A method of strengthening forging of 316L austenitic stainless steel forging

Abstract

This invention relates to the field of stainless steel forging technology, specifically to a strengthening forging process for 316L austenitic stainless steel forgings. The process includes sequentially heating, forging, solution treatment, and machining a 316L steel ingot, followed by warm pressing deformation at 600–650°C with a deformation amount of 55–65%, and then rapid water cooling to produce 316L austenitic stainless steel forgings. The tensile strength of these forgings is ≥600 MPa, and the yield strength is ≥500 MPa. This invention improves the tensile strength and yield strength of 316L austenitic stainless steel forgings.

Description

Technical Field

[0001] This invention relates to the field of stainless steel forging technology, specifically to a strengthening forging process for 316L austenitic stainless steel forgings. Background Technology

[0002] 316L austenitic stainless steel forgings possess good toughness, high plasticity, strong corrosion resistance, and excellent comprehensive processing and welding properties, making them widely used in nuclear industry, petrochemical industry, marine engineering, structural decoration, cryogenic technology, and even daily life. However, due to the relatively low strength of 316L austenitic stainless steel forgings, they cannot meet the requirements of some applications that require higher tensile strength and higher yield strength, thus limiting their use to some extent.

[0003] Therefore, it is necessary to address the above-mentioned problems by improving the existing forging process of 316L austenitic stainless steel to increase the tensile strength and yield strength of 316L austenitic stainless steel. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a strengthening forging process for 316L austenitic stainless steel forgings, aiming to improve the tensile strength and yield strength of 316L austenitic stainless steel forgings. The specific technical solution is as follows:

[0005] A strengthening forging process for 316L austenitic stainless steel forgings involves sequentially heating, forging, solution treatment, and machining a 316L steel ingot, followed by warm pressing deformation at 600–650°C with a deformation amount of 55–65%, and then rapid water cooling to produce 316L austenitic stainless steel forgings. The tensile strength of the 316L austenitic stainless steel forgings is ≥600 MPa, and the yield strength is ≥500 MPa.

[0006] Preferably, the strengthening forging process specifically includes the following steps:

[0007] (1) Billet preparation: 316L electroslag ingots are selected as the billet for forging.

[0008] (2) Heating: The billet is heated in the furnace to 1180℃ and held for 4-5 hours;

[0009] (3) Forging: Forging is carried out using a hydraulic press, and the final forging temperature is controlled at 850℃; the billet is kept warm before being taken out of the furnace, and then lightly rolled into an outer circle, upset, flattened, and drawn into a strip forging.

[0010] (4) Solution treatment: Heat the forging to 1040-1050℃, hold for 4-5 hours, remove from the furnace and quickly cool in water;

[0011] (5) Machining: The upper and lower planes of the strip forging are machined to form a flat plane with the upper and lower planes parallel to each other;

[0012] (6) Warm-press deformation: After the strip forging is machined, it is loaded into the furnace, heated to 600-650℃, and kept at that temperature for 4-5 hours before being taken out of the furnace. The forging is then warm-pressed using a hydraulic press and a warm-press deformation mold. Under pressure, the strip forging is elongated and its height is reduced in the warm-press deformation mold. After the deformation reaches 55-65%, it is quickly cooled in water to obtain a long strip rectangular section forging.

[0013] Preferably, the hydraulic press is a 45MN to 100MN forging hydraulic press.

[0014] Preferably, a grain size inspection process is set between the machining in step (5) and the warm-pressing deformation in step (6) to ensure that the grain size of the forging reaches level 3 and the hardness is 110-135HB.

[0015] Preferably, during the warm pressing deformation process in step (6), the deformation amount of the strip forging is 60%.

[0016] Preferably, the 316L steel ingot is an electroslag ingot with a diameter of Φ360mm×743mm. In the forging process of step (3), it is upset to 520mm×520mm×280mm, flattened to 400mm×300mm×630mm, and drawn into a strip forging of 350mm×200mm×1080mm. In the machining process of step (5), the strip forging is machined to 340mm×200mm×1080mm. In step (6), the temperature and pressure transformation... During the forming process, the strip forging is warm-pressed to 135mm×200mm×2720mm; the resulting elongated rectangular cross-section forging has a longitudinal tensile strength of 767MPa, a longitudinal yield strength of 713MPa, a longitudinal elongation after fracture of 20%, and a longitudinal reduction of area of ​​75%; the resulting elongated rectangular cross-section forging has a transverse tensile strength of 688MPa, a transverse yield strength of 643MPa, a transverse elongation after fracture of 17.5%, and a transverse reduction of area of ​​54%.

[0017] In this invention, during the rapid water cooling process after solution treatment in step (4) and rapid water cooling after thermoforming in step (6), a Venturi rapid water cooling device is used to rapidly water cool the forging. The Venturi rapid water cooling device includes an open cuboid water tank and a Venturi rapid water flow channel disposed within the cuboid water tank. The Venturi rapid water flow channel is surrounded by a pair of irregularly shaped baffles disposed within the cuboid water tank and arranged symmetrically. The front and rear ends of the pair of irregularly shaped baffles are respectively connected to the inner walls of the front and rear ends of the cuboid water tank, thereby blocking the cooling water stored in the water tank. The left and right sides inside the tank; each of the irregularly shaped partitions includes a first inclined plate, a middle vertical plate and a second inclined plate connected sequentially from top to bottom. A cooling water inlet channel with a larger upper part and a smaller lower part is formed between a pair of first inclined plates in the pair of irregularly shaped partitions. A cooling water straight channel through which cooling water flows is formed between a pair of middle vertical plates in the pair of irregularly shaped partitions. A cooling water outlet channel with a smaller upper part and a larger lower part is formed between a pair of second inclined plates in the pair of irregularly shaped partitions. A pair of electric drain gates are correspondingly provided on the pair of first inclined plates for allowing the cooling water located outside the irregularly shaped partition in the water tank to enter the cooling water inlet channel.

[0018] When rapidly water-cooling a forging, the forging is horizontally hoisted into the cooling water direct channel (for strip-shaped or long strip-shaped forgings are horizontally arranged), and then a pair of electric drain gates are opened to allow cooling water to enter the Venturi-type rapid water flow channel. The cooling water flows rapidly from top to bottom through the cooling water direct channel, thereby achieving rapid water cooling of the forging.

[0019] Preferably, a pair of diverter plates are also provided in the Venturi rapid water flow channel at the position of the cooling water outlet channel. The front and rear ends of the pair of diverter plates are respectively connected to the inner walls of the front and rear ends of the cuboid water tank, and a cooling water diversion channel with a larger upper part and a smaller lower part is formed between the pair of diverter plates.

[0020] By setting up a pair of diverter plates, a portion of the cooling water flowing downward from the direct cooling water channel can be discharged downward from the diverter channel, ensuring that the lower part of the forging is completely immersed in the flowing cooling water, thereby guaranteeing the water cooling effect of all parts of the forging.

[0021] Preferably, the manifold is an arc-shaped manifold.

[0022] In this invention, the pair of irregularly shaped partitions further includes a pair of horizontal plates correspondingly connected to the lower end of the pair of second inclined plates, and a pair of lower vertical plates correspondingly connected to the other end of the pair of horizontal plates. The front and rear ends of the pair of horizontal plates and the front and rear ends of the pair of lower vertical plates are respectively connected to the inner walls of the front and rear ends of the cuboid water tank. The space enclosed by the pair of horizontal plates and the pair of lower vertical plates forms a water collection cavity for receiving cooling water in the Venturi-type rapid water flow channel.

[0023] Preferably, a serpentine tube heat exchanger can also be installed in the water accumulation cavity to quickly remove heat from the cooling water.

[0024] Preferably, the serpentine tube heat exchanger is connected to a cooling tower or cooling water tank via a circulating cooling pipeline.

[0025] Preferably, a number of cooling water circulation pipes are provided between the cooling water storage chamber located outside the irregularly shaped partition and the water accumulation chamber in the water tank, and a cooling water circulation pump is provided on the cooling water circulation pipes.

[0026] By setting up cooling water circulation pipelines and cooling water circulation pumps, continuous water flow cooling of forgings can be ensured.

[0027] The beneficial effects of this invention are:

[0028] First, the present invention provides a strengthening forging process for 316L austenitic stainless steel forgings. By subjecting the forgings after forging and solution treatment to warm pressing deformation at 600-650°C, the 316L austenitic stainless steel forgings are strengthened, and the tensile strength and yield strength of the forgings are greatly improved, thereby meeting the application requirements of special high-strength performance.

[0029] Secondly, the strengthening forging process of 316L austenitic stainless steel forgings of the present invention can improve the rapid water cooling effect after solution treatment and after hot pressing deformation by setting a Venturi rapid water cooling device, thereby further improving the corrosion resistance and other properties of 316L austenitic stainless steel forgings. Attached Figure Description

[0030] Figure 1 This is a schematic flow diagram of a strengthening forging process for 316L austenitic stainless steel forgings according to the present invention.

[0031] Figure 2 This is a schematic diagram of a Venturi rapid water cooling device.

[0032] Figure 3 It is a longitudinal metallographic inspection image of the completed forging (including non-metallic inclusions, grain size and metallographic structure);

[0033] Figure 4It is a transverse metallographic inspection diagram of the completed forging (including non-metallic inclusions, grain size and metallographic structure).

[0034] In the diagram: 1. Rectangular water tank; 2. Venturi-type rapid water flow channel; 3. Irregularly shaped baffle; 4. First inclined plate; 5. Middle vertical plate; 6. Second inclined plate; 7. Cooling water inlet channel; 8. Cooling water straight channel; 9. Cooling water outlet channel; 10. Electric drain gate; 11. Diverter plate; 12. Horizontal plate; 13. Lower vertical plate; 14. Water accumulation chamber; 15. Cooling water storage chamber; 16. Cooling water circulation pipeline; 17. Cooling water circulation pump; 18. Forging. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0036] Example 1:

[0037] like Figures 1 to 4 The illustration shows an embodiment of a strengthening forging process for 316L austenitic stainless steel forgings according to the present invention. A 316L steel ingot is sequentially heated, forged, solution-treated, and machined. Then, it undergoes warm pressing deformation at 600–650°C with a deformation amount of 55–65%. Finally, it is rapidly cooled in water to produce 316L austenitic stainless steel forgings. The tensile strength of the 316L austenitic stainless steel forgings is ≥600 MPa, and the yield strength is ≥500 MPa.

[0038] Preferably, the strengthening forging process specifically includes the following steps:

[0039] (1) Billet preparation: 316L electroslag ingots are selected as the billet for forging.

[0040] (2) Heating: The billet is heated in the furnace to 1180℃ and held for 4-5 hours;

[0041] (3) Forging: Forging is carried out using a hydraulic press, and the final forging temperature is controlled at 850℃; the billet is kept warm before being taken out of the furnace, and then lightly rolled into an outer circle, upset, flattened, and drawn into a strip forging.

[0042] (4) Solution treatment: Heat the forging to 1040-1050℃, hold for 4-5 hours, remove from the furnace and quickly cool in water;

[0043] (5) Machining: The upper and lower planes of the strip forging are machined to form a flat plane with the upper and lower planes parallel to each other;

[0044] (6) Warm-press deformation: After the strip forging is machined, it is loaded into the furnace, heated to 600-650℃, and kept at that temperature for 4-5 hours before being taken out of the furnace. The forging is then warm-pressed using a hydraulic press and a warm-press deformation mold. Under pressure, the strip forging is elongated and its height is reduced in the warm-press deformation mold. After the deformation reaches 55-65%, it is quickly cooled in water to obtain a long strip rectangular section forging.

[0045] Preferably, the hydraulic press is a 45MN to 100MN forging hydraulic press.

[0046] Preferably, a grain size inspection process is set between the machining in step (5) and the warm-pressing deformation in step (6) to ensure that the grain size of the forging reaches level 3 and the hardness is 110-135HB.

[0047] Preferably, during the warm pressing deformation process in step (6), the deformation amount of the strip forging is 60%.

[0048] Preferably, the 316L steel ingot is an electroslag ingot with a diameter of Φ360mm×743mm. In the forging process of step (3), it is upset to 520mm×520mm×280mm, flattened to 400mm×300mm×630mm, and drawn into a strip forging of 350mm×200mm×1080mm. In the machining process of step (5), the strip forging is machined to 340mm×200mm×1080mm. In step (6), the temperature and pressure transformation... During the forming process, the strip forging is warm-pressed to 135mm×200mm×2720mm; the resulting elongated rectangular cross-section forging has a longitudinal tensile strength of 767MPa, a longitudinal yield strength of 713MPa, a longitudinal elongation after fracture of 20%, and a longitudinal reduction of area of ​​75%; the resulting elongated rectangular cross-section forging has a transverse tensile strength of 688MPa, a transverse yield strength of 643MPa, a transverse elongation after fracture of 17.5%, and a transverse reduction of area of ​​54%.

[0049] In this embodiment, during the rapid water cooling process after solution treatment in step (4) and rapid water cooling after thermoforming in step (6), a Venturi rapid water cooling device is used to rapidly water cool the forging. The Venturi rapid water cooling device includes a rectangular water tank 1 with an open upper part and a Venturi rapid water flow channel 2 disposed in the rectangular water tank 1. The Venturi rapid water flow channel 2 is surrounded by a pair of irregularly shaped baffles 3 disposed in the rectangular water tank 1 and arranged symmetrically. The front and rear ends of the pair of irregularly shaped baffles 3 are respectively connected to the inner walls of the front and rear ends of the rectangular water tank 1, thereby blocking the cooling water stored in the water tank 1 from entering the water tank 1. Left and right sides; each of the irregularly shaped partitions 3 includes a first inclined plate 4, a middle vertical plate 5 and a second inclined plate 6 connected sequentially from top to bottom. A cooling water inlet channel 7 with a larger upper part and a smaller lower part is formed between a pair of first inclined plates 4 in the pair of irregularly shaped partitions 3. A cooling water straight channel 8 through which cooling water flows is formed between a pair of middle vertical plates 5 in the pair of irregularly shaped partitions 3. A cooling water outlet channel 9 with a smaller upper part and a larger lower part is formed between a pair of second inclined plates 6 in the pair of irregularly shaped partitions 3. A pair of electric drain gates 10 are correspondingly provided on the pair of first inclined plates 4 for allowing the cooling water located outside the irregularly shaped partitions 3 in the water tank 1 to enter the cooling water inlet channel 7.

[0050] When rapidly water-cooling the forging 18, the forging 18 is horizontally hoisted into the cooling water direct channel 8 (the strip forging or long strip forging is set horizontally), and then a pair of electric drain gates 10 are opened to allow cooling water to enter the Venturi rapid water flow channel 2. The cooling water flows rapidly from top to bottom through the cooling water direct channel 8, thereby achieving rapid water cooling of the forging.

[0051] Preferably, a pair of diverter plates 11 are also provided in the Venturi rapid water flow channel at the position of the cooling water outlet channel 9. The front and rear ends of the pair of diverter plates 11 are respectively connected to the inner walls of the front and rear ends of the cuboid water tank 1, and the pair of diverter plates 11 form a cooling water diversion channel with a larger upper part and a smaller lower part.

[0052] By setting a pair of diverter plates 11, a portion of the cooling water flowing downward from the cooling water direct channel 8 can be discharged downward from the cooling water diversion channel, so that the lower part of the forging is completely immersed in the flowing cooling water, thereby ensuring the water cooling effect of each part of the forging.

[0053] Preferably, the manifold 11 is an arc-shaped manifold.

[0054] In this embodiment, the pair of irregularly shaped partitions 3 also include a pair of horizontal plates 12 correspondingly connected to the lower end of the pair of second inclined plates 6, and a pair of lower vertical plates 13 correspondingly connected to the other end of the pair of horizontal plates 12. The front and rear ends of the pair of horizontal plates 12 and the front and rear ends of the pair of lower vertical plates 13 are respectively connected to the inner walls of the front and rear ends of the cuboid water tank 1. The space enclosed by the pair of horizontal plates 12 and the pair of lower vertical plates 13 forms a water collection cavity 14 for collecting cooling water in the Venturi-type rapid water flow channel.

[0055] Preferably, a serpentine tube heat exchanger (not shown in the figure) can also be installed in the water accumulation cavity 14 to quickly remove heat from the cooling water.

[0056] Preferably, the serpentine tube heat exchanger is connected to a cooling tower or cooling water tank via a circulating cooling pipeline.

[0057] Preferably, a number of cooling water circulation pipes 16 are provided between the cooling water storage chamber 15 located outside the irregular partition 3 in the water tank 1 and the water accumulation chamber 14, and a cooling water circulation pump 17 is provided on the cooling water circulation pipes 16.

[0058] By setting up cooling water circulation pipe 16 and cooling water circulation pump 17, continuous water flow cooling of the forging can be ensured.

[0059] Example 2:

[0060] Intermediate process testing was conducted on the 316L austenitic stainless steel forgings in Example 1. Specifically, the grain size and hardness of the forgings after machining were tested. The measured results were: grain size of grade 3, and hardness (measured at 3 locations) of 118HB, 112HB and 128HB.

[0061] Example 3:

[0062] The surface hardness of the 316L austenitic stainless steel forging prepared in Example 1 was tested (at 3 locations), and the measured hardness values ​​were 256HB, 260HB and 270HB.

[0063] Example 4:

[0064] The chemical composition of the 316L austenitic stainless steel forging (ingot blank) from Example 1 was tested, and the measured results are shown in the table below (%):

[0065] C Si Mn P S Cr Mo Ni Cu 0.01 0.17 0.47 0.002 0.001 17..56 2.60 13.63 0.004 Co Al Nb Ti N V W Ca Fe 0.009 0.004 0.005 0.001 0.041 0.008 0.020

[0066] Example 5:

[0067] Samples were taken from the 316L austenitic stainless steel forging prepared in Example 1. The sampling location was 30 mm below the surface of the forging. Mechanical property samples and metallographic samples were taken and subjected to mechanical property testing and metallographic testing, respectively.

[0068] (1) The tensile test results are as follows:

[0069] a. Sampling location: 30 cm longitudinally under the skin

[0070] b. Required value:

[0071] Tensile strength Rm (MPa): ≥760

[0072] Yield strength Rp0.2 (MPa): ≥650

[0073] Elongation after fracture A (%): ≥16

[0074] c. Measured value:

[0075] Vertical:

[0076] Tensile strength Rm (MPa): 767

[0077] Yield strength Rp0.2 (MPa): 713

[0078] Elongation at break A (%): 20.0

[0079] Reduction of area Z (%): 75

[0080] Horizontal:

[0081] Tensile strength Rm (MPa): 688

[0082] Yield strength Rp0.2 (MPa): 643

[0083] Elongation at break A (%): 17.5

[0084] Reduction of area Z (%): 54

[0085] (2) The metallographic test results are shown in the table below.

[0086]

[0087] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A strengthening forging process for 316L austenitic stainless steel forgings, characterized in that, The 316L steel ingot is heated, forged, solution treated and machined in sequence, and then subjected to warm pressing deformation at 600-650℃ with a deformation amount of 55-65%. Then it is rapidly cooled in water to produce 316L austenitic stainless steel forgings. The tensile strength of the 316L austenitic stainless steel forgings is ≥600Mpa and the yield strength is ≥500Mpa. The strengthening forging process specifically includes the following steps: (1) Billet preparation: 316L electroslag ingots are selected as the billet for forging; (2) Heating: The billet is heated in the furnace to 1180℃ and held for 4 to 5 hours; (3) Forging: Forging is carried out using a hydraulic press, and the final forging temperature is controlled at 850℃; the billet is kept warm before being taken out of the furnace, and then lightly rolled into an outer circle, upset, flattened, and drawn into a strip forging. (4) Solution treatment: Heat the forging to 1040-1050℃, hold for 4-5 hours, remove from the furnace, and quickly cool in water; (5) Machining: The upper and lower planes of the strip forging are machined to form a flat plane with the upper and lower planes parallel to each other; (6) Warm-press deformation: After the strip forging is machined, it is loaded into the furnace and heated to 600-650℃. After holding for 4-5 hours, it is taken out of the furnace and subjected to warm-press deformation using a hydraulic press and a warm-press deformation mold. Under pressure, the strip forging is elongated and its height is reduced in the warm-press deformation mold. After the deformation reaches 55-65%, it is quickly immersed in water for cooling to obtain a long strip rectangular section forging.

2. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 1, characterized in that, The hydraulic press is a 45MN to 100MN forging hydraulic press.

3. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 1, characterized in that, Between step (5) machining and step (6) warm pressing deformation, a grain size inspection process is also set to ensure that the grain size of the forging reaches level 3 and the hardness is 110-135HB.

4. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 1, characterized in that, During the warm pressing deformation process in step (6), the deformation amount of the strip forging is 60%.

5. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 1, characterized in that, The 316L steel ingot is an electroslag ingot with a diameter of Φ360mm×743mm. In step (3) forging, it is upset to 520mm×520mm×280mm, flattened to 400mm×300mm×630mm, and drawn into a strip forging of 350mm×200mm×1080mm. In step (5) machining, the strip forging is machined to 340mm×200mm×1080mm. In step (6) warm pressing deformation, the strip forging is warm pressed to 135mm×200mm×2720mm. The longitudinal tensile strength of the elongated rectangular cross-section forging is 767 MPa, the longitudinal yield strength is 713 MPa, the longitudinal elongation after fracture is 20%, and the longitudinal reduction of area is 75%. The transverse tensile strength of the elongated rectangular cross-section forging is 688 MPa, the transverse yield strength is 643 MPa, the transverse elongation after fracture is 17.5%, and the transverse reduction of area is 54%.

6. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 1, characterized in that, In the rapid water cooling process after solution treatment in step (4) and rapid water cooling after thermoforming in step (6), a Venturi rapid water cooling device is used to rapidly cool the forging. The Venturi rapid water cooling device includes an open cuboid water tank and a Venturi rapid water flow channel set inside the cuboid water tank. The Venturi rapid water flow channel is surrounded by a pair of irregularly shaped baffles arranged symmetrically inside the cuboid water tank. The front and rear ends of the pair of irregularly shaped baffles are respectively connected to the front and rear inner walls of the cuboid water tank, thereby blocking the cooling water stored in the water tank from entering the water tank. The left and right sides; each of the irregularly shaped partitions includes a first inclined plate, a middle vertical plate and a second inclined plate connected sequentially from top to bottom. A cooling water inlet channel with a larger upper part and a smaller lower part is formed between a pair of first inclined plates in the pair of irregularly shaped partitions. A cooling water straight channel through which cooling water flows is formed between a pair of middle vertical plates in the pair of irregularly shaped partitions. A cooling water outlet channel with a smaller upper part and a larger lower part is formed between a pair of second inclined plates in the pair of irregularly shaped partitions. A pair of electric drain gates are correspondingly provided on the pair of first inclined plates for allowing the cooling water located outside the irregularly shaped partition in the water tank to enter the cooling water inlet channel.

7. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 6, characterized in that, A pair of diverter plates are also provided in the Venturi rapid water flow channel at the location of the cooling water outlet channel. The front and rear ends of the pair of diverter plates are respectively connected to the inner walls of the front and rear ends of the cuboid water tank, and a cooling water diversion channel with a larger upper part and a smaller lower part is formed between the pair of diverter plates.

8. The strengthening forging process for 316L austenitic stainless steel forgings according to claim 6, characterized in that, The pair of irregularly shaped partitions also include a pair of horizontal plates corresponding to the lower end of the pair of second inclined plates and a pair of lower vertical plates corresponding to the other end of the pair of horizontal plates. The front and rear ends of the pair of horizontal plates and the front and rear ends of the pair of lower vertical plates are respectively connected to the inner walls of the front and rear ends of the cuboid water tank. The space enclosed by the pair of horizontal plates and the pair of lower vertical plates forms a water collection cavity for receiving cooling water in the Venturi rapid water flow channel.

9. A strengthening forging process for 316L austenitic stainless steel forgings according to claim 8, characterized in that, A number of cooling water circulation pipes are provided between the cooling water storage chamber located outside the irregularly shaped partition and the water accumulation chamber inside the water tank, and a cooling water circulation pump is provided on the cooling water circulation pipes.

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