A dual-phase stainless steel plate for engineering equipment having excellent toughness and a method of manufacturing the same

Abstract

The present application relates to the technical field of special stainless steel plate rolling, and particularly relates to a dual-phase stainless steel plate for engineering equipment with excellent toughness and a manufacturing method thereof, wherein the chemical composition of the steel plate is as follows in terms of percentage by weight: C≤0.03%, Si≤0.80%, Mn≤1.50%, P≤0.03%, S≤0.02%, Cr: 24.0%-26.0%, Ni: 4.0%-6.0%, Mo: 2.5%-3.5%, Cu≤0.30%, Al: 0.01%-0.10%, N: 0.20%-0.30%, W: 0.5%-0.8%, Ce: 0.02%-0.04%, Nb: 0.2%-0.4%, and the balance of Fe and inevitable impurities. The dual-phase stainless steel plate for engineering equipment has excellent low-temperature toughness, high fatigue performance and high flatness. The yield strength of the steel plate is greater than or equal to 510 MPa, the tensile strength is greater than or equal to 650 MPa, the high-cycle fatigue limit stress at-20 DEG C is greater than or equal to 320 MPa, PREN is greater than or equal to 33, the low-temperature impact toughness in the transverse direction at 46 DEG C is greater than or equal to 47 J, the elongation is greater than or equal to 23%, and the flatness is less than 5 mm / 2 m.

Description

Technical Field

[0001] This invention relates to the field of special stainless steel plate rolling technology, and in particular to a duplex stainless steel medium-thick plate with excellent strength and toughness for engineering equipment and its manufacturing method. Background Technology

[0002] In just a few years, duplex stainless steel has swept through shipbuilding, marine engineering, and seawater desalination industries in powder form. Duplex stainless steel (DSS) is a type of stainless steel with approximately 50% ferrite and 50% austenitic phase, and the content of the lesser phase is generally at least 30%. This type of steel combines the characteristics of both austenitic and ferritic stainless steels, possessing approximately twice the yield strength of ordinary stainless steel. However, due to the influence of the service environment, such as floating production storage and offloading vessels, it needs to have excellent resistance to wind and waves, withstand extremely cold conditions, and have high resistance to seawater corrosion. Currently, the performance requirements of existing marine-grade 2205 duplex stainless steel cannot meet the following specifications (under extreme service conditions at 46℃, for medium-thick plates with a thickness of 6-20mm, transverse low-temperature impact toughness ≥47J, yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33 (PREN=%Cr + 3.3×%Mo + 16×%N)). The composition design, smelting, rolling, and heat treatment processes of duplex stainless steel all affect its strength and toughness. In actual manufacturing, it has been found that under special service conditions, the low-temperature impact toughness and strength of existing marine-grade 2205 duplex stainless steel cannot meet the standard requirements for this technical requirement. To solve these problems, domestic and foreign stainless steel manufacturers have optimized existing 2205 stainless steel production processes, such as optimizing the traditional 2205 stainless steel heat treatment process by increasing the solution temperature to promote the dissolution of the brittle phase and improve toughness. However, the improvement in strength indicators is limited, especially fatigue performance. Therefore, it is evident that a key issue that urgently needs to be addressed in the mass production of thick plates of duplex stainless steel for engineering equipment with excellent low-temperature toughness and high fatigue performance, using continuously cast billets with cross sections of 200mm and below to roll into thicknesses of 6-20mm, in order to continuously meet the major demands of downstream industries (such as shipbuilding).

[0003] Compared with existing technologies: To date, there has been very little research, both domestically and internationally, on the manufacture of medium-thick duplex stainless steel plates with a thickness of 6-20mm for engineering equipment, exhibiting excellent low-temperature toughness, high flatness, and high fatigue performance. Prior to this invention, patent application number 202310643964.7, "A duplex stainless steel for offshore floating production storage and offloading vessels and its preparation method," employed upsetting and forging of steel ingots to obtain tube blanks, followed by a graded heat treatment process. However, this method is not suitable for the production requirements of medium-thick plates, and the requirements for low-temperature toughness and fatigue performance of the steel plates are not clearly defined. The technical solution provided by this invention effectively overcomes these shortcomings, enabling the production of medium-thick duplex stainless steel plates with a rolling thickness of 6-20mm, exhibiting excellent low-temperature toughness and high fatigue performance, using continuously cast billets with a cross-section of 200mm or less. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a medium-thick duplex stainless steel plate for engineering equipment with a thickness of 6-20mm, exhibiting excellent low-temperature toughness and high fatigue performance, as well as its manufacturing method. This method employs optimized composition design, steelmaking, heating, rolling, online solution treatment, and straightening processes, ultimately achieving the production of duplex stainless steel plates for engineering equipment with excellent low-temperature toughness, high fatigue performance, and high straightness using continuously cast billets with a cross-section of 200mm or less. The steel plate has a yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33, transverse low-temperature impact toughness at 46℃ ≥47J, elongation ≥23%, and straightness below 5mm / 2m.

[0005] To achieve the above objectives, the present invention employs the following technical solution: A medium-thick duplex stainless steel plate for engineering equipment with excellent strength and toughness, wherein the chemical composition of the steel by weight percentage is: C≤0.07%, Si≤0.80%, Mn≤1.50%, P≤0.03%, S≤0.02%, Cr: 24.0%~26.0%, Ni: 4.0%~6.0%, Mo: 2.5%~3.5%, Cu≤0.30%, Al: 0.01%~0.10%, N: 0.20%~0.30%, W: 0.5%~0.8%, Ce: 0.02%~0.04%, Nb: 0.2%~0.4%, with the balance being Fe and unavoidable impurities.

[0006] The thickness of the steel plate is 6-20mm.

[0007] The steel plate has a yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33, transverse low-temperature impact toughness at 46℃ ≥47J, elongation ≥23%, and flatness below 5mm / 2m.

[0008] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows: Nitrogen (N) is a key element in the formation and stabilization of the austenite phase. Since nitrogen exists as interstitial solid solution atoms, increasing the nitrogen mass fraction enhances the austenite solid solution strengthening effect, thereby increasing the strength of the steel. Although nitrogen can dissolve at grain boundaries to reduce the precipitation of intermetallic phases, increasing the nitrogen mass fraction in the steel increases the risk of surface cracking. Therefore, this invention sets the N content range to 0.20%–0.30%.

[0009] Cr: Chromium is the most abundant metallic element in duplex stainless steel. When the chromium content in steel reaches a certain range, a passivation film will spontaneously form to inhibit corrosion. However, as the mass fraction of chromium gradually increases, it will promote the precipitation of intermetallic compounds, which will seriously affect the mechanical properties and corrosion resistance of the steel. Therefore, this invention sets the Cr content range to 24.0% to 26.0%.

[0010] Ni: In traditional stainless steel, nickel plays a crucial role because it controls the balance between the two phases. The addition of nickel not only improves the toughness of the steel but also lowers its brittle transition temperature. However, excessive nickel content is detrimental; too much nickel can disrupt the balance between the two phases, increasing the precipitation of ferrite phases and reducing the steel's plasticity. Therefore, this invention sets the Ni content range to 4.0%–6.0%.

[0011] Mo: The addition of molybdenum increases the stability of the passivation film on the steel surface, thus enhancing the steel's resistance to pitting corrosion. With increasing molybdenum mass fraction, the pitting potential and critical pitting temperature of the steel both increase. Due to the high affinity between molybdenum and carbon, the addition of molybdenum can inhibit the precipitation of chromium carbide. Therefore, the present invention sets the Mo content range to 2.5%–3.5%.

[0012] Mn: Manganese plays a similar role in steel as nickel, both expanding the equilibrium ratio of the two phases in the austenite region. Although manganese's effect is only 50% that of nickel, its lower price makes it a common substitute for nickel in steel. The addition of manganese increases the solubility of nitrogen in steel; therefore, this invention sets the Mn content range to 1.5% and below.

[0013] The addition of Ce reduces the number and size of inclusions, transforming the inclusions in the steel from Al2O3, MnS, and their composites to CeAlO3, Ce2O2S, CeS, and a small amount of Ce2O3. Rare earth inclusions also simultaneously increase the proportion of austenite phase, improving the low-temperature toughness of the steel plate. In this invention, the Ce content is set at 0.02%–0.04%.

[0014] Niobium (Nb): Niobium is a strong carbide-forming element, with a higher affinity for carbon than chromium. In duplex stainless steel, niobium preferentially combines with carbon to form stable niobium carbide (NbC), thus preventing carbon from combining with chromium at grain boundaries to form chromium carbide (Cr). 23 C6). This avoids the formation of "chromium-depleted zones" near grain boundaries, effectively improving the steel's resistance to intergranular corrosion. At the same time, the size of the precipitated phase is also refined, improving the strength and toughness of the steel plate. The Nb content in this invention is set at 0.2% to 0.4%.

[0015] Tungsten (W) significantly improves the pitting corrosion resistance equivalent (PREN) of duplex stainless steel, enhancing its resistance to pitting and crevice corrosion in chloride environments. As a strong ferrite-forming element, tungsten helps regulate the phase balance between austenite and ferrite. Appropriate addition of W can refine grains and delay the precipitation of brittle phases, thereby maintaining the material's toughness and structural stability. However, when the tungsten content exceeds 1.0%, it may actually promote the precipitation of brittle intermetallic phases, increasing the risk of material embrittlement and reducing weldability and corrosion resistance. Therefore, this invention sets the W content range to 0.5%–0.8%.

[0016] This invention provides a method for manufacturing medium-thick duplex stainless steel plates for engineering equipment with a rolled thickness of 6-20mm, exhibiting excellent low-temperature toughness and high fatigue performance. The method includes electric arc furnace steelmaking → AOD decarburization → VOD deoxidation → continuous casting → slab reheating → controlled rolling and online solution treatment → cold straightening. Specifically, it includes the following steps: Steelmaking and continuous casting: Smelted according to the following composition, with the following chemical composition by weight percentage: C≤0.07%, Si≤0.80%, Mn≤1.50%, P≤0.03%, S≤0.02%, Cr 24.0%~26.0%, Ni 4.0%~6.0%, Mo 2.5%~3.5%, Cu≤0.30%, Al 0.01%~0.10%, N 0.20%~0.30%, W 0.5%~0.8%, Ce 0.02%~0.04%, Nb 0.2%~0.4%, with the remainder being Fe and unavoidable impurities. After primary refining, the molten steel is transferred to an argon-oxygen decarburization furnace (AOD). By blowing in a mixture of argon and oxygen, the carbon content is efficiently reduced to extremely low levels (e.g., 0.03% or below) under conditions of reduced carbon monoxide partial pressure, while maximizing the retention of easily oxidized chromium. Desulfurization and composition fine-tuning are also performed. Argon blowing lasts for at least 35 minutes during tapping. Argon blowing and killing before continuous casting promotes the removal of inclusions in the molten steel and improves the uniformity of its composition. Next, a vacuum oxygen decarburization furnace (VOD) process is performed, maintaining a vacuum for at least 40 minutes for deep degassing. This more effectively removes harmful gases such as hydrogen, oxygen, and nitrogen, further reducing carbon and nitrogen content. An Fe-Ce alloy is added. With the addition of rare earth elements, the inclusions in the steel change from Al2O3, MnS, and their composites to CeAlO3, Ce2O2S, CeS, and a small amount of Ce2O, with a size within 0.2 μm. After degassing, slab continuous casting is carried out. The superheating temperature is 8–12℃, and the casting speed is 0.9–1.2 m / min (reducing the superheating temperature and casting speed can improve the macroscopic segregation of the slab, reduce the spacing of secondary dendrite arms in the solidification structure of the slab, and help reduce slab segregation and internal structural defects). Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage, with alternating forward and reverse stirring. The forward stirring time is 20–30 s, the reverse stirring time is 40–60 s, the current is 500–600 A, and the frequency is 20–30 Hz. This ensures that the molten steel passes through the continuous casting process to obtain a slab with an isometric crystal ratio of not less than 70%. Simultaneously, a strong cooling process is used in the fan-shaped section of the continuous casting process. The total cooling water flow rate for sections 1-2 is 1000-1200 L / min, for sections 3-4 it is 200-300 L / min, and for sections 5-8 it is 800-1000 L / min. This ensures a large temperature gradient with a temperature difference of >500℃ between the surface and core at the end of the continuous casting solidification process. Then, heavy pressure is applied with a reduction of 20-25 mm. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuous casting billet is significantly improved. The fan-shaped section uses strong cooling and controls the cooling water flow rate without interruption to ensure the temperature gradient in the thickness direction of the billet. At the same time, heavy pressure is applied, which helps to reduce billet segregation and ensure that strain penetrates to the core of the billet. This can significantly improve the central defects of the billet and provide a guarantee for the strength and toughness of the subsequent steel plate.After the casting billet comes off the production line, it is stacked for slow cooling at a temperature greater than 600℃. At the same time, other hot billet materials are used to cover the upper and lower surfaces of the duplex stainless steel continuous casting billet and to surround it. There are no less than 3 blocks covering the upper and lower surfaces and no less than 12 blocks surrounding it. The slow cooling time is no less than 72 hours. The continuous casting billet is covered on the upper and lower surfaces and surrounded, and the stacking temperature and time are controlled to promote the diffusion of elements such as Mn, C, Nb, H, and W, reduce the impact of component segregation on the microstructure and properties, and ensure that the H content is no more than 5 ppm.

[0017] 2) Billet Heating: Continuous casting billets with a thickness of 200mm or less are fed into a walking beam furnace for heating. The preheating section temperature range is 950–970℃ (to promote the partial solidification of carbonitrides of Cr and Nb in the matrix and allow for sufficient diffusion), the heating section temperature range is 1210–1230℃, and the soaking section temperature range is 1190–1210℃. The total time spent in the furnace during the preheating, heating, and soaking sections is controlled at 3.5–4.0 hours, with the heating and soaking sections taking 3.0–3.5 hours. The opening degree of the upper and lower burners in the furnace is adjusted to control the air-fuel ratio at 1:1.7–1:2.1. 2–4 empty spaces are reserved at the furnace head to ensure that the temperature difference between the upper and lower surfaces of the billet is within 10℃. The heating and soaking sections provide high-temperature heating, and the air-fuel ratio and furnace head clearance are controlled to ensure the temperature difference on the upper surface of the billet and prevent the lower surface of the billet from being affected by the water beam at the bottom of the furnace, which would result in a lower temperature on the lower surface and affect the uniformity of rolling deformation on the upper and lower surfaces. This ensures the uniformity of metal flow in the transverse and longitudinal directions on the steel plate surface. The total furnace time is controlled to effectively promote the full diffusion of elements such as C, Mn, Cr, W and N, while also ensuring that the original ferrite and austenite grain size is controlled within 60-150 μm. When the ferrite and austenite size is large, the resistance to crack propagation is reduced, resulting in a loss of steel plate toughness.

[0018] 3) Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1.5–2.0 min, with a descaling machine pressure of 20–25 MPa; a two-stage rolling process is adopted: the first stage is "austenite deformation recrystallization + ferrite two-phase zone rolling", the roughing and finishing rolling temperature range is 1000–1030℃, and the reduction rate of each of the last three passes in the roughing stage is not less than 15%. At the same time, descaling water is sprayed into each of the last three passes in the roughing stage, with a descaling time of 10–30 s and a descaling machine pressure of 15–20 MPa. (In the last three passes of the roughing stage, a large reduction rate is used to utilize the high-temperature deformation-induced effect to promote the precipitation of Nb-rich phases within the ferrite and austenite grains, improving strength and toughness; at the same time, it promotes dynamic recrystallization of austenite grains, refining the grain size. The thickness of the resulting intermediate billet is 1.5 to 2.5 times the thickness of the finished steel plate. The second stage is "rolling in the two-phase region of austenite without recrystallization + ferrite," with an initial rolling temperature range of 955–970℃, a bite and rolling speed of 0.8–1.1 m / s, and a final rolling temperature range of 870–850℃.) At 90℃, the first two passes of the finishing rolling stage should have a reduction rate of over 20%, with no more than five finishing passes. The final finishing pass should be a leveling pass with a small reduction rate of 0.2%–0.5%. (By controlling the reduction speed and the reduction rate in the finishing rolling stage, the density of dislocations, vacancies, and deformation bands in the austenite body are ensured, promoting the precipitation of Nb-rich phases, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate. The final passing pass uses a leveling pass with a small reduction rate to flatten the shape of the steel plate and reduce internal stress.) To prevent water accumulation in the steel plate after controlled cooling due to broken waves and curling ends, which would affect the uniformity of microstructure and properties, the steel plate is subjected to slow throwing after rolling, with a throwing speed of 1.5 to 2.0 m / s. After rolling, the steel plate enters an insulated box for reheating at a temperature of 980 to 1020℃. The speed of the steel plate through the insulated box is 0.3 to 0.5 m / s. This reheating of the steel plate ensures the uniformity of temperature in all positions along the width and length of the rolled steel plate when it enters the water, thereby ensuring the full solid solution of the brittle phase and guaranteeing its low-temperature toughness.

[0019] 4) Online solution treatment: UFC (Ultra-Fast Cooling System) is used for cooling. The UFC cooling process employs fully automatic controlled cooling. The initial cooling temperature of the steel plate is 970–1005℃, and the final cooling temperature is 25–50℃. The cooling rate is controlled at 25–35℃ / s. The roller speed of the steel plate conveyor in the controlled cooling area is 1.5–2.0 m / s, and the acceleration is 0.004–0.006 m / s². 2 After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water volume are 15-20 MPa and 150-200 m³ / h, respectively. 3 / h, with a purge pressure of 10-15MPa. During UFC cooling, the starting and ending cooling temperatures and cooling rates are controlled to maintain the austenite and ferrite grain sizes in the steel plate at 30-60μm and 40-80μm, respectively. This ensures sufficient solid solution of the brittle phase, enhances toughness, and lowers the ductile-brittle transition temperature of the steel plate. Side spraying and purge are used to control the shape of the steel plate, improve the uniformity of its properties, and reduce the likelihood of misshapen or uneven plate shapes. The rolled steel plate undergoes three cold straightening processes with a straightening force between 5500KN and 6800KN. The inlet roller position is -1.0 to -2.0mm, the outlet roller position is -3.6 to -4.6mm, and the tilt value is set to 7 to 10mm. Using a cold straightening machine with appropriate roller gaps and tilt values, multiple passes of small bending are employed to ensure that the cumulative strain at 10% to 20% of the steel plate surface exceeds the elastic limit strain. This promotes the full release of internal stress and ensures good flatness of the straightened plate.

[0020] Compared with the prior art, the beneficial effects of the present invention are: 1. By blowing a mixture of argon and oxygen into the AOD (Anaerobic Degassing) system, the carbon content is efficiently reduced to an extremely low level under the condition of lowering the partial pressure of carbon monoxide, while retaining the easily oxidized chromium element to the maximum extent. Desulfurization and composition fine-tuning are also performed. The argon blowing time is controlled to ensure the uniformity of the steel composition. The VOD (Vacuum Degassing) system can more effectively remove harmful gases such as hydrogen, oxygen, and nitrogen, and further reduce the carbon and nitrogen content. After adding rare earth elements, the inclusions in the steel change from Al2O3, MnS, and their composite inclusions to CeAlO3, Ce2O2S, CeS, and a small amount of Ce2O with a size within 0.2μm. This reduces superheat and continuous casting speed, which can improve the macroscopic segregation of the continuous casting billet. Reducing the spacing of secondary dendrite arms in the solidification structure of continuously cast billets helps to reduce billet segregation and internal structural defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed grain ratio of the continuously cast billet is significantly improved, and its grain size is refined, ensuring surface quality control during subsequent rolling. Strong cooling is used in the fan-shaped section, and the cooling water volume in different sections is controlled to ensure the temperature gradient in the thickness direction of the billet. Simultaneously, applying heavy pressure helps to reduce billet segregation and ensures that strain penetrates to the core of the billet, which can significantly improve central defects in the billet and ensure the strength and toughness of the subsequent steel plate. Covering the upper and lower surfaces of the continuously cast billet and surrounding it with barriers, and controlling the stacking temperature and time, promotes the diffusion of elements such as Mn, C, Nb, H, and W, reducing the impact of their compositional segregation on the microstructure and properties, and ensuring that the H content is not greater than 5 ppm.

[0021] 2. The composition of this invention is rationally designed, using continuously cast billets with a cross-section of 200mm or less for low-cost rolling, which greatly reduces alloy costs and resistance to high-temperature deformation during roughing and finishing rolling. This is beneficial for increasing the reduction per pass and ensuring the comprehensive performance of the steel plate. Furthermore, W, as a strong ferrite-forming element, helps to regulate the phase balance between austenite and ferrite. Appropriate addition (≤1.0%) can refine grains and delay the precipitation of brittle phases, thereby maintaining the toughness and structural stability of the material. The addition of N can improve the austenite solid solution strengthening effect, thereby increasing the strength of the steel; the addition of Ce also improves the size of inclusions, and the addition of Nb promotes the precipitation of Nb-rich phases, improving the strength and toughness of the steel plate.

[0022] 3. The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to promote the partial dissolution of Cr and Nb carbonitrides in the matrix and their full diffusion. The heating and soaking zones provide high-temperature heating, and the air-fuel ratio and furnace head clearance are controlled to ensure the temperature difference on the upper surface of the billet and prevent the lower surface of the billet from being affected by the water beam at the furnace bottom, which would result in a lower temperature on the lower surface and affect the uniformity of rolling deformation on both the upper and lower surfaces. This ensures the uniformity of transverse and longitudinal metal flow on the steel plate surface. The total furnace time is controlled to effectively promote the full diffusion of elements such as C, Mn, Cr, W, and N, while also ensuring that the original ferrite and austenite grain size is controlled within 60–150 μm. When the ferrite and austenite size is large, the resistance to crack propagation is reduced, resulting in a loss of steel plate toughness.

[0023] 4. Two-stage rolling: The reduction and rolling temperature of each pass in each rolling stage are controlled separately, and the intermediate billet thickness is optimized. This promotes plastic deformation during the first stage, "austenite deformation recrystallization + ferrite two-phase rolling," which refines austenite grains through recrystallization. This, in turn, promotes the formation of a fine ferrite and austenite dual-phase structure during subsequent rolling and cooling, thus improving the material's strength and toughness. The last three passes of the roughing stage employ a large reduction rate, utilizing the high-temperature deformation-induced effect to promote the precipitation of Nb-rich phases within the ferrite and austenite grains, improving strength and toughness. Simultaneously, it induces dynamic recrystallization of the austenite grains, refining the grain size. The second stage is "rolling in the two-phase region of austenite without recrystallization and ferrite". By controlling the reduction speed and the reduction rate in the finishing rolling stage, the density of faults, vacancies and deformation bands in the austenite body are ensured, promoting the precipitation of Nb-rich phase, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate. The last pass uses a low reduction rate leveling pass to flatten the shape of the steel plate, reduce the internal stress of the steel plate, and avoid water retention in the steel plate due to broken waves and curling after controlled cooling, which would affect the uniformity of the structure and properties. The steel plate is reheated after rolling to ensure the temperature uniformity of all positions in the rolling width and length direction when the steel plate enters the water, thereby ensuring the full solid solution of the brittle phase of the steel plate and ensuring its low-temperature toughness.

[0024] 5. During the UFC cooling process, the starting cooling temperature, the reheating temperature, and the cooling rate are controlled to maintain the austenite and ferrite grain sizes in the steel plate at 30–60 μm and 40–80 μm, respectively. This ensures sufficient solid solution of the brittle phase, enhances toughness, and lowers the ductile-brittle transition temperature of the steel plate. Side spraying and air purging facilitate the control of the steel plate shape, improve the uniformity of steel plate performance, and reduce the probability of problems with the top and bottom of the plate. Using a cold straightener, appropriate roll gap and tilt values ​​are set, and multiple passes with small bending amounts are adopted to ensure that the cumulative strain at 10%–20% of the steel plate surface exceeds the elastic limit strain, promoting further and full release of internal stress and ensuring good flatness of the straightened plate.

[0025] 6. Using the composition and process scheme of this invention, a medium-thick duplex stainless steel plate with excellent low-temperature toughness and high fatigue performance in thicknesses of 6-20mm for engineering equipment is obtained, along with its manufacturing method. This method employs optimized steelmaking, heating, rolling, online solution treatment, and straightening processes, ultimately achieving the production of medium-thick duplex stainless steel plates with thicknesses of 6-20mm using continuously cast billets with cross-sections of 200mm and below. These plates exhibit excellent low-temperature toughness and high fatigue performance. The resulting steel plates have a yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33, transverse low-temperature impact toughness at 46℃ ≥47J, elongation ≥23%, and straightness below 5mm / 2m. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0027] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0028] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting process of the steel in the examples; Table 3 shows the heating process of the billet of the steel in the examples; Table 4 shows the rolling method of the steel in the examples; Table 5 shows the online solution treatment and straightening process of the steel in the examples; and Table 6 shows the dimensions, properties and flatness of the steel in the examples.

[0029] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0030] Note: Impurity elements in steel: P≤0.03%, S≤0.02%.

[0031] Table 2. Smelting process of steel in the examples

[0032] Table 3 Heating regime of steel billets in the examples

[0033] Table 4 Rolling method of steel in the examples

[0034] Table 5. Online solution treatment and straightening processes for steels in the examples.

[0035] Table 6. Steel dimensions, properties, and straightness in the examples.

[0036] Therefore, compared with the prior art, the present invention provides a method for manufacturing medium-thick duplex stainless steel plates for engineering equipment with excellent low-temperature toughness and high fatigue performance (6-20mm thickness). This method adopts optimized steelmaking, heating, rolling, online solution treatment, and straightening processes, ultimately realizing the production of medium-thick duplex stainless steel plates for engineering equipment with a thickness of 6-20mm using continuous casting billets with a cross-section of 200mm or less. The steel plates have a yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33, transverse low-temperature impact toughness at 46℃ ≥47J, elongation ≥23%, and flatness below 5mm / 2m.

[0037] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A medium-thick duplex stainless steel plate for engineering equipment with excellent strength and toughness, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C≤0.07%, Si≤0.80%, Mn≤1.50%, P≤0.03%, S≤0.02%, Cr: 24.0%~26.0%, Ni: 4.0%~6.0%, Mo: 2.5%~3.5%, Cu≤0.30%, Al: 0.01%~0.10%, N: 0.20%~0.30%, W: 0.5%~0.8%, Ce: 0.02%~0.04%, Nb: 0.2%~0.4%, with the balance being Fe and unavoidable impurities.

2. The duplex stainless steel medium-thick plate for engineering equipment with excellent strength and toughness according to claim 1, characterized in that, The thickness of the steel plate is 6-20mm.

3. The duplex stainless steel medium-thick plate for engineering equipment with excellent strength and toughness according to claim 1, characterized in that, The steel plate has a yield strength ≥510MPa, tensile strength ≥650MPa, high-cycle fatigue limit stress at -20℃ ≥320MPa, PREN ≥33, transverse low-temperature impact toughness at 46℃ ≥47J, elongation ≥23%, and flatness below 5mm / 2m.

4. A method for manufacturing a medium-thick duplex stainless steel plate for engineering equipment with excellent strength and toughness as described in any one of claims 1-3, characterized in that, Specifically, the methods and steps are as follows: 1) Steelmaking and Continuous Casting: After primary refining, the molten steel is transferred to an argon-oxygen decarburization furnace, where a mixture of argon and oxygen is blown in. Argon is blown for at least 35 minutes during tapping. This is followed by a vacuum oxygen decarburization furnace process, where the vacuum is maintained for at least 40 minutes. After degassing, slab continuous casting is performed. The superheat is 8–12°C, and the casting speed is 0.9–1.2 m / min. During the controlled continuous casting stage, electromagnetic stirring is used in the secondary cooling zone. The stirring method is alternating forward and reverse rotation, with forward stirring time of 20–30 s and reverse stirring time of 40–60 s. The current is 500–600 A, and the frequency is 20–30 Hz. After continuous casting, a continuously cast billet is obtained. In the fan-shaped section of the continuous casting, the total cooling water volume of the first and second sections is 1000-1200 L / min, the total cooling water volume of the third and fourth sections is 200-300 L / min, and the total cooling water volume of the fifth and eighth sections is 800-1000 L / min. Then, heavy pressure is applied with a reduction of 20-25 mm. After the billet is removed from the line, it is stacked for slow cooling with a stacking temperature greater than 600℃. At the same time, other hot billet materials are used to cover the upper and lower surfaces of the duplex stainless steel continuously cast billet and to surround it. The number of covering blocks on the upper and lower surfaces is not less than 3, and the number of surrounding blocks is not less than 12. The slow cooling time is not less than 72 hours. 2) Billet heating: Continuous casting billets with a thickness of 200mm or less are heated. The temperature range of the preheating section is 950-970℃, the temperature range of the heating section is 1210-1230℃, and the temperature range of the soaking section is 1190-1210℃. The total time in the furnace for the preheating, heating, and soaking sections is controlled at 3.5-4.0 hours, of which the time in the heating and soaking sections is 3.0-3.5 hours. The opening degree of the upper and lower burners in the heating furnace is adjusted to control the air-fuel ratio at 1:1.7-1:2.

1. 2-4 empty spaces are reserved in the furnace head of the heating furnace to ensure that the temperature difference between the upper and lower surfaces of the billet is within 10℃. 3) Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1.5–2.0 min, with a descaling machine pressure of 20–25 MPa; a two-stage rolling process is adopted: the first stage is "austenite deformation recrystallization + ferrite two-phase region rolling", with a roughing and finishing rolling temperature range of 1000–1030℃. The reduction rate of each of the last three passes in the roughing stage is not less than 15%, and descaling water is sprayed into each of the last three passes in the roughing stage for 10–30 s, with a descaling machine pressure of 15–20 MPa. The second stage is "austenite without recrystallization + ferrite two-phase region rolling". "Two-phase rolling" is used, with an initial rolling temperature range of 955–970℃, a bite and rolling speed of 0.8–1.1 m / s, and a final rolling temperature range of 870–890℃. The first two passes in the finishing rolling stage ensure a reduction rate of over 20%, and the finishing rolling stage has no more than five passes. The final finishing rolling pass is a leveling pass with a small reduction rate of 0.2%–0.5%. After rolling, slow steel ejection is adopted, with an ejection speed of 1.5–2.0 m / s. After rolling, the steel plate enters an insulated box for reheating, with a reheating temperature of 980–1020℃ and a steel plate speed of 0.3–0.5 m / s through the insulated box. 4) Online solution treatment: UFC cooling is used, with the initial cooling temperature of the steel plate at 970-1005℃ and the final cooling temperature at 25-50℃. The cooling rate is controlled at 25-35℃ / s. The roller speed of the steel plate conveyor in the controlled cooling zone is 1.5-2.0m / s, and the acceleration is 0.004-0.006m / s. 2 After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water volume are 15-20 MPa and 150-200 m³ / h, respectively. 3 / h, the air blowing pressure is 10~15MPa, the rolled steel plate is cold straightened in three stages, the straightening force is between 5500KN~6800KN, the position of the inlet roller is -1.0~-2.0mm, the position of the outlet roller is -3.6~-4.6mm, and the tilt value is set to 7~10mm.

5. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, In the vacuum oxygen decarburization furnace process, after adding Fe-Ce alloy, the size of inclusions in the steel is within 0.2μm.

6. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, The isometric crystal ratio of the continuously cast billet is not less than 70%.

7. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, After cooling, the temperature difference between the surface and the core of the continuously cast solidified end of the fan-shaped section is >500℃; the H content of the billet after slow cooling is not greater than 5ppm.

8. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, After the billet is heated, the original ferrite and austenite grain size in the steel is controlled at 60-150 μm.

9. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, The thickness of the intermediate billet obtained after rough rolling is 1.5 to 2.5 times the thickness of the finished steel plate.

10. The method for manufacturing a medium-thick duplex stainless steel plate with excellent strength and toughness for engineering equipment according to claim 4, characterized in that, After UFC cooling, the grain sizes of austenite and ferrite phases in the steel are 30–60 μm and 40–80 μm, respectively; after three cold straightening processes, the cumulative strain of the steel plate at a distance of 10%–20% from the surface exceeds the elastic limit strain.

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