1000MPa-grade high-strength steel for pressure steel pipe of hydropower station and production method of 1000MPa-grade high-strength steel

Abstract

The high-grade steel comprises the following chemical components in percentage by weight: 0.039 to 0.059 percent of C, 1.20 to 1.60 percent of Mn, less than or equal to 0.15 percent of Si, less than or equal to 0.015 percent of P, less than or equal to 0.003 percent of S, 0.040 to 0.060 percent of Nb, 0.030 to 0.060 percent of V, 0.010 to 0.020 percent of Ti, 0.001 to 0.003 percent of B, 0.010 to 0.060 percent of Alt, 1.20 to 2.00 percent of Ni, 0.30 to 0.60 percent of Cr, 0.40 to 0.70 percent of Mo, less than or equal to 0.55 percent of Ceq, less than or equal to 0.22 percent of Pcm and the balance of Fe and impurities. The ultra-low carbon C is smaller than 0.06%, the ultra-low welding crack sensitivity coefficient Pcm is smaller than or equal to 0.22%, Nb + V + Ti microalloying and Ni + Cr + Mo alloying design are adopted, Ti + B elements are adopted for improving the hardenability of the steel plate, the technologies of rolling under large pressure, low-temperature controlled rolling, ultra-fast cooling of the rolled steel plate and the like are adopted in the controlled rolling and controlled cooling process, and the properties that the yield strength is larger than or equal to 890 MPa, the tensile strength is 950-1100 MPa, and the transverse low-temperature impact energy of the steel plate at the temperature of 60 DEG C below zero is larger than or equal to 150 J are achieved.

Description

Technical Field

[0001] This invention relates to a steel plate and its production method, specifically to a 1000MPa grade high-strength steel for hydropower station pressure steel pipes and its production method, belonging to the field of metallurgical technology. Background Technology

[0002] Under the new circumstances of accelerating the low-carbon transformation and green development of energy, renewable energy is the main force in building a new power system, with wind and solar power developing on a large scale and in a high proportion. Based on the national strategy of serving the hydropower development in the lower reaches of the Yarlung Tsangpo River and the urgent need for pumped storage power station construction, in order to improve power generation efficiency, the design of generator units in the hydropower industry is developing towards high head (HD), high speed, high efficiency, and large capacity. Large-scale hydropower projects are increasingly using 800MPa grade low-weld-crack-sensitivity quenched and tempered high-strength steel plates. To reduce the wall thickness of pressure steel pipes, volutes, and branch pipes, and to reduce construction and welding difficulties, the research and engineering application of higher-strength steel for large-scale hydropower projects is imminent. 1000MPa grade high-strength hydropower steel plates have a yield strength ≥890MPa, a tensile strength of 950~1100MPa, and a transverse low-temperature impact energy ≥70J at -60℃. However, to ensure high strength and toughness in steel plates, designs with a carbon content ≥0.08% and high alloy composition are generally used, along with quenching + tempering or online quenching + tempering heat treatment processes. This results in a welding cold crack sensitivity coefficient (Pcm) of 0.25%-0.27%, severely impacting the welding quality and efficiency of the steel plates. To achieve better weldability, methods that reduce carbon content are also employed, but these methods suffer from complex heat treatment processes and high alloy content.

[0003] CN119663121A discloses "a 1000MPa grade hydroelectric steel with optimized low-temperature toughness and its preparation method. The composition design of this invention is C: 0.03~0.09%. Although it adopts low carbon composition and low alloy content, the production adopts a double quenching + tempering heat treatment process, including first quenching, second quenching and tempering, with many production steps and complex production process."

[0004] CN119843166A discloses "a 1000MPa tensile strength grade hydroelectric steel plate and its preparation method". The composition of this invention is designed with C≤0.08%, but the Ni content is 2.50-3.00%, resulting in high alloy cost. At the same time, the steel plate needs to be relaxed and air-cooled online after rolling, which results in a slow production pace and high requirements for production process equipment. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a 1000MPa grade high-strength steel for hydropower station pressure steel pipes and its production method.

[0006] To solve the above technical problems, the present invention provides a 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations. The high-strength steel, by weight percentage, comprises: C 0.039-0.059%, Mn 1.20-1.60%, Si≤0.15%, P≤0.015%, S≤0.003%, Nb 0.040-0.060%, V 0.030-0.060%, Ti 0.010-0.020%, B 0.001-0.003%, Alt 0.010-0.060%, Ni 1.20-2.00%, Cr 0.30-0.60%, Mo 0.40-0.70%, with the remainder being Fe and impurities.

[0007] C is the second most important element after iron, and it directly affects the strength, plasticity, toughness and weldability of steel. To reduce the sensitivity of steel to welding cracks, the C content in steel is generally controlled to not exceed 0.12%. Reducing the C content to below 0.06% can significantly improve the low-temperature toughness and weldability of steel. Si is an important reducing agent and deoxidizer in the steelmaking process. Si can dissolve in ferrite and austenite to increase the hardness and strength of steel. However, a high Si content will reduce the low-temperature toughness and weldability of steel. Generally, the Si content is required to be no more than 0.30%. Mn: Improves the strength of steel. Because Mn is relatively inexpensive and can be infinitely dissolved in Fe, it can improve the strength of steel while having a relatively small impact on plasticity. Therefore, Mn is widely used as a strengthening element in steel. Al: When added to steel as a deoxidizer or alloying element, Al's deoxidizing ability is much stronger than that of silicon and manganese. The main role of Al in steel is to refine grains and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing its tendency to brittleness and aging. Niobium (Nb) partially dissolves into the solid solution, acting as a solid solution strengthening agent. When dissolved in austenite, it significantly improves the hardenability of steel. Trace amounts of niobium can increase the strength of steel without affecting its plasticity or toughness. Due to its grain-refining effect, it can improve the impact toughness of steel and lower its brittle transition temperature, making it widely used in hot-rolled steel sheets. V: It mainly exists in the form of carbides in steel. In quenched and tempered steel, it mainly refines the grains, improves the strength and yield ratio of steel, and increases the tempering stability of quenched steel. Ti has a strong affinity for nitrogen, oxygen, and carbon, and a stronger affinity for sulfur than iron. It is an excellent deoxidizer and degassing agent, as well as an effective element for fixing nitrogen and carbon. Adding trace amounts of Ti can improve the strength of steel, and the alloy cost is very low. However, higher Ti levels will reduce the low-temperature toughness of steel. Generally, the Ti content should not exceed 0.050% for low-temperature toughness at -40℃ and below. Ni: Its lattice constant is similar to that of γ-iron, so it can form a continuous solid solution. This is beneficial for improving the hardenability of steel. Ni can lower the critical point and increase the stability of austenite. Generally, Ni-added steel is used for impact requirements at -60℃ and below. On the one hand, it greatly improves the strength of steel, and on the other hand, it always keeps the toughness of iron at an extremely high level. Cr: It can increase the hardenability of steel and has the effect of secondary hardening. Its main role in quenching and tempering is to improve hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering. Mo: In steel, molybdenum can improve hardenability and hot strength, prevent temper brittleness, improve the tempering resistance or tempering stability of steel, and allow parts to be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and improving plasticity. B: Its main function in steel is to increase the hardenability of steel, thereby saving other rarer and more expensive metals. It is used together with nickel, chromium, molybdenum, etc., and its content is generally specified in the range of 0.001% to 0.005%.

[0008] The technical solution further defined in this invention is: Furthermore, the chemical composition of the high-grade steel also includes satisfying Ceq≤0.55% and Pcm≤0.22%.

[0009] Furthermore, the high-strength steel has a yield strength ≥890MPa, a tensile strength of 950~1100MPa, a transverse low-temperature impact energy of -60℃ ≥150J, and a maximum thickness of 60mm.

[0010] This invention provides a method for producing 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations, characterized by the following steps: (1) Steelmaking process: molten steel is smelted according to the chemical composition described in claim 1 and continuously cast into slabs. The continuous casting process adopts dynamic light reduction technology, and the center segregation of the slab is C≤1.0 grade; (2) Heating process: The billet is heated in a heating furnace with a heating coefficient of 10.0-14.0 min / cm and a heating temperature of 1180-1220℃; (3) Rolling process: A two-stage controlled rolling process is adopted; (4) Cooling process: After rolling, the steel plate is cooled in an ultra-fast cooling system with a cooling rate of 10-20℃ / s and a reddening temperature of 250-350℃.

[0011] Furthermore, in the steelmaking process, when producing high-strength steel with a thickness of 20mm or 40mm, the thickness of the continuously cast slab is 260mm; when producing high-strength steel with a thickness of 60mm, the thickness of the continuously cast slab is 320mm.

[0012] Furthermore, in the rolling process, the reduction rate of the last two passes of the first stage rough rolling is ≥22%, and the rolling end temperature is ≥1000℃; the starting rolling temperature of the second stage is 800℃-850℃, and the finishing rolling temperature is 720℃-780℃.

[0013] Furthermore, the controlled temperature for re-heating after cooling is 250-350℃; when producing high-strength steel with a thickness of 20mm, the controlled temperature for re-heating after cooling is 320-350℃; when producing high-strength steel with a thickness of 40mm, the controlled temperature for re-heating after cooling is 280-320℃; and when producing high-strength steel with a thickness of 60mm, the controlled temperature for re-heating after cooling is 250-280℃.

[0014] The beneficial effects of this invention are: This invention employs ultra-low carbon (C < 0.06%) and ultra-low welding crack sensitivity coefficient (Pcm ≤ 0.22%), Nb+V+Ti microalloying, Ni+Cr+Mo alloying design, and Ti+B elements to improve the hardenability of the steel plate. The controlled rolling and cooling process utilizes technologies such as high rolling reduction, low-temperature controlled rolling, and ultra-rapid cooling of the rolled steel plate. This has resulted in the development of a 1000MPa-grade high-strength hydropower steel plate with a maximum thickness of 60mm. The plate exhibits a yield strength ≥ 890MPa, tensile strength 950–1100MPa, and a transverse low-temperature impact energy ≥ 150J at -60℃. Because the welding cold crack sensitivity coefficient (Pcm) is ≤ 0.22%, compared to 0.25-0.27% for existing 1000MPa-grade high-strength hydropower steel plates, it enables low-temperature preheating welding of ultra-high-strength steel plates, improving welding quality and production efficiency at large-scale hydropower project construction sites.

[0015] In the continuous casting stage, the invention employs dynamic light reduction to control center segregation to ≤C 1.0 grade, solving the problem of central porosity in thick plates. In the rolling stage, the austenite grains are refined by using a high reduction rate (≥22%) in the last two passes of rough rolling, combined with low-temperature finishing rolling (720-780℃) to control the phase transformation structure. In the cooling stage, an ultra-fast cooling rate of 10-20℃ / s and a reddening temperature of 250-350℃ are used to achieve a uniform distribution of martensite + bainite composite structure, balancing strength and toughness. Detailed Implementation

[0016] To make the content of this invention easier to understand, the invention will be further described in detail below based on specific embodiments. Example 1

[0017] This embodiment provides a 1000MPa grade high-strength steel for hydropower station pressure steel pipes, with a thickness of 20mm. The chemical composition of this high-grade steel, by weight percentage, includes: C: 0.043%, Mn: 1.59%, Si: 0.08%, P: 0.012%, S: 0.002%, Nb: 0.058%, V: 0.051%, Ti: 0.013%, B: 0.0015%, Alt: 0.018%, Ni: 1.25%, Cr: 0.39%, Mo: 0.48%, satisfying Ceq: 0.54%, Pcm: 0.21%, with the remainder being Fe and impurities.

[0018] This embodiment provides a method for producing 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations, specifically including the following steps: (1) Steelmaking process: Smelting according to the composition in Table 1, the continuous casting process adopts dynamic light reduction technology, and the casting is a 260mm continuous casting slab with C 1.0 grade segregation in the center of the slab; (2) Heating process: The heating process for the steel billet has a heating coefficient of 10.5 min / cm and a heating temperature of 1207℃; (3) Rolling process: A two-stage controlled rolling process is adopted. In the first stage, the reduction rate of the last two passes is 26% and 25%, and the rolling end temperature is 1008℃. In the second stage, the starting rolling temperature is 847℃ and the ending rolling temperature is 776℃. (4) Cooling process: The rolled steel plate is rapidly cooled in an ultra-fast cooling device at a rate of 19℃ / s and a reddening temperature of 336℃. Example 2

[0019] This embodiment provides a 1000MPa grade high-strength steel for hydropower station pressure steel pipes, with a thickness of 40mm. The chemical composition of this high-grade steel, by weight percentage, includes: C: 0.050%, Mn: 1.55%, Si: 0.09%, P: 0.009%, S: 0.002%, Nb: 0.053%, V: 0.055%, Ti: 0.015%, B: 0.0013%, Alt: 0.032%, Ni: 1.51%, Cr: 0.42%, Mo: 0.45%, satisfying Ceq: 0.55%, Pcm: 0.22%, with the remainder being Fe and impurities.

[0020] This embodiment provides a method for producing 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations, specifically including the following steps: (1) Steelmaking process: Smelting according to the composition in Table 1, the continuous casting process adopts dynamic light reduction technology, and the casting is a 260mm continuous casting slab with C 1.0 grade segregation in the center of the slab; (2) Heating process: The heating process for the steel billet has a heating coefficient of 10.9 min / cm and a heating temperature of 1198℃; (3) Rolling process: A two-stage controlled rolling process is adopted. In the first stage, the reduction rate of the last two passes is 25% and 23%, and the rolling end temperature is 1013℃. In the second stage, the starting rolling temperature is 827℃ and the ending rolling temperature is 764℃. (4) Cooling process: The rolled steel plate is rapidly cooled in an ultra-fast cooling device at a rate of 16℃ / s and a reddening temperature of 311℃. Example 3

[0021] This embodiment provides a 1000MPa grade high-strength steel for hydropower station pressure steel pipes, with a thickness of 60mm. The chemical composition of this high-grade steel, by weight percentage, includes: C: 0.055%, Mn: 1.48%, Si: 0.11%, P: 0.007%, S: 0.001%, Nb: 0.049%, V: 0.047%, Ti: 0.011%, B: 0.0012%, Alt: 0.04%, Ni: 1.43%, Cr: 0.45%, Mo: 0.43%, satisfying Ceq: 0.54%, Pcm: 0.22%, with the remainder being Fe and impurities.

[0022] This embodiment provides a method for producing 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations, specifically including the following steps: (1) Steelmaking process: Smelting according to the composition in Table 1, the continuous casting process adopts dynamic light reduction technology, and the casting is a 320mm continuous casting slab with C 1.0 grade segregation in the center of the slab; (2) Heating process: The heating process for the steel billet has a heating coefficient of 11.3 min / cm and a heating temperature of 1186℃; (3) Rolling process: A two-stage controlled rolling process is adopted. In the first stage, the reduction rate of the last two passes is 24% and 23%, and the rolling end temperature is 1023℃. In the second stage, the starting rolling temperature is 817℃ and the finishing rolling temperature is 736℃. (4) Cooling process: The rolled steel plate is rapidly cooled in an ultra-fast cooling device at a rate of 13℃ / s and a reddening temperature of 287℃.

[0023] The mechanical properties of the steel plates in Examples 1-3 are shown in Table 1; Table 1

[0024] As shown in Table 1, the 1000MPa grade high-strength steel used in the 60mm and below thickness hydropower station pressure steel pipes of the present invention has a yield strength of 933-998MPa, a tensile strength of 958-1011MPa, an elongation after fracture of 16%-17.5%, and an average transverse impact energy of 194J-253J at -60℃.

[0025] 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 type of 1000MPa grade high-strength steel for pressure steel pipes in hydropower stations, characterized in that, The chemical composition of this high-grade steel, by weight percentage, includes: C 0.039-0.059%, Mn 1.20-1.60%, Si≤0.15%, P≤0.015%, S≤0.003%, Nb 0.040-0.060%, V 0.030-0.060%, Ti 0.010-0.020%, B 0.001-0.003%, Alt 0.010-0.060%, Ni 1.20-2.00%, Cr 0.30-0.60%, Mo 0.40-0.70%, with the remainder being Fe and impurities.

2. The 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to claim 1, characterized in that: The chemical composition of the high-grade steel also includes Ceq≤0.55% and Pcm≤0.22%.

3. The 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to claim 1, characterized in that: The high-strength steel has a yield strength ≥890MPa, a tensile strength of 950~1100MPa, a transverse low-temperature impact energy of -60℃ ≥150J, and a maximum thickness of 60mm.

4. A method for producing 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to any one of claims 1-3, characterized in that: Specifically, the following steps are included: (1) Steelmaking process: molten steel is smelted according to the chemical composition described in claim 1 and continuously cast into slabs. The continuous casting process adopts dynamic light reduction technology, and the center segregation of the slab is C≤1.0 grade; (2) Heating process: The billet is heated in a heating furnace with a heating coefficient of 10.0-14.0 min / cm and a heating temperature of 1180-1220℃; (3) Rolling process: A two-stage controlled rolling process is adopted; (4) Cooling process: After rolling, the steel plate is cooled in an ultra-fast cooling system with a cooling rate of 10-20℃ / s and a reddening temperature of 250-350℃.

5. The method for producing 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to claim 1, characterized in that: In the steelmaking process, when producing high-strength steel with a thickness of 20mm or 40mm, the thickness of the continuously cast slab is 260mm; when producing high-strength steel with a thickness of 60mm, the thickness of the continuously cast slab is 320mm.

6. The method for producing 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to claim 3, characterized in that: In the rolling process, the reduction rate of the last two passes of the first stage rough rolling is ≥22%, and the rolling end temperature is ≥1000℃; the starting rolling temperature of the second stage is 800℃-850℃, and the finishing rolling temperature is 720℃-780℃.

7. The method for producing 1000MPa grade high-strength steel for hydropower station pressure steel pipes according to claim 3, characterized in that: The controlled temperature for re-heating after cooling is 250-350℃; when producing high-strength steel with a thickness of 20mm, the controlled temperature for re-heating after cooling is 320-350℃; when producing high-strength steel with a thickness of 40mm, the controlled temperature for re-heating after cooling is 280-320℃; and when producing high-strength steel with a thickness of 60mm, the controlled temperature for re-heating after cooling is 250-280℃.

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