A solution heat treatment process for large-size duplex stainless steel

Abstract

The present invention relates to the technical field of duplex stainless steel, and specifically discloses a solution heat treatment process for large-scale duplex stainless steel. By employing a process involving alternating cooling in water and air to a specific temperature followed by stress relief annealing in the furnace, and adding a heating step before exiting the furnace after heating and holding, the present invention effectively addresses issues such as imbalance in the two-phase structure, precipitation of harmful phases, and excessive stress during the solution treatment of large-scale duplex stainless steel. After solution treatment, the ferrite phase ratio is 45% to 55%, and the harmful precipitated phase is ≤1% (area percentage). Both hot working performance and mechanical properties are significantly improved, effectively expanding the application of duplex stainless steel in the manufacturing of high-end equipment such as pressure vessels and marine engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of duplex stainless steel, and in particular to a solution heat treatment process for large-scale duplex stainless steel. Background Art

[0002] Duplex stainless steel refers to stainless steel with a solid solution structure consisting of approximately 50% ferrite and 50% austenite. Typical grades include S32750 (F53). Its chemical composition is C ≤ 0.02%, Si ≤ 0.6%, Mn 0.7%–0.9%, S ≤ 0.001%, P ≤ 0.030%, Ni 6.5%–8.0%, Cr 25.0%–26.0%, Mo 3.5%–4.5%, and N 0.25%–0.32%. Duplex stainless steel combines the good toughness and weldability of austenitic stainless steel with the high strength and stress corrosion resistance of ferritic stainless steel, making it widely used in high-end manufacturing applications such as pressure vessels and offshore engineering.

[0003] In the production and processing of duplex stainless steel, solution heat treatment is a core step in determining its properties. The heating and cooling phases of this process are accompanied by microstructural transformations between the austenite and ferrite phases, as well as the dissolution and precipitation of deleterious phases. Effectively controlling the two-phase microstructure ratio and the state of the precipitated phases is crucial for improving the mechanical and processing properties of duplex stainless steel. Given the defined chemical composition and forging process, the solution heat treatment method directly determines the two-phase microstructure and the distribution of deleterious phases in duplex stainless steel.

[0004] The traditional solution heat treatment method involves heating and holding the forged bars at 1050°C to 1150°C, followed by water cooling. This method is subject to factors such as water entry conditions and cooling rate. Especially when processing large-sized bars, it can easily lead to an imbalance in the ratio of austenite to ferrite phases, a large amount of residual precipitates in the steel, and significant structural and thermal stresses. These problems not only increase the risk of cracking in the forged bars during the solution treatment process, but also negatively impact their mechanical properties and subsequent processing. Therefore, it is urgent to develop a new solution heat treatment process that can precisely control process parameters and optimize the distribution of the two-phase structure and harmful phases. Summary of the Invention

[0005] To address the problems of existing large-scale duplex stainless steels prone to two-phase ratio imbalance and the residual precipitation of large amounts of precipitated phases during solution heat treatment, resulting in solution cracking, poor workability, and poor mechanical properties, the present invention provides a solution heat treatment process for large-scale duplex stainless steel. This process utilizes a process that alternates water and air cooling to a specific temperature before returning to the furnace for stress relief annealing, and adds a heating process before exiting the furnace after heating and holding to control the structural transformation and precipitation of harmful phases during the solution process. This solves the problem of solution cracking in large-scale duplex stainless steel, improves hot workability and mechanical properties, and meets the increasingly stringent performance requirements for duplex stainless steel in high-end manufacturing.

[0006] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0007] A solution heat treatment process for large-size duplex stainless steel comprises the following steps:

[0008] S1, keep the forged round steel at 1050℃~1100℃, raise the furnace temperature by 20℃~40℃ 20min~30min before taking out of the furnace, then cool it down to 300℃~400℃ by alternating water and air cooling after taking out of the furnace;

[0009] S2, put the cooled forged round steel into a furnace at 300℃~400℃ for insulation, then cool it down to below 200℃ along with the furnace, and take it out of the furnace for air cooling.

[0010] When large-sized bars are solution heat treated, the time required for heat to transfer to the center of the bar increases significantly due to the increased cross-sectional size. During the heating and cooling stages, a large temperature difference will occur between the surface and the center of the bar, resulting in inconsistent phase transformation processes between austenite and ferrite. In addition, the larger the bar size, the more difficult it is to evenly diffuse alloying elements within the bar, further exacerbating the complexity of controlling the two-phase structure. At the same time, uneven structural transformation will induce structural stress. The inconsistency of structural transformation in large-sized bars makes the structural stress distribution more complex. These stresses are superimposed on each other, easily forming stress concentration areas within the bar, increasing the risk of cracking in the bar during the solution process, and seriously affecting product quality and yield. In addition, when large-sized bars are solution treated, due to the large overall heat capacity, both the heating and cooling processes require longer time. During the heating or cooling process, the bars stay in the sensitive temperature range for harmful phase precipitation for a longer time, which increases the probability of harmful phase nucleation. Moreover, the internal component segregation phenomenon of large-sized bars is relatively more serious, and the concentration of alloy elements in local areas varies greatly, which also creates more favorable conditions for the precipitation of harmful phases and increases the difficulty of controlling the precipitation of harmful phases.

[0011] Compared with the prior art, the solution heat treatment process of large-scale duplex stainless steel provided by the present invention first heats the forged round steel at 1050℃~1100℃ to dissolve the alloying elements in the austenite and ferrite phases, laying the foundation for obtaining an ideal duplex structure; the furnace temperature is increased by 20℃~40℃ 20min~30min before being taken out of the furnace, which can further promote the dissolution of harmful phases and avoid the precipitation of harmful phases in large quantities during transportation or subsequent cooling; at the same time, the heating process is also conducive to fine-tuning the phase transformation process of austenite and ferrite, and better controlling the ratio of the two-phase structure; then water is used to heat the steel. Alternating air cooling to a material temperature of 300℃~400℃. Compared with the traditional single water cooling method, alternating water and air cooling can more accurately control the cooling rate. The rapid water cooling stage can inhibit the precipitation of harmful phases, and the air cooling stage can relieve the thermal stress caused by too rapid water cooling, avoid the concentration of structural stress caused by uneven cooling, and effectively reduce the risk of cracking of large-size bars; the cooled forged round steel is placed in a 300℃~400℃ furnace for insulation, and the temperature is cooled to below 200℃ with the furnace before being taken out of the furnace for air cooling, which can fully release the residual structural stress and thermal stress in the steel and further reduce the internal stress level.

[0012] The present invention fully eliminates internal structural transformation stress and thermal stress by adopting a water-air alternating cooling method and re-furnace stress relief annealing, as well as adding a heating process before being discharged from the furnace. At the same time, it also effectively regulates the duplex structure ratio, which not only solves the problem of easy cracking of large-scale duplex stainless steel, but also improves the hot working performance and mechanical properties of the steel, which is conducive to expanding the application of duplex stainless steel in the field of high-end manufacturing and has high practical value.

[0013] It should be noted that the specifications of the large-size duplex stainless steel described in the present invention are Ф300mm~Ф580mm.

[0014] In addition, it should be noted that the above-mentioned large-size duplex stainless steel is S32750.

[0015] Furthermore, in step S1, the temperature is increased to 1050°C~1100°C at a rate of 80°C / h~120°C / h.

[0016] Specifically, in step S1, the specific heating process is: keeping the forged round steel at 150℃~200℃ for 2h~3h, then heating it to 500℃~600℃ at a rate of 80℃ / h~120℃ / h and keeping it for 2h~3h, and then heating it to 1050℃~1100℃ at a rate of 80℃ / h~120℃ / h.

[0017] Furthermore, in step S1, the insulation time is 5h~10h.

[0018] The optimal heating rate and holding time can make the alloy elements diffuse evenly, promote the full transformation and equilibrium of the austenite and ferrite phases, lay a good foundation for obtaining the ideal dual-phase structure, and help reduce tissue stress and avoid the risk of cracking during subsequent processing.

[0019] Furthermore, in step S1, the number of cycles of the water-air alternating cooling is 3 to 4 times, the water cooling time and the air cooling time in each cycle are the same, and the cooling time of each cycle increases gradually with the increase in the number of cycles.

[0020] Furthermore, in step S1, the number of cycles of the water-air alternating cooling is 3 times, wherein the first water cooling time and air cooling time are T1=D / (150~200) mm / min; the second water cooling time and air cooling time are T2=D / (100~150) mm / min; the third water cooling time and air cooling time are T3=D / (80~120) mm / min; D is the diameter of the round steel after forging, mm.

[0021] In the initial cooling stage, a shorter water cooling time is used to quickly cross the temperature range where harmful phases precipitate rapidly, reducing the nucleation rate of harmful phases. Subsequently, by precisely controlling the water cooling and air cooling times, the precipitation and aggregation of harmful phases can be further suppressed, ensuring that the residual amount of harmful phases after solution treatment remains at a low level. In addition, the rapid cooling during the water cooling stage promotes the transformation of austenite to ferrite, and the cooling rate is appropriately slowed during the air cooling stage to allow sufficient time for the diffusion of alloying elements and phase transformation to occur, thereby avoiding insufficient phase transformation or imbalance in the two-phase ratio caused by excessive cooling. The gradient design of the number of cycles and cooling time can make the two-phase structure closer to the ideal ratio of ferrite and austenite each accounting for approximately 50% during multiple phase transformation adjustments. Moreover, compared with a single water cooling method, alternating water and air cooling effectively alleviates the stress concentration problem caused by rapid cooling.

[0022] Furthermore, in step S1, the time from when the forged round steel is taken out of the furnace to when it enters the water is controlled to be 60s to 100s.

[0023] It should be noted that the above time refers to the time from opening the furnace door to the round billet entering the water for the first water cooling after forging, which is 60s~100s.

[0024] Furthermore, in step S2, the insulation time is 20h~40h.

[0025] Holding the steel at 300°C to 400°C for 20h to 40h can provide sufficient activity energy for the atoms in the steel, promoting dislocation slip and rearrangement, and fully recovering the lattice distortion, thereby effectively releasing residual stress. In addition, during the holding process, tiny harmful phase particles that may exist in the steel will dissolve due to atomic diffusion and return to the solid solution state, thereby reducing the number and size of harmful phases.

[0026] Furthermore, in step S2, the cooling rate of the furnace is 40°C / h to 60°C / h.

[0027] Controlling the cooling rate of the furnace to 40℃ / h~60℃ / h can reduce the thermal stress caused by rapid cooling, ensuring that a stable microstructure with low internal stress is finally obtained, thereby significantly improving the hot working performance and mechanical properties of large-size duplex stainless steel.

[0028] The solution heat treatment process for large-size duplex stainless steel provided by the present invention effectively solves the problems of two-phase structure imbalance, harmful phase precipitation and excessive stress during the solution treatment of large-size duplex stainless steel. After solution treatment, the ferrite phase ratio is 45% to 55%, and the harmful precipitated phase is ≤1% (area percentage). The hot working performance and mechanical properties are significantly improved, effectively expanding the application of duplex stainless steel in the manufacturing fields of high-end equipment such as pressure vessels and marine engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the metallographic structure diagram of the duplex stainless steel after solution treatment in Example 1 of the present invention, magnified at 100X;

[0030] Figure 2 This is a metallographic structure diagram of the duplex stainless steel after solution treatment in Example 2 of the present invention, with a magnification of 100X;

[0031] Figure 3 This is a metallographic structure diagram of the duplex stainless steel after solution treatment in Example 3 of the present invention, with a magnification of 100X;

[0032] Figure 4 This is a photo of the appearance of the duplex stainless steel after solution treatment in Comparative Example 1 of the present invention;

[0033] Figure 5 This is the metallographic structure diagram of the duplex stainless steel after solution treatment in Comparative Example 1 of the present invention, magnified at 100X;

[0034] Figure 6 This is the metallographic structure diagram of the duplex stainless steel after solution treatment in Comparative Example 2 of the present invention, magnified at 100X;

[0035] Figure 7 This is a transverse cross-sectional photograph of the duplex stainless steel after solution treatment in Comparative Example 3 of the present invention;

[0036] Figure 8 This is the metallographic structure diagram of the duplex stainless steel after solution treatment in Comparative Example 3 of the present invention, with a magnification of 100X. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] In order to better illustrate the present invention, further examples are given below.

[0039] Example 1

[0040] This embodiment provides a solution heat treatment process for large-scale duplex stainless steel, comprising the following steps:

[0041] S1, the forged Φ580mm S32750 round steel is kept at 200℃ for 2h, then heated to 580℃ at 100℃ / h and kept for 3h, and finally heated to 1080℃ at a rate of 100℃ / h and kept for 6h. The furnace temperature is raised by 40℃ 30min before being taken out of the furnace, and then the furnace door is opened. The round steel is lifted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water is controlled to 80s. The material temperature is alternately cooled by water and air to 320℃, specifically: water cooling 180s → air cooling 180s → water cooling 240s → air cooling 240s → water cooling 300s → air cooling 300s. The material temperature is measured at 320℃.

[0042] S2: Place the round steel into a furnace preheated to 350℃ and keep it at this temperature for 30h. Then slowly cool it down to 185℃ at a rate of 50℃ / h and take it out of the furnace for air cooling.

[0043] After being taken out of the furnace, the surface of the round steel is visually inspected to ensure that there are no crack defects. After the surface is polished, ultrasonic testing is carried out according to GB / T 4162-2022 Ultrasonic Testing Method for Forged and Rolled Steel Bars to ensure that there are no cracks.

[0044] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. The metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution and the austenite phase ratio was 54.5%, the ferrite phase ratio was 45%, and the σ phase content was 0.5% (area percentage). Figure 1 shown.

[0045] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 607MPa / 594MPa, Rm: 824MPa / 830MPa, A: 44% / 46.5%, Z: 83% / 83%.

[0046] Example 2

[0047] This embodiment provides a solution heat treatment process for large-scale duplex stainless steel, comprising the following steps:

[0048] S1: The forged Φ450mm S32750 round steel was kept at 180℃ for 2.5h, then heated to 580℃ at a rate of 120℃ / h and kept for 2h, and finally heated to 1050℃ at a rate of 80℃ / h and kept for 10h. The furnace temperature was raised by 40℃ 20min before being taken out of the furnace. Then the furnace door was opened, and the round steel was hoisted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water was controlled to 70s. The material temperature was alternately cooled by water and air to 340℃, specifically: water cooling for 150s → air cooling for 150s → water cooling for 210s → air cooling for 210s → water cooling for 270s → air cooling for 270s. The material temperature was measured to be 340℃.

[0049] S2: Place the round steel into a furnace preheated to 390°C and keep it there for 23 hours. Then slowly cool it down to 185°C at a rate of 40°C / h and take it out of the furnace for air cooling.

[0050] After being taken out of the furnace, the surface of the round steel is visually inspected to ensure that there are no crack defects. After the surface is polished, ultrasonic testing is carried out according to GB / T 4162-2022 Ultrasonic Testing Method for Forged and Rolled Steel Bars to ensure that there are no cracks.

[0051] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. The metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution and the austenite phase ratio was 56%, the ferrite phase ratio was 43.7%, and the σ phase content was 0.3% (area percentage). Figure 2 shown.

[0052] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 587MPa / 584MPa, Rm: 820MPa / 823MPa, A: 48% / 47.5%, Z: 80% / 79.5%.

[0053] Example 3

[0054] This embodiment provides a solution heat treatment process for large-scale duplex stainless steel, comprising the following steps:

[0055] S1, the forged Φ300mm S32750 round steel was kept at 150℃ for 3h, then heated to 500℃ at 80℃ / h and kept for 3h, and finally heated to 1100℃ at a rate of 120℃ / h and kept for 5h. The furnace temperature was raised by 20℃ 25min before being taken out of the furnace, and then the furnace door was opened. The round steel was lifted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water was controlled to 100s. The material temperature was alternately cooled by water and air to 320℃, specifically: water cooling 120s → air cooling 120s → water cooling 180s → air cooling 180s → water cooling 210s → air cooling 210s. The material temperature was measured to be 320℃;

[0056] S2: Place the round steel into a furnace preheated to 300°C and keep it at this temperature for 38 hours. Then slowly cool it down to 185°C at a rate of 60°C / h and take it out of the furnace for air cooling.

[0057] After being taken out of the furnace, the surface of the round steel is visually inspected to ensure that there are no crack defects. After the surface is polished, ultrasonic testing is carried out according to GB / T 4162-2022 Ultrasonic Testing Method for Forged and Rolled Steel Bars to ensure that there are no cracks.

[0058] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. After the metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution, the austenite phase ratio was 54%, the ferrite phase ratio was 45.8%, and the σ phase content was 0.2% (area percentage). Figure 3 shown.

[0059] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 597MPa / 604MPa, Rm: 812MPa / 826MPa, A: 48.5% / 48%, Z: 80% / 80%.

[0060] Comparative Example 1

[0061] This comparative example provides a solution heat treatment process for large-scale duplex stainless steel, which differs from Example 1 only in that water cooling is used instead of water-air cooling, and includes the following steps:

[0062] S1: The forged Φ580mm S32750 round steel is kept at 200℃ for 2 hours, then heated to 580℃ at a rate of 100℃ / h and kept for 3 hours. Finally, the temperature is raised to 1100℃ at a rate of 100℃ / h and kept for 6 hours. 20 minutes before being taken out of the furnace, the furnace temperature is raised by 30℃. Then the furnace door is opened, and the round steel is lifted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water is controlled to 80 seconds, and the steel is water-cooled to 320℃.

[0063] S2: Place the round steel into a furnace preheated to 350℃ and keep it warm for 30h. Then slowly cool it down to 190℃ at a rate of 50℃ / h and take it out of the furnace for air cooling.

[0064] After leaving the furnace, visually inspect the round steel surface for crack defects, such as Figure 4 shown.

[0065] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. The metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution and the austenite phase ratio was 55%, the ferrite phase ratio was 30%, and the σ phase content was 15% (area percentage). Figure 5 shown.

[0066] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 518MPa / 517MPa, Rm: 737MPa / 736MPa, A: 24% / 26.5%, Z: 53% / 53%.

[0067] Comparative Example 2

[0068] This comparative example provides a solution heat treatment process for large-scale duplex stainless steel, which differs from Example 1 only in that water-air cooling followed by water cooling to room temperature is used, comprising the following steps:

[0069] S1, the forged Φ580mm S32750 round steel is kept at 200℃ for 2h, then heated to 580℃ at 100℃ / h and kept for 3h, and finally heated to 1100℃ at a rate of 100℃ / h and kept for 6h. The furnace temperature is raised by 30℃ 20min before being taken out of the furnace, and then the furnace door is opened. The round steel is lifted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water is controlled to be 80s, and the material temperature is alternately cooled by water and air until it reaches room temperature. Specifically, the process is as follows: water cooling for 180s → air cooling for 180s → water cooling for 240s → air cooling for 240s → water cooling for 300s → air cooling for 300s → water cooling to room temperature;

[0070] S2: Place the round steel into a furnace preheated to 350℃ and keep it warm for 30h. Then slowly cool it down to 190℃ at a rate of 50℃ / h and take it out of the furnace for air cooling.

[0071] After taking out of the furnace, visually inspect the round steel surface for crack defects.

[0072] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. After the metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution, the austenite phase ratio was 54.5%, the ferrite phase ratio was 45.2%, and the σ phase content was 0.3% (area percentage). Figure 6 shown.

[0073] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 532MPa / 539MPa, Rm: 747MPa / 746MPa, A: 26% / 26.5%, Z: 55% / 55%.

[0074] Comparative Example 3

[0075] This comparative example provides a solution heat treatment process (conventional process) for large-scale duplex stainless steel. The only difference from Example 1 is that the temperature is not raised before being discharged from the furnace, and water cooling to room temperature and no secondary holding annealing are used. The specific steps are as follows:

[0076] The forged Φ580mm S32750 round steel is kept at 200℃ for 2h, then heated to 580℃ at 100℃ / h and kept for 3h, and finally heated to 1100℃ at a rate of 100℃ / h and kept for 6h. Then the furnace door is opened, and the round steel is lifted by an overhead crane and quickly put into water. The time from opening the furnace door to entering the water is controlled to be 80s, and then cooled to room temperature by water.

[0077] After the round steel is taken out of the furnace, the surface is visually inspected to ensure that there are no cracks. After the surface is polished, ultrasonic testing is performed according to GB / T 4162-2022 forged steel bars. Cracks are found inside the steel after transverse cutting. Figure 7 shown.

[0078] A test piece was cut from the end of the round steel for metallographic and room temperature tensile property testing. The metallographic specimen was corroded by potassium permanganate-sulfuric acid-water solution and tested for austenite phase ratio of 55%, ferrite phase ratio of 20%, and σ phase content of 25% (area percentage). Figure 8 shown.

[0079] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Tests on Metallic Materials": Rp0.2: 522MPa / 518MPa, Rm: 736MPa / 731MPa, A: 25% / 26.5%, Z: 53% / 53.5%.

[0080] In summary, the embodiments of the present invention suppress the precipitation of harmful phases, regulate the ratio of the two-phase structure, and reduce thermal stress through alternating water-air cooling, promote the dissolution of harmful phases and fine-tune the ratio of the two phases in conjunction with heating before being discharged from the furnace, and perform annealing in conjunction with secondary remelting to fully release the tissue stress and thermal stress. This effectively solves the problems of imbalance of the two-phase structure, precipitation of harmful phases, and excessive stress in the solid solution process of large-size duplex stainless steel, improves the hot working performance and mechanical properties of the material, lays a good foundation for subsequent processing, improves the product yield and production efficiency, and has high promotion and application value.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A solution heat treatment process for large-size duplex stainless steel, characterized in that: The following steps are involved: S1, keep the forged round steel at 1050℃~1100℃, raise the furnace temperature by 20℃~40℃ 20min~30min before taking out of the furnace, then cool it down to 300℃~400℃ by alternating water and air cooling after taking out of the furnace; S2, put the cooled forged round steel into a furnace at 300℃~400℃ for insulation, then cool it down to below 200℃ along with the furnace, and take it out of the furnace for air cooling; In step S1, the number of cycles of the water-air alternating cooling is 3, wherein the first water cooling time and air cooling time are T1=D / (150-200) mm / min; the second water cooling time and air cooling time are T2=D / (100-150) mm / min; the third water cooling time and air cooling time are T3=D / (80-120) mm / min; D is the diameter of the round steel after forging, mm.

2. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: In step S1, the temperature is increased to 1050°C~1100°C at a rate of 80°C / h~120°C / h.

3. The solution heat treatment process for large-size duplex stainless steel according to claim 1 or 2, characterized in that: In step S1, the insulation time is 5h~10h.

4. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: In step S1, the time from when the forged round steel is taken out of the furnace to when it enters the water is controlled to be 60s to 100s.

5. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: In step S2, the insulation time is 20h~40h.

6. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: In step S2, the cooling rate of the furnace is 40°C / h to 60°C / h.

7. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: The specifications of the large-size duplex stainless steel are Ф300mm~Ф580mm.

8. The solution heat treatment process for large-size duplex stainless steel according to claim 1, characterized in that: The large-size duplex stainless steel is S32750.

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