High-toughness 80-120mm-thick q420qD bridge plate and preparation method thereof

Abstract

The application provides a high-toughness 80-120mm-thick Q420qD bridge plate and a preparation method thereof, and belongs to the technical field of metal smelting.The bridge plate comprises the following elements in percentage by mass:C 0.05-0.08%, Mn 1.20-1.30%, Nb 0.020-0.025%, V 0.015-0.020%, Cr 0.05-0.10%, Mo 0.03-0.06%, S≤0.005%, P≤0.015%, and the balance of Fe and inevitable impurities.The application realizes the uniformity of the mechanical properties of the whole section of the steel plate by optimizing the component system and the preparation process, not only makes the yield strength, the tensile strength, the elongation and the yield strength ratio all higher than the national standard by 20%, but also ensures that the impact toughness at the position of 1 / 2 of the plate thickness is reduced by not more than 20% compared with the matrix sample, the average impact toughness reaches more than 160J, and the use requirements of large bridge engineering are met.

Description

Technical Field

[0001] This invention belongs to the field of metal smelting technology, and particularly relates to a high-toughness 80-120mm thick Q420qD bridge plate and its preparation method. Background Technology

[0002] As bridge engineering develops towards longer spans and heavier loads, higher requirements are placed on the mechanical properties and service safety of thicker bridge decks. Among them, 80-120mm thick Q420qD bridge decks have become the core material for key parts such as main beams and webs of large bridges due to their strong load-bearing capacity and wide adaptability.

[0003] Currently, a key technical challenge exists in the fabrication of 80-120mm thick Q420qD bridge plates using existing technologies: due to the large plate thickness, the deformation penetration depth during rolling is insufficient, failing to effectively transfer to the half-thickness area (i.e., the central region of the plate). This results in coarse grains and uneven microstructure in this region, leading to substandard impact toughness and failing to meet the mechanical performance requirements of thick plates across the entire cross-section in bridge engineering. Furthermore, existing composition systems and fabrication processes struggle to balance strength and toughness. Simply pursuing increased strength leads to decreased toughness, particularly severe impact performance degradation at the half-thickness area; conversely, prioritizing toughness optimization sacrifices strength, failing to meet the mechanical requirements of Q420qD.

[0004] According to the GB / T 714-2015 standard "Structural Steel for Bridges," Q420qD bridge plates with a thickness of 100-120mm should have a yield strength ≥380MPa, tensile strength 520-660MPa, elongation ≥17%, yield-to-tensile strength ratio ≤0.85, and impact energy ≥34J at -20℃. Existing technologies for preparing 80-120mm thick Q420qD bridge plates not only fail to simultaneously achieve yield strength, tensile strength, elongation, and yield-to-tensile strength ratio that meet national standards, but more importantly, they exhibit large fluctuations in impact toughness at half the plate thickness, often falling below the national standard requirements. This severely impacts the service safety and lifespan of bridge structures, limiting the widespread application of thicker Q420qD bridge plates. Therefore, developing an 80-120mm thick Q420qD bridge plate that solves the problem of substandard impact toughness at half the plate thickness and possesses excellent comprehensive mechanical properties, along with its preparation method, has become a pressing technical challenge in this field. Summary of the Invention

[0005] To address the technical shortcomings of existing 80-120mm thick Q420qD bridge plates, which suffer from insufficient deformation penetration leading to substandard impact performance at half the thickness and difficulty in achieving comprehensive mechanical properties, this invention provides a high-toughness 80-120mm thick Q420qD bridge plate and its preparation method. By optimizing the composition system and preparation process, the mechanical properties of the steel plate are made uniform across the entire cross-section. This not only makes the yield strength, tensile strength, elongation, and yield ratio 20% higher than the national standard, but also ensures that the impact toughness at half the thickness is no more than 20% lower than that of the matrix sample, with an average impact toughness of over 160J, meeting the requirements for use in large-scale bridge engineering.

[0006] The high-toughness 80-120mm thick Q420qD bridge plate of this invention comprises the following elements by mass percentage: C 0.05-0.08%, Mn 1.20-1.30%, Nb 0.020-0.025%, V 0.015-0.020%, Cr 0.05-0.10%, Mo 0.03-0.06%, S≤0.005%, P≤0.015%, with the balance being Fe and unavoidable impurities.

[0007] In this invention, carbon (C) is 0.05-0.08%. Carbon is the foundation for ensuring the strength of steel plates. Adopting a low-carbon design can avoid the decrease in toughness caused by excessive carbon content. At the same time, controlling it within the range of 0.05-0.08% can not only meet the strength requirements of Q420qD level, but also provide a basis for microalloying elements to play their role, reduce segregation defects in the center area of ​​the plate thickness, and improve impact toughness.

[0008] Manganese (Mn): 1.20-1.30%; Manganese can enhance the strength of steel plates through solid solution strengthening, while also improving the toughness and weldability of steel. Controlling the content within the range of 1.20-1.30% can avoid uneven microstructure caused by excessive manganese content. Combined with low-carbon design, it can achieve a synergistic improvement in strength and toughness, while also promoting deformation penetration and improving the microstructure at 1 / 2 of the plate thickness.

[0009] Niobium (Nb): 0.020-0.025%; Niobium is a core microalloying element that can improve the strength and toughness of steel plates through grain refinement and precipitation strengthening. During heating, it forms carbonitrides, effectively preventing austenite grain growth. During rolling, it inhibits austenite recrystallization, refines ferrite grains, and particularly improves grain size at half the plate thickness, thus enhancing impact toughness. Controlling the content within the 0.020-0.025% range avoids the decrease in weldability caused by excessive niobium content, while ensuring its grain refinement effect is fully realized.

[0010] Vanadium (V): 0.015-0.020%; Vanadium works synergistically with niobium to further enhance the strength of steel plates through precipitation strengthening, while refining the grains and improving the toughness and plasticity of the steel plates. When controlled in the range of 0.015-0.020%, it can form a synergistic strengthening effect with niobium, avoid the performance shortcomings caused by the use of a single microalloying element, and further optimize the impact performance at 1 / 2 of the plate thickness.

[0011] Chromium (Cr): Appropriate amount, preferably 0.05-0.10%; Chromium can improve the hardenability and strength of steel plates, improve the corrosion resistance of steel, and appropriate addition can help improve the comprehensive mechanical properties of steel plates. Avoid excessive addition, which will lead to a decrease in toughness and an increase in cost.

[0012] Molybdenum (Mo): Appropriate amount, preferably 0.03-0.06%; Molybdenum can improve the high-temperature strength and hardenability of steel plates, suppress temper brittleness, and work synergistically with chromium to further optimize the strength and toughness of steel plates. In particular, it can improve the uniformity of the microstructure during the rolling process of thick plates and enhance the impact toughness at 1 / 2 of the plate thickness.

[0013] The balance consists of iron (Fe) and unavoidable impurities; the content of impurity elements (such as S and P) is controlled within the national standard allowable range, with S≤0.005% and P≤0.015%, to avoid the adverse effects of impurity elements on the toughness and impact performance of the steel plate.

[0014] The core advantage of the above composition system is that it adopts a low-carbon microalloying design, and through the synergistic effect of niobium, vanadium, chromium and molybdenum, it can effectively refine the grains and improve the microstructure at 1 / 2 of the plate thickness while ensuring the high strength of the steel plate. This solves the problem of unqualified impact performance caused by insufficient deformation penetration, while also taking into account the weldability and corrosion resistance of the steel plate.

[0015] The preparation method of the high-toughness 80-120mm thick Q420qD bridge plate of the present invention includes the following steps: (1) Hot metal pretreatment: Hot metal is pretreated using KR desulfurization process to control the sulfur content of hot metal to ≤0.005% and the phosphorus content to ≤0.015%; (2) Converter smelting: The pretreated molten iron is fed into the converter and blown using a one-time carbon pulling process. The final carbon content is 0.04-0.07% and the final temperature is 1650-1680℃. The steel is pre-alloyed during the tapping process to obtain molten steel. (3) LF refining: The molten steel is fed into the LF refining furnace. During the refining process, the stirring intensity is increased so that the exposed diameter of the molten steel is controlled at 40-50cm. After strong stirring for 1-2 minutes, it is stirred at an appropriate intensity for 3-4 minutes to promote the full floating of inclusions. At the same time, the content of each alloying element is precisely adjusted to ensure that the final composition meets the composition requirements of the present invention. The refining endpoint temperature is controlled at 1580-1600℃ to obtain refined molten steel A. (4) RH refining: The refined molten steel A is sent to the RH refining furnace for vacuum degassing treatment. The vacuum degree is ≤110Pa, the vacuum time is 15-20min, the pure degassing time is ≥6min, and the circulating gas flow rate is >90m³. 3 / h, hydrogen mass fraction ≤2×10 -6 Refined molten steel B is obtained; (5) Continuous casting: The refined steel B is cast under full protection. The RH refined steel is sent to the continuous casting machine for continuous casting. A covering agent is used in the tundish, and protective slag is added to the crystallizer. The liquid level fluctuation is controlled within ±3mm, the superheat is set to 15-30℃, and the casting speed is controlled at 0.8-1.0m / min to prepare a continuous casting slab with a thickness of 200-350mm. (6) Slow cooling: The continuously cast slab is sent into a slow cooling pit and slowly cooled for 48 hours, with the cooling rate controlled at 5-10℃ / h. (7) Heating: The slowly cooled slab is sent into a heating furnace for heating at a temperature of 1150-1200℃ for 3.5-4.5 hours. (8) Differential temperature rolling: Differential temperature rolling process is adopted. Taking into account the deformation penetration characteristics of 80-120mm thick steel plates, the temperature difference between the surface and center of the steel plate is controlled to be 50-80℃, the surface temperature is 950-1000℃, and the center temperature is 870-920℃. The rolling process is divided into rough rolling and finish rolling. The total reduction rate of rough rolling is ≥60%, the initial rolling temperature of finish rolling is ≤950℃, and the final rolling temperature is 830-880℃. (9) Rapid cooling: After rolling, the steel plate is rapidly cooled at a rate of 15-20℃ / s until it reaches 300-350℃. (10) Slow cooling for 24 hours: The rapidly cooled steel plate is sent back into the slow cooling pit for 24 hours of slow cooling at a rate of 3-8℃ / h. (11) Normalizing: The slowly cooled steel plate is sent into a normalizing furnace for normalizing treatment. The normalizing temperature is 900-920℃, and the holding time is related to the thickness of the steel plate by 1.3-1.5 min / mm. Then it is air-cooled to room temperature.

[0016] Preferably, in step (2), 1-2 kg / t of barium aluminum silicon is added for pre-deoxidation during the steel tapping process, and 17-20 kg / t of ferromanganese, 4-5 kg / t of ferrochrome, and 6.0-7.5 kg / t of ferromolybdenum are added for preliminary alloying to obtain molten steel.

[0017] Preferably, the argon flow rate used to increase the stirring intensity in step (3) is 250-350 NL / min; the suitable stirring intensity is 120-200 NL / min.

[0018] Preferably, the covering agent in step (5) is a CaO-Al2O3 system covering agent, with the following specific composition: CaO 45%~50%, Al2O3 35%~40%, SiO2≤4.0%, Cf≤1.5%; The protective slag has a hemispherical point temperature of 1090±45℃, a viscosity of 0.22~0.34Pa.s at 1300℃, an alkalinity of 0.84~0.98, and Cf≤2.0%; the amount of carbon-free low-silicon covering agent added is based on completely covering the intermediate tundish, and the amount of protective slag added is 0.55-0.65kg / t.

[0019] Preferably, the heating rate in step (7) is 5-7℃ / min.

[0020] Preferably, the differential temperature rolling in step (8) is as follows: surface temperature 950℃, center temperature 870℃, total reduction rate of rough rolling 60%, 10 rough rolling passes, 6 finishing rolling passes, finishing rolling temperature 830℃, and rolled into an 80mm thick steel plate.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) It solves the core technical pain points of 80-120mm thick bridge plates: Through the low carbon micro-alloying composition system (C, Mn, Nb, V, Cr, Mo synergistic ratio) and differential temperature rolling process, the deformation penetration depth is effectively improved, which solves the problem of unqualified impact performance caused by insufficient deformation at 1 / 2 of the plate thickness. It ensures that the impact toughness at 1 / 2 of the plate thickness is reduced by no more than 20% compared with the matrix sample, and the average impact toughness reaches more than 160J, which is far higher than the national standard requirement of 34J, significantly improving the service safety of thick plates.

[0022] (2) Excellent comprehensive mechanical properties: The prepared Q420qD bridge plate has yield strength, tensile strength, elongation and yield strength ratio that are 20% higher than the national standard. Specifically, the yield strength is ≥456MPa (national standard ≥380MPa), tensile strength is ≥624MPa (national standard ≥520MPa), elongation is ≥20.4% (national standard ≥17%), and yield strength ratio is ≤0.68 (national standard ≤0.85), which fully meets the mechanical requirements of thick bridge plates for large bridge projects, while also taking into account the weldability and corrosion resistance of the steel plate.

[0023] (3) High process stability: The preparation process of this invention is reasonable, and the parameters of each process are precise and controllable. The cleanliness of the molten steel is ensured by iron pretreatment and LF-RH double refining. Internal stress is eliminated by two slow coolings. The microstructure is optimized by differential rolling and normalizing treatment. It can stably produce 80-120mm thick Q420qD bridge plates with high product qualification rate and is suitable for large-scale industrial production. Detailed Implementation

[0024] Example 1 A method for preparing an 80mm thick Q420qD bridge plate, comprising the following steps: (1) Ingredient design: C: 0.05%, Mn: 1.20%, Nb: 0.020%, V: 0.015%, Cr: 0.05%, Mo: 0.03%, S: 0.004%, P: 0.012%, balance Fe and unavoidable impurities.

[0025] (2) Hot metal pretreatment: Hot metal pretreatment is carried out using KR desulfurization process, and the S content of hot metal is controlled to be ≤0.005% and P content to be ≤0.015%; (3) Converter smelting: The pretreated molten iron is placed in a converter and smelted using a one-time carbon pulling process. The final carbon content is 0.04% and the final temperature is 1650℃. During the tapping process, 1.0 kg / t of silicon aluminum barium is added for pre-deoxidation, and 18 kg / t of ferromanganese, 4.2 kg / t of ferrochrome and 6.8 kg / t of ferromolybdenum are added for preliminary alloying to obtain molten steel. (4) LF refining: The molten steel is fed into the LF refining furnace, the stirring intensity is increased to 260NL / min, the exposed diameter of the molten steel is 40-50cm, and strong stirring is carried out for 1-2 minutes. Then, the stirring intensity is increased to 180NL / min for 3-4 minutes to promote the full floating of inclusions. At the same time, the content of each alloying element is precisely adjusted to ensure that the final composition meets the composition requirements of step (1). The refining endpoint temperature is controlled at 1580℃ to obtain refined molten steel A. (5) RH refining: The refined molten steel A is fed into the RH refining furnace for vacuum degassing treatment. The vacuum degree is 95 Pa, the low vacuum time is controlled at 15 min, the pure degassing time is 6 min, and the circulating gas flow rate is >90 m³ / min. 3 / h, the hydrogen content of molten steel is detected to be ≤2×10 -6 Refined molten steel B was obtained; (6) Continuous casting: Refined steel B is cast under full protective conditions. The RH-refined molten steel is fed into a continuous casting machine for continuous casting. A covering agent is used in the tundish, and protective slag is added to the crystallizer. The liquid level fluctuation is controlled within ±3mm, the superheat is set to 15℃, and the casting speed is controlled at 1.0m / min to prepare a continuous casting slab with a thickness of 200mm. The covering agent is a CaO-Al2O3 system covering agent with the following specific composition: CaO 50%, Al2O3 40%, SiO2 3.5%, C f 1.1%, the remainder is impurities; The protective slag has a hemispherical point temperature of 1090±45℃, a viscosity of 0.26 Pa·s at 1300℃, an alkalinity of 0.87, and a C0.05. f 1.6%; (7) Slow cooling for 48 hours: cooling rate 5℃ / h, to eliminate casting stress; (8) Heating: The continuously cast slab is heated to 1150℃ (heating rate is 6℃ / min) for 3.5h to ensure that the microalloying elements are fully dissolved; (9) Differential temperature rolling: surface temperature 950℃, center temperature 870℃, total reduction rate of rough rolling 60%, 10 passes of rough rolling, 6 passes of finishing rolling, finishing rolling temperature 830℃, rolled into 80mm thick steel plate. (10) Rapid cooling: Cooling rate 15℃ / s, stop when cooling reaches 300℃; (11) Slow cooling for 24 hours: cooling rate 3℃ / h, to eliminate rolling internal stress; (12) Normalizing: The steel plate after slow cooling is held at 900℃ for 104 min (1.3 min / mm×80 mm) and then air-cooled to room temperature to obtain the bridge plate.

[0026] The performance of the bridge deck was tested, and the results are as follows: Yield strength: 460MPa (better than the national standard GB / T714-2025 420MPa, 21.1% higher); Tensile strength: 630MPa (better than the national standard of 540MPa, 21.2% higher); Elongation: 21% (better than the national standard of 19%, 23.5% higher); Yield-to-tensile strength ratio: 0.73 (14.1% higher than the national standard); Impact toughness at 1 / 2 plate thickness: average value 165J, which is 16.7% lower than that of the matrix sample (198J) (≤20%), but is significantly better than the national standard of 34J.

[0027] All performance indicators meet the requirements of this invention and the standards for use in bridge engineering.

[0028] Example 2 A method for preparing a 100mm thick Q420qD bridge plate, comprising the following steps: (1) Ingredient design: C: 0.065%, Mn: 1.25%, Nb: 0.022%, V: 0.018%, Cr: 0.07%, Mo: 0.045%, S: 0.003%, P: 0.010%, balance Fe and unavoidable impurities.

[0029] (2) Hot metal pretreatment: Hot metal pretreatment is carried out using KR desulfurization process, and the S content of hot metal is controlled to be ≤0.005% and P content to be ≤0.015%; (3) Converter smelting: The pretreated molten iron is placed in a converter and blown using a one-time carbon pulling process. The final carbon content is 0.055% and the final temperature is 1665℃. During the tapping process, 1.2 kg / t of silicon aluminum barium pre-deoxidation is used, and 18.5 kg / t of ferromanganese, 4.3 kg / t of ferrochrome and 7.0 kg / t of ferromolybdenum are added for preliminary alloying to obtain molten steel. (4) LF refining: The molten steel is fed into the LF refining furnace, the stirring intensity is increased to 280NL / min, the exposed diameter of the molten steel is 40-50cm, and after strong stirring for 1-2min, it is stirred for 4min at a suitable intensity and flow rate of 160NL / min to promote the full floating of inclusions; at the same time, the content of each alloy element is precisely adjusted to ensure that the final composition meets the composition requirements of step (1), and the refining endpoint temperature is controlled at 1590℃ to obtain refined molten steel A; (5) RH refining: Refined steel A is fed into the RH refining furnace for vacuum degassing treatment. The vacuum degree is 90 Pa, the low vacuum time is controlled at 18 min, the pure degassing time is 7 min, and the circulating gas flow rate is >90 m³ / min. 3 / h, the hydrogen content of molten steel is detected to be ≤2×10 -6 Refined molten steel B was obtained; (6) Continuous casting: Refined steel B was cast under protective conditions. The superheat of the steel was 22°C and the casting speed was 0.9 m / min. A 220 mm thick continuous casting slab was prepared. The other parameters were the same as in Example 1. (7) Slow cooling for 48 hours: cooling rate 8℃ / h, to eliminate casting stress; (8) Heating: The continuously cast slab is heated to 1180℃ (heating rate is 5℃ / min) for 4.0h to ensure that the microalloying elements are fully dissolved; (9) Differential temperature rolling: surface temperature 970℃, center temperature 890℃, total reduction rate of rough rolling 62%, rough rolling 10 passes, finish rolling 6 passes, finish rolling final temperature 850℃, rolled into 100mm thick steel plate. (10) Rapid cooling: Cooling rate 18℃ / s, stop when cooling to 320℃; (11) Slow cooling for 24 hours: cooling rate 5℃ / h to eliminate rolling internal stress; (12) Normalizing: The steel plate after slow cooling is held at 910℃ for 140 min (1.4 min / mm×100mm) and then air-cooled to room temperature to obtain the bridge plate.

[0030] The performance of the bridge deck was tested, and the results are as follows: Yield strength: 475MPa (better than the national standard of 420MPa, 25.0% higher); Tensile strength: 645MPa (better than the national standard of 540MPa, 24.0% higher); Elongation: 22% (better than the national standard of 19%, 29.4% higher); Yield-to-tensile strength ratio: 0.74 (12.9% higher than the national standard); Impact toughness at 1 / 2 plate thickness: average value 172J, which is 18.1% lower than that of the matrix sample (210J) (≤20%), but is significantly better than the national standard of 34J.

[0031] All performance indicators meet the requirements of this invention and the standards for use in bridge engineering.

[0032] Example 3 A method for preparing a 120mm thick Q420qD bridge plate, the steps of which are as follows: (1) Ingredient design: C: 0.08%, Mn: 1.30%, Nb: 0.025%, V: 0.020%, Cr: 0.10%, Mo: 0.06%, S: 0.005%, P: 0.015%, balance Fe and unavoidable impurities.

[0033] (2) Hot metal pretreatment: Hot metal pretreatment is carried out using KR desulfurization process, and the S content of hot metal is controlled to be ≤0.005% and P content to be ≤0.015%; (3) Converter smelting: The pretreated molten iron is placed in a converter and blown using a one-time carbon pulling process. The final carbon content is 0.07% and the final temperature is 1680℃. During the tapping process, 1.3 kg / t of silicon aluminum barium pre-deoxidation is used, and 18.6 kg / t of ferromanganese, 4.8 kg / t of ferrochrome and 7.1 kg / t of ferromolybdenum are added for preliminary alloying to obtain molten steel. (4) LF refining: The molten steel is fed into the LF refining furnace, the stirring intensity is increased to 300NL / min, the exposed diameter of the molten steel is 40-50cm, and after strong stirring for 1-2min, it is stirred for 4min at a suitable intensity and flow rate of 180NL / min to promote the full floating of inclusions; at the same time, the content of each alloying element is precisely adjusted to ensure that the final composition meets the composition requirements of step (1), and the refining endpoint temperature is controlled at 1600℃ to obtain refined molten steel A; (5) RH refining: The refined molten steel A is fed into the RH refining furnace for vacuum degassing treatment. The vacuum degree is 100 Pa, the low vacuum time is 20 min, the pure degassing time is 8 min, and the circulating gas flow rate is >90 m³ / min. 3 / h, the hydrogen content of molten steel is detected to be ≤2×10 -6 Refined molten steel B was obtained; (6) Continuous casting: Refined steel B was cast under protective conditions with a superheat of 30°C and a casting speed of 0.8 m / min to prepare a 350 mm thick continuous casting slab; other parameters were the same as in Example 1. (7) Slow cooling for 48 hours: cooling rate 10℃ / h, to eliminate casting stress; (8) Heating: The continuously cast slab is heated to 1200℃ (heating rate is 7℃ / min) for 4.5h to ensure that the microalloying elements are fully dissolved; (9) Differential temperature rolling: surface temperature 1000℃, center temperature 920℃, total reduction rate of rough rolling 65%, rough rolling 10 passes, finishing rolling 6 passes, finishing rolling final rolling temperature 880℃, rolled into 120mm thick steel plate. (10) Rapid cooling: Cooling rate 20℃ / s, stop when cooling reaches 350℃; (11) Slow cooling for 24 hours: cooling rate 8℃ / h to eliminate rolling internal stress; (12) Normalizing: The steel plate after slow cooling is held at 920℃ for 180 min (1.5 min / mm×120 mm) and then air-cooled to room temperature to obtain the bridge plate.

[0034] The performance of the bridge deck was tested, and the results are as follows: Yield strength: 480MPa (better than the national standard of 420MPa, 26.3% higher); Tensile strength: 650MPa (better than the national standard of 540MPa, 25.0% higher); Elongation: 21.5% (better than the national standard of 19%, and higher than 26.5%). Yield-to-tensile strength ratio: 0.74 (12.9% higher than the national standard); Impact toughness at 1 / 2 plate thickness: average value 168 J, a decrease of 18.0% (≤20%) compared to the matrix sample (205 J), but significantly better than the national standard of 34 J.

[0035] All performance indicators meet the requirements of this invention and the standards for use in bridge engineering.

[0036] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-toughness 80-120mm thick Q420qD bridge plate, characterized in that, Elements including the following mass percentages: C 0.05-0.08%, Mn 1.20-1.30%, Nb 0.020-0.025%, V 0.015-0.020%, Cr 0.05-0.10%, Mo 0.03-0.06%, S≤0.005%, P≤0.015%, with the balance being Fe and unavoidable impurities.

2. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 1, characterized in that, Includes the following steps: (1) Hot metal pretreatment: Hot metal is pretreated using KR desulfurization process to control the sulfur content of hot metal to ≤0.005% and the phosphorus content to ≤0.015%; (2) Converter smelting: The pretreated molten iron is fed into the converter and blown using a one-time carbon pulling process. The final carbon content is 0.04-0.07% and the final temperature is 1650-1680℃. The steel is pre-alloyed during the tapping process to obtain molten steel. (3) LF refining: The molten steel is fed into the LF refining furnace. During the refining process, the stirring intensity is increased so that the exposed diameter of the molten steel is controlled at 40-50cm. After strong stirring for 1-2 minutes, it is stirred at an appropriate intensity for 3-4 minutes to promote the full floating of inclusions. At the same time, the content of each alloying element is precisely adjusted to ensure that the final composition meets the composition requirements of the present invention. The refining endpoint temperature is controlled at 1580-1600℃ to obtain refined molten steel A. (4) RH refining: The refined molten steel A is sent to the RH refining furnace for vacuum degassing treatment. The vacuum degree is ≤110Pa, the vacuum time is 15-20min, the pure degassing time is ≥6min, and the circulating gas flow rate is >90m³. 3 / h, hydrogen mass fraction ≤2×10 -6 Refined molten steel B is obtained; (5) Continuous casting: The refined steel B is cast under full protection. The RH refined steel is sent to the continuous casting machine for continuous casting. A covering agent is used in the tundish, and protective slag is added to the crystallizer. The liquid level fluctuation is controlled within ±3mm, the superheat is set to 15-30℃, and the casting speed is controlled at 0.8-1.0m / min to prepare a continuous casting slab with a thickness of 200-350mm. (6) Slow cooling: The continuously cast slab is sent into a slow cooling pit and slowly cooled for 48 hours, with the cooling rate controlled at 5-10℃ / h. (7) Heating: The slowly cooled slab is sent into a heating furnace for heating at a temperature of 1150-1200℃ for 3.5-4.5 hours. (8) Differential temperature rolling: Differential temperature rolling process is adopted. Taking into account the deformation penetration characteristics of 80-120mm thick steel plates, the temperature difference between the surface and center of the steel plate is controlled to be 50-80℃, the surface temperature is 950-1000℃, and the center temperature is 870-920℃. The rolling process is divided into rough rolling and finish rolling. The total reduction rate of rough rolling is ≥60%, the initial rolling temperature of finish rolling is ≤950℃, and the final rolling temperature is 830-880℃. (9) Rapid cooling: After rolling, the steel plate is rapidly cooled at a rate of 15-20℃ / s until it reaches 300-350℃. (10) Slow cooling for 24 hours: The rapidly cooled steel plate is sent back into the slow cooling pit for 24 hours of slow cooling at a rate of 3-8℃ / h. (11) Normalizing: The slowly cooled steel plate is sent into a normalizing furnace for normalizing treatment. The normalizing temperature is 900-920℃, and the holding time is related to the thickness of the steel plate by 1.3-1.5 min / mm. Then it is air-cooled to room temperature.

3. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 2, characterized in that, In step (2), 1-2 kg / t of barium aluminum silicon is added for pre-deoxidation during the tapping process, while 17-20 kg / t of ferromanganese, 4-5 kg / t of ferrochrome, and 6.0-7.5 kg / t of ferromolybdenum are added for preliminary alloying to obtain molten steel.

4. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 2, characterized in that, In step (3), the argon flow rate used to increase the stirring intensity is 250-350 NL / min; the suitable stirring intensity is 120-200 NL / min.

5. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 2, characterized in that, The covering agent in step (5) is a CaO-Al2O3 system covering agent; the hemispherical point temperature of the protective slag is 1090±45℃, the viscosity at 1300℃ is 0.22~0.34Pa.s, the basicity is 0.84~0.98, and Cf≤2.0%; the amount of carbon-free and low-silicon covering agent added is based on the requirement of completely covering the intermediate tundish, and the amount of protective slag added is 0.55-0.65kg / t.

6. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 2, characterized in that, The heating rate in step (7) is 5-7℃ / min.

7. The method for preparing a high-toughness 80-120mm thick Q420qD bridge plate according to claim 2, characterized in that, The differential temperature rolling process described in step (8) is as follows: surface temperature 950℃, center temperature 870℃, total reduction rate of rough rolling 60%, 10 rough rolling passes, 6 finishing rolling passes, and finishing rolling temperature 830℃.