Large-diameter superaustenitic stainless steel pierced billet for nuclear power station and hot rolling perforation process thereof

Abstract

The large-diameter super austenitic stainless steel pierced billet comprises the following chemical components in percentage by mass: less than or equal to 0.03% of C, less than or equal to 1.0% of Si, less than or equal to 1.0% of Mn, 20-22% of Cr, 23.5-25.5% of Ni, 6-7% of Mo, 0.5-1.0% of Cu, 0.15-0.25% of N, less than or equal to 0.010% of S, less than or equal to 0.03% of P and the balance of Fe, and comprises the following steps: step 1, preparing a tube blank; 2, a heating process system is formulated; 3, a rolling angle is additionally arranged; 4, the stabilization tendency of the roller and the top head is designed; 5, perforating operation is carried out; and 6, cooling. The preparation method has the advantages that the weight of a single piece exceeds 2300 kg, and the hot extrusion method is adopted for preparation.

Description

Technical Field

[0001] The invention relates to the field of stainless steel rough pipes, in particular to a large-diameter super austenitic stainless steel rough pipe for nuclear power plants and a hot rolling and piercing process thereof. Background Art

[0002] The global nuclear power industry is currently experiencing rapid growth. Developed countries are upgrading the design of nuclear power plants and increasing investment in the research and development of nuclear power equipment and materials. my country, in turn, is promoting and supporting high-end equipment manufacturing and new materials as strategic emerging industries. Nuclear power plants in my country currently utilize third-generation nuclear power technology and will spearhead the large-scale development of fourth-generation nuclear power. This technology utilizes an advanced, non-powered pressurized water reactor (PWR) reactor. Fission of uranium fuel within the reactor generates significant heat, which is removed using high-pressure water. Therefore, pipelines operating under high pressure and high temperature require high strength and resistance to heat and corrosion. N08367 super austenitic stainless steel is a preferred choice over other 300 series stainless steels. N08367 is a typical 6% molybdenum super austenitic stainless steel. Its resistance to chloride pitting, crevice corrosion, and stress corrosion is far superior to that of ordinary stainless steel in harsh environments such as high pressure, high temperature, and seawater. Its cost is lower than that of corrosion-resistant nickel-based alloys. It exhibits excellent corrosion resistance in various severe corrosive environments and is suitable for high-chloride aqueous solutions and chloride-corrosive environments. Nuclear power plant seawater corrosion-resistant piping is designed using N08367 super austenitic stainless steel seamless pipes. The maximum outer diameter used in nuclear power plant seawater systems is 711mm, the wall thickness is 25mm, the length is 6000-8000mm, and the minimum single pipe weight is 2572.5kg.

[0003] While N08367 finished tubes are ultimately cold-rolled, the raw material (or blank tube) used is hot-formed. The high alloy content of N08367, particularly the 6% molybdenum content, has long limited the use of hot rolling and piercing for the preparation of basic tube blanks. Taigang Stainless Steel previously used hot extrusion to produce blank tubes, which met the required standards. However, due to the processing characteristics of hot extrusion and mold limitations, domestic production is currently limited to tubes weighing less than 2,300 kg. Products weighing above this range are difficult to produce using hot extrusion. To address this, our company undertook the research and development of materials and performance optimization for basic tube blanks. Our R&D team has successfully trial-produced large-diameter, 2,870 kg single-piece N08367 super austenitic stainless steel blank tubes (Ø520×42mm) using the piercing process, filling a domestic and international gap. This product has successfully passed inspection and acceptance by downstream manufacturers and third parties. Summary of the Invention

[0004] The purpose of the present invention is to provide a large-diameter super austenitic stainless steel rough pipe for nuclear power plants to solve the problems raised in the above background technology. The pipe has the advantages of a single pipe weight exceeding 2300 kg and is prepared by hot extrusion.

[0005] To achieve the above-mentioned object, the present invention provides the following technical solution: a large-diameter super austenitic stainless steel rough pipe for nuclear power plants, comprising the following chemical components and their mass percentages: C: ≤0.03%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 20-22%, Ni: 23.5-25.5%, Mo: 6-7%, Cu: 0.5-1.0%, N: 0.15-0.25%, S, ≤0.010%, P: ≤0.03%, and the balance being Fe.

[0006] Another object of the present invention is to provide a hot rolling and piercing process for a large-diameter super austenitic stainless steel rough pipe for a nuclear power plant, comprising the following steps:

[0007] Step 1: Prepare the tube blank and select the tube blank with the above chemical composition;

[0008] Step 2: Develop a heating process system and set multiple heating zones, including the first heating zone, the second heating zone, the third heating zone, and the fourth heating zone. The temperature of the first heating zone is set to 1180°C for 60 minutes, the temperature of the second heating zone is set to 1050°C for 120 minutes, the temperature of the third heating zone is set to 960°C for 120 minutes, and the temperature of the fourth heating zone is set to 800°C for 60 minutes.

[0009] Step 3: Add a rolling angle, use a Ф720 tapered piercing machine, set the feed angle to 8°-15°, set the rolling angle to 12°-15°, and set the initial speed to 15r / min-18r / min;

[0010] Step 4: Design the stabilization tendency of the roll and the plug,

[0011] Roller, the rolling strip width is 8mm, the roller surface is added with "knurling" process, the modulus of the mesh is m0.6, the depth is 1.2mm, the surface roughness is Ra12.5µm,

[0012] The plug is made of molybdenum powder with a purity of 99.96% and a density of 9.5g / cmᵌ, and is mixed with rare earth elements (La2O3) to form the plug;

[0013] Step 5: Punching operation, using the punching unit to punch the tube into shape at 1150℃-1180℃;

[0014] Step 6: Cooling. After the rough pipe is successfully perforated, the water temperature in the quenching pool is set to 12°C lower than the room temperature, and the pipe is cooled at a cooling rate of more than 500°C / min to obtain the finished product.

[0015] Preferably, the initial speed of the cone punch is set to 12 r / min.

[0016] Preferably, during the heating process of the tube blank, the temperature should be quickly passed in the range of 600°C-900°C, and should be slowly heated after 900°C.

[0017] Preferably, the feed angle is set to 8.5° and the rolling angle is set to 15°.

[0018] Preferably, the plug size is set as follows: the maximum diameter (cylinder) is 456 mm, with an arc-shaped design; the length is 460 mm; the surface turning roughness Ra1.0 µm, the surface hardness 260 (HV), the ovality (guide plate spacing / roller spacing) is 1.06, and the plug extension is 90 mm.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. Formulate a heating process system and adopt appropriate heating temperature and time to comprehensively solve the four existing problems in tube billet heating, specifically, (1) temperature and time control for optimal plasticity; (2) uniform heating of the tube billet with a temperature difference of no more than 10°C; (3) temperature control for precipitation and precipitation of σ (χ) intermetallic phase; (4) control to prevent coarse grains in the core of the tube billet, and standardize operating procedures to adapt to the perforation performance of N08367 super austenitic stainless steel.

[0021] 2. Compared with the traditional piercing machine, it has an additional rolling angle. In hot rolling piercing, especially in the production of highly alloyed materials, its advantages are more obvious. The specific effects are: (1) the rotary forging effect is suppressed; (2) when piercing in the three-dimensional arrangement of the tapered rollers, the metal flow is reasonable; (3) the tapered rollers are very suitable for piercing stainless steel and highly alloyed steel;

[0022] 3. Since N08367 super austenitic stainless steel has strong resistance to plastic deformation, achieving the first bite is very critical, and the warp (pressing down) deformation in the deformation zone is improved. The axial resistance is increased during the second bite (when the plug enters the tube blank), achieving smooth bite. Specific feed angles and rolling angles are selected to achieve the above effects.

[0023] 4. The design of the stabilization tendency of the roll and the plug is adopted to improve the perforation performance of N08367 super austenitic stainless steel. Specifically, the "knurling" process is added to the surface of the roll. DETAILED DESCRIPTION

[0024] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0025] In the embodiment of the present invention,

[0026] Example 1: A large-diameter super austenitic stainless steel rough pipe for a nuclear power plant, comprising the following chemical components and their mass percentages: C: ≤0.03%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 20%, Ni: 23.5%, Mo: 6%, Cu: 0.5%, N: 0.15%, S, ≤0.010%, P: ≤0.03%, and the remainder being Fe.

[0027] A hot rolling and perforating process for preparing large-diameter super austenitic stainless steel rough pipes for nuclear power plants comprises the following steps:

[0028] Step 1: Prepare the tube blank and select the tube blank with the above chemical composition;

[0029] Step 2: Develop a heating process system and set multiple heating zones, including the first heating zone, the second heating zone, the third heating zone, and the fourth heating zone. The temperature of the first heating zone is set to 1180°C for 60 minutes, the temperature of the second heating zone is set to 1050°C for 120 minutes, the temperature of the third heating zone is set to 960°C for 120 minutes, and the temperature of the fourth heating zone is set to 800°C for 60 minutes.

[0030] Step 3: Add a rolling angle, use a Ф720 tapered piercing machine, set the feed angle to 8°-15°, set the rolling angle to 12°-15°, and set the initial speed to 15r / min-18r / min;

[0031] Step 4: Design the stabilization tendency of the roll and the plug,

[0032] Roller, the rolling strip width is 8mm, the roller surface is added with "knurling" process, the modulus of the mesh is m0.6, the depth is 1.2mm, the surface roughness is Ra12.5µm,

[0033] The plug is made of molybdenum powder with a purity of 99.96% and a density of 9.5g / cmᵌ, and is mixed with rare earth elements (La2O3) to form the plug;

[0034] Step 5: Punching operation, using the punching unit to punch the tube into shape at 1150℃-1180℃;

[0035] Step 6: Cooling. After the rough pipe is successfully perforated, the water temperature in the quenching pool is set to 12°C lower than the room temperature, and the pipe is cooled at a cooling rate of more than 500°C / min to obtain the finished product.

[0036] The initial speed of the cone punch was set to 12 r / min.

[0037] During the heating process of the tube blank, the temperature should be quickly passed in the range of 600℃-900℃, and should be heated slowly after 900℃.

[0038] The feed angle is set to 8.5° and the rolling angle is set to 15°.

[0039] The plug dimensions are set as follows: the maximum diameter (cylindrical) is 456mm, with an arc-shaped design; the length is 460mm; the surface turning roughness is Ra1.0µm, the surface hardness is 260 (HV), the ovality (guide plate spacing / roller spacing) is 1.06, and the plug extension is 90mm.

[0040] Example 2: A large-diameter super austenitic stainless steel rough pipe for a nuclear power plant, comprising the following chemical components and their mass percentages: C: ≤0.03%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 21%, Ni: 24%, Mo: 6-7%, Cu: 0.8%, N: 0.2%, S, ≤0.010%, P: ≤0.03%, and the remainder being Fe.

[0041] A hot rolling and perforating process for preparing large-diameter super austenitic stainless steel rough pipes for nuclear power plants comprises the following steps:

[0042] Step 1: Prepare the tube blank and select the tube blank with the above chemical composition;

[0043] Step 2: Develop a heating process system and set multiple heating zones, including the first heating zone, the second heating zone, the third heating zone, and the fourth heating zone. The temperature of the first heating zone is set to 1180°C for 60 minutes, the temperature of the second heating zone is set to 1050°C for 120 minutes, the temperature of the third heating zone is set to 960°C for 120 minutes, and the temperature of the fourth heating zone is set to 800°C for 60 minutes.

[0044] Step 3: Add a rolling angle, use a Ф720 tapered piercing machine, set the feed angle to 8°-15°, set the rolling angle to 12°-15°, and set the initial speed to 15r / min-18r / min;

[0045] Step 4: Design the stabilization tendency of the roll and the plug,

[0046] Roller, the rolling strip width is 8mm, the roller surface is added with "knurling" process, the modulus of the mesh is m0.6, the depth is 1.2mm, the surface roughness is Ra12.5µm,

[0047] The plug is made of molybdenum powder with a purity of 99.96% and a density of 9.5g / cmᵌ, and is mixed with rare earth elements (La2O3) to form the plug;

[0048] Step 5: Punching operation, using the punching unit to punch the tube into shape at 1150℃-1180℃;

[0049] Step 6: Cooling. After the rough pipe is successfully perforated, the water temperature in the quenching pool is set to 12°C lower than the room temperature, and the pipe is cooled at a cooling rate of more than 500°C / min to obtain the finished product.

[0050] The initial speed of the cone punch was set to 12 r / min.

[0051] During the heating process of the tube blank, the temperature should be quickly passed in the range of 600℃-900℃, and should be heated slowly after 900℃.

[0052] The feed angle is set to 8.5° and the rolling angle is set to 15°.

[0053] The plug dimensions are set as follows: the maximum diameter (cylindrical) is 456mm, with an arc-shaped design; the length is 460mm; the surface turning roughness is Ra1.0µm, the surface hardness is 260 (HV), the ovality (guide plate spacing / roller spacing) is 1.06, and the plug extension is 90mm.

[0054] Example 3: A large-diameter super austenitic stainless steel rough pipe for a nuclear power plant, comprising the following chemical components and their mass percentages: C: ≤0.03%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 22%, Ni: 25.5%, Mo: 7%, Cu: 1.0%, N: 0.25%, S, ≤0.010%, P: ≤0.03%, and the remainder being Fe.

[0055] A hot rolling and perforating process for preparing large-diameter super austenitic stainless steel rough pipes for nuclear power plants comprises the following steps:

[0056] Step 1: Prepare the tube blank and select the tube blank with the above chemical composition;

[0057] Step 2: Develop a heating process system and set multiple heating zones, including the first heating zone, the second heating zone, the third heating zone, and the fourth heating zone. The temperature of the first heating zone is set to 1180°C for 60 minutes, the temperature of the second heating zone is set to 1050°C for 120 minutes, the temperature of the third heating zone is set to 960°C for 120 minutes, and the temperature of the fourth heating zone is set to 800°C for 60 minutes.

[0058] Step 3: Add a rolling angle, use a Ф720 tapered piercing machine, set the feed angle to 8°-15°, set the rolling angle to 12°-15°, and set the initial speed to 15r / min-18r / min;

[0059] Step 4: Design the stabilization tendency of the roll and the plug,

[0060] Roller, the rolling strip width is 8mm, the roller surface is added with "knurling" process, the modulus of the mesh is m0.6, the depth is 1.2mm, the surface roughness is Ra12.5µm,

[0061] The plug is made of molybdenum powder with a purity of 99.96% and a density of 9.5g / cmᵌ, and is mixed with rare earth elements (La2O3) to form the plug;

[0062] Step 5: Punching operation, using the punching unit to punch the tube into shape at 1150℃-1180℃;

[0063] Step 6: Cooling. After the rough pipe is successfully perforated, the water temperature in the quenching pool is set to 12°C lower than the room temperature, and the pipe is cooled at a cooling rate of more than 500°C / min to obtain the finished product.

[0064] The initial speed of the cone punch was set to 12 r / min.

[0065] During the heating process of the tube blank, the temperature should be quickly passed in the range of 600℃-900℃, and should be heated slowly after 900℃.

[0066] The feed angle is set to 8.5° and the rolling angle is set to 15°.

[0067] The plug dimensions are set as follows: the maximum diameter (cylindrical) is 456mm, with an arc-shaped design; the length is 460mm; the surface turning roughness is Ra1.0µm, the surface hardness is 260 (HV), the ovality (guide plate spacing / roller spacing) is 1.06, and the plug extension is 90mm.

[0068] Principle analysis:

[0069] 1. The high degree of alloying of N08367 makes the thermoplasticity worse, and the heating temperature and time become the difficulty in whether the hot rolling piercing can be carried out smoothly.

[0070] Molybdenum and chromium are both elements that form and stabilize ferrite and expand the ferrite phase. Molybdenum also promotes the precipitation of intermetallic phases, such as σ and χ phases, in austenitic stainless steel, adversely affecting the steel's mechanical properties, particularly reducing toughness. The addition of molybdenum to austenitic stainless steel primarily improves the steel's corrosion resistance in reducing media and its resistance to pitting and crevice corrosion. These benefits become more pronounced with increasing molybdenum content. Furthermore, increasing molybdenum content significantly improves high-temperature strength. For example, properties such as rupture and creep are significantly improved, leading to the widespread use of molybdenum-containing stainless steel in high-temperature applications. However, the addition of molybdenum increases the steel's resistance to high-temperature deformation. Furthermore, due to the frequent presence of small amounts of δ-ferrite in the steel, the hot workability of molybdenum-containing stainless steel is inferior to that of non-molybdenum-containing steel, with higher molybdenum content leading to a decrease in hot workability. Furthermore, molybdenum-containing stainless steel is prone to σ (χ) phase precipitation, significantly deteriorating the steel's plasticity and toughness. N08367 super austenitic stainless steel contains 6% molybdenum, far more than typical austenitic stainless steel (304 contains no molybdenum, 316 contains only 2%, and even the more corrosion-resistant 317L contains only 3%). Stainless steel with high molybdenum content is difficult to control even for small-diameter billets, let alone very large diameters. Precisely because of its high resistance to high-temperature deformation, uniform heating temperatures create temperature stresses that exacerbate hot-rolled piercing. Too low a temperature can prevent penetration and damage the piercing machine. Too high a temperature can cause the billet to shred and crack, rendering it unusable. Therefore, when hot-rolling N08267 billet piercing, four issues must be addressed:

[0071] (1) Temperature and time control for optimal plasticity;

[0072] (2) Heat the tube evenly with a temperature difference of no more than 10°C;

[0073] (3) Temperature control of σ(χ) intermetallic phase precipitation and precipitation;

[0074] (4) Prevent the control of coarse grains in the core of the tube blank.

[0075] The present invention utilizes appropriate heating temperature and time to comprehensively solve the above four problems and standardize the operating procedures to adapt to the perforation performance of N08367 super austenitic stainless steel. The specific practices and the reasons before adopting the corresponding practices are analyzed as follows:

[0076] In the production of stainless steel and special alloys, the heating temperature is often determined according to their special properties. The heating process system is the key to whether the final product can meet the standard technical requirements. Therefore, the formulation of the heating process system of N08367 super austenitic stainless steel should also take its special properties as a basis.

[0077] N08367 is a highly alloyed super-austenitic stainless steel. This high alloying increases the heated interface, significantly impacting the uniformity of tube heating and its thermoplastic properties. Excessively high temperatures can increase the core grain size of the tube, significantly compromising its toughness. The piercing process creates alternating stresses, causing the raw tube to break upon exit. Conversely, excessively low temperatures increase the deformation resistance, resulting in poor penetration and, more seriously, fracture of the equipment's transmission coupling.

[0078] First, Mo itself is a refractory metal, with a thermal sensitivity exceeding 600°C and a melting point of 2610°C. While extremely stable at room temperature, Mo maintains its high strength and hardness at elevated temperatures. Therefore, the hot working properties of stainless steel with a high Mo content are essentially a function of the Mo's heating properties. To ensure that Mo is not only highly stable during heating, but also exhibits a certain degree of plasticity to reduce its resistance to deformation, Molybdenum alone cannot exhibit plasticity; its unique properties only emerge when alloyed with other metals. N08367 super austenitic stainless steel is a high-Mo alloy with a melting point of approximately 1370°C. While alloys can generally be hot worked below 150°C below the melting point, this grade's high content of ferrite, such as chromium and molybdenum, may lead to the precipitation of brittle phases as the temperature rises, reducing the plasticity of the tube billet. Therefore, the maximum control temperature is set at approximately 200°C below the melting point.

[0079] Secondly, affected by the thermal sensitivity of molybdenum, the thermal sensitivity of N08367 super austenitic stainless steel is much higher than that of general stainless steel. When the temperature entering the austenite γ phase zone is higher than the Ac3 line by 100~150℃, the grains begin to form or grow slowly. Therefore, the temperature of the austenite stable zone is stable along with the thermal stability of molybdenum.

[0080] A hot working map was established for a single N08367 tube billet test. The high-temperature deformation behavior of super-austenitic stainless steel N08367 was investigated by observing its microstructural deformation process. In metal plastic deformation, deformation is caused by dislocation motion, and the deformation stress is determined by the resistance to dislocation motion. This resistance consists of resistance that can be overcome by thermal activation and resistance that cannot be thermally activated. In the hot testing of N08367, the processing stress was significantly affected by deformation temperature and strain rate. At the same strain rate, the deformation resistance decreased with increasing deformation temperature. At the same deformation temperature, the higher the strain rate, the greater the deformation resistance. This indicates that plastic deformation is a thermally activated process, and the energy required during this process is the thermal deformation activation energy. The higher this energy, the lower the deformation resistance. The high-temperature deformation behavior of N08367 was investigated by microstructural observations of the hot working map. Results indicate that within the deformation parameter range where the instability zone lies, the material exhibits poor processing properties, concentrated deformation, and non-uniform microstructure after deformation. This results in localized plastic deformation instability, leading to plastic instability. During deformation in the instability zone, the energy consumed by the structural changes in hot working deformation of metal materials is very low, with most of the energy converted into heat. At high strain rates, the large amount of heat cannot dissipate quickly enough, causing localized temperature increases in the material, leading to concentrated deformation and a high risk of rheological instability. In this situation, cracks and distortion will appear on the inner wall of N08367 super austenitic stainless steel after forming the rough tube. Therefore, hot piercing should be avoided in low-temperature, high-speed areas.

[0081] Conclusion 1: Based on the above analysis, we conclude that the σ(χ) intermetallic phase precipitation temperature lies between 600°C and 900°C. Heating should be performed rapidly, without dwelling time, and after 900°C, heating should be slow. This reduces the thermal instability zone and allows harmful intermetallic phases to disperse rather than precipitate. Although austenite recrystallization begins at 750°C, and grain growth is slow, grain growth accelerates with increasing temperature and heating time. After 1050°C, nuclei form and gradually begin to grow, reaching grain saturation at 1150°C. Thereafter, growth continues rapidly with increasing temperature, reaching its peak. Coarse grains not only reduce plasticity during hot rolling and piercing, but can also cause defects such as delamination and end shredding. Therefore, around 1150°C is considered the optimal temperature for hot rolling and piercing. N08367 super austenitic stainless steel exhibits strong deformation resistance. Lower temperatures can make piercing difficult and overload equipment, leading to production accidents. Finally, according to the characteristics of N08367 super austenitic stainless steel, the hot rolling piercing temperature is determined to be between 1150℃ and 1180℃.

[0082] Adopt inclined bottom heating furnace, basic situation of inclined bottom heating furnace:

[0083] Furnace effective dimensions: 27000 (length) × 5200 (width) × 1500 (height) mm;

[0084] Burner model and arrangement: 23 top-mounted direct-injection burners, (2×4) + (5×3);

[0085] Steel turning opening: 18 steel turning openings (doors) on one side; the fuel is natural gas.

[0086] Combined with the situation of inclined bottom heating furnace, the heating process system for hot perforation of N8367 super austenitic stainless steel was developed.

[0087] Hot rolling piercing heating process of N08367 super austenitic stainless steel (Table 1)

[0088]

[0089] The following requirements are put forward for production operators:

[0090] (1) If the deviation between the target temperature and the furnace temperature is greater than 10°C, stop the material at the 4# door and keep it warm for 20 minutes. Before taking the material out of the furnace, confirm again that the temperature difference between the inside and outside of the billet is within 10°C and that the target temperature is consistent with the actual billet temperature.

[0091] (2) The temperature given above refers to the steel temperature. Use an optical thermometer to measure the temperature at the connection between the two tube billets to ensure that the tube billet temperature reaches uniformity.

[0092] (3) When turning the steel, the time must be evenly arranged, and two guarantees must be made: first, the section travel time must be guaranteed; second, the steel must be turned 90° at a time.

[0093] 2. The three-dimensional layout technology of the oblique rolling space of the tapered piercing machine and the control of metal flow stress and strain

[0094] The piercing unit is a 720mm tapered cross-rolling piercing unit with a maximum production diameter of 800mm. Two tapered rollers are driven by a 2000kW main machine. The rollers are arranged horizontally, with a maximum elongation coefficient of 6. The tapered piercing unit differs from conventional piercing units in the shape of the rollers. While maintaining the same feed angle, it has an additional rolling angle. Its advantages are particularly evident in hot-rolling piercing, particularly when producing highly alloyed materials. Its features are as follows:

[0095] First, the rotary forging effect is suppressed. To make the roll arrangement suitable for the piercing process, in addition to the feed angle, a rolling angle is also set. The rolls are tapered and double-supported. Experiments have shown that the mechanical properties, elongation, and cross-sectional compression of the tube blank and rough pipe are largely dependent on the roll angle and feed angle of the rolls. The larger the two values, the greater the elongation and compression. Further microscopic observations using a scanning electron microscope (SEM) show that the rotary forging effect is significantly suppressed, and cavity defects are almost invisible. In contrast, when piercing with relatively small feed and rolling angles, the rotary forging effect is very pronounced, almost as obvious as with conventional cross-rolling piercing machines, and cavity defects are clearly visible.

[0096] Second, metal flow is optimized during piercing within a three-dimensional arrangement of tapered rollers. Piercing experiments were conducted with varying rolling and feed angles to examine their effects on circumferential shear deformation. The experiments clearly showed that the feed angle significantly impacted circumferential shear deformation. As the feed angle increased proportionally, the circumferential shear deformation value decreased significantly; similarly, as the rolling angle increased, the circumferential shear deformation value also decreased significantly. This can be expressed as follows:

[0097] Circumferential shear deformation value = (blank pipe radius × shear deformation displacement) / blank pipe wall thickness.

[0098] Particularly noticeable is that when the rolling angle is 15° and the feed angle is greater than 14°, or when the rolling angle is 20° and the feed angle is greater than 10°, circumferential shear deformation completely disappears, i.e., its value is zero. The influence of the roll rolling and feed angles on surface distortion is also significant, especially at high rolling rates, where metal flow does not produce surface distortion that would otherwise cause shear deformation. When piercing at large rolling and feed angles, circumferential shear and surface distortion are strictly controlled, even to zero, maintaining essentially the same metal flow velocity.

[0099] Third, tapered rollers are well-suited for perforating stainless steel and highly alloyed steel. When a large cross-rolling angle and feed angle are used for perforation, the accompanying release of shear forces can prevent the increase in central porosity that can lead to microcavity defects. Highly alloyed and ultra-highly alloyed stainless steels often develop microcracks on the inner surface of the rough pipe using traditional oblique rolling perforation methods. Severe microcracks can extend to the surface and cause cracking of the pipe wall. Experiments have shown that when the perforation temperature is appropriate, tapered roller perforators with large cross-angles can accommodate the perforation of these materials without causing internal cracking defects.

[0100] The perforation performance of N08367 super austenitic stainless steel is relatively complex. The selection of the feed angle and rolling angle during tapered roll perforation is very important. On a two-roller oblique rolling mill, if the rolls are arranged left and right, when viewed from top to bottom, the roll axis and the rolling centerline are parallel in the top view of the rolls; when viewed from the side, the roll axis and the rolling centerline intersect in the side view of the rolls. This angle is called the feed angle, also called the bite angle. The feed angle is generally between 8° and 15°. The size of the feed angle is related to the following factors: (1) The larger the feed angle, the higher the outlet speed of the blank tube and the shorter the perforation time; (2) The smaller the feed angle, the easier it is for the tube blank to bite; (3) The size of the feed angle determines the size of the rolling power. The larger the angle, the greater the mill load. If the rolls are arranged left and right, when viewed from top to bottom, the roll axis and the rolling centerline are not parallel in the top view of the rolls, but intersect. This intersection angle is called the rolling angle. The rolling angle of a double-supported tapered roller piercing mill is a key characteristic, and a fixed value of 15° is generally considered reasonable. A larger rolling angle increases the weight of the main drive and rollers, as well as the space and plane occupied, resulting in bulkier equipment. Our company's Ø720 tapered piercing mill offers an adjustable rolling angle of 12° to 15°. The rolling angle is closely related to the circumferential speed of the rollers. The angle is selected to ensure a regular increase in the circumferential speed of the rollers along the outlet direction, matching the speed of the tube billet. This creates a stretching or resistance effect on the tube billet. This minimizes the rotary forging effect, surface distortion, and circumferential shear deformation during cross-roll piercing, significantly reducing internal and external surface defects. Due to the strong resistance of N08367 super-austenitic stainless steel to plastic deformation, achieving the first bite is crucial. This also reduces the longitudinal (reduction) deformation within the deformation zone, increasing the axial resistance during the second bite (when the plug enters the tube billet), ensuring smooth engagement. In practice, we choose a feed angle of 8.5° and a rolling angle of 15°.

[0101] The speed of the tapered rollers depends on the circumferential speed (i.e., linear velocity) of each roller segment. Generally, there are three sections: the inlet, the rolling, and the outlet. Different diameters dictate different linear velocities. The slow inlet facilitates entry, while the fast outlet facilitates pull-out. This allows the wall thickness of the rough pipe to be naturally adjusted during the intermediate rolling process, with compression followed by tensioning. This process is critically influenced by the initial roll speed. While the initial speed of a typical stainless steel tapered piercing mill is designed to be between 15 and 18 rpm, the N08367 is designed for 12 rpm due to its unique characteristics.

[0102] 3. The design of the stabilization tendency of the roll and the plug is one of the measures to ensure the improvement of the perforation performance of N08367 super austenitic stainless steel.

[0103] If the tube billet does not slip within the roll pass, its theoretical axial and tangential speeds would be the same as those at corresponding points on the roll surfaces. However, in reality, from the initial bite to the narrowest throat in the deformation zone (where the roll gap is minimum), the cross-sectional area of ​​the tube billet decreases as it advances, and the total extension of the metal in this section can reach a maximum of more than 4°. Therefore, within the deformation zone, the metal flow velocity accelerates, and the advance speed of the tube billet at the inlet and the blank tube at the outlet differ significantly. Consequently, slip is inevitable between the tube billet, the blank tube, and the roll surfaces. Whether caused by tangential or axial slip, the metal flow rate within the deformation zone is maintained in a dynamic equilibrium under the influence of the roll's positive pressure and friction, the axial thrust of the plug, and the friction of the guide plate. Tangential slip (distortion of the tube billet) or increased axial slip (delayed bite) disrupts this dynamic equilibrium, making final forming of the blank tube difficult or causing internal bore defects. The greater the resistance to metal flow in the deformation zone, the smaller the slip and the larger the slip coefficient. Therefore, factors such as the friction coefficient between the tool and tube contact surface, rolling speed and temperature, tool and tube shape and dimensions, plug extension and mandrel usage (ours is fixed), and feed angle all influence the tangential and axial slip coefficients. Excessive slip during the rolling process is detrimental, reducing production efficiency, accelerating tool wear, increasing energy consumption, and deteriorating rough tube quality. Therefore, optimal design should maximize the slip coefficient.

[0104] First, in roll design, we took into account the characteristics of the difficult-to-deform N08367 super austenitic stainless steel. After the temperature was determined, we reduced the width of the rolling strip to reduce the neutral zone and thus the slip area. The typical rolling strip width is 12mm, but we designed it to be 8mm. Most importantly, to increase the sliding coefficient between the roll surface and the tube, we needed to increase the dynamic friction coefficient of the roll surface. The relationship is:

[0105] Dynamic friction coefficient (µ) = friction force (f) / normal pressure (N)

[0106] The varying molecular structures of various material surfaces influence the magnitude of the kinetic friction coefficient. For example, the kinetic friction coefficient of steel-on-steel is 0.25. The rougher the surface, the greater the kinetic friction coefficient. Therefore, surface roughness directly influences the magnitude of the kinetic friction coefficient. Highly alloyed N08367 super-austenitic stainless steel, with its high Cr, Mo, and Ni content, exhibits a smooth, low-oxidation surface and high hardness at temperatures of 1180°C. If friction is not increased to increase the kinetic friction coefficient between the surface and the roll, excessive slippage will occur, while the sliding coefficient will decrease, making slip inevitable and, in severe cases, preventing engagement. To prevent this, we have added a "knurling" process to the roll surface. The modulus of the texture is m0.6, and its depth is twice the modulus, at 1.2 mm, resulting in a surface roughness of Ra12.5µm.

[0107] Secondly, the cross-rolling process subjects the metal to a distinct triaxial stress and strain state. This spatial stress-strain state, simplified to a planar or axisymmetric problem, becomes a three-dimensional equilibrium and stability problem. In reality, cross-rolling is spiral rolling, where the metal undergoes cyclical and continuous action within the same deformation zone, causing changes in shape and size. A closed pass is formed between the plug, the rolls, and the guide plate, where the metal undergoes continuous cyclic rolling. During cross-rolling piercing without the intervention of the plug, the entrance cone should be the tube diameter, known as the first bite, and also the roll reduction applied to the tube. When the plug intervenes (the second bite) and begins to penetrate the center of the tube, the plug area represents the rough tube wall thickness. The absolute reduction between the plug, rolls, and guide plate at this point is twice the radial reduction. Reducing the slip surface and maintaining a stable spiral motion within three-dimensional equilibrium requires, in addition to roll bite (increasing the dynamic friction coefficient) and the support provided by the guide plate, the appropriate plug shape, surface smoothness, and hardness. When piercing N08367 super austenitic stainless steel, we use high-purity molybdenum powder FMo-1 plugs. The molybdenum powder has a purity of 99.96% and a density of 9.5g / cmᵌ. It is mixed with a rare earth element (La2O3) to improve the high-temperature creep resistance and ductility of the molybdenum plug.

[0108] Plug dimensions: The maximum diameter (cylindrical) is 456mm, with a curved design; the length is 460mm; the surface roughness is Ra1.0µm, and the surface hardness is 260 (HV). The plug reach (the distance between the front end of the plug and the rolled strip) is adjusted based on the ovality of the roll and guide plate spacing. A greater ovality results in a greater plug reach, and vice versa, in a proportional relationship. The actual adjustment parameters are an ovality (guide plate spacing / roll spacing) of 1.06 and a plug reach of 90mm.

[0109] After the rough pipe was successfully perforated, it needed to be cooled rapidly. The cooling water was replaced with a cooling tower that accelerated cooling. The water temperature in the quenching tank was 12°C below room temperature, and the cooling rate was above 500°C / min.

[0110] Data results:

[0111] The dimensional deviation of the N08367 super austenitic stainless steel rough pipe is: maximum outer diameter 530mm, minimum 515mm; maximum wall thickness 43.56mm, minimum 41.25mm, which fully complies with the dimensional deviation specified in 5.1.2 of the China Iron and Steel Association's T / CISA439-2024 "Stainless Steel and Nickel-Based Alloy Rough Pipe" standard.

[0112] Mechanical properties test:

[0113]

[0114] Grain size: 4-5.5. Downstream user requirements: ≥3.5.

[0115] Conclusion: It fully meets the requirements of N08367 super austenitic stainless steel pipe for nuclear power plant pipes.

[0116] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A large-diameter super austenitic stainless steel rough pipe for nuclear power plants, characterized by: The chemical components and their mass percentages are: C: ≤0.03%, Si: ≤1.0%, Mn: ≤1.0%, Cr: 20~22%, Ni: 23.5~25.5%, Mo: 6~7%, Cu: 0.5~1.0%, N: 0.15~0.25%, S, ≤0.010%, P: ≤0.03%, and the balance is Fe.

2. A hot rolling and perforation process for preparing the large-diameter super austenitic stainless steel rough pipe for nuclear power plants as claimed in claim 1, characterized in that: The steps include: Step 1: Prepare the tube blank and select the tube blank with the above chemical composition; Step 2: Develop a heating process system and set multiple heating zones, including the first heating zone, the second heating zone, the third heating zone, and the fourth heating zone. The temperature of the first heating zone is set to 1180°C for 60 minutes, the temperature of the second heating zone is set to 1050°C for 120 minutes, the temperature of the third heating zone is set to 960°C for 120 minutes, and the temperature of the fourth heating zone is set to 800°C for 60 minutes. Step 3: Add a rolling angle, use a Ф720 tapered piercing machine, set the feed angle to 8°-15°, set the rolling angle to 12°-15°, and set the initial speed to 15r / min-18r / min; Step 4: Design the stabilization tendency of the roll and the plug, Roller, rolling strip width is 8mm, the roller surface is added with "knurling" process, the modulus of the mesh is m0.6, the depth is 1.2mm, the surface roughness is Ra12.5µm, The plug is made of molybdenum powder with a purity of 99.96% and a density of 9.5g / cmᵌ, and is mixed with rare earth elements (La2O3) to form the plug; Step 5: Punching operation, using the punching unit to punch the tube into shape at 1150℃-1180℃; Step 6: Cooling. After the rough pipe is successfully perforated, the water temperature in the quenching pool is set to 12°C lower than the room temperature, and the pipe is cooled at a cooling rate of more than 500°C / min to obtain the finished product.

3. The large-diameter super austenitic stainless steel rough pipe for nuclear power plants and the hot rolling and piercing process thereof according to claim 2, characterized in that: The initial speed of the cone punch was set to 12 r / min.

4. The large-diameter super austenitic stainless steel rough pipe for nuclear power plants and the hot rolling and piercing process thereof according to claim 2, characterized in that: During the heating process of the tube blank, the temperature should be quickly passed in the range of 600℃-900℃, and should be heated slowly after 900℃.

5. The large-diameter super austenitic stainless steel rough pipe for nuclear power plants and the hot rolling and piercing process thereof according to claim 2, characterized in that: The feed angle is set to 8.5° and the rolling angle is set to 15°.

6. The large-diameter super austenitic stainless steel rough pipe for nuclear power plants and the hot rolling and piercing process thereof according to claim 2, characterized in that: The plug dimensions are set as follows: the maximum diameter (cylindrical) is 456mm, with an arc-shaped design; the length is 460mm; the surface turning roughness is Ra1.0µm, the surface hardness is 260 (HV), the ovality (guide plate spacing / roller spacing) is 1.06, and the plug extension is 90mm.

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