A duplex stainless steel and a method of manufacturing the same

Abstract

The application belongs to the field of alloy material forging technology, and particularly relates to a duplex stainless steel and a manufacturing method thereof. The duplex stainless steel is subjected to upsetting forging and elongation forging. The upsetting ratio of the upsetting forging is 1.5-1.7. The upsetting forging comprises single-fire-time upsetting forging for several times. The deformation of the single-fire-time upsetting forging is 20-30%. The elongation ratio of the elongation forging is 1.5-1.7. The elongation forging comprises single-fire-time elongation forging for several times. The deformation of the single-fire-time elongation forging is 10-20%. The application effectively reduces the cracking degree of the duplex stainless steel in the forging process.

Description

Technical Field

[0001] This invention belongs to the field of alloy material forging technology, specifically relating to a duplex stainless steel and its manufacturing method. Background Technology

[0002] Duplex stainless steel is a type of stainless steel that contains both ferrite and austenite structures in its solid solution structure. Therefore, it has the dual characteristics of austenitic and ferritic stainless steel. Compared with ferritic stainless steel, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved resistance to intergranular corrosion and weldability. It also has superplasticity. Compared with austenitic stainless steel, it has higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. It is widely used in deep-sea applications.

[0003] However, when duplex stainless steel forgings are performed on a press, the slow press stroke and long forging process lead to cracking, especially at the end faces. Therefore, optimizing and improving the manufacturing method of duplex stainless steel to reduce cracking during forging is a pressing technical challenge in this field. Summary of the Invention

[0004] Therefore, the present invention aims to provide a duplex stainless steel and a method for manufacturing the same.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] According to an embodiment of the present invention, in a first aspect, the present invention provides a method for manufacturing duplex stainless steel, comprising the following steps:

[0007] Upsetting and drawing forging are performed on duplex stainless steel.

[0008] The upsetting ratio of the upsetting forging is 1.5 to 1.7, the upsetting forging includes several single-fire upsetting forgings, and the deformation of the single-fire upsetting forging is 20% to 30%.

[0009] The drawing ratio of the drawing forging is 1.5 to 1.7, and the drawing forging includes several single-fire drawing forgings, with a deformation of 10 to 20% in each single-fire drawing forging. In this invention, the order of the upsetting and drawing forging steps for duplex stainless steel is not required.

[0010] When the height of the duplex stainless steel is not less than its diameter or length, the manufacturing process includes the following steps: sequentially upsetting and drawing forging the duplex stainless steel.

[0011] When the height of the duplex stainless steel is less than its diameter or length, the manufacturing process includes the following steps: sequentially performing elongation forging and upsetting forging on the duplex stainless steel;

[0012] The relationship between height and diameter or length is determined by the cross-sectional shape of duplex stainless steel. When duplex stainless steel is cylindrical, the cross-sectional shape is circular, and the order of upsetting and drawing forging is determined by the relationship between the height of the duplex stainless steel billet and its cross-sectional diameter. When duplex stainless steel is in other shapes, such as cubes or cuboids, the cross-sectional shape is rectangular or square, and the order of upsetting and drawing forging is determined by the relationship between the height of the duplex stainless steel billet and its cross-sectional length. Length refers to the longest side of the cross-section.

[0013] According to an embodiment of the present invention, in a single upsetting forging and drawing forging process, the upsetting forging may include several single-fire upsetting forgings until the upsetting ratio is satisfied; the drawing forging may include several single-fire drawing forgings until the drawing ratio is satisfied; in a single upsetting forging and drawing forging process, the deformation amount of the single-fire upsetting forging is consistent; the deformation amount of the single-fire drawing forging is consistent.

[0014] When upsetting requires several single-fire upsetting forgings, in actual production, during the last single-fire upsetting forging, once the upsetting ratio for upsetting is met, the drawing forging begins after the upsetting forging is completed. That is, the last single-fire upsetting forging does not need to be fully completed. Drawing forging is similar.

[0015] According to an embodiment of the present invention, a cooling step is also included after forging.

[0016] According to an embodiment of the present invention, the cooling method includes air cooling or water cooling.

[0017] According to an embodiment of the present invention, if the effective heat treatment wall thickness of the forging is ≤250mm, the product is subjected to air cooling treatment, which does not affect the internal microstructure of the forging. Therefore, the air cooling post-forging cooling method can be directly adopted.

[0018] According to an embodiment of the present invention, if the effective heat treatment wall thickness of the forging is >250mm, water cooling treatment is performed. If water cooling is not used, the internal cooling rate of the thick-walled product is slow. When the internal temperature of the product is at 850-900℃ for a long time, chromite is easily precipitated, which will cause crack defects inside the product and result in the product failing ultrasonic testing (UT).

[0019] According to an embodiment of the present invention, if the mass of duplex stainless steel is ≥2t, upsetting forging and drawing forging are performed at least twice, i.e., the upsetting forging and drawing forging steps are repeated. Single F51 duplex stainless steel products weighing 2t or more are mainly produced using steel ingots as raw materials, which can significantly remove defects such as porosity and shrinkage cavities existing inside the steel ingots and ensure the quality of the ingot billet end face, reducing the probability of crack formation. According to an embodiment of the present invention, if the mass of duplex stainless steel is <2t, the upsetting forging and drawing forging are performed at least once. Single F51 duplex stainless steel products weighing less than 2t are mainly produced using forged billets, produced using a non-fast forging press, which can significantly improve the production efficiency of small-scale products.

[0020] Furthermore, when the manufacturing method includes several upsetting and drawing forging steps, regardless of whether the billet of duplex steel is a steel ingot or a steel billet, based on the present invention, when the billet meets the following condition: 2 ≤ height-to-diameter ratio ≤ 3, one drawing process can be omitted, or the drawing process can be continued, but other requirements of the present invention still need to be met. For example, when the mass of duplex stainless steel is ≥ 2t, upsetting and drawing forging must still be performed at least twice; when the mass of duplex stainless steel is < 2t, upsetting and drawing forging must still be performed at least once. For ease of understanding, an example is given here. When the mass of duplex stainless steel is ≥2t, in a certain specific manufacturing method, upsetting forging and drawing forging need to be performed 3 times. When the height-to-diameter ratio of the duplex stainless steel meets 2-3, one drawing forging can be reduced. That is, after upsetting forging and drawing forging 2 times, drawing forging can be performed after the third upsetting forging, or drawing forging can be omitted. When the height-to-diameter ratio of the duplex stainless steel does not meet 2-3, the process of this invention shall be followed.

[0021] According to an embodiment of the present invention, during upsetting and drawing forging of the duplex stainless steel, the surface temperature of the billet is not lower than 960°C. If the surface temperature of the stainless steel is lower than 960°C, it needs to be reheated to 1150-1250°C at a rate of 60-90°C / h. The holding time is: effective heat-treated wall thickness × 0.45 < T < effective heat-treated wall thickness × 0.45 + 120, with the holding time in minutes. Subsequent processes are then performed. The effective heat-treated wall thickness is calculated based on the billet before each reheating of the forging product. The shape of the billet changes before each reheating, resulting in a change in the effective heat-treated wall thickness. This invention simplifies the billet shape to a round bar shape, i.e., the billet height ≥ cross-sectional diameter, then the effective heat-treated wall thickness is the radius of the duplex stainless steel billet; if the billet height < cross-sectional diameter, then the effective heat-treated wall thickness is the billet height.

[0022] According to an embodiment of the present invention, after the duplex stainless steel is subjected to single-fire upsetting forging, the outer diameter of the duplex stainless steel is further subjected to rolling treatment to prevent the outer diameter of the billet from cracking during upsetting.

[0023] According to an embodiment of the present invention, after the duplex stainless steel is drawn and forged, a step of flattening the end face is included to prevent the end face of the billet from cracking when it is drawn out of the furnace.

[0024] According to an embodiment of the present invention, a pretreatment step is further included before forging.

[0025] According to an embodiment of the present invention, when the duplex stainless steel is a forging billet, the pretreatment step includes sawing; when forging after normal heating, the outer end face is basically free of defects such as porosity and shrinkage cavities, and can be directly sawed and then heated and forged without hot cutting.

[0026] According to an embodiment of the present invention, when duplex stainless steel is ingot, the entire ingot is used as raw material for production of a single product, and the sprue cannot be sawn off. This is because there are many defects inside the ingot, and their locations are difficult to determine. Only by upsetting and drawing with the riser attached, and then hot-cutting the riser, can the quality of the riser end face of the ingot body be guaranteed. Furthermore, hot-cutting the riser can increase the weight of the ingot body and improve the utilization rate of the ingot. In addition, the sprue end is generally not directly cut off. It is only necessary to grind the small boss before heating in the furnace. In the punching process after multiple upsetting and drawing, the part with a small number of defects in the center of the sprue end can be punched out as the core material.

[0027] According to an embodiment of the present invention, when the duplex stainless steel is ingot, the pretreatment step includes heating the ingot to 1200-1250°C, holding it at that temperature for 3-7 hours, performing upsetting forging and drawing forging, and then removing the riser. The deformation amount of upsetting forging is 20-30%, and the deformation amount of drawing forging is 10-20%, followed by removal of the riser. Preferably, the heating rate is 60-90°C / h.

[0028] According to an embodiment of the present invention, a heating step is also included before forging.

[0029] According to an embodiment of the present invention, the duplex stainless steel is a forged billet, the heating temperature is 1150~1200℃, the heating rate is 60~90℃ / h, and the holding time is: effective heat-treated wall thickness × 0.45 < T < effective heat-treated wall thickness × 0.45 + 120, with the holding time in minutes.

[0030] According to an embodiment of the present invention, if the duplex stainless steel is a steel ingot, the heating temperature is 1200-1250℃, the heating rate is 60-90℃ / h, and the holding time is: effective heat-treated wall thickness × 0.45 < T < effective heat-treated wall thickness × 0.45 + 120, and the holding time is in minutes.

[0031] Furthermore, when the duplex stainless steel is in the form of an ingot, the manufacturing method includes the following steps: pretreatment of the duplex stainless steel, heating, upsetting and drawing forging, cooling, etc.

[0032] When duplex stainless steel is in the form of a forging billet, the manufacturing method includes the following steps: heating the duplex stainless steel, upsetting and drawing forging, and cooling.

[0033] The order of upsetting forging and drawing forging is determined by the relationship between height and diameter or length.

[0034] Secondly, the present invention provides a duplex stainless steel manufactured by the above-described manufacturing method.

[0035] The single-fire upsetting and drawing method used in this invention is mainly suitable for ordinary forging presses with a slow anvil pressing speed. Since the plasticity of F51 duplex stainless steel is not as good as that of carbon steel and ordinary alloy steel, if it is forged with an ordinary forging press, when the billet temperature drops to 950°C, continuing to forge will cause cracks on the product surface to extend to the core of the forging, causing the forging to crack completely. It will also cause the forging press to be unable to press, thus affecting subsequent processes.

[0036] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0037] 1. The present invention provides a method for manufacturing duplex stainless steel, which involves upsetting and drawing forging of the duplex stainless steel; wherein the upsetting ratio of the upsetting forging is 1.5 to 1.7, the upsetting forging includes several single-fire upsetting forgings, and the deformation amount of the single-fire upsetting forging is 20 to 30%; the drawing ratio of the drawing forging is 1.5 to 1.7, the drawing forging includes several single-fire drawing forgings, and the deformation amount of the single-fire drawing forging is 10 to 20%. The present invention effectively reduces the degree of cracking of duplex stainless steel during the forging process while ensuring the physical and chemical properties of the forging.

[0038] Controlling the upsetting ratio in upset forging and the drawing ratio in drawing forging to be greater than 1.5 allows sufficient movement of atoms within the metal to reshape the grains and ensures the flaw detection requirements of the finished forgings; controlling the upsetting ratio and drawing ratio to be less than 1.7 ensures the efficiency of the entire process.

[0039] If the single-fire upsetting deformation and single-fire drawing deformation are too small while ensuring the same forging ratio, the number of forging operations will increase indefinitely, thus affecting production efficiency; if the single-fire upsetting deformation is too large, cracks will appear on the outer circle; if the single-fire drawing deformation is too large, cracks will appear on the end face.

[0040] 2. The method for manufacturing duplex stainless steel provided by this invention is applicable to... Forging of a series of F51 duplex steels. Attached Figure Description

[0041] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0042] Figure 1 This is a schematic diagram of the crack defects in the forging prepared in Embodiment 1 of the present invention.

[0043] Figure 2 This is a schematic diagram of the crack defects in the forging prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0044] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0045] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0046] Upsetting ratio = height before upsetting in a single process / height after upsetting in a single process; or, upsetting ratio = cross-sectional area after upsetting in a single process / cross-sectional area before upsetting in a single process.

[0047] Lengthening ratio = Height after lengthening in a single process / Height before lengthening in a single process; or, Lengthening ratio = Cross-sectional area before lengthening in a single process / Cross-sectional area after lengthening in a single process.

[0048] The density of the duplex stainless steel in this invention is 7.85 g / mm³. 3 .

[0049] Example 1

[0050] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0051] (1) Blanking process: F51 duplex stainless steel forging billet (the cross-sectional dimensions of the forging billet are as follows) The calculated height is 460mm and the height-to-diameter ratio (height / diameter) is 1.10. The sawn blank weighs 0.5t and has an effective heat-treated wall thickness of 210mm.

[0052] (2) Pre-forging heating: The billet is placed in a forging heating furnace and heated to 1175℃ at a rate of 75℃ / h, and held for 2 hours. The cross-sectional dimensions of the forged billet are as follows:

[0053] (3) Forging: The billet heated in step (2) is transferred to a press for forging. Two upsetting forgings are performed with an upsetting ratio of 1.6 and a deformation of 25% per forging, resulting in a forging with an effective heat-treated wall thickness of 287.5 mm. Then, four drawing forgings are performed with a drawing ratio of 1.6 and a deformation of 15% per forging, until a forging with an effective heat-treated wall thickness of 210 mm is obtained. After each upsetting, the outer diameter of the billet needs to be rolled to prevent cracking of the outer diameter of the billet during the next upsetting. After each drawing, the end face needs to be flattened to prevent cracking of the end face of the billet during the next drawing. When performing the last single-fire upsetting forging, the upsetting ratio is met, and the upsetting forging ends, the drawing forging begins. That is, the last single-fire upsetting forging does not need to be fully performed. The drawing forging is similar. During upsetting and drawing forging, if the surface temperature of stainless steel is below 960℃, it is reheated to 1175℃ at a rate of 60℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated from the billet before each reheating and holding process, before proceeding with subsequent steps.

[0054] (4) Cooling after forging: The forgings from step (3) are air-cooled.

[0055] Example 2

[0056] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0057] (1) Blanking process: The F51 duplex stainless steel forging billet (the forging billet section specification height is 450mm, the calculated diameter is 600mm, and the height-to-diameter ratio (height / diameter) is 0.75) is sawn to obtain a billet weighing 1t with an effective heat-treated wall thickness of 450mm.

[0058] (2) Pre-forging heating: The billet is placed in a forging heating furnace and heated to 1150℃ at 70℃ / h and held for 4.5h. The cross-sectional specification of the forging billet is 450mm.

[0059] (3) Forging: The billet heated in step (2) is transferred to a press for forging. It undergoes three-stage drawing forging with a drawing ratio of 1.5 and a single-stage deformation of 20%, resulting in a forging with an effective heat-treated wall thickness of 245mm. Then, it undergoes two-stage upsetting forging with an upsetting ratio of 1.5 and a single-stage deformation of 20%, resulting in a forging with an effective heat-treated wall thickness of 450mm. After each upsetting stage, the outer diameter of the billet needs to be rolled to prevent cracking of the outer diameter during the next upsetting stage. After each drawing stage, the end face needs to be flattened to prevent cracking of the end face during the next drawing stage. The raw materials are prepared according to... The specifications are calculated to be a round disc with a height of 450mm, so it must be drawn first and then upset. During the last single-fire drawing forging, once the drawing ratio is met, the upsetting forging begins after the drawing forging is completed. That is, the last single-fire drawing forging does not need to be fully completed. Upsetting forging is similar. During upsetting and drawing forging, if the stainless steel surface temperature is below 960℃, it is reheated to 1150℃ at a rate of 65℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated for the billet before each reheating and holding process, before proceeding to subsequent steps.

[0060] (4) Cooling after forging: The forgings from step (3) are subjected to water cooling treatment.

[0061] Example 3

[0062] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0063] (1) Blanking process: F51 duplex stainless steel forging billet (forging billet specifications are as follows) The calculated height is 676mm and the height-to-diameter ratio (height / diameter) is 1.13. The sawn blank weighs 1.5t and has an effective heat-treated wall thickness of 300mm.

[0064] (2) Pre-forging heating: The billet is placed in a forging heating furnace and heated to 1200℃ at a rate of 80℃ / h, and held for 4 hours. The cross-sectional dimensions of the forging billet are as follows:

[0065] (3) Forging: The billet heated in step (2) is transferred to a press for forging. Two upsetting forgings are performed with an upsetting ratio of 1.7 and a deformation of 30% per forging, resulting in a forging with an effective heat-treated wall thickness of 398mm. Then, five drawing forgings are performed with a drawing ratio of 1.5 and a deformation of 10% per forging, resulting in a forging with an effective heat-treated wall thickness of 596mm. After each upsetting, the outer diameter of the billet needs to be rolled to prevent cracking of the outer diameter of the billet during the next upsetting. After each drawing, the end face needs to be flattened to prevent cracking of the end face of the billet during the next drawing. When performing the last single-fire upsetting forging, the upsetting ratio is met, and the upsetting forging ends, the drawing forging begins. That is, the last single-fire upsetting forging does not need to be fully performed. The drawing forging is similar. During upsetting and drawing forging, if the surface temperature of stainless steel is below 960℃, it is reheated to 1200℃ at a rate of 70℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated from the billet before each reheating and holding process, before proceeding with subsequent steps.

[0066] (4) Cooling after forging: The forgings from step (3) are subjected to water cooling treatment.

[0067] Example 4

[0068] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0069] (1) Blanking process: The F51 duplex stainless steel ingot with a total weight of 2.2t and an effective heat-treated wall thickness of 440mm (the ingot has a cross-sectional specification of 440mm in length × 440mm in width, a calculated height of 1447mm, and a height-to-diameter ratio (height / length) of 3.29) is placed in the forging heating furnace and heated to 1250℃ at 65℃ / h and held for 4.5h before being taken out of the furnace for upsetting forging. The deformation of upsetting forging is 25%. Then, elongation forging is carried out, and the deformation of elongation forging is 15%. When hot cutting the riser, it is necessary to start hot cutting 40mm inward along the end face of the riser. This can ensure that the riser is hot cut cleanly, and the end face of the riser after cutting must be flat. The sprue cannot be sawn off, and the pre-treated duplex stainless steel billet is obtained.

[0070] (2) Pre-forging heating: The pretreated billet is placed in a forging furnace at 1225℃ and held for 3 hours. The cross-sectional specifications of the duplex stainless steel ingot are as follows:

[0071] (3) Forging: The billet heated in step (2) is transferred to the press for a first upsetting forging and a first drawing forging. The upsetting ratio of the billet is 1.6, including 2 upsetting passes, and the upsetting deformation per pass is 25%; the drawing ratio is 1.6, including 4 upsetting passes, and the drawing deformation per pass is 15%; a forging with an effective heat-treated wall thickness of 259mm is obtained.

[0072] Then, a second upsetting forging and a second drawing forging are performed. The upsetting ratio is 1.6, including 2 heats, with a single heat upsetting deformation of 25%; the drawing ratio is 1.6, including 4 heats, with a single heat drawing deformation of 15%; a forging with an effective heat-treated wall thickness of 259mm is obtained.

[0073] After each upsetting process, the outer diameter of the billet must be rolled to prevent cracking of the outer diameter during the next upsetting process. After each drawing process, the end face must be flattened to prevent cracking of the end face of the billet during the next drawing process. During upsetting and drawing forging, if the surface temperature of the stainless steel is below 960℃, it is reheated to 1225℃ at a rate of 75℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated from the billet before each reheating. Then, subsequent processes are carried out.

[0074] During the final single-fire forging of each upsetting forging process, once the upsetting ratio for upsetting is met, the upsetting forging ends and the drawing forging begins. In other words, the final single-fire upsetting forging does not need to be fully completed. The drawing forging process is similar.

[0075] (4) Cooling after forging: The forgings from step (3) are subjected to water cooling treatment.

[0076] Example 5

[0077] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0078] (1) Blanking process: The F51 duplex stainless steel ingot with a total weight of 3.5t and an effective heat-treated wall thickness of 550mm (the cross-sectional specifications of the square ingot are 550mm long × 550mm wide, the calculated height is 1474mm, and the height-to-diameter ratio (height / length) is 2.63) is placed in the forging heating furnace and heated to 1250℃ at 60℃ / h and held for 5h before being taken out of the furnace for upsetting forging. The deformation amount of upsetting forging is 25%. Then, elongation forging is carried out, and the deformation amount of elongation forging is 15%. When hot cutting the riser, it is necessary to start hot cutting 40mm inward along the end face of the riser. This can ensure that the riser is hot cut cleanly, and the end face of the riser after cutting must be flat. The sprue cannot be sawn off to obtain the pre-treated duplex stainless steel billet.

[0079] (2) Furnace holding: The pretreated billet is placed in a forging furnace at 1200℃ and held for 4 hours. The cross-sectional specifications of the duplex stainless steel ingot are as follows:

[0080] (3) Forging: The billet heated in step (2) is transferred to the press for a first upsetting forging and a first drawing forging. The upsetting ratio of the billet is 1.5, including 2 upsettings, with a single upsetting deformation of 20%; the drawing ratio is 1.7, including 3 upsettings, with a single drawing deformation of 20%; a forging with an effective heat-treated wall thickness of 304mm is obtained.

[0081] Subsequently, a second upsetting forging and a second drawing forging were carried out. The upsetting ratio was 1.7, including 2 heats, with a single heat upsetting deformation of 30%; the drawing ratio was 1.6, including 4 heats, with a single heat drawing deformation of 15%; a forging with an effective heat-treated wall thickness of 313 mm was obtained.

[0082] The forging is then subjected to three upsetting forgings and three drawing forgings. The upsetting ratio is 1.6, including two upsetting cycles, with a single upsetting deformation of 25%. The drawing ratio is 1.5, including four upsetting cycles, with a single drawing deformation of 13%. A forging with an effective heat-treated wall thickness of 323.5 mm is obtained.

[0083] After each upsetting process, the outer diameter of the billet must be rolled to prevent cracking of the outer diameter during the next upsetting process. After each drawing process, the end face must be flattened to prevent cracking of the end face of the billet during the next drawing process. During upsetting and drawing forging, if the surface temperature of the stainless steel is below 960℃, it is reheated to 1200℃ at 80℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated from the billet before each reheating. Then, subsequent processes are carried out.

[0084] During the final single-fire forging of each upsetting forging process, once the upsetting ratio for upsetting is met, the upsetting forging ends and the drawing forging begins. In other words, the final single-fire upsetting forging does not need to be fully completed. The drawing forging process is similar.

[0085] (4) Cooling after forging: The forgings from step (3) are subjected to water cooling treatment.

[0086] Example 6

[0087] The method for manufacturing duplex stainless steel provided in this embodiment includes the following steps:

[0088] (1) Blanking process: F51 duplex stainless steel ingots weighing 5t and with an effective heat-treated wall thickness of 600mm (the ingot cross-section is 600mm long × 600mm wide, with a calculated height of 1769mm and a height-to-diameter ratio (height / length) of 2.95) are placed in a forging heating furnace and heated to 1250℃ at 85℃ / h and held for 5h before being taken out of the furnace for upsetting forging. The deformation of upsetting forging is 25%. Then, elongation forging is carried out, with a deformation of 15%. When hot-cutting the riser, it is necessary to start hot-cutting 40mm inward along the end face of the riser. This can ensure that the riser is cleanly hot-cut and the end face of the riser after cutting must be flat. The sprue cannot be sawn off to obtain pre-treated duplex stainless steel billet.

[0089] (2) Furnace holding: The pretreated billet is placed in a forging furnace at 1250℃ and held for 4 hours. The cross-sectional specifications of the duplex stainless steel ingot are as follows:

[0090] (3) Forging: The billet heated in step (2) is transferred to the press for a first upsetting forging and a first drawing forging. The upsetting ratio of the billet is 1.7, including 2 upsettings, with a single upsetting deformation of 30%; the drawing ratio is 1.5, including 3 upsettings, with a single drawing deformation of 20%, to obtain a forging with an effective heat-treated wall thickness of 376mm.

[0091] Subsequently, a second upsetting forging and a second drawing forging were carried out. The upsetting ratio was 1.5, including 2 heats, with a single heat upsetting deformation of 23%; the drawing ratio was 1.7, including 4 heats, with a single heat drawing deformation of 18%, resulting in a forging with an effective heat-treated wall thickness of 353mm.

[0092] The forging process is then subjected to three more upsetting forgings with an upsetting ratio of 1.6, including two upsetting cycles, with a single upsetting deformation of 27%; resulting in a forging with an effective heat-treated wall thickness of 447 mm.

[0093] After each upsetting process, the outer diameter of the billet must be rolled to prevent cracking of the outer diameter of the billet during the next upsetting process. After each drawing process, the end face must be flattened to prevent cracking of the end face of the billet during the next drawing process. During upsetting and drawing forging, if the surface temperature of the stainless steel is lower than 960℃, it is reheated to 1250℃ at a rate of 90℃ / h. The holding time is calculated based on the effective heat-treated wall thickness, which is calculated from the billet before each reheating. Then, subsequent processes are carried out.

[0094] During the final single-fire forging of each upsetting forging process, once the upsetting ratio for upsetting is met, the upsetting forging ends and the drawing forging begins. In other words, the final single-fire upsetting forging does not need to be fully completed. The drawing forging process is similar.

[0095] (4) Cooling after forging: The forgings from step (3) are subjected to water cooling treatment.

[0096] Comparative Example 1

[0097] The manufacturing method of duplex stainless steel provided in this comparative example is basically the same as the steps in Example 1, except that the upsetting ratio is 3.

[0098] Comparative Example 2

[0099] The manufacturing method of duplex stainless steel provided in this comparative example is basically the same as the steps in Example 1, except that the drawing ratio is 2.5.

[0100] Comparative Example 3

[0101] The manufacturing method of duplex stainless steel provided in this comparative example is basically the same as the steps in Example 1, except that the deformation amount of the single-pass upsetting forging is 35%.

[0102] Comparative Example 4

[0103] The manufacturing method of duplex stainless steel provided in this comparative example is basically the same as the steps in Example 1, except that the single-fire deformation amount of the drawing forging is 30%.

[0104] Experimental Example

[0105] The duplex stainless steels manufactured in the above embodiments and comparative examples were subjected to cracking tests, and the results are shown in Table 1.

[0106] Table 1. Performance test results of duplex stainless steel prepared in each embodiment and comparative example.

[0107] Cracking Example 1 none Example 2 none Example 3 none Example 4 none Example 5 none Example 6 none Comparative Example 1 One longitudinal crack on the outer circle Comparative Example 2 Two places in the middle of the end face Comparative Example 3 There are two longitudinal cracks on the outer circle. Comparative Example 4 Two cracks in the core of the end face

[0108] Figure 1 and Figure 2The figures shown are schematic diagrams of forging crack defects in Examples 1 and 1 (Comparative Example 1). As can be seen from Table 1, the forgings manufactured in Examples 1 through 6 were free of cracks. The forging manufactured in Comparative Example 1 showed one longitudinal crack on the outer circumference, caused by an excessively high upsetting ratio in a single process. The upsetting ratio in a single process needs to be controlled to be no greater than 2, but considering forging efficiency, it should be controlled to be no greater than 1.7. The forging manufactured in Comparative Example 2 showed two cracks in the middle of the end face, caused by an excessively high drawing ratio in a single process. The drawing ratio in a single process needs to be controlled to be no greater than 2, but considering forging efficiency, it should be controlled to be no greater than 1.7. The forging manufactured in Comparative Example 3 showed two longitudinal cracks on the outer circumference, caused by excessive upsetting in a single heat. The forging manufactured in Comparative Example 4 showed two cracks on the end face, caused by excessive drawing in a single heat, leading to core cracking. This demonstrates that the forging method of the present invention for F51 material products can effectively reduce forging cracking.

[0109] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for manufacturing duplex stainless steel, characterized in that, Includes the following steps: Upsetting and drawing forging are performed on duplex stainless steel. The upsetting ratio of the upsetting forging is 1.5 to 1.7, the upsetting forging includes several single-fire upsetting forgings, and the deformation amount of the single-fire upsetting forging is 20% to 30%. The drawing ratio of the drawing forging is 1.5 to 1.7, and the drawing forging includes several single-fire drawing forgings, with a deformation amount of 10 to 20% in each single-fire drawing forging. When upsetting and drawing the duplex stainless steel, the surface temperature of the billet shall not be lower than 960°C. When the surface temperature of the billet is below 960℃, it is reheated to 1150-1250℃ at a rate of 60-90℃ / h and then held at that temperature. The heat treatment time is calculated as effective heat treatment wall thickness × 0.45 < T < effective heat treatment wall thickness × 0.45 + 120, and the heat treatment time is in minutes. Before forging, a heating step is also included; The duplex stainless steel is a forged billet, and the heating temperature is 1150–1200℃; or… The duplex stainless steel is a steel ingot, and the heating temperature is 1200-1250℃.

2. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, When the height of the duplex stainless steel is not less than its diameter or length, the manufacturing process includes the following steps: sequentially upsetting and drawing forging the duplex stainless steel. When the height of the duplex stainless steel is less than its diameter or length, the manufacturing process includes the following steps: sequentially performing elongation forging and upsetting forging on the duplex stainless steel.

3. The method for manufacturing duplex stainless steel according to claim 1 or 2, characterized in that, The forging process also includes a cooling step.

4. The method for manufacturing duplex stainless steel according to claim 3, characterized in that, The cooling methods include air cooling or water cooling.

5. The method for manufacturing duplex stainless steel according to claim 4, characterized in that, If the effective heat-treated wall thickness of the forging is ≤250mm, then air cooling treatment should be performed; If the effective heat-treated wall thickness of the forging is greater than 250 mm, then water cooling treatment should be performed.

6. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, If the mass of duplex stainless steel is <2t, the upsetting forging and drawing forging shall be performed at least once; or, If the mass of duplex stainless steel is ≥2t, the upsetting forging and drawing forging shall be performed at least twice.

7. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, The process includes single-fire upsetting of the duplex stainless steel followed by rolling of the outer diameter of the duplex stainless steel.

8. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, The process of single-fire drawing forging of the duplex stainless steel also includes a step of flattening the end face.

9. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, The forging process also includes a pretreatment step.

10. The method for manufacturing duplex stainless steel according to claim 9, characterized in that, When the duplex stainless steel is a forged billet, the pretreatment step includes sawing the billet; When duplex stainless steel is ingot, the pretreatment steps include heating the ingot to 1200-1250°C, holding it at that temperature for 3-7 hours, performing upsetting forging and drawing forging, and then removing the riser.

11. The method for manufacturing duplex stainless steel according to claim 10, characterized in that, The temperature is increased at a rate of 60–90 °C / h.

12. The method for manufacturing duplex stainless steel according to claim 1, characterized in that, The heating rate is 60–90 °C / h.

13. A duplex stainless steel manufactured by the manufacturing method according to any one of claims 1 to 12.

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