A forging method for achieving homogenized microstructure in large-diameter 316H bars

Abstract

This invention relates to the field of forging technology, specifically disclosing a forging method for homogenizing the microstructure of large-diameter 316H bars. The invention employs a three-stage forging process, with the heating temperature gradually decreasing with each forging stage, and the fourth stage using the same temperature as the third stage. Water cooling is performed after the fourth stage, effectively refining the grain size of the 316H alloy core. Furthermore, the fifth stage uses low-temperature, slow-frequency forging, effectively reducing the internal temperature rise of the bar, which further refines the grain size of the 316H alloy, resulting in a homogenized microstructure. The sixth stage involves holding at 1000℃ to 1020℃, ensuring the recrystallization of elongated grains at the edges, guaranteeing the uniformity of grain size at the edges. Simultaneously, it allows the surface grains of the forged billet to undergo a static recrystallization process, thereby ensuring the grain uniformity of the entire 316H cross-section.

Description

Technical Field

[0001] This invention relates to the field of forging technology, and in particular to a forging method for achieving uniform microstructure in large-diameter 316H bars. Background Technology

[0002] 316H alloy is a Cr-Ni-Mo iron-based alloy that exhibits excellent high-temperature strength, ductility, toughness, corrosion resistance, and weldability below 650℃, making it widely used in various industries, especially in the nuclear power sector. Due to its inherent composition, 316H alloy does not undergo microstructural transformation during heat treatment, and its grains cannot be uniformly refined. Therefore, the performance of 316H alloy primarily depends on the forging process.

[0003] Currently, the conventional forging process for producing 316H alloy often involves forging at 1180℃, followed by three upsetting and drawing processes to obtain the finished product. At this temperature, the core grain size of the alloy grows rapidly, typically reaching grade 0-1.5, resulting in coarse core grains. Furthermore, this forging process suffers from a series of problems, including poor uniformity of the resulting bar structure and elongated surface grains, which negatively impact product performance and prevent effective performance improvement. Therefore, it is necessary to develop a novel production process for 316H alloy to improve its microstructure uniformity, refine the grain size, and thus effectively enhance the performance of the 316H alloy. Summary of the Invention

[0004] To address the problems of poor microstructure uniformity, coarse grain size, and ineffective performance improvement in existing large-size 316H alloy forging processes, this invention provides a forging method for achieving microstructure uniformity in large-size 316H bars.

[0005] To solve the above-mentioned technical problems, the technical solution provided by the embodiments of the present invention is as follows:

[0006] A forging method for achieving uniform microstructure in large-diameter 316H bars includes the following steps:

[0007] S1, 316H electroslag ingot is heated to the first heat treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and drawn to an octagonal length of 880mm to 900mm to obtain a first heat forging billet; wherein, the first heat treatment temperature is 1170℃ to 1190℃;

[0008] S2, the first-fire forging billet is heated to the second-fire treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and drawing to obtain the second-fire forging billet; wherein, the second-fire treatment temperature is 20℃~30℃ lower than the first-fire treatment temperature;

[0009] S3, the two-heat forging billet is heated to the three-heat treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and deformed to obtain a three-heat forging billet; wherein, the three-heat treatment temperature is 50℃~70℃ lower than the two-heat treatment temperature;

[0010] S4, the three-heat forging billet is heated to the four-heat treatment temperature, kept heated through, then taken out of the furnace for upsetting and drawing, and water-cooled to below 600°C to obtain a four-heat forging billet; wherein, the temperature of the four-heat treatment is the same as the temperature of the three-heat treatment.

[0011] S5, the four-heat forging billet is first heated through at 980℃~1000℃, then heated to the five-heat treatment temperature, held for 0.5h~1h, and then forged to the finished product specifications at a frequency of 50%~70% to obtain the five-heat forging billet; wherein, the five-heat treatment temperature is 30℃~50℃ lower than the four-heat treatment temperature.

[0012] S6. Heat the five-fired forging billet to 1000℃~1020℃, hold for 40min~60min, remove from the furnace and air cool to obtain 316H alloy bar.

[0013] Compared to existing technologies, the forging method for homogenizing the microstructure of large-size 316H bars provided by this invention employs a method of gradually decreasing the heating temperature with each forging pass from the first to the third forging pass, and using the same temperature for the fourth forging pass as for the third forging pass, followed by water cooling after the fourth forging pass, which effectively refines the grain size of the 316H alloy core. In addition, the fifth forging pass uses a low-temperature, slow-frequency forging method, which effectively reduces the temperature rise inside the bar, which is beneficial for further refining the grain size inside the 316H alloy and making the microstructure homogenized. Finally, the fifth forging billet is held at 1000℃~1020℃ for a specific time to ensure that the elongated grains at the edge are restored and recrystallized, ensuring the uniformity of the grain size at the edge, and allowing the grain size of the surface layer of the forging billet to complete a static recrystallization process, thereby ensuring the grain uniformity of the entire cross-section of 316H.

[0014] Specifically, the forging method for homogenizing the microstructure of large-diameter 316H bars provided by the present invention is applicable to the preparation of bars with a diameter of Ф757mm to Ф760mm.

[0015] Furthermore, in S1, the 316H electroslag ingot is cylindrical with a diameter of 920mm to 1020mm.

[0016] It should be noted that the 316H electroslag ingot described in this invention can be prepared by conventional methods in the art, such as electric furnace smelting, AOD refining, LF refining, VD vacuum refining, casting to obtain a Ф680mm electrode, and electroslag remelting to obtain a Ф920mm electroslag ingot.

[0017] Furthermore, in S1, the heat preservation and heat penetration time is 6h to 8h.

[0018] Furthermore, in S1, the elongation deformation is 35% to 45%.

[0019] Specifically, in S1, the diameter is first thickened to 1140mm-1160mm and then drawn to 880mm-900mm (octagonal).

[0020] Furthermore, in S2, the heat preservation and heat penetration time is 2h to 3h.

[0021] Furthermore, in S2, the material is first uptaken to a diameter of 1140mm to 1160mm, and then drawn to an octagonal diameter of 880mm to 900mm, with a drawing deformation of 35% to 45%.

[0022] Furthermore, in S3, the deformation of the upsetting is 30% to 40%.

[0023] Specifically, in S3, the pier is thickened to a diameter of 1090mm to 1110mm.

[0024] Furthermore, in S3, the heat preservation and heat penetration time is 4h to 6h.

[0025] Furthermore, in S4, the material is first uptaken to a diameter of 1210mm to 1240mm, and then drawn to a diameter of 1150mm to 1180mm. The total deformation of uptake and drawing is 20% to 40%.

[0026] Furthermore, in S4, the heat preservation and heat penetration time is 2h to 3h.

[0027] Furthermore, in S5, the heat preservation and heat penetration time is 4h to 5h.

[0028] Furthermore, in S5, the deformation during forging is 45% to 60%.

[0029] This invention employs a forging method in which the deformation amount is greater than 25% each time, accompanied by an upsetting process. This ensures that the internal as-cast structure of the forging billet is fully broken up, and that the core and radius have sufficient deformation to ensure that the grains in the core and radius do not grow due to reheating in the next furnace.

[0030] Specifically, the present invention can use a 6000-ton fast forging machine to produce bars with uniform structure through the above-mentioned 6-fire process. The 316H alloy bars forged by this process have a full cross-section grain size ≥ 3.0 grade and a range ≤ 2.0 grade.

[0031] It should be noted that the above-mentioned deformation amounts in this invention all refer to the ratio of the cross-sectional area before and after plastic deformation.

[0032] The present invention also provides a large-size 316H bar stock, characterized in that it is prepared by the forging method for homogenizing the microstructure of the large-size 316H bar stock described in any one of the above claims.

[0033] The forging method for large-size 316H bars provided by this invention adopts a six-fire forging process, with the temperature gradually decreasing from the first to the third fire, the fourth fire being at the same temperature as the third fire, followed by water cooling after the fourth fire, and the fifth fire using a low-temperature, slow-frequency forging method. This effectively improves the microstructure uniformity of the 316H bars, refines the grain size, and achieves a full-section grain size ≥3.0 grade and a range ≤2.0 grade. This effectively solves the problem that the existing 316H alloy grain size is generally between 0 and 1.5 grade, making it difficult to achieve microstructure uniformity. Furthermore, the improved process is simple to operate and has high practical value. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the grain size sampling location in the embodiment;

[0035] Figure 2 Metallographic images of the head and tail edges, half-radius position, and center of the 316H rod prepared in Example 1; wherein, A1-head edge position, A2-head half-radius position, A3-head center position, B1-tail edge position, B2-tail half-radius position, B3-tail center position.

[0036] Figure 3 Metallographic images of the head and tail edges, half-radius position, and center of the 316H rod prepared for Comparative Example 1; where A1 - head edge position, A2 - head half-radius position, A3 - head center position, B1 - tail edge position, B2 - tail half-radius position, and B3 - tail center position. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] To better illustrate the present invention, further examples are provided below.

[0039] Example 1

[0040] This invention provides a forging method for achieving uniform microstructure in large-diameter 316H bars, comprising the following steps:

[0041] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1180℃ and held at that temperature for 7 hours to ensure uniform and thorough heating. After being taken out of the furnace, it is first upheaded to a diameter of 1150mm and then drawn to an octagonal shape of 900mm to obtain a forged billet. During the upheading process, the billet is first upset by 300mm and then held for 20s before continuing to uphead.

[0042] S2. The first-fire forging billet is heated to 1150℃ and held for 2 hours to ensure that the steel ingot is evenly heated through. After being taken out of the furnace, it is first uprooted to a diameter of 1150mm, and then drawn and forged to 900mm octagonal to obtain the second-fire forging billet.

[0043] S3. The second-fired forging billet is heated to 1100℃ and held for 5 hours to ensure that the steel ingot is heated evenly and thoroughly. After being removed from the furnace, it is roughened to a diameter of 1100mm to obtain the third-fired forging billet.

[0044] S4. Continue heating the three-fire forging billet to 1100℃, hold it for 2 hours to ensure that the steel ingot is evenly heated through, then remove it from the furnace, first uplift it to a diameter of 1220mm, then draw it to a diameter of 1150mm, and then water cool it for 2 hours after forging to make the billet temperature ≤600℃, thus obtaining the four-fire forging billet.

[0045] S5, the four-fire forging billet is reheated to 1000℃ and held for 4 hours, then heated to 1050℃ and held for 0.5 hours. After being taken out of the furnace, it is stretched and deformed at 50% of the forging frequency to obtain a round bar with a diameter of Ф757mm.

[0046] S6. The Ф757mm round steel is reheated to 1010℃ and held for 50 minutes, then air-cooled to obtain 316H bar stock.

[0047] Example 2

[0048] This invention provides a forging method for achieving uniform microstructure in large-diameter 316H bars, comprising the following steps:

[0049] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1170℃ and held at that temperature for 8 hours to ensure that the ingot is heated evenly and thoroughly. After being taken out of the furnace, it is first uprooted to a diameter of 1140mm and then drawn to an octagonal shape of 880mm to obtain a fire-forged billet. During the uprooting process, the billet is first upset by 300mm and then held for 20s before continuing to uproot.

[0050] S2. The first-fire forging billet is heated to 1140℃ and held for 3 hours to ensure that the steel ingot is evenly heated through. After being taken out of the furnace, it is first uprooted to a diameter of 1140mm, and then drawn and forged to an octagonal length of 880mm to obtain the second-fire forging billet.

[0051] S3. The second-fired forging billet is heated to 1070℃ and held for 6 hours to ensure that the steel ingot is evenly heated through. After being removed from the furnace, it is roughened to a diameter of 1090mm to obtain the third-fired forging billet.

[0052] S4. Continue heating the three-fire forging billet to 1070℃, hold it for 3 hours to ensure the steel ingot is evenly heated through, then remove it from the furnace, first uplift it to a diameter of 1210mm, then draw it to a diameter of 1160mm, and then water cool it for 2 hours after forging to make the billet temperature ≤600℃, thus obtaining the four-fire forging billet.

[0053] S5, reheat the four-fire forging billet to 980℃ and hold for 5 hours, then raise the temperature to 1020℃ and hold for 1 hour. After taking it out of the furnace, it is stretched and deformed at 70% forging frequency to obtain a round bar with a diameter of Ф760mm.

[0054] S6. The Ф760mm round steel is reheated to 1000℃ and held for 60 minutes, then air-cooled to obtain 316H bar stock.

[0055] Example 3

[0056] This invention provides a forging method for achieving uniform microstructure in large-diameter 316H bars, comprising the following steps:

[0057] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1190℃ and held at that temperature for 6 hours to ensure that the ingot is heated evenly and thoroughly. After being taken out of the furnace, it is first uprooted to a diameter of 1160mm and then drawn to an octagonal shape of 890mm to obtain a fire-forged billet. During the uprooting process, the billet is first upset by 300mm and then held for 20s before continuing to uproot.

[0058] S2. The first-fire forging billet is heated to 1170℃ and held for 2 hours to ensure that the steel ingot is evenly heated through. After being taken out of the furnace, it is first uprooted to a diameter of 1160mm, and then drawn and forged to an octagonal shape of 890mm to obtain the second-fire forging billet.

[0059] S3. The second-fired forging billet is heated to 1120℃ and held for 4 hours to ensure that the steel ingot is evenly heated through. After being removed from the furnace, it is roughened to a diameter of 1110mm to obtain the third-fired forging billet.

[0060] S4. Continue heating the three-fire forging billet to 1120℃, hold it for 2 hours to ensure that the steel ingot is evenly heated through, then remove it from the furnace, first uplift it to a diameter of 1240mm, then draw it to a diameter of 1180mm, and then water cool it for 2 hours after forging to make the billet temperature ≤600℃, thus obtaining the four-fire forging billet.

[0061] S5, the four-fire forging billet is reheated to 990℃ and held for 4 hours, then heated to 1090℃ and held for 0.5 hours. After being taken out of the furnace, it is stretched and deformed at 60% forging frequency to obtain a round bar with a diameter of Ф758mm.

[0062] S6. The Ф758mm round steel is reheated to 1020℃ and held for 40 minutes, then air-cooled to obtain 316H bar stock.

[0063] Comparative Example 1

[0064] This comparative example provides a forging method for large-diameter 316H bars. The only differences from Example 1 are the temperatures of the second and fourth heating processes (i.e., this comparative example uses isothermal first and second heating processes, with progressively decreasing temperatures from the third to the fifth heating process), and the absence of water cooling after the fourth heating process. The specific steps are as follows:

[0065] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1180℃ and held at that temperature for 7 hours to ensure uniform and thorough heating. After being taken out of the furnace, it is first upheaded to a diameter of 1150mm and then drawn to an octagonal shape of 900mm to obtain a forged billet. During the upheading process, the billet is first upset by 300mm and then held for 20s before continuing to uphead.

[0066] S2. The first-fire forging billet is heated to 1180℃ and held for 2 hours to ensure that the steel ingot is evenly heated through. After being taken out of the furnace, it is first uprooted to a diameter of 1150mm, and then drawn and forged to 900mm octagonal to obtain the second-fire forging billet.

[0067] S3. The second-fired forging billet is heated to 1100℃ and held for 5 hours to ensure that the steel ingot is heated evenly and thoroughly. After being removed from the furnace, it is roughened to a diameter of 1100mm to obtain the third-fired forging billet.

[0068] S4. Continue heating the three-fire forging billet to 1080℃, hold for 2 hours to ensure the steel ingot is evenly heated through, then remove it from the furnace, first uplift it to a diameter of 1220mm, and then draw it to a diameter of 1150mm to obtain the four-fire forging billet.

[0069] S5, the four-fire forging billet is heated to 1050℃, held for 3 hours, and then taken out of the furnace and elongated to obtain a round bar with a diameter of Ф757mm by drawing deformation at a forging frequency of 50%.

[0070] S6. The Ф757mm round steel is reheated to 1010℃ and held for 50 minutes, then air-cooled to obtain 316H bar stock.

[0071] Comparative Example 2

[0072] This comparative example provides a forging method for large-diameter 316H bars, which differs from Example 1 only in that water cooling is not performed after the fourth heating. The specific steps are as follows:

[0073] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1180℃ and held at that temperature for 7 hours to ensure uniform and thorough heating. After being taken out of the furnace, it is first upheaded to a diameter of 1150mm and then drawn to an octagonal shape of 900mm to obtain a forged billet. During the upheading process, the billet is first upset by 300mm and then held for 20s before continuing to uphead.

[0074] S2. The first-fire forging billet is heated to 1150℃ and held for 2 hours to ensure that the steel ingot is evenly heated through. After being taken out of the furnace, it is first uprooted to a diameter of 1150mm, and then drawn and forged to 900mm octagonal to obtain the second-fire forging billet.

[0075] S3. The second-fired forging billet is heated to 1100℃ and held for 5 hours to ensure that the steel ingot is heated evenly and thoroughly. After being removed from the furnace, it is roughened to a diameter of 1100mm to obtain the third-fired forging billet.

[0076] S4. Continue heating the three-fire forging billet to 1100℃, hold for 2 hours to ensure the steel ingot is evenly heated through, then remove it from the furnace, first uplift it to a diameter of 1220mm, and then draw it to a diameter of 1150mm to obtain the four-fire forging billet.

[0077] S5, the four-fire forging billet is reheated to 1000℃ and held for 4 hours, then heated to 1050℃ and held for 0.5 hours. After being taken out of the furnace, it is stretched and deformed at 50% of the forging frequency to obtain a round bar with a diameter of Ф757mm.

[0078] S6. The Ф757mm round steel is reheated to 1010℃ and held for 50 minutes, then air-cooled to obtain 316H bar stock.

[0079] Comparative Example 3

[0080] This comparative example provides a forging method for large-diameter 316H bars, which differs from Example 1 only in that an air cooling step is added after the second and fourth heating processes. The specific steps are as follows:

[0081] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1180℃ and held at that temperature for 7 hours to ensure uniform and thorough heating. After being taken out of the furnace, it is first upheaded to a diameter of 1150mm and then drawn to an octagonal shape of 900mm to obtain a forged billet. During the upheading process, the billet is first upset by 300mm and then held for 20s before continuing to uphead.

[0082] S2. The first-fire forging billet is heated to 1150℃ and held for 2 hours to ensure that the steel ingot is evenly heated. After being taken out of the furnace, it is first uprooted to a diameter of 1150mm, then drawn and forged to 900mm octagonal, and then air-cooled until the surface temperature of the forging billet reaches 600~700℃ to obtain the second-fire forging billet.

[0083] S3. The second-fired forging billet is heated to 1100℃ and held for 5 hours to ensure that the steel ingot is heated evenly and thoroughly. After being removed from the furnace, it is roughened to a diameter of 1100mm to obtain the third-fired forging billet.

[0084] S4. Continue heating the three-fire forging billet to 1100℃, hold it for 2 hours to ensure that the steel ingot is evenly heated through, and then remove it from the furnace. First, it is uprooted to a diameter of 1220mm, then drawn to a diameter of 1150mm, and then air-cooled until the surface temperature of the forging billet reaches 600-700℃ to obtain the four-fire forging billet.

[0085] S5, the four-fire forging billet is reheated to 1000℃ and held for 4 hours, then heated to 1050℃ and held for 0.5 hours. After being taken out of the furnace, it is stretched and deformed at 50% of the forging frequency to obtain a round bar with a diameter of Ф757mm.

[0086] S6. The Ф757mm round steel is reheated to 1010℃ and held for 50 minutes, then air-cooled to obtain 316H bar stock.

[0087] Comparative Example 4

[0088] This comparative example provides a forging method for large-diameter 316H bars. Unlike Example 1, the first, second, and third forging processes are carried out isothermally, and an air cooling step is added after the second forging. The specific steps are as follows:

[0089] S1. A 316H electroslag ingot with a diameter of 920mm is heated to 1180℃ and held at that temperature for 7 hours to ensure uniform and thorough heating. After being taken out of the furnace, it is first upheaded to a diameter of 1150mm and then drawn to an octagonal shape of 900mm to obtain a forged billet. During the upheading process, the billet is first upset by 300mm and then held for 20s before continuing to uphead.

[0090] S2. The first-fire forging billet is heated to 1180℃ and held for 2 hours to ensure that the steel ingot is evenly heated. After being taken out of the furnace, it is first uprooted to a diameter of 1150mm, then drawn and forged to 900mm octagonal, and then air-cooled until the surface temperature of the forging billet reaches 600~700℃ to obtain the second-fire forging billet.

[0091] S3. The second-heat forging billet is heated to 1180℃ and held for 2 hours to ensure that the steel ingot is heated evenly and thoroughly. After being removed from the furnace, it is roughened to a diameter of 1100mm to obtain the third-heat forging billet.

[0092] S4, the three-fire forging billet is further heated to 1050℃, held for 3 hours, and then taken out of the furnace and elongated to obtain a Ф760mm round bar by drawing deformation at a forging frequency of 50%.

[0093] S6. The Ф760mm round steel is reheated to 1010℃ and held for 50 minutes, then air-cooled to obtain 316H bar stock.

[0094] Grain size test

[0095] The grain size of 316H bars with a diameter of 757 mm to 760 mm prepared in Examples 1 to 3 and Comparative Examples 1 to 4 was tested at multiple points. To ensure the accuracy and reliability of the test results, the following procedures were followed: Figure 1 The sampling method shown is to take samples from multiple points along the radius of the bar in a fan shape. The test results are shown in Table 1.

[0096] Table 1. Grain size detection results

[0097]

[0098]

[0099]

[0100] Note: The following explains the grain size representation X(Y) and XY in the table: ① X(Y) means that more than 90% of the grain size is grade X and less than 10% is grade Y; ② X-Y means that there are mixed crystals in the structure, the matrix is ​​grade X, accounting for more than 50%, and the rest are grade Y grain size.

[0101] As can be seen from the table above, although the grain size of the core was improved in Comparative Examples 1 to 3, large grains were still present in the core of the finished products. The embodiments of this invention employ a unique six-fire forging process. By controlling the temperature of each fire treatment, adding water cooling after the fourth fire, and using low-temperature, slow-frequency forging in the fifth fire, followed by low-temperature holding in the sixth fire, the internal structure of large-size 316H alloy bars can be guaranteed to be uniform, with a grain size of grade 3.0 or higher and a range ≤ 2.0.

[0102] Figure 2 The images show the metallographic structure at the edges of the head and tail, at half the radius, and at the center of the 316H bar prepared in Example 1. Figure 3 Metallographic images of the head and tail edges, half radius, and center of the 316H bar prepared for Comparative Example 1.

[0103] As can be seen from the figure, in Comparative Example 1, elongated grains with a grade greater than 0 are present at the head and tail edges, with fine grains reaching grade 7.0. The grain size at half the radius mostly meets the requirement of grade ≥3.0, but some large grains exist. The grain size at the center is grade 0–1.5. In contrast, the 316H rod prepared in Example 1 has a uniform grain size distribution at the head and tail edges, half the radius, and the center, all within the range of grade 3.0–4.0, meeting the requirements.

[0104] 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 within the protection scope of the present invention.

Claims

1. A forging method for achieving homogeneous microstructure in large-diameter 316H bars, characterized in that, Includes the following steps: S1, 316H electroslag ingot is heated to the first heat treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and drawn to an octagonal length of 880mm to 900mm to obtain a first heat forging billet; wherein, the first heat treatment temperature is 1170℃ to 1190℃; S2, the first-fire forging billet is heated to the second-fire treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and drawing to obtain the second-fire forging billet; wherein, the second-fire treatment temperature is 20℃~30℃ lower than the first-fire treatment temperature; S3, the two-heat forging billet is heated to the three-heat treatment temperature, and after being kept at the temperature until it is thoroughly heated, it is taken out of the furnace and upsetting and deformed to obtain a three-heat forging billet; wherein, the three-heat treatment temperature is 50℃~70℃ lower than the two-heat treatment temperature; S4, the three-heat forging billet is heated to the four-heat treatment temperature, kept heated through, then taken out of the furnace for upsetting and drawing, and water-cooled to below 600°C to obtain a four-heat forging billet; wherein, the temperature of the four-heat treatment is the same as the temperature of the three-heat treatment. S5, the four-heat forging billet is first heated through at 980℃~1000℃, then heated to the five-heat treatment temperature, held for 0.5h~1h, and then forged to the finished product specifications at a frequency of 50%~70% to obtain the five-heat forging billet; wherein, the five-heat treatment temperature is 30℃~50℃ lower than the four-heat treatment temperature. S6. Heat the five-fired forging billet to 1000℃~1020℃, hold for 40min~60min, remove from the furnace and air cool to obtain 316H alloy bar.

2. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S1, the 316H electroslag ingot is cylindrical with a diameter of 920mm to 1020mm; and / or In S1, the heat preservation and heat penetration time is 6h to 8h; and / or In S1, the elongation deformation is 35% to 45%.

3. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S2, the heat preservation and heat penetration time is 2h to 3h; and / or In S2, the material is first uptaken to a diameter of 1140mm to 1160mm, and then drawn to an octagonal diameter of 880mm to 900mm, with a drawing deformation of 35% to 45%.

4. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S3, the deformation of the upsetting is 30%–40%; and / or In S3, the heat preservation and heat penetration time is 4h to 6h.

5. The forging method for achieving homogenization of microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S4, the material is first uptaken to a diameter of 1210mm to 1240mm, and then drawn to a diameter of 1150mm to 1180mm. The total deformation of uptake and drawing is 20% to 40%.

6. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S4, the heat preservation and heat penetration time is 2h to 3h.

7. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S5, the heat preservation and heat penetration time is 4h to 5h.

8. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 1, characterized in that, In S5, the diameter of the finished product is 757mm to 760mm.

9. The forging method for achieving uniform microstructure in large-diameter 316H bars as described in claim 8, characterized in that, In S5, the deformation during forging is 45% to 60%.

10. A large-size 316H bar, characterized in that, It is prepared by the forging method for homogenizing the microstructure of large-size 316H bars as described in any one of claims 1 to 9.

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