A 1000MPa grade high-toughness steel for hydropower and its production method

Abstract

The invention relates to a 1000MPa grade high-toughness steel for hydropower and a production method thereof. The chemical components of the steel are as follows by weight: C 0.03%-0.10%, Si 0.85%-1.0%, Mn 0.5%-1.0%, P≤0.008%, S≤0.002%, Ni 0.1%-0.5%, Cr 0.91%-1.1%, Mo 0.2%-0.6%, V 0.09%-0.16%, N 0.006%-0.012%, Zr 0.005%-0.01%, Al 0.02%-0.05%, [O]≤15ppm, [H]≤3ppm, and the balance is Fe and unavoidable impurities. The production process of the present invention is stable and feasible, and the produced steel plate has an ultra-high strength of 1000MPa, excellent impact toughness at -60°C, and good welding performance.

Description

Technical Field

[0001] The invention belongs to the field of metal materials, and in particular relates to a 1000MPa-grade high-toughness hydropower steel and a production method thereof. Background Art

[0002] With the continuous development of hydropower in my country and the trend of large-scale engineering construction, the demand for 1000MPa hydropower steel has also been raised. 1000MPa hydropower steel must not only have ultra-high strength, but also have excellent low-temperature toughness to meet the operating requirements of hydropower stations in harsh natural environments.

[0003] The present invention provides a 1000MPa high-toughness hydropower steel with a thickness of no more than 70mm and a production method thereof. The produced steel plate not only has an ultra-high strength of 1000MPa, but also has excellent impact toughness at -60°C, which can fully meet the use requirements of large-scale hydropower engineering construction for the high toughness of 1000MPa hydropower steel.

[0004] The present invention is different from the existing domestic public technology regarding this type of steel in terms of the chemical composition of the steel and the production process design. Typical examples are as follows.

[0005] CN108504960A discloses "a 1000MPa grade low crack hydropower steel plate for large hydropower projects and its production method". The steel plate composition design adopts high Ni (1.0-2.0%), and adds elements such as Cr, Mo, Nb, V, and Ti; its production method adopts die casting + blanking + reheating controlled rolling and controlled cooling + offline tempering process, the production process is complicated, the alloy element content is high, and the production cost is high.

[0006] CN 108359879A discloses "a DQ-T method for a 1000MPa grade hydropower steel plate with a thickness of no more than 60mm". The steel plate composition is designed to be C: 0.09-0.18, Mn: 0.80-1.60%, Ni (0.9-2.6%), Mo (0.3-0.65%), and a certain amount of Cu, Cr, Mo, Nb, V, Ti, B and other elements are added to control Pcm≤0.29. The alloy content is relatively high and the Pcm is relatively high. The production process is two-stage controlled rolling + online quenching + high temperature tempering, which is different from the present invention.

[0007] CN 108193137A discloses “a DQ-QT production method for a 1000MPa grade hydropower steel plate with a thickness of no more than 80mm”. The steel plate composition is designed to have a high carbon content and a high Pcm, which is not conducive to welding. The production process adopts two-stage controlled rolling + online quenching + offline tempering, which is different from the present invention. Summary of the invention

[0008] The present invention provides a 1000MPa high-toughness steel for hydropower and a production method thereof. The production process is stable and feasible. The produced steel plate has an ultra-high strength of 1000MPa, excellent impact toughness at -60°C, and good welding performance.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A 1000MPa grade high-toughness steel for hydropower, wherein the chemical composition of the steel is as follows by weight: C 0.03%-0.10%, Si 0.85%-1.0%, Mn 0.5%-1.0%, P≤0.008%, S≤0.002%, Ni 0.1%-0.5%, Cr 0.91%-1.1%, Mo 0.2%-0.6%, V 0.09%-0.16%, N 0.006%-0.012%, Zr0.005%-0.01%, Al 0.02%-0.05%, [O]≤15ppm, [H]≤3ppm, and the balance is Fe and unavoidable impurities. Carbon equivalent Ceq≤0.55%, welding crack sensitivity coefficient Pcm≤0.25%; where: Ceq(%)=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14; Pcm(%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0011] The reasons for adopting the above-mentioned composition design are as follows: (Mechanism refinement of main components)

[0012] C: It is used to form fine carbides with Cr, V, and Zr, and it refines the grains and stabilizes the structure and performance of the steel plate while playing a certain precipitation strengthening role; carbon has the effect of increasing the cold brittleness and aging sensitivity of steel, so the present invention appropriately reduces the carbon content, which is beneficial to improving the aging impact performance of the steel plate; in addition, the lower carbon content control is conducive to the formation of bainite. Therefore, the C content of the present invention is designed to be 0.03-0.10%.

[0013] Si: Silicon is an important alloying element in the steel of the present invention. Its main functions are: 1) dissolving in ferrite and austenite, playing a good solid solution strengthening role; 2) improving the hardenability of the steel plate; 3) inhibiting or delaying the decomposition of carbides of supercooled austenite, and promoting the formation of a continuous austenite film or strip-shaped MA island structure between bainitic ferrite strips. Due to the presence of austenite and its morphology and distribution, cracks can be branched and passivated, or phase transformation-induced plasticity can be generated, thereby effectively improving the low-temperature toughness of the steel plate. If the silicon content is too high, it is easy to form a banded structure in the steel, resulting in uneven performance and being unfavorable to the welding performance of the steel. Therefore, the Si content of the present invention is designed to be 0.85-1.0%.

[0014] Mn: has the effects of deoxidation and desulfurization, and also plays a certain role in solid solution strengthening. In the present invention, it is used in combination with Si to reduce band segregation in steel and improve the uniformity of steel plate performance and welding performance. The Mn content is controlled to be 0.5% to 1.0%.

[0015] P and S: Both are harmful elements. P will cause cold brittleness of steel, and S will cause hot brittleness of steel and is easy to segregate, which has obvious adverse effects on the low-temperature toughness and welding performance of steel. Therefore, the lower the content, the better. The present invention requires that P≤0.008% and S≤0.002% in steel be controlled.

[0016] Ni: used to improve the low temperature toughness and hardenability of the steel plate; used to improve the structural stability, improve the corrosion resistance of hydropower steel, and thus improve the safety of engineering applications. Since nickel expands the austenite region of iron, too high a content is not conducive to the formation of a certain proportion of bainite in the steel of the present invention, so according to the needs, the Ni content of the present invention is designed to be 0.1-0.5%.

[0017] Cr: used to improve the strength and hardenability of steel; increase the activation energy of carbon diffusion in steel, reduce the tendency of grain coarsening when steel is heated; increase the corrosion resistance of steel; used in combination with Mo, Mn, etc., to make the steel plate have good comprehensive properties after heat treatment. Therefore, the Cr content in the steel of the present invention is controlled to be 0.91-1.1%.

[0018] Mo: used in combination with Cr and Mn, it improves the tempering resistance of the steel plate and improves the comprehensive performance of the steel plate after heat treatment; it improves the thermal stability of carbonitrides and promotes the dispersion distribution of fine precipitates. The Mo content of the present invention is designed to be 0.2-0.6%.

[0019] V: A strong carbonitride-forming element. The formation of fine V (CN) can produce a strong precipitation strengthening effect; pinning the grain boundary during controlled rolling, refining the grains, and improving the toughness of the steel plate; inhibiting the strain aging embrittlement of the steel, ensuring that the steel plate has good strain aging low-temperature impact performance. The V content of the present invention is designed to be 0.09-0.16%.

[0020] N: promotes the precipitation of V (C, N), refines the structure of steel, and improves or improves the comprehensive performance of steel; forms high-melting-point ZrN with Zr, which is used to improve the cast structure of steel and inhibit the coarsening of high-temperature austenite grains during reheating. According to the requirements of the present invention, the N content is controlled to be 0.006-0.012%.

[0021] Zr: used to form nano-scale high melting point ZrC and ZrN in steel, refine the cast structure, improve the quality of the ingot, reduce the strain aging tendency and temper brittleness of the steel, and improve the low temperature toughness and strain aging impact performance of the steel. The present invention controls the Zr content to 0.005-0.01%.

[0022] Al: plays a certain deoxidation role and can form AlN with N, refine austenite grains and improve the low-temperature toughness of steel. According to needs, the Al content of the present invention is controlled at 0.02-0.05%.

[0023] H and O: Hydrogen and oxygen are harmful elements in the steel of the present invention. In order to reduce the influence of H and O on the properties of the steel and improve the purity and homogenization of the steel, the present invention controls [H]≤3ppm and [O]≤15ppm.

[0024] Ceq, Pcm: are two important indicators for measuring the welding performance of hydropower steel, the lower the better. The present invention is ultra-high strength steel, and by controlling Ceq≤0.55%, Pcm≤0.25%, it ensures that the steel plate has good welding performance while having ultra-high strength and toughness.

[0025] The room temperature tensile strength of the steel plate is greater than 950MPa, the yield strength is greater than 890MPa, and the elongation after fracture is ≥20%; the impact absorption energy at -60°C is greater than 250J; and the strain aging impact absorption energy at -60°C is ≥200J.

[0026] A method for producing 1000MPa grade high-toughness hydropower steel, the method comprising:

[0027] The steel plate is produced by continuous casting process. The double-linked method is used for deep desulfurization and dephosphorization of molten steel to control P≤0.008% and S≤0.002% in steel; [H]≤3ppm and [O]≤15ppm are reduced by refining outside the furnace; non-metallic inclusions in steel are controlled to be ≤1.0 level.

[0028] During casting, the superheat of the tundish is controlled at 31-40°C, the secondary cooling water temperature is 20-30°C, and electromagnetic stirring and light pressure are used at the end of continuous casting. The electromagnetic stirring current is 360-400A and the frequency is 10-13H. Z , the light reduction is 7-10mm, and the billet drawing speed is controlled to be 0.6-0.8m / min. Through process control, the proportion of equiaxed crystals in the billet is increased, and the segregation is reduced to below level 1.0, which effectively improves the as-cast structure and improves the uniformity of the structure, laying the foundation for the steel plate to obtain excellent performance. The preferred thickness of the continuous casting billet is 200-300mm, and the stacking and slow cooling for more than 24 hours after the billet is off the line further promotes the homogenization of the structure and precipitates and reduces the component segregation.

[0029] The continuous casting ingot is heated in a walking beam heating furnace at a temperature of 1250-1300°C for 3-5 hours. During heating, the high melting point Zr (CN) and the high content of Cr formed in the steel are used to inhibit the growth of austenite grains, while ensuring that the ingot is fully austenitized and the grain size is uniform.

[0030] After the ingot is cast out of the furnace, it is descaled by high-pressure water. The rolling is carried out in two stages. The rolling temperature in the first stage is 1180-1210℃, the single-pass reduction rate is ≥15%, the thickness of the intermediate ingot is >2t, t is the thickness of the finished steel plate, and the final rolling temperature is 1000-1050℃.

[0031] The second stage rolling temperature is 880-920℃, the total reduction ratio is >60%, and the final rolling temperature is 750-780℃; the two-stage controlled rolling makes the steel plate grains fully refined, increases the dislocation density, and promotes the dispersion and precipitation of carbonitrides. The thickness of the rolled steel plate is ≤70mm.

[0032] After the steel plates come off the production line, they are stacked and slowly cooled for no less than 20 hours. Through long-term slow cooling at a low cooling rate, the rolled structure and precipitates are further homogenized, while reducing or eliminating segregation in the steel.

[0033] The steel plate is subjected to offline quenching and tempering heat treatment, in which the quenching and holding temperature is 780-820℃, the holding time is 3-6min / mm, and the quenching cooling rate is 25-30℃ / s, which promotes the formation of lath bainite and martensite and increases the proportion of bainite structure. The tempering holding temperature is 550-600℃, the holding time is 150-300min, and air cooling is performed after leaving the furnace. The tempered bainite and tempered bainite complex structures are obtained, in which the tempered bainite accounts for 70%-85% and the tempered bainite accounts for 15%-30%. After a long period of tempering, the further precipitation and dispersion distribution of carbides are also promoted, so that the steel plate obtains excellent strength and toughness.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. The steel of the present invention adopts low carbon, low manganese, Si+Ni+Cr+Mo+V low alloy composition design to obtain a 1000MPa grade, high toughness complex phase structure hydropower steel (tempered troostite accounts for 70-85%, tempered bainite accounts for 15-30%), and by controlling Ceq≤0.55%, Pcm≤0.25%, the good welding performance of the steel plate is guaranteed.

[0036] 2. The present invention realizes homogenization, low segregation and clean production of continuous casting billets through effective control of key process parameters of smelting and continuous casting processes, thus laying a good foundation for obtaining excellent performance of steel plates.

[0037] 3. The present invention adopts a two-stage controlled rolling + dual-phase zone quenching + high-temperature tempering process to achieve the production of a 1000MPa grade hydropower steel. The production process is stable and feasible. The produced steel plate has excellent -60°C low-temperature impact performance (>250J) while maintaining ultra-high strength.

[0038] 4. The present invention obtains a 1000MPa grade high-toughness hydropower steel with a thickness of no more than 70mm. The room temperature tensile strength of the steel plate is greater than 950MPa, the yield strength is greater than 890MPa, the elongation after fracture is ≥20%; the impact absorption energy at -60℃ is greater than 250J; the impact absorption energy after strain aging at -60℃ is ≥200J. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the metallographic structure diagram of Example 1. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] The chemical composition of each embodiment is shown in Table 1. The process parameters of the steel of each embodiment of the present invention are shown in Table 2. The mechanical properties of the steel plates of each embodiment of the present invention are shown in Table 3. Figure 1 .

[0042] Table 1 Chemical composition of steel smelting in each example (%)

[0043]

[0044]

[0045] Table 2 Process parameters of each example steel

[0046]

[0047]

[0048] Table 3 Microstructure and properties of quenched and tempered steel plate

[0049]

Claims

1. A 1000MPa grade high toughness steel for hydropower, characterized in that: The chemical composition of the steel is calculated by weight percentage: C0.03%~0.09%, Si 0.85%~1.0%, Mn 0.5%~0.93%, P≤0.008%, S≤0.002%, Ni 0.31%~0.5%, Cr 0.91%~1.1%, Mo 0.2%~0.6%, V 0.09%~0.16%, N 0.006%~0.012%, Zr 0.005%~0.01%, Al 0.02%~0.05%, and [O]≤15ppm, [H]≤3ppm, the balance is Fe and unavoidable impurities; carbon equivalent Ceq≤0.55%, welding crack sensitivity coefficient Pcm≤0.25%; where: Ceq (%) = C+ Mn / 6+ Si / 24+ Ni / 40+Cr / 5+ Mo / 4+ V / 14; Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B; the complex phase structure in the steel is: tempered bainite accounts for 70% to 85%, and tempered bainite accounts for 15% to 30%.

2. The 1000MPa high-toughness steel for hydropower according to claim 1, characterized in that: The room temperature tensile strength of the steel plate is greater than 950MPa, the yield strength is greater than 890MPa, and the elongation after fracture is ≥20%; the impact absorption energy at -60℃ is greater than 250J; the impact absorption energy after strain aging at -60℃ is ≥200J.

3. A 1000MPa grade high toughness steel for hydropower according to claim 1 or 2, characterized in that: The thickness of the steel plate shall not exceed 70mm.

4. A method for producing 1000MPa high-toughness hydropower steel as claimed in any one of claims 1 to 3, characterized in that: include: 1) During casting, control the tundish superheat to 31-40°C, the secondary cooling water temperature to 20-30°C, and the casting speed to 0.6-0.8m / min; 2) The continuous casting billet heating temperature is 1250-1300°C and the heating time is 3-5h; 3) The rolling adopts two-stage controlled rolling. The first stage rolling temperature is 1180-1210℃, the single-pass reduction rate is ≥15%, the thickness of the rolling intermediate billet is >2t, t is the thickness of the finished steel plate, and the final rolling temperature is 1000-1050℃; The second stage rolling temperature is 880-920℃, the total reduction ratio is >60%, and the final rolling temperature is 750-780℃; 4) After the steel plates come off the production line, they are stacked and slowly cooled for no less than 20 hours; 5) The steel plate is subjected to offline quenching and tempering heat treatment, wherein the quenching and holding temperature is 780-820°C, the holding time is 3-6 min / mm, the quenching cooling rate is 25-30°C / s, the tempering and holding temperature is 550-600°C, the holding time is 150-300 min, and air cooling is performed after leaving the furnace.

5. The method for producing a 1000MPa high-toughness steel for hydropower according to claim 4, characterized in that: The double-linked method is used for deep desulfurization and dephosphorization in molten steel to control P≤0.008% and S≤0.002% in steel; [H]≤3ppm and [O]≤15ppm are reduced through off-furnace refining; and non-metallic inclusions in steel are controlled to ≤1.0 level.

6. The method for producing a 1000MPa high-toughness steel for hydropower according to claim 4, characterized in that: At the end of continuous casting, electromagnetic stirring and light pressure are used. The electromagnetic stirring current is 360-400A and the frequency is 10-13H. Z , the light pressure reduction is 7 to 10 mm.

7. The method for producing 1000MPa high-toughness steel for hydropower according to claim 4, characterized in that: The thickness of the continuous casting billet is 200 to 300 mm, and it is stacked and slowly cooled for more than 24 hours after leaving the production line.

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