A large nitrogen-strengthened austenitic stainless steel forging and a method of making the same

Abstract

The present application relates to a kind of large nitrogen reinforced austenitic stainless steel forgings and its preparation method, belong to large stainless steel forging manufacturing technical field, solve the problem that large nitrogen reinforced austenitic stainless steel forgings in prior art is prone to cracking, grain size control is difficult.The method comprises the following steps: (1) first homogenization heat treatment is carried out to electroslag ingot and is kept warm;(2) high temperature large deformation mode is used to make blank;(3) heating and keeping warm below the temperature of secondary recrystallization temperature and precision forging to finished product size;(4) the forged piece of finished product size is again homogenized heat treatment and is kept warm.The method effectively controls the generation of crack in forging process, so that the precipitated phase of finished product forging is effectively improved, there is no σ phase and ferrite in microstructure, and the grain is small and uniform.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of large-scale stainless steel forging manufacturing, and particularly relates to a large-scale nitrogen-strengthened austenitic stainless steel forging and a preparation method thereof. BACKGROUND

[0002] A fusion reactor is an important way to ultimately solve the energy and environmental problems of human beings because of its rich fuel resources, no pollution to the environment and no radioactive nuclear waste. The fusion reactor equipment forgings generally adopt low-temperature-resistant non-magnetic austenitic stainless steel materials, mainly 316LN materials. However, the 316LN material still cannot meet the low-temperature performance requirements of some key parts. At present, the mainstream alternative material is high-nitrogen austenitic stainless steel N50. The low-temperature strength and toughness of N50 forgings are required to be high. Because of the high nitrogen content and high alloy content, the forgeable temperature range of N50 is narrow, the deformation resistance is large, the forging is prone to cracking, and the grain size control is difficult. At present, the manufacturing plant does not have the engineering manufacturing capability of N50 large forgings. SUMMARY

[0003] In view of the above analysis, the embodiments of the present application aim to provide a large-scale nitrogen-strengthened austenitic stainless steel forging and a preparation method thereof, so as to solve the problems of easy cracking and difficult grain size control of the existing large-scale nitrogen-strengthened austenitic stainless steel forging.

[0004] In one aspect, the embodiments of the present application provide a preparation method of a large-scale nitrogen-strengthened austenitic stainless steel forging, and the method comprises the following steps:

[0005] (1) performing first homogenization heat treatment on the electroslag ingot and keeping the temperature;

[0006] (2) performing blank making by using a high-temperature large deformation method;

[0007] (3) performing heating and keeping the temperature below the secondary recrystallization temperature and performing finish forging to the finished product size;

[0008] (4) performing second homogenization heat treatment on the finished product size forging and keeping the temperature.

[0009] Preferably, the high-temperature large deformation method comprises the following steps:

[0010] (201) upsetting treatment;

[0011] (202) intermediate secondary homogenization heat treatment and keeping the temperature;

[0012] (203) square bar drawing treatment.

[0013] Preferably, in step (1), the first homogenization heat treatment adopts a stepwise heating method, including: the first temperature step is 840-860℃, the second temperature step is 1055-1075℃, and the third temperature step is 1190-1210℃.

[0014] Preferably, in step (201), the upsetting ratio is greater than or equal to 1.7, and the final forging temperature is higher than 900 DEG C.

[0015] Preferably, in step (202), the intermediate sub-uniformizing heat treatment is performed in a stepwise heating mode, including: the first temperature step is 1055-1075 DEG C, and the second temperature step is 1190-1210 DEG C.

[0016] Preferably, in step (203), the square upsetting is performed on an upper and lower wide flat anvil, the anvil width ratio is 0.6-0.7, the reduction per anvil is greater than or equal to 20%, the forging ratio is greater than or equal to 1.7, and the final forging temperature is higher than 900 DEG C.

[0017] Preferably, step (3) comprises: heating and holding at a temperature below the secondary recrystallization temperature and then performing finish forging, and if the first heating and holding at a temperature below the secondary recrystallization temperature and then performing finish forging does not reach the finished product size, then heating and holding at a temperature below the secondary recrystallization temperature and then performing finish forging again until the finished product size requirement is met.

[0018] Preferably, in step (4), the temperature of the secondary uniformizing heat treatment is 1055-1075 DEG C.

[0019] Preferably, the preparation method further comprises: before the first uniformizing heat treatment, pre-treating the electroslag ingot to remove surface slag grooves.

[0020] In another aspect, the present application also provides a large nitrogen-strengthened austenitic stainless steel forging prepared by the above preparation method.

[0021] Compared with the prior art, the present application can achieve at least one of the following beneficial effects:

[0022] 1. The present application adopts a forging method combining high-temperature large deformation and finish forging at a temperature below the secondary recrystallization temperature, and performs first uniformizing heat treatment before forging and secondary uniformizing heat treatment on the forged blank after forging, thereby solving a series of problems in the engineering manufacturing process of the large nitrogen-strengthened austenitic stainless steel forging, effectively controlling the generation of cracks in the forging process, and effectively improving the precipitated phase of the finished forging, so that the microstructure is free of sigma phase and ferrite, and the grains are fine and uniform.

[0023] 2. The electroslag ingot is pre-treated to remove surface slag grooves before the first uniformizing heat treatment, thereby effectively avoiding cracks caused by the slag grooves in the subsequent forging process.

[0024] 3. The first homogenization heat treatment adopts a step heating mode, and the segregation elements are pre-diffused by keeping the temperature at the first temperature step and the second temperature step for a period of time, so as to avoid the initial melting of the grain boundary, and then the temperature is increased to the third step and kept for a period of time, so as to gradually eliminate the segregation and dendrite.

[0025] 4. The upper and lower wide flat anvil can effectively avoid the cracking caused by the large tensile stress in the deformation process of the blank, and the chamfering treatment of the forged blank after the square bar drawing can avoid the cracking caused by the stress concentration of the subsequent sharp edges.

[0026] 5. The fine forging is formed below the secondary recrystallization temperature, so as to effectively solve the problems of grain coarsening and mixed crystal caused by repeated thermal cycles.

[0027] 6. The large nitrogen-strengthened austenitic stainless steel forging obtained by the preparation method has no cracks on the surface, the precipitated phase of the forging is effectively improved, there is no sigma phase and ferrite in the microstructure, and the grains are small and uniform.

[0028] In the present application, the above technical solutions can be combined with each other to realize more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification or be understood by implementing the present application. The purposes and other advantages of the present application can be realized and obtained from the contents specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings are included to provide a further understanding of the application and are incorporated herein and constitute a part of the application. The drawings illustrate embodiments of the application and, together with the description, serve to explain the principles of the application. In the drawings:

[0030] Figure 1 It is a first homogenization heat treatment and holding curve diagram;

[0031] Figure 2 It is a middle secondary homogenization heat treatment and holding curve diagram;

[0032] Figure 3 It is a heating and holding curve diagram at a temperature below the secondary recrystallization temperature;

[0033] Figure 4 It is a third homogenization heat treatment and holding curve diagram;

[0034] Figure 5 It is a structure size diagram of the electroslag ingot of Example 1;

[0035] Figure 6 It is a structure size diagram of the electroslag ingot of Example 1 after processing and pretreatment;

[0036] Figure 7 It is a schematic diagram of the forged blank after upsetting of Example 1;

[0037] Figure 8 Drawing of the elongated and forged blank of Example 1 (the left is a front view and the right is a sectional view) ;

[0038] Figure 9 Drawing of the chamfered blank of Example 1;

[0039] Figure 10 Drawing of the gas cutting of the finished forging of Example 1;

[0040] Figure 11 Drawing of the finished forging of Example 1;

[0041] Figure 12a Metallographic drawing of the finished forging of Example 1;

[0042] Figure 12b Another metallographic drawing of the finished forging of Example 1. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present application will be described in detail below with reference to the drawings, which constitute a part of this application, and are used to explain the principles of the application together with the embodiments of the application, but are not used to limit the scope of the application.

[0044] In one aspect, the present application provides a method for preparing a large nitrogen strengthened austenitic stainless steel forging, characterized in that the method comprises the following steps:

[0045] (1) performing first homogenization heat treatment and holding on the electroslag ingot;

[0046] (2) performing blanking by high-temperature large deformation;

[0047] (3) heating and holding at a temperature below the second recrystallization temperature and precision forging to the finished size; as shown in Figure 3 ;

[0048] (4) performing second homogenization heat treatment and holding on the finished size forging.

[0049] Compared with the prior art, the present application adopts the forging method combining high-temperature large deformation and precision forging at a temperature below the second recrystallization temperature, and performs first homogenization heat treatment before forging and second homogenization heat treatment on the blank after forging, which solves a series of problems in the engineering manufacturing process of the large nitrogen strengthened austenitic stainless steel forging, effectively controls the generation of cracks in the forging process, and effectively improves the precipitated phase of the finished forging, so that there is no σ phase and ferrite in the microstructure, the grains are small and uniform, and the final technical requirements are met.

[0050] Exemplarily, the nitrogen strengthened austenitic stainless steel forging is high-nitrogen austenitic stainless steel N50.

[0051] It should be noted that the weight of the electroslag ingot is greater than or equal to 5 tons, and the nitrogen content is 2000 ppm-4000 ppm.

[0052] Specifically, the preparation method further comprises: pretreating the electroslag ingot before the first homogenization heat treatment, and machining to remove the surface slag groove after annealing the electroslag ingot, which can effectively avoid cracks caused by the slag groove in the subsequent forging process.

[0053] Specifically, the high-temperature large deformation mode comprises the following steps:

[0054] (201) upsetting treatment;

[0055] (202) intermediate secondary homogenization heat treatment and holding;

[0056] (203) square bar drawing treatment.

[0057] Exemplarily, in step (1), the first homogenization heat treatment adopts a stepwise heating mode, as shown in the following table: Figure 1 As shown, the first temperature step is 840-860 DEG C, the second temperature step is 1055-1075 DEG C, and the third temperature step is 1190-1210 DEG C.

[0058] It should be noted that holding is performed at each temperature step, and the holding time is determined according to the minimum position section of the forging, the minimum holding time is determined according to one hour per 80 mm, and the maximum holding time is determined according to one hour per 50 mm.

[0059] Specifically, the first temperature step is a thermal expansion coefficient mutation point, after holding and homogenizing at this temperature, the temperature is raised to the second temperature step, and holding and homogenizing at the second temperature step will not cause obvious grain growth, and then holding at the third temperature step at high temperature provides necessary conditions for large deformation. In addition, the first homogenization heat treatment adopts a stepwise heating mode, and holding for a period of time at the first temperature step and the second temperature step can make the segregation elements pre-diffuse, avoid the initial melting of the grain boundary, and then raise to the third step and hold for a certain period of time, which can gradually eliminate segregation and dendrites.

[0060] In the high-temperature large deformation forging mode of the application, in step (201), the upsetting ratio in the upsetting treatment is greater than or equal to 1.7 (for example, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, etc.), and the final forging temperature is higher than 900 DEG C (for example, 1000 DEG C, 1100 DEG C, 1200 DEG C, 1300 DEG C, 1400 DEG C, 1500 DEG C, etc.). The forging ratio reaches 1.7 or more to ensure sufficient large deformation and meet the requirement of fine and uniform grain; the final forging temperature is higher than 900 DEG C to avoid cracking, and the cracking tendency will increase if it is lower than 900 DEG C.

[0061] It should be noted that the upset treatment after the forging blank is returned to the furnace, the forging blank is wrapped with an asbestos anvil, the purpose is to keep warm forging, to avoid the temperature drop too fast in the forging process, can prolong the effective forging time. After returning to the furnace, the intermediate sub-uniformization heat treatment is carried out.

[0062] Specifically, in step (202), the intermediate sub-uniformization heat treatment also adopts a stepped heating mode, as shown in the table, including: the first temperature step is 1055-1075℃, and the second temperature step is 1190-1210℃. The intermediate sub-uniformization heat treatment adopts a stepped heating mode, which is also to refine the grains. Figure 2

[0063] It should be noted that the holding time is determined according to the minimum position section of the forging, the minimum holding time is determined according to one hour per 80mm, and the maximum holding time is determined according to one hour per 50mm.

[0064] Exemplarily, in step (203), in the square bar drawing process, an upper and lower wide flat anvil is used for square bar drawing, the anvil width ratio is 0.6-0.7, the reduction rate of each anvil is ≥20% (for example, 25%, 30%, 35%, 40%, etc.), the forging ratio is 1.7 or more (for example, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, etc.), and the final forging temperature is higher than 900℃ (for example, 1000℃, 1100℃, 1200℃, 1300℃, 1400℃, 1500℃, etc.). The use of an upper and lower wide flat anvil can effectively avoid cracking caused by large tensile stress during blank deformation; the anvil width ratio, the reduction rate and the forging ratio in the above range can ensure sufficient large deformation; and the final forging temperature higher than 900℃ is to avoid cracking, and the cracking tendency will increase if it is lower than 900℃.

[0065] It should be noted that after the square bar drawing process is completed, the edge is chamfered, for example, octagonal chamfering, to avoid cracking caused by stress concentration of the relatively sharp edge in the subsequent process.

[0066] It should be noted that the forging blank after the square bar drawing process is returned to the furnace, the forging blank is wrapped with an asbestos anvil, the purpose is to keep warm forging, to avoid the temperature drop too fast in the forging process, can prolong the effective forging time. After returning to the furnace, the intermediate sub-uniformization heat treatment is carried out.

[0067] Exemplarily, the secondary recrystallization temperature of the austenitic stainless steel is 1100℃.

[0068] The temperature below the secondary recrystallization temperature can be 1070℃, 1060℃, 1050℃, etc. Heating and holding at a temperature below the secondary recrystallization temperature can ensure sufficient degree of component homogenization, and the grain size will not grow significantly.

[0069] ​It should be noted that the holding time at the temperature below the secondary recrystallization temperature is determined according to the minimum cross section of the forging, the minimum holding time is determined as one hour per 80 mm, and the maximum holding time is determined as one hour per 50 mm.

[0070] Exemplarily, the step (3) comprises: heating and holding at a temperature below the secondary recrystallization temperature and performing finish forging, and if the first heat finish forging is not forged to the finished product size, heating and holding at a temperature below the secondary recrystallization temperature again and performing finish forging until the forging meets the finished product size requirement. The final forging forming at the temperature below the secondary recrystallization temperature can effectively solve the problems of grain coarsening and mixed crystal caused by repeated thermal cycles.

[0071] Exemplarily, as shown in Figure 4 , in the step (4), the temperature of the second homogenization heat treatment is 1055-1075℃. The degree of component homogenization can be ensured to be sufficient while the grain size does not obviously grow.

[0072] The forging blank is air-cooled to room temperature after holding at the temperature of the second homogenization heat treatment. The holding time is determined according to the minimum cross section of the forging, the minimum holding time is determined as one hour per 80 mm, and the maximum holding time is determined as one hour per 50 mm.

[0073] On the other hand, the present application also provides the large nitrogen strengthened austenitic stainless steel forging obtained by the above preparation method. The large nitrogen strengthened austenitic stainless steel forging has no cracks on the surface, the precipitated phase of the forging is effectively improved, there is no σ phase and ferrite and other organizations in the microstructure, and the grain is fine and uniform.

[0074] Hereinafter, the large nitrogen strengthened austenitic stainless steel forging and the preparation method thereof of the present application will be further described through specific examples.

[0075] Example 1

[0076] The present embodiment provides a preparation method of a large nitrogen strengthened austenitic stainless steel forging, and the preparation method comprises:

[0077] (1) The high-nitrogen austenitic stainless steel N50 electroslag ingot is pretreated, the electroslag ingot is annealed and then processed to remove surface slag grooves, and the electroslag ingot is subjected to first homogenization heat treatment and holding; as shown in Figure 5 , the weight of the electroslag ingot is 8900 kg, and the content of nitrogen element is 3000 ppm; the structural size after removing the surface slag grooves is as shown in Figure 6 ; the first homogenization heat treatment adopts a stepwise heating mode, which comprises: the first temperature step is 850℃, the second temperature step is 1065℃, and the third temperature step is 1200℃; holding is performed at each temperature step, and the holding time is determined as one hour per 70 mm of the minimum cross section of the forging.

[0078] (2) using high temperature and large deformation to make the blank, including:

[0079] (201) upsetting treatment, upsetting ratio is 1.7, and the final forging temperature is 1000℃; such as Figure 7 ;

[0080] (202) after the upsetting treatment, the forged blank is wrapped with an asbestos anvil before returning to the furnace, and the intermediate secondary homogenization heat treatment is carried out after returning to the furnace and insulation; insulation is carried out at each temperature step, and the insulation time is determined according to the minimum position section of the forging per 70mm per hour;

[0081] (203) square bar drawing treatment, using upper and lower wide flat anvil for square bar drawing, anvil width ratio is 0.6, the reduction of each anvil is 20%, the forging ratio is 1.7, and the final forging temperature is 1000℃; such as Figure 8 ; after the square bar drawing treatment, the edge is processed into an inverted octagon; such as Figure 9 .

[0082] (3) after the square bar drawing treatment, the forged blank is wrapped with an asbestos anvil before returning to the furnace, and heating and insulation are carried out at a temperature below the secondary recrystallization temperature (1065℃) and the forging is finished to the finished product size; the insulation time is determined according to the minimum position section of the forging per 70mm per hour; if the first time of the first fire is not forged to the finished product size, heating and insulation are carried out again at a temperature below the secondary recrystallization temperature and the forging is finished until the forging meets the finished product size requirements.

[0083] Figure 10 The abandoned material is removed from the ingot end and the water end after the finish forging; Figure 11 The finished product forging structure size is (330mm x 605mm x 5300mm).

[0084] (4) the finished product size forging is carried out again at 1065℃ homogenization heat treatment and insulation, and the insulation time is determined according to the minimum position section of the forging per 70mm per hour; after insulation, air cooling to room temperature.

[0085] The large nitrogen strengthened austenitic stainless steel forging obtained in the embodiment has a uniform surface and no cracks.

[0086] Figure 12a And 12b The metallographic diagram of the large nitrogen strengthened austenitic stainless steel forging obtained in the embodiment can be seen that the grain size of the forging is 4-5 level, the difference is less than 2 level, the grain is small and uniform, and there is no harmful phase such as sigma phase and ferrite.

[0087] Example 2

[0088] The embodiment provides a preparation method of a large nitrogen strengthened austenitic stainless steel forging similar to example 1, except that the surface slag groove of the electroslag ingot is not removed before the first homogenization heat treatment.

[0089] The large nitrogen strengthened austenitic stainless steel forging obtained in this embodiment has a small amount of cracks on the surface.

[0090] Example 3

[0091] This embodiment provides a preparation method of a large nitrogen strengthened austenitic stainless steel forging similar to that of Example 1, except that the final forging temperature of upsetting and square bar drawing is 800℃.

[0092] The large nitrogen strengthened austenitic stainless steel forging obtained in this embodiment has a small amount of cracks on the surface.

[0093] Example 4

[0094] This embodiment provides a preparation method of a large nitrogen strengthened austenitic stainless steel forging similar to that of Example 1, except that the first homogenization heat treatment is not a step heating mode, but is directly heated to 1200℃.

[0095] The large nitrogen strengthened austenitic stainless steel forging obtained in this embodiment has a grain size of 1-3 levels, and a small amount of mixed crystals and coarse crystals.

[0096] Example 5

[0097] This embodiment provides a preparation method of a large nitrogen strengthened austenitic stainless steel forging similar to that of Example 1, except that the intermediate secondary homogenization heat treatment is not a step heating mode, but is directly heated to 1200℃.

[0098] The large nitrogen strengthened austenitic stainless steel forging obtained in this embodiment has a grain size of 1-3 levels, and a small amount of mixed crystals and coarse crystals.

[0099] The above description is only the preferred embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. A method for preparing large nitrogen-strengthened austenitic stainless steel forgings, characterized in that, The method includes the following steps: (1) Before the first homogenization heat treatment, the electroslag ingot is pretreated and the surface slag groove is removed after the electroslag ingot is annealed. (2) Perform initial homogenization heat treatment on the electroslag ingot and keep it at a constant temperature; The initial homogenization heat treatment adopts a stepped heating method, including: a first temperature step of 840-860℃, a second temperature step of 1055-1075℃, and a third temperature step of 1190-1210℃. The first temperature step is the point of abrupt change in the coefficient of thermal expansion. After holding and homogenizing at this temperature, the temperature is raised to the second temperature step. Holding and homogenizing at the second temperature step will not cause significant grain growth. Then, holding at high temperature at the third temperature step provides the necessary conditions for large deformation. (3) The billet is prepared by high temperature and large deformation. The high temperature and large deformation includes upsetting, intermediate homogenization heat treatment and heat preservation, and square drawing. In the square drawing, the anvil width ratio is adjusted to 0.6-0.7, the reduction rate of each anvil is ≥20%, and the forging ratio is ≥1.7, which can ensure sufficient large deformation and meet the requirements of fine and uniform grains. The intermediate homogenization heat treatment adopts a stepped heating method, including: the first temperature step is 1055-1075℃, and the second temperature step is 1190-1210℃, which can refine the grains. (4) Heating and holding the product at a temperature below the secondary recrystallization temperature and then precision forging it to the finished size can effectively solve the problem of coarse grains and mixed crystals caused by repeated thermal cycling. (5) The forgings of finished size are subjected to a second homogenization heat treatment and held at a temperature of 1055-1075℃ to ensure sufficient homogenization of composition and prevent significant grain growth.

2. The preparation method according to claim 1, characterized in that, In the upsetting process, the upsetting ratio is above 1.7, and the final forging temperature is above 900℃.

3. The preparation method according to claim 1, characterized in that, In the square drawing process, a wide and flat anvil is used for square drawing, with an anvil width ratio of 0.6-0.7, a reduction rate of ≥20% per anvil, a forging ratio of ≥1.7, and a final forging temperature of ≥900℃.

4. The preparation method according to claim 1, characterized in that, Step (4) includes: heating and holding at a temperature below the secondary recrystallization temperature and then precision forging. If the first precision forging does not reach the finished product size, the forging is heated and held at a temperature below the secondary recrystallization temperature again and then precision forged until the forging meets the finished product size requirements.

5. The preparation method according to claim 1, characterized in that, In step (5), the temperature of the second homogenization heat treatment is 1055-1065℃.

6. A large nitrogen-strengthened austenitic stainless steel forging, characterized in that, It is prepared by any one of the preparation methods described in claims 1-5.

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