A low-cost, ultra-high-strength, ultra-thick steel for hydropower and its production method

Abstract

The present invention relates to a low-cost, ultra-high-strength, ultra-thick steel for hydropower generation and a production method thereof. The chemical composition of the steel, calculated by weight percentage, is as follows: C 0.09% to 0.14%, Si 0.2% to 0.45%, Mn 2.1% to 2.9%, P ≤ 0.010%, S ≤ 0.005%, Cr 0.51% to 0.85%, V 0.075% to 0.15%, N 0.028% to 0.055%, with 2 < V / N < 3; Re 0.01% to 0.04%, [O] ≤ 10 ppm, with the remainder being Fe and unavoidable impurities. The present invention not only has a low production cost and a short production cycle, but also provides excellent and stable steel plate performance, fully meeting the application requirements of large-scale hydropower projects.

Description

Technical Field

[0001] The present invention relates to the field of metal materials, and in particular to low-cost, ultra-high-strength and ultra-thick hydropower steel and a production method thereof. Background Art

[0002] Currently, the production of 1000MPa-grade hydropower steel generally utilizes a higher alloy composition combined with offline quenching and tempering (quenching and high-temperature tempering) heat treatment. However, this production method is not only costly but also complex and time-consuming. The key challenge facing practitioners in this field is to achieve both cost savings and superior performance.

[0003] Existing invention patents for steel of this strength level mostly adopt designs with higher alloy compositions and offline tempering heat treatment processes, which not only result in high production costs but also long production cycles.

[0004] Example 1: Invention patent application CN 108504960 A discloses "a 1000MPa-grade low-crack hydropower steel plate for large-scale hydropower projects and its production method." The invented steel contains 1.0-2.0% Ni and 0.4-0.7% Mo. The production process adopts controlled rolling and controlled cooling + offline tempering heat treatment. Not only is the alloy cost high, but the production cycle is also long.

[0005] Example 2: Invention patent application CN 113652607 A discloses "a 1000MPa grade quenched and tempered steel plate for hydropower and its production method". The invented steel contains 1.0-1.7% Ni and 0.4-0.6% Mo. The production process adopts a two-stage controlled rolling + offline quenching and tempering heat treatment method, resulting in high alloy cost and long production cycle.

[0006] Example 3: Invention patent application CN 108359879 A discloses a "DQ-T method for 1000 MPa grade hydropower steel plate with a thickness not exceeding 60 mm". 0.9-2.6% Ni and 0.3-0.65% Mo are added to the steel, resulting in a relatively high alloy cost. Summary of the Invention

[0007] The present invention provides a low-cost ultra-high-strength and extra-thick steel for hydropower and a production method thereof. The produced steel plate has a thickness of 60 to 130 mm and a tensile strength of 1000 MPa. Not only is the production cost low and the production cycle short, but the steel plate also has excellent and stable performance, good low-temperature toughness, resistance to lamellar tearing and cold bending performance, and can meet the requirements of large-scale hydropower projects for the use of ultra-high-strength and extra-thick plates.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] Disclosed is a low-cost, ultra-high-strength, and extra-thick steel for hydropower use. The chemical composition of the steel is, by weight percentage, as follows: C 0.09%-0.14%, Si 0.2%-0.45%, Mn 2.1%-2.9%, P≤0.010%, S≤0.005%, Cr 0.51%-0.85%, V 0.075%-0.15%, N 0.028%-0.055%, with 2<V / N<3; Re 0.01%-0.04%, [O]≤10ppm, and the balance being Fe and unavoidable impurities.

[0010] The thickness of steel plates for hydropower is 60 to 130 mm.

[0011] The reasons for adopting the above components are as follows:

[0012] Carbon (C): Carbon is the most fundamental strengthening element in steel. Furthermore, in the present invention, it forms fine carbides with Cr and V, pinning grain boundaries and refining grains, thereby improving the plasticity and toughness of the steel plate. If the carbon content is too low, the steel plate's strength cannot be guaranteed. If it is too high, it promotes martensite formation, affecting the desired amount of bainite and retained austenite. Therefore, the carbon content in the present invention is designed to be 0.09-0.14%.

[0013] Si: In this invention, Si is primarily used for deoxidation, coordinating with the smelting process to reduce [O] in the steel to ≤ 10 ppm, thereby preventing [O] from affecting the effect of Mn. Excessive Si content can reduce the plasticity and toughness of the steel. Therefore, the Si content in the steel of this invention is designed to be 0.2-0.45%.

[0014] Mn: Manganese is added as an alloying element in this invention to lower the martensitic transformation temperature and phase transformation rate of the steel, improve the steel's hardenability, promote the formation of a mixed bainite and martensite structure, and increase the amount of retained austenite in the steel. Manganese also improves the strength of the steel plate by lowering the phase transformation temperature, refining the martensite and bainite matrix structures, and allowing more precipitates to precipitate at lower temperatures, reducing the size of the precipitates and fully utilizing the precipitation strengthening effect. Therefore, the designed Mn content is 2.1-2.9%.

[0015] P and S: Both are harmful elements. P will cause cold brittleness of steel, and S will cause hot brittleness of steel. Both have obvious adverse effects on various properties of steel plates. Therefore, the lower the content, the better. However, considering the production cost, according to the needs, the present invention controls P≤0.010% and S≤0.005% in steel.

[0016] Cr: Chromium can significantly improve the hardenability of steel plates. In the steel of the present invention, by combining it with Mn, it effectively ensures that the steel plates of the present invention form a uniform and consistent structure along the thickness section during rapid cooling after rolling, thereby improving the uniformity of the steel plate performance and enhancing the safety of the steel plate in engineering applications. As an important alloying element in the steel of the present invention, chromium is also used to increase the orientation difference between the lath martensite and bainite, increase the crack propagation resistance, and improve the plasticity and toughness of the lath structure; at the same time, it refines the lath structure during the cooling process and forms small and stable chromides, thereby ensuring that the steel plate has ultra-high strength. In addition, since a relatively high content of V is added to the steel of the present invention, chromium can inhibit the aggregation and coarsening of vanadium by improving the thermodynamic stability of vanadium. Excessive chromium content will promote the formation of martensite, increase the ductile-brittle transition temperature of the steel plate, and is not conducive to welding performance. Therefore, the Cr content in the steel of the present invention is designed to be 0.51-0.85%.

[0017] V: In the steel of this invention, V is induced to precipitate through deformation during rolling. Under high nitrogen conditions, this forms fine, nitrogen-rich VN and V(CN), which pin grain boundaries, inhibit austenite grain growth, and refine grains. During cooling, large amounts of V, VN, and V(CN) precipitate and become dispersed throughout the structure, exerting a strong precipitation strengthening effect. Furthermore, by controlling the ratio 2 < V / N < 3, this invention significantly improves the strength and toughness of the steel plate while ensuring the efficient utilization of V and N. The V content of this invention is designed to be 0.075-0.15%.

[0018] Nitrogen forms VN and V(CN) with V during rolling, pinning grain boundaries and inhibiting austenite grain growth, refining the grains. During cooling, it also provides precipitation strengthening. Furthermore, the large amount of nano-sized nitrides precipitated along the grain boundaries in the present steel not only strengthens the grain boundary energy but also inhibits the decomposition of retained austenite, ensuring that the steel plate retains an appropriate amount of retained austenite after tempering. This results in high strength and good low-temperature toughness. Therefore, the present invention aims for a Nitrogen content of 0.028-0.055%.

[0019] Re: The addition of an appropriate amount of Re to the present steel primarily serves the following purposes: 1) Refining the solidified steel structure and improving as-cast properties; 2) Reducing the compositional segregation caused by the high Mn content in the present steel, thereby improving the uniformity of the as-cast structure. 3) Reducing the activity of C and N, increasing their solubility and reducing their dissolution and diffusion into crystal defects or areas of internal stress concentration, thereby reducing the number of interstitial atoms pinned by dislocations and improving the toughness and ductility of the steel. Depending on the needs, the Re content in the present steel is designed to be 0.01-0.04%.

[0020] [O]: In order to avoid the adverse effects of [O] on Mn and the generation of excessive oxide inclusions, [O] is controlled to be ≤10ppm in the steel of the present invention.

[0021] A low-cost method for producing ultra-high-strength, ultra-thick steel for hydropower applications. The steel plate manufacturing process comprises: smelting - continuous casting - heating - controlled rolling and controlled cooling - ultrasonic flaw detection - performance testing, specifically including the following methods:

[0022] The steel of the present invention is smelted by a converter + external refining method. In order to effectively control the [O] content in the steel, vacuum degassing treatment is performed during external refining. The cycle degassing time is 22 to 27 minutes, and the standing time before pouring is 10 to 17 minutes to control [O] ≤ 10 ppm.

[0023] During casting, the tundish superheat is controlled at 14-22°C. While ensuring the fluidity of the molten steel, the lower superheat helps reduce segregation in the ingot. During continuous casting, electromagnetic stirring is applied at the 42%-47% unsolidified portion of the ingot, and heavy pressure is applied at the end of the continuous casting process. The electromagnetic stirring current is controlled at 420-450A, the frequency is 3-5Hz, the heavy pressure reduction is 12-20mm, and the casting speed is controlled at 0.5-0.8m / min. Through appropriate electromagnetic stirring and heavy pressure processes, the growth of columnar crystals can be suppressed, the central equiaxed crystal zone can be expanded, and a uniform and dense equiaxed crystal structure can be obtained. This effectively reduces the compositional segregation caused by high Mn content, and controls the central segregation and central porosity to no more than level 1.0.

[0024] In order to ensure sufficient reduction and improve rolling penetration, the preferred thickness of the continuous casting slab is 250 to 400 mm.

[0025] The continuous casting billet heating rate is 5-9 min / cm, the holding temperature is 1220-1260°C, and the holding time is 2.5-4h.

[0026] The rolling start temperature is 1150-1190°C, the single-pass reduction rate is greater than 10%, the final rolling temperature is 920-960°C, and the thickness of the rolled steel plate is 60-130 mm; rolling with a large reduction rate in the high temperature zone is beneficial to improving rolling penetration, promoting sufficient recrystallization of the deformed austenite in the full-thickness section of the ingot, and improving the uniformity of grain size.

[0027] After rolling, it is directly water-cooled at a cooling rate of 16-23°C / s, with a final cooling temperature of 370-440°C, and then air-cooled to room temperature.

[0028] Rapid cooling after rolling ensures the formation of lath martensite, lath bainite, and a small amount of retained austenite while also refining the microstructure. Controlling the final cooling temperature adjusts the martensite and bainite lath substructures, stabilizes the retained austenite structure, improves the steel plate's plasticity and toughness, and reduces structural stress during cooling.

[0029] The final microstructure is: 23% to 37% lath martensite, 62% to 76% lath bainite, and 0.7% to 1.4% retained austenite.

[0030] The steel plate of the present invention has room temperature tensile strength of ≥1000MPa, yield strength of ≥900MPa, elongation after fracture of ≥15%, impact absorption energy of -40°C>100J, Z-direction stretching of>50%, and 180° cold bending qualified.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The steel of the present invention adopts a low-carbon, Mn+Cr+V+N+Re low-alloy composition design without adding precious metals such as Ni, Mo, and Nb, thus realizing a low-cost, ultra-high-strength and ultra-thick hydropower steel alloy composition design.

[0033] 2. No need for offline quenching and tempering heat treatment. Through online controlled cooling process, while ensuring the performance of the steel plate, it greatly shortens the process flow, shortens the production cycle, reduces heat treatment costs, and reduces energy consumption.

[0034] 3. The present invention realizes the high efficiency and stable production of extra-thick and ultra-high-strength hydropower steel through innovative design of chemical composition and process, and the obtained steel plate has uniform and good mechanical properties.

[0035] 4. The present invention has obtained a 60mm to 130mm thick ultra-high strength and extra-thick hydropower steel, the room temperature tensile strength of the steel plate is ≥1000MPa, the yield strength is ≥900MPa, the elongation after fracture is ≥15%; the impact absorption energy at -40℃ is >100J; the Z direction is >50%; and the 180° cold bending is qualified. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the metallographic structure diagram of martensite + bainite + a small amount of retained austenite of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the specific implementation methods of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0038] The examples are shown in Tables 1 to 3, wherein Table 1 shows the chemical composition of each example; Table 2 shows the process parameters of the steel of each example; and Table 3 shows the properties and structure of the steel plates of each example.

[0039] Table 1 Chemical composition of steel smelting in each example (%)

[0040]

[0041] Table 2 Process parameters of each example steel

[0042]

[0043]

[0044] Table 3 Performance and structure of steel plates in Example

[0045]

[0046]

Claims

1. A low-cost, ultra-high-strength, ultra-thick steel for hydropower, characterized by: The chemical composition of the steel is calculated by weight as follows: C 0.09% to 0.14%, Si 0.2% to 0.45%, Mn 2.1% to 2.9%, P ≤ 0.010%, S ≤ 0.005%, Cr 0.51% to 0.85%; V 0.075% to 0.15%, N 0.028% to 0.055% and 2 < V / N < 3; Re 0.01% to 0.04%, [O] ≤ 10ppm; the balance is Fe and unavoidable impurities; The low-cost production method of ultra-high-strength and ultra-thick hydropower steel includes the following contents: During casting, the tundish superheat is controlled at 14-22°C. During continuous casting, electromagnetic stirring is applied when the unsolidified ratio of the billet is 42%-47%, and the billet drawing speed is controlled at 0.5-0.8 m / min. The continuous casting billet heating rate is 5-9 min / cm, the holding temperature is 1220-1260℃, and the holding time is 2.5-4h; The rolling start temperature is 1150-1190℃, the single-pass reduction rate is greater than 10%, and the final rolling temperature is 920-960℃; After rolling, it is directly water-cooled at a cooling rate of 16-23°C / s, with a final cooling temperature of 370-440°C, and then air-cooled to room temperature.

2. The low-cost, ultra-high-strength, ultra-thick steel for hydropower according to claim 1, characterized in that: The thickness of steel plates for hydropower is 60 to 130 mm.

3. The low-cost, ultra-high-strength, ultra-thick steel for hydropower according to claim 1, characterized in that: The structure of hydropower steel is: lath martensite 23%~37%, lath bainite 62%~76%, and retained austenite 0.7%~1.4%.

4. The low-cost, ultra-high-strength, ultra-thick steel for hydropower according to claim 1, characterized in that: The room temperature tensile strength of the steel plate is ≥1000MPa, the yield strength is ≥900MPa, and the elongation after fracture is ≥15%.

5. The low-cost, ultra-high-strength, ultra-thick steel for hydropower according to claim 1, characterized in that: The steel plate's impact absorption energy at -40°C is >100J; the Z-axis tensile strength is >50%; and the 180° cold bending test is qualified.

6. A method for producing low-cost, ultra-high-strength, ultra-thick hydropower steel according to any one of claims 1 to 4, characterized in that: Includes the following: During casting, the tundish superheat is controlled at 14-22°C. During continuous casting, electromagnetic stirring is applied when the unsolidified ratio of the billet is 42%-47%, and the billet drawing speed is controlled at 0.5-0.8 m / min. The continuous casting billet heating rate is 5-9 min / cm, the holding temperature is 1220-1260℃, and the holding time is 2.5-4h; The rolling start temperature is 1150-1190℃, the single-pass reduction rate is greater than 10%, and the final rolling temperature is 920-960℃; After rolling, it is directly water-cooled at a cooling rate of 16-23°C / s, with a final cooling temperature of 370-440°C, and then air-cooled to room temperature.

7. The method for producing low-cost, ultra-high-strength, and ultra-thick hydropower steel according to claim 6, characterized in that: The smelting method is converter + external refining. The external refining is vacuum degassing. The cycle degassing time is 22 to 27 minutes. The standing time before pouring is 10 to 17 minutes.

8. The method for producing low-cost, ultra-high-strength, and ultra-thick hydropower steel according to claim 6, characterized in that: The electromagnetic stirring current is controlled at 420-450A and the frequency is 3-5Hz.

9. The method for producing low-cost, ultra-high-strength, and ultra-thick hydropower steel according to claim 6, characterized in that: At the end of continuous casting, heavy pressure is used, and the amount of heavy pressure reduction is 12 to 20 mm.

10. The method for producing low-cost, ultra-high-strength, and ultra-thick hydropower steel according to claim 6, characterized in that: The thickness of the continuous casting slab is 250-400 mm.

Images