Production process of high-nitrogen austenitic steel forged plate

Abstract

The invention discloses a production process of a high-nitrogen austenitic steel forged plate. The production process comprises the steps of electroslag remelting, heating before forging and forging. In the forging step, seven-heating-number forging is adopted, and the pressing amount of forging of each heating number is controlled as follows: the pressing amount of the first heating number is 5-8 mm; according to the second heating number, the third heating number and the fourth heating number, the pressing amount of each time of upsetting ranges from 20 mm to 25 mm, and the total length of each heating number of upsetting ranges from 130 mm to 150 mm; the pressing amount of the single side of each time of drawing in the fifth heating number is 5mm-8mm, and the thickness of the heating number is 300mm-315mm; the pressing amount of the single side of each time of drawing in the sixth heating number is 5-8 mm, and the heating number is pressed until the thickness of the wide surface is 295-305 mm; and the single-side pressing amount of each time of drawing in the seventh heating number is 5-8 mm, and the heating number is pressed until the thickness of the wide surface is 245-250 mm. According to the process, the problems that an existing process is prone to cracking and uneven in structure are effectively solved, the obtained steel is excellent in performance in all aspects, and the material requirements of high-precision tip projects can be met.

Description

Technical Field

[0001] The invention relates to a plate production method, in particular to a high nitrogen austenitic steel forging plate production process. Background Art

[0002] High-strength stainless steel is a large-scale cryogenic project, such as future interstellar high-speed flight stainless steel thin-walled tanks, magnetic confinement nuclear fusion, hydrogen energy, etc., which have huge application demands for high-strength and high-toughness stainless steel for temperatures below 20K. For example, due to the huge electromagnetic force of magnetic confinement superconducting magnets, the yield strength of structural stainless steel materials at 4K is required to be greater than 1300MPa, while the yield strength of 316LN stainless steel with the best low-temperature performance at this temperature is only 1000MPa, which cannot meet the application requirements. In summary, it is crucial to conduct in-depth research on the strengthening and toughening mechanism of stainless steel materials in a lower temperature range and the development and application research of low-temperature stainless steel used in the 20K temperature range. High nitrogen austenitic steel has a wide range of applications in energy fields such as nuclear power due to its high corrosion resistance, good weldability and machinability. Its high strength and high toughness at low temperatures meet the requirements of the new generation of superconducting coil armor materials. The development of nitrogen-containing austenitic stainless steel in my country started late. In the 1950s, with the help of the research of the Institute of Metal Research, Chinese Academy of Sciences, my country successfully smelted a new type of stainless steel with a composition of 0Cr17Mn14Mo2N. In the 1960s, with the development of my country's aerospace and nuclear power industries, the performance requirements for new austenitic stainless steels were further improved, so my country developed a new type of chromium-manganese-nitrogen 1Cr18Mn14Mo2N steel. In the 1980s, my country began to develop nitrogen-controlled and nitrogen-alloyed austenitic stainless steels. For example, Lang Yuping and others from the Beijing Iron and Steel Research Institute developed high-nitrogen stainless steels with a nitrogen content of more than 0.7% through vacuum smelting equipment. Subsequently, my country began to explore the strengthening effect of nitrogen.

[0003] The function of forging is to make the metal blank produce plastic deformation to obtain forgings with certain mechanical properties, certain shapes and sizes. Forging can eliminate defects such as loose cast metal produced during the smelting process, optimize the microstructure, and preserve the complete metal flow line. Free forging is a processing method that uses impact force or pressure to make the metal deform freely in all directions between the upper and lower anvil surfaces to obtain forgings with certain mechanical properties of the required shape and size without any restrictions. At the current forging level, most of them use normal forging processes, and the deformation mode and pressure cannot be quantified specifically. After forging, there are many cracks in the corners, fish-scale cracks on the surface, and cracks in the core due to imperfect smelting process. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a production process for high nitrogen austenitic steel forging plates with good product quality.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: it includes electroslag remelting, pre-forging heating and forging steps; The forging step adopts seven-fire forging, and the pressing amount of each fire is controlled as follows: the pressing amount of the first fire is 5mm-8mm; the pressing amount of each upsetting in the second, third and fourth fires is 20mm-25mm, and the total upsetting length of each fire is 130mm-150mm; the pressing amount of each side of the fifth fire is 5mm-8mm for each elongation, and the thickness of the fire is pressed to 300mm-315mm; the pressing amount of each side of the sixth fire is 5mm-8mm for each elongation, and the thickness of the wide surface is 295mm-305mm; the pressing amount of each side of the seventh fire is 5mm-8mm for each elongation, and the thickness of the wide surface is 245mm-250mm.

[0006] Furthermore, the final forging temperature of the workpiece during each forging is ≥1000°C, and the insulation time between each forging is ≥1h.

[0007] Furthermore, the pre-forging heating step includes: heating to 1200-1250° C. and keeping the temperature for 20-26 hours; cooling to 1180-1220° C. and keeping the temperature.

[0008] Furthermore, the pre-forging heating step is: the billet is put into the furnace with a furnace temperature of ≤450°C and kept warm for 1.5 to 3 hours; then the temperature is increased by 75 to 110°C per hour to 850 to 900°C, and kept warm for 1.5 to 3 hours; then the temperature is increased to 1000 to 1050°C at maximum power, and kept warm for 3.5 to 5.5 hours; then the temperature is increased to 1200 to 1250°C at maximum power, and kept warm for 20 to 26 hours; finally, the furnace is cooled to 1180 to 1220°C, and kept warm for 4 to 5 hours.

[0009] The beneficial effect of adopting the above technical solution is that the present invention adopts the forging method of "seven-fire roughing and then drawing, with a cross section of eight sides and four flat arcs" combined with strict temperature control to effectively overcome the problems of easy cracking and uneven structure of high-nitrogen austenitic steel in the existing forging process. The forged steel has excellent performance in all aspects and can meet the material requirements of high-precision and cutting-edge projects. The high-nitrogen austenitic steel forging plate obtained by the present invention has a yield strength of more than 1400Mpa in the helium temperature zone and a fracture toughness of 180Mpa / cm 2 , elongation after fracture is greater than 30%. The key core technologies such as hot forging cracking mechanism and prevention and control technology of high nitrogen steel, grain control technology and deformation strengthening control technology have been successfully broken through.

[0010] In view of the serious cracking problem of hot forging of high-nitrogen austenitic steel, the present invention has developed effective prevention and control measures, such as adopting special forging die design, optimizing forging process route, etc., to reduce the risk of cracking; through grain control technology, the grain size is made more uniform and fine, and the strength and toughness of the material are improved; in terms of deformation strengthening control, the deformation degree and the coordination of heat treatment process are accurately controlled to achieve precise regulation of material properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0012] Figure 1 It is a schematic diagram of the forging process of the first forging step of the present invention. DETAILED DESCRIPTION

[0013] The chemical composition and mass percentage of the high nitrogen austenitic steel described in the high nitrogen austenitic steel forging plate production process are as follows: C < 0.05%, Si 0.150% ~ 0.550%, Mn 3.80% ~ 6.20%, P < 0.040%, S < 0.040%, Ni12.80 ~ 14.80%, Al < 0.05%, Cr 20.00% ~ 22.50%, Mo 1.50% ~ 3.00%, V 0.100% ~ 0.300%, Nb < 0.15%, N 0.20% ~ 0.35%, Ti < 0.002%, and the balance is Fe and unavoidable impurities; The functions of the main elements of the high nitrogen austenitic steel are as follows: N is a very strong austenite-forming element, which can expand the austenite phase area in steel. The ability of N to stabilize austenite can be characterized by the nickel equivalent calculation formula represented by the modified Schaffler phase organization diagram: Nieg=%Ni+%Co+0.1%Mn²+18%N+30%C; From the formula, it can be seen that the ability of N to stabilize austenite is second only to carbon, which is 18 times that of Ni. Therefore, nitrogen is an ideal austenitizing element. Adding N to high-nitrogen austenitic steel can reduce the use of expensive Ni, thereby saving costs without significantly reducing material properties. The strengthening effect of N in steel is determined by its state in the steel. N exists in the octahedral interstitial position in the lattice structure in the form of interstitial solid solution. At the same time, N can combine with other elements to form a second phase with precipitation strengthening effect. The strengthening effect of N element is strongly related to temperature. When 0.1% nitrogen is added, the room temperature yield strength of austenitic stainless steel increases by about 50MPa. When the temperature is reduced to 4K, the strength can be increased by 250Mpa. The higher the N content, the higher the strength. In addition, N element can also improve the pitting corrosion resistance, stress corrosion resistance and intergranular corrosion resistance of austenitic stainless steel.

[0014] Cr is a ferrite-forming element that can promote the formation of ferrite and reduce the austenite phase. Therefore, austenitic steel with a high Cr content needs to add a large amount of austenite-forming elements to stabilize its austenite structure. The most important role of Cr for austenitic stainless steel is to improve the corrosion resistance of stainless steel. Cr combines with O to form a dense oxide film on the metal surface. This oxide film can protect the steel and reduce its corrosion by acid and alkali media.

[0015] The Ni element is the main structural stabilizing element of austenitic stainless steel. Its main function is to form and stabilize the austenite structure, so that the steel obtains a good match of strength and toughness. Ni can reduce the ferrite formation temperature, reduce the precipitation of ferrite, and can significantly reduce the formation tendency of σ phase. At the same time, the Ni element can reduce the solubility of the N element, thereby enhancing the precipitation of nitrides. However, the Ni element is expensive, and some steel grades can replace nickel with nitrogen to obtain good mechanical properties.

[0016] Nb, V, and Ti are the most commonly used microalloying elements. Their main function in steel is to combine with C and N elements to form carbonitrides. The fine dispersed precipitated carbonitrides can pin the grain boundaries, inhibit the growth of austenite grains during heating, and have a precipitation strengthening effect on austenitic steel. The grain refining effect of Nb is stronger, while the precipitation strengthening effect of V is stronger, and Ti is between the two. The atomic radius of Nb atoms is larger than that of iron atoms. In addition to existing in the form of precipitated phases, Nb atoms can also exist in the matrix as substitutional solid solution atoms, and they are easily segregated on dislocation lines, exerting a strong drag on dislocation climb, thereby strengthening the steel.

[0017] In low-temperature austenitic steels with low Ni content, Mn is the main alloying element. Its austenite forming ability is relatively weak, less than one-tenth of Ni. In Fe-Cr-Mn alloys, when the Cr content is greater than 14%, a single austenite structure cannot be obtained by simply increasing the Mn content, and a certain amount of other austenite forming elements such as Ni and N must be added. In general, the Mn element cooperates with the Cr element to increase the solubility of the N element. The Mn element does not have a great effect on the corrosion resistance of austenitic stainless steel. In austenitic steel, Mn is easy to form MnS inclusions with S, thereby reducing the chloride corrosion resistance of Cr-Ni-Mn austenitic stainless steel.

[0018] The production process of high nitrogen austenitic steel forging plate adopts smelting, electroslag remelting, pre-forging heating and forging steps; the process of each step is as follows: (1) Smelting steps: including pre-smelting preparation, smelting and casting process; (1.1) Preparation process before smelting: clean the new ladle and place it under the ladle baking device to bake it to 750℃~780℃, waiting for low-temperature steel smelting. The ingot mold baking is carried out according to the current baking process, the ingot mold is placed in the slow cooling pit for insulation, and the ingot mold is placed 1.5~2 hours before pouring steel. The ingot mold temperature out of the pit is ≥80℃. The casting refractory is dried in the low-temperature drying room for more than 24 hours. The alloy material and pre-molten slag baking are carried out according to the current baking process, and the ferrochromium nitride is not baked.

[0019] Raw material preparation: Steel material: industrial pure iron. Alloy material: low aluminum ferrosilicon, metallic manganese, electrolytic nickel plate, metallic chromium, molybdenum bar, ferrovanadium, niobium block and ferrochrome nitride. Auxiliary materials: aluminum granules, aluminum wire, 622 pre-melted slag; aluminum granules are used for diffusion deoxidation, and aluminum wire is used for precipitation deoxidation.

[0020] (1.2) Smelting process: When the temperature of molten steel rises to 1540℃~1580℃, take samples for analysis. When the temperature of molten steel rises to 1570℃~1585℃, adjust the power to keep warm and remove part of the slag according to the slag situation. Adjust the bottom blowing pressure to 600kpa~620kpa and the bottom blowing flow rate to 6.8~7.2L / min, start the bottom blowing system and start blowing. Adjust the medium frequency power and start adding ferrochromium nitride to match nitrogen. When adding ferrochromium nitride, add it slowly with a shovel and pay close attention to the situation in the furnace to prevent gas enrichment and splashing. Adjust the appropriate power according to the speed of adding ferrochromium nitride to ensure that the temperature of molten steel remains at 1575℃~1585℃. After the ferrochromium nitride alloy is completely melted, take samples for analysis.

[0021] If the nitrogen element in the composition meets the target requirements, pull out all the slag and start making white slag for deoxidation again. When making slag, add slag-making materials, including lime, fluorite, etc. first; after the slag is made, start inserting aluminum wire for deoxidation, and add 5.5-6.3kg of aluminum wire for the first time; start sampling after deoxidation, and start adding various alloy materials to the target composition after the oxygen is appropriate. The added alloy materials include low-aluminum ferrosilicon. After the alloy materials are added and the alloy is melted, take samples for analysis, and start heating and tapping if the composition is appropriate.

[0022] Before tapping, remove all the slag. The tapping temperature is controlled at 1610-1630℃. Take an appropriate amount of carbon-free protective slag from the low-temperature drying chamber in advance and hang it 200-400mm away from the bottom of the ingot mold; the amount of protective slag is 0.8-1.2kg / t; the pouring temperature is controlled at 1490℃-1500℃.

[0023] (1.3) Casting process: After pouring, cast at full flow. After the steel flow is stable and the molten steel flows into the ingot mold, stabilize the injection speed. Pay close attention to the steel flow and the amount of molten steel in the ladle in the later stage of casting, make good riser shrinkage compensation, and reasonably control the injection margin. Use N2 gas to protect the casting. It is strictly forbidden to add exothermic agents and carbonized rice husks into the ingot mold after casting. After pouring for 1.5 to 2 hours, the mold is hoisted into the slow cooling pit, and the slow cooling time is more than 12 hours.

[0024] (2) Electroslag remelting steps: The electrode ingot needs to be baked at 280℃~320℃ for 50min~70min, and the surface treatment is carried out using a grinder, peeling lathe or shot blasting machine. The riser end of the electrode ingot is cut flat, and the electrode ingot and the dummy electrode are welded using carbon dioxide shielded welding. The pre-molten slag baking temperature is ≥650℃ and the temperature is kept for more than 6 hours to achieve sufficient baking. The arc-starting plate uses the arc-starting plate of Bengang with a thickness of 30±5mm. The protective plate uses thick carbon steel plate; the edges of the arc-starting plate and the protective plate are chamfered and deburred, and rust-free polishing is carried out; the arc-starting plate is placed in the middle of the protective plate, and welding is carried out in the form of interrupted welding. After welding is completed, the flux skin and splashing are removed, and the protective plate is not warped. Before loading the furnace, the welded arc-starting plate and the protective plate need to be baked at a baking temperature of 100~150℃ and a baking time of 15min~25min.

[0025] The whole remelting process is nitrogen-filled, and nitrogen replacement before smelting, nitrogen filling during the whole slag adding process, and nitrogen filling process are performed; the nitrogen replacement time is 9 to 10 minutes, and the flow rate is 180 to 260 LPM (liters / minute); during the slag adding process, the nitrogen filling flow rate is 60 to 100 LPM; during the operation process, the nitrogen filling flow rate is 140 to 200 LPM; electroslag ingots with a size of Φ285 to Φ310 mm and a length of 950 to 1000 mm are produced.

[0026] (3) Pre-forging heating steps: Masterbatch requirements: The masterbatch uses the above-mentioned electroslag ingots with a diameter of Φ285~Φ310mm and a length of 950mm~1000mm; the composition meets the requirements of the steel grade range, the surface quality has no obvious cracks or bulges, and the ends have no loose cracks. The masterbatch is first polished to a bright surface without cracks, and then the riser is sawed. After sawing, flaw detection is performed to ensure that both ends are cut cleanly. Forging specifications: Forging is a forging plate with a specification of (245~250)×(245~250)×L (mm) (thickness×width×length).

[0027] Heating process: when the furnace temperature is ≤450℃, the billet is put into the furnace and kept warm for 1.5 to 3 hours; then the temperature is increased by 75 to 110℃ per hour to 850 to 900℃, and kept warm for 1.5 to 3 hours; then the temperature is increased to 1000℃ to 1050℃ at the maximum power, and kept warm for 3.5 to 5.5 hours; then the temperature is increased to 1200 to 1250℃ at the maximum power, and kept warm for 20 to 26 hours; finally, the furnace is cooled to 1180℃ to 1220℃, and kept warm for 4 to 5 hours.

[0028] (4) Forging steps: Pre-forging state: the riser of the base material is removed, and the surface is turned and polished to remove surface defects such as oxide scale; before forging this high nitrogen austenitic steel, at least two pieces of die steel are forged to ensure that the anvil and hammer are fully preheated.

[0029] Since high nitrogen austenitic steel contains high alloy elements and is prone to cracking during forging, a reasonable forging process has been developed to ensure its forging feasibility. The forging process uses seven fires to first roughen and then draw. The roughening uses a four-sided four-square arc forging process to ensure its forging feasibility. The first fire forges the base material into an octagonal cross-section. Figure 1 As shown in the figure, due to the low forging temperature of the steel, the second to fourth fires are used to roughen the blank; the fifth to seventh fires are used to lengthen the blank and finally reach the target size. The final forging temperature of the workpiece during each forging is ≥1000℃, and the insulation time between each forging is ≥1 hour; forging is stopped when the forging temperature is lower than 1000℃, and subsequent forging is carried out after returning to the furnace. During the process, attention should be paid to the surface quality of the forgings. Forging should be stopped in time when cracks appear to prevent cracks.

[0030] (4.1) First fire: The operating machine clamps the riser end of the forging and uses a flat anvil to perform axial forging on the forging; first forge four flat surfaces and four curved surfaces; then forge the four curved surfaces to forge the forging into an octagon; then stretch and reduce the forging, and the four surfaces correspond to the four curved surfaces. The steel contains a high amount of alloy elements and is prone to cracking during the forging process. The amount of reduction during the forging process is very important, and the single-sided reduction is controlled at 5mm to 8mm. Figure 1 As shown, first forge the AD-BC surface, flip 180 degrees to forge the BD-AC surface, flip 45 degrees to forge the BC surface, and flip 180 degrees to forge the CD surface.

[0031] (4.2) Second fire: After the billet is out of the furnace, it is forged at high temperature to be roughened. The operator clamps the middle part of the forging, and the front commander instructs the operator to put the billet straight, press the press lightly, and after the billet is stabilized, release the jaws, and the press starts to press down. The target is to upset to 1 / 2 of the length of the electroslag ingot. When upsetting, use the slow gear. This fire adopts multiple upsettings, and the down pressure of each upsetting is 20mm~25mm, and then the subsequent times of upsetting are carried out, and the upsetting length is 130mm~150mm.

[0032] (4.3) The third and fourth fires: the same process requirements as the second fire are adopted: the down pressure of each upsetting is 20mm~25mm, and the upsetting length is 130mm~150mm.

[0033] (4.4) Fifth fire: Perform the stretching operation. The single-sided pressing amount is 5mm to 8mm. The operator clamps the forging and uses the stretching anvil to stretch the forging. After it comes out of the furnace, one side is pressed down by 5mm to 8mm, and then pressed down for the second time, and so on, until the thickness is 300mm to 315mm. During the forging process, an infrared thermometer is used to measure the surface temperature of the forging. If the forging temperature is lower than 1000℃, the pressing is stopped in time. The commander should pay attention to the surface quality of the forging during the forging process, and stop forging in time if there are signs of cracks to prevent cracks.

[0034] (4.5) The sixth fire: Perform the stretching operation, the jaws of the operating machine are 90 degrees, the blank is clamped and turned 90 degrees, so that the blank changes to the wide side and starts to press down. The single-sided press amount is 5mm~8mm, and then the second press is performed, and so on, the press is pressed down to the wide side of 295mm~305mm. During the forging process, the infrared thermometer is used to detect the surface temperature of the forging for measurement. When the temperature reaches 1000℃, the press is stopped in time to prevent cracks. The commander should pay attention to the surface quality of the forging during the forging process, and stop forging in time if there are signs of cracks to prevent cracks.

[0035] (4.6) The seventh fire: Perform the stretching operation, the jaws of the operating machine are 90 degrees, the blank is clamped and turned 90 degrees, and the blank is replaced with the thick side and pressed down. The single-side press is 5mm to 8mm, and then the second press is performed, and so on, the press is pressed down to the wide side of 245mm to 250mm. During the forging process, the infrared thermometer is used to detect the surface temperature of the forging for measurement. If the forging temperature is lower than 1000℃, stop pressing in time to prevent cracks.

[0036] The commander shall pay attention to the forging steps mentioned above; the head and tail of the forged billet shall be clearly distinguished and marked. Forging with cracks is strictly prohibited, except for cracks at the riser; after the surface is obviously dark red, forging is prohibited, and the forging shall be carried out again after the temperature returns to the forging temperature or directly returned to the furnace; inverted forging is prohibited; when the cumulative deformation of large deformation in the later stage is large, when bright star-shaped patterns appear on the end face, any forging operation is prohibited, and the chamfering is directly carried out with a light hammer, and the star-shaped patterns are lightened when the core temperature returns, and then forging is carried out again; large-section forging is prohibited, steps of >25mm are strictly prohibited, and small steps are forged with half-coverage; when forging the connection part of the clamp, it is prohibited to extend the forging to the undeformed position of the clamp, and the temperature of the undeformed position is controlled to be lower than the forging temperature; when there are signs of blackening on the edge of the end, it is prohibited to approach forging and return to the furnace. Example

[0037] (1) Smelting steps: including pre-smelting preparation, smelting and casting process; (1.1) Preparation process before smelting: clean the new ladle and place it under the ladle baking device to bake it to 780℃, waiting for low-temperature steel smelting. Put the ingot mold into the slow cooling pit for insulation, place the ingot mold 2 hours before pouring steel, and the ingot mold temperature out of the pit is 100℃. The casting refractory is dried in the low-temperature drying room for 30 hours. The alloy material and pre-molten slag are baked according to the current baking method, and the chromium nitride is not baked.

[0038] Raw material preparation: Steel material: industrial pure iron. Alloy material: low aluminum ferrosilicon, metallic manganese, electrolytic nickel plate, metallic chromium, molybdenum bar, ferrovanadium, niobium block and ferrochrome nitride. Auxiliary materials: aluminum granules, aluminum wire, 622 pre-melted slag; aluminum granules are used for diffusion deoxidation, and aluminum wire is used for precipitation deoxidation.

[0039] (1.2) Smelting process: When the temperature of molten steel rises to 1558℃, samples are taken for analysis. When the temperature of molten steel rises to 1578℃, some slag is removed. The bottom blowing pressure is adjusted to 615kpa and the bottom blowing flow rate is adjusted to 7L / min. The bottom blowing system is started and blowing begins. The medium frequency power is adjusted and nitrogen is added to ferrochromium nitride. The appropriate power is adjusted according to the speed of adding ferrochromium nitride. The temperature of molten steel is maintained at 1585℃. After the ferrochromium nitride alloy is completely melted, samples are taken for analysis.

[0040] After the nitrogen element meets the target requirement, pull out all the slag and start making white slag for deoxidation again. When making slag, add lime, fluorite and other materials first. After the slag is made, start inserting aluminum wire for deoxidation. The first time, 6.2kg of aluminum wire is added. After deoxidation, start sampling. After the oxygen is appropriate, start adding various alloy materials to the target composition. When the composition is appropriate, start heating and steelmaking.

[0041] Before tapping, remove all slag. The tapping temperature is controlled at 1630℃. Take an appropriate amount of carbon-free protective slag from the low-temperature drying chamber in advance and hang it 270mm away from the bottom of the ingot mold; the amount of protective slag is 0.9kg / t; the pouring temperature is controlled at 1490℃.

[0042] (1.3) Casting process: After pouring, pour at full flow, make sure to compensate for shrinkage at the riser, and control the excess amount. Use N2 gas to protect the casting. After pouring for 1.5 hours, hoist the mold into the slow cooling pit for 19 hours.

[0043] (2) Electroslag remelting steps: The electrode ingot is baked at 320℃ for 57min and surface treated with a grinder. The riser end of the electrode ingot is cut flat, and the electrode ingot and the dummy electrode are welded using carbon dioxide shielded welding. The pre-slag baking temperature is 699℃ and the temperature is kept for more than 6 hours to achieve sufficient baking. The arc-starting plate uses the arc-starting plate of Bengang with a thickness of 30mm. The protective plate uses thick carbon steel plate; the edges of the arc-starting plate and the protective plate are chamfered and deburred, and rust-free polishing is performed; the arc-starting plate is placed in the middle of the protective plate, and welding is performed in the form of interrupted welding. After welding, the flux skin and splashing are removed, and the protective plate is free of warping. Before loading the furnace, the welded arc-starting plate and the protective plate need to be baked at a baking temperature of 150℃ and a baking time of 25min. Electroslag ingots with a diameter of Φ300mm and a length of 1000mm are made. Nitrogen is filled throughout the remelting process, and nitrogen replacement before smelting, nitrogen filling during the slag addition process, and nitrogen filling during operation are performed. The nitrogen replacement time is 10 minutes and the flow rate is 248LPM (liters / minute); during the slag adding process, the nitrogen filling flow rate is 77LPM; during the operation process, the nitrogen filling flow rate is 152LPM.

[0044] (3) Pre-forging heating steps: Master material requirements: The master material uses Φ300mm electroslag ingots and a length of 1000mm; the composition meets the requirements of the steel grade range, the surface quality has no obvious cracks or bulges, and the ends have no loose cracks. The parent material is first polished to a bright surface without cracks, and then the riser is sawed. After sawing, flaw detection is performed to ensure that both ends are cut cleanly. Forging specifications: Forging is a forging plate with a specification of (245~250)×(245~250)×L (mm) (thickness×width×length).

[0045] Heating process: put the billet into the furnace at 58℃ and keep it warm for 3 hours; then heat it up to 900℃ at 75℃ per hour and keep it warm for 3 hours; then heat it up to 1020℃ at maximum power and keep it warm for 4.5 hours; then heat it up to 1210℃ at maximum power and keep it warm for 22 hours; finally cool it to 1180℃ with the furnace and keep it warm for 5 hours.

[0046] (4) Forging steps: (4.1) First fire: The operator clamps the riser end of the forging and uses a flat anvil to forge the forging axially; first forge four planes and four arc-shaped surfaces; then forge the four arc-shaped surfaces to forge the forging into an octagon; then draw and reduce the length, and the four surfaces correspond to the four arc-shaped surfaces, and control the single-side reduction at 5mm. The final forging temperature is 1010℃, and the billet is upright and returned to the furnace. Forge the AD-BC surface first, turn 180 degrees to forge the BD-AC surface, turn 45 degrees to forge the BC surface, and turn 180 degrees to forge the CD surface.

[0047] (4.2) Second fire: After the billet is out of the furnace, it is roughened. The operator clamps the middle part of the forging, and the front commander instructs the operator to put the billet straight, press the press lightly, and after the billet is stabilized, release the jaws, and the press starts to press down. The target is to use a slow gear when upsetting to 1 / 2 of the length of the electroslag ingot. Each time the upsetting pressure is 25mm, and then the second upsetting is carried out. The upsetting length of the fire is 140mm.

[0048] (4.3) Third and fourth fires: adopt the same process requirements as the second fire; the upsetting length of each fire is 140mm.

[0049] (4.4) Fifth firing: Perform the drawing operation. The single-side pressing amount is 5 mm. The manipulator clamps the forging and uses the drawing anvil to draw the forging. After it comes out of the furnace, one side is first pressed down by 5 mm, and then pressed down for the second time until the thickness reaches 300 mm.

[0050] (4.5) The sixth fire: perform the lengthening operation, operate the machine jaws to 90 degrees, clamp the blank and turn it 90 degrees, so that the blank changes to the wide side and starts to press down. The single-side press amount is 5mm, and then press down for the second time, and so on; press down to 300mm on the wide side.

[0051] (4.6) The seventh fire: Perform the drawing operation, the jaws of the operating machine are 90 degrees, the blank is clamped and turned 90 degrees, and the blank is changed to the thick side and starts to press down. The single side press is 5mm. Then press down for the second time, and so on. Press down to the wide side 246mm square steel.

[0052] After forging, the properties of this high nitrogen austenitic steel are as follows: yield strength in the helium temperature zone is 1408Mpa, fracture toughness is 191Mpa / cm 2 , elongation after break is 31%. Meets product quality goals.

[0053] The chemical composition and mass percentage of high nitrogen austenitic steel are: C 0.04%, Si 0.150%, Mn 4.2%, P0.027%, S 0.025%, Ni 13.80%, Al 0.05%, Cr 22.50%, Mo 2.13%, V 0.178%, Nb 0.11%, N0.23%, Ti 0.002%, and the balance is Fe and unavoidable impurities.

Claims

1. A process for producing high nitrogen austenitic steel forging plates, characterized in that: It includes electroslag remelting, pre-forging heating and forging steps; The forging step adopts seven-fire forging, and the pressing amount of each fire is controlled as follows: the pressing amount of the first fire is 5mm-8mm; the pressing amount of each upsetting in the second, third and fourth fires is 20mm-25mm, and the total upsetting length of each fire is 130mm-150mm; the pressing amount of each side of the fifth fire is 5mm-8mm for each elongation, and the thickness of the fire is pressed to 300mm-315mm; the pressing amount of each side of the sixth fire is 5mm-8mm for each elongation, and the thickness of the wide surface is 295mm-305mm; the pressing amount of each side of the seventh fire is 5mm-8mm for each elongation, and the thickness of the wide surface is 245mm-250mm.

2. The process for producing a high nitrogen austenitic steel forged plate according to claim 1, characterized in that: The final forging temperature of the workpiece during each forging is ≥1000℃, and the insulation time between each firing is ≥1h.

3. A process for producing a high nitrogen austenitic steel forged plate according to claim 1 or 2, characterized in that: The pre-forging heating step comprises heating to 1200-1250° C. and keeping the temperature for 20-26 hours; cooling to 1180-1220° C. and keeping the temperature.

4. The process for producing a high nitrogen austenitic steel forging plate according to claim 3, characterized in that: The pre-forging heating steps are as follows: the blank is put into the furnace with a furnace temperature of ≤450°C and kept warm for 1.5 to 3 hours; then the temperature is increased by 75 to 110°C per hour to 850 to 900°C and kept warm for 1.5 to 3 hours; then the temperature is increased by maximum power to 1000 to 1050°C and kept warm for 3.5 to 5.5 hours; then the temperature is increased by maximum power to 1200 to 1250°C and kept warm for 20 to 26 hours; finally, the blank is cooled with the furnace to 1180 to 1220°C and kept warm for 4 to 5 hours.

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