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

Abstract

The invention relates to the technical field of duplex stainless steel, and particularly discloses a solid solution heat treatment process of large-specification duplex stainless steel. According to the method, the process that water and air are alternately cooled to the specific temperature and then returned to the furnace for stress relief annealing is adopted, and the heating process before discharging is added after heating and heat preservation are conducted, so that the problems of two-phase structure imbalance, harmful phase precipitation, overlarge stress and the like in the solid solution process of the large-specification duplex stainless steel are effectively solved, the ferrite phase proportion after solid solution treatment is 45%-55%, and the yield of the large-specification duplex stainless steel is greatly improved. And the hot working performance and the mechanical performance are remarkably improved, and the application of the duplex stainless steel in the manufacturing field of high-end equipment such as pressure containers and ocean engineering is effectively expanded.

Description

Technical Field

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

[0002] Duplex stainless steel refers to stainless steel in which the ferrite and austenite phases each account for approximately 50% in the solution structure. Representative grades include S32750 (F53), etc., and the 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%, N 0.25% - 0.32%. Duplex stainless steel combines the advantages of good toughness and weldability of austenitic stainless steel and high strength and stress corrosion resistance of ferritic stainless steel, and has been widely used in high-end manufacturing fields such as pressure vessels and offshore engineering.

[0003] In the production and processing of duplex stainless steel, solution heat treatment is the core link determining its properties. During the heating and cooling stages of this process, there are phase transformations between austenite and ferrite, as well as the re-dissolution and precipitation of harmful phases. How to effectively control the proportion of the two-phase structure and the state of the precipitated phases has become the key to improving the mechanical properties and processing performance of duplex steel. On the premise of determining the chemical composition and forging process, the solution heat treatment method directly determines the two-phase structure morphology and harmful phase distribution of duplex stainless steel.

[0004] The traditional solution heat treatment method is to heat and hold the forged bar at 1050°C - 1150°C and then cool it with water. This method is restricted by factors such as water entry conditions and cooling rate. Especially when processing large-sized bars, it is extremely easy to cause an imbalance in the proportion of austenite and ferrite phases, a large amount of precipitated phases to remain in the steel, and significant tissue stress and thermal stress to be generated. These problems not only increase the risk of cracking of the forged bar during the solution process but also have a negative impact on its mechanical properties and subsequent processing. Therefore, it is extremely urgent to develop a new solution heat treatment process that can precisely control process parameters and optimize the two-phase structure and harmful phase distribution. Summary of the Invention

[0005] In view of the fact that existing large-sized duplex stainless steel is prone to two-phase ratio imbalance and a large amount of precipitated phases remain in the steel during the solution heat treatment process, resulting in solid solution cracking, poor processing performance and poor mechanical properties, the present invention provides a large-sized duplex stainless steel solution heat treatment process. The present invention adopts a process of alternately cooling water and air to a specific temperature and then returning to the furnace for stress relief annealing, and adds a heating process before being discharged from the furnace after heating and heat preservation to control the transformation of the solid solution process and the precipitation of harmful phases, thereby solving the problem of solid solution cracking of large-sized duplex stainless steel, improving the hot processing performance and mechanical properties, and meeting the increasingly stringent performance requirements of duplex stainless steel in the field of high-end manufacturing.

[0006] In order to solve the above technical problems, the technical solution provided by the present invention is: A solution heat treatment process for large-size duplex stainless steel comprises the following steps: S1, keep the forged round steel at 1050℃~1100℃, raise the furnace temperature by 20℃~40℃ 20min~30min before taking out of the furnace, and then cool it alternately with water and air to 300℃~400℃ after taking out of the furnace; S2, put the cooled forged round steel into a 300℃~400℃ furnace for heat preservation, then cool it down to below 200℃ along with the furnace, and then take it out of the furnace for air cooling.

[0007] When large-sized bars are subjected to solution heat treatment, the time required for heat transfer to the center of the bar is significantly extended due to the increase in 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 of austenite and ferrite. In addition, the larger the size, the more difficult it is for alloy elements to diffuse evenly inside the bar, further exacerbating the complexity of two-phase microstructure control. At the same time, uneven microstructure transformation will induce microstructure stress. The inconsistency of microstructure transformation in large-sized bars makes the distribution of microstructure stress more complicated. These stresses are superimposed on each other, which easily forms stress concentration areas inside the bar, increasing the risk of cracking of the bar during the solution process, seriously affecting product quality and yield rate. 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, increasing the probability of nucleation of harmful phases. 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.

[0008] Compared with the prior art, for the solution heat treatment process of the large-sized duplex stainless steel provided by the present invention, first, the forged round steel is held at 1050°C to 1100°C to dissolve alloying elements into the austenite and ferrite phases, laying a foundation for obtaining an ideal duplex structure; 20 to 30 minutes before discharging the furnace, the furnace temperature is raised by 20°C to 40°C, which can further promote the re-dissolution of harmful phases and avoid the massive precipitation of harmful phases during transportation or subsequent cooling; at the same time, this 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; subsequently, it is cooled alternately by water and air to a material temperature of 300°C to 400°C. Compared with the traditional single water-cooling method, water-air alternate 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 generated by too fast water-cooling, avoiding the concentration of tissue stress caused by uneven cooling and effectively reducing the cracking risk of large-sized bars; the cooled forged round steel is put into a furnace at 300°C to 400°C for heat preservation and then cooled with the furnace to below 200°C before being taken out of the furnace for air-cooling, which can promote the full release of the residual tissue stress and thermal stress in the steel and further reduce the internal stress level.

[0009] By adopting the water-air alternate cooling method, re-heating stress relief annealing, and increasing the heating process before discharging the furnace, the present invention fully eliminates the internal tissue transformation stress and thermal stress. At the same time, it effectively regulates the ratio of the duplex structure, not only solves the problem of easy cracking of large-sized duplex stainless steel, but also improves the hot working performance and mechanical properties of the steel, which is beneficial to expanding the application of duplex stainless steel in the high-end manufacturing field and has high practical value.

[0010] It should be noted that the specification of the large-sized duplex stainless steel described in the present invention is Ф300mm to Ф580mm.

[0011] In addition, it should be noted that the above-mentioned large-sized duplex stainless steel is S32750.

[0012] Further, in step S1, it is heated to 1050°C to 1100°C at a rate of 80°C / h to 120°C / h.

[0013] Specifically, in step S1, the specific heating process is: the forged round steel is held at 150°C to 200°C for 2 to 3 hours, then heated to 500°C to 600°C at a rate of 80°C / h to 120°C / h and held for 2 to 3 hours, and then heated to 1050°C to 1100°C at a rate of 80°C / h to 120°C / h.

[0014] Further, in step S1, the holding time is 5 to 10 hours.

[0015] The preferred heating rate and holding time can enable the alloying elements to diffuse uniformly, promote the full transformation of the austenite and ferrite phases and tend towards equilibrium, lay a good foundation for obtaining an ideal duplex structure, and are conducive to reducing the tissue stress and avoiding the cracking risk during subsequent processing.

[0016] Further, in step S1, the number of cycles of water-air alternative cooling is 3 to 4 times. In each cycle, the water cooling time and the air cooling time are the same, and the cooling time of each cycle increases in a gradient manner with the increase in the number of cycles.

[0017] Further, in step S1, the number of cycles of water-air alternative cooling is 3 times. Among them, 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 forged round steel, in mm.

[0018] In the initial stage of cooling, adopting a shorter water cooling time can quickly cross the temperature range where harmful phases precipitate rapidly, reduce 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 inhibited, ensuring that the residual amount of harmful phases after solution treatment remains at a low level. In addition, the rapid temperature drop in the water cooling stage promotes the transformation of austenite to ferrite, and the appropriate slowdown of the cooling rate in the air cooling stage gives sufficient time for the diffusion of alloying elements and phase transformation to proceed, avoiding incomplete phase transformation or imbalance in the proportion of the two phases caused by too fast cooling. The gradient design of the number of cycles and the cooling time can make the duplex structure closer to the ideal proportion of about 50% for both ferrite and austenite in multiple phase transformation adjustments. And the water-air alternative cooling effectively alleviates the stress concentration problem caused by rapid cooling compared with the single water cooling method.

[0019] Further, in step S1, the time from the forged round steel out of the furnace to entering the water is controlled to be 60 s to 100 s.

[0020] It should be noted that the above time refers to the time from opening the furnace door to the forged round billet entering the water for the first water cooling, which is 60 s to 100 s.

[0021] Further, in step S2, the holding time is 20 h to 40 h.

[0022] Holding at a temperature of 300°C to 400°C for 20 h to 40 h can provide sufficient activity energy for the atoms in the steel, promote the slip and rearrangement of dislocations, and fully recover the lattice distortion, thereby effectively releasing the residual stress; in addition, during the holding process, the 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.

[0023] Further, in step S2, the cooling rate of furnace cooling is 40°C / h to 60°C / h.

[0024] Controlling the cooling rate of furnace cooling to be 40°C / h to 60°C / h can reduce the thermal stress generated by rapid cooling, ensure a stable tissue state with relatively small internal stress is finally obtained, and thus significantly improve the hot working performance and mechanical properties of large-sized duplex stainless steel.

[0025] The solution heat treatment process of the large-sized duplex stainless steel provided by the present invention effectively solves problems such as two-phase tissue imbalance, harmful phase precipitation, and excessive stress during the solution treatment of large-sized duplex stainless steel. After solution treatment, the ferrite phase ratio is 45% to 55%, and the harmful precipitated phase is ≤1% (area percentage). Both 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 ocean engineering. Description of the Drawings

[0026] Figure 1 It is the metallographic structure diagram of the duplex stainless steel after solution treatment in Example 1 of the present invention, with a magnification of 100X; Figure 2 It is the metallographic structure diagram of the duplex stainless steel after solution treatment in Example 2 of the present invention, with a magnification of 100X; Figure 3 It is the metallographic structure diagram of the duplex stainless steel after solution treatment in Example 3 of the present invention, with a magnification of 100X; Figure 4 It is the appearance photo of the duplex stainless steel after solution treatment in Comparative Example 1 of the present invention; Figure 5 It is the metallographic structure diagram of the duplex stainless steel after solution treatment in Comparative Example 1 of the present invention, with a magnification of 100X; Figure 6 It is the metallographic structure diagram of the duplex stainless steel after solution treatment in Comparative Example 2 of the present invention, with a magnification of 100X; Figure 7 It is the cross-sectional photo of the duplex stainless steel after solution treatment in the transverse direction in Comparative Example 3 of the present invention; Figure 8 It 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 Embodiments

[0027] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] To better illustrate the present invention, further examples are given below through embodiments.

[0029] Example 1 This embodiment provides a solution heat treatment process for large-sized duplex stainless steel, including the following steps: S1. The Φ580mm S32750 round steel formed by forging is kept at 200°C for 2h, then heated to 580°C at a rate of 100°C / h and kept for 3h, and finally heated to 1080°C at a rate of 100°C / h and kept for 6h. 30 minutes before discharging, the furnace temperature is increased by 40°C, then the furnace door is opened, and the round steel is quickly lifted by the overhead crane and put into water. The time from opening the furnace door to putting into water is controlled to be 80s, and it is cooled by water-air alternation until the material temperature reaches 320°C. Specifically: water cooling for 180s → air cooling for 180s → water cooling for 240s → air cooling for 240s → water cooling for 300s → air cooling for 300s, and the material temperature is measured to be 320°C; S2. The round steel is put into a furnace preheated to 350°C and kept for 30h, then slowly cooled to 185°C at a rate of 50°C / h, and discharged for air cooling.

[0030] After discharging, visually inspect the surface of the round steel for no crack defects. After the surface is turned smooth, ultrasonic flaw detection is carried out according to GB / T 4162-2022 Method for Ultrasonic Testing of Forged and Rolled Steel Bars, and no crack indication is shown.

[0031] Specimens are cut from the end of the round steel for metallographic and room temperature tensile property tests. After the metallographic specimens are corroded by potassium permanganate-sulfuric acid-aqueous solution, the austenite phase ratio is 54.5%, the ferrite phase ratio is 45%, and the σ phase content is 0.5% (area percentage), as Figure 1 shown.

[0032] According to GB / T 228.1-2010 Metallic Materials - Tensile Testing, the room temperature tensile properties are tested. Rp0.2: 607MPa / 594MPa, Rm: 824MPa / 830MPa, A: 44% / 46.5%, Z: 83% / 83%.

[0033] Example 2 This embodiment provides a solution heat treatment process for large-sized duplex stainless steel, including the following steps: S1. Heat the forged S32750 round steel with a diameter of Φ450mm to 180°C and hold for 2.5h, then heat it to 580°C at a rate of 120°C / h and hold for 2h, and finally heat it to 1050°C at a rate of 80°C / h and hold for 10h. Raise the furnace temperature by 40°C 20 minutes before discharging. Then open the furnace door, and use the overhead crane to quickly lift the round steel into the water. Control the time from opening the furnace door to entering the water to be 70s, and cool it alternately with water and air until the material temperature reaches 340°C. 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, and measure the material temperature of 340°C; S2. Load the round steel into a furnace preheated to 390°C and hold for 23h, then slowly cool it to 185°C at a rate of 40°C / h, and discharge it for air cooling.

[0034] After discharging, visually inspect the surface of the round steel and find no crack defects. After turning the surface smooth, perform ultrasonic flaw detection according to GB / T 4162-2022 Method for Ultrasonic Testing of Forged and Rolled Steel Bars, and no crack indication is found.

[0035] Cut specimens at the end of the round steel for metallographic and room temperature tensile property tests. After making metallographic specimens and corroding them with potassium permanganate - sulfuric acid - aqueous solution, detect the austenite phase ratio of 56%, ferrite phase ratio of 43.7%, and σ phase content of 0.3% (area percentage), as Figure 2 shown.

[0036] Detect the room temperature tensile properties according to GB / T 228.1-2010 Metallic Materials - Tensile Testing. Rp0.2: 587MPa / 584MPa, Rm: 820MPa / 823MPa, A: 48% / 47.5%, Z: 80% / 79.5%.

[0037] Example 3 This example provides a solution heat treatment process for large - sized duplex stainless steel, including the following steps: S1. Heat the forged S32750 round steel with a diameter of Φ300mm to 150°C and hold for 3h, then heat it to 500°C at a rate of 80°C / h and hold for 3h, and finally heat it to 1100°C at a rate of 120°C / h and hold for 5h. Raise the furnace temperature by 20°C 25 minutes before discharging. Then open the furnace door, and use the overhead crane to quickly lift the round steel into the water. Control the time from opening the furnace door to entering the water to be 100s, and cool it alternately with water and air until the material temperature reaches 320°C. Specifically: water cooling for 120s → air cooling for 120s → water cooling for 180s → air cooling for 180s → water cooling for 210s → air cooling for 210s, and measure the material temperature of 320°C; S2. Load the round steel into a furnace preheated to 300°C and hold for 38h, then slowly cool it to 185°C at a rate of 60°C / h, and discharge it for air cooling.

[0038] After the round steel is taken out of the furnace, visually inspect the surface of the round steel for no crack defects. After the surface is turned smooth, ultrasonic flaw detection is carried out in accordance with GB / T 4162-2022 "Ultrasonic Testing Method for Forged and Rolled Steel Bars", and no crack indication is found.

[0039] Take specimens at the end of the round steel for metallographic and room temperature tensile property tests. After making the metallographic specimens corroded by potassium permanganate - sulfuric acid - aqueous solution, the austenite phase ratio is detected to be 54%, the ferrite phase ratio is 45.8%, and the σ phase content is 0.2% (area percentage), as Figure 3 shown.

[0040] Detect the room temperature tensile properties in accordance with GB / T 228.1-2010 "Metallic Materials - Tensile Testing". Rp0.2: 597MPa / 604MPa, Rm: 812MPa / 826MPa, A: 48.5% / 48%, Z: 80% / 80%.

[0041] Comparative Example 1 This comparative example provides a solution heat treatment process for large - sized duplex stainless steel. The only difference from Example 1 is that water cooling is used instead of water - air cooling, and it includes the following steps: S1. Heat the Φ580mm - sized S32750 round steel formed by forging at 200°C for 2h, then heat it to 580°C at a rate of 100°C / h and hold for 3h, and finally heat it to 1100°C at a rate of 100°C / h and hold for 6h. Raise the furnace temperature by 30°C 20min before taking the round steel out of the furnace, then open the furnace door, and use the overhead crane to quickly lift the round steel into water, controlling the time from opening the furnace door to entering the water to be 80s, and water - cool it to 320°C; S2. Load the round steel into a furnace pre - heated to 350°C and hold for 30h, then slowly cool it to 190°C at a rate of 50°C / h, and take it out of the furnace and air - cool.

[0042] After taking the round steel out of the furnace, visually inspect that there are crack defects on the surface of the round steel, as Figure 4 shown.

[0043] Take specimens at the end of the round steel for metallographic and room temperature tensile property tests. After making the metallographic specimens corroded by potassium permanganate - sulfuric acid - aqueous solution, the austenite phase ratio is detected to be 55%, the ferrite phase ratio is 30%, and the σ phase content is 15% (area percentage), as Figure 5 shown.

[0044] Detect the room temperature tensile properties in accordance with GB / T 228.1-2010 "Metallic Materials - Tensile Testing". Rp0.2: 518MPa / 517MPa, Rm: 737MPa / 736MPa, A: 24% / 26.5%, Z: 53% / 53%.

[0045] Comparative Example 2 This comparative example provides a solution heat treatment process for large-sized duplex stainless steel. The only difference from Example 1 is that after water-air cooling, it is water-cooled to room temperature, and the process includes the following steps: S1. The Φ580mm S32750 round steel formed by forging is held at 200°C for 2h, then heated to 580°C at a rate of 100°C / h and held for 3h, and finally heated to 1100°C at a rate of 100°C / h and held for 6h. 20 minutes before discharging, the furnace temperature is raised by 30°C, then the furnace door is opened, and the round steel is quickly lifted by the overhead crane and immersed in water. The time from opening the furnace door to immersing in water is controlled to be 80s, and it is cooled by water-air alternation until the material temperature reaches room temperature. Specifically: water-cooled for 180s → air-cooled for 180s → water-cooled for 240s → air-cooled for 240s → water-cooled for 300s → air-cooled for 300s → water-cooled to room temperature; S2. The round steel is loaded into a furnace preheated to 350°C and held for 30h, then slowly cooled to 190°C at a rate of 50°C / h, and discharged for air-cooling.

[0046] After discharging, visually inspect the surface of the round steel and find crack defects.

[0047] Take specimens at the end of the round steel for metallographic and room temperature tensile property tests. After the metallographic specimens are corroded by potassium permanganate-sulfuric acid-aqueous solution, the austenite phase ratio is detected to be 54.5%, the ferrite phase ratio is 45.2%, and the σ phase content is 0.3% (area percentage), as Figure 6 shown.

[0048] According to GB / T 228.1-2010 "Metallic materials - Tensile testing", the room temperature tensile properties are detected: Rp0.2: 532MPa / 539MPa, Rm: 747MPa / 746MPa, A: 26% / 26.5%, Z: 55% / 55%.

[0049] Comparative Example 3 This comparative example provides a solution heat treatment process (traditional process) for large-sized duplex stainless steel. The only difference from Example 1 is that there is no pre-discharge temperature increase, and the steps of water-cooling to room temperature and no secondary annealing are adopted. The specific steps are as follows: The Φ580mm S32750 round steel formed by forging is held at 200°C for 2h, then heated to 580°C at a rate of 100°C / h and held for 3h, and finally heated to 1100°C at a rate of 100°C / h and held for 6h. Then the furnace door is opened, and the round steel is quickly lifted by the overhead crane and immersed in water. The time from opening the furnace door to immersing in water is controlled to be 80s, and it is water-cooled to room temperature.

[0050] After discharging, visually inspect the surface of the round steel and find no crack defects. After the surface is turned smooth, ultrasonic flaw detection is carried out according to GB / T 4162-2022 "Ultrasonic testing method for forged and rolled steel bars", and crack indications are found. After cutting transversely, cracks are found inside the steel, as Figure 7 shown.

[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 made and corroded by potassium permanganate-sulfuric acid-water solution to test the austenite phase ratio of 55%, the ferrite phase ratio of 20%, and the σ phase content of 25% (area percentage). Figure 8 shown.

[0052] The room temperature tensile properties are tested according to GB / T 228.1-2010 "Tensile Test of Metal Materials": Rp0.2: 522MPa / 518MPa, Rm: 736MPa / 731MPa, A: 25% / 26.5%, Z: 53% / 53.5%.

[0053] In summary, the embodiments of the present invention suppress the precipitation of harmful phases, adjust the ratio of two-phase structures, and reduce thermal stress through alternating water-air cooling, promote the dissolution of harmful phases and fine-tune the ratio of two phases in conjunction with heating before being discharged from the furnace, and carry out annealing in conjunction with secondary re-melting to fully release the structural stress and thermal stress, effectively solve the problems of imbalance of two-phase structures, precipitation of harmful phases, and excessive stress in the solid solution process of large-size duplex stainless steel, improve the thermal processing performance and mechanical properties of the material, lay a good foundation for subsequent processing, improve product yield and production efficiency, and have high promotion and application value.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent substitution or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A solution heat treatment process for large-sized duplex stainless steel, characterized in that, It includes the following steps: S1. Keep the forged round steel at 1050°C - 1100°C for heat preservation. Raise the furnace temperature by 20°C - 40°C 20 min - 30 min before discharging, and then discharge and cool it by water-air alternative cooling until the material temperature reaches 300°C - 400°C; S2. Load the cooled forged round steel into a furnace at 300°C - 400°C for heat preservation, then cool it with the furnace to below 200°C and discharge it for air cooling.

2. The solution heat treatment process of the large-sized duplex stainless steel according to claim 1, characterized in that, In step S1, heat it up to 1050°C - 1100°C at a rate of 80°C / h - 120°C / h.

3. The solution heat treatment process of the large-sized duplex stainless steel according to claim 1 or 2, characterized in that, In step S1, the heat preservation time is 5 h - 10 h.

4. The solution heat treatment process of the large-size duplex stainless steel according to claim 1, characterized in that, In step S1, the number of water-air alternative cooling cycles is 3 - 4 times. The water cooling time and air cooling time in each cycle are the same, and the cooling time of each cycle increases gradiently with the increase of the number of cycles.

5. The solution heat treatment process of the large-size duplex stainless steel according to claim 1 or 4, characterized in that, In step S1, the number of water-air alternative cooling cycles is 3 times. Among them, 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 forged round steel, in mm.

6. The solution heat treatment process of the large-sized duplex stainless steel according to claim 1, characterized in that, In step S1, the time from discharging the forged round steel to putting it into water is controlled within 60 s - 100 s.

7. The solution heat treatment process of the large-sized duplex stainless steel according to claim 1, characterized in that, In step S2, the heat preservation time is 20 h - 40 h.

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

9. The solution heat treatment process of the large-size duplex stainless steel according to claim 1, characterized in that, The specification of the large-sized duplex stainless steel is Ф300mm - Ф580mm.

10. The solution heat treatment process of the large-sized duplex stainless steel according to claim 1, characterized in that, The large-sized duplex stainless steel is S32750.

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