1000mpa grade extra-thick hydroelectric steel with low strain age sensitivity and preparation method thereof

Abstract

The application discloses a 1000MPa-grade extra-thick hydroelectric steel with low strain aging sensitivity, and chemical components and mass percentages of the steel include the following: C: 0.05%-0.13%, Si: 0.15%-0.35%, Mn: 0.8%-1.2%, Ni: 1.8%-2.2%, Cr: 0.3%-0.6%, Mo: 0.3%-0.6%, V: 0.03%-0.07%, Ti: 0.01%-0.03%, Nb: 0.02%-0.05%, Alt: 0.02-0.05%, S≤0.002%, P≤0.008%, N≤0.008%, and the rest is Fe and inevitable impurity elements. The application has the advantages that the steel plate can still maintain extremely low strain aging sensitivity after cold working deformation, and a stable dislocation network and nanometer precipitated phase are introduced by using a specific thermal mechanical treatment process.

Description

Technical Field

[0001] This invention belongs to the field of hydropower steel manufacturing technology, and particularly relates to a 1000MPa grade extra-thick hydropower steel with low strain aging sensitivity and its preparation method. Background Technology

[0002] As the heart of the power station, the water diversion system's key components, such as pressure steel pipes and branch pipes, must not only withstand enormous hydrostatic pressure but also resist the severe water hammer impact caused by frequent start-ups and shutdowns and load changes. To reduce pipe wall thickness, lighten the structure's weight, and simplify welding and installation, the use of 1000MPa-grade ultra-high-strength steel plates has become an inevitable trend. For some giant branch pipes operating under extreme conditions, the designed wall thickness even exceeds 120mm.

[0003] However, with the leap in steel plate strength, the material's toughness reserve inevitably decreases, while its sensitivity to strain aging significantly increases. During the heavy-duty assembly of extra-thick steel plates into pipe sections and branch pipes, the steel plate body and the heat-affected zone of the welded joints undergo 2%–5% deformation. For 1000MPa grade low-carbon bainitic / martensitic steel, this pre-strain generates extremely high dislocation density. During subsequent natural placement, solar heating, or welding thermal cycling, interstitial atoms such as C or N in the steel matrix possess extremely high diffusion driving forces, rapidly segregating around dislocation lines to form "Cochrie-laden atmospheres," strongly pinning dislocations. This microscopic "pinning effect" manifests macroscopically as "strain-aging embrittlement": the material's yield strength further increases, but its plasticity and toughness deteriorate sharply, and the brittle transition temperature rises. Because the circumferential stress on the surface of pressure steel pipes is the greatest during service, once severe strain-aging embrittlement occurs near the surface, cracks are very likely to initiate from the surface and rapidly propagate inward under low temperature environment or water hammer impact, causing catastrophic accidents.

[0004] Currently, existing technologies mainly focus on addressing the weld crack susceptibility issue of 1000MPa grade steel plates through low Pcm design, or ensuring "hardenability" of large thicknesses through alloying. While these technologies have enabled the engineering application of high-strength steel to some extent, they often neglect the deep-seated control of strain aging mechanisms. For 120mm thick 1000MPa grade steel plates produced using conventional quenching and tempering processes, the sharp deterioration of near-surface toughness after undergoing 5% engineering pre-strain and artificial aging has become a key bottleneck restricting the safe construction of next-generation ultra-large pumped storage power stations. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of severe strain-aging embrittlement caused by large surface deformation during cold rolling and service of existing 1000MPa grade extra-thick steel plates. It provides a 1000MPa grade extra-thick hydroelectric steel with low strain-aging sensitivity, which can maintain extremely low strain-aging sensitivity after cold working deformation and improve toughness retention rate. Starting from the physical metallurgical mechanism of strain aging, it "cuts off" the source of aging by maximally reducing the solid solution content of interstitial atoms C and N, and at the same time, it introduces a stable dislocation network and nano-precipitates by using a specific thermomechanical treatment process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity has the following chemical composition and mass percentage: C: 0.05%~0.13%, Si: 0.15%~0.35%, Mn: 0.8%~1.2%, Ni: 1.8%~2.2%, Cr: 0.3%~0.6%, Mo: 0.3%~0.6%, V: 0.03%~0.07%, Ti: 0.01%~0.03%, Nb: 0.02%~0.05%, Alt: 0.02~0.05%, S≤0.002%, P≤0.008%, N≤0.008%, with the remainder being Fe and unavoidable impurity elements.

[0007] Furthermore, the thickness of the extra-thick hydroelectric steel is 120mm, and its metallographic structure is fine tempered martensite.

[0008] To further achieve the objectives of this invention, a method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity is also provided, the specific steps of which include: S1, Clean steel smelting and calcium treatment: The process of converter smelting + LF refining + RH vacuum degassing is adopted to strictly control the gas content in the steel, H≤1.2ppm, N≤40ppm; after vacuum treatment, calcium wire is fed in for inclusion modification treatment, and Ca / S≥2.0 is controlled, followed by soft blowing of argon gas for ≥20min. S2, Continuous casting and slow cooling: The slab is cast into an extra-thick slab with a thickness of ≥350mm using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below; the slab is immediately slow-cooled after it comes off the production line, and the slow cooling time is ≥60h to eliminate thermal stress and fully diffuse hydrogen atoms. S3, Heating: Heat the slab to 1180~1240℃, with a soaking time ≥2.0min / mm, to ensure that the alloying elements are fully dissolved; S4, Differential Temperature Rolling and Relaxation: A two-stage rolling process is adopted. The first stage is high-reduction rolling in the austenite recrystallization zone, with a single-pass reduction rate of ≥15%. The second stage adopts differential temperature rolling technology. Rolling begins when the surface temperature of the steel plate drops to 820~860℃ and the core temperature is maintained above 900℃. The temperature gradient is used to allow deformation to penetrate into the core. After the second stage of rolling, a temperature-controlled relaxation is performed for 30~90s to promote the static recovery of austenite. S5, Online Quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate ≥20℃ / s and the reddening temperature controlled at 300~400℃; S6, Offline tempering: After the steel plate cools to room temperature, it is heated to 600~640℃ for high-temperature tempering, with a holding time ≥3.0min / mm, and then air-cooled after being taken out of the furnace.

[0009] Furthermore, the mechanical properties of the steel plate prepared in steps S1 to S6 are as follows: yield strength ≥ 950 MPa, tensile strength 1000~1150 MPa, elongation after fracture ≥ 16%, and transverse impact energy at -60℃ ≥ 180 J; after aging treatment at 5% strain + 250℃, the transverse impact energy at -60℃ ≥ 150 J, and the impact energy attenuation rate ≤ 20%.

[0010] The control mechanism for low strain aging sensitivity adopted in this invention is as follows: By using ultra-low N combined with Ti and Alt to fix free nitrogen atoms, N≤40ppm; and by using high Ni and Mo alloying to improve the tempering resistance of the matrix, high-temperature tempering processes above 600℃ are allowed, thereby eliminating dislocation stress fields to the greatest extent and reducing the pinning effect of interstitial C atoms on dislocations while maintaining a strength of 1000MPa.

[0011] Compared with the prior art, the advantages of the technical solution of the present invention are as follows: (1) This invention cuts off the aging source from the source and passivates the newly formed dislocations by combining the strategy of "ultra-low gap atomic control + high temperature tempering secondary hardening". In particular, it effectively suppresses the strong pinning of dislocations by the Cotillard atmosphere in the near-surface layer of the steel plate that bears the maximum plastic deformation. (2) After undergoing 5% engineering pre-strain and 250℃ artificial aging, the attenuation rate of the near-surface low-temperature impact energy at -60℃ is strictly controlled within 20%, the impact energy is ≥150J, and the attenuation rate is extremely low, which greatly improves the anti-brittle fracture ability of the pressure steel pipe during cold rolling and service process, and is suitable for the manufacture of pressure steel pipes for ultra-high head pumped storage power stations. (2) This invention is designed for 120mm thick hydropower steel of 1000 MPa grade. It adopts a composition design of low C, low N and high Ni-Mo, combined with calcium treatment, slab stacking slow cooling, differential rolling + relaxation, online quenching and high temperature tempering process. By controlling the multi-scale precipitated phase pinning dislocations and fixing interstitial atoms, the strain aging sensitivity of the material is significantly reduced. While ensuring the ultra-high strength of 1000MPa grade, the core has excellent low temperature toughness. (3) In response to the pain points of difficult core deformation and coarse microstructure of 1000MPa grade extra-thick plates, the present invention adopts the "differential temperature rolling + post-rolling relaxation" process. By utilizing the temperature gradient difference between the surface and the core, the rolling deformation is forced to penetrate into the core. Combined with the static recovery during the relaxation process to refine the substructure, it ensures that fine and uniform low-carbon tempered martensite microstructure is obtained throughout the 120mm thickness range, avoiding the appearance of coarse upper bainite or severe central segregation in the core, and achieving a high degree of consistency between the mechanical properties of the core and the surface of the extra-thick plate. Attached Figure Description

[0012] Figure 1 This is a SEM microstructure image of the extra-thick hydroelectric steel near the surface in Embodiment 1 of the present invention; Figure 2 This is a SEM microstructure image of the extra-thick hydroelectric steel at 1 / 4 thickness in Embodiment 1 of the present invention; Figure 3 This is a SEM microstructure image of the extra-thick hydroelectric steel at 1 / 2 thickness in Embodiment 1 of the present invention; Figure 4 This table shows the mechanical properties of steel plates with low weld crack sensitivity in Embodiments 1 to 4 of the present invention. Detailed Implementation Example 1

[0013] To make the present invention clearer, the following further describes a 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity and its preparation method. The specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0014] A 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity has the following chemical composition and mass percentage: C: 0.07%, Si: 0.25%, Mn: 1.1%, Ni: 2.2%, Cr: 0.5%, Mo: 0.55%, V: 0.06%, Ti: 0.015%, Nb: 0.035%, Alt: 0.035%, S: 0.001%, P: 0.005%, N: 0.003%, with the remainder being Fe and unavoidable impurity elements.

[0015] The preparation method of the above-mentioned 120mm extra-thick hydroelectric steel with low strain aging sensitivity for 1000MPa grade specifically includes the following steps: (1) Clean steel smelting and calcium treatment: The converter smelting + LF refining + RH vacuum degassing process is adopted to strictly control the gas content in the steel, H=0.8ppm, N=30ppm; after vacuum treatment, calcium wire is fed in for inclusion modification treatment, Ca / S=2.5, followed by soft blowing of argon gas for 25min. (2) Continuous casting and slow cooling: The 400mm thick slab is cast using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below. The slab is immediately slow cooled by stacking after it comes off the production line. The slow cooling time is 72h to eliminate thermal stress and fully diffuse hydrogen atoms. (3) Heating: Heat the slab to 1210℃ and soak it for 2.5 min / mm to ensure that the alloying elements are fully dissolved; (4) Differential temperature rolling and relaxation: A two-stage rolling process is adopted. In the first stage, high reduction rolling is carried out in the austenite recrystallization zone, with a single-pass reduction rate of 18%; in the second stage, differential temperature rolling technology is adopted, and rolling is started when the surface temperature of the steel plate drops to 840℃ and the core temperature is about 950℃, so that the deformation can penetrate into the core by utilizing the temperature gradient; after the second stage of rolling is completed, a temperature relaxation is carried out, with a relaxation time of 45s, to promote the static recovery of austenite; (5) Online quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate of 22℃ / s and the reddening temperature controlled at 360℃; (6) Offline tempering: After the steel plate is cooled to room temperature, it is heated to 620℃ for high-temperature tempering, and the holding time is 3.1min / mm. After being taken out of the furnace, it is air-cooled.

[0016] In this embodiment, the 120mm steel plate has a microstructure of fine tempered martensite. SEM micrographs of the microstructure at different thickness locations are shown below. Figures 1-3 As shown, its mechanical properties are as follows Figure 4 As shown. Example 2

[0017] A 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity has the following chemical composition and mass percentage: C: 0.05%, Si: 0.18%, Mn: 1.25%, Ni: 1.95%, Cr: 0.65%, Mo: 0.45%, V: 0.05%, Ti: 0.02%, Nb: 0.045%, Alt: 0.045%, S: 0.002%, P: 0.006%, N: 0.002%, with the remainder being Fe and unavoidable impurity elements.

[0018] The preparation method of the above-mentioned 120mm extra-thick hydroelectric steel with low strain aging sensitivity for 1000MPa grade specifically includes the following steps: (1) Clean steel smelting and calcium treatment: The converter smelting + LF refining + RH vacuum degassing process is adopted to strictly control the gas content in the steel, H=0.9ppm, N=30ppm; after vacuum treatment, calcium wire is fed in for inclusion modification treatment, Ca / S=3.0, followed by soft blowing of argon gas for 30min. (2) Continuous casting and slow cooling: The 410mm thick slab is cast using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below. The slab is immediately slow cooled by stacking after it comes off the production line. The slow cooling time is 65h to eliminate thermal stress and fully diffuse hydrogen atoms. (3) Heating: Heat the slab to 1190℃ and soak it for 3.0 min / mm to ensure that the alloying elements are fully dissolved; (4) Differential temperature rolling and relaxation: A two-stage rolling process is adopted. In the first stage, high reduction rolling is carried out in the austenite recrystallization zone, with a single-pass reduction rate of 16%; in the second stage, differential temperature rolling technology is adopted, and rolling is started when the surface temperature of the steel plate drops to 830°C and the core temperature is about 940°C, so that the deformation can penetrate into the core by utilizing the temperature gradient; after the second stage of rolling is completed, a temperature relaxation is carried out, with a relaxation time of 60s, to promote the static recovery of austenite; (5) Online quenching: After relaxation, a high-strength online quenching system is used for cooling at a rate of 25℃ / s and the reddening temperature is controlled at 340℃; (6) Offline tempering: After the steel plate is cooled to room temperature, it is heated to 605℃ for high-temperature tempering, and the holding time is 3.5min / mm. After being taken out of the furnace, it is air-cooled.

[0019] In this embodiment, the 120mm steel plate has a microstructure of fine tempered martensite, and its mechanical properties are as follows: Figure 4 As shown. Example 3

[0020] A 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity has the following chemical composition and mass percentage: C: 0.08%, Si: 0.3%, Mn: 0.95%, Ni: 2.05%, Cr: 0.5%, Mo: 0.35%, V: 0.07%, Ti: 0.012%, Nb: 0.025%, Alt: 0.025%, S: 0.001%, P: 0.004%, N: 0.004%, with the remainder being Fe and unavoidable impurity elements.

[0021] The preparation method of the above-mentioned 120mm extra-thick hydroelectric steel with low strain aging sensitivity for 1000MPa grade specifically includes the following steps: (1) Clean steel smelting and calcium treatment: The converter smelting + LF refining + RH vacuum degassing process is adopted to strictly control the gas content in the steel, H=1.1ppm, N=25ppm; after vacuum treatment, calcium wire is fed in for inclusion modification treatment, Ca / S=2.0 is controlled, followed by soft blowing of argon gas for 35min. (2) Continuous casting and slow cooling: The slab is cast into an extra-thick slab with a thickness of 370mm using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below. The slab is immediately slow-cooled after it comes off the production line, and the slow cooling time is 80h to eliminate thermal stress and fully diffuse hydrogen atoms. (3) Heating: Heat the slab to 1230℃ and soak it for 2.2 min / mm to ensure that the alloying elements are fully dissolved; (4) Differential temperature rolling and relaxation: A two-stage rolling process is adopted. In the first stage, high reduction rolling is carried out in the austenite recrystallization zone, with a single-pass reduction rate of 20%; in the second stage, differential temperature rolling technology is adopted, and rolling is started when the surface temperature of the steel plate drops to 850°C and the core temperature is about 960°C, so that the deformation can penetrate into the core by utilizing the temperature gradient; after the second stage of rolling is completed, a temperature relaxation is carried out for 30s to promote the static recovery of austenite; (5) Online quenching: After relaxation, a high-strength online quenching system is used for cooling at a rate of 20℃ / s and the reddening temperature is controlled at 380℃; (6) Offline tempering: After the steel plate is cooled to room temperature, it is heated to 635℃ for high-temperature tempering, and the holding time is 2.8min / mm. After being taken out of the furnace, it is air-cooled.

[0022] In this embodiment, the 120mm steel plate has a microstructure of fine tempered martensite, and its mechanical properties are as follows: Figure 4 As shown.

[0023] This invention is based on "interstitial atom source blocking + thermomechanical treatment to passivate dislocations". Unlike traditional technologies that only focus on reducing the amount of components to improve weldability, this invention starts from the physical metallurgical mechanism of strain aging. It "cuts off" the aging source by maximally reducing the amount of solid solution of interstitial atoms. At the same time, it uses a specific thermomechanical treatment process to introduce a stable dislocation network and nano-precipitates, so that the extra-thick hydroelectric steel plate can still maintain extremely low strain aging sensitivity after undergoing cold working deformation.

[0024] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity, characterized in that: The chemical composition and mass percentage of the hydroelectric steel include: C: 0.05%~0.13%, Si: 0.15%~0.35%, Mn: 0.8%~1.2%, Ni: 1.8%~2.2%, Cr: 0.3%~0.6%, Mo: 0.3%~0.6%, V: 0.03%~0.07%, Ti: 0.01%~0.03%, Nb: 0.02%~0.05%, Alt: 0.02~0.05%, S≤0.002%, P≤0.008%, N≤0.008%, with the remainder being Fe and unavoidable impurity elements.

2. The 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 1, characterized in that: The chemical composition and mass percentage of the hydroelectric steel include: C: 0.07%, Si: 0.25%, Mn: 1.1%, Ni: 2.2%, Cr: 0.5%, Mo: 0.55%, V: 0.06%, Ti: 0.015%, Nb: 0.035%, Alt: 0.035%, S: 0.001%, P: 0.005%, N: 0.003%, with the remainder being Fe and unavoidable impurity elements.

3. The 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 1, characterized in that: The chemical composition and mass percentage of the hydroelectric steel include: C: 0.05%, Si: 0.18%, Mn: 1.25%, Ni: 1.95%, Cr: 0.65%, Mo: 0.45%, V: 0.05%, Ti: 0.02%, Nb: 0.045%, Alt: 0.045%, S: 0.002%, P: 0.006%, N: 0.002%, with the remainder being Fe and unavoidable impurity elements.

4. The 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 1, characterized in that: The chemical composition and mass percentage of the hydroelectric steel include: C: 0.08%, Si: 0.3%, Mn: 0.95%, Ni: 2.05%, Cr: 0.5%, Mo: 0.35%, V: 0.07%, Ti: 0.012%, Nb: 0.025%, Alt: 0.025%, S: 0.001%, P: 0.004%, N: 0.004%, with the remainder being Fe and unavoidable impurity elements.

5. The 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to any one of claims 1 to 4, characterized in that: The extra-thick hydroelectric steel has a thickness of 120mm and its metallographic structure consists of fine tempered martensite.

6. A method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity as described in claim 1, characterized in that: S1, Clean steel smelting and calcium treatment: The process of converter smelting + LF refining + RH vacuum degassing is adopted to strictly control H in steel ≤ 1.2ppm and N ≤ 40ppm; after vacuum treatment, calcium wire is fed in for inclusion modification treatment, and Ca / S ≥ 2.0 is controlled, followed by soft blowing of argon gas ≥ 20min. S2, Continuous casting and slow cooling: The slab is cast into an extra-thick slab with a thickness of ≥350mm using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below; the slab is immediately stacked and slow cooled after it comes off the production line, and the slow cooling time is ≥60h. S3, Heating: Heat the slab to 1180~1240℃, with a soaking time ≥2.0min / mm; S4, Differential Temperature Rolling and Relaxation: A two-stage rolling process is adopted. The first stage is high-reduction rolling in the austenite recrystallization zone, with a single-pass reduction rate of ≥15%. The second stage adopts differential temperature rolling technology. Rolling begins when the surface temperature of the steel plate drops to 820~860℃ and the core temperature is maintained above 900℃. The temperature gradient is used to allow deformation to penetrate to the core. After the second stage of rolling, a temperature relaxation is performed for 30~90s. S5, Online Quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate ≥20℃ / s and the reddening temperature controlled at 300~400℃; S6, Offline tempering: After the steel plate cools to room temperature, it is heated to 600~640℃ for high-temperature tempering, with a holding time ≥3.0min / mm, and then air-cooled after being taken out of the furnace.

7. The method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 6, characterized in that: S1, Clean steel smelting and calcium treatment: The process of converter smelting + LF refining + RH vacuum degassing is adopted to control H=0.8ppm and N=30ppm in steel; after vacuum treatment, calcium wire is fed in for inclusion modification treatment to control Ca / S=2.5, followed by soft blowing of argon gas for 25min. S2, Continuous casting and slow cooling: The 400mm thick slab is cast using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below; the slab is immediately slow-cooled after it comes off the production line, and the slow cooling time is 72 hours. S3, Heating: Heat the slab to 1210℃, with a soaking time of 2.5 min / mm; S4, Differential Temperature Rolling and Relaxation: A two-stage rolling process is adopted. The first stage is high-reduction rolling in the austenite recrystallization zone, with a single-pass reduction rate of 18%. The second stage adopts differential temperature rolling technology. Rolling starts when the surface temperature of the steel plate drops to 840°C and the core temperature is about 950°C. The temperature gradient is used to allow deformation to penetrate into the core. After the second stage of rolling is completed, a temperature relaxation is performed for 45 seconds. S5, Online Quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate of 22℃ / s and the reddening temperature controlled at 360℃; S6, Offline Tempering: After the steel plate cools to room temperature, it is heated to 620℃ for high-temperature tempering, with a holding time of 3.1 min / mm, and then air-cooled after being taken out of the furnace.

8. The method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 6, characterized in that: S1, Clean steel smelting and calcium treatment: The process of converter smelting + LF refining + RH vacuum degassing is adopted to control H=0.9ppm and N=30ppm in steel; after vacuum treatment, calcium wire is fed in for inclusion modification treatment to control Ca / S=3.0, followed by soft blowing of argon gas for 30min. S2, Continuous casting and slow cooling: The 410mm thick slab is cast using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below; the slab is immediately slow-cooled after it comes off the production line, and the slow cooling time is 65 hours. S3, Heating: Heat the slab to 1190℃, with a soaking time of 3.0 min / mm; S4, Differential Temperature Rolling and Relaxation: A two-stage rolling process is adopted. The first stage is high-reduction rolling in the austenite recrystallization zone, with a single-pass reduction rate of 16%. The second stage adopts differential temperature rolling technology. Rolling starts when the surface temperature of the steel plate drops to 830°C and the core temperature is about 940°C. The temperature gradient is used to allow deformation to penetrate into the core. After the second stage of rolling is completed, a temperature relaxation is performed for 60 seconds. S5, Online Quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate of 25℃ / s and the reddening temperature controlled at 340℃; S6, Offline Tempering: After the steel plate cools to room temperature, it is heated to 605℃ for high-temperature tempering, with a holding time of 3.5min / mm, and then air-cooled after being taken out of the furnace.

9. The method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to claim 6, characterized in that: S1, Clean steel smelting and calcium treatment: The process of converter smelting + LF refining + RH vacuum degassing is adopted to control H=1.1ppm and N=25ppm in the steel; after vacuum treatment, calcium wire is fed in for inclusion modification treatment to control Ca / S=2.0, followed by soft blowing of argon gas for 35min. S2, Continuous casting and slow cooling: The slab is cast into an extra-thick slab with a thickness of 370mm using dynamic light reduction technology, and the center segregation is controlled to Class C 1.0 and below; the slab is immediately slow-cooled after it comes off the production line, and the slow cooling time is 80h. S3, Heating: Heat the slab to 1230℃, with a soaking time of 2.2 min / mm; S4, Differential Temperature Rolling and Relaxation: A two-stage rolling process is adopted. The first stage is high-reduction rolling in the austenite recrystallization zone, with a single-pass reduction rate of 20%. The second stage adopts differential temperature rolling technology. Rolling starts when the surface temperature of the steel plate drops to 850°C and the core temperature is about 960°C. The temperature gradient is used to allow deformation to penetrate into the core. After the second stage of rolling is completed, a temperature relaxation is performed for 30 seconds. S5, Online Quenching: After relaxation, a high-strength online quenching system is used for cooling, with a cooling rate of 20℃ / s and the reddening temperature controlled at 380℃; S6, Offline Tempering: After the steel plate cools to room temperature, it is heated to 635℃ for high-temperature tempering, with a holding time of 2.8 min / mm, and then air-cooled after being taken out of the furnace.

10. The method for preparing 1000MPa grade extra-thick hydroelectric steel with low strain aging sensitivity according to any one of claims 6 to 9, characterized in that: The mechanical properties of the steel plates prepared in steps S1 to S6 are as follows: yield strength ≥ 950 MPa, tensile strength 1000~1150 MPa, elongation after fracture ≥ 16%, and transverse impact energy at -60℃ ≥ 180 J; after aging treatment at 5% strain + 250℃, the transverse impact energy at -60℃ ≥ 150 J, and the impact energy attenuation rate ≤ 20%.

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