A method for smelting and casting high-nitrogen stainless steel

Abstract

The present application is a high-nitrogen stainless steel smelting and casting method, which realizes stable control of nitrogen content of high-nitrogen stainless steel. The vacuum induction furnace is used for smelting, and the steel ingot is directly cast under vacuum condition, including: 1) furnace charge and charging; 2) vacuumizing and melting: when the vacuum degree in the furnace reaches ≤5 Pa, the furnace charge is heated by power supply, and the activity oxygen in the molten steel is reduced to below 0.0015% by using the carbon-oxygen reaction under vacuum condition; 3) gas charging and alloying: argon-nitrogen mixed gas is charged into the furnace, the partial pressure of nitrogen gas is calculated through the composition of the steel and the temperature of the molten steel, the activity coefficient of nitrogen in the steel and the saturation solubility of nitrogen at the smelting temperature are calculated according to the composition of the steel and the requirement of nitrogen content, the nitrogen partial pressure and the addition amount of nitrogen-increasing alloy at the time of gas charging are determined according to the target nitrogen content, so that the saturation solubility of nitrogen is higher than the target nitrogen content; 4) tapping and casting: the steel ingot mold is preheated and placed in the vacuum furnace before the furnace is combined, and the position is below the smelting crucible, and the molten steel is directly poured into the steel ingot mold from the furnace mouth.

Description

Technical Field

[0001] This invention relates to the technical field of stainless steel smelting and casting methods, and particularly to a high-nitrogen stainless steel smelting and casting method. Background Technology

[0002] With the development of my country's economy and stainless steel industry, and the increasing demand for stainless steel materials in various fields, my country has become a veritable world-class producer and consumer of stainless steel. In recent years, my country's total stainless steel consumption has continued to grow annually. Due to increased emphasis on life and property safety, the requirements for steel quality in various sectors are also becoming increasingly stringent. Nitrogen, due to its solid solution strengthening effect, can not only improve the strength and wear resistance of steel, but also enhance its corrosion resistance. Therefore, adding nitrogen to stainless steel can improve its performance. However, due to the high vacuum level and very low nitrogen partial pressure in vacuum induction furnaces, smelting nitrogen-containing stainless steel is quite difficult. Currently, the main methods for smelting nitrogen-containing steel are vapor-phase nitriding and the addition of nitrogen-enhancing alloys. Since nitrogen cannot be blown into molten steel in vacuum induction furnaces, vapor-phase nitriding is not applicable. When smelting with argon gas in a vacuum induction furnace followed by the addition of nitrogen-enhancing alloys, the average nitrogen recovery rate drops to below 80%, and precise control of nitrogen levels becomes difficult. For high-nitrogen stainless steel, nitrogen precipitation during the casting of steel ingots can easily form nitrogen pores. At the same time, the steel ingots are also prone to forming defects such as shrinkage cavities and shrinkage porosity during solidification, resulting in a low yield of stainless steel, usually only 60-80%, or even lower.

[0003] Chinese Patent CN101372721A discloses a method for increasing nitrogen content in nitrogen-containing steel smelting using a high-vacuum induction furnace. This method is applicable to steel grades requiring a nitrogen content greater than a certain value or within a controlled range. The aim is to achieve minimal oxidation of the molten steel and a high and stable nitrogen recovery rate. The process involves first loading clean, dry scrap steel or pure iron / ferroalloy into the high-vacuum induction furnace; calculating the required amount of nitriding alloy based on the required nitrogen content in the steel; then evacuating the furnace and electrically heating the furnace charge; melting and refining the charge; and finally, charging the furnace with nitrogen. Under this nitrogen-protected atmosphere, after adjusting other components besides nitrogen to meet the requirements of the smelting grade, chromium nitride, ferromanganese nitride, or other nitrogen-enhancing alloys are added according to the calculated amount of nitrogen-enhancing alloy. The steel is then tapped and cast. The patent describes filling the furnace with nitrogen at a pressure higher than the required nitrogen partial pressure at the steel temperature. This increases the nitrogen content in the steel, making it difficult to control. Furthermore, when simply filling with nitrogen, the furnace remains under negative pressure, which allows air to be drawn into the furnace, causing the steel to oxidize.

[0004] Chinese Patent CN105642844A discloses a casting method for 2205 duplex stainless steel ingots, characterized by: I. a baking temperature range of 60-65℃ before casting; II. a temperature range of 1560-1565℃ during steel molten casting; and III. casting flow reduction requirements, with the first flow reduction occurring 200mm below the cap line and the second flow reduction occurring 100mm below the cap line. The cast 2205 duplex stainless steel ingots achieve a centerless shrinkage cavity in 2205 duplex stainless steel round bars with diameters ranging from Ф250mm to Ф432mm, significantly improving the yield of round bars. The patent describes a casting temperature range of 1560-1565℃, which is relatively high. During solidification, nitrogen is easily precipitated, forming nitrogen pores. At the same time, excessively high casting temperatures will exacerbate the tendency for shrinkage cavities and porosity to form in the steel ingot. The patent eliminates the central shrinkage cavity of Ф250mm-Ф432mm, but only to eliminate the shrinkage cavity in the steel ingot body from the bottom of the steel ingot to the cap line. The cap cannot be effectively utilized, so the yield of round steel is still not high.

[0005] In summary, existing technologies for producing high-nitrogen stainless steel ingots suffer from significant fluctuations in nitrogen content, unstable composition control, high total oxygen content, and poor cleanliness. This leads to defects such as porosity, shrinkage cavities, and shrinkage porosity, resulting in low stainless steel yield. Therefore, there is an urgent need to develop a smelting and casting method for high-nitrogen stainless steel that ensures stable nitrogen content control, low total oxygen content, high steel cleanliness, and the absence of defects like porosity, shrinkage cavities, and shrinkage porosity, thereby guaranteeing a high yield of stainless steel sheets. Summary of the Invention

[0006] To address the technical problems in the background art, the present invention provides a method for smelting and casting high-nitrogen stainless steel, thereby achieving stable control of the nitrogen content in high-nitrogen stainless steel.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A method for smelting and casting high-nitrogen stainless steel, wherein the method employs a vacuum induction furnace for smelting and directly casts the steel ingots under vacuum conditions. The process includes furnace charge preparation and charging, vacuuming and melting, gas filling and alloying, and tapping and casting, wherein:

[0009] 1) Charge and charging: The phosphorus content of the scrap steel should be ≤0.02% and the sulfur content should be ≤0.002%; the nitrogen-enhancing alloy should be ferrochrome nitride, with a chromium content of 55-65% and a nitrogen content of 7.5-8.5%, the remainder being iron and unavoidable impurities;

[0010] 2) Vacuuming and melting: After closing the vacuum furnace, vacuuming begins. When the vacuum level inside the furnace reaches ≤5Pa, the furnace charge is heated by electricity. The carbon-oxygen reaction under vacuum conditions is used to reduce the active oxygen in the molten steel to below 0.0015%.

[0011] 3) Gas charging and alloying: An argon-nitrogen mixture is charged into the furnace. The partial pressure of nitrogen is calculated based on the steel composition (including nitrogen content) and the temperature of the molten steel.

[0012]

[0013]

[0014] In the formula: The nitrogen partial pressure is the ratio of nitrogen filling pressure to atmospheric pressure, expressed in atm. N [N] is the activity coefficient of nitrogen, calculated based on the steel composition and the interaction coefficients of each element with nitrogen, and is dimensionless; [%N] is the solubility of nitrogen in molten steel, i.e., the required nitrogen content; K N This is the equilibrium constant for nitrogen dissolution in molten steel, calculated from the temperature of the molten steel;

[0015] Based on the composition of the steel and the nitrogen content requirements, the activity coefficient of nitrogen in the steel and the saturated solubility of nitrogen at the melting temperature are calculated. Based on the target nitrogen content, the nitrogen partial pressure during gas charging and the amount of nitrogen-enhancing alloy added are determined so that the nitrogen saturated solubility is higher than the target nitrogen content.

[0016] 4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before the furnace is closed, positioned below the smelting crucible. Steel is tapped from the furnace opening and poured directly into the steel ingot mold. The temperature of the molten steel is controlled at 50-70℃ above the liquidus line. After the steel ingot with riser is heated in a heating furnace, it is directly rolled into steel plate.

[0017] Furthermore, in the aforementioned furnace charge and charging process: the length direction of the furnace charge is consistent with the height direction of the crucible, the maximum cross-sectional dimension of the furnace charge is 60-80% of the true diameter of the crucible, and the furnace charge extends ≤100mm above the crucible in the height direction.

[0018] Furthermore, during the vacuuming and melting process: 8-15 minutes before powering on, 40-60% power is applied to prevent excessive current fluctuations. Then, as the current stabilizes, 80-100% power is used for melting until the furnace charge is completely melted. The carbon-oxygen reaction under vacuum conditions is used to reduce the active oxygen in the molten steel to below 0.0015%.

[0019] Furthermore, during the gas filling and alloying process, nitrogen and argon are mixed, and the flow rate of nitrogen is adjusted so that the nitrogen partial pressure is controlled within a narrow range of the nitrogen content required by the molten steel, so as to prevent nitrogen from escaping from the molten steel and to prevent the addition of nitrogen to the molten steel, thereby achieving stable control of the nitrogen content in the molten steel.

[0020] Furthermore, during the gas filling and alloying process, the total pressure of the argon-nitrogen mixed gas reaches 0.93-0.99 atm during gas filling. This can prevent the furnace from still having a certain vacuum when nitrogen is filled alone, thus reducing the oxidation of the molten steel.

[0021] Furthermore, in the gas filling and alloying process: after the protective gas is filled, an alloy including silicon-manganese and ferrochromium nitride are added sequentially from the alloy silo to achieve nitrogen enrichment and alloying of the molten steel.

[0022] Furthermore, in the gas filling and alloying process: the purity of the nitrogen gas used is ≥99%, and the oxygen content in the nitrogen gas is less than 0.05%; the purity of the argon gas used is ≥99%, and the oxygen content in the argon gas is less than 0.01%.

[0023] Furthermore, during the steel casting process, the temperature of the molten steel is controlled at 50-70°C above the liquidus line so that when the molten steel is poured into the ingot mold, it solidifies rapidly and enters the austenite through high-temperature ferrite, preventing nitrogen from precipitating out of the molten steel and forming pores, while ensuring the fluidity of the molten steel.

[0024] Furthermore, in the steel tapping and casting process: molten steel is poured from the top. In order to ensure the temperature of the molten steel in the furnace during tapping and casting, an electric pouring method is adopted to ensure that the temperature of the molten steel poured into the ingot mold later is slightly higher than that of the molten steel poured earlier, so as to achieve the effect of sequential solidification. At the same time, the molten steel poured later reduces the speed and stops after exceeding the height of the riser, thereby reducing the depth of the shrinkage cavity in the riser.

[0025] Furthermore, in the steel casting process described above: when the molten steel is poured to the bottom of the riser, the flow rate of the molten steel is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 50-100mm above the bottom of the riser, the pouring is stopped for 5-15 seconds. When the casting is resumed, the flow rate is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 150-200mm above the bottom of the riser, the pouring is stopped for 20-30 seconds, and the flow rate of the molten steel is reduced to 2 / 8-3 / 8 of the original flow rate until the pouring is completed. This can reduce the depth of the shrinkage cavity in the riser by 70-80%.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1) The nitrogen content of steel ingots produced by this smelting and casting method is stable. The fluctuation value between the actual nitrogen content and the target value is: Actual nitrogen value - target nitrogen value / Target nitrogen value is less than 2%, total oxygen content is less than 0.0010%, the steel is clean and free from defects such as porosity, shrinkage cavities, and shrinkage porosity, and the rolled steel plates are of good quality with a yield of up to 90-96%.

[0028] 2) Steel ingots produced using this smelting and casting method have low total oxygen content and high steel cleanliness. There are no defects such as porosity, shrinkage cavities, or shrinkage porosity in the steel ingots, ensuring a high yield of stainless steel plates.

[0029] 3) By using a nitrogen-argon mixture to fill the vacuum furnace, the nitrogen partial pressure is stably controlled, and a stable composition is achieved by utilizing nitriding alloys despite large fluctuations in nitrogen content.

[0030] 4) The carbon-oxygen reaction under vacuum conditions generates gas, which does not form inclusions in the steel. At the same time, the negative pressure is not formed during gas filling, which inhibits gas absorption, resulting in clean molten steel with low total oxygen content.

[0031] 5) During the casting process, the superheat of the molten steel is controlled, and the casting speed is reduced and interrupted to improve the feeding effect during the solidification of the molten steel and reduce defects such as porosity, shrinkage cavities and shrinkage porosity.

[0032] 6) Steel ingots with risers are directly rolled after heating, resulting in high yield, saving resources and reducing production costs;

[0033] 7) The high-nitrogen stainless steel smelting method of the present invention is carried out in a vacuum furnace, employing vacuum carbon deoxidation. The generated gas is discharged outside the furnace, avoiding the formation of oxide inclusions, improving the cleanliness of the molten steel, and reducing the burn-off of easily oxidized alloys such as aluminum, thus achieving precise control of elements such as aluminum and silicon. During the smelting process, a nitrogen-argon mixed gas is used to fill the vacuum furnace, reducing the oxidation of the molten steel caused by air being drawn in due to the negative pressure inside the furnace. Furthermore, precise control of nitrogen element is achieved when adding chromium nitride alloys. Through carbon deoxidation and the introduction of a nitrogen-argon mixed gas into the furnace, the cleanliness of the steel is improved.

[0034] 8) The casting method of the high-nitrogen stainless steel of the present invention controls the superheat of the molten steel so that during the solidification process of the molten steel, the steel quickly passes through the high-temperature ferrite phase region with low nitrogen solubility and enters the austenite phase region with significantly increased nitrogen solubility. This avoids the formation of nitrogen pores due to the decrease in nitrogen solubility in the steel caused by the temperature drop. At the same time, in the later stage of casting, the casting speed of the molten steel is controlled to avoid casting defects such as shrinkage cavities and shrinkage porosity in the steel ingot, thereby improving the yield of steel plates. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below:

[0036] This invention produces high-nitrogen stainless steel through appropriate smelting and casting processes, forming a method for smelting and casting high-nitrogen stainless steel. In the production of high-nitrogen stainless steel, the external and internal quality of the steel ingot, as well as the stability of its composition, directly determine the stability of the product's performance. This invention employs a vacuum induction furnace for smelting and directly casts the steel ingots under vacuum conditions.

[0037] A method for smelting and casting high-nitrogen stainless steel includes furnace charge and charging, vacuuming and melting, gas filling and alloying, tapping and casting.

[0038] (1) Charge and Loading: To reduce gas and inclusions in the molten steel, the scrap steel charge surface must be free of rust and oil. The charge size should not be too large or too small, ensuring a compact packing inside the furnace to facilitate magnetic and electrical conductivity. The length of the charge should align with the height of the crucible, and the maximum cross-sectional dimension of the charge should be 60-80% of the crucible's true diameter. The charge should extend ≤100mm above the crucible in the height direction. The phosphorus content of the scrap steel should be ≤0.02%, and the sulfur content ≤0.002%. Before loading, clean the crucible in the vacuum furnace, calculate the amount of scrap steel and various alloys to be added, and simultaneously load the clean, dry scrap steel and non-oxidizing and non-volatile ferroalloys into the crucible of the vacuum induction furnace. Add the nitrogen-enhancing alloys and other alloys that need to be added later in the melting process into the alloy hopper. For smelting high-nitrogen stainless steel, the nitrogen-enhancing alloy uses ferrochrome nitride conforming to national standards, with a chromium content of 55-65%, a nitrogen content of 7.5-8.5%, and the remainder being iron and unavoidable impurities.

[0039] (2) Vacuuming and melting: After closing the vacuum furnace, start vacuuming. When the vacuum degree in the furnace reaches ≤5Pa, turn on the power to heat the furnace charge. Use 40-60% power for the first 10 minutes to prevent excessive current fluctuations. Then, when the current stabilizes, use high power to melt until the furnace charge is completely melted. By utilizing the carbon-oxygen reaction under vacuum conditions, the active oxygen in the molten steel can be reduced to below 0.0015%.

[0040] (3) Gas filling and alloying: Argon-nitrogen mixed gas is introduced into the furnace. The partial pressure of nitrogen can be calculated from the composition of the steel (including nitrogen content) and the temperature of the molten steel.

[0041]

[0042]

[0043] In the formula: The nitrogen partial pressure is the ratio of nitrogen filling pressure to atmospheric pressure, expressed in atm. N [N] is the activity coefficient of nitrogen, which can be calculated based on the composition of the steel and the interaction coefficients of each element with nitrogen; it is dimensionless. [%N] represents the solubility of nitrogen in molten steel, i.e., the required content. K NThe equilibrium constant for nitrogen dissolution in molten steel can be calculated from the temperature of the steel. Based on the steel composition and nitrogen content requirements, the activity coefficient of nitrogen in the steel and the saturated solubility of nitrogen at the smelting temperature are calculated. The nitrogen partial pressure during charging and the amount of nitrogen-enhancing alloy added are determined according to the target nitrogen content, ensuring that the saturated solubility of nitrogen is higher than the target nitrogen content. Nitrogen and argon are mixed, and the nitrogen flow rate is adjusted to control the nitrogen partial pressure within a narrow range required for the nitrogen content in the molten steel. This prevents nitrogen from escaping from the molten steel while also preventing the addition of nitrogen to it, thus achieving stable control of the nitrogen content in the molten steel. The total pressure of the argon-nitrogen mixture during charging reaches 0.93-0.99 atm. This prevents air from being drawn into the furnace due to a certain vacuum when charging with nitrogen alone, thus reducing the oxidation of the molten steel. If nitrogen is introduced alone and the pressure inside the furnace is increased, it will cause nitrogen to be added to the molten steel, making it difficult to control the nitrogen content in the molten steel. By accurately calculating the required nitrogen partial pressure based on the nitrogen content and temperature of the molten steel, nitrogen in the steel is not lost, and the gas does not add nitrogen to the molten steel, reducing nitrogen content fluctuations. After the protective gas is introduced, silicon-manganese alloys and ferrochrome nitride are added sequentially from the alloy hopper to achieve nitrogen addition and alloying of the molten steel. The purity of the nitrogen used is ≥99%, and the oxygen content in the nitrogen is less than 0.05%. The purity of the argon used in this invention is ≥99%, and the oxygen content in the argon is less than 0.01%.

[0044] (4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before smelting, positioned below the smelting crucible. Steel is tapped directly from the furnace opening and poured into the steel ingot mold. The temperature of the molten steel is controlled at 50-70°C above the liquidus line so that the molten steel can solidify rapidly upon pouring into the steel ingot mold, allowing the high-temperature ferrite to enter the austenite, preventing nitrogen from precipitating from the molten steel and forming pores, while also ensuring the fluidity of the molten steel. Molten steel is poured using a top-pouring method. To ensure the temperature of the molten steel in the furnace during tapping and pouring, an electrified pouring method is adopted to ensure that the temperature of the molten steel poured into the ingot mold later is slightly higher than that of the molten steel poured earlier, achieving a sequential solidification effect. At the same time, the molten steel poured later is slowed down and paused after exceeding the height of the riser, effectively reducing the depth of shrinkage cavities in the riser. When the molten steel is poured to the bottom of the riser, the flow rate is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 50-100mm above the bottom of the riser, pouring is stopped for 5-15 seconds. When pouring is resumed, the flow rate is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 150-200mm above the bottom of the riser, pouring is stopped for 20-30 seconds, and the flow rate is reduced to 2 / 8-3 / 8 of the original flow rate until the pouring is completed. The depth of shrinkage cavities in the riser can be reduced by 70-80%.

[0045] After being heated in a furnace, steel ingots with risers are directly rolled into steel plates. The nitrogen content of steel ingots produced by this smelting and casting method is stably controlled. The fluctuation value between the actual nitrogen content and the target value is: |actual nitrogen value - target nitrogen value| / target nitrogen value is less than 2%, the total oxygen content is less than 0.0010%, the steel is clean, and there are no defects such as porosity, shrinkage cavities, or shrinkage porosity. The rolled steel plates are of good quality, with a yield rate as high as 90-96%.

[0046] Example 1

[0047] The steel was smelted in a 200 kg vacuum furnace. The steel grade was duplex stainless steel S32205. The composition control range and target are shown in Table 1. The nitrogen activity coefficient was calculated based on the interaction coefficient of each element with nitrogen, and the nitrogen partial pressure was calculated according to the control temperature of the molten steel and the target nitrogen content.

[0048] Table 1. Composition and target percentage of S32205 stainless steel

[0049] element C Si Mn P S scope ≤0.03 ≤1.0 ≤2.0 ≤0.035 ≤0.015 Target 0.02 0.5 1.2 0.03 0.001 element Cr Ni Mo N / scope 21 / 23 4 / 6 2.5 / 3.5 0.10 / 0.22 / Target 22 5 3 0.17 /

[0050] From the formula Calculations show that the nitrogen partial pressure is 0.14 atm at a temperature of 1808 K. The stainless steel smelting and casting process includes furnace charge preparation and charging, vacuuming and melting, gas filling and alloying, tapping and casting.

[0051] (1) Charge and Loading: To reduce gas and inclusions in the molten steel, the scrap steel charge surface must be free of rust and oil. The charge size should not be too large or too small, ensuring a tight packing inside the furnace to facilitate magnetic and electrical conductivity. The length of the charge should be aligned with the height of the crucible, and the maximum cross-sectional dimension of the charge should be 60% of the crucible's true diameter. The charge should extend 100mm above the crucible in the height direction. The scrap steel should have a phosphorus content of 0.018% and a sulfur content of 0.0018%. Before loading, clean the crucible in the vacuum furnace, calculate the amount of scrap steel and various alloys to be added, and simultaneously load the clean, dry scrap steel and non-oxidizing and non-volatile ferroalloys into the crucible of the vacuum induction furnace. Add the nitrogen-enhancing alloys and other alloys that need to be added later in the melting process into the alloy hopper. For smelting high-nitrogen stainless steel, the nitrogen-enhancing alloy uses ferrochrome nitride conforming to national standards, with a chromium content of 65%, a nitrogen content of 7.5%, and the remainder being iron and unavoidable impurities.

[0052] (2) Vacuuming and melting: After closing the vacuum furnace, start vacuuming. When the vacuum degree in the furnace reaches 4Pa, power is supplied to heat the furnace charge. 40% power is supplied 10 minutes before powering on to prevent excessive current fluctuations. Then the current tends to stabilize, and high power is used for melting until the furnace charge is completely melted. By utilizing the carbon-oxygen reaction under vacuum conditions, the active oxygen in the molten steel can be reduced to 0.0014%.

[0053] (3) Gas Filling and Alloying: Argon-nitrogen mixed gas is filled into the furnace, with the nitrogen partial pressure controlled at 0.14 atm to prevent nitrogen from escaping from the molten steel and to avoid adding nitrogen to the molten steel, thus achieving stable control of the nitrogen content in the molten steel. The total pressure of the argon-nitrogen mixed gas reaches 0.99 atm during gas filling. This prevents air from being drawn into the furnace due to a certain vacuum when nitrogen is filled alone, thus reducing the oxidation of the molten steel. If nitrogen is filled alone and the pressure of nitrogen in the furnace is increased, it will cause nitrogen to be added to the molten steel, making it difficult to control the nitrogen content in the molten steel. Based on the nitrogen content and temperature of the molten steel, the required nitrogen partial pressure is accurately calculated to ensure that the nitrogen element in the steel is not lost, and that the gas does not add nitrogen to the molten steel, reducing the fluctuation of nitrogen content. After the protective gas is filled, silicon manganese and other alloys and ferrochrome nitride are added sequentially from the alloy hopper to achieve nitrogen addition and alloying of the molten steel. The purity of the nitrogen used is 99.5%, and the oxygen content in the nitrogen is 0.018%. The argon gas used in this invention has a purity of 99.8% and an oxygen content of 0.006%.

[0054] (4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before smelting, positioned below the smelting crucible. Steel is tapped directly from the furnace opening and poured into the steel ingot mold. The temperature of the molten steel is controlled at 70°C above the liquidus line so that the molten steel can solidify rapidly upon pouring into the steel ingot mold, allowing the high-temperature ferrite to enter the austenite, preventing nitrogen from precipitating from the molten steel and forming pores, while also ensuring the fluidity of the molten steel. Molten steel is poured using a top-pouring method. To ensure the temperature of the molten steel in the furnace during tapping and pouring, an electrified pouring method is adopted to ensure that the temperature of the molten steel poured into the ingot mold later is slightly higher than that of the molten steel poured earlier, achieving a sequential solidification effect. At the same time, the molten steel poured later is slowed down and paused after exceeding the height of the riser, effectively reducing the depth of shrinkage cavities in the riser. When the molten steel is poured to the bottom of the riser, the flow rate is reduced to 3 / 8 of the original flow rate. When the molten steel is poured to 100mm above the bottom of the riser, pouring is stopped for 15 seconds. When pouring is resumed, the flow rate is reduced to 3 / 8 of the original flow rate. When the molten steel is poured to 150mm above the bottom of the riser, pouring is stopped for 20 seconds, and the flow rate is reduced to 2 / 8 of the original flow rate until the pouring is completed. The depth of shrinkage cavities in the riser can be reduced by 70%.

[0055] Steel ingots with risers are heated in a furnace and then directly rolled into steel plates. The nitrogen content of the steel ingots produced using this smelting and casting method is kept stable, and the nitrogen content in the steel plates is 0.169%. The actual nitrogen content differs from the target value in the following ways:

[0056] |Actual Nitrogen Value - Target Nitrogen Value| / The target nitrogen value is 0.59%, the total oxygen content is 0.0008%, the steel is clean, and there are no defects such as porosity, shrinkage cavities, or shrinkage porosity. The rolled steel plate is of good quality, with a yield rate as high as 93%.

[0057] Example 2

[0058] The steel was smelted in a 500 kg vacuum furnace, and the steel grade was stainless steel S20430. The composition control range and target are shown in Table 2. The nitrogen activity coefficient was calculated based on the interaction coefficient of each element with nitrogen, and the nitrogen partial pressure was calculated according to the control temperature of the molten steel and the target nitrogen content.

[0059] Table 2. Composition and Target Values ​​of S20430 Stainless Steel (%)

[0060] element C Si Mn P S scope ≤0.15 ≤1.0 6.5 / 9.0 ≤0.06 ≤0.03 Target 0.08 0.5 8.0 0.03 0.005 element Cr Ni Mo Cu N scope 15.5 / 17.5 1.5 / 3.5 0.5 / 1.5 2.0 / 4.0 0.05 / 0.25 Target 16.5 2.5 1.0 3.0 0.15

[0061] From the formula Calculations show that the nitrogen partial pressure is 0.43 atm at a temperature of 1776 K. The stainless steel smelting and casting process includes furnace charge preparation and charging, vacuuming and melting, gas filling and alloying, tapping and casting.

[0062] (1) Charge and Loading: To reduce gas and inclusions in the molten steel, the scrap steel charge surface must be free of rust and oil. The charge size should not be too large or too small, ensuring a compact packing inside the furnace to facilitate magnetic and electrical conductivity. The length of the charge should align with the height of the crucible, and the maximum cross-sectional dimension of the charge should be 70% of the crucible's true diameter. The charge should extend 50mm above the crucible in the height direction. The scrap steel should have a phosphorus content of 0.015% and a sulfur content of 0.0016%. Before loading, clean the crucible in the vacuum furnace, calculate the amount of scrap steel and various alloys to be added, and simultaneously load the clean, dry scrap steel and non-oxidizing and non-volatile ferroalloys into the crucible of the vacuum induction furnace. Add the nitrogen-enhancing alloys and other alloys that need to be added later in the melting process into the alloy hopper. For smelting high-nitrogen stainless steel, the nitrogen-enhancing alloy uses ferrochrome nitride conforming to national standards, with a chromium content of 60%, a nitrogen content of 8.0%, and the remainder being iron and unavoidable impurities.

[0063] (2) Vacuuming and melting: After closing the vacuum furnace, start vacuuming. When the vacuum degree in the furnace reaches 3Pa, turn on the power to heat the furnace charge. 10 minutes before turning on the power, turn on the power at 60% power to prevent the current from fluctuating too much. Then the current tends to stabilize and high power is used for melting until the furnace charge is melted. By using the carbon-oxygen reaction under vacuum conditions, the active oxygen in the molten steel can be reduced to 0.0012%.

[0064] (3) Gas Filling and Alloying: Argon-nitrogen mixed gas is filled into the furnace, with the nitrogen partial pressure controlled at 0.43 atm to prevent nitrogen from escaping from the molten steel and to avoid adding nitrogen to the molten steel, thus achieving stable control of the nitrogen content in the molten steel. The total pressure of the argon-nitrogen mixed gas reaches 0.95 atm during gas filling. This prevents the furnace from still having a certain vacuum when nitrogen is filled alone, which would cause air to be drawn into the furnace and reduce the oxidation of the molten steel. If nitrogen is filled alone and the pressure of nitrogen in the furnace is increased, it will cause nitrogen to be added to the molten steel, making it difficult to control the nitrogen content in the molten steel. Based on the nitrogen content and temperature of the molten steel, the required nitrogen partial pressure is accurately calculated to ensure that the nitrogen element in the steel is not lost, and that the gas does not add nitrogen to the molten steel, reducing the fluctuation of nitrogen content. After the protective gas is filled, silicon manganese and other alloys and ferrochrome nitride are added sequentially from the alloy hopper to achieve nitrogen addition and alloying of the molten steel. The purity of the nitrogen used is 99.7%, and the oxygen content in the nitrogen is 0.015%. The argon gas used in this invention has a purity of ≥99.7% and an oxygen content of 0.005%.

[0065] (4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before smelting, positioned below the smelting crucible. Steel is tapped directly from the furnace opening and poured into the steel ingot mold. The temperature of the molten steel is controlled at 60°C above the liquidus line so that the molten steel can solidify rapidly upon pouring into the steel ingot mold, allowing the high-temperature ferrite to enter the austenite, preventing nitrogen from precipitating from the molten steel and forming pores, while also ensuring the fluidity of the molten steel. Molten steel is poured using a top-pouring method. To ensure the temperature of the molten steel in the furnace during tapping and pouring, an electrified pouring method is adopted to ensure that the temperature of the molten steel poured into the ingot mold later is slightly higher than that of the molten steel poured earlier, achieving a sequential solidification effect. At the same time, the molten steel poured later is slowed down and paused after exceeding the height of the riser, effectively reducing the depth of shrinkage cavities in the riser. When the molten steel is poured to the bottom of the riser, the flow rate is reduced to 4 / 8 of the original flow rate. When the molten steel is poured to 75mm above the bottom of the riser, pouring is stopped for 10 seconds. When pouring is resumed, the flow rate is reduced to 4 / 8 of the original flow rate. When the molten steel is poured to 180mm above the bottom of the riser, pouring is stopped for 25 seconds, and the flow rate is reduced to 3 / 8 of the original flow rate until the pouring is completed. The depth of shrinkage cavities in the riser can be reduced by 75%.

[0066] Steel ingots with risers are heated in a furnace and then directly rolled into steel plates. The nitrogen content of steel ingots produced by this smelting and casting method is kept stable. The nitrogen content in the steel plates is 0.152%, and the fluctuation value between the actual nitrogen content and the target value is 1.3% (actual nitrogen value - target nitrogen value) / target nitrogen value. The total oxygen content is 0.0007%, the steel is clean, and there are no defects such as porosity, shrinkage cavities, or shrinkage porosity. The rolled steel plates are of good quality, with a yield rate as high as 90%.

[0067] Example 3

[0068] A 1-ton vacuum furnace was used for smelting, and the steel grade was duplex stainless steel S32101. The composition control range and target are shown in Table 3. The nitrogen activity coefficient was calculated based on the interaction coefficient of each element with nitrogen, and the nitrogen partial pressure was calculated according to the control temperature of the molten steel and the target nitrogen content.

[0069] Table 3. Composition and target percentage of S32101 stainless steel

[0070] element C Si Mn P S scope 0.15 / 0.35 0.45 / 0.75 4.7 / 5.5 ≤0.03 ≤0.005 Target 0.023 0.65 5.0 0.02 0.002 element Cr Ni Mo Cu N scope 21 / 21.8 1.5 / 1.7 0.15 / 0.33 0.15 / 0.35 0.20 / 0.23 Target 21.5 1.6 0.24 0.25 0.21

[0071] From the formula Calculations show that the nitrogen partial pressure is 0.17 atm at a temperature of 1788 K. The stainless steel smelting and casting process includes furnace charge preparation and charging, vacuuming and melting, gas filling and alloying, tapping and casting.

[0072] (1) Charge and Loading: To reduce gas and inclusions in the molten steel, the scrap steel charge surface must be free of rust and oil. The charge size should not be too large or too small, ensuring a tight packing inside the furnace to facilitate magnetic and electrical conductivity. The length of the charge should be aligned with the height of the crucible, and the maximum cross-sectional dimension of the charge should be 80% of the crucible's true diameter. The charge should be 0 mm above the crucible in the height direction. The scrap steel should have a phosphorus content of 0.016% and a sulfur content of 0.0019%. Before loading, clean the crucible in the vacuum furnace, calculate the amount of scrap steel and various alloys to be added, and simultaneously load the clean, dry scrap steel and non-oxidizing and non-volatile ferroalloys into the crucible of the vacuum induction furnace. Add the nitrogen-enhancing alloys and other alloys that need to be added later in the melting process into the alloy hopper. For smelting high-nitrogen stainless steel, the nitrogen-enhancing alloy uses ferrochrome nitride conforming to national standards, with a chromium content of 55%, a nitrogen content of 8.5%, and the remainder being iron and unavoidable impurities.

[0073] (2) Vacuuming and melting: After closing the vacuum furnace, start vacuuming. When the vacuum degree in the furnace reaches 2Pa, turn on the power to heat the furnace charge. 10 minutes before turning on the power, turn on the power at 50% power to prevent the current from fluctuating too much. Then the current tends to stabilize and high power is used for melting until the furnace charge is melted. By using the carbon-oxygen reaction under vacuum conditions, the active oxygen in the molten steel can be reduced to 0.0013%.

[0074] (3) Gas Filling and Alloying: Argon-nitrogen mixed gas is filled into the furnace, with the nitrogen partial pressure controlled at 0.17 atm to prevent nitrogen from escaping from the molten steel and to avoid adding nitrogen to the molten steel, thus achieving stable control of the nitrogen content in the molten steel. The total pressure of the argon-nitrogen mixed gas reaches 0.93 atm during gas filling. This prevents the furnace from still having a certain vacuum when nitrogen is filled alone, which would cause air to be drawn into the furnace and reduce the oxidation of the molten steel. If nitrogen is filled alone and the pressure of nitrogen in the furnace is increased, it will cause nitrogen to be added to the molten steel, making it difficult to control the nitrogen content in the molten steel. Based on the nitrogen content and temperature of the molten steel, the required nitrogen partial pressure is accurately calculated to ensure that the nitrogen element in the steel is not lost and that the gas does not add nitrogen to the molten steel, reducing the fluctuation of nitrogen content. After the protective gas is filled, silicon manganese and other alloys and ferrochrome nitride are added sequentially from the alloy hopper to achieve nitrogen addition and alloying of the molten steel. The purity of the nitrogen used is 99.8%, and the oxygen content in the nitrogen is 0.014%. The argon gas used in this invention has a purity of 99.3% and an oxygen content of 0.007%.

[0075] (4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before smelting, positioned below the smelting crucible. Steel is tapped directly from the furnace opening and poured into the steel ingot mold. The temperature of the molten steel is controlled at 50°C above the liquidus line so that the molten steel can solidify rapidly upon pouring into the steel ingot mold, allowing the high-temperature ferrite to enter the austenite, preventing nitrogen from precipitating from the molten steel and forming pores, while also ensuring the fluidity of the molten steel. Molten steel is poured using a top-pouring method. To ensure the temperature of the molten steel in the furnace during tapping and pouring, an electrified pouring method is adopted to ensure that the temperature of the molten steel poured into the ingot mold later is slightly higher than that of the molten steel poured earlier, achieving a sequential solidification effect. At the same time, the molten steel poured later is slowed down and paused after exceeding the height of the riser, effectively reducing the depth of shrinkage cavities in the riser. When the molten steel is poured to the bottom of the riser, the flow rate is reduced to 5 / 8 of the original flow rate. When the molten steel is poured to 50mm above the bottom of the riser, pouring is stopped for 5 seconds. When pouring is resumed, the flow rate is reduced to 5 / 8 of the original flow rate. When the molten steel is poured to 200mm above the bottom of the riser, pouring is stopped for 30 seconds, and the flow rate is reduced to 2 / 8 of the original flow rate until the pouring is completed. The depth of shrinkage cavities in the riser can be reduced by 80%.

[0076] Steel ingots with risers are heated in a furnace and then directly rolled into steel plates. The nitrogen content of steel ingots produced by this smelting and casting method is stably controlled. The nitrogen content in the steel plates is 0.207%, and the fluctuation value between the actual nitrogen content and the target value is 1.43% (actual nitrogen value - target nitrogen value) / target nitrogen value. The total oxygen content is 0.0009%, the steel is clean, and there are no defects such as porosity, shrinkage cavities, or shrinkage porosity. The rolled steel plates are of good quality, with a yield rate as high as 96%.

[0077] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for smelting and casting high-nitrogen stainless steel, characterized in that, The method employs a vacuum induction furnace for smelting and directly casts steel ingots under vacuum conditions. The process includes furnace charge preparation and charging, vacuuming and melting, gas filling and alloying, and tapping and casting, wherein: 1) Charge and charging: The phosphorus content of the scrap steel should be ≤0.02% and the sulfur content should be ≤0.002%; the nitrogen-enhancing alloy should be ferrochrome nitride, with a chromium content of 55-65% and a nitrogen content of 7.5-8.5%, the remainder being iron and unavoidable impurities; 2) Vacuuming and melting: After closing the vacuum furnace, vacuuming begins. When the vacuum level inside the furnace reaches ≤5Pa, the furnace charge is heated by electricity. The carbon-oxygen reaction under vacuum conditions is used to reduce the active oxygen in the molten steel to below 0.0015%. 3) Gas filling and alloying: An argon-nitrogen mixed gas is introduced into the furnace. The partial pressure of nitrogen is calculated based on the steel composition and the temperature of the molten steel. ； ； In the formula: This is the ratio of nitrogen filling pressure to atmospheric pressure, i.e., nitrogen partial pressure, with units of atm; [N] is the activity coefficient of nitrogen, calculated based on the composition of the steel and the interaction coefficients of each element with nitrogen, and is dimensionless; [%N] is the solubility of nitrogen in molten steel, i.e., the required content. This is the equilibrium constant for nitrogen dissolution in molten steel, calculated from the temperature of the molten steel; Based on the composition of the steel and the nitrogen content requirements, the activity coefficient of nitrogen in the steel and the saturated solubility of nitrogen at the melting temperature are calculated. Based on the target nitrogen content, the nitrogen partial pressure during gas charging and the amount of nitrogen-enhancing alloy added are determined so that the nitrogen saturated solubility is higher than the target nitrogen content. 4) Steel tapping and casting: After preheating, the steel ingot mold is placed in a vacuum furnace before the furnace is closed, positioned below the smelting crucible. Steel is tapped from the furnace opening and poured directly into the steel ingot mold. The temperature of the molten steel is controlled at 50-70℃ above the liquidus line. After the steel ingot with riser is heated in a heating furnace, it is directly rolled into steel plate. During the steel casting process, the temperature of the molten steel is controlled at 50-70°C above the liquidus line. The molten steel is poured into the ingot mold and solidifies rapidly. The high-temperature ferrite enters the austenite, preventing nitrogen from precipitating out of the molten steel and forming pores, while ensuring the fluidity of the molten steel. During the steel tapping and casting process, molten steel is poured from the top. In order to ensure the temperature of the molten steel in the furnace, an electric pouring method is adopted during tapping and casting to ensure that the temperature of the molten steel poured into the ingot mold later is higher than that of the molten steel poured earlier, so as to achieve the effect of sequential solidification. At the same time, the molten steel poured later reduces the speed and stops after exceeding the height of the riser, thereby reducing the depth of the shrinkage cavity in the riser. During the steel casting process described above: when the molten steel is poured to the bottom of the riser, the flow rate of the molten steel is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 50-100 mm above the bottom of the riser, the pouring is stopped for 5-15 seconds. When pouring is resumed, the flow rate is reduced to 3 / 8-5 / 8 of the original flow rate. When the molten steel is poured to 150-200 mm above the bottom of the riser, the pouring is stopped for 20-30 seconds, and the flow rate of the molten steel is reduced to 2 / 8-3 / 8 of the original flow rate until the pouring is completed. This process can reduce the depth of shrinkage cavities in the riser by 70-80%.

2. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the furnace charge and charging process, the length direction of the furnace charge is consistent with the height direction of the crucible, the maximum cross-sectional dimension of the furnace charge is 60-80% of the true diameter of the crucible, and the furnace charge extends ≤100mm above the crucible in the height direction.

3. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the vacuuming and melting process: 8-15 minutes before powering on, power is supplied at 40-60% to prevent excessive current fluctuations. Then, as the current stabilizes, 80-100% power is used for melting until the furnace charge is completely melted. The carbon-oxygen reaction under vacuum conditions is used to reduce the active oxygen in the molten steel to below 0.0015%.

4. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the gas filling and alloying process, nitrogen and argon are mixed, and the flow rate of nitrogen is adjusted to control the nitrogen partial pressure within a narrow range of the required nitrogen content in the molten steel, so that nitrogen does not escape from the molten steel and does not add nitrogen to the molten steel, thereby achieving stable control of the nitrogen content in the molten steel.

5. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the gas filling and alloying process, the total pressure of the argon-nitrogen mixed gas reaches 0.93-0.99 atm during gas filling to prevent air from being drawn into the furnace due to the vacuum in the furnace when nitrogen is filled alone, thus reducing the oxidation of the molten steel.

6. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the gas filling and alloying process: after the protective gas is filled, an alloy including silicon-manganese and ferrochromium nitride are added sequentially from the alloy silo to achieve nitrogen enrichment and alloying of the molten steel.

7. The method for smelting and casting high-nitrogen stainless steel according to claim 1, characterized in that, During the gas filling and alloying process: the purity of the nitrogen gas used is ≥99%, the oxygen content in the nitrogen gas is less than 0.05%, the purity of the argon gas used is ≥99%, and the oxygen content in the argon gas is less than 0.01%.

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