A wide-thin high-flatness dual-phase stainless steel for engineering equipment and a manufacturing method thereof

Abstract

The present application relates to the field of special stainless steel plate rolling technology, and particularly relates to a wide and thin gauge high flatness duplex stainless steel for engineering equipment and a manufacturing method thereof, wherein the chemical components in the steel are as follows in percentage by weight: C: 0.01-0.030%, Si: 0.5-0.8%, Mo: 0.1-0.3%, Mn: 5-6%, Cr: 21-22%, Ni: 1.5-1.7%, W: 0.5-1%, P: ≤0.020%, S: ≤0.003%, N: 0.20-0.25%, Ce: 0.02-0.04%, V: 0.01-0.05%, O: ≤0.005%, Al: ≤0.05%, and the balance of Fe and inevitable impurities. The steel plate has excellent strength and toughness and high flatness. The yield strength of the steel plate is ≥460 MPa, the tensile strength is ≥630 MPa, the transverse low-temperature impact toughness at 40 DEG C is ≥120 J, the elongation is ≥20%, the flatness is below 5 mm / 2 m, and the surface crack waste product rate is reduced from more than 2% in the initial stage to below 0.5%.

Description

Technical Field

[0001] This invention relates to the field of special stainless steel plate rolling technology, and in particular to a wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment and its manufacturing method. Background Technology

[0002] Ferritic and austenitic duplex stainless steels possess excellent strength, toughness, and corrosion resistance at both high and low temperatures, making them widely used in shipbuilding, petroleum, chemical, and nuclear power industries for the manufacture of large ships, pressure vessels, and critical equipment. Currently, the width of domestic duplex stainless steel plates is primarily 2000-3000 mm, with a thickness of 5-10 mm. The use of large-width duplex stainless steel plates reduces weld seams, decreases the workload of welding material inspection during manufacturing, lowers equipment maintenance costs, and increases equipment safety. Simultaneously, it significantly reduces factory prefabrication costs, shortens the procurement cycle for cost-effective duplex stainless steel, and accelerates project construction, demonstrating broad application prospects. Therefore, the market demand for ultra-thin and ultra-wide duplex stainless steel plates is increasing. However, due to limitations in rolling processes, mass production needs cannot be met. This is mainly reflected in the following points: 1. While a few steel companies' wide and thick plate rolling mills can meet the width requirements, using these mills to produce thin-width, wide duplex stainless steel is difficult to control in terms of plate shape. Production failures such as frame scraping and bending are prone to occur during the rolling process, easily leading to equipment damage. 2. Due to the rapid temperature drop of the steel plate during the rolling process, it is difficult to control the temperature stably, resulting in large fluctuations in thickness and performance, and low pass rate in mass production. 3. When rolling 4mm ultra-thin steel plates, due to their high alloy content, high deformation resistance, and rapid temperature drop, the phenomenon of adding passes to the mold occurs in the later stages of rolling. This leads to the rolling force exceeding the mill's design limit due to the rapid temperature drop in the later stages of rolling, making further rolling impossible. Therefore, it is necessary to develop a method for producing ultra-thin and ultra-wide extreme specification engineering structural steel using a wide and thick plate rolling mill. 4. Currently, duplex stainless steel continuously cast billets have a high nitrogen content, and the matrix is ​​a duplex structure. Therefore, during hot working, thin specifications experience rapid temperature drop. Rolling within the precipitation temperature range of brittle phases, when the proportion of brittle phases is high, the duplex grain boundary stress is large, and deformation is not coordinated, it easily leads to the generation of hot-rolled edge cracks and surface cracks. To address the challenges in manufacturing thin-gauge, wide-width duplex stainless steel plates, domestic and international stainless steel producers have optimized existing duplex stainless steel production processes. This includes controlling process parameters such as the deformation per pass in roughing and finishing rolling, and the transverse and longitudinal rolling processes to reduce cracking. However, plate shape control remains unclear. Therefore, a key issue that urgently needs to be addressed in the mass production of wide-gauge, thin-gauge duplex stainless steel plates for engineering equipment—characterized by thicknesses of 5-10mm and widths >3200mm from continuously cast billets with cross-sections of 150mm and below—to continuously meet the significant demands of downstream industries.

[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 for engineering equipment with excellent strength and toughness, high flatness, and a thickness of 5-10mm and a width >3200mm. Prior to this invention, patent document CN101450353A, "A method for controlling the shape of a steel plate," provided a method for controlling the shape of medium-thick plates, but this method was only for the rolled shape of the steel plate, and the thickness was 6mm or more. CN116875907 B, "An ultra-wide duplex stainless steel medium-thick plate and its manufacturing method," provided a method for manufacturing wide duplex stainless steel, but it did not specify the width and thickness during the manufacturing process. Using the technical solution provided by this invention, the above deficiencies can be effectively overcome, realizing the rolling of duplex stainless steel for engineering equipment with a thickness of 5-10mm and a width >3200mm using continuous casting billets with a cross-section of 150mm 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 wide-width, thin-gauge, high-straightness duplex stainless steel for engineering equipment and its manufacturing method. This method employs optimized composition design, steelmaking, heating, rolling, controlled cooling, and straightening processes, ultimately achieving the production of wide-width, thin-gauge duplex stainless steel for engineering equipment with excellent strength and toughness, and high straightness, using continuously cast billets with a cross-section of 150mm or less. The steel plate has a yield strength ≥460MPa, tensile strength ≥630MPa, transverse low-temperature impact toughness at 40℃ ≥120J, elongation ≥20%, straightness below 5mm / 2m, and the surface crack scrap rate is reduced from over 2% initially to below 0.5%.

[0005] To achieve the above objectives, the present invention employs the following technical solution: A wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment, wherein the chemical composition of the steel by weight percentage is: C: 0.05%~0.080%, Si: 0.5%~0.8%, Mo: 0.1%~0.3%, Mn: 5%~6%, Cr: 21%~22%, Ni: 1.5%~1.7%, W: 0.5%~1%, P≤0.020%, S≤0.003%, N: 0.20%~0.25%, Ce: 0.02%~0.04%, V: 0.01%~0.05%, O≤0.005%, Als≤0.05%, with the balance being Fe and unavoidable impurities.

[0006] The steel plate has a thickness of 5-10 mm and a width of >3200 mm. The steel plate has a yield strength ≥460 MPa, tensile strength ≥630 MPa, transverse low-temperature impact toughness at -40℃ ≥120 J, elongation ≥20%, and flatness below 5 mm / 2 m.

[0007] 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%–0.25%.

[0008] 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 21.0% to 22.0%.

[0009] 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 1.5%–1.7%.

[0010] 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 0.1%–0.3%.

[0011] 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% of 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 5%–6%.

[0012] 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%.

[0013] Tungsten (W): 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%–1%.

[0014] This invention also provides a method for manufacturing duplex stainless steel plates for engineering equipment with a rolling thickness of 5-10 mm and a width > 3200 mm, exhibiting excellent strength, toughness, and high flatness. The method utilizes a heavy plate rolling mill to produce ultra-thin, ultra-wide, and high-flatness duplex stainless steel plates for engineering equipment, using continuously cast billets with a thickness of 150 mm or less on a medium-thick plate reciprocating rolling mill. The process includes electric arc furnace steelmaking → AOD decarburization → VOD deoxidation → continuous casting → billet reheating → controlled rolling and online solution treatment → cold straightening. Specifically, the method includes the following steps: 1) Steelmaking and continuous casting: Smelted according to the following composition, with the following chemical composition by weight percentage: C 0.05%~0.080%, Si 0.5%~0.8%, Mo 0.1%~0.3%, Mn 5%~6%, Cr 21%~22%, Ni 1.5%~1.7%, W 0.5%~1%, P≤0.020%, S≤0.003%, N 0.20%~0.25%, Ce 0.02%~0.04%, V 0.01%~0.05%, O≤0.005%, Als≤0.05%, 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 that lower the partial pressure of carbon monoxide, while maximizing the retention of easily oxidized chromium. Desulfurization and composition fine-tuning are also performed. Argon is blown for more than 40 minutes during tapping. Argon blowing and killing before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition. Next, a vacuum oxygen decarburization furnace (VOD) process is carried out, with vacuum maintained for more than 30 minutes. Deep degassing can more effectively remove harmful gases such as hydrogen, oxygen, and nitrogen, and further reduce the carbon and nitrogen content. Fe-Ce alloy is added. 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. After degassing, slabs are continuously cast. The superheat of the continuous casting is 10-15℃, and the casting speed is 0.9-1.1m / min. Reducing the superheat and casting speed can improve the macroscopic segregation of the continuous casting slab, reduce the spacing of secondary dendrite arms in the solidification structure of the continuous casting slab, and help reduce the segregation of the steel billet and reduce 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 30–45 s, and the reverse stirring time is 40–60 s. The current is 500–700 A, and the frequency is 20–40 Hz. This ensures that the molten steel is continuously cast into a billet with an isometric crystal ratio of not less than 90%. Simultaneously, a strong cooling process is used in the fan-shaped section of the continuous casting, with a total cooling water flow of 600–800 L / min for sections 1–4. This ensures a large temperature gradient field with a surface-to-core temperature difference >200°C at the end of the solidification process. Light reduction of 3–5 mm is also applied. The total cooling water volume for sections 5-8 is 1000-1500 L / min to ensure a large temperature gradient field with a temperature difference of >500℃ between the surface and core at the end of continuous casting solidification. Then, heavy pressure is applied with a reduction of 10-15 mm. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuous casting billet is significantly improved. Strong cooling is used in the fan-shaped section, and the cooling water volume is controlled without interruption to ensure the temperature gradient in the thickness direction of the billet. At the same time, light and heavy pressure are applied in different sections, which helps to reduce billet segregation and ensure that strain penetrates to 1 / 4 of the billet (the location improved under light pressure) and 1 / 2 of the core (the location improved under heavy pressure). This can significantly improve defects in different locations of the billet cross-section and provide a guarantee for the strength and toughness of the subsequent steel plate. After the billet comes off the production line, it is kept in an electrically heated bell-type furnace for heat preservation, homogenization and dehydrogenation treatment. The heat preservation temperature is between 700 and 800℃ and the heat preservation time is not less than 48 hours. The continuous casting billet is kept in a heated bell-type furnace for heat preservation, homogenization and dehydrogenation treatment, and the stacking temperature and time are controlled to promote the diffusion of elements such as Mn, C, W, H, Ni and N, reduce the impact of their component segregation on the microstructure and properties, and ensure that the H content is not greater than 5 ppm.

[0015] Billet grinding and heating: The stacked continuous casting billets are ground, with full grinding of both the upper and lower surfaces. After grinding the upper and lower surfaces by 0.5-0.6 mm, dye penetrant testing is performed. Grinding the upper and lower surfaces removes surface porosity and microcracks, ensuring that no microcrack defects are generated on the surface of the billet before it enters the furnace, thereby avoiding the propagation of surface cracks during the billet rolling process. The ground continuous casting billets with a thickness of 150 mm or less are sent into a walking beam furnace for heating. The preheating temperature range is 960-980℃ (to promote the partial dissolution of Cr carbonitrides in the matrix and their full diffusion), and the heating temperature range is 1250-1... At 270℃, with a soaking zone temperature range of 1210~1230℃ and a main gas pressure of 7~9KPa, the tapping rhythm increases to ≥180s, the soaking zone and heating time are 3.5~4.0 hours, and the total furnace time is 4~4.5 hours; when the main gas pressure is <7KPa, the tapping rhythm increases to ≥230s, the soaking zone and heating time are 4.0~4.5 hours, and the total furnace time is 4.5~5 hours. At the same time, the opening degree of the upper and lower burners of the heating furnace is adjusted, the air-fuel ratio is controlled at 1:1.7~1:2.1, and 2~4 empty spaces are reserved at the furnace head to ensure that the temperature difference between the upper and lower surfaces of the steel billet is within 10℃. The heating and soaking sections provide high-temperature heating, ensuring gas pressure, controlling the tapping rhythm, air-fuel ratio, and furnace head clearance, thus maintaining the temperature difference on the upper surface of the billet and preventing 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. Controlling the total furnace time effectively promotes the full diffusion of elements such as C, Mn, V, Cr, W, and N, while also ensuring that the original ferrite and austenite grain size is controlled within 60–100 μm. When the ferrite and austenite size is large, the resistance to crack propagation is reduced, resulting in a loss of steel plate toughness and exacerbating the formation of surface microcracks.

[0016] Controlled rolling: Before rolling ultra-thin and ultra-wide steel plates, roll at least 10 steel plates of similar specifications to the target product as transition plates. The finished width of the transition plates must be at least 300mm wider than the target product, and the thickness is allowed to deviate by no more than 10mm. Before production scheduling, the transition production of similar specifications ensures sufficient heat treatment of the roller table and rolls, preventing the steel plate from being affected by excessively rapid temperature drop during the formal rolling of ultra-thin and ultra-wide duplex stainless steel plates, thus maintaining the desired plate shape. For subsequent rolling, the thickness accuracy of the steel plate model is controlled; before rolling, the billet after exiting the furnace is descaled with high-pressure water for 0.5-1.0 min, with a descaling machine pressure of 15-20 MPa; the rolling roll cycle is selected to be 1500-2000 tons after roll change, and the descaling machine pressure and time are controlled to avoid excessive temperature drop of the rolled steel plate; the production roll cycle is controlled to control the roll crown and ensure the shape of the rolled steel plate; during rolling, the cooling water of the front and rear roller tables of the mill is turned off, and the rolling roll cooling water is reduced to 100-200 m³. 3 / h, while reducing the contact between the steel plate and the cooling medium, to avoid the steel plate temperature dropping too quickly during rolling, which would cause brittle phase precipitation and microcracks on the steel plate surface. Two-stage rolling is adopted: the first stage is "austenite deformation recrystallization + ferrite two-phase region rolling", the roughing and finishing rolling temperature range is 1010~1050℃, and the reduction rate of the last three passes in the roughing stage is not less than 12% per pass; the last three passes in the roughing stage use a large reduction rate to promote dynamic recrystallization of austenite grains and refine the grains. The thickness of the intermediate billet is 1.5 to 2.5 times that of the finished steel plate. During the intermediate billet's warming process, a pre-straightening machine is used to hot-straighten the billet, with a straightening force between 2000KN and 3000KN. The inlet roll position is -0.5 to -1.0 mm, and the outlet roll position is -1.5 to -2.5 mm, ensuring the billet's shape before rolling and providing a foundation for subsequent rolling shape control. The second stage is "rolling in the austenite-non-recrystallized + ferrite two-phase region," with an initial rolling temperature range of 970 to 980℃, a bite and rolling speed of 0.86 to 1.2 m / s, and a final rolling temperature range of 870 to 890℃. The first two passes of the second stage ensure a reduction rate of over 20%. The rolling process involves no more than five passes, with the final pass being a leveling pass and a small reduction rate of 0.2% to 0.5%. The length of the rolled steel plate should be less than 25m. By controlling the reduction speed and the reduction rate in the finishing stage, the density of faults, vacancies, and deformation bands in the austenitic body are ensured, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate. The final pass, a leveling pass with a small reduction rate, optimizes the billet shape and effectively controls the length of the rolled piece during the rolling process. This avoids large temperature fluctuations caused by excessively long rolled pieces, which would make subsequent plate shape control difficult. At the same time, it reduces the internal stress of the steel plate and prevents water retention in the steel plate after controlled cooling due to breakage and curling, which would affect the uniformity of the microstructure and properties. After rolling, the steel is thrown out at a slow speed of 1.0 to 1.5 m / s. After rolling, the steel plate enters a heat preservation box for reheating at a temperature of 1010 to 1030℃. The speed of the steel plate passing through the heat preservation box is 0.4 to 0.6 m / s. By reheating the steel plate, the temperature uniformity of each position in the rolling width and length direction is ensured when the steel plate enters the water, thereby ensuring that the brittle phase of the steel plate is fully dissolved to ensure its low-temperature toughness.

[0017] 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 990–1015℃, and the final cooling temperature is 25–50℃, with the cooling rate controlled at 25–35℃ / s. The steel plate is inserted into the UFC area with the beginning and end shielded. The upper manifold opening correction value is 400–600mm, and the lower manifold opening correction value is 800–1000mm. 8–12 sets of water are opened in both the upper and lower manifolds, with an opening water flow of 250–280m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.1–2.4; the roller speed of the steel plate conveyor in the controlled cooling zone is 1.3–1.5 m / s, and the acceleration is 0.003–0.005 m / 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 100-150 m³ / h, respectively. 3 / h, with a wind purging pressure of 10-15MPa, during the UFC cooling process, the starting and ending cooling temperatures, cooling rates, and the head and tail shielding parameters of the controlled-cooling steel plate are controlled. The purpose is to effectively regulate the temperature uniformity at different positions of the head, middle, and tail of the steel plate to ensure the strength, toughness, and shape of the steel plate. The grain sizes of austenite and ferrite phases in the steel plate are controlled at 35-70μm and 50-85μm, respectively, to ensure sufficient solid solution of brittle phases, strength, and toughness, and to reduce the ductile-brittle transition temperature of the steel plate. The side spraying and wind purging are used to control the shape of the steel plate, improve the uniformity of the steel plate performance, and reduce the probability of problems with the shape of the head and tail. The rolled steel plate undergoes three cold straightening passes with a straightening force between 5500KN and 6800KN. The inlet roll position is -1.5 to -2.5mm, the outlet roll position is -3.5 to -4.5mm, and the tilt value is set to 8 to 12mm. Using a cold straightening machine with appropriate roll gaps and tilt values, multiple passes with small bending amounts are employed to ensure that the cumulative strain at 15% to 25% of the steel plate surface exceeds the elastic limit strain. This promotes further and full release of internal stress, ensuring good flatness of the straightened plate.

[0018] 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 extremely low levels while minimizing the partial pressure of carbon monoxide. Simultaneously, easily oxidized chromium is preserved to the maximum extent. Desulfurization and composition fine-tuning are also performed. Controlling the argon blowing time ensures the uniformity of the molten steel composition. Using VOD (Vacuum Degassing) further effectively removes harmful gases such as hydrogen, oxygen, and nitrogen, further reducing 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, and CeS. The addition of a small amount of Ce2O, with a size within 0.2μm, reduces superheat and casting speed, which can improve macroscopic segregation of the continuously cast billet, reduce the spacing of secondary dendrite arms in the solidification structure of the billet, and help reduce billet segregation and internal structural defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuously cast billet is significantly improved. Strong cooling is used in the fan-shaped section, and the cooling water volume is controlled without interruption to ensure the temperature gradient in the thickness direction of the billet. At the same time, light and heavy pressure are applied to different sections, which can help reduce billet segregation and ensure that strain penetrates to 1 / 4 of the billet (the improved position under light pressure) and 1 / 2 of the core (the improved position under heavy pressure). This can significantly improve defects in different positions of the billet cross-section and provide a guarantee for the strength and toughness of the subsequent steel plate. After the billet is taken off the line, it is kept warm, homogenized and dehydrogenated, and the stacking temperature and time are controlled to promote the diffusion of elements such as Mn, C, W, H, Ni and N, reduce the impact of their compositional segregation on the microstructure and properties, and ensure that the H content is not greater than 5ppm.

[0019] 2. The composition of this invention is reasonable, and the amount of precious alloy (Ni) added is low. It utilizes continuously cast billets with a cross-section of 150mm or less for low-cost rolling, significantly reducing 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 overall performance of the steel plate. Furthermore, W, as a strong ferrite-forming element, helps 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 material's toughness and structural stability. Ni addition enhances the austenite solid solution strengthening effect, thus increasing the steel's strength; Ce addition also improves inclusion size, enhancing the steel plate's strength and toughness.

[0020] 3. Grind the upper and lower surfaces of the continuously cast billet to remove surface porosity and microcracks, ensuring that the billet surface is free of microcrack defects before entering the furnace. This prevents surface crack propagation during the billet rolling process. The heating and soaking sections provide high-temperature heating, ensuring gas pressure, controlling the tapping rhythm, air-fuel ratio, and furnace head clearance to maintain the temperature difference on the upper surface of the billet. This prevents 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 uniform transverse and longitudinal metal flow on the steel plate surface. Controlling the total furnace time effectively promotes the full diffusion of elements such as C, Mn, V, Cr, W, and N, while also ensuring that the original ferrite and austenite grain size is controlled within 60–100 μm. When the ferrite and austenite size is large, the resistance to crack propagation is reduced, resulting in a loss of steel plate toughness and exacerbating the formation of surface microcracks.

[0021] 4. Before production scheduling, conduct transitional production of similar specifications to ensure sufficient heat treatment of the roller table and rolls. This prevents the steel plate from cooling down too quickly during the formal rolling of ultra-thin and ultra-wide duplex stainless steel plates, which could affect the plate shape. This is beneficial for subsequent thickness accuracy control of the rolled steel plate model, controlling the descaling machine pressure and time to avoid excessive temperature drop in the rolled steel plate. Control the production scheduling roll cycle to control the roll crown and ensure the shape of the rolled steel plate. During rolling, shut off the cooling water in the front and rear roller tables of the mill and control the flow rate of the rolling roll cooling water to reduce the contact between the steel plate and the cooling medium, avoiding excessive temperature drop in the rolled steel plate, which could cause brittle phase precipitation and microcracks on the steel plate surface. By controlling the reduction amount and rolling temperature of each rolling stage and optimizing the thickness of the intermediate billet, plastic deformation such as rolling is promoted in the first stage "austenite deformation recrystallization + ferrite two-phase region rolling". The recrystallization can refine the austenite grains, thereby promoting the formation of fine ferrite and austenite dual-phase structure during the subsequent rolling and cooling process, which helps to improve the strength and toughness of the material. Pre-straightening of the intermediate billet ensures its shape before rolling, providing a foundation for subsequent shape control. The second stage is "rolling in the austenite-non-recrystallized + ferrite two-phase region." 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 ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate. The final pass uses a low reduction rate leveling pass. The optimization of the billet shape enables effective control of the length of the rolled piece during rolling, avoiding large temperature fluctuations caused by excessively long rolled pieces, which would make subsequent shape control difficult. At the same time, it reduces the internal stress of the steel plate and avoids water retention in the steel plate after controlled cooling due to broken waves and curling ends, which would affect the uniformity of the structure and properties. The steel plate is reheated after rolling to ensure temperature uniformity in all positions along the width and length of the rolled plate when it enters the water, thereby ensuring sufficient solid solution of the brittle phase and ensuring its low-temperature toughness.

[0022] 5. During the UFC cooling process, controlling the initial cooling temperature, final cooling temperature, cooling rate, and the head and tail shielding parameters of the controlled-cooling steel plate aims to effectively regulate the temperature uniformity at different positions of the steel plate (head, middle, and tail) to ensure the steel plate's strength, toughness, and shape. It also controls the austenite and ferrite grain sizes in the steel plate to be 35–70 μm and 50–85 μm respectively, ensuring sufficient solid solution of the brittle phase, maintaining strength and toughness, and lowering the ductile-brittle transition temperature. Side spraying and air purging facilitate control of the steel plate shape, improve the uniformity of steel plate performance, and reduce the likelihood of head and tail shape issues. Using a cold straightener, setting appropriate roll gaps and tilt values, and employing multi-pass small-bending straightening ensures that the cumulative strain at 15%–25% 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.

[0023] 6. A method for manufacturing medium-thick duplex stainless steel plates for engineering equipment with excellent strength and toughness, high flatness, and a thickness of 5-10mm and a width of >3200mm, using the composition and process scheme of this invention. This method adopts optimized composition design, steelmaking, heating, rolling, online solution treatment, and straightening processes, and realizes the rolling of duplex stainless steel plates with a thickness of 5-10mm and a width of >3200mm using continuous casting billets with a cross section of 150mm and below. The plates have excellent strength and toughness, high flatness, yield strength ≥460MPa, tensile strength ≥630MPa, transverse low-temperature impact toughness at 40℃ ≥120J, elongation ≥20%, flatness below 5mm / 2m, and the surface crack scrap rate is reduced from more than 2% initially to less than 0.5%. Detailed Implementation

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0028] Note: Impurity elements in steel: P≤0.02%, O≤0.005%, S≤0.003%.

[0029] Table 2. Smelting process of steel in the examples

[0030] Table 3 Heating regime of steel billets in the examples

[0031] Table 4 Rolling method of steel in the examples

[0032] Table 5. Online solution treatment and straightening processes for steels in the examples.

[0033] Table 6. Steel dimensions, properties, and straightness in the examples.

[0034] 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 strength and toughness, high flatness, and a thickness of 5-10mm and a width >3200mm. This method employs optimized composition design, steelmaking, heating, rolling, online solution treatment, and straightening processes. It achieves the production of duplex stainless steel plates for engineering equipment with excellent strength and toughness, high flatness, and a thickness of 5-10mm and a width >3200mm using continuously cast billets with a cross-section of 150mm or less. The steel plates have a yield strength ≥460MPa, tensile strength ≥630MPa, transverse low-temperature impact toughness at 40℃ ≥120J, elongation ≥20%, flatness below 5mm / 2m, and a surface crack scrap rate reduced from over 2% initially to below 0.5%. 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 this invention shall be covered within the scope of protection of this invention.

Claims

1. A wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.05%–0.080%, Si: 0.5%–0.8%, Mo: 0.1%–0.3%, Mn: 5%–6%, Cr: 21%–22%, Ni: 1.5%–1.7%, W: 0.5%–1%, P≤0.020%, S≤0.003%, N: 0.20%–0.25%, Ce: 0.02%–0.04%, V: 0.01%–0.05%, O≤0.005%, Als≤0.05%, with the balance being Fe and unavoidable impurities.

2. The wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment according to claim 1, characterized in that, The steel plate has a thickness of 5-10mm and a width of >3200mm.

3. The wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment according to claim 1, characterized in that, The steel plate has a yield strength ≥460MPa, tensile strength ≥630MPa, transverse low-temperature impact toughness ≥120J at -40℃, elongation ≥20%, and flatness below 5mm / 2m.

4. A method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment 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 40 minutes during tapping. This is followed by a vacuum oxygen decarburization furnace process, where the vacuum is maintained for at least 30 minutes. After degassing, slab continuous casting is performed. The superheating temperature is 10–15°C, and the casting speed is 0.9–1.1 m / min. During the controlled continuous casting stage, electromagnetic stirring is used in the secondary cooling zone, with alternating forward and reverse stirring. The forward stirring time is 30–45 s, and the reverse stirring time is 40–60 s. The current is 500-700A and the frequency is 20-40Hz. The molten steel is continuously cast to obtain a continuously cast billet. The total cooling water flow rate of the first to fourth sections of the continuous casting fan-shaped section is 600-800L / min. Light reduction is applied with a reduction of 3-5mm. The total cooling water flow rate of the fifth to eighth sections is 1000-1500L / min. Then heavy reduction is applied with a reduction of 10-15mm. After the billet is removed from the line, it is held in an electrically heated bell furnace for heat preservation, homogenization and dehydrogenation treatment. The heat preservation temperature is between 700-800℃ and the heat preservation time is not less than 48h. 2) Billet grinding and heating: The upper and lower surfaces of the billet are fully ground to a thickness of 0.5–0.6 mm. After grinding, dye penetrant testing is performed. Continuous casting billets with a thickness of 150 mm or less after grinding are heated. The preheating temperature range is 960–980℃, the heating temperature range is 1250–1270℃, and the soaking temperature range is 1210–1230℃. When the main gas pressure is 7–9 kPa, the tapping rhythm is increased to ≥180 seconds. The soaking and heating time is 3.5 to 4.0 hours, and the total time in the furnace is 4 to 4.5 hours. When the main gas pressure is <7 kPa, the tapping rhythm is increased to ≥230 seconds, the soaking and heating time is 4.0 to 4.5 hours, and the total time in the furnace is 4.5 to 5 hours. At the same time, the opening degree of the upper and lower burners of the heating furnace is adjusted to control the air-fuel ratio of 1:1.7 to 1:2.

1. 2 to 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 steel billet is within 10℃. 3) Controlled Rolling: Before production scheduling, roll at least 10 steel plates of similar specifications to the target product as transition steel plates. The finished width of the transition steel plates must be at least 300mm wider than the target product, and the thickness is allowed to deviate by no more than 10mm. Before rolling, use high-pressure water to descale the cast billet after it exits the furnace for 0.5-1.0 minutes, with a descaling machine pressure of 15-20MPa. The rolling roll cycle should be 1500-2000 tons after the roll change. During rolling, shut off the cooling water in the front and rear roller tables of the mill, and reduce the rolling roll cooling water to 100-200m³. 3 / h, employing a two-stage rolling process: The first stage is "austenite deformation recrystallization + ferrite two-phase region rolling," with a roughing and finishing rolling temperature range of 1010~1050℃. The reduction rate in the last three passes of the roughing stage is no less than 12% per pass. During the intermediate billet warming process, a pre-straightening machine is used to hot-straighten the billet, with a straightening force between 2000KN and 3000KN. The inlet roll position is -0.5~-1.0mm, and the outlet roll position is -1.5~-2.5mm. The second stage is "austenite without recrystallization + ferrite two-phase region rolling," with an initial rolling temperature range of... The rolling temperature is 970–980℃, the rolling speed is 0.86–1.2 m / s, and the final rolling temperature range is 870–890℃. The first two passes of the second stage should ensure a reduction rate of more than 20%, and the second stage should not exceed five passes. The final pass of the finishing rolling should be a leveling pass with a small reduction rate of 0.2%–0.5%. The length of the rolled steel plate should be less than 25 m. After rolling, the steel plate should be slowly ejected at a speed of 1.0–1.5 m / s. After rolling, the steel plate should be placed in an insulated box for reheating at a temperature of 1010–1030℃. The speed of the steel plate passing through the insulated box should be 0.4–0.6 m / s. 4) Online solution treatment: UFC cooling is used, with the initial cooling temperature of the steel plate at 990-1015℃ and the final cooling temperature at 25-50℃, and the cooling rate controlled at 25-35℃ / s; the steel plate is inserted into the UFC area with the beginning and end shielded, the upper manifold opening correction value is 400-600mm, and the lower manifold opening correction value is 800-1000mm; 8-12 sets of water are opened in both the upper and lower manifolds, with an opening water flow of 250-280m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.1–2.4; the roller speed of the steel plate conveyor in the controlled cooling zone is 1.3–1.5 m / s, and the acceleration is 0.003–0.005 m / 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 100-150 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 -1.5~-2.5mm, the position of the outlet roller is -3.5~-4.5mm, and the tilt value is set to 8~12mm.

5. The method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel 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 0.20μm.

6. The method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment according to claim 4, characterized in that, The isometric crystal ratio of the continuously cast billet is not less than 90%.

7. The method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel for engineering equipment according to claim 4, characterized in that, In the continuous casting sector, after cooling of sections 1-4, the temperature difference between the surface and the core of the solidified end of the continuous casting is >200℃, and after cooling of sections 5-8, the temperature difference between the surface and the core of the solidified end of the continuous casting is >500℃. The H content of the billet after slow cooling is not greater than 5ppm.

8. The method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel 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 60-100 μm.

9. The method for manufacturing wide-width, thin-gauge, high-flatness duplex stainless steel 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 wide-width, thin-gauge, high-flatness duplex stainless steel 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 35–70 μm and 50–85 μm, respectively; after three cold straightening processes, the cumulative strain of the steel plate at a distance of 15%–25% from the surface of the steel plate exceeds the elastic limit strain.

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