960mpa grade bridge structural steel with low internal stress and excellent fracture toughness and a preparation method thereof

Abstract

The present application relates to a kind of 960MPa grade bridge structural steel with low internal stress fracture toughness and excellent and its preparation method, the steel contains the following weight percentage content chemical composition: C:0.055~0.105%, Si:0.05~0.15%, Mn:1.65~1.75%, P≤0.010%, S≤0.003%, Ni:0.80~1.80%, Cr:0.45~0.75%, Mo:0.20~0.30%, Nb:0.060~0.080%, Ti:0.035~0.050%, Als:0.015~0.045%, the balance is Fe and inevitable impurity element;Wherein chemical composition needs to satisfy:0.188≤Pcm≤0.275, 0.513≤CEV≤0.717;Production step includes: smelting, slab heating, rolling, ACC controlled cooling, intercritical quenching, stress relief annealing heat treatment;The finished steel plate prepared by the present application has yield strength ≥985MPa, tensile strength ≥1100MPa, excellent low temperature toughness, longitudinal KV2 at-40 ℃ is ≥120J, fracture toughness, the fracture toughness of material under room temperature condition is ≥0.26mm, and the absolute value of steel plate residual internal stress is ≤18MPa;The 960MPa grade high-performance bridge steel plate prepared by the method of the present application has high strength and toughness, low internal stress, excellent welding performance, meets the application requirements of bridge, high-rise building and other steel structures.

Description

Technical Field

[0001] This invention relates to the field of low alloy steel manufacturing technology, and in particular to a 960MPa grade structural steel for bridges with excellent fracture toughness under low internal stress and its preparation method. Background Technology

[0002] In recent years, bridge engineering has rapidly developed towards high speed, heavy load, and long span, accompanied by higher requirements and functionalization of materials. High strength helps reduce structural self-weight, improving bridge spanning capacity and load-bearing capacity; high toughness enhances the low-temperature toughness, fatigue fracture resistance, seismic resistance, and impact resistance of structural materials; functionalization includes corrosion resistance, ease of welding, and high heat input welding, etc. In recent years, the market demand for high-performance bridge steel has been increasing, and its application prospects are promising. High-performance bridge steel is usually produced using TMCP + tempering stress relief process. In order to ensure the strength and yield strength ratio of the steel plate, the tempering temperature is not high and the tempering time is short. The steel plate still has large structural stress and thermal stress problems, which brings difficulties to users' deep processing and poses hidden dangers to bridge safety. Secondly, the actual production thickness is also limited by the online cooling capacity, making it difficult to improve the strength of the steel plate. Finally, under normal circumstances, as the strength of the steel plate increases, the low-temperature toughness, fracture toughness, and weldability of the material will decrease.

[0003] Chinese patent application CN200510024775.3, entitled "Ultra-high strength steel plate with yield strength above 960MPa and its manufacturing method", discloses an ultra-high strength steel plate with yield strength above 960MPa and its manufacturing method. Its composition (by weight percentage) is as follows: C 0.08~0.18%; Si≤0.6%; Mn 0.5~2.0%; Al≤0.018%; N≤0.008%; B≤0.0025%; Ca 0~0.006%; P≤0.015%; S≤0.005%; one or more of Ni≤1.0%, Cr≤0.8%, Cu≤0.5%, and Mo≤0.6%; one or more of Ti 0.01~0.03%, V≤0.1%, and Nb 0.01~0.1%; residual iron and unavoidable impurities. The production method involves heating a steel billet to 1100–1250°C; rolling the billet into a steel plate in the austenite recrystallization zone; rolling the steel plate into a final thickness in the non-recrystallization zone of austenite at a final rolling temperature of 860–920°C; cooling the steel plate to a cooling termination temperature below Ms to Ms+100°C at a cooling rate of not less than approximately 5°C / s; and tempering the cooled steel plate to improve its properties. This invention's steel has high C and Mn content, which is detrimental to center segregation, increases carbon equivalent, and is unfavorable for weldability; its high Si content is detrimental to impact toughness; while adding Cu alloying element can improve strength, Cu element tends to accumulate on the steel plate surface and at grain boundaries, generating microcracks; and when V alloying element exists in the steel in the form of carbides, it is detrimental to the hardenability of the steel. In addition, the steel of this invention is produced by online direct quenching and tempering process. Due to the limitations of production equipment capacity, the thickness of the steel plate is restricted. When a certain thickness is reached, the cooling rate of the steel plate cannot be increased, and the strength cannot be guaranteed.

[0004] Chinese patent application CN201310218152.4, entitled "Ultra-high strength high-titanium steel plate with a yield strength of 960MPa and its manufacturing method," discloses an ultra-high strength high-titanium steel plate with a yield strength of 960MPa and its manufacturing method. The steel plate is composed of the following components by weight percentage: C: 0.06–0.09%, Si: 0.10–0.50%, Mn: 1.00–1.60%, P: <0.01%. The composition is as follows: 5%, S: <0.005%, Cr: 0.20-0.50%, Mo: 0.15-0.30%, Nb: 0.02-0.05%, Ti: 0.06-0.20%, B: 0.0005-0.003%, Cu: ≤0.10%, Al: 0.01-0.05%, N: <0.005%, O: <0.003%, Ni: 0-0.30%, with the remainder being Fe and unavoidable impurities. The manufacturing method includes smelting and casting; heating of continuously cast billets; two-stage rolling; and tempering. The steel plate of this invention possesses good toughness and cold bending performance. The alloy composition of this invention's steel is relatively economical. However, the low C content of the Ti-containing titanium steel makes it prone to defects due to the high viscosity of the molten steel, which makes it difficult for non-metallic inclusions to separate and float. The hardness of the quenched titanium steel decreases with increasing Ti content. Ti has a strong affinity for N and O, making it highly susceptible to forming non-metallic inclusions such as TiN and TiO2, as well as subcutaneous pores and other defects. In addition, the TMCP + tempering process used in the production of this invention's steel is limited by the capacity of the production equipment, restricting the thickness of the steel plate. Once a certain thickness is reached, the cooling rate of the steel plate cannot be increased, and the strength cannot be guaranteed.

[0005] Chinese patent application CN201610313584.7, entitled "A steel for engineering machinery with a yield strength ≥960MPa and a production method thereof," discloses a steel for engineering machinery with a yield strength ≥960MPa and a production method thereof. The composition and wt% of the steel are: C: 0.04~0.20%, Si: 0.15~0.40%, Mn: 1.0~10%, N: 0.01~0.08%, Ti: 0.010~0.050%, Als: 0.01~0.06%, P: ≤0.020%, S: ≤0.010%, Cr: 0.05~3%. The production steps are: smelting and vacuum treatment; tapping and casting into billets and heating; hot rolling; and cold rolling. This invention proposes an engineering steel with a yield strength ≥960MPa, elongation ≥15.0%, AKV not less than 60J at -20℃, dimple characteristics on the impact fracture surface, and excellent strength-toughness matching. The strength of the steel is ensured by adding a suitable amount of nitrogen (N) to the chemical composition, eliminating the need for expensive microalloying elements such as Mo, V, and Nb. The rolling process of this steel is simple and easy to implement, requiring controlled cooling and eliminating the need for heat treatment. However, when used in the engineering machinery field, this steel is produced using the TMCP process without heat treatment. This results in high internal stress and low fracture toughness, making it unsuitable for bridge manufacturing. Furthermore, the high economic efficiency of this steel, without the addition of expensive alloying elements, leads to insufficient toughness, failing to meet the 120J impact toughness requirement for bridge steel.

[0006] Chinese patent application CN201810310107.4, entitled "A Method for Producing Low-Inclusion Pipeline Steel," provides a method for producing low-inclusion pipeline steel. The method is characterized by the following composition of the pipeline steel: C 0.062-0.065%, Si 0.25-0.30%, Mn 1.75-1.78%, P≤0.015%, S≤0.005%, Nb 0.070-0.095%, Ti 0.010-0.012%, Al≤0.050%, Cr 0.50-0.65%, Mo 0.55-0.60%, Ni 0.20-0.25%, Cu 0.15-0.19%, W 0.005-0.008%, Zr 0.0001-0.0004%, and Ta... The composition is as follows: 0.0001-0.005% rare earth (Sc+Y), 0.0001-0.0005% Sc:Y mass ratio is 4-5:1, B 0.0001-0.0005%, N 0.001-0.005%, with the balance being Fe and unavoidable impurity elements. The final microstructure consists of 90-92% acicular ferrite, 3-5% polygonal ferrite, and 3-5% masonry, with an average grain size of 2-5 μm. The yield strength is 930-960 MPa, the tensile strength is 1050-1250 MPa, the impact toughness at -40℃ is 150-170 J, and the elongation is 17-19%. The process route includes: proportioning and preparation → hot metal pretreatment and desulfurization → converter smelting → LF refining → RH refining → continuous casting → rolling → coiling. This type of steel is produced using a hot continuous rolling line and the TMCP process, limiting the thickness to within 25mm. This cannot meet the production demand for thicker steel plates. Furthermore, the hot continuous rolling line rolls the steel plate from bottom to top, resulting in high internal stress and significant anisotropy in performance, which is detrimental to the processing of bridge steel structures and the safety requirements throughout their entire life cycle. The high Si content in the steel is also unfavorable to impact toughness. Although adding Cu alloying element can improve strength, Cu element tends to accumulate on the surface of the steel plate and at grain boundaries, generating microcracks. Summary of the Invention

[0007] The purpose of this invention is to develop a 960MPa grade structural steel for bridges with excellent fracture toughness under low internal stress, based on the current development needs of large-span, heavy-load, single / double-layer, and multi-lane steel structure bridge projects, and its preparation method. The steel plate prepared by this invention has low internal stress, high fracture toughness, excellent comprehensive performance, and simple manufacturing process, which meets the development needs of a resource-saving and environmentally friendly society.

[0008] This invention discloses a 960MPa grade bridge structural steel with excellent low internal stress fracture toughness. The steel contains the following chemical composition by weight percentage: C: 0.055~0.105%, Si: 0.05~0.15%, Mn: 1.65~1.75%, P: ≤0.010%, S: ≤0.003%, Ni: 0.80~1.80%, Cr: 0.45~0.75%, Mo: 0.20~0.30%, Nb: 0.060~0.080%, Ti: 0.035~0.050%, Als: 0.015~0.045%, with the balance being Fe and unavoidable impurity elements. The chemical composition must meet the following requirements:

[0009] 0.188≤Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.275;

[0010] 0.513≤CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15≤0.717.

[0011] The finished steel plate obtained by this invention has a yield strength ≥985MPa, tensile strength ≥1100MPa, excellent low-temperature toughness, longitudinal KV2 ≥120J at -40℃, fracture toughness, fracture toughness of the material at room temperature ≥0.26mm, and absolute value of residual internal stress of the steel plate ≤18MPa.

[0012] The present invention discloses a method for preparing 960MPa grade bridge structural steel with excellent low internal stress fracture toughness, comprising the following steps:

[0013] 1) Smelting: Hot metal desulfurization + converter LD + LF desulfurization and temperature control + RH vacuum degassing + continuous casting CC, smelting and producing continuous casting slabs according to the above steel composition design.

[0014] 2) Slab heating: The slab is heated in the furnace with a temperature control range of 600~1280℃. The first heating control range is 600~1100℃, with rapid heating; the second heating control range is 1130~1230℃, with a heating rate of 10~11 min / cm; the third heating control range is 1220~1280℃, with a heating rate of 9~10 min / cm; the soaking zone temperature control range is 1200~1240℃, with a soaking time ≥20 min; and the tapping temperature range is 1160~1200℃.

[0015] 3) Rolling: Finishing rolling is carried out in the non-recrystallized austenite region, with an initial rolling temperature of 830-950℃ and an intermediate billet thickness of 36-130mm; the cumulative reduction rate in the finishing rolling stage is ≥60%, and the final rolling temperature is 760-860℃;

[0016] 4) ACC controlled cooling: After the steel plate is rolled, it is straightened by a pre-straightening machine, and then the steel plate is cooled. The cooling rate is controlled at 3-5℃ / second, and the final cooling temperature is controlled at 600-650℃.

[0017] 5) Sub-temperature quenching: The steel plate is subjected to sub-temperature quenching. The quenching temperature is 760~860℃ and the furnace time is (h+30)min, where h is the thickness of the finished steel plate in mm.

[0018] 6) Stress-relieving annealing heat treatment: Stress-relieving annealing heat treatment is performed on quenched steel plates at a temperature of 520~600℃; the holding time at the temperature is 2~4h; the heating rate is unlimited below 400℃; the heating rate is 90~120℃ / h above 400℃; the cooling rate is 90~120℃ / h above 350℃, and unlimited below 350℃.

[0019] Furthermore, in step (1) above, the temperature difference between the molten steel and the liquidus is controlled at 15~30℃ during the casting process. Electromagnetic stirring is adopted in the second cooling section. Through the generated electromagnetic force, the equiaxed crystal ratio of the billet is improved, and a billet with a good solidification structure is obtained. Dynamic light pressing is adopted at the end of the billet solidification, and the amount of light pressing is controlled at 1~3mm.

[0020] Furthermore, after the final rolling of step (3) above is completed, an additional leveling pass is added, with the reduction controlled at 2~12mm.

[0021] The reasons for limiting the main chemical composition of the steel in this invention are as follows:

[0022] The steel composition design of this invention is economical. It adopts a low C-low Si-medium Mn-micro (Nb, V, Ti) treatment, rationally adds Mo, Ni, and Cr alloying elements, is produced by controlled rolling process, and supplemented by sub-temperature quenching and stress-relieving annealing heat treatment to obtain a 960MPa grade high-performance bridge steel with excellent comprehensive performance, including high strength, high fracture toughness, low yield strength ratio, low internal stress, and excellent weldability.

[0023] Carbon (C) is essential for the strength of steel. However, C affects the center segregation of the cast billet, as well as the toughness, plasticity, and weldability of the steel plate. Therefore, a lower C content in the steel is preferable. Taking into account the strength, yield strength ratio, toughness, plasticity, weldability, and microstructure control of the steel plate, the carbon content of the steel in this invention is designed to be 0.055~0.095%.

[0024] In steelmaking, silicon (Si) is generally used as a reducing agent and deoxidizer. Silicon can significantly improve the fatigue strength, fatigue ratio (σ⁻¹ / σb, the ratio of fatigue strength to tensile strength; a higher σb indicates better fatigue resistance), elastic limit, yield point, and tensile strength of steel. Excessive Si addition leads to poor surface quality of the steel plate; increasing Si content promotes the formation of island-like martensite, which is detrimental to the toughness of the weld heat-affected zone. Furthermore, Si easily promotes solidification segregation in molten steel, worsening the low-temperature impact toughness of high-strength steel plates and negatively impacting fracture toughness. Considering all these factors, the Si content in the steel of this invention is controlled within the range of 0.05% to 0.15%.

[0025] Mn is an important element for strength and toughness in steel, but it tends to agglomerate and segregate during the solidification of molten steel, which can be inherited by the core of the steel plate, resulting in a decrease in low-temperature toughness. To ensure both strength and toughness of the steel plate, the Mn content of the steel in this invention is designed to be 1.65~1.75%.

[0026] Phosphorus (P) is one of the harmful elements in steel. Increased phosphorus content increases the strength of steel but decreases its plasticity and toughness, reducing its machinability. For high-strength steel, machinability is already difficult due to its high strength. Therefore, the P content of the steel in this invention is designed to be ≤0.010%.

[0027] Sulfur (S) is an impurity element in steel. S readily segregates and accumulates in steel, forming elongated MnS inclusions that reduce the impact toughness of the steel plate. S also contributes to hot brittleness, leading to decreased weldability, impact toughness, fatigue resistance, and corrosion resistance. The steel of this invention strictly controls the sulfur content level, i.e., S ≤ 0.003%.

[0028] Cr is an effective element for improving the strength of steel plates, and its effect is even more significant when other elements are added. Cr slows down the decomposition rate of austenite, significantly improving the hardenability of steel. However, when the Cr content is high, the impact toughness decreases sharply. Taking all factors into consideration, the Cr content in this invention is controlled at 0.45~0.75%.

[0029] Ni can reduce the resistance to dislocation movement in steel and decrease internal stress, thereby improving low-temperature toughness. For high-performance bridge steel, adding Ni to improve low-temperature toughness is even more necessary. Taking all factors into consideration, the Ni content of the steel in this invention is controlled at 0.80~1.80%.

[0030] Mo plays a crucial role in steel. It can increase the strength and hardness of steel, improve its ductility and toughness, and enhance its hardenability (slightly less than chromium). Mo can also improve the creep resistance of ferrite, effectively inhibit cementite aggregation, and improve thermal stability. The Mo content of the steel in this invention is controlled at 0.20~0.30%.

[0031] Nitrogen (Nb) is a strong carbide-forming element that can expand the austenite non-crystallization region in controlled-rolled steel and inhibit the recrystallization and growth of austenite in high-temperature regions. Nb precipitates in ferrite, increasing the strength of the steel while preventing HAZ grain coarsening during welding. However, excessively high Nb content can reduce the weldability of the steel plate. This invention controls the Nb content to 0.060~0.080%.

[0032] Ti is a strong carbide and nitride forming element, which can maximally refine austenite and ferrite grains. Due to its high decomposition temperature and low growth rate, the precipitates exhibit strong precipitation strengthening effect. Combined with the rolling process, strain-induced precipitation improves its high-temperature strength and low-temperature toughness. Ti nitride precipitates can effectively pin austenite grain boundaries, inhibit austenite grain growth, and greatly improve the low-temperature toughness of the weld heat-affected zone of the steel. However, excessively high Ti content can lead to a decrease in the plasticity and toughness of the steel. This invention controls the Ti element content to 0.035~0.050%.

[0033] Al deoxidation significantly reduces the oxygen content in steel. It also refines grains, fixes nitrogen, and improves the low-temperature impact toughness of the base metal and the weld heat-affected zone. The ALs content of the steel in this invention is controlled at 0.015~0.045%.

[0034] The present invention further defines:

[0035] 1) 0.188≤Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.275; This limiting relationship ensures that the steel plate has high strength while also having good resistance to cracking. It meets the requirement that no cracks appear in the welded joints after manual welding and submerged arc welding tests, meaning the steel plate has good resistance to thermal cracking.

[0036] 2) 0.513≤CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15≤0.717; Besides carbon, alloying elements in steel also play an important role in the strength and weldability of steel. This limiting relationship ensures that the steel plate has high strength while the welded joint has a low hardenability tendency, and the maximum hardness of the joint meets the application requirements; it can be welded at room temperature without preheating or at a low preheating temperature, and has good weldability.

[0037] The rationale for setting up the steel production process in this invention:

[0038] (1) Smelting and casting

[0039] Hot metal desulfurization + converter LD + LF desulfurization and temperature control + RH vacuum degassing + continuous casting CC, according to the steel composition design of this invention, continuously cast slabs are produced.

[0040] The steel of this invention is smelted in a top-and-bottom blown converter, refined in an LF+RH ladle, and produced by continuous casting. During casting, the temperature difference between the molten steel and the liquidus is controlled at 15~30℃. Electromagnetic stirring is used in the secondary cooling section. The generated electromagnetic force increases the equiaxed crystal ratio of the billet, resulting in a billet with a good solidification structure. At the end of the billet solidification, dynamic light reduction is applied, with the reduction amount controlled at 1~3mm. The pressure is applied to deform the billet shell to compensate for the solidification shrinkage in the two-phase region, promoting a denser central structure and reducing central porosity and segregation.

[0041] (2) Slab heating

[0042] The slab is heated in the furnace with a temperature control range of 600~1280℃. The first heating control range is 600~1100℃, with rapid heating. The second heating control range is 1130~1230℃, with a heating rate of 10~11 min / cm. The third heating control range is 1220~1280℃, with a heating rate of 9~10 min / cm. The soaking zone temperature control range is 1200~1240℃, with a soaking time of ≥20 min. The target tapping temperature is 1160~1200℃.

[0043] To ensure sufficient solid solution of microalloying elements, prevent coarse austenite grains, and avoid overheating, burning, and cracking during heating, appropriate slab heating temperatures, heating rates at each stage, and soaking times must be adopted.

[0044] 3) Rolling: Finish rolling is carried out in the non-recrystallized austenite region, with an initial rolling temperature of 830-950℃ and an intermediate billet thickness of 36-130mm; the cumulative reduction rate in the finish rolling stage is ≥60%, and the final rolling temperature is 760-860℃. After the final rolling is completed, an additional leveling pass is added, with the reduction controlled at 2-12mm.

[0045] The steel of this invention adopts a controlled rolling mode, which controls the finishing rolling start temperature, intermediate billet thickness, cumulative reduction rate and final rolling temperature, so that the austenite grains can be fully recovered and refined; in order to improve the plate shape of high-strength steel, a leveling pass is added in the last pass, without empty passing, and the reduction of the leveling pass is controlled at 2~12mm.

[0046] 4) ACC Controlled Cooling: After the steel plate is rolled, it is straightened by a pre-straightening machine, with the pre-straightening reduction controlled at 2~4mm. The steel plate is then cooled. The cooling rate is controlled at 3~5℃ / second, and the final cooling temperature is controlled at 600~650℃.

[0047] After rolling, the steel plate is subjected to appropriate ACC cooling to improve the density and continuity of the iron oxide scale on the surface of the steel plate. The steel plate is pre-straightened before entering ACC to improve the plate shape and increase the flatness qualification rate.

[0048] 5) Sub-temperature quenching: The steel plate is subjected to sub-temperature quenching. Quenching temperature: 760~860℃, furnace time: (h+30)min.

[0049] Sub-critical quenching, typically performed between AC1 and AC3 temperatures, significantly improves the toughness of steel and lowers the brittle transition temperature. It refines the grain size, resulting in a suitable amount of uniformly distributed fine grains. Sub-critical quenching preserves a small amount of stable material, reducing crack formation. It also reduces the segregation of harmful impurities, purifying the steel. Furthermore, the low internal stress generated by sub-critical quenching allows for the use of lower stress-relieving tempering temperatures, effectively eliminating internal stress. In summary, sub-critical quenching endows steel plates with both high strength and high toughness, giving high-strength steel good fracture toughness.

[0050] 6) Stress-relieving annealing heat treatment: Stress-relieving annealing heat treatment is performed on quenched steel plates. Treatment temperature: 520~600℃; holding time at temperature: 2~4h; heating rate: below 400℃, heating rate is unlimited; above 400℃, heating rate is 90~120℃ / h; cooling rate: below 350℃, cooling rate is unlimited; above 350℃, cooling rate is 90~120℃ / h.

[0051] For medium- and low-carbon sub-temperature quenched bridge steel, residual stress generated during cooling can lead to problems such as warping, deformation, softening, and cracking during processing and use. Therefore, the steel of this invention employs stress-relieving annealing heat treatment. The steel plate is heated to the target temperature at a limited heating rate above 400°C; after reaching the target temperature, it is held at that temperature for a period of time to allow relaxation within the steel plate; then, it is cooled to 350°C at a limited cooling rate, and finally arbitrarily cooled to room temperature. Through this process, the internal stress of the steel plate is low and uniform, and all mechanical properties meet the technical requirements. Simultaneously, after sufficient stress relief, the steel plate exhibits excellent comprehensive performance of the base material, strong adaptability to welding processes, and can be welded manually, using CO2+Ar shielded welding, and high-intensity submerged arc welding. Furthermore, the welded joints exhibit excellent performance and superior toughness in the heat-affected zone.

[0052] To achieve excellent overall performance and ensure high strength, high toughness, low internal stress, high fracture toughness, and excellent weldability of the steel plate, this invention employs a low-carbon, micro-alloyed composition design and a sub-critical quenching and stress-relieving annealing process to produce high-performance structural steel for bridges. The rational composition design and sub-critical quenching + stress-relieving annealing heat treatment enable the steel to acquire high strength and toughness. Simultaneously, the high annealing temperature and long time fully release residual stress, resulting in stable and excellent overall performance of the steel plate, including high strength, high toughness, low internal stress, high fracture toughness, and good weldability.

[0053] The steel of this invention has the following advantages:

[0054] 1) The steel manufacturing process of this invention is simple, has low production cost, is environmentally friendly, and is highly operable, and can be implemented in general metallurgical enterprises and steel structure manufacturing plants.

[0055] 2) The 960MPa grade high-performance bridge steel plate manufactured by the method of the present invention has high strength and toughness, low internal stress and excellent welding performance, which meets the application requirements of steel structures such as bridges and high-rise buildings. Detailed Implementation

[0056] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way. The sequence numbers of the following embodiments are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0057] Table 1 below lists the chemical composition values ​​for each embodiment of the present invention;

[0058] Table 2 below lists the main process parameters for the slab heating process in various embodiments of the present invention;

[0059] Table 3 below lists the main process parameters for rolling and cooling processes in various embodiments of the present invention;

[0060] Table 4 below lists the main process parameters for sub-temperature quenching and annealing heat treatment in various embodiments of the present invention.

[0061] Table 5 below lists the test results of the mechanical properties of various embodiments of the present invention.

[0062] A method for preparing 960MPa grade bridge structural steel with excellent low internal stress fracture toughness according to various embodiments of the present invention includes the following steps:

[0063] 1) Smelting: Hot metal desulfurization + converter LD + LF desulfurization and temperature control + RH vacuum degassing + continuous casting CC, smelting and producing continuously cast slabs according to the steel composition design of claim 1;

[0064] 2) Slab heating: The slab is heated in the furnace with a temperature control range of 600~1280℃. The first heating control range is 600~1100℃, with rapid heating; the second heating control range is 1130~1230℃, with a heating rate of 10~11 min / cm; the third heating control range is 1220~1280℃, with a heating rate of 9~10 min / cm; the soaking zone temperature control range is 1200~1240℃, with a soaking time ≥20 min; and the tapping temperature range is 1160~1200℃.

[0065] 3) Rolling: Finishing rolling is carried out in the non-recrystallized austenite region, with an initial rolling temperature of 830-950℃ and an intermediate billet thickness of 36-130mm; the cumulative reduction rate in the finishing rolling stage is ≥60%, and the final rolling temperature is 760-860℃;

[0066] 4) ACC controlled cooling: After the steel plate is rolled, it is straightened by a pre-straightening machine, and then the steel plate is cooled. The cooling rate is controlled at 3-5℃ / second, and the final cooling temperature is controlled at 600-650℃.

[0067] 5) Sub-temperature quenching: The steel plate is subjected to sub-temperature quenching. The quenching temperature is 760~860℃ and the furnace time is (h+30)min, where h is the thickness of the finished steel plate in mm.

[0068] 6) Stress-relieving annealing heat treatment: Stress-relieving annealing heat treatment is performed on quenched steel plates at a temperature of 520~600℃; the holding time at the temperature is 2~4h; the heating rate is unlimited below 400℃; the heating rate is 90~120℃ / h above 400℃; the cooling rate is 90~120℃ / h above 350℃, and unlimited below 350℃.

[0069] Furthermore, in step (1) above, the temperature difference between the molten steel and the liquidus is controlled at 15~30℃ during the casting process. Electromagnetic stirring is adopted in the second cooling section. Through the generated electromagnetic force, the equiaxed crystal ratio of the billet is improved, and a billet with a good solidification structure is obtained. Dynamic light pressing is adopted at the end of the billet solidification, and the amount of light pressing is controlled at 1~3mm.

[0070] Furthermore, after the final rolling of step (3) above is completed, an additional leveling pass is added, with the reduction controlled at 2~12mm.

[0071] Table 1. List of chemical composition values ​​(wt, %) for various embodiments of the present invention

[0072]

[0073] Table 2. List of main process parameters for slab heating process in various embodiments of the present invention

[0074]

[0075] Table 3. List of main process parameters for rolling and cooling processes in various embodiments of the present invention.

[0076]

[0077] Table 4. List of main process parameters for sub-temperature quenching and annealing heat treatment in various embodiments of the present invention.

[0078]

[0079] Table 5. List of mechanical property test results for various embodiments of the present invention

[0080]

[0081] As shown in Tables 1-5, the steel of this invention has a simple manufacturing process and the following physical properties: yield strength ≥ 985 MPa, tensile strength ≥ 1100 MPa, excellent low-temperature toughness (longitudinal KV2 ≥ 120 J at -40℃), and fracture toughness (fracture toughness ≥ 0.26 mm at room temperature). The absolute value of residual internal stress in the steel plate is ≤ 18 MPa. In summary, this invention possesses good physical and application properties, its production process is simple, environmentally friendly, and low-cost, making it suitable for the manufacturing needs of bridge steel structures.

[0082] The above embodiments are merely specific examples exemplified to explain the present invention and do not limit the present invention in any way. Any non-substantial changes made by any person based on the above content and form that do not depart from the scope of protection of the claims of the present invention should be considered to fall within the scope of protection of the claims of the present invention. The present invention is not limited to the specific embodiments described above.

Claims

1. A 960MPa grade bridge structural steel with excellent fracture toughness under low internal stress, characterized in that... The steel contains the following chemical composition by weight percentage: C: 0.055~0.105%, Si: 0.05~0.15%, Mn: 1.65~1.75%, P≤0.010%, S≤0.003%, Ni: 0.80~1.80%, Cr: 0.45~0.75%, Mo: 0.20~0.30%, Nb: 0.060~0.080%, Ti: 0.035~0.050%, Als: 0.015~0.045%, with the balance being Fe and unavoidable impurity elements. The chemical composition must meet the following requirements: 0.188≤Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.275, 0.513≤CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15≤0.717; The method for preparing the steel includes the following steps: 1) Smelting: Hot metal desulfurization + converter LD + LF desulfurization and temperature control + RH vacuum degassing + continuous casting CC, smelting and producing continuous casting slabs according to the steel composition design. 2) Slab heating: The slab is heated in the furnace with a temperature control range of 600~1280℃. The first heating control range is 600~1100℃, with rapid heating; the second heating control range is 1130~1230℃, with a heating rate of 10~11 min / cm; the third heating control range is 1220~1280℃, with a heating rate of 9~10 min / cm; the soaking zone temperature control range is 1200~1240℃, with a soaking time ≥20 min; and the tapping temperature range is 1160~1200℃. 3) Rolling: Finishing rolling is carried out in the non-recrystallized austenite region, with an initial rolling temperature of 830-950℃ and an intermediate billet thickness of 36-130mm; the cumulative reduction rate in the finishing rolling stage is ≥60%, and the final rolling temperature is 760-860℃; 4) ACC controlled cooling: After the steel plate is rolled, it is straightened by a pre-straightening machine, and then the steel plate is cooled. The cooling rate is controlled at 3-5℃ / second, and the final cooling temperature is controlled at 600-650℃. 5) Sub-temperature quenching: The steel plate is subjected to sub-temperature quenching. The quenching temperature is 760~860℃ and the furnace time is (h+30)min, where h is the thickness of the finished steel plate in mm. 6) Stress-relieving annealing heat treatment: Stress-relieving annealing heat treatment is performed on quenched steel plates at a temperature of 520~600℃; the holding time at the temperature is 2~4h; the heating rate is unlimited below 400℃; the heating rate is 90~120℃ / h above 400℃; the cooling rate is 90~120℃ / h above 350℃, and unlimited below 350℃.

2. The 960MPa grade bridge structural steel with excellent low internal stress fracture toughness according to claim 1, characterized in that: The finished steel plate has a yield strength ≥985MPa, tensile strength ≥1100MPa, excellent low-temperature toughness, longitudinal KV2 ≥120J at -40℃, fracture toughness ≥0.26mm at room temperature, and absolute value of residual internal stress ≤18MPa.

3. The 960MPa grade bridge structural steel with excellent low internal stress fracture toughness according to claim 1, characterized in that: Step (1) During the casting process, the temperature difference between the molten steel and the liquidus is controlled at 15~30℃. Electromagnetic stirring is adopted in the second cooling section. The electromagnetic force generated improves the equiaxed crystal ratio of the billet and obtains a billet with a good solidification structure. Dynamic light pressing is adopted at the end of the billet solidification, and the amount of light pressing is controlled at 1~3mm.

4. The 960MPa grade bridge structural steel with excellent low internal stress fracture toughness according to claim 1, characterized in that: After the final rolling in step (3), an additional leveling pass is added, with the reduction controlled at 2~12mm.

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