Ultra-high-strength steel plate for hydropower applications and production method therefor

Abstract

An ultra-high-strength steel plate for hydropower applications, the chemical composition thereof, in percent by weight, being: 0.10-0.14% of C, 0.90-1.20% of Mn, 0.10-0.50% of Si, ≤ 0.012% of P, ≤ 0.003% of S, 0.020-0.050% of Nb, 0.040-0.060% of V, 0.015-0.025% of Ti, 0.001-0.003% of B, 0.010-0.060% of Al, 1.55-2.55% of Ni, 0.40-0.60% of Cr, 0.40-0.70% of Mo, ≤ 0.28% of Pcm, 0.025% ≤ Ti + 10*B ≤ 0.035%, and the balance being Fe and impurities. The steel plate has a thickness of up to 60 mm, yield strength ≥ 960 MPa, tensile strength ≥1040 MPa, excellent low-temperature impact toughness and welding performance at -60°C, and high production efficiency.

Description

Ultra-high strength steel plate for hydropower and production method thereof Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and in particular relates to an ultra-high strength steel plate for hydropower and a production method thereof. Background Art

[0002] Pumped hydropower storage, as the most technologically mature, economically viable, and scalable green, low-carbon, and flexible power source with the longest lifecycle, is a key means of integrating large-scale renewable energy into the grid. To improve power generation efficiency, generator set designs within the hydropower industry are evolving toward high head, high speed, high efficiency, and large capacity. Large hydropower projects are increasingly utilizing quenched and tempered high-strength steel plates with low weld crack sensitivity, including 690MPa yield strength and 800MPa tensile strength, and 890MPa yield strength and 1000MPa tensile strength. To reduce the wall thickness of penstocks, volutes, and bifurcations, and to ease construction and welding challenges, research and application of higher-strength steel for large hydropower projects is urgently needed. The yield strength of ultra-high-strength steel plates for hydropower use is ≥960MPa, the tensile strength is 1040~1200MPa, and the transverse low-temperature impact energy of the steel plates at -60℃ is ≥120J. Since the cold crack sensitivity coefficient Pcm of the steel plate welding is ≤0.28%, low-preheat welding of ultra-high-strength steel plates can be achieved, thereby improving the production efficiency at the construction site of large-scale hydropower projects.

[0003] Existing national and industry standards for hydropower steel plate materials have a maximum strength rating of 690 MPa yield strength and 800 MPa tensile strength, which has been widely used in projects. Group and enterprise standards have a maximum strength rating of 890 MPa yield strength and 1000 MPa tensile strength, and major companies are actively promoting their use in projects. However, material standards currently do not include ultra-high-strength hydropower steel plates with yield strengths ≥960 MPa and tensile strengths between 1040 and 1200 MPa.

[0004] Currently, the highest strength grade for steel plates in domestic and international hydropower steel standards is 690MPa yield strength and 800MPa tensile strength. Hydropower steel with a yield strength of 890MPa and a tensile strength of 1000MPa has been successfully developed and is currently undergoing engineering deployment. However, the higher yield strength of 960MPa and 1040MPa is not specified in material standards. When using the current highest yield strength of 890MPa and 1000MPa steel plates, the maximum wall thickness of the penstock exceeds 60mm. This increased thickness introduces a host of challenges in fabrication, such as rolling, welding, non-destructive testing, transportation, and installation. To reduce the wall thickness of the penstock, volute, and bifurcated pipes, and to ease construction and welding challenges, research and application of higher-strength steel for large-scale hydropower projects is urgently needed.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to solve the problems of the inability to balance the strength and thickness of existing steel plates, the large wall thickness of steel pipes and the high difficulty of construction welding. The present invention provides an ultra-high strength steel plate for hydropower, the maximum thickness of the steel plate can reach 60mm, the performance reaches a yield strength ≥960MPa, a tensile strength of 1040~1200MPa, and the transverse low-temperature impact energy of the steel plate at -60℃ ≥120J. Since the cold crack sensitivity coefficient Pcm of the steel plate welding is ≤0.28%, the low-preheat welding of ultra-high strength steel plates can be realized, thereby improving the production efficiency of the construction site of large-scale hydropower projects.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] Disclosed is an ultra-high-strength steel plate for hydropower use, the chemical composition of which is, by weight percentage, 0.10-0.14% of C, 0.90-1.20% of Mn, 0.10-0.50% of Si, P≤0.012%, S≤0.003%, 0.020-0.050% of Nb, 0.040-0.060% of V, 0.015-0.025% of Ti, 0.001-0.003% of B, 0.010-0.060% of Al, 1.55-2.55% of Ni, 0.40-0.60% of Cr, 0.40-0.70% of Mo, Pcm≤0.28%, wherein 0.025%≤Ti+10*B≤0.035%, and the remainder is Fe and impurities.

[0009] Furthermore, the thickness of the ultra-high strength steel plate for hydropower is 25 mm to 60 mm.

[0010] The reasons for limiting the chemical composition of the present invention are as follows:

[0011] C is the main element after iron, which directly affects the strength, plasticity, toughness and welding performance of steel. In order to reduce the welding crack sensitivity of steel, the C content in steel is generally controlled not to exceed 0.14%. Reducing the C content to below 0.12% can further improve the low-temperature toughness of steel.

[0012] 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, but a higher Si content will reduce the welding performance of steel. Generally, the Si content is required to not exceed 0.50%.

[0013] Mn can improve the strength of steel. Since Mn is relatively cheap and can be infinitely dissolved in Fe, it has a relatively small effect on the plasticity while improving the strength of steel. Therefore, Mn is widely used as a strengthening element in steel.

[0014] Al is added to steel as a deoxidizer or alloying element. Its deoxidizing ability is much stronger than that of silicon and manganese. The main function of Al in steel is to refine the grain size and fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel and reducing its tendency to cold brittleness and aging.

[0015] Niobium partially dissolves into solid solution, acting as a solid solution strengthening agent. When dissolved in austenite, it significantly improves the hardenability of the steel. Trace amounts of niobium can increase the strength of the steel without affecting its plasticity or toughness. By refining the grains, it can improve the steel's impact toughness and lower its brittle transition temperature.

[0016] V exists in steel mainly in the form of carbides. In quenched and tempered steel, it mainly refines the grains, improves the strength and yield ratio of the steel, and increases the tempering stability of quenched steel.

[0017] Ti has a strong affinity for nitrogen, oxygen, and carbon, and its affinity for sulfur is stronger than that of iron. It is a good deoxidizer and degassing agent and an effective element for fixing nitrogen and carbon. Adding 0.015% or more Ti can significantly improve the strength of steel, and the alloy cost is very low; however, higher Ti content will reduce the low-temperature toughness of steel. Generally, the low-temperature toughness requirement at -40°C and below requires a Ti content of no more than 0.025%.

[0018] The lattice constant of nickel is similar to that of γ-iron, so it can form a continuous solid solution. This helps improve the hardenability of steel. Ni can lower the critical point and increase the stability of austenite. Generally, nickel-added steel is used for impact requirements at -60°C and below. On the one hand, it greatly improves the strength of the steel, and on the other hand, it always keeps the toughness of the iron at an extremely high level.

[0019] Cr can increase the hardenability of steel and has a secondary hardening effect. Its main function in the quenched and tempered structure is to improve the hardenability, so that the steel has better comprehensive mechanical properties after quenching and tempering.

[0020] Mo in steel can improve hardenability and heat resistance, prevent temper brittleness, improve tempering resistance or tempering stability of steel, and enable parts to be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stress and improving plasticity.

[0021] The main function of B in steel is to increase the hardenability of steel, thereby saving other rare and precious metals. It is used together with nickel, chromium, molybdenum, etc., and its content is generally specified to be within the range of 0.001% to 0.005%.

[0022] To further achieve the purpose of the present invention, a method for producing ultra-high-strength steel plates for hydropower is provided, which includes a steelmaking process, a billet heating process, a rolling process, a controlled cooling process, and a heat treatment process. The specific requirements are as follows:

[0023] (1) Steelmaking process: Molten steel is smelted according to the designed chemical composition and continuously cast into slabs. The chemical composition is calculated by weight percentage, namely, C: 0.10-0.14%, Mn: 0.90-1.20%, Si: 0.10-0.50%, P≤0.012%, S≤0.003%, Nb: 0.020-0.050%, V: 0.040-0.060%, Ti: 0.015-0.025%, B: 0.001-0.003%, Al: 0.010-0.060%, Ni: 1.55-2.55%, Cr: 0.40-0.60%, Mo: 0.40-0.70%, Pcm≤0.28%, of which 0.025%≤Ti+10*B≤0.035%, and the rest is Fe and impurities.

[0024] (2) Heating process: The billet is heated in a heating furnace with a heating coefficient of 9.0 to 14.0 min / cm and a heating temperature of 1150 to 1220°C to ensure uniform heating of the billet.

[0025] (3) Rolling process: A two-stage controlled rolling process is adopted. The first stage of rough rolling adopts high-temperature large-reduction technology, that is, the reduction of the last two passes is ≥35mm, and the final rolling temperature is ≥980℃; the second stage adopts low-temperature rolling technology, with the starting rolling temperature of 800℃~850℃ and the final rolling temperature of 750℃~600℃.

[0026] (4) Cooling process: The rolled steel plate enters the ultra-fast cooling system for online quenching and cooling to room temperature at a cooling rate of 20 to 35 °C / s.

[0027] (5) Heat treatment process: The steel plate is put into the heating furnace for tempering heat treatment, the tempering temperature is 570-610℃, and the time in the furnace is 2.5-4.0 min / mm.

[0028] Furthermore, in the step (1), the center segregation of the ingot does not exceed level C1.0.

[0029] Furthermore, in step (2), the heating coefficient is 9.8 to 12.4 min / cm, and the heating temperature is 1167 to 1211°C.

[0030] Furthermore, in the step (4), the cooling rate is 23 to 30°C / s.

[0031] Furthermore, in the step (5), the tempering temperature is 591-606°C.

[0032] Compared with the prior art, the advantages of the technical solution of the present invention are:

[0033] 1. The ultra-high-strength steel plate for hydropower use of the present invention has a yield strength of ≥960 MPa and a tensile strength of 1040-1200 MPa. The strength level is higher than the highest level of 690 MPa in the national and industry standards for existing hydropower steel plate materials, which is a yield strength of 800 MPa and a tensile strength of 890 MPa, and higher than the highest level of 1000 MPa in the group and enterprise standards. It can further reduce the wall thickness of the penstock, volute and bifurcated pipe of hydropower projects, reduce the difficulty of construction and welding, and help the hydropower industry develop towards high head (HD), high speed, high efficiency and large capacity.

[0034] 2. While meeting the high strength requirement of yield strength ≥960MPa, the steel plate of the present invention adopts peritectic steel, Nb microalloying, Ni+Cr+Mo alloying design, and Ti+B elements to improve the hardenability of the steel plate. The controlled cooling process adopts large rolling reduction, low-temperature controlled rolling, and post-rolling steel plate residual temperature online quenching + offline tempering heat treatment and other technologies. The maximum thickness reaches 60mm, and the cold crack sensitivity coefficient Pcm of the steel plate welding is achieved ≤0.28%, which meets the low preheating welding use of ultra-high strength steel plates and improves the production efficiency at the construction site of large-scale hydropower projects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG1 is a microstructure image of a steel plate at a quarter thickness in Example 2 of the present invention;

[0036] FIG2 is a table of chemical compositions of steel plates in Examples 1 to 4 of the present invention;

[0037] FIG3 is a table showing the mechanical properties of the steel plates in Examples 1 to 4 of the present invention. DETAILED DESCRIPTION

[0038] Example 1

[0039] To make the present invention more clear, an ultra-high strength steel plate for hydropower and a production method thereof of the present invention are further described below in conjunction with the accompanying drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0040] Taking the ultra-high strength steel plate for hydropower with a thickness of 60 mm and the component content (wt) as shown in FIG2 as an example, the production method of the steel plate is as follows, which is characterized by:

[0041] (1) Steelmaking process: The steel was smelted according to the composition shown in the table in Figure 2. The continuous casting process adopted dynamic soft reduction technology and the casting was 370 mm continuous casting slab with a center segregation of C1.0.

[0042] (2) Heating process: The heating coefficient of the steel billet is 10.5 min / cm and the heating temperature is 1167°C.

[0043] (3) Rolling process: A two-stage controlled rolling process was adopted. The reduction of the last two passes in the first stage was 39 mm and 38 mm, and the final rolling temperature was 1012 °C. The start rolling temperature in the second stage was 807 °C, and the final rolling temperature was 797 °C.

[0044] (4) Cooling process: The rolled steel plate is quenched online in an ultra-fast cooling device and cooled to room temperature at a cooling rate of 23°C / s.

[0045] (5) Heat treatment process: The steel plate is put into the furnace for offline tempering heat treatment, the tempering temperature is 591℃, and the time in the furnace is 251min.

[0046] In this embodiment, the yield strength of the 60 mm thick ultra-high strength hydropower steel plate is 996 MPa, the tensile strength is 1045 MPa, and the elongation after fracture is 14.5%. The specific mechanical properties are shown in the table of FIG3 .

[0047] Example 2

[0048] Taking the ultra-high strength steel plate for hydropower with a thickness of 50 mm and the component content (wt) as shown in FIG2 as an example, the production method of the steel plate is as follows, which is characterized by:

[0049] (1) Steelmaking process: According to the composition shown in the table in Figure 2, the continuous casting process adopts dynamic soft reduction and electromagnetic stirring technology, and the casting is 320 mm continuous casting slab with a center segregation of C0.5.

[0050] (2) Heating process: The heating coefficient of the steel billet is 12.4 min / cm and the heating temperature is 1181 °C.

[0051] (3) Rolling process: A two-stage controlled rolling process was adopted. The reduction of the last two passes in the first stage was 36 mm and 35 mm, and the final rolling temperature was 1024 °C. The start rolling temperature in the second stage was 827 °C, and the final rolling temperature was 789 °C.

[0052] (4) Cooling process: The rolled steel plate is quenched online in an ultra-fast cooling device and cooled to room temperature at a cooling rate of 24°C / s.

[0053] (5) Heat treatment process: The steel plate is put into the furnace for offline tempering heat treatment, the tempering temperature is 595℃, and the furnace time is 244min.

[0054] In this embodiment, the yield strength of the 50 mm thick ultra-high strength hydropower steel plate is 1019 MPa, the tensile strength is 1052 MPa, and the elongation after fracture is 14.5%. The specific mechanical properties are shown in the table of FIG3 , and the microstructure image of the steel plate is shown in FIG1 .

[0055] Example 3

[0056] Taking the ultra-high strength steel plate for hydropower with a thickness of 40 mm and the component content (wt) as shown in FIG2 as an example, the production method of the steel plate is as follows, which is characterized by:

[0057] (1) Steelmaking process: According to the composition shown in the table in Figure 2, the continuous casting process adopts dynamic soft reduction and electromagnetic stirring technology, and the casting is 260 mm continuous casting slab with a center segregation of C1.0.

[0058] (2) Heating process: The heating coefficient of the steel billet is 11.7 min / cm and the heating temperature is 1208°C.

[0059] (3) Rolling process: A two-stage controlled rolling process was adopted. The reduction of the last two passes in the first stage was 34 mm and 33 mm, and the final rolling temperature was 1027 °C. The start rolling temperature in the second stage was 833 °C, and the final rolling temperature was 794 °C.

[0060] (4) Cooling process: The rolled steel plate is quenched online in an ultra-fast cooling device and cooled to room temperature at a cooling rate of 27°C / s.

[0061] (5) Heat treatment process: The steel plate is put into the furnace for offline tempering heat treatment, the tempering temperature is 598℃, and the time in the furnace is 205min.

[0062] In this embodiment, the yield strength of the 40 mm thick ultra-high strength hydropower steel plate is 1022 MPa, the tensile strength is 1053 MPa, and the elongation after fracture is 14.5%. The specific mechanical properties are shown in the table of FIG3 .

[0063] Example 4

[0064] Taking the ultra-high strength steel plate for hydropower with a thickness of 25 mm and the component content (wt) as shown in FIG2 as an example, the production method of the steel plate is as follows, which is characterized by:

[0065] (1) Steelmaking process: The steel was smelted according to the composition in Table 1. The continuous casting process adopted dynamic soft reduction and electromagnetic stirring technology. The casting was 260 mm continuous casting slab, and the center segregation of the slab was C1.0.

[0066] (2) Heating process: The heating coefficient of the steel billet is 9.8 min / cm and the heating temperature is 1211°C.

[0067] (3) Rolling process: A two-stage controlled rolling process was adopted. The reductions of the last two passes in the first stage were 33 mm and 32 mm, and the final rolling temperature was 1031 °C. The start rolling temperature in the second stage was 842 °C, and the final rolling temperature was 792 °C.

[0068] (4) Cooling process: The rolled steel plate is quenched and cooled to room temperature in an ultra-fast cooling device at a cooling rate of 30°C / s.

[0069] (5) Heat treatment process: The steel plate is put into the furnace for offline tempering heat treatment, the tempering temperature is 606℃, and the furnace time is 198min.

[0070] In this embodiment, the yield strength of the 25 mm thick ultra-high strength hydropower steel plate is 1019 MPa, the tensile strength is 1052 MPa, and the elongation after fracture is 14.5%. The specific mechanical properties are shown in the table of FIG3 .

[0071] The steel plate of the present invention has a yield strength of 960MPa and a tensile strength of 1040MPa. It also has the characteristics of excellent low-temperature toughness and good welding performance. It adopts an online quenching process and does not require offline quenching heat treatment. It has fewer production processes and can achieve economical and rapid production of steel plates with good economic benefits. The gross profit per ton of steel is expected to be more than 1,500 yuan / ton.

[0072] In addition to the above embodiments, the present invention may also have other implementations. Any technical solution formed by equivalent replacement or equivalent transformation falls within the scope of protection required by the present invention.

Claims

1. An ultra-high strength steel plate for hydropower, characterized in that: Its chemical composition includes, by weight percentage, 0.10-0.14% C, 0.90-1.20% Mn, 0.10-0.50% Si, P≤0.012%, S≤0.003%, 0.020-0.050% Nb, 0.040-0.060% V, 0.015-0.025% Ti, 0.001-0.003% B, 0.010-0.060% Al, 1.55-2.55% Ni, 0.40-0.60% Cr, 0.40-0.70% Mo, Pcm≤0.28%, of which 0.025%≤Ti+10*B≤0.035%, and the rest is Fe and impurities.

2. The ultra-high strength steel plate for hydropower according to claim 1, characterized in that: The thickness of the ultra-high strength steel plate for hydropower is 25 mm to 60 mm.

3. A method for producing an ultra-high strength steel plate for hydropower as claimed in claim 1, comprising a steelmaking process, a billet heating process, a rolling process, a controlled cooling process, and a heat treatment process, wherein the specific steps are as follows, and the method is characterized in that: (1) Steelmaking process: molten steel is smelted according to the designed chemical composition and continuously cast into slabs; (2) Heating process: The billet is heated in a heating furnace with a heating coefficient of 9.0 to 14.0 min / cm and a heating temperature of 1150 to 1220°C to ensure uniform heating of the billet; (3) Rolling process: A two-stage controlled rolling process is adopted. The first stage of rough rolling adopts high temperature and large reduction technology, that is, the reduction of the last two passes is ≥35mm, and the final rolling temperature is ≥980℃; the second stage adopts low temperature rolling technology, the starting rolling temperature is 800℃~850℃, and the final rolling temperature is 750℃~600℃; (4) Cooling process: The rolled steel plate enters the ultra-fast cooling system for online quenching and cooling to room temperature at a cooling rate of 20 to 35 °C / s; (5) Heat treatment process: The steel plate is put into a heating furnace for tempering heat treatment, the tempering temperature is 570-610°C, and the furnace time is 2.5-4.0 min / mm.

4. The method for producing ultra-high strength steel plate for hydropower according to claim 3, characterized in that: In the step (1), the center segregation of the ingot does not exceed level C1.

0.

5. The method for producing ultra-high strength steel plate for hydropower according to claim 3, characterized in that: In the step (2), the heating coefficient is 9.8 to 12.4 min / cm, and the heating temperature is 1167 to 1211°C.

6. The method for producing ultra-high strength steel plate for hydropower according to claim 3, characterized in that: In the step (4), the cooling rate is 23-30°C / s.

7. The method for producing ultra-high strength steel plate for hydropower according to claim 3, characterized in that: In the step (5), the tempering temperature is 591-606°C.

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