Forging process of high-strength superaustenitic stainless steel bar

Abstract

The invention relates to a forging process of a high-strength super austenitic stainless steel bar, which is used for a 22Cr-18Ni-6Mo-6Mn type super austenitic stainless steel bar with the diameter of 75-320mm and the yield strength higher than 931MPa, and comprises the following chemical components in percentage by mass: less than or equal to 0.03% of C, less than or equal to 0.80% of Si, 5.00-8.00% of Mn, less than or equal to 0.03% of P, less than or equal to 0.01% of S, 20.0-23.0% of Cr, 17.0-20.0% of Ni, 4.0-7.0% of Mo, 0.25-0.65% of N, less than or equal to 0.10% of Cu and the balance of Fe. The process comprises the steps of pretreatment of a cast ingot or a casting blank, heat treatment before forging and forging, oxidation loss in heating before forging and surface crack defects in the forging process are reduced through pretreatment of the cast ingot or the casting blank, and the forging blank obtains uniform components and microstructures through homogenization treatment in the heat treatment before forging; through solution treatment and warm forging strengthening treatment in the forging process, the austenitic stainless steel rod which is fine and uniform in structure and excellent in mechanical property and corrosion resistance is obtained.

Description

Technical Field

[0001] This invention belongs to the field of forging technology, specifically relating to a forging process for high-strength super austenitic stainless steel bars, which is particularly suitable for forging 22Cr-18Ni-6Mo-6Mn type super austenitic stainless steel bars. Background Technology

[0002] Super austenitic stainless steel, due to its higher chromium (Cr), molybdenum (Mo), and nitrogen (N) content, exhibits superior resistance to uniform corrosion, pitting corrosion, crevice corrosion, and stress corrosion compared to conventional austenitic stainless steel. Simultaneously, nitrogen alloying significantly enhances its strength and toughness, making it widely applicable in harsh environments such as oil and gas extraction, seawater desalination, chemical equipment, and flue gas desulfurization. However, the high alloying element content, especially nitrogen, presents challenges to the hot working of super austenitic stainless steel. The main problems include: severe dendritic segregation in the as-cast microstructure, which easily induces cracks during subsequent processing; and a narrow hot working temperature window (typically 950–1180℃), where excessively high temperatures can lead to coarse grains and the precipitation of harmful phases (such as high-temperature ferrite and σ phase), while excessively low temperatures can easily cause cracking due to insufficient plasticity, or due to Cr2N and M... 23 The precipitation of C6 and other phases impairs corrosion resistance; it is difficult to achieve the comprehensive requirements of high strength, fine grains, uniform structure and excellent corrosion resistance simultaneously through conventional heat treatment processes.

[0003] Currently, some research has been conducted in China on the forging processes of certain super austenitic stainless steels. For example, Chinese patent "A method for high-temperature homogenization treatment of super austenitic stainless steel, publication number CN106893831A" describes a homogenization treatment of S32654 super austenitic stainless steel at 1240-1280℃, and controls the cooling process to obtain a uniform microstructure without Sigma precipitates; Chinese patent "A high-nitrogen, low-molybdenum super austenitic stainless steel and its preparation method, publication number CN115976417A" discloses a 25Cr-16Ni-5Mo-7Mn type super austenitic stainless steel with added rare earth elements and its manufacturing process. One method involves obtaining high-N, low-Mo super austenitic stainless steel ingots through pressure smelting, followed by homogenization and hot working to achieve super austenitic stainless steel materials with suitable grain size and good matching of corrosion resistance and mechanical properties. Chinese patent "A forging process for nitrogen-containing austenitic stainless steel, publication number CN109732034A" discloses a forging process for ordinary nitrogen-containing austenitic stainless steel of type 21Cr-4Ni-9Mn. This process uses a temperature compensation device on the forging press to control the final forging temperature, avoiding forging cracks while achieving a uniform microstructure and improving forging efficiency. However, there are currently no reports on the production process of 22Cr-18Ni-6Mo-6Mn super austenitic stainless steel forging billets, and existing reports mainly focus on super austenitic stainless steel used in the solution-treated state, with few reports on obtaining high-strength super austenitic stainless steel through warm forging. Existing literature on super austenitic stainless steel shows that there are significant differences between super austenitic stainless steels with different chemical compositions and production processes. The segregating elements and their degree of segregation, the type of precipitated phases, high-temperature plasticity, and the properties of the forged products all differ significantly. Key technical parameters such as the pre-forging heating regime, forging deformation method, and heating temperature and time during forging also vary. Therefore, it is necessary to explore a forging process for high-strength super austenitic stainless steel bars.

[0004] Forging processes need to overcome problems such as compositional segregation, hot working cracks, precipitation of harmful phases, coarse grain size, and uneven microstructure that are common in super austenitic stainless steel. Simultaneously, the strength must be improved through work hardening. Therefore, a suitable pre-forging heating regime, combined with appropriate forging deformation processes, intermediate billet heat treatment, and cold deformation strengthening forging, is required to obtain high-strength super austenitic stainless steel bars with uniform microstructure, fine grains, and excellent corrosion resistance. However, existing technologies either focus on homogenization treatment, the preparation of specific steel grades, or are geared towards ordinary nitrogen-containing steels used in the solution state. They do not address a complete solution for obtaining high-strength (especially yield strength ≥931 MPa), fine-grained, and corrosion-resistant bars through forging process control for specific high-alloy super austenitic stainless steels like 22Cr-18Ni-6Mo-6Mn. In particular, there is a lack of publicly available reports on how to effectively combine homogenization, solution treatment, and warm deformation strengthening through forging process design to systematically solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a forging process for high-strength super austenitic stainless steel bars. This process can effectively eliminate billet segregation, control the precipitation of harmful phases, refine grains, and utilize the warm forging work hardening effect to stably produce super austenitic stainless steel bars with a diameter of 75~320mm, yield strength ≥931MPa, uniform microstructure, fine grains (grain size ≥6.5 grade), and excellent corrosion resistance.

[0006] The objective of this invention is achieved as follows: A forging process for a high-strength super austenitic stainless steel bar, wherein the high-strength super austenitic stainless steel bar comprises the following chemical composition by mass percentage (wt%): C≤0.03, Si≤0.80, Mn: 5.00~8.00, P≤0.03, S≤0.01, Cr: 20.0~23.0, Ni: 17.0~20.0, Mo: 4.0~7.0, N: 0.25~0.65, Cu≤0.10, with the balance being Fe and unavoidable impurities; the forging process for the high-strength super austenitic stainless steel bar includes the following steps: Step 1) Surface pretreatment of billet: Surface finishing of super austenitic stainless steel ingots or billets to remove surface cracks and pinhole defects, prevent surface defects from expanding inward during forging, and ensure the surface quality of forgings. Step 2) Spraying an anti-high temperature oxidation coating: Spraying a nano-silicate anti-high temperature oxidation coating on the surface of the finished ingot or billet to prevent excessive oxidation during the high temperature homogenization process before forging and improve the forging yield. Step 3) Stepped high-temperature homogenization treatment: The ingot or billet is placed in a heating furnace for stepped high-temperature homogenization treatment. After preheating at 300-400℃, the temperature is increased to 1100-1180℃ at a rate of ≤100℃ / h and held for 5-20h. Then, the temperature is increased to 1180-1250℃ in the heating furnace and held for 15-40h. Step 4) Forging: The homogenized ingot or billet is transferred to forging equipment for forging. The ingot or billet is forged to the size of an intermediate billet using a precision forging machine with a tonnage of 14MN or above. The diameter of the intermediate billet is 1.10 to 1.3 times the diameter of the finished forging billet, and the final forging temperature is guaranteed to be greater than 950℃. Step 5) Solution treatment of intermediate billet: Return the intermediate billet after billet opening to the heating furnace for solution treatment, heat the billet to 1050-1150℃ and hold it at that temperature for 0.5-2 hours; Step 6) Quenching and rapid cooling: After the intermediate billet has completed solution treatment and heat preservation, it is quickly taken out of the furnace and immersed in water for quenching. The quenching time is controlled between 140 and 450 seconds. Step 7) Warm forging strengthening: Take the quenched intermediate billet out of the water and let it stand in the air to allow the residual heat in the core to be conducted to the surface of the billet. When the surface temperature of the intermediate billet reaches 400-450℃, use a precision forging machine to warm forge the intermediate billet to the finished size of the forging billet. Step 8) Post-forging cooling: Cool the finished bar stock after warm forging with water.

[0007] In step 2), the nano-silicate high-temperature oxidation resistant coating is uniformly sprayed onto the surface of the blank to form a continuous coating.

[0008] In step 3), during the heat preservation stage of 1100–1180℃, the Sigma phase and Cr2N phase precipitated in the as-cast structure are fully dissolved, and the segregated elements in the microsegregation region are initially diffused; during the heat preservation stage of 1180–1250℃, the segregated elements are fully and uniformly diffused.

[0009] In step 5), the solution treatment ensures that the Sigma phase precipitated during forging is redissolved.

[0010] In step 6), the quenching is performed using water immersion cooling, which rapidly cools the Sigma phase, Cr2N phase, and M phase to prevent the formation of these phases. 23 Precipitation of the C6 phase.

[0011] In step 7), when warm forging is carried out in the temperature range of 400 to 450°C, the work hardening effect generated by deformation at this temperature is used to improve the yield strength of the bar.

[0012] In step 8), the water cooling time is no less than 10 minutes to fix the tissue state.

[0013] After step 8), finished product processing is carried out: after the bar is cooled, it is cut to length, cleaned, and tested for physical and chemical properties.

[0014] The finished diameter of the super austenitic stainless steel bar is 75-320 mm, its yield strength is ≥931 MPa, its grain size is ≥6.5 grade, and it has excellent resistance to intergranular corrosion.

[0015] The beneficial effects of this invention are as follows: 1. It can effectively improve the yield and quality: By finishing the billet and pre-spraying anti-oxidation coating, the risk of forging cracking is reduced and the heating oxidation loss is reduced.

[0016] 2. Optimize the uniformity of the microstructure: The two-stage homogenization process of "lower temperature pre-diffusion + high temperature full diffusion" is adopted, which not only avoids over-burning in the high segregation zone, but also effectively eliminates dendrite segregation, laying a foundation for uniform chemical composition for subsequent processing.

[0017] 3. It can effectively control harmful phases and refine grains: By combining the rapid deformation of the precision forging machine, the solution treatment and quenching of the intermediate billet, the precipitation of harmful phases such as Sigma phase and Cr2N phase is effectively suppressed, and fine austenite grains are obtained through recrystallization and deformation control.

[0018] 4. It can achieve a balance between high strength and corrosion resistance: The unique "solution treatment + warm forging" process route suppresses harmful phases and ensures excellent corrosion resistance (especially resistance to intergranular corrosion). At the same time, it utilizes the work hardening effect generated by warm forging to significantly increase the yield strength of the material to over 931 MPa, achieving a good match between high strength and high corrosion resistance. Attached Figure Description

[0019] Picture 1 The image shows the metallographic structure of the super austenitic stainless steel bar prepared using the process described in Example 1 of this invention, revealing a uniform and fine all-austenitic structure. Detailed Implementation

[0020] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] Example 1: The forging material in this example is 22Cr-18Ni-6Mo-6Mn type super austenitic stainless steel, and its specific chemical composition is shown in Table 1. The raw material is φ510mm / φ560mm electroslag ingot produced by electric furnace + ladle refining + electroslag remelting process.

[0022] Table 1. Chemical composition (wt%) of the stainless steel used in Example 1 C Si Mn Ni Cr Mo N P S 0.012 0.30 6.85 17.52 21.95 6.25 0.43 0.020 0.003 The forging process is carried out according to the following steps: Step 1) Surface pretreatment of billet: The surface of the electroslag ingot is thoroughly ground to remove all visible defects; to prevent surface defects from expanding inward during the forging process and to ensure the surface quality of the forging; Step 2) Spraying a high-temperature oxidation resistant coating: The surface is uniformly sprayed with a nano-silicate high-temperature oxidation resistant coating to prevent excessive oxidation during the high-temperature homogenization process before forging and to improve the forging yield. Step 3) Stepped high-temperature homogenization treatment: Furnace heating: After the billet is preheated at 350℃ for 5 hours, it is heated to 1150℃ at a rate of 80℃ / h and held for 15 hours; then it is heated to 1200℃ at maximum power and held for 25 hours. Step 4) Forging: Using a 16MN precision forging machine, the heated ingot is forged into an intermediate billet with a diameter of φ180mm. The final forging temperature is controlled at 980℃. Step 5) Solution treatment of intermediate billet: Put the intermediate billet back into the heating furnace and hold it at 1110℃ for 1 hour; Step 6) Quenching and rapid cooling: Quench in water within 1 minute after taking it out of the furnace for 300 seconds; Step 7) Warm forging strengthening: After water removal, let it cool in the air for 6 minutes. When the surface temperature rises to about 420℃, use the same precision forging machine to warm forge it to the finished size φ155mm. Step 8) Post-forging cooling: Immediately water cool the finished bar after warm forging for 10 minutes; Step 9) Material feeding and physicochemical testing.

[0023] Testing revealed that the yield strength of the super austenitic stainless steel bar sample at a distance of 25.4 mm from the outer surface was 1080 MPa, the tensile strength was 1197 MPa, the elongation after fracture was 23%, the reduction of area was 67%, and the impact energy AK(V) was 226 / 220 / 231 J. Intergranular corrosion resistance test A showed a stepped structure, while intergranular corrosion resistance test E and weldability tests showed no intergranular cracks. The metallographic microstructure of the super austenitic stainless steel bar is as follows: Picture 1 As shown, it has a fully austenitic structure with a grain size of 6.5.

[0024] The above embodiments illustrate only one implementation of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. The scope of protection of this invention patent should be determined by the appended claims.

Claims

1. A forging process for high-strength super austenitic stainless steel bars, characterized in that, The high-strength super austenitic stainless steel bar comprises the following chemical composition by mass percentage (wt%): C≤0.03, Si≤0.80, Mn: 5.00~8.00, P≤0.03, S≤0.01, Cr: 20.0~23.0, Ni: 17.0~20.0, Mo: 4.0~7.0, N: 0.25~0.65, Cu≤0.10, with the balance being Fe and unavoidable impurities; The forging process of the high-strength super austenitic stainless steel bar includes the following steps: Step 1) Surface pretreatment of billet: Surface finishing of super austenitic stainless steel ingots or billets to remove surface cracks and pinhole defects, prevent surface defects from expanding inward during forging, and ensure the surface quality of forgings. Step 2) Spraying an anti-high temperature oxidation coating: Spraying a nano-silicate anti-high temperature oxidation coating on the surface of the finished ingot or billet to prevent excessive oxidation during the high temperature homogenization process before forging and improve the forging yield. Step 3) Stepped high-temperature homogenization treatment: The ingot or billet is placed in a heating furnace for stepped high-temperature homogenization treatment. After preheating at 300-400℃, the temperature is increased to 1100-1180℃ at a rate of ≤100℃ / h and held for 5-20h. Then, the temperature is increased to 1180-1250℃ in the heating furnace and held for 15-40h. Step 4) Forging: The homogenized ingot or billet is transferred to forging equipment for forging. The ingot or billet is forged to the size of an intermediate billet using a precision forging machine with a tonnage of 14MN or above. The diameter of the intermediate billet is 1.10 to 1.3 times the diameter of the finished forging billet, and the final forging temperature is guaranteed to be greater than 950℃. Step 5) Solution treatment of intermediate billet: Return the intermediate billet after billet opening to the heating furnace for solution treatment, heat the billet to 1050-1150℃ and hold it at that temperature for 0.5-2 hours; Step 6) Quenching and rapid cooling: After the intermediate billet has completed solution treatment and heat preservation, it is quickly taken out of the furnace and immersed in water for quenching. The quenching time is controlled between 140 and 450 seconds. Step 7) Warm forging strengthening: Take the quenched intermediate billet out of the water and let it stand in the air to allow the residual heat in the core to be conducted to the surface of the billet. When the surface temperature of the intermediate billet reaches 400-450℃, use a precision forging machine to warm forge the intermediate billet to the finished size of the forging billet. Step 8) Post-forging cooling: Cool the finished bar stock after warm forging with water.

2. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 2), the nano-silicate high-temperature oxidation resistant coating is uniformly sprayed onto the surface of the blank to form a continuous coating.

3. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 3), during the heat preservation stage of 1100–1180℃, the Sigma phase and Cr2N phase precipitated in the as-cast structure are fully dissolved, and the segregated elements in the microsegregation region are initially diffused; during the heat preservation stage of 1180–1250℃, the segregated elements are fully and uniformly diffused.

4. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 5), the solution treatment ensures that the Sigma phase precipitated during forging is redissolved.

5. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 6), the quenching is performed using water immersion cooling, which rapidly cools the Sigma phase, Cr2N phase, and M phase to prevent the formation of these phases. 23 Precipitation of the C6 phase.

6. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 7), when warm forging is carried out in the temperature range of 400 to 450°C, the work hardening effect generated by deformation at this temperature is used to improve the yield strength of the bar.

7. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: In step 8), the water cooling time is no less than 10 minutes to fix the tissue state.

8. The forging process for high-strength super austenitic stainless steel bars according to claim 1, characterized in that: After step 8), finished product processing is carried out: after the bar is cooled, it is cut to length, cleaned, and tested for physical and chemical properties.

9. The forging process for high-strength super austenitic stainless steel bars according to any one of claims 1 to 8, characterized in that: The finished diameter of the super austenitic stainless steel bar is 75-320 mm, its yield strength is ≥931 MPa, its grain size is ≥6.5 grade, and it has excellent resistance to intergranular corrosion.

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