Smelting method of low-temperature austenitic steel

Abstract

According to the smelting method, a medium-frequency induction furnace is adopted for smelting, and nitrogen control, white slag making and soft blowing processes are carried out in a heat preservation smelting step; in the nitrogen control process, nitrogen nitride ferrochrome is added to serve as a nitrogen increasing agent for first nitrogen increasing, the nitrogen adding amount is increased according to the upper limit during nitrogen increasing, bottom blowing nitrogen serves as a protective atmosphere, and the nitrogen increasing time is controlled to range from 15 min to 20 min; in the white slag making process, after nitrogen is increased for the first time, 10-25 kg of small lime and 5-15 kg of fluorite are supplemented according to the slag amount for slagging, and 1-3 kg of aluminum particles and 2-15 kg of low-aluminum silicon iron are added for diffusion deoxidation. According to the method, a medium-frequency induction furnace is adopted as smelting equipment, the nitrogen control process is operated in a temperature interval under the nitrogen bottom blowing condition, [N] in molten steel is accurately controlled through boiling nitrogen increasing and slag layer covering, [O] and non-metallic inclusions in the molten steel are adsorbed through white slag making, the alloying rate is controlled through time, and the oxygen adsorption capacity is reduced.

Description

Technical Field

[0001] The invention relates to a smelting method, in particular to a smelting method for low-temperature austenitic steel. Background Art

[0002] Aiming at the national strategic needs and the major market demands, the company develops low-temperature austenitic steel for nuclear fusion devices. Low-temperature austenitic steel has sufficient strength and toughness in low-temperature environments, good corrosion resistance, and can adapt to low-temperature environments below -196°C; if ordinary steel is used in low-temperature environments, it is easy to reduce plastic toughness and brittle fracture, which poses a great threat to people and the environment and causes safety accidents.

[0003] The requirements for chemical elements in low-temperature steel are very strict, with a narrow composition range and low C content requirements, especially the gas content and steel purity requirements in low-temperature steel are almost harsh. The composition design of high-strength and high-toughness stainless steel materials should reduce the C content as much as possible and increase the content of Mn and Ni to ensure that the ductile-brittle transition temperature is effectively reduced; at the same time, it is necessary to consider adding appropriate amounts of alloying elements such as Nb, V, and N to the steel. Through the fine grain strengthening and precipitation strengthening effects produced by these elements, the strength of the steel is further improved, which also helps to improve the plasticity and toughness of the steel.

[0004] In the prior art, due to strict requirements and narrow range of composition, low-temperature austenitic steel needs to be refined, vacuum treated or electroslag remelted after smelting, which results in a complicated preparation process and high production cost. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for smelting low-temperature austenitic steel without refining.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: using a medium frequency induction furnace for smelting, and performing nitrogen control, white slag making and soft blowing processes in the heat preservation smelting step;

[0007] The nitrogen control process: adding ferrochrome nitride as a nitrogen increasing agent for the first nitrogen increase, adding nitrogen according to the upper limit during nitrogen increase, bottom blowing nitrogen as a protective atmosphere, and controlling the nitrogen increase time to be 15 to 20 minutes;

[0008] The white slag making process is as follows: after the first nitrogen addition, 10-25 kg of lime and 5-15 kg of fluorite are added according to the amount of slag to make slag, and 1-3 kg of aluminum particles and 2-15 kg of low-aluminum ferrosilicon are added to cooperate with diffusion deoxidation.

[0009] Furthermore, in the nitrogen control process, after the first nitrogen increase, the bottom blowing flow rate is adjusted to 40-50 L / min, and the black oxide slag in the upper slag layer is scraped out.

[0010] Furthermore, the total duration of the first nitrogen addition and deoxygenation is controlled within 30 to 50 minutes.

[0011] Furthermore, after soft blowing, additional alloy material is added based on the component sample analysis to determine whether to add nitrogen for the second time.

[0012] Furthermore, in the low-temperature austenitic steel, C≤0.02%, and N 0.2%-0.35%.

[0013] The beneficial effects of the above technical solution are as follows: the present invention adopts a medium frequency induction furnace as smelting equipment, the nitrogen control process is operated within a temperature range and under bottom blowing nitrogen conditions, nitrogen is increased by boiling, and the slag layer is covered to accurately control [N] in the molten steel, and [O] and non-metallic inclusions in the molten steel are adsorbed by making white slag, and the alloying rate is controlled by time to reduce the ability to adsorb oxygen. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0015] Figure 1 It is a process flow chart of the present invention;

[0016] Figure 2 It is a process flow chart of an embodiment of the present invention. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below in conjunction with specific implementation modes.

[0018] The invention discloses a smelting method for low-temperature austenitic steel. The components and mass percentages of the low-temperature austenitic steel are as follows: C≤0.02%, Si 0.5%-0.6%, Mn 5.5%-6.0%, P≤0.01%, S≤0.002%, Cr 21.5%-22.5%, Ni 14%-14.5%, Mo 1.9%-2.1%, V 0.2%-0.25%, Nb 0.08%-0.1%, N0.2%-0.35%, Al 0.010%-0.030%, and the balance is Fe and a small amount of impurities.

[0019] Figure 1 As shown, the smelting method of low-temperature austenitic steel adopts a medium-frequency induction furnace for smelting, including the steps of material distribution, power melting, heat preservation melting and steel pouring; the process of each step is as follows:

[0020] (1) Material laying step: first carry out material batching, and then lay the materials in the medium frequency induction furnace.

[0021] (1.1) Material preparation process: The raw materials include main raw materials and slag materials. The main raw materials include pure iron, metallic chromium, metallic manganese, low-aluminum ferrosilicon, nickel plates, molybdenum bars, ferrovanadium, niobium blocks and ferrochromium nitride. The iron content in the pure iron is ≥99.8wt%; the chromium content in the metallic chromium is ≥99.8wt%; the manganese content in the metallic manganese is ≥97.5wt%; the Si content in the low-aluminum ferrosilicon is ≥72.5wt%, and the Al content is ≥0.065wt%; the nickel content in the metallic nickel is ≥99.8wt%; the molybdenum content in the metallic molybdenum is ≥99.9wt%; the vanadium content in the ferrovanadium is ≥80wt%; the niobium content in the niobium block is ≥65wt%; the Al content in the aluminum particles is ≥99.5wt%; the nitrogen content in the ferrochromium nitride alloy is ≥8%, and the Cr content is ≥65%, and the particle size is between 5 and 20mm, with few impurities. The ingredients are prepared according to the target composition, wherein the amount of low-aluminum ferrosilicon added is prepared according to the ratio of 0.15% to 0.20% of the mass percentage of silicon in the steel grade. This can promote the removal of some [O] in the molten steel during the melting process. During the melting process, according to the oxidizability of the slag (the color of the slag: black, gray-black, white-ash), a partial aluminum content deoxidizer of 0.3kg / t to 1kg / t should be appropriately added to obtain the target [O] content of 20 to 40ppm in the melted steel.

[0022] The main raw material is kept at a temperature of 350°C or below for 4.5 hours or more before being laid, and then slowly dropped to 100°C or below after 2 hours or more. The slag-making material includes lime and fluorite, and the effective CaO in the lime is ≥80wt%; the amount of lime added is 10-20kg, and the amount of fluorite added is 5-10kg. The bottom slag material is baked at 100°C-150°C for 4 hours or more before being laid.

[0023] (1.2) Material distribution process: The main raw materials are added in batches and slag materials into the pre-cleaned medium frequency induction furnace crucible. The filling method is as follows: the first material distribution in the furnace is to add slag materials such as lime and fluorite, metal materials such as nickel plate, metal chromium with a chromium content ratio of 6% to 8%, metal manganese, low-aluminum ferrosilicon with a silicon content ratio of 0.15% to 0.20%, ferrovanadium, molybdenum bars, niobium blocks, and 800kg to 1000kg of pure iron. The material distribution should not exceed the upper part of the magnetic coil, and the material distribution should be loose at the top and tight at the bottom to prevent bridging and hanging bridges.

[0024] (2) Power supply melting: After the charging is completed, power supply and bottom blowing of nitrogen are started. In the early stage of power supply, low power supply is used, and then the power is gradually increased until the charge is melted. As the charge melts, the remaining pure iron is added. It is required to add a small amount in multiple batches, and the amount added in each batch is preferably 150kg to prevent hanging and crusting; power supply melting is used to add all the pure iron. During the melting process of the charge, slag-making materials including lime and fluorite are added again. The amount of slag is to foam the slag layer, so that the slag layer has a certain fluidity and good coverage, to ensure that the molten steel does not contact the air during the melting process, and at the same time, the oxidized black slag is indirectly scraped off during the melting process, so that the melted steel has a lower [O] content. After all the steel is melted, the temperature reaches 1560℃~1580℃, and the slag is sampled for testing and analysis. The melting process is based on the oxidizability of the slag, and the oxidizability of the slag is judged according to the color of the slag, whether it is black, gray-black or white-ash. A portion of aluminum deoxidizer 0.3kg / t to 1kg / t should be appropriately added to obtain the target [O] content requirement of 20 to 40ppm in the melted steel liquid. In the power supply melting step, the bottom blowing nitrogen flow rate is controlled to be 5L / min to 35L / min to ensure that small bubbles float up and part of the [O] in the molten steel liquid is removed. The steel liquid is prohibited from large tumbling and air inhalation. Due to the large power, a certain eddy current will be generated, which is also very likely to cause the molten steel liquid to absorb oxygen twice. The slag layer coverage should be increased in time, and the location of adding pure iron should be adjusted to control the eddy current and avoid oxygen absorption.

[0025] (3) Insulated smelting: including nitrogen control, white slag making and soft blowing process.

[0026] (3.1) Nitrogen control process: After the power supply melting step is completed, the nitrogen control and deoxidation step is entered, and the bottom blowing flow rate is increased to 25L / min~40L / min to ensure that small bubbles on the slag layer float and do not overturn. Add ferrochrome nitride as a nitrogen-increasing agent for the first nitrogen increase, and the nitrogen amount is supplemented according to the upper limit. The particle size of ferrochrome nitride is required to be 5mm~20mm. Ferrochrome nitride should be added in batches for multiple times, with an interval of 15s~1min between each batch (depending on the boiling of molten steel). Bottom blowing nitrogen is used as a protective atmosphere; the nitrogen increase time is controlled at 15~20min; the temperature range of the nitrogen increase process is controlled at 1550℃~1580℃, and the nitrogen absorption rate in this temperature range is relatively stable. The nitrogen amount is supplemented according to the upper limit during the first nitrogen increase, because the subsequent additional alloy will dilute a certain amount of nitrogen content, which can prevent secondary molten steel from being contaminated by multiple nitrogen supplements; after the nitrogen supplement is completed, the bottom blowing flow rate is adjusted to 40~50L / min for maximum tumbling and exhaust absorption, and the black oxide slag in the upper slag layer is scraped out at the same time.

[0027] (3.2) White slag making process: After the first nitrogen addition and slag removal, add 10-25kg of white ash and 5-15kg of fluorite according to the amount of slag for slag making, add 1-3kg of aluminum particles and 2-15kg of low-aluminum ferrosilicon for diffusion deoxidation, and appropriately add a small batch of ferrosilicon for white slag deoxidation. The amount of ferrosilicon added does not exceed the silicon component requirements, and it is best to increase the silicon content in the molten steel by 0.03% to 0.05%. The white slag should be formed quickly, and the time should not be prolonged to cause accidental air inhalation and nitrogen leakage. Always pay attention to the coverage of the slag layer, and replenish the slag in time until the slag becomes grayish white. The total time for the first nitrogen addition and deoxidation is 30-50 minutes.

[0028] (3.3) Soft blowing process: soft blowing is carried out after the white slag is made. During the soft blowing, the power is reduced to stabilize the temperature, and the temperature of the molten steel is controlled at 1550℃~1580℃; sample the composition for testing and analysis; observe the slag formation at any time and take samples for testing; add alloy materials according to the test results, and the alloy materials include metallic chromium and low-aluminum ferrosilicon; because the first nitrogen increase is added according to the upper limit composition, if the absorption is good and the sampling test results meet the nitrogen content requirements of the finished product, the second nitrogen increase is not required. If the sampling test results do not meet the nitrogen content requirements of the finished product, the second nitrogen increase is required; when the second nitrogen increase is required, the additional nitrided chromium iron is added together with the alloy material; after the alloy composition is qualified, soft blowing is carried out for 15 to 20 minutes to sample and release the steel.

[0029] (4) Steel tapping and pouring steps: Before tapping, hang carbon-free protective slag in the ingot mold, with a carbon-free protective slag dosage of 0.8kg to 1kg / t, and hang it 200 to 400mm from the bottom of the ingot mold; during the tapping process, the tapping port should be kept round and not scattered, and the speed should be fast to prevent secondary air inhalation; the pouring temperature should be 70 to 80°C above the liquidus line, and during the pouring process, a protective gas atmosphere with a pressure of 0.05MPa to 0.20MPa around the nozzle should be maintained; argon protection pouring is used for pouring, and the pressure around the nozzle should be maintained at 0.05MPa to 0.20MPa during protective pouring to prevent tertiary air inhalation; full flow casting is performed after pouring begins, and the pouring speed is stabilized after the steel flow is stable and the molten steel flows into the ingot mold. In the later stage of casting, pay close attention to the steel flow and the amount of molten steel in the ladle, do a good job of shrinkage compensation at the riser, and reasonably control the injection surplus; after the casting is completed, it is strictly forbidden to add exothermic agents and carbonized rice husks into the ingot mold; the steel ingot is taken out after it is completely solidified in the ingot mold.

[0030] Example: Figure 2 As shown, this example uses the following steps.

[0031] (1) Material distribution steps: First, add 20kg of lime and 5kg of fluorite to the bottom of the medium frequency induction furnace; start preparing the alloy material at 12:20, and add the prepared alloy material into the furnace. The amount of alloy added is shown in Table 1:

[0032] Table 1: Alloy addition amount (kg)

[0033] Low Aluminum Ferrosilicon Chromium Manganese Metal Nickel Plate Molybdenum Bar Ferrovanadium Niobium Block Pure Iron 13.2 300 145.5 350.5 52.1 9.8 2.6 1350

[0034] (2) Power melting: 300 kg of metallic chromium, 1 / 3 or 100 kg was added first; 1350 kg of pure iron, 800 kg was added first; at 12:40, power was turned on for melting. When the liquid level melted to 1 / 2, the bottom blowing nitrogen flow rate of 10 L / min was turned on, and 2 kg of aluminum wire was inserted. The remaining pure iron was added at a rate of ≤ 150 kg each time, and the remaining metallic chromium was reserved and not added; after all was melted, the temperature was measured at 1503°C. During the melting process, it was found that a small part of the molten steel was exposed, and 8 kg of lime was added and 2 kg of fluorite was covered; the temperature was continued to rise and measured at 1568°C, and the temperature was reduced to the insulation power to start the first sampling. The composition of the sample is shown in Table 2, and the composition of the gas sample is shown in Table 3:

[0035] Table 2: First sampling composition (wt%)

[0036] C Si Mn P S Cr Ni Mo V Nb Al 0.013 0.23 6.93 0.008 0.001 11.8 17.59 2.69 0.23 0.092 0.0005

[0037] Table 3: Gas sample composition (wt%)

[0038] O(％) N(％) H(％) 0.0043 0.0108 0.0008

[0039] (3) Insulated smelting: After the first sampling at 13:26, 1 kg of aluminum wire was inserted, and the above steps were completed. The first nitrogen addition operation was started and supplemented according to the upper limit, and the bottom blowing nitrogen flow rate was adjusted to 30 L / min; 290 kg of ferrochrome nitride was added in small batches, and metallic chromium was added during the period, which was controlled at 150 kg to prevent the boiling of the ferrochrome nitride caused by the over-compression of the finished chromium. The process temperature was controlled and measured six times, which were 1578°C, 1565°C, 1558°C, 1569°C, 1572°C, and 1556°C respectively; after the nitrogen supplement was completed, part of the black slag was removed, 1 kg of aluminum wire was inserted, and sampling and analysis were performed; at the same time, the bottom blowing nitrogen flow rate was adjusted to 45 L / min, 10 kg of small white ash, 10 kg of fluorite, 0.5 kg of aluminum particles sprinkled on the slag surface, 5 kg of low-aluminum ferrosilicon, and 2 kg of ferrosilicon were added to quickly make white slag. After the white slag was formed, the bottom blowing nitrogen flow rate was adjusted to 10 L / min for soft blowing. The composition of the sample is shown in Table 4, and the composition of the gas sample is shown in Table 5:

[0040] Table 4: Composition of white slag after sampling (wt%)

[0041] C Si Mn P S Cr Ni Mo V Nb Al 0.014 0.42 6.06 0.01 0.001 20.62 14.60 2.04 0.20 0.092 0.07

[0042] Table 5: Gas sample composition (wt%)

[0043] O(％) N(％) H(％) 0.0029 0.33 0.0004

[0044] At 14:35, because the first nitrogen supplementation was well absorbed according to the upper limit composition, no secondary nitrogen supplementation was required. Only the second batch of alloys was supplemented: 50kg of metallic chromium and 2kg of low-aluminum ferrosilicon. After 20 minutes of soft blowing, the steel samples were sampled and the composition of the steel samples was shown in Table 6, and the composition of the gas samples was shown in Table 7:

[0045] Table 6: Composition of steel samples (wt%)

[0046]

[0047]

[0048] Table 7: Gas sample composition (wt%)

[0049] O(％) N(％) H(％) 0.0028 0.30 0.00046

[0050] (4) Steel pouring: After steel is tapped, add 2 bags of 10kg carbon-free rice husks to cover the liquid surface, and quickly lift them to the pouring platform. Before pouring, take 3kg of carbon-free protective slag from the low-temperature drying room and hang them in 2 260mm ingot molds, 200-400mm from the bottom of the ingot mold, 1.5kg each. Pouring temperature control: 1485-1495℃, liquidus line of molten steel 1415℃. Pouring is protected by argon to prevent three times of inhalation. After pouring, full flow casting is carried out. After the steel flow is stable and the molten steel flows into the ingot mold, the injection speed is stabilized. In the later stage of casting, pay close attention to the steel flow and the amount of molten steel in the bag, do a good job of shrinkage compensation at the riser, and reasonably control the injection surplus. After the casting is completed, it is strictly forbidden to add exothermic agents and carbonized rice husks to the ingot mold. The composition of the molten steel in the ladle is shown in Table 8, and the gas composition of the molten steel in the ladle is shown in Table 9:

[0051] Table 8: Composition of molten steel in ladle (wt%)

[0052] C Si Mn P S Cr Ni Mo V Nb Al 0.014 0.58 5.90 0.01 0.001 21.8 14.4 2.07 0.21 0.095 0.028

[0053] Table 9: Gas composition of molten steel in ladle (wt%)

[0054] O(％) N(％) H(％) 0.004 0.30 0.00046 .

Claims

1. A method for smelting low-temperature austenitic steel, characterized in that: A medium frequency induction furnace is used for smelting, and nitrogen control, white slag making and soft blowing processes are carried out in the heat preservation smelting step; The nitrogen control process: adding ferrochrome nitride as a nitrogen increasing agent for the first nitrogen increase, adding nitrogen according to the upper limit during nitrogen increase, bottom blowing nitrogen as a protective atmosphere, and controlling the nitrogen increase time to be 15 to 20 minutes; The white slag making process is as follows: after the first nitrogen addition, 10-25 kg of lime and 5-15 kg of fluorite are added according to the amount of slag to make slag, and 1-3 kg of aluminum particles and 2-15 kg of low-aluminum ferrosilicon are added to cooperate with diffusion deoxidation.

2. The method for smelting low-temperature austenitic steel according to claim 1, characterized in that: In the nitrogen control process, after the first nitrogen increase, the bottom blowing flow rate is adjusted to 40-50 L / min, and the black oxide slag in the upper slag layer is scraped out.

3. The method for smelting low-temperature austenitic steel according to claim 1, characterized in that: The total duration of the first nitrogen addition and deoxygenation is controlled within 30 to 50 minutes.

4. The method for smelting low-temperature austenitic steel according to claim 1, characterized in that: The soft blowing process: after the soft blowing, additional alloy material is added according to the component sample analysis and whether the second nitrogen addition is required.

5. A method for smelting low-temperature austenitic steel according to any one of claims 1 to 4, characterized in that: In the low-temperature austenitic steel, C≤0.02%, and N 0.2%-0.35%.

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