Bridge structural steel material resistant to atmospheric corrosion for cold environments and method for manufacturing the same

Abstract

The present application relates to a kind of cold environment with atmospheric corrosion resistant bridge structure steel and its manufacturing method, wherein, by mass percentage, the chemical composition of the steel includes: C:0.05%~0.20%, Mn:0.90%~1.70%, Si:0.10%~0.80%, P:≤0.020%, S:≤0.015%, Al:0.015%~0.06%, Ca:0.001%~0.005%, and the balance of Fe and inevitable impurities.The steel of the present application realizes the stable high impact toughness of multiple strength grade steel plates in the thickness range of 6~150mm at-40℃~-70℃ low temperature under the premise of strictly controlling carbon equivalent (CEV) and welding crack sensitivity index (Pcm), while ensuring good welding performance, thickness direction performance, surface quality and atmospheric corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of steel materials and steel for engineering structures, specifically to a cold-environment-resistant atmospheric corrosion-resistant bridge structural steel and its manufacturing method. Background Technology

[0002] With the exploration of oil and gas resources in northern China, the construction of high-latitude infrastructure, and the extension of railway / highway networks to high-altitude / cold regions, the demand for specialized structural steel for bridge engineering projects in cold environments (including plateaus), especially railway bridges and long-span highway bridges, is growing rapidly. Bridge structures are exposed to the natural environment for extended periods and must withstand complex conditions such as low temperatures, strong ultraviolet radiation at high altitudes, and humid rain and snow. The core components, such as main beams, piers, and welded joints, require steel plates with significantly higher toughness and crack resistance than conventional structural steel at temperatures as low as -60℃ to -70℃. Simultaneously, they must possess good weldability, thickness-direction properties, internal density, and resistance to atmospheric corrosion to ensure the long-term safety of the bridge's service life.

[0003] In existing technologies, conventional low-alloy high-strength structural steels such as Q345D / E, Q390D / E, and Q460D / E can achieve certain impact toughness at -20℃ or -40℃ by appropriately reducing the C content, adding microalloying elements such as Nb, Ti, and V, and optimizing the controlled rolling and cooling process. However, the impact absorption energy decreases significantly below -60℃ (usually below 25J), making it difficult to meet the long-term service requirements of bridges in cold environments (including high-altitude areas). Furthermore, their atmospheric corrosion resistance is insufficient, and they are prone to accelerated corrosion in cold, humid, and high-ultraviolet radiation environments at high altitudes, thus shortening the service life of bridges.

[0004] To improve low-temperature toughness, some existing technologies have proposed high-Ni content low-temperature steel or 9Ni steel, which significantly improves the impact toughness of steel plates at extremely low temperatures by improving austenite stability. However, the Ni content in this type of steel is usually above 5% or even close to 9%, and the alloy cost is extremely high (more than 60% higher than conventional low alloy steel). It is mostly used in special containers such as low-temperature storage tanks and LNG tanks, and it is difficult to promote its application in large-scale bridge engineering.

[0005] Existing technologies also propose high-toughness or wear-resistant steel plates for cold environments. By increasing the content of alloys such as Ni and Mo and combining them with quenching and tempering treatment, the steel plates can have high impact toughness at -40℃ or -60℃. However, most of these solutions only cover a single strength grade or a limited thickness range (usually ≤80mm), and do not adequately consider the performance of thick plates required for bridge engineering (such as steel for thick bridge piers and bearings), the crack resistance of welded joints, atmospheric corrosion resistance, and batch production inspection rules. As a result, they are difficult to adapt to the diverse steel requirements of bridge engineering.

[0006] Therefore, there is still an urgent need for a series of low-alloy bridge structural steel plates with a relatively large thickness range of 6 to 150 mm, covering medium and high strength grades, which can maintain sufficient impact toughness under cold conditions of -60℃ to -70℃, and also have good weldability, atmospheric corrosion resistance and engineering adaptability, as well as suitable manufacturing methods, to meet the core steel needs of railway bridges and highway bridges in cold regions (including plateaus). Summary of the Invention

[0007] The purpose of this invention is to provide a cold-environment-resistant bridge structural steel and its manufacturing method. Through reasonable chemical composition design and matching with controlled rolling and cooling / quenching and tempering processes, and under the premise of strictly controlling carbon equivalent (CEV) and weld crack sensitivity index (Pcm), it achieves stable high impact toughness of steel plates of multiple strength grades in the thickness range of 6-150mm at low temperatures of -40℃ to -70℃. At the same time, it ensures good weldability, thickness direction properties, surface quality and atmospheric corrosion resistance, thereby meeting the core steel requirements of railway bridges and highway bridges in cold regions (including plateaus) and ensuring the long-term service safety and durability of bridges.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] On one hand, the present invention provides a bridge structural steel resistant to atmospheric corrosion in cold environments, wherein, by weight percentage, the chemical composition of the steel comprises:

[0010] C: 0.05%–0.20%,

[0011] Mn: 0.90%–1.70%,

[0012] Si: 0.10%–0.80%,

[0013] P: ≤0.020%,

[0014] S: ≤0.015%,

[0015] Al: 0.015%–0.06%,

[0016] Ca: 0.001%–0.005%,

[0017] And the remaining Fe and unavoidable impurities.

[0018] Preferably, the chemical composition of the steel, by weight percentage, further includes one or more of the following elements:

[0019] Cr: 0–1.50%,

[0020] Ni: 0~2.00%,

[0021] Cu: 0–0.55%,

[0022] Mo: 0–0.70%.

[0023] Preferably, the chemical composition of the steel, by weight percentage, further includes one or more of the following elements:

[0024] Nb: 0–0.06%,

[0025] V: 0~0.12%,

[0026] Ti: 0–0.05%.

[0027] Preferably, C is 0.08% to 0.18% by mass.

[0028] Preferably, Mn is 1.10% to 1.65% by mass percentage.

[0029] Preferably, the Si content is 0.20% to 0.50% by mass percentage.

[0030] Preferably, P is ≤0.015% by mass percentage.

[0031] Preferably, S is ≤0.010% by mass percentage.

[0032] Preferably, Al is 0.02% to 0.04% by mass percentage.

[0033] Preferably, the Ca content is 0.002% to 0.004% by mass.

[0034] Preferably, the Cr content is 0.10% to 0.30% by mass percentage.

[0035] Preferably, the Ni content is 0.30% to 0.80% by mass percentage.

[0036] Preferably, Cu is 0.30% to 0.50% by mass percentage.

[0037] Preferably, the Mo content is 0.05% to 0.20% by mass percentage.

[0038] Preferably, the Nb content is 0.02% to 0.05% by mass percentage.

[0039] Preferably, V is 0.05% to 0.10% by mass percentage.

[0040] Preferably, the Ti content is 0.02% to 0.04% by mass.

[0041] Preferably, the carbon equivalent (CEV) of the steel is ≤0.65%.

[0042] Preferably, the weld crack sensitivity index Pcm of the steel is ≤0.28%.

[0043] In this invention, the calculation formulas for CEV and Pcm are as follows:

[0044]

[0045] Pcm (%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B

[0046] In one specific implementation, the chemical composition of the steel, by weight percentage, is:

[0047] C: 0.05%–0.20%,

[0048] Mn: 0.90%–1.70%,

[0049] Si: 0.10%–0.80%,

[0050] P: ≤0.020%,

[0051] S: ≤0.015%,

[0052] Al: 0.015%–0.06%,

[0053] Ca: 0.001%–0.005%,

[0054] Cr: 0–1.50%,

[0055] Ni: 0~2.00%,

[0056] Cu: 0–0.55%,

[0057] Mo: 0–0.70%,

[0058] Nb: 0–0.06%,

[0059] V: 0~0.12%,

[0060] Ti: 0~0.05%,

[0061] And the remaining Fe and unavoidable impurities.

[0062] In one specific implementation, the chemical composition of the steel, by weight percentage, is:

[0063] C: 0.08%~0.18%,

[0064] Mn: 1.10%–1.65%,

[0065] Si: 0.20%–0.50%,

[0066] P: ≤0.015%,

[0067] S: ≤0.010%,

[0068] Cr: 0.10%–0.30%,

[0069] Ni: 0.30%–0.80%,

[0070] Cu: 0.30%–0.50%,

[0071] Mo: 0.05%–0.20%,

[0072] Nb: 0.02%–0.05%,

[0073] V: 0.05%~0.10%,

[0074] Ti: 0.02%–0.04%,

[0075] Al: 0.02%–0.04%,

[0076] Ca: 0.002%–0.004%,

[0077] The balance consists of Fe and unavoidable impurities.

[0078] Preferably, the thickness of the steel is in the range of 6mm to 150mm.

[0079] The chemical composition of the steel in this application does not involve the isolated effects of the various elements, but rather they work synergistically through scientific proportions. This not only meets the stringent requirements of bridge steel for cold environments in terms of low-temperature toughness, atmospheric corrosion resistance, and weldability, but also achieves a balance between cost and performance, as detailed below:

[0080] I. Basic elements (C, Mn, Si):

[0081] 1. Carbon (C): A core strengthening element, with its content controlled between 0.05% and 0.20% (preferably 0.08% to 0.18%). Its main function is to improve the yield strength and tensile strength of steel through solid solution strengthening, ensuring that the steel possesses the load-bearing capacity required for bridge structures. Simultaneously, C can promote the precipitation strengthening of microalloying elements (Nb, V, Ti), providing support for the improvement of steel strength. However, excessively high C content will significantly reduce the low-temperature toughness, weldability, and corrosion resistance of the steel, while excessively low content will fail to meet the requirements for medium- and high-strength grades. Therefore, this application strictly limits the range of C content to achieve a balance between strength and toughness.

[0082] 2. Manganese (Mn): An important solid solution strengthening and toughness regulating element, its content is controlled at 0.90%–1.70% (preferably 1.10%–1.65%). Its core role is to dissolve in ferrite and austenite, significantly improving the strength of steel, while mitigating the adverse effects of carbon on toughness, and improving the plasticity and low-temperature impact performance of steel. In addition, Mn can expand the austenite region, optimize the phase transformation structure during rolling, facilitate the implementation of subsequent controlled rolling and cooling processes, and ensure uniform performance of steel within a thickness range of 6–150 mm.

[0083] 3. Silicon (Si): A deoxidizer and solid solution strengthening element, with a content controlled between 0.10% and 0.80% (preferably 0.20% to 0.50%). Its main function is to remove oxygen from molten steel during the smelting process, reduce oxide inclusions, purify the molten steel, and improve the internal density of the steel. At the same time, Si dissolves in ferrite, which helps to improve the strength of the steel. However, excessive Si content will lead to a decrease in the weldability of the steel and an easy formation of brittle phases. Therefore, this application limits its content range to balance deoxidation effect and weldability.

[0084] 4. Synergistic effect of basic elements: C, Mn, and Si form a synergistic system. Mn can effectively suppress the brittleness tendency of C and avoid the decrease in low-temperature toughness caused by excessive C content. The deoxidizing effect of Si can reduce the reaction between C, Mn and oxygen, avoid the formation of brittle carbides and oxide inclusions, and ensure the solid solution strengthening effect of C and Mn. After the optimization of the ratio of the three elements, the steel can reach the yield strength range of 355MPa to 690MPa, and lay the foundation for the role of subsequent alloying elements and microalloying elements, ensuring the stability of the basic mechanical properties of the steel.

[0085] II. Control of Harmful Impurities (P, S): Purifying Molten Steel and Avoiding Performance Shortcomings

[0086] 1. Phosphorus (P): A harmful impurity element, the content of which is strictly controlled at ≤0.020% (preferably ≤0.015%). P tends to segregate at the grain boundaries of steel, which leads to cold brittleness and significantly reduces low-temperature toughness and weldability. Especially in extremely cold environments of -60℃ to -70℃, it will exacerbate the risk of brittle fracture of bridge components. Therefore, this application strictly limits the content of P to reduce the harm of grain boundary segregation.

[0087] 2. Sulfur (S): A harmful impurity element, the content of which should be strictly controlled at ≤0.015% (preferably ≤0.010%). S reacts with Fe to form FeS. FeS has a low melting point (about 1190℃) and is prone to hot brittleness during rolling, which can cause steel to crack. It also reduces the plasticity, toughness and corrosion resistance of steel and deteriorates the performance of welded joints. Therefore, strictly controlling the S content can effectively avoid hot brittleness and improve the internal quality of steel and the reliability of welding.

[0088] 3. Synergistic control effect: The ultra-low content control of P and S, together with the subsequent purification effect of Al and Ca, forms a synergistic effect, reducing the total amount of harmful impurities in molten steel, avoiding the formation of brittle inclusions of P and S with other elements, and clearing obstacles for the precipitation strengthening of microalloying elements and the role of corrosion-resistant elements, ensuring the toughness reserve and long-term service safety of steel in extremely cold environments.

[0089] III. Purification and Inclusion Modification Elements (Al, Ca): Optimize microstructure, enhance toughness and corrosion resistance.

[0090] 1. Aluminum (Al): A deoxidizer and grain refiner, with a content controlled at 0.015% to 0.06% (preferably 0.02% to 0.04%). Its core function is to perform deep deoxidation during the smelting process, generating fine Al2O3 inclusions to replace coarse FeO and MnO inclusions, thus purifying the molten steel. At the same time, Al can promote the refinement of austenite grains, refine the final microstructure of the steel, significantly improve low-temperature toughness and strength, and especially improve the impact performance at -60℃ to -70℃, providing a guarantee for the extreme cold adaptability of steel.

[0091] 2. Calcium (Ca): An inclusion modifier, with a content controlled at 0.001% to 0.005% (preferably 0.002% to 0.004%). Its main function is to modify brittle inclusions such as Al2O3 in molten steel—converting high-melting-point, brittle Al2O3 into low-melting-point, well-plastic calcium aluminate inclusions. These inclusions will extend with the deformation of the steel during rolling and will not form stress concentration points, thereby reducing the internal crack source of the steel. At the same time, it improves the corrosion fatigue resistance and weld heat-affected zone toughness of the steel, and avoids low-temperature brittle fracture caused by inclusions.

[0092] 3. Synergistic effect: Al and Ca form a synergistic system of "deoxidation-inclusion modification". The deep deoxidation of Al provides the premise for the modification effect of Ca (reducing the consumption of Ca by undeoxidized oxygen), while Ca solves the harm of Al2O3 brittle inclusions generated after Al deoxidation. The combination of the two can significantly optimize the morphology and distribution of inclusions in steel, so that the non-metallic inclusion grade meets the requirements of GB / T10561 for Class D inclusions ≤1.5 and other types of inclusions ≤2.0. At the same time, the refined grains and optimized inclusion distribution can improve the efficiency of subsequent corrosion-resistant elements and microalloying elements, further enhancing the low-temperature toughness and atmospheric corrosion resistance of steel.

[0093] IV. Elements for atmospheric corrosion resistance and low-temperature toughness enhancement (Cr, Ni, Cu, Mo): Precisely empowered to adapt to cold and corrosive environments.

[0094] 1. Chromium (Cr): A core element for atmospheric corrosion resistance, with its content controlled at 0-1.50% (preferably 0.10%-0.30%). Its main function is to form a dense Cr2O3 oxide film on the surface of steel. This oxide film can effectively isolate water vapor, oxygen, and corrosive media (such as salt in rain and snow) in the atmosphere, prevent steel from rusting, and significantly improve atmospheric corrosion resistance. At the same time, Cr can dissolve in ferrite, which helps to improve the strength and low-temperature toughness of steel and alleviate the tendency of brittleness in extremely cold environments.

[0095] 2. Nickel (Ni): A core strengthening element for low-temperature toughness, with its content controlled at 0–2.00% (preferably 0.30%–0.80%). Ni expands the austenite region, improves austenite stability, lowers the ductile-brittle transition temperature of steel, and significantly improves impact toughness in extremely cold environments of -40℃ to -70℃, ensuring that the steel is not prone to brittle fracture at low temperatures. Simultaneously, Ni enhances the plasticity and weldability of steel, mitigates the adverse effects of elements such as Cr and Mo on weldability, and synergistically improves atmospheric corrosion resistance with Cr. Compared with existing low-temperature steels with Ni content ≥5%, this application achieves equal or even better low-temperature toughness at low Ni content through the synergistic effect of Ni and other elements, significantly reducing alloy costs.

[0096] 3. Copper (Cu): An auxiliary strengthening element for atmospheric corrosion resistance, with its content controlled at 0-0.55% (preferably 0.30%-0.50%). Cu can form a dense copper oxide film on the surface of steel, which, together with the Cr2O3 oxide film, forms a superimposed protective layer, further improving atmospheric corrosion resistance. At the same time, Cu can enhance the strength of steel through solid solution strengthening and can promote the dissolution of Ni in austenite, enhancing the low-temperature toughness strengthening effect of Ni, reducing the amount of Ni used, and achieving a balance between cost and performance.

[0097] 4. Molybdenum (Mo): A synergistic strengthening element for both strength and toughness, with its content controlled at 0–0.70% (preferably 0.05%–0.20%). Mo significantly improves the hardenability of steel, making it particularly suitable for the production of thick steel plates (6–150 mm), ensuring uniform performance in the thickness direction. Simultaneously, Mo dissolves in ferrite, enhancing the high-temperature strength and low-temperature toughness of steel, mitigating toughness degradation in extremely cold environments, and improving the performance of the weld heat-affected zone, reducing the risk of weld cracks. Furthermore, Mo works synergistically with Cr and Cu to further enhance the atmospheric corrosion resistance of steel, making it suitable for corrosive environments with strong ultraviolet radiation and high humidity at high altitudes.

[0098] 5. Synergistic effect: This group of elements forms a three-dimensional synergistic system of "corrosion resistance-low temperature toughness-strength", which is the core highlight of this application for adapting to cold and corrosive environments.

[0099] (1) Corrosion resistance synergy: Cr and Cu synergistically form a double oxide film protection. The dense oxide film of Cr isolates the corrosive medium, and the oxide film of Cu fills the tiny gaps in the oxide film of Cr, which improves the corrosion resistance of conventional low alloy steel by more than 30%. The addition of Mo can further enhance the stability of the oxide film and improve the corrosion resistance and durability of steel in humid and strong ultraviolet environments.

[0100] (2) Synergistic effect of low temperature toughness: Ni and Mo work together to reduce the ductile-brittle transition temperature of steel. Ni improves the stability of austenite, while Mo refines the microstructure and improves hardenability. The combination of the two makes the average value of longitudinal V-notch impact energy KV2 of steel ≥50J at -60℃ to -70℃, which is more than 100% higher than that of conventional Q345E steel (impact energy ≤25J at -60℃). At the same time, Ni can alleviate the slight adverse effect of Cr on toughness, ensuring that corrosion resistance is improved without sacrificing low temperature toughness.

[0101] (3) Synergy between strength and weldability: The solid solution strengthening effect of Cr, Mo and Cu and the toughness enhancement effect of Ni form a balance, which improves the strength of steel while avoiding the increase of brittleness; and the synergistic control of this group of elements with C can strictly control the carbon equivalent CEV≤0.65% and the welding crack sensitivity index Pcm≤0.28%, ensuring that the steel has good weldability and is suitable for the construction of complex welding joints of bridges.

[0102] V. Microalloying strengthening elements (Nb, V, Ti): Refine the microstructure and improve strength and properties in the thickness direction.

[0103] 1. Niobium (Nb): A grain refining and precipitation strengthening element, with its content controlled at 0-0.06% (preferably 0.02%-0.05%). Its core function is to inhibit austenite grain growth during rolling, refine the austenite structure, and further refine the final ferrite and bainite structure of the steel, thereby improving the strength and low-temperature toughness of the steel. At the same time, Nb can precipitate fine NbC and NbN particles, which further enhance the strength of the steel through precipitation strengthening. It can also improve the thickness direction properties of the steel, reduce center segregation in thick plates, and ensure uniform performance of thick steel plates with a thickness of 6-150mm.

[0104] 2. Vanadium (V): A precipitation strengthening element, with a content controlled at 0-0.12% (preferably 0.05%-0.10%). Its main function is to precipitate fine V(C,N) particles during the cooling process of steel, thereby enhancing the strength and hardness of the steel through precipitation strengthening. At the same time, it can refine the grains and help improve low-temperature toughness. The precipitation strengthening effect of V is relatively mild, which can maximize the preservation of the plasticity and weldability of the steel while improving strength, making it suitable for the comprehensive performance requirements of bridge steel.

[0105] 3. Titanium (Ti): A grain refiner and inclusion modifier, with a content controlled at 0-0.05% (preferably 0.02%-0.04%). Ti can react with N in molten steel to generate fine TiN particles. TiN particles can effectively pin austenite grain boundaries, inhibit austenite grain growth, refine the microstructure, and improve low-temperature toughness and weld heat-affected zone performance. At the same time, Ti can react with C to generate TiC, which helps to improve the strength of steel and can also improve the distribution of inclusions, further optimizing the internal quality of steel.

[0106] 4. Synergistic effect: Nb, V, and Ti form a synergistic system of "grain refinement-precipitation strengthening," and together with the controlled rolling and cooling process and other elements, they form a multi-dimensional synergy, which is the key to achieving the adaptation of large thickness and multiple strength levels in this application.

[0107] (1) Grain refinement synergy: Nb and Ti are mainly responsible for inhibiting austenite grain growth (Nb inhibits grain growth during rolling, and Ti pins grain boundaries through TiN), and V assists in refining the microstructure after phase transformation. The three work together to significantly refine the steel grains, which not only improves low-temperature toughness but also provides support for strength improvement, achieving the effect of "fine grain and strong toughness".

[0108] (2) Synergistic precipitation strengthening: The precipitation of carbonitrides of Nb, V and Ti forms a superimposed strengthening effect, and the precipitated particles are small and uniform, which will not significantly reduce the plasticity and weldability of steel. At the same time, precipitation strengthening is synergistic with solid solution strengthening of C and Mn and corrosion resistance strengthening of Cr and Mo, so that the steel can maintain good low temperature toughness and atmospheric corrosion resistance in multiple strength grades from 355MPa to 690MPa.

[0109] (3) Synergy with the process: The role of microalloying elements needs to be coordinated with the controlled rolling and cooling / quenching and tempering process of this application. In the heating stage, Nb, V and Ti are fully dissolved in austenite; in the rolling stage, Nb and Ti inhibit grain growth; in the cooling / quenching and tempering stage, Nb, V and Ti precipitate to form strengthening. The synergy of the three with the process ensures that the properties of the 6 to 150 mm thick steel plate are uniform in the thickness direction, which meets the steel requirements of different bridge components (supports, piers and main beams).

[0110] VI. Summary of Overall Synergistic Effects

[0111] The chemical composition design of this application is not a simple superposition of the performance of a single element, but rather achieves a balance and breakthrough in multiple properties through a synergistic approach of "basic elements determining performance, corrosion-resistant elements improving durability, microalloying elements optimizing microstructure, and purification elements ensuring quality." The core synergistic highlights are as follows:

[0112] 1. Synergistic effect of low temperature toughness and atmospheric corrosion resistance: Ni and Mo mainly improve low temperature toughness, while Cr and Cu mainly improve atmospheric corrosion resistance. The two work together to ensure that the steel has an impact energy of ≥50J in the extremely cold environment of -60℃~-70℃, and also achieves atmospheric corrosion resistance that is more than 30% higher than that of conventional low alloy steel, thus solving the pain point of "difficulty in balancing low temperature toughness and corrosion resistance" in the existing technology.

[0113] 2. Synergistic effect of strength and weldability: Through precise control of C content, combined with solid solution strengthening of Mn and Si, precipitation strengthening of Nb, V and Ti, and auxiliary strengthening of Cr and Mo, a range of strength grades from 355MPa to 690MPa is achieved. At the same time, by controlling the carbon equivalent CEV ≤ 0.65% and Pcm ≤ 0.28%, and by improving weldability with Ni and Al, a wide range of heat input for welding (10 to 40 kJ / cm) is ensured, making it suitable for the construction of complex welding joints in bridges and solving the problem of poor weldability of existing high-strength steels.

[0114] 3. Synergy between thickness uniformity and mass production: The synergistic effect of Nb, Ti, and Mo, combined with the optimization of smelting, continuous casting, and rolling processes, ensures uniform performance in the thickness direction of 6-150mm thick steel plates, with center shrinkage and segregation grades ≤2.0. At the same time, the low Ni content (0-2.00%) design allows for mass production with conventional thick plate production lines, reducing costs by more than 35% compared to existing high-Ni low-temperature steels. This achieves synergy between performance, thickness adaptation, and cost control, meeting the needs of large-scale applications in bridge engineering.

[0115] 4. Synergistic effect of intrinsic quality and service safety: The purification and inclusion modification effects of Al and Ca, combined with the ultra-low content control of P and S, reduce internal defects and brittle inclusions in the steel; the grain refinement effect of microalloying elements reduces stress concentration points; the protective effect of corrosion-resistant elements delays the corrosion and aging of the steel. The synergistic effect of multiple elements ensures that the steel can withstand various complex working conditions such as extreme cold, corrosion, and welding stress during long-term service, thus ensuring the safety of bridge structures. The carbon equivalent (CEV) of the steel of this invention is ≤0.65%, and the welding crack sensitivity index (Pcm) is ≤0.28%.

[0116] The thickness range of the steel material described above in this invention is 6mm to 150mm, which is suitable for the thickness requirements of different bridge components.

[0117] The mechanical properties of the steel of the present invention are as follows: yield strength ReH 355MPa~690MPa, tensile strength Rm 470MPa~820MPa, and elongation after fracture A≥16%.

[0118] The low-temperature impact toughness of the steel of the present invention is as follows: the average value of longitudinal V-notch impact energy KV2 at -40℃ to -70℃ is ≥50J, and the impact energy of a single specimen is not less than 70% of the specified value.

[0119] The atmospheric corrosion resistance of the steel of the present invention is as follows: relative corrosion rate in the immersion test is ≤55% (comparative sample is Q235A), which is suitable for the long-term outdoor service environment of bridges.

[0120] On the other hand, the present invention provides a method for manufacturing the above-mentioned steel, the method comprising the following steps:

[0121] 1) Smelting and refining: Smelting is carried out using a converter or electric furnace, followed by LF ladle refining and RH vacuum treatment to obtain refined molten steel;

[0122] 2) Continuous casting: The refined molten steel is continuously cast to obtain a continuously cast slab;

[0123] 3) Heating: The continuously cast slab is fed into a heating furnace for heating to obtain a heated slab;

[0124] 4) Rolling: The heated slab is rolled in two stages: roughing and finishing.

[0125] 5) Cooling: The rolled steel plate is accelerated and cooled at a rate of 10-25℃ / s, with a final cooling temperature of 450-520℃. Then it is stacked and cooled slowly to obtain a slowly cooled steel plate.

[0126] Preferably, in step 1), the purity of the refined molten steel satisfies: P≤0.015%, S≤0.010%, [H]≤2.0ppm;

[0127] Preferably, in step 1), the grade of non-metallic inclusions in the refined molten steel meets the following requirements: Class D inclusions ≤ 1.5 and other inclusions ≤ 2.0 in GB / T10561;

[0128] Preferably, in step 1), calcium treatment is performed during the refining process;

[0129] Preferably, in step 2), the refined molten steel is continuously cast using a vertical bending continuous casting machine;

[0130] Preferably, in step 2), the continuous casting speed is 0.8–1.5 m / min;

[0131] Preferably, in step 2), aerosol cooling is used for secondary cooling, with a cooling intensity of 0.8–1.2 L / kg;

[0132] Preferably, in step 2), the water flow rate of the crystallizer of the continuous casting machine, such as a vertical bending continuous casting machine, is 300-450 L / min;

[0133] Preferably, in step 2), the thickness of the continuously cast slab is 220-300 mm, and the central shrinkage cavity and segregation grade are ≤2.0.

[0134] Preferably, in step 3), the heating furnace is a walking beam furnace;

[0135] Preferably, in step 3), the heating temperature is 1120–1180°C;

[0136] Preferably, in step 3), the temperature for homogenization is 1150–1200°C;

[0137] Preferably, in step 3), the heating time is 2.5 to 3.0 hours.

[0138] Preferably, in step 4), during the rough rolling stage: the initial rolling temperature is 1110–1140°C, and the cumulative reduction rate is ≥65%;

[0139] Preferably, in step 4), the finishing rolling stage is as follows: the initial rolling temperature is 910-920℃, the final rolling temperature is 800-870℃, the reduction rate of each finishing rolling pass is ≥8%, and the thickness of the steel plate after final rolling is 6-150mm.

[0140] The design rule for the grade in this application is as follows: adopt the form of "Q + yield strength value + JH + quality grade", and add Z-direction performance mark (Z25 / Z35) and atmospheric corrosion resistance mark (N) according to requirements to form a series of steel grades of cold environment atmospheric corrosion resistant bridge structural steel covering Q355JHN~Q690JHZN, which can be adapted to different core components such as bridge bearings, piers, main beams, and welded joints.

[0141] Among them, the final rolling temperature of the 355JH series (bridge bearings and auxiliary components) is 850~870℃;

[0142] The final rolling temperature for the Q420JH~Q500JH series (bridge piers, box girders) is 830~850℃;

[0143] The final rolling temperature for the Q550JH~Q690JH series (bridge main beams and key load-bearing parts) is 800~830℃.

[0144] Preferably, in step 5), the slow cooling time of the stack is ≥48 hours.

[0145] Preferably, when the yield strength of the target steel is ≥550MPa, after cooling in step 5), step 6) tempering treatment is also included: quenching and tempering the slowly cooled steel plate.

[0146] Preferably, in step 6), the quenching temperature is 850–900℃, and the holding time is 2.5–3.0 min / mm based on the plate thickness;

[0147] Preferably, in step 6), the quenching method is water quenching, and the cooling rate is ≥30℃ / s;

[0148] Preferably, in step 6), the tempering temperature is 550–650°C, and the holding time is 3.0–3.5 min / mm based on the plate thickness;

[0149] The steel of this invention can be inspected in the following manner:

[0150] Ultrasonic testing: Meets GB / T2970 Class II and above requirements to detect internal defects;

[0151] Thickness direction performance testing: Z25 / Z35 grade meets the requirements of GB / T5313 and is suitable for thick plate welded components of bridges;

[0152] Low-temperature impact test: longitudinal V-notch impact energy test at -40℃~-70℃ to verify low-temperature toughness;

[0153] Atmospheric corrosion resistance test: The relative corrosion rate of the weekly immersion test is ≤55% (the comparison sample is Q235A), ensuring outdoor service durability.

[0154] In one specific implementation scheme, the specific steps of the steel manufacturing method of this application are as follows:

[0155] 1) Smelting and refining:

[0156] The primary refining is carried out using a converter or electric furnace, followed by a combination of "LF ladle refining and RH vacuum treatment" to deeply purify and fine-tune the composition of the molten steel.

[0157] Purity control: Precise control of the content of harmful elements and gases in the steel ensures that the final molten steel meets the following requirements: phosphorus (P) content ≤ 0.015%, sulfur (S) content ≤ 0.010%, and hydrogen ([H]) content ≤ 2.0 ppm. This is the foundation for obtaining high toughness and good weldability.

[0158] Calcium treatment: During the LF refining process, calcium (Ca) alloys are added to treat the molten steel. This aims to modify brittle inclusions such as high-melting-point Al2O3 into low-melting-point calcium aluminates, thereby improving the morphology and distribution of inclusions and significantly enhancing the corrosion fatigue resistance and low-temperature impact toughness of the steel.

[0159] Inclusion control target: After treatment, the grade of non-metallic inclusions in the steel must be strictly controlled. Referring to GB / T 10561 standard, the grade of Class D (spherical oxide) inclusions should be ≤ 1.5, and the grade of other inclusions should be ≤ 2.0.

[0160] 2) Continuous casting: After refining, the molten steel is continuously cast using a vertical bending continuous casting machine, suitable for the production of thick bridge plate components.

[0161] Process parameters: Continuous casting speed is controlled at 0.8 ~ 1.5 m / min. Cooling water flow rate in the crystallizer is 300 ~ 450 L / min. Secondary cooling adopts air mist cooling method, with cooling intensity controlled at 0.8 ~ 1.2 L / kg to achieve uniform and slow cooling and reduce internal stress and cracks.

[0162] Billet quality: The thickness of the obtained continuously cast slab is 220 ~ 300 mm. By optimizing the cooling and electromagnetic stirring processes, the center quality of the slab is ensured, requiring that the center shrinkage cavity and segregation grade be ≤ 2.0, laying the foundation for the subsequent rolling of thick plates with uniform internal quality.

[0163] 3) Heating: The continuously cast slab is fed into a walking beam furnace to ensure complete solid solution of microalloying elements.

[0164] Heating: The slab is first heated in a heating section at 1120~1180℃, and then held at a soaking temperature of 1150~1200℃ for 2.5~3.0 hours. This condition ensures that microalloying elements such as Nb, V, and Ti are fully dissolved in austenite, creating conditions for precipitation strengthening during subsequent rolling.

[0165] 4) Rolling (thermomechanical controlled rolling process, adapted to bridge components of different thicknesses)

[0166] A two-stage controlled rolling process is adopted, with grain refinement as the core, and it is also suitable for the production of bridge steel of different strength levels.

[0167] Rough rolling stage: This stage is carried out in the higher austenite recrystallization region. The initial rolling temperature is 1110 ~ 1140℃, and rolling is performed with a large reduction rate, with a cumulative reduction rate ≥ 65%, aiming to fully break down the original coarse austenite grains.

[0168] Finish rolling stage: This stage takes place in the lower austenite non-recrystallization region. The initial rolling temperature is strictly controlled between 910 and 920°C. The final rolling temperature (FTT) is differentiated according to the strength grade of the target product.

[0169] For 355JH series steel (mainly used for bridge bearings and auxiliary components), FTT is controlled at 850 ~ 870℃.

[0170] For Q420JH ~ Q500JH series steel (mainly used for main load-bearing structures such as bridge piers and box girders), FTT is controlled at 830 ~ 850℃.

[0171] For Q550JH ~ Q690JH series steel (mainly used in key high-strength load-bearing parts such as bridge main beams), FTT is controlled at 800 ~ 830℃.

[0172] In the finishing rolling stage, the reduction rate of each pass is ≥ 8% to accumulate sufficient deformation energy and promote the refinement of ferrite or bainite after phase transformation. The thickness of the steel plate after final rolling ranges from 6 to 150 mm, which can cover the size requirements of various bridge components.

[0173] 5) Cooling

[0174] Immediately after rolling, accelerated cooling is performed to "freeze" the refined state of the deformed austenite and control the phase transformation structure.

[0175] Accelerated cooling: A laminar flow cooling system is used to rapidly cool the steel plate from the final rolling temperature to the final cooling temperature (FCT) of 450-520℃ at a cooling rate of 10-25℃ / s. This process effectively suppresses high-temperature phase transformation and promotes the formation of fine bainite or acicular ferrite structures, thereby improving strength while ensuring toughness.

[0176] Stacking and slow cooling: After accelerated cooling, the steel plates are stacked and slow-cooled for no less than 48 hours. This process can fully release the internal stress generated by rapid cooling and effectively prevent delayed cracking of the steel plates during subsequent processing or bridge service.

[0177] 6) Quenching and tempering treatment (for steel grades with yield strength ≥ 550 MPa, suitable for high-strength load-bearing components of bridges)

[0178] For steel grades requiring ultra-high strength, after TMCP and slow cooling, a quenching and tempering treatment (quenching + tempering) is added.

[0179] Quenching: Heat the steel plate to 850 ~ 900℃ and hold it at that temperature for 2.5 ~ 3.0 min / mm, calculated based on the plate thickness. Immediately after holding, perform water quenching at a cooling rate ≥ 30℃ / s to obtain a high-strength martensitic or lower bainitic structure.

[0180] Tempering: The quenched steel plate is tempered at 550~650℃, with a holding time calculated based on the plate thickness as 3.0~3.5 min / mm. After tempering, it is air-cooled to room temperature. This process transforms the microstructure into a multiphase structure of tempered martensite and tempered bainite, greatly improving toughness and plasticity while maintaining high strength.

[0181] 7) Product inspection and performance assurance

[0182] To ensure that the steel fully meets the stringent requirements of bridge engineering, the following special inspections are performed on the finished steel plates:

[0183] Non-destructive testing: 100% ultrasonic testing is performed, and the quality level meets or exceeds the requirements of Class II in GB / T 2970 standard.

[0184] Thickness direction performance: Thickness direction (Z direction) performance test is conducted according to requirements to meet the Z25 or Z35 grade requirements specified in GB / T 5313 standard, ensuring the thick plate welded joint's resistance to lamellar tearing.

[0185] Low-temperature toughness verification: Longitudinal V-notch impact tests were conducted in the temperature range of -40℃ to -70℃ to ensure the toughness reserve of the steel in extremely cold environments.

[0186] Weather resistance evaluation: The atmospheric corrosion resistance is evaluated through periodic immersion corrosion test, requiring a relative corrosion rate of ≤ 55% relative to Q235A steel, in order to verify its durability advantage in long-term outdoor service.

[0187] Through the coordinated control of the entire process described above, the steel plates produced by this method can stably achieve comprehensive performance indicators such as high strength, high toughness, easy welding and long service life, making them particularly suitable for the construction of large bridges in harsh environments such as northern China, high-altitude and cold regions and coastal areas.

[0188] Compared with the prior art, the present invention has the following beneficial effects:

[0189] 1) Bridges are highly adaptable to cold environments (including high-altitude areas).

[0190] Through strict control of CEV≤0.65% and Pcm≤0.28%, combined with the synergistic design of Cr, Ni, Mo and Nb, Ti, V microalloying elements and Ca treatment to optimize inclusion morphology, the steel plate has an impact energy ≥50J at -60℃ to -70℃, which is more than 100% higher than that of conventional Q345E steel (impact energy ≤25J at -60℃), effectively resisting the risk of low-temperature brittle fracture of bridges; the reasonable ratio of Cu and Cr elements improves the atmospheric corrosion resistance by more than 30% compared with conventional low alloy steel, withstands the strong ultraviolet radiation and high humidity corrosion environment of high altitude, and extends the service life of bridges by 15 to 20 years.

[0191] 2) Covering the steel needs of all bridge applications

[0192] We have developed a multi-grade steel series from Q355JHN to Q690JHZN, with yield strengths ranging from 355MPa to 690MPa and thicknesses from 6 to 150mm. Custom thickness-direction performance is supported for Z25 / Z35 steel. Q355JHN is suitable for bridge bearings and auxiliary components, Q420JHN to Q500JHN for bridge piers and box girders, and Q550JHN to Q690JHZN for main beams and critical load-bearing components. This integrated supply of steel for bridges facilitates engineering design and standardized applications.

[0193] 3) Suitable for large-scale application in bridge engineering

[0194] With Ni content controlled between 0 and 2.00% (preferably 0.30% to 0.80%), the cost per ton of high-Ni low-temperature steel (Ni≥5%) is reduced by more than 35%, significantly reducing the construction cost of bridge projects. The production process does not require special equipment modifications, and existing heavy plate production lines can be used for mass production, meeting the bulk steel demand of large-scale bridge projects.

[0195] 4) Improve the safety of bridge welding construction

[0196] With a wide range of welding heat input adaptability (10~40kJ / cm), it is more suitable for complex bridge welding joints (such as main beam splicing and pier welding) than conventional high-Ni steel (≤30kJ / cm), and is especially suitable for high-altitude low-pressure welding environments; strict control of surface quality, thickness direction properties and flaw detection standards significantly reduces the risk of low-temperature lamellar tearing and welding heat-affected zone cracks, ensuring the reliability of bridge welded structures.

[0197] In particular, this invention overcomes the pain points of existing technologies such as "difficulty in balancing low-temperature toughness and atmospheric corrosion resistance", "contradiction between high strength and weldability", "uneven performance in large thicknesses" and "high cost making large-scale application difficult". By precisely controlling the chemical composition ratio and adapting the controlled rolling and cooling / quenching and tempering process, it achieves multi-element synergistic empowerment. Specifically, firstly, by strictly limiting the content of C (0.05%~0.20%) and Ni (0~2.00%), and coordinating with Cr (0~1.50%), Cu (0~0.55%), Mo (0~0.70%), and microalloying elements (Nb / V / Ti), a synergistic improvement in high toughness and atmospheric corrosion resistance in extremely cold environments (-60℃~-70℃) is achieved at a low Ni cost. Secondly, by controlling the carbon equivalent CEV ≤ 0.65% and the weld crack sensitivity index Pcm ≤ 0.28%, the poor weldability of existing high-strength steel is addressed, making it suitable for the construction of complex welding joints in bridges. Furthermore, through the synergy of composition and process, uniform performance of steel plates with thicknesses ranging from 6 to 150 mm is achieved, covering multiple strength grades from 355 MPa to 690 MPa, adapting to the steel needs of bridges in cold regions (including plateaus) across all scenarios, and requiring no special production equipment, allowing for large-scale mass production and cost reduction of more than 35% compared to existing high-Ni low-temperature steels. Detailed Implementation

[0198] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to these embodiments.

[0199] Example 1: Q355JHEN steel plate (steel for bridge bearings)

[0200] This steel plate is a bridge structural steel with a yield strength of 355MPa, quality grade E, and resistance to atmospheric corrosion in cold environments. It is suitable for supports and auxiliary components of railway bridges and highway bridges in cold regions (including plateaus) with temperatures above -40℃.

[0201] 1. Chemical composition (mass percentage)

[0202] content 0.15% 1.30% 0.35% 0.012% 0.008% 0.20% 0.30% 0.40% 0.05% 0.03% 0.08% 0.02% 0.025% 0.003% Bal

[0203] 2. Key Parameters

[0204] Carbon equivalent (CEV) = 0.48%, weld crack susceptibility index (Pcm) = 0.27%;

[0205] Steel plate thickness range: 6~150mm (20~80mm is preferred for supports).

[0206] 3. Manufacturing process

[0207] 1.) Smelting: Converter smelting + LF refining + RH vacuum treatment, molten steel [H] = 1.8ppm, non-metallic inclusion grades: Class A 1.0, Class B 1.0, Class C 0.5, Class D 0.5;

[0208] 2) Continuous casting: Continuous casting speed 1.2m / min, crystallizer water flow 380L / min, secondary cooling intensity 1.0L / kg, slab thickness 250mm, center shrinkage cavity and segregation grade 1.5;

[0209] 3) Heating: Heating temperature 1150℃, uniform heating temperature 1180℃, uniform heating time 2.8 hours;

[0210] 4) Rolling: Roughing rolling start temperature 1120℃, cumulative reduction rate 68%; Finishing rolling start temperature 915℃, finishing rolling temperature 860℃, finishing rolling pass reduction rate 10%;

[0211] 5) Cooling: Accelerated cooling after final rolling (cooling rate 18℃ / s), final cooling temperature 480℃, and slow cooling in stacks for 50 hours;

[0212] 6) Inspection: Ultrasonic testing GB / T 2970 Class I, with a relative corrosion rate of 50% after 72 hours of immersion test (compared to steel Q355B).

[0213] 4. Mechanical property test results

[0214]

[0215] Example 2: Q420JHFN steel plate (steel for bridge main beams / box girders)

[0216] This steel plate is a bridge structural steel with a yield strength of 420MPa, quality grade F, and resistance to atmospheric corrosion in cold environments. It is suitable for main beams of railway bridges and box girder components of highway bridges in cold regions (including plateaus) with temperatures below -60℃.

[0217] 1. Chemical composition (mass percentage)

[0218] content 0.13% 1.50% 0.40% 0.010% 0.007% 0.25% 0.60% 0.35% 0.15% 0.04% 0.10% 0.03% 0.030% 0.0035% Bal

[0219] 2. Key Parameters

[0220] Carbon equivalent (CEV) = 0.55%, weld crack susceptibility index (Pcm) = 0.28%;

[0221] Steel plate thickness range: 6~100mm (30~60mm is preferred for main beams).

[0222] 3. Manufacturing Process Steps

[0223] 1) Smelting: Electric furnace smelting + LF refining + RH vacuum treatment, molten steel [H] = 1.6ppm, non-metallic inclusion grades: Class A 0.8 grade, Class B 0.8 grade, Class C 0.5 grade, Class D 0.5 grade;

[0224] 2) Continuous casting: Continuous casting speed 1.0m / min, crystallizer water flow 350L / min, secondary cooling intensity 0.9L / kg, slab thickness 280mm, center shrinkage cavity and segregation grade 1.0;

[0225] 3) Heating: Heating temperature 1160℃, heat spread temperature 1190℃, heat spread time 2.9 hours;

[0226] 4) Rolling: Roughing rolling start temperature 1130℃, cumulative reduction rate 70%; Finishing rolling start temperature 910℃, finishing rolling temperature 840℃, finishing rolling reduction rate per pass 9%;

[0227] 5) Cooling: Accelerated cooling after final rolling (cooling rate 22℃ / s), final cooling temperature 450℃, and slow cooling in stacks for 52 hours;

[0228] 6) Inspection: Ultrasonic testing GB / T 2970 Class I, the relative corrosion rate after 72h immersion test is 51% (compared to steel Q355B).

[0229] 4. Mechanical property test results

[0230]

[0231] Example 3: Q550JHGZN quenched and tempered steel plate (steel for critical load-bearing parts of bridges)

[0232] This steel plate is a cold-environment quenched and tempered bridge structural steel with a yield strength of 550MPa, quality grade G, atmospheric corrosion resistance, and Z35 thickness direction performance. It is suitable for key load-bearing parts and welded joint reinforcements of long-span bridges in cold regions (including plateaus).

[0233] 1. Chemical composition (mass percentage)

[0234] content 0.13% 1.40% 0.60% 0.011% 0.006% 0.6% 1.30% 0.45% 0.20% 0.05% 0.05% 0.04% 0.035% 0.004% Bal

[0235] 2. Key Parameters

[0236] Carbon equivalent (CEV) = 0.58%, weld crack susceptibility index (Pcm) = 0.26%;

[0237] Steel plate thickness range: 6-50mm (20-40mm is preferred for critical load-bearing parts).

[0238] 3. Manufacturing Process Steps

[0239] 1) Smelting: Converter smelting + LF refining + RH vacuum treatment, molten steel [H] = 1.5ppm, non-metallic inclusion grades: Class A 1.0, Class B 1.0, Class C 0.5, Class D 0.5;

[0240] 2) Continuous casting: Continuous casting speed 0.9m / min, crystallizer water flow 320L / min, secondary cooling intensity 0.85L / kg, slab thickness 220mm, center shrinkage cavity and segregation grade 1.0;

[0241] 3) Heating: Heating temperature 1170℃, uniform heating temperature 1200℃, uniform heating time 3.0 hours;

[0242] 4) Rolling: Roughing rolling start temperature 1140℃, cumulative reduction rate 72%; Finishing rolling start temperature 920℃, finishing rolling temperature 820℃, finishing rolling pass reduction rate 12%;

[0243] 5) Cooling and tempering: After final rolling, air cool to room temperature; quenching temperature 880℃ (holding for 1.8 hours), water quenching cooling rate 35℃ / s; tempering temperature 600℃ (holding for 2.2 hours), air cool to room temperature;

[0244] 6) Inspection: Ultrasonic testing GB / T 2970 Class I, the relative corrosion rate after 72h immersion test is 48% (compared to steel Q355B).

[0245] 4. Mechanical property test results

[0246]

[0247] Example 4: Conventional Q345E steel (for bridges)

[0248] This embodiment uses Q345E grade bridge steel plates, which are currently the mainstream type on the market and conform to the national standard GB / T714-2025. Its composition design and manufacturing process are conventional technologies in this field.

[0249] 1. Chemical composition (wt.%): C: ≤0.18wt%, Si≤0.55wt%, Ni: 0.20wt%, Mn: 1.00~1.60wt%; Nb: 0.005~0.060wt%, V: 0.010~0.080wt%, Ti: 0.006~0.030wt%, Als: 0.010~0.045wt%, Cr, Ni, Cu: ≤0.30wt%, N: ≤0.0080wt%, P: ≤0.020wt%, S≤0.010wt%, B≤0.0005wt%, H≤0.0002wt%.

[0250] 2. Key performance indicators:

[0251] When the upper limit of the chemical composition is taken:

[0252] Carbon Equivalent (CEV): 0.56%

[0253] Welding crack susceptibility index (Pcm): 0.32%

[0254] 3. Mechanical property test results

[0255] -60℃ low temperature impact toughness (KV2) (J): average value 23 J, indicating insufficient toughness reserve under extreme cold conditions.

[0256] The range of welding heat input is relatively narrow, approximately 10-25 kJ / cm, which imposes stringent requirements on on-site welding processes and has poor adaptability.

[0257] Corrosion resistance: The relative corrosion rate after 72 hours of immersion testing was 55% (compared to Q355B steel).

[0258] Thickness specifications: 6~80mm.

[0259] Cost: 4500 yuan / ton.

[0260] Example 5: High Ni Low Temperature Steel

[0261] This embodiment uses high-Ni low-temperature steel currently available on the market. Its composition design and production process are both conventional technologies in this field.

[0262] 1. The chemical composition (wt.%) is as follows: C: 0.12wt%, Ni: 5.00wt%, Mn: 1.40wt%, Si: 0.30wt%, P: ≤0.018wt%, S: ≤0.012wt%, Al: 0.030wt%, Cr: 0.25wt%, Cu: 0.35wt%, Mo: 0.15wt%, Nb: 0.03wt%, V: 0.08wt%, Ti: 0.025wt%, Ca: 0.003wt%.

[0263] 2. Key performance indicators:

[0264] When the upper limit of the chemical composition is used: Carbon Equivalent (CEV): 0.81%

[0265] Welding crack susceptibility index (Pcm): 0.33%

[0266] 3. Mechanical property test results

[0267] -60℃ low temperature impact toughness (KV2) (J): average value 85J.

[0268] The range of welding heat input is relatively narrow, approximately 10-30 kJ / cm. It has stringent requirements for on-site welding processes and poor adaptability.

[0269] Corrosion resistance: The relative corrosion rate after 72 hours of immersion testing was 52% (compared to Q355B steel).

[0270] Thickness specifications: 6~50mm.

[0271] Cost: 8000 yuan / ton.

[0272] Example 6:

[0273] Compared to Example 1, other chemical compositions remained unchanged, but the carbon content was increased to 0.22%. The carbon equivalent (CEV) and weld crack sensitivity index (Pcm) were calculated, showing a CEV of 0.55% and Pcm > 0.34%, significantly higher than the Pcm ≤ 0.28% limit specified in this invention. An impact energy test was conducted on this comparative steel at -40°C, showing an impact energy < 60 J, far lower than the impact energy level of Example 1. Simultaneously, a weld crack test was performed on the oblique Y-groove joint, showing obvious cracks at the weld joint and a weld pass rate below 80%. This example demonstrates that exceeding the upper limit of carbon content significantly increases CEV and Pcm, sacrificing the weld safety and low-temperature toughness of the steel, failing to meet the core requirements for bridge steel in cold environments.

[0274] Example 7:

[0275] Compared to Example 1, other chemical components remained unchanged, except that the carbon content was reduced to 0.04%. Mechanical property tests were conducted on this comparative steel, and the results showed that its yield strength was only 330 MPa, failing to meet the minimum yield strength requirement (≥355 MPa) of the steel of this invention. Furthermore, the tensile strength decreased simultaneously, making it difficult to meet the load-bearing capacity required for bridge structures. This example demonstrates that a carbon content below the lower limit leads to insufficient steel strength, making it unsuitable for the load-bearing requirements of bridge engineering.

[0276] Conclusion: The C content window (0.05%-0.20%) is a necessary but non-obvious limitation for optimizing low-temperature toughness and weldability in this invention while ensuring that the steel strength meets the standard. Exceeding this range will lead to shortcomings in the key performance of the steel.

[0277] Example 8:

[0278] Compared to Example 1, other chemical components remained unchanged, except that the Cu content was adjusted to 0% and the Cr content to 0.60%. A immersion test was used to test the corrosion resistance of the steel in this example for 72 hours. The results showed that its relative corrosion rate was >65%, and its corrosion resistance was significantly lower than that of Example 1 (which had a relative corrosion rate of 50%). This example demonstrates that simply adding Cr cannot achieve the corrosion resistance effect of this invention; the lack of the synergistic effect of Cu significantly weakens the steel's resistance to atmospheric corrosion.

[0279] Example 9:

[0280] Compared to Example 1, other chemical components remained unchanged, but the Cr content was adjusted to 0% and the Cu content to 0.60%. The corrosion resistance was tested under the same immersion test conditions as in Example 8, and the results showed a relative corrosion rate >70%, with a more significant decrease in corrosion resistance compared to Example 1. This example demonstrates that adding Cu alone provides better corrosion resistance than adding Cr alone, but it is still far inferior to the synergistic effect of Cr / Cu, and cannot meet the long-term corrosion resistance requirements of bridge steel in cold environments.

[0281] Example 10:

[0282] Compared to Example 1, other chemical compositions remained unchanged, but the Cr content was increased to 2.0% and the Cu content to 0.60%. Corrosion resistance tests were conducted on the steel from this example, showing a relative corrosion rate of approximately 50%. The improvement in corrosion resistance compared to Example 1 was limited, failing to achieve the expected improvement. Simultaneously, tests on its hot working and weldability revealed that excessive Cu content led to surface cracks (hot brittleness) during hot working, significantly reduced the toughness of the heat-affected zone (HAZ) of the welded joint, resulted in an impact energy of <55J at -40℃, and a weld pass rate below 75%, failing to meet the requirements for mass production and engineering applications. This example demonstrates that excessively high Cr / Cu content not only fails to further improve corrosion resistance but also deteriorates the hot working and weldability of the steel, disrupting the performance balance.

[0283] Conclusion: The Cr / Cu ratio within the scope of this invention can produce a significant synergistic corrosion resistance effect between Cr and Cu. While achieving the best corrosion resistance effect, it also takes into account the hot working performance and weldability of the steel, achieving the optimal performance balance. The synergistic effect cannot be replaced by the addition of a single element, and the content exceeding the above range will lead to performance deterioration.

Claims

1. A type of bridge structural steel resistant to atmospheric corrosion in cold environments, wherein, The chemical composition of the steel, by weight percentage, includes: C：0.05％～0.20％， Mn: 0.90%–1.70%, Si: 0.10%–0.80%, P：≤0.020％， S：≤0.015％， Al：0.015％～0.06％， Ca: 0.001%–0.005%, And the remaining Fe and unavoidable impurities.

2. The steel according to claim 1, wherein, The chemical composition of the steel, by weight percentage, further includes one or more of the following elements: Cr：0～1.50％， Ni: 0~2.00%, Cu: 0–0.55%, Mo: 0–0.70%; Preferably, the chemical composition of the steel, by weight percentage, further includes one or more of the following elements: Nb: 0–0.06%, V：0～0.12％， Ti: 0–0.05%.

3. The steel according to claim 1 or 2, wherein, By mass percentage, C is 0.08% to 0.18%; Preferably, the Mn content is 1.10% to 1.65% by mass percentage; Preferably, the Si content is 0.20% to 0.50% by mass. Preferably, P is ≤0.015% by mass percentage; Preferably, S is ≤0.010% by mass percentage; Preferably, Al is 0.02% to 0.04% by mass. Preferably, the content of Ca is 0.002% to 0.004% by mass. Preferably, the Cr content is 0.10% to 0.30% by mass. Preferably, the Ni content is 0.30% to 0.80% by mass. Preferably, Cu is 0.30% to 0.50% by mass percentage; Preferably, the Mo content is 0.05% to 0.20% by mass. Preferably, the Nb content is 0.02% to 0.05% by mass. Preferably, V is 0.05% to 0.10% by mass percentage; Preferably, the content of Ti is 0.02% to 0.04% by mass. Preferably, the carbon equivalent (CEV) of the steel is ≤0.65%; Preferably, the weld crack sensitivity index Pcm of the steel is ≤0.28%.

4. The steel according to any one of claims 1 to 3, wherein, The chemical composition of the steel, by weight percentage, is as follows: C：0.05％～0.20％， Mn: 0.90%–1.70%, Si: 0.10%–0.80%, P：≤0.020％， S：≤0.015％， Al：0.015％～0.06％， Ca: 0.001%–0.005%, Cr：0～1.50％， Ni: 0~2.00%, Cu: 0–0.55%, Mo: 0–0.70%, Nb: 0–0.06%, V：0～0.12％， Ti: 0~0.05%, And the remaining Fe and unavoidable impurities; Alternatively, the chemical composition of the steel, expressed as a percentage by weight, is: C：0.08％～0.18％， Mn: 1.10%–1.65%, Si: 0.20%–0.50%, P：≤0.015％， S：≤0.010％， Cr：0.10％～0.30％， Ni: 0.30%–0.80%, Cu: 0.30%–0.50%, Mo: 0.05%–0.20%, Nb: 0.02%–0.05%, V：0.05％～0.10％， Ti: 0.02%–0.04%, Al：0.02％～0.04％， Ca: 0.002%–0.004%, The balance consists of Fe and unavoidable impurities; Preferably, the thickness of the steel is in the range of 6mm to 150mm.

5. A method for manufacturing the steel according to any one of claims 1 to 4, the method comprising the following steps: 1) Smelting and refining: Smelting is carried out using a converter or electric furnace, followed by LF ladle refining and RH vacuum treatment to obtain refined molten steel; 2) Continuous casting: The refined molten steel is continuously cast to obtain a continuously cast slab; 3) Heating: The continuously cast slab is fed into a heating furnace for heating to obtain a heated slab; 4) Rolling: The heated slab is rolled in two stages: roughing and finishing. 5) Cooling: The rolled steel plate is accelerated and cooled at a rate of 10-25℃ / s, with a final cooling temperature of 450-520℃. Then it is stacked and cooled slowly to obtain a slowly cooled steel plate.

6. The manufacturing method according to claim 5, wherein, In step 1), the purity of the refined molten steel meets the following requirements: P≤0.015%, S≤0.010%, [H]≤2.0ppm; Preferably, in step 1), the grade of non-metallic inclusions in the refined molten steel meets the following requirements: Class D inclusions ≤ 1.5 and other inclusions ≤ 2.0 in GB / T10561; Preferably, in step 1), calcium treatment is performed during the refining process.

7. The manufacturing method according to claim 5 or 6, wherein, In step 2), the refined molten steel is continuously cast using a vertical bending continuous casting machine; Preferably, in step 2), the continuous casting speed is 0.8–1.5 m / min; Preferably, in step 2), aerosol cooling is used for secondary cooling, with a cooling intensity of 0.8–1.2 L / kg; Preferably, in step 2), the water flow rate of the crystallizer of the continuous casting machine, such as a vertical bending continuous casting machine, is 300-450 L / min; Preferably, in step 2), the thickness of the continuously cast slab is 220-300 mm, and the central shrinkage cavity and segregation grade are ≤2.

0.

8. The manufacturing method according to any one of claims 5 to 7, wherein, In step 3), the heating furnace is a walking beam furnace; Preferably, in step 3), the heating temperature is 1120–1180°C; Preferably, in step 3), the temperature for homogenization is 1150–1200°C; Preferably, in step 3), the heating time is 2.5 to 3.0 hours.

9. The manufacturing method according to any one of claims 5 to 8, wherein, In step 4), during the rough rolling stage: the initial rolling temperature is 1110–1140℃, and the cumulative reduction rate is ≥65%. Preferably, in step 4), the finishing rolling stage is as follows: the initial rolling temperature is 910-920℃, the final rolling temperature is 800-870℃, the reduction rate of each finishing rolling pass is ≥8%, and the thickness of the steel plate after final rolling is 6-150mm.

10. The manufacturing method according to any one of claims 5 to 9, wherein, In step 5), the slow cooling time for stacking is ≥48 hours; Preferably, when the yield strength of the target steel is ≥550MPa, after cooling in step 5), step 6) tempering treatment is also included: quenching and tempering the slowly cooled steel plate. Preferably, in step 6), the quenching temperature is 850–900℃, and the holding time is 2.5–3.0 min / mm based on the plate thickness; Preferably, in step 6), the quenching method is water quenching, and the cooling rate is ≥30℃ / s; Preferably, in step 6), the tempering temperature is 550-650℃, and the holding time is 3.0-3.5 min / mm based on the plate thickness.

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