nitrogen-containing alloy is added to the molten steel. After circulating for 2 minutes, the vacuum pump is switched to a fourth or fifth stage pump for nitrogen addition. After adding nitrogen for 15-25 minutes, the vacuum is broken. After the RH treatment is completed, molten steel with S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-13.5%, and N content 0.03-0.08% is obtained.

[0021] S4. Conventional continuous casting: The billet is continuously cast into a billet according to conventional slab casting. The entire continuous casting process is protected to maintain the stability of the composition.

[0022] Furthermore, in S1, the amount of lime used is 1.7-2.0 kg / ton of molten steel, the amount of aluminum blocks used is 0.9-1.5 kg / ton of molten steel, the amount of low-carbon ferrochrome used is 44-47 kg / ton of molten steel, and the amount of top slag modifier used is 0.8-1.5 kg / ton of molten steel.

[0023] Furthermore, the chemical composition of the top slag modifier in S1 has the following weight percentages: Al content 35-50%, Al2O3 content 12-25%, CaO content 18-28%, SiO2 content 2.5-7.5%, and MgO content 1.5-5.5%.

[0024] Furthermore, in S2, the amount of lime used is 5-10 kg / ton of molten steel, the amount of low-silicon refining pre-melting slag used is 0.5-2.5 kg / ton of molten steel, the amount of aluminum blocks used is 0.4-0.8 kg / ton of molten steel, and the amount of ferrochrome alloy used is 91-98 kg / ton of molten steel.

[0025] Furthermore, the chemical composition of the low-silicon refined pre-melted slag in S2 has the following weight percentages: Al2O3 content 30-45%, CaO content 40-55%, SiO2 content 2-5%, and MgO content 1.5-10.5%.

[0026] Furthermore, the amount of nitrogen-containing alloy used in S3 is 1.2-4.2 kg / ton of molten steel.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. This invention provides a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel, grade GC4011A0, and its smelting method. The method controls the Alt, S, Cr, and N content in the molten steel through nitrogen blowing throughout the converter smelting process, converter top slag modification, LF furnace slag formation and desulfurization, LF furnace ferrochrome alloying, RH vacuum treatment with nitrogen blowing throughout the process, and nitrogen-containing alloying. This eliminates the need for additional equipment, ensuring the steel composition meets the requirements for low-aluminum, low-sulfur, high-chromium, and high-nitrogen steel. It guarantees deep desulfurization under low-Alt conditions and optimizes LF refining parameters, employing an LF furnace to smelt high-N, high-Cr steel and stably control each component.

[0029] 2. The smelting process of the present invention is simple, highly operable, effectively controls the refining cycle, and ensures stable production rhythm. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] A pressure vessel steel with low aluminum, low sulfur, high chromium, and high nitrogen content (steel grade GC4011A0) has the following chemical composition and weight percentage content: C content 0.06-0.12%, Si content 0.2-0.4%, Mn content 0.3-0.6%, P content ≤0.025%, S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-10.5%, Mo content 0.8-1.1%, V content 0.16-0.25%, Nb content 0.05-0.07%, N content 0.03-0.06%, with the balance being Fe and other unavoidable impurities.

[0032] A smelting method for a 210-ton-class low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel (steel grade GC4011A0) includes the following steps:

[0033] S1. Converter smelting:

[0034] 1) 160±5 tons of molten iron and 50±5 tons of scrap steel (containing 3500-3550 kg of ferromolybdenum) are fed into a BOF converter for smelting, with oxygen blowing via an oxygen lance at a flow rate of 45000 Nm³. 3 / h, after blowing for 14min, lift the gun, the final blowing temperature ≤1650℃, the final C content ≤0.055% and the O content ≤800ppm;

[0035] 2) After slag removal, the molten steel is tapped. During tapping, lime is added sequentially at a rate of 1.7-2.0 kg / ton of molten steel for slag washing, aluminum blocks are added at a rate of 0.9-1.5 kg / ton of molten steel for pre-deoxidation, and then low-carbon ferrochrome is added at a rate of 44-47 kg / ton of molten steel for alloying. Nitrogen is bottom-blown from the bottom of the ladle during tapping, with a nitrogen flow rate of 400 Nm³. 3 / h, molten steel is transferred to the argon station and top slag modifier (chemical composition by weight percentage: Al content 35-50%, Al2O3 content 12-25%, CaO content 18-28%, SiO2 content 2.5-5.5%, MgO content 1.5-5.5%) is added to the top slag at a rate of 0.8-1.5 kg / ton of molten steel to modify the top slag and obtain the required raw material molten steel;

[0036] S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, lime is added in batches at a rate of 5-10 kg / ton of steel, and low-silicon refining pre-melted slag (with a chemical composition of 30-45% Al2O3, 40-55% CaO, 2-5% SiO2, and 1.5-6.5% MgO by weight percentage) is added at a rate of 0.5-2.5 kg / ton of steel. After heating the raw steel to ≥1605℃, aluminum blocks are added at a rate of 0.4-0.8 kg / ton of steel. While stirring and desulfurizing, ferrochrome alloy is added in batches at a rate of 91-98 kg / ton of steel to obtain steel with S content ≤0.003%, Alt content 0.01-0.018%, Cr content 8.0-13.5%, and N content ≥0.03%.

[0037] S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. The circulating gas during the entire RH treatment process uses nitrogen blowing mode. The RH treatment lasts 25-35 minutes. For the first 10 minutes, a single-stage vacuum pump is used; for the following 15-25 minutes, a four-stage or five-stage pump is used. The circulating gas flow rate is 140 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h-180Nm 3 / h, after 2 minutes adjust the circulation flow rate to 180Nm 3 / h-220Nm 3 After processing for 10 minutes, nitrogen-containing alloy is added to the molten steel at a rate of 1.2-4.2 kg / ton of molten steel. After circulating for 2 minutes, the vacuum pump is switched to a fourth-stage or fifth-stage pump for nitrogen addition. After adding nitrogen for 15-25 minutes, the vacuum is broken. After the RH treatment is completed, molten steel with S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-13.5%, and N content 0.03-0.08% is obtained.

[0038] S4. Conventional Continuous Casting: The billet is continuously cast from conventional slabs. Protective casting is implemented throughout the continuous casting process to maintain the stability of the composition. The final steel billet contains 0.06-0.12% C, 0.2-0.4% Si, 0.3-0.6% Mn, ≤0.025% P, ≤0.003% S, 0.005-0.015% Alt, 8.0-10.5% Cr, 0.8-1.1% Mo, 0.16-0.25% V, 0.05-0.07% Nb, and 0.03-0.06% N, with the balance being Fe and other unavoidable impurities.

[0039] Example 1

[0040] A smelting method for a 210-ton-class low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel (steel grade GC4011A0) includes the following steps:

[0041] S1. Converter smelting:

[0042] 1) 160 tons of molten iron and 50 tons of scrap steel (containing 3500 kg of ferromolybdenum) are fed into a BOF converter for smelting, with oxygen blowing via an oxygen lance at a flow rate of 45000 Nm³. 3 / h, after blowing for 14 minutes, the gun was lifted. The final blowing temperature was 1613℃, and the final C content was 0.026% and the O content was 765ppm.

[0043] 2) After slag removal, the molten steel is tapped. During the tapping process, 400 kg of lime is added sequentially for slag washing, 300 kg of aluminum blocks are added for pre-deoxidation, and then 9827 kg of low-carbon ferrochrome is added for alloying. Nitrogen is bottom-blown from the bottom of the ladle at a flow rate of 400 Nm³. 3 / h, the temperature of the alloyed molten steel is 1522℃, the C content of the molten steel is 0.0283%, the molten steel is transferred to the argon station and 200kg of top slag modifier (chemical composition by weight percentage: Al content 39.63%, Al2O3 content 18.52%, CaO content 24.96%, SiO2 content 3.86%, MgO content 3.53%) is added to the top slag to modify the top slag and obtain the required raw material molten steel;

[0044] S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, a total of 1850 kg of lime and 215 kg of low-silicon refining pre-melted slag (with the following chemical composition by weight percentage: Al2O3 content 40.32%, CaO content 47.52%, SiO2 content 3.58%, MgO content 4.75%) are added in three batches. After heating the raw steel to ≥1605℃, 125 kg of aluminum blocks are added. While stirring and desulfurizing, 19583 kg of ferrochrome alloy is added in five batches to obtain steel with S content of 0.0017%, Alt content of 0.0155%, Cr content of 8.512%, and N content of 0.0345%.

[0045] S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. The circulating gas during the entire RH treatment process uses nitrogen blowing mode. The RH treatment lasts 28 minutes, with a single-stage vacuum pump for the first 10 minutes and a four-stage pump for the following 18 minutes. The circulating gas flow rate is 145 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h, after 2 minutes adjust the circulation flow rate to 190Nm 3After processing for 10 minutes, 615 kg of nitrogen-containing alloy was added to the molten steel. After circulating for 2 minutes, the vacuum pump was switched to a fourth-stage pump for nitrogen enrichment. After nitrogen enrichment for 18 minutes, the vacuum was broken. After the RH treatment was completed, molten steel with S content of 0.0023%, Alt content of 0.0137%, Cr content of 8.472%, and N content of 0.0374% was obtained.

[0046] S4. Conventional Continuous Casting: The billet is continuously cast from conventional slabs. Protective casting is implemented throughout the continuous casting process to maintain the stability of the composition. The final steel billet contains 0.0907% C, 0.324% Si, 0.474% Mn, 0.012% P, 0.0023% S, 0.0137% Alt, 8.472% Cr, 0.952% Mo, 0.206% V, 0.0638% Nb, and 0.0374% N, with the balance being Fe and other unavoidable impurities.

[0047] Example 2

[0048] A smelting method for 210 tons of low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel (grade GC4011A0) includes the following steps:

[0049] S1. Converter smelting:

[0050] 1) 162 tons of molten iron and 48 tons of scrap steel (containing 3550 kg of ferromolybdenum) were fed into a BOF converter for smelting, with oxygen blowing via an oxygen lance at a flow rate of 45000 Nm³. 3 / h, after blowing for 14 minutes, the gun was lifted. The final blowing temperature was 1625℃, and the final C content was 0.035% and the O content was 695ppm.

[0051] 2) After slag removal, the molten steel is tapped. During tapping, 400 kg of lime is added sequentially for slag washing, 250 kg of aluminum blocks are added for pre-deoxidation, and then 9397 kg of low-carbon ferrochrome is added for alloying. Nitrogen is bottom-blown from the bottom of the ladle at a flow rate of 400 Nm³. 3 / h, the temperature of the alloyed molten steel is 1537℃, the C content of the molten steel is 0.0379%, the molten steel is transferred to the argon station and 200kg of top slag modifier (chemical composition by weight percentage: Al content 42.62%, Al2O3 content 19.75%, CaO content 21.2%, SiO2 content 3.41%, MgO content 3.49%) is added to the top slag to modify the top slag and obtain the required raw material molten steel;

[0052] S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, a total of 2030 kg of lime and 198 kg of low-silicon refining pre-melted slag (with the following chemical composition by weight percentage: Al2O3 content 41.78%, CaO content 42.36%, SiO2 content 3.26%, MgO content 4.83%) are added in four batches. After heating the raw steel to ≥1605℃, 118 kg of aluminum blocks are added. While stirring and desulfurizing, 20353 kg of ferrochrome alloy is added in four batches to obtain steel with S content of 0.0013%, Alt content of 0.0175%, Cr content of 8.572%, and N content of 0.0379%.

[0053] S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. Nitrogen blowing is used for the entire RH treatment process. The RH treatment lasts 31 minutes, with a single-stage vacuum pump for the first 10 minutes and a five-stage pump for the remaining 23 minutes. The flow rate of the circulating gas is 160 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h, after 2 minutes adjust the circulation flow rate to 210Nm 3 After processing for 10 minutes, 608 kg of nitrogen-containing alloy was added to the molten steel. After circulating for 2 minutes, the vacuum pump was switched to a fourth-stage pump for nitrogen enrichment. After nitrogen enrichment for 22 minutes, the vacuum was broken. After the RH treatment was completed, molten steel with S content of 0.0019%, Alt content of 0.0145%, Cr content of 8.517%, and N content of 0.0396% was obtained.

[0054] S4. Conventional Continuous Casting: The billet is continuously cast from conventional slabs. Protective casting is implemented throughout the continuous casting process to maintain the stability of the composition. The final steel billet contains 0.0997% C, 0.284% Si, 0.514% Mn, 0.024% P, 0.0019% S, 0.0145% Alt, 8.517% Cr, 1.077% Mo, 0.184% V, 0.0541% Nb, and 0.0396% N, with the balance being Fe and other unavoidable impurities.

[0055] Comparative Example 1

[0056] The chemical composition and weight percentage content of steel grade GC4011A0 are as follows: C: 0.35-0.42%, Si: 0.3-0.50%, Mn: 0.2-0.5%, P≤0.025%, S≤0.005%, Alt: 0.01-0.03%, Cr: 12.2-13.5%, V: 0.05-0.08%, Nb: 0.01-0.02%, N≤0.03%.

[0057] A smelting method for 185-ton grade steel (steel grade GC4011A0) includes the following steps:

[0058] S1. Converter smelting:

[0059] 1) 150 tons of molten iron and 35 tons of scrap steel are fed into a BOF converter for smelting, with oxygen blowing via an oxygen lance at a flow rate of 45,000 Nm³. 3 / h, after blowing for 14 minutes, the gun was lifted. The final blowing temperature was 1640℃, and the final C content was 0.029% and the O content was 712ppm.

[0060] 2) After slag removal, the molten steel is tapped. During the tapping process, 400 kg of lime is added sequentially for slag washing, followed by 14,550 kg of low-carbon ferrochrome for alloying. Nitrogen is bottom-blown from the bottom of the ladle at a flow rate of 400 Nm³. 3 / h, the temperature of the alloyed molten steel is 1515℃, the C content of the molten steel is 0.198%, and the raw material molten steel is obtained;

[0061] S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, a total of 1685 kg of lime is added in three batches. After the raw steel is heated to ≥1600℃, 215 kg of aluminum blocks are added. While stirring to desulfurize, 37675 kg of ferrochrome alloy is added in five batches to obtain steel with an S content of 0.0035%, an Alt content of 0.00235%, a Cr content of 13.35%, and a N content of 0.0158%.

[0062] S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. The circulating gas during the entire RH treatment process uses nitrogen blowing mode. The RH treatment lasts 23 minutes, with a single-stage vacuum pump for the first 10 minutes and a three-stage pump for the remaining 13 minutes. The circulating gas flow rate is 75 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h, after 2 minutes adjust the circulation flow rate to 190Nm 3 After RH treatment, molten steel with S content of 0.0038%, Alt content of 0.0215%, Cr content of 13.125%, and N content of 0.0118% was obtained.

[0063] S4. Conventional Continuous Casting: The billet is continuously cast from conventional slabs. Protective casting is implemented throughout the continuous casting process to maintain the stability of the composition. The final steel billet contains 0.391% C, 0.462% Si, 0.357% Mn, 0.019% P, 0.0038% S, 0.0215% Alt, 12.925% Cr, 0.064% V, 0.0155% Nb, and 0.0118% N, with the balance being Fe and other unavoidable impurities.

[0064] As can be seen from Example 1, Example 2 and Comparative Example 1:

[0065] (1) Different steel composition systems have a wider requirement for S content in the comparative example, do not require extremely deep desulfurization, have a lower N content, have no lower limit requirement, and can be achieved by ordinary processes; while the Cr content in the example is higher, which increases the difficulty of stable control of LF furnace composition.

[0066] (2) Different converter smelting processes result in different effects of top slag modification;

[0067] (3) Different LF furnace processes: Comparative Example 1 does not use N-containing alloys to increase N, and the N content can only be controlled at a low level.

[0068] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0069] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for smelting low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel, characterized in that, Includes the following steps: S1. Converter smelting: Molten iron and scrap steel are fed into a BOF converter for smelting, with a molten iron to scrap steel ratio of 16±0.5 tons: 5±0.5 tons; oxygen lance blowing is performed, with an oxygen supply flow rate of 45000 Nm³. 3 / h, after blowing for 14 minutes, the lance is lifted. The final blowing temperature is ≤1650℃, and the final carbon content is ≤0.055% and the oxygen content is ≤800ppm. After slag removal, the molten steel is tapped. During tapping, lime is added sequentially for slag washing, followed by the addition of aluminum blocks for pre-deoxidation, and finally, low-carbon ferrochrome is added for alloying. Nitrogen is bottom-blown from the bottom of the ladle during tapping, with a nitrogen flow rate of 400Nm. 3 / h, molten steel is transferred to the argon station and top slag modifier is added to the top slag of the ladle to modify the top slag and obtain the required raw material molten steel; S2.LF Refining: The raw steel is transferred to the LF furnace for LF refining. During the LF refining process, lime and low-silicon refining pre-melted slag are added in batches. After the raw steel is heated to ≥1605℃, aluminum blocks are added. While stirring to desulfurize, ferrochrome alloy is added in batches to obtain steel with S content ≤0.003%, Alt content 0.01-0.015%, Cr content 8.0-13.5%, and N content ≥0.03%. S3. RH Vacuum Treatment: The molten steel is transferred to an RH furnace for RH vacuum treatment. The circulating gas during the entire RH treatment process uses nitrogen blowing mode. The RH treatment lasts 25-35 minutes. For the first 10 minutes, a single-stage vacuum pump is used, followed by a four-stage or five-stage pump for the next 15-25 minutes. The circulating gas flow rate is 140 Nm³ / min for the first 2 minutes after the RH vacuum main valve is opened. 3 / h-180Nm 3 / h, after 2 minutes adjust the circulation flow rate to 180Nm 3 / h-220Nm 3 After processing for 10 minutes, a nitrogen-containing alloy is added to the molten steel. After circulating for 2 minutes, the vacuum pump is switched to a fourth or fifth stage pump for nitrogen addition. After adding nitrogen for 15-25 minutes, the vacuum is broken. After the RH treatment is completed, molten steel with S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-13.5%, and N content 0.03-0.08% is obtained. S4. Conventional continuous casting: The billet is continuously cast into a billet according to the conventional slab casting method, and protective pouring is implemented throughout the continuous casting process; The chemical composition and weight percentage of the low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel are as follows: C content 0.06-0.15%, Si content 0.2-0.5%, Mn content 0.3-0.6%, P content ≤0.025%, S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-13.5%, Mo content 0.8-1.2%, V content 0.15-0.3%, Nb content 0.05-0.1%, N content 0.03-0.08%, with the balance being Fe and other unavoidable impurities. The amount of lime used in S1 is 1.7-2.0 kg / ton of molten steel, the amount of aluminum blocks is 0.9-1.5 kg / ton of molten steel, the amount of low-carbon ferrochrome is 44-47 kg / ton of molten steel, and the amount of top slag modifier is 0.8-1.5 kg / ton of molten steel. The chemical composition of the top slag modifier in S1 is as follows (by weight percentage): Al content 35-50%, Al2O3 content 12-25%, CaO content 18-28%, SiO2 content 2.5-7.5%, and MgO content 1.5-5.5%. The amount of lime used in S2 is 5-10 kg / ton of molten steel, the amount of low-silicon refining pre-melted slag is 0.5-2.5 kg / ton of molten steel, the amount of aluminum blocks is 0.4-0.8 kg / ton of molten steel, and the amount of ferrochrome alloy is 91-98 kg / ton of molten steel. The chemical composition of the low-silicon refined pre-melted slag in S2 is as follows (by weight percentage): Al2O3 content 30-45%, CaO content 40-55%, SiO2 content 2-5%, and MgO content 1.5-10.5%.

2. The smelting method for a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel according to claim 1, characterized in that, The chemical composition and weight percentage of the low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel are as follows: C content 0.06-0.12%, Si content 0.2-0.4%, Mn content 0.3-0.6%, P content ≤0.025%, S content ≤0.003%, Alt content 0.005-0.015%, Cr content 8.0-10.5%, Mo content 0.8-1.1%, V content 0.16-0.25%, Nb content 0.05-0.07%, N content 0.03-0.06%, with the balance being Fe and other unavoidable impurities.

3. The smelting method for a low-aluminum, low-sulfur, high-chromium, and high-nitrogen pressure vessel steel according to claim 1, characterized in that... The amount of nitrogen-containing alloy used in S3 is 1.2-4.2 kg / ton of molten steel.