A short process preparation technology of high-strength corrosion-resistant duplex stainless steel flange

Abstract

The application discloses a kind of highly creative short process preparation process of high-strength corrosion-resistant duplex stainless steel flange.It breaks through the double technical prejudice of "casting state must be repeatedly forged" and "850-950 DEG C is the forbidden zone of heat treatment", initiates the short process route of centrifugal casting blank-near-final shape ring piece casting-rolling integration, and initiates the abnormal active isothermal controlled cooling treatment in the "forbidden zone" temperature zone.By accurately coupling deformation and phase change, the disadvantages are turned into advantages, and the optimal organization with no harmful phase, ideal two-phase ratio and gradient distribution of elements is obtained under the extremely simple process.The yield strength of the obtained flange is greater than or equal to 550 MPa, the tensile strength is greater than or equal to 750 MPa, the elongation is greater than or equal to 25%, the critical pitting temperature is greater than or equal to 50 DEG C, and the performance is superior to that of traditional long process, and unexpected technical effects are obtained.

Description

Technical Field

[0001] This invention relates to the field of metal forming and heat treatment technology, specifically to a flange manufacturing process, and more particularly to a short-process manufacturing process for high-strength, corrosion-resistant duplex stainless steel flanges that combines casting and rolling with gradient controlled cooling. Background Technology

[0002] Duplex stainless steel flanges hold an irreplaceable position in harsh environments such as marine engineering and petrochemicals due to their excellent mechanical properties and resistance to chloride ion corrosion. The 2205 type duplex stainless steel, represented by UNS S32205, ideally has a microstructure of approximately 50% ferrite and 50% austenite, which requires precise hot working and heat treatment processes to achieve.

[0003] Currently, the mainstream manufacturing process for this type of flange is still the traditional long process of "ingot casting → forging → multi-pass hot forging → solution treatment". However, this process has the following recognized technical challenges and cognitive biases:

[0004] Strong process dependence: It is generally believed in the industry that the as-cast structure is coarse and has serious segregation. It is necessary to break the as-cast structure through at least 3-5 rounds of upsetting and drawing forging in order to achieve uniform structure. This understanding has led to the current situation of long process and extremely high energy consumption.

[0005] Solution treatment for solidification: To achieve ideal two-phase equilibrium, the conventional practice for those skilled in the art is to perform high-temperature solution treatment at 1050-1120℃ followed by rapid water quenching. This method is based on the thermodynamic principle that "elements diffuse rapidly at high temperatures to reach equilibrium, and rapid cooling can fix this high-temperature equilibrium structure." However, this method is prone to the precipitation of harmful phases such as σ and χ during the cooling process due to insufficient cooling rate, leading to a sharp deterioration in toughness and corrosion resistance.

[0006] Technological Bias: For a long time, there has been a technological bias that considers any processing within the 850-950℃ temperature range, a sensitive temperature range for harmful phase precipitation, to be a process no-go zone. Therefore, all traditional processes emphasize extremely rapid cooling within this range, and this mindset has limited the exploration of new process routes.

[0007] Cracking problem: 2205 duplex stainless steel has a narrow hot working window. Once it enters the two-phase region (where the strength difference between ferrite and austenite is large) during the forging temperature drop process, it is very easy to crack, resulting in a low yield.

[0008] The aforementioned technical challenges and cognitive biases constitute insurmountable technical obstacles for those skilled in the art when facing the goal of "shortening processes and improving performance." This invention is precisely based on overcoming these biases and obstacles. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing technologies and long-standing cognitive biases, and to provide an unconventional, short-process manufacturing process for high-strength, corrosion-resistant duplex stainless steel flanges. This process boldly adopts near-net-shape casting and rolling integration technology, completely overturning the traditional route that requires repeated forging, and innovatively introduces active isothermal treatment in the "process forbidden zone" (850-950℃), turning "avoiding harm" into "seeking benefit," and obtaining flanges with comprehensive performance far exceeding that of traditional processes with extremely simplified procedures.

[0010] A short-process manufacturing procedure for high-strength, corrosion-resistant duplex stainless steel flanges includes the following steps:

[0011] S1. Melting and centrifugal casting: After melting the raw materials weighed according to the target composition, centrifugal casting is performed to obtain a near-net-shape ring billet;

[0012] S2. High-temperature demolding and homogenization: Demold the casting at a core temperature of 1250-1320℃ and homogenize at 1250-1280℃ for 30-60 minutes.

[0013] S3. Ring casting and rolling: The ring forging is formed by casting and rolling at an initial rolling temperature of 1100-1180℃ and a final rolling temperature of 950-1020℃ with a total reduction rate of 20-50%.

[0014] S4. Unconventional multi-stage gradient controlled cooling: The rolled ring is subjected to the following steps in sequence: First stage, rapid cooling to 850-950℃ at a cooling rate of 5-15℃ / s; Second stage, isothermal holding for 10-30 minutes within the traditionally recognized sensitive temperature range for harmful phase precipitation of 850-950℃, in order to actively utilize this temperature range for beneficial redistribution of alloying elements, while preventing the precipitation of harmful phases; Third stage, slow cooling to room temperature at a rate of 1-5℃ / s.

[0015] S5. Finishing and Inspection.

[0016] Specifically, the composition of the duplex stainless steel in step S1, by mass percentage, is: C≤0.03%, Cr 22.0-23.0%, Ni 4.5-6.5%, Mo 3.0-3.5%, N 0.14-0.20%, Mn≤2.0%, Si≤1.0%, P≤0.03%, S≤0.02%, with the balance being Fe and unavoidable impurities.

[0017] Specifically, the centrifugal casting speed in step S1 is 300-800 rpm.

[0018] Specifically, in step S2, the core temperature of the demolded casting is 1280-1310℃, and the homogenization treatment is carried out at 1255-1270℃ for 40-50 minutes.

[0019] Specifically, the casting and rolling in step S3 is radial-axial ring rolling, with an initial rolling temperature of 1130-1170℃, a final rolling temperature of 960-1000℃, and a total reduction rate of 30-45%.

[0020] Specifically, in step S4, the rapid cooling in the first stage is achieved by a mixed cooling method of water mist and forced air cooling, with a cooling rate of 8-12℃ / s and a rapid cooling termination temperature of 880-920℃.

[0021] Specifically, in step S4, the temperature of the second stage of intentional isothermal maintenance is 890-910℃, and the maintenance time is 15-25 minutes. Precise control within this range allows alloying elements Cr and Mo to be enriched in the ferrite phase, while N and Ni are enriched in the austenite phase. Furthermore, no σ, χ, or Cr2N precipitates can be observed under a transmission electron microscope.

[0022] Specifically, the slow cooling rate in the third stage described in step S4 is 2-4℃ / s, and this slow cooling step helps to further relax the residual stress introduced by casting and rapid cooling without the precipitation of harmful phases.

[0023] Specifically, the finishing in step S5 includes machining the cooled annular forging to the final flange dimensions, and the inspection includes ultrasonic non-destructive testing, room temperature tensile testing, and critical pitting temperature testing according to ASTM G48 A method.

[0024] Specifically, its microstructure consists of 40-60% ferrite and 60-40% austenite by volume, with a clean interface between the two phases, and no harmful intermetallic compounds such as σ phase and χ phase precipitate throughout the grain, grain boundaries and phase boundaries.

[0025] The short-process manufacturing process for high-strength, corrosion-resistant duplex stainless steel flanges of the present invention has the following advantages compared with the prior art:

[0026] (1) A fundamental subversion of the technical route: The process of "ingot casting-multi-fire forging" which is considered absolutely necessary has been completely abandoned. The "centrifugally cast near-net-shape ring billet" is directly subjected to one or a few ring rolling passes, which solves the huge technical obstacle that "the as-cast structure cannot be directly used for key components without forging". The key technology lies in coupling high-temperature demolding with homogenization and precise casting and rolling thermodynamic parameters. This achieves a finer and more uniform structure in the as-cast structure than in the forged state, even with a deformation amount much lower than that of traditional forging. This is something that those skilled in the art could not have foreseen based on their existing knowledge.

[0027] (2) Direct overcoming of technical bias: It bravely ventured into the "forbidden zone of heat treatment" of 850-950℃ and pioneered the introduction of static isothermal treatment into it, transforming it from a "temperature zone for harmful phase precipitation" into a "functional temperature zone for tissue regulation and performance optimization". This reverse thinking method of "turning disadvantages into advantages" and the unexpected technical effects achieved make the inventiveness of this invention highly significant;

[0028] (3) Unexpected synergistic effect: "Casting and rolling" provides a fine-grained, high-density dislocation "activated state" structure for subsequent controlled cooling, while "forbidden zone isothermal" makes full use of the rapid diffusion channels of this "activated state" structure, realizing the redistribution of elements in a very short time that would take a long time to achieve in traditional processes. This synergistic strengthening effect, which is stimulated by the strong coupling of "deformation" and "phase transformation" under a special thermodynamic window, brings about a simultaneous and significant improvement in strength, plasticity, and corrosion resistance. This effect far exceeds the simple superposition of individual processes. Detailed Implementation

[0029] The specific embodiments of the present invention will be described in further detail below.

[0030] To further illustrate the present invention, the following detailed description is provided in conjunction with embodiments and comparative examples. The nominal composition of the raw materials used in the following embodiments and comparative examples is controlled within the same target range: C 0.02%, Cr 22.5%, Ni 5.5%, Mo 3.2%, N 0.18%, Mn 1.2%, Si 0.5%, with the balance being Fe.

[0031] Example 1

[0032] An unconventional, short-process preparation method, comprising the following steps:

[0033] S1. The raw materials according to the target composition ratio are melted in an induction furnace. When the superheat of the molten steel is controlled at 50°C, it is poured into a rotary mold with a rotation speed of 500 rpm for centrifugal casting to obtain a dense ring billet.

[0034] S2. When the temperature of the ring blank core drops to 1290℃, demold and quickly transfer it to a heating furnace at 1250℃ for 45 minutes.

[0035] S3. The homogenized ring billet is transferred to a radial-axial ring rolling mill and rolled at an initial rolling temperature of 1150℃, with a single-pass reduction of 35% and a final rolling temperature of 980℃, to form a flange ring of the target size.

[0036] S4. Immediately after rolling, the ring is subjected to unconventional multi-stage gradient controlled cooling: first, it is rapidly cooled to 900℃ at a gentle cooling rate of 10℃ / s; then, it is deliberately kept at 900℃ for 20 minutes in the sensitive area for the precipitation of harmful phases; finally, it is slowly cooled to room temperature at a cooling rate of 3℃ / s.

[0037] S5. After finishing to the final dimensions and passing ultrasonic flaw detection, samples are taken for testing.

[0038] Example 2

[0039] An unconventional, short-process preparation method, comprising the following steps:

[0040] S1. The raw materials according to the target composition ratio are melted in an induction furnace. When the superheat of the molten steel is controlled at 40°C, it is poured into a rotary mold with a rotation speed of 600 rpm for centrifugal casting to obtain a dense ring billet.

[0041] S2. When the temperature of the ring blank core drops to 1270℃, demold and quickly transfer it to a heating furnace at 1260℃ for 50 minutes.

[0042] S3. The homogenized ring billet is transferred to a radial-axial ring rolling mill and rolled at an initial rolling temperature of 1130℃, with a total reduction of 45% in two passes and a final rolling temperature of 960℃, to form a flange ring of the target size.

[0043] S4. Immediately after rolling, the ring is subjected to unconventional multi-stage gradient controlled cooling: first, it is rapidly cooled to 920℃ at a gentle cooling rate of 12℃ / s; then, it is deliberately held at 920℃ for 15 minutes in the sensitive area for the precipitation of harmful phases; finally, it is slowly cooled to room temperature at a cooling rate of 2℃ / s.

[0044] S5. After finishing to the final dimensions and passing ultrasonic flaw detection, samples are taken for testing.

[0045] Example 3

[0046] An unconventional, short-process preparation method, comprising the following steps:

[0047] S1. The raw materials according to the target composition ratio are melted in an induction furnace. When the superheat of the molten steel is controlled at 55°C, it is poured into a rotary mold with a rotation speed of 400 rpm for centrifugal casting to obtain a dense ring billet.

[0048] S2. When the temperature of the ring blank core drops to 1310℃, demold and quickly transfer it to a heating furnace at 1270℃ for 35 minutes.

[0049] S3. The homogenized ring billet is transferred to a radial-axial ring rolling mill and rolled at an initial rolling temperature of 1170℃, with a single-pass reduction of 25% and a final rolling temperature of 1010℃, to form a flange ring of the target size.

[0050] S4. Immediately after rolling, the ring is subjected to unconventional multi-stage gradient controlled cooling: first, it is rapidly cooled to 880°C at a gentle cooling rate of 8°C / s; then, it is deliberately held at 880°C for 28 minutes in the sensitive area for the precipitation of harmful phases; finally, it is slowly cooled to room temperature at a cooling rate of 4°C / s.

[0051] S5. After finishing to the final dimensions and passing ultrasonic flaw detection, samples are taken for testing.

[0052] Comparative Example 1 (representing conventional processes with existing technological biases)

[0053] Using ingots with the same composition as in the examples, the blanks were repeatedly upset and forged at 1200°C, with a final forging temperature of approximately 950°C, to form ring-shaped blanks. Subsequently, a solution treatment was performed at 1050°C for 2 hours according to conventional solution treatment methods in the art, followed by forced water quenching to room temperature, and finally finishing and sampling.

[0054] Comparative Example 2 (representing conventional thinking that follows process taboos)

[0055] The preparation steps are basically the same as in Example 1. The difference is that in order to avoid the harmful phase precipitation zone of 850-950℃, step S4 adopts a common fast cooling method in the field: the rolled ring is directly sprayed with water to cool to room temperature (average cooling rate of about 50℃ / s), without isothermal stage and slow cooling stage.

[0056] Comparative Example 3 (A comparative example demonstrating the criticality of the controlled cooling zone)

[0057] The preparation steps are basically the same as in Example 1, except that in step S4, the temperature of the second-stage isothermal treatment is set at 850°C, the recognized most sensitive temperature for σ-phase precipitation. All other conditions remain unchanged.

[0058] Test data and analysis

[0059] The flanges prepared in the above embodiments and comparative examples were subjected to microstructural observation, phase ratio determination (ferrite content), room temperature tensile testing, and critical pitting temperature (CPT) testing according to ASTM G48 A method. The results are shown in Table 1.

[0060] Data Analysis:

[0061]

[0062] Comparing Examples 1-3 with Comparative Example 1, it is evident that all embodiments of the present invention comprehensively surpass the traditional long-process technology in terms of strength, plasticity, and pitting resistance. This powerfully demonstrates that the short-process route of the present invention, which replaces forging with casting and rolling, not only does not reduce performance but also, due to the special gradient controlled cooling system, stimulates a synergistic strengthening effect of deformation-phase transformation, resulting in a finer and better microstructure and overcoming the technical bias of requiring repeated forging.

[0063] Comparing Examples 1-3 with Comparative Example 2, the data reveals a severe reversal between high plasticity and corrosion resistance. Comparative Example 2, representing the conventional "harm avoidance" approach (rapid cooling), resulted in a catastrophic imbalance in performance. In contrast, the embodiments of this invention, by taking the opposite approach and precisely insulating in the "forbidden zone," achieved a perfect balance of strength, plasticity, and corrosion resistance. This departure from conventional process methods yields a transformative and unexpected technical effect.

[0064] Comparing Example 1 with Comparative Example 3, the key comparison is the isothermal temperature. Example 1 achieved ideal results by holding at 900°C (within the preferred range of this invention), while Comparative Example 3, by simply lowering the holding temperature by 50°C to 850°C, where the σ-phase precipitation kinetics are fastest, resulted in the precipitation of a large amount of harmful phase and performance degradation. This indicates that the "forbidden zone isotherm" of this invention is not a simple temperature selection, but a high-precision process window discovered through in-depth research, defying common sense but undeniably existing, further highlighting its non-obviousness.

[0065] In summary, this invention possesses significant inventiveness through its disruptive technical approach (integrated casting and rolling), unconventional heat treatment thinking (active isothermal control in the sensitive range of harmful phases), and the unexpected comprehensive performance advantages it achieves.

[0066] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A short-process manufacturing process for high-strength, corrosion-resistant duplex stainless steel flanges, characterized in that, Includes the following steps: S1. Melting and centrifugal casting: After melting the raw materials weighed according to the target composition, centrifugal casting is performed to obtain a near-net-shape ring billet; S2. High-temperature demolding and homogenization: Demold the casting at a core temperature of 1250-1320℃ and homogenize at 1250-1280℃ for 30-60 minutes. S3. Ring casting and rolling: The ring forging is formed by casting and rolling at an initial rolling temperature of 1100-1180℃ and a final rolling temperature of 950-1020℃ with a total reduction rate of 20-50%. S4. Unconventional multi-stage gradient controlled cooling: The rolled ring is subjected to the following steps in sequence: First stage, rapid cooling to 850-950℃ at a cooling rate of 5-15℃ / s; Second stage, isothermal holding for 10-30 minutes within the traditionally recognized sensitive temperature range for harmful phase precipitation of 850-950℃, in order to actively utilize this temperature range for beneficial redistribution of alloying elements, while preventing the precipitation of harmful phases; Third stage, slow cooling to room temperature at a rate of 1-5℃ / s. S5. Finishing and Inspection.

2. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S1, the composition of the duplex stainless steel by mass percentage is: C≤0.03%, Cr 22.0-23.0%, Ni 4.5-6.5%, Mo 3.0-3.5%, N 0.14-0.20%, Mn≤2.0%, Si≤1.0%, P≤0.03%, S≤0.02%, with the balance being Fe and unavoidable impurities.

3. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S1, the centrifugal casting speed is 300-800 rpm.

4. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S2, the core temperature of the demolded casting is 1280-1310℃, and the homogenization treatment is carried out at 1255-1270℃ for 40-50 minutes.

5. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S3, the casting and rolling is radial-axial ring rolling, with an initial rolling temperature of 1130-1170℃, a final rolling temperature of 960-1000℃, and a total reduction rate of 30-45%.

6. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S4, the rapid cooling in the first stage is achieved by a mixed cooling method of water mist and forced air cooling, with a cooling rate of 8-12℃ / s and a rapid cooling termination temperature of 880-920℃.

7. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S4, the temperature at which the second stage is intentionally maintained isothermally is 890-910℃, and the maintenance time is 15-25 minutes.

8. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S4, the slow cooling rate in the third stage is 2-4℃ / s.

9. The short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to claim 1, characterized in that, In step S5, the finishing includes machining the cooled annular forging to the final flange dimensions, and the inspection includes ultrasonic non-destructive testing, room temperature tensile testing, and critical pitting temperature testing according to ASTM G48 A method.

10. A short-process manufacturing process for a high-strength, corrosion-resistant duplex stainless steel flange according to any one of claims 1 to 9, characterized in that, Its microstructure consists of 40-60% ferrite and 60-40% austenite by volume. The two-phase interface is clean, and there are no harmful intermetallic compounds such as σ phase and χ phase precipitated throughout the grain, grain boundaries and phase boundaries. At the same time, its yield strength is ≥550MPa, tensile strength is ≥750MPa, elongation is ≥25%, and critical pitting temperature is ≥50℃.

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