400MPa-grade low-temperature corrosion-resistant steel bar for frozen soil engineering and preparation method of low-temperature corrosion-resistant steel bar

Abstract

The invention relates to the technical field of low-temperature corrosion-resisting steel, in particular to a 400 MPa grade low-temperature corrosion-resisting steel bar for frozen soil engineering and a preparation method of the 400 MPa grade low-temperature corrosion-resisting steel bar, and the low-temperature corrosion-resisting steel bar comprises, by weight, 0.03%-0.25% of C, 0.1%-0.5% of Si, 0.2%-1.5% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.015% of S, 0.01%-0.30% of Ni, 0.01%-0.60% of Cr, 0.01%-0.30% of Cu, 0.001%-0.015% of Ce + La, 0.001%-0.050% of V, 0.001%-0.050% of Ti and the balance Fe and inevitable trace inclusions. By adding nickel, chromium, copper and other elements, a compact oxidation film is formed on the surface of the steel bar, and the corrosion resistance is improved; by adding rare earth elements and utilizing the purification effect of the rare earth elements, inclusions in steel are reduced, and meanwhile the corrosion resistance and oxidation resistance of the steel bar are improved; vanadium and titanium are added, and the fine grain strengthening and precipitation strengthening effects of vanadium and titanium are utilized, so that the strength and low-temperature toughness of the steel bar are improved, and the problems that in the prior art, low-temperature brittleness is large, low-temperature corrosion resistance is insufficient, and low-temperature toughness and corrosion resistance are difficult to consider at the same time are solved.

Description

Technical Field

[0001] This invention relates to the field of low-temperature corrosion-resistant steel technology, and in particular to a 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering and its preparation method. Background Technology

[0002] Engineering construction in permafrost regions faces numerous technical challenges. These areas typically exhibit extreme environmental characteristics such as high altitude, high radiation, and frequent freeze-thaw cycles. Due to permafrost degradation, the foundations of permafrost engineering projects are prone to settlement, leading to brittle fracture of reinforcing steel and significantly reducing the service life of the project. Therefore, developing low-temperature corrosion-resistant reinforcing steel suitable for permafrost regions is of great significance for improving the durability of engineering structures.

[0003] Existing steel reinforcement technology has shortcomings in terms of low-temperature performance and corrosion resistance. For example, ordinary steel reinforcement is prone to brittle fracture in low-temperature environments and cannot meet the requirements for use in permafrost regions with extreme low temperatures (such as -20℃ or -45℃). In addition, the soil in permafrost regions is rich in chloride and sulfate ions, which can accelerate the corrosion rate of steel reinforcement, leading to the failure of reinforced concrete structures and shortening the service life of projects.

[0004] In the prior art, although some patents involve production methods for low-temperature steel bars or high-strength steel bars, these technologies mainly focus on low-temperature toughness or high-strength performance, without fully considering the corrosion resistance and relative corrosion rate control of steel bars in frozen soil environments.

[0005] Therefore, existing technologies lack a type of steel reinforcement and its preparation method that can simultaneously meet the requirements of low-temperature toughness and corrosion resistance in permafrost regions. This invention addresses this technological gap by proposing a 400MPa-grade low-temperature corrosion-resistant steel reinforcement for permafrost engineering and its preparation process, aiming to solve the problem that existing steel reinforcement cannot simultaneously meet the requirements of low-temperature performance and corrosion resistance. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering and its preparation method, in order to solve at least one of the problems in the prior art, such as high low-temperature brittleness, insufficient low-temperature corrosion resistance, and difficulty in balancing low-temperature toughness and corrosion resistance.

[0007] The objective of this invention is mainly achieved through the following technical solutions: A low-temperature corrosion-resistant steel bar for frozen soil engineering with a pressure rating of 400MPa has the following chemical composition by weight percentage: C: 0.03-0.25%, Si: 0.1-0.5%, Mn: 0.2-1.5%, P: ≤0.015%, S: ≤0.015%, Ni: 0.01-0.30%, Cr: 0.01-0.60%, Cu: 0.01-0.30%, Ce+La: 0.001-0.015%, V: 0.001-0.050%, Ti: 0.001-0.050%, with the balance being Fe and unavoidable trace inclusions.

[0008] Preferably, the content of S is ≤0.011%.

[0009] Preferably, the Ni content is controlled between 0.018 and 0.29%.

[0010] Preferably, the Cr content is controlled between 0.01% and 0.38%.

[0011] Preferably, the Cu content is controlled between 0.01% and 0.30%.

[0012] Preferably, the total mass fraction of Ni, Cr and Cu is ≥0.4%.

[0013] Preferably, the total mass fraction of Ce and La is ≥0.01%.

[0014] Preferably, the total mass fraction of V and Ti is ≥0.03%.

[0015] Preferably, the grain size of the 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering is grade 9 to 12.

[0016] Preferably, the 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering has a low-temperature impact energy of -45℃. ≥27J Preferably, the average corrosion rate of the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering is between 1.63g / m²·h and 1.76g / m²·h.

[0017] A method for preparing low-temperature corrosion-resistant steel bars for 400MPa grade frozen soil engineering, comprising: S1. Smelting: Steel is obtained by smelting raw materials in a converter or electric furnace. S2, Refining: The molten steel is refined using an LF furnace, and rare earth elements are added to the molten steel after refining. S3, Continuous Casting: The refined molten steel is continuously cast to form steel billets; S4. Heating: The steel billet obtained by continuous casting is heated to 1000℃~1150℃; S5. Controlled rolling: The heated steel billet is subjected to two stages of rolling: rough rolling and finish rolling. The finish rolling is carried out at low temperature in the non-recrystallization zone. S6. Cooling Control: The rolled steel billet is subjected to controlled cooling at a rate of 0.2℃ / s to 30℃ / s.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: (1) This invention improves the corrosion resistance of steel bars by adding elements such as nickel (Ni), chromium (Cr), and copper (Cu) to form a dense oxide film on the surface of the steel bars; by adding rare earth elements (such as cerium Ce and lanthanum La), the purification effect of rare earth elements is used to reduce inclusions in the steel, while improving the corrosion resistance and oxidation resistance of the steel bars; by adding vanadium (V) and titanium (Ti), the strength and low-temperature toughness of the steel bars are improved by using their fine grain strengthening and precipitation strengthening effects; the invention solves the problem of insufficient corrosion resistance of ordinary steel bars in permafrost areas due to chloride and sulfate ion corrosion, and solves the problem of brittle fracture of existing steel bars in extreme low-temperature environments, resulting in a significant reduction in the relative corrosion rate of the steel bars, a significant improvement in corrosion resistance, a significant improvement in the low-temperature toughness of the steel bars, and a reduction in the brittle transition temperature.

[0019] (2) The present invention adopts low-temperature large deformation controlled rolling. The rough rolling adopts a large deformation process, and the finish rolling is carried out in the non-recrystallization zone at low temperature. After rolling, controlled cooling is adopted with a cooling rate of 0.2~30℃ / s to refine the grain structure. The grain size of the steel bar reaches grade 9~12, the structure is fine-grained ferrite and pearlite, the ferrite content is ≥60%, the yield strength of the steel bar is ≥400MPa, the tensile strength is ≥540MPa, the yield ratio is <0.8, the elongation is ≥9%, and the low-temperature impact toughness is significantly improved. The low-temperature impact energy KV2 at -45℃ is ≥27J, which solves the problem that the strength and toughness of existing steel bars are difficult to balance in low-temperature environment and the problem that the toughness of ordinary steel bars is insufficient due to coarse grains in low-temperature environment.

[0020] (3) In this invention, rare earth elements (Ce or La) are added to molten steel after LF refining, which reduces the oxidation loss of rare earth elements, increases the yield (>50%), increases the solid solubility of rare earth elements in steel, enhances its corrosion resistance, reduces the formation of rare earth oxide inclusions, and further purifies the steel. This solves the problems of easy oxidation and low yield of rare earth elements during the addition process and the problem that rare earth elements cannot fully exert their purification and corrosion resistance in the prior art.

[0021] (4) By strictly controlling the P and S content: P≤0.015%, S≤0.011%, and oxygen content≤30ppm, the present invention reduces the influence of impurity elements on low-temperature toughness and corrosion resistance, and also reduces the negative impact of oxide inclusions on performance, thereby significantly improving the low-temperature toughness of steel bars, reducing the brittle transition temperature, further enhancing the corrosion resistance of steel bars, and reducing the corrosion rate. This solves the problems of insufficient low-temperature toughness of existing steel bars due to the enrichment of impurity elements and the increase of inclusions and performance decline due to high oxygen content in steel.

[0022] (5) This invention optimizes the entire process of steel bars through full-process control, from converter (or electric furnace) smelting, LF furnace refining, continuous casting of steel billets, continuous casting billet heating to controlled rolling, controlled cooling, slow cooling on the cooling bed and steel bar finishing. It further refines the process parameters, with rough rolling start temperature of 950℃~1100℃, finish rolling finish temperature of 800℃~930℃, and heating temperature of 1000℃~1150℃. This enables the industrial continuous production of steel bars, improves production efficiency, ensures the uniformity of steel bar structure and the consistency of performance, and solves the problem of difficulty in achieving industrial continuous production in existing preparation processes and the problem of uneven steel bar structure and performance in existing processes.

[0023] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained through the embodiments described and the accompanying drawings, which are particularly pointed out. Attached Figure Description

[0024] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 Metallographic image of the low-temperature corrosion-resistant steel reinforcement used in 400MPa grade frozen soil engineering in Example 1; Figure 2 The actual product after salt spray test of the 400MPa grade frozen soil engineering low temperature corrosion resistant steel bar in Example 1; Figure 3 This is the actual product of the low-temperature corrosion-resistant steel bar used in the 400MPa grade frozen soil engineering after salt spray test, as shown in Comparative Example 1. Detailed Implementation

[0025] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0026] On one hand, the present invention discloses a low-temperature corrosion-resistant steel bar for frozen soil engineering with a pressure rating of 400MPa. The chemical composition, by weight percentage, is as follows: C: 0.03-0.25%, Si: 0.1-0.5%, Mn: 0.2-1.5%, P: ≤0.015%, S: ≤0.015%, Ni: 0.01-0.30%, Cr: 0.01-0.60%, Cu: 0.01-0.30%, Ce+La: 0.001-0.015%, V: 0.001-0.050%, Ti: 0.001-0.050%, with the balance being Fe and unavoidable trace inclusions.

[0027] Preferably, the hot-rolled microstructure of the low-temperature corrosion-resistant steel bar is a fine-grained ferrite and pearlite microstructure, wherein the content of fine-grained ferrite is >60%.

[0028] It should be noted that the microstructure of the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering is mainly composed of fine-grained ferrite. This is because fine-grained ferrite enables the low-temperature corrosion-resistant steel to have a high elongation after fracture and a low yield strength ratio, meeting the performance requirements of steel bars for high-altitude frozen soil engineering to resist large deformations. However, pearlite will inevitably be present during the preparation process. If there is too much pearlite, although the strength will increase, the low-temperature toughness will be significantly reduced. Therefore, the ferrite content in the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering is >60%.

[0029] It should be noted that carbon (C) is an important strengthening element in steel. The steel grade of this invention has a fine-grained ferrite as its final main microstructure, requiring strict control of the C content. The addition of an appropriate amount of C can play a solid solution strengthening role in the steel, improving the strength of the matrix. Strict control of the C content can effectively reduce the sensitivity of the Mn / S ratio in the continuously cast billet, reducing the probability of continuous casting cracks. A relatively low C content can improve the uniform elongation of the steel; therefore, the carbon content of the steel in this invention is controlled at 0.03–0.25%.

[0030] Preferably, the carbon content of the steel of the present invention is controlled to be 0.03 to 0.09%.

[0031] Silicon: Si not only provides solid solution strengthening but also improves the corrosion resistance of steel. Excessive silicon content negatively impacts the ductility and toughness of steel; therefore, the Si content should be controlled within the range of 0.1% to 0.5%.

[0032] Manganese: Mn is the most effective element for expanding the austenite phase region, refining grains, and ensuring the comprehensive performance of steel. 0.1% solid solution Mn can increase the tensile strength of the matrix by 8 MPa. Considering all factors, the Mn content in steel should be controlled within the range of 0.2% to 1.5%.

[0033] Phosphorus and sulfur: Phosphorus and sulfur have a significant impact on the low-temperature toughness of steel. The enrichment or segregation of phosphorus and sulfur will seriously reduce the low-temperature toughness of steel. Therefore, it is necessary to strictly control the phosphorus and sulfur content in steel. The phosphorus content should be controlled at ≤0.015%, and the sulfur content should be controlled at ≤0.015%.

[0034] Preferably, the content of S is ≤0.011%.

[0035] Nickel: Ni is an important element for reducing the brittle transition temperature of steel bars. Nickel forms an infinite solid solution with iron, which can expand the austenite phase region, stabilize the austenite structure and refine the grain structure, thereby inhibiting the brittle transition. The content should be controlled at 0.01-0.30%.

[0036] More preferably, the Ni content is controlled at 0.018–0.29%.

[0037] Chromium: Cr is an important element for improving the corrosion resistance of steel. After Cr oxidizes on the surface of steel bars, it forms a relatively dense oxide film, which prevents the further oxidation of iron on the surface of the steel bar matrix. The content is controlled between 0.01% and 0.60%.

[0038] More preferably, the Cr content is controlled between 0.01% and 0.38%.

[0039] Copper: Cu is a solid solution strengthening element that can improve the strength of the steel matrix. The addition of Cu can enhance the corrosion resistance of steel, making it more durable in corrosive environments such as acids, alkalis, and salts. Therefore, the Cu content in steel is controlled within the range of 0.01% to 0.30%.

[0040] More preferably, the Cu content is controlled at 0.19–0.30%.

[0041] Preferably, the total mass fraction of Ni, Cr and Cu satisfies: [Ni]+[Cr]+[Cu]≥0.4%.

[0042] The applicant's research found that the synergistic effect of nickel, chromium, and copper can significantly improve the overall performance of reinforcing steel. Nickel is primarily responsible for low-temperature toughness, chromium is primarily responsible for corrosion resistance, while copper plays a balancing role between the two, improving both strength and corrosion resistance. This invention ensures that the reinforcing steel possesses sufficient performance in low-temperature environments and corrosive soils by setting a minimum content requirement of ≥0.4% of the total mass of Ni, Cr, and Cu. The low-temperature impact strength and corrosion resistance of the reinforcing steel are significantly better than those with lower content, while the above content range effectively balances cost and performance.

[0043] Cerium (Ce) and lanthanum (La) can effectively improve the characteristics of inclusions in steel, purify molten steel, refine grains, and enhance the corrosion resistance and oxidation resistance of steel, effectively resisting corrosion from high-altitude soils. Therefore, the total mass fraction of Ce and La ([Ce]+[La]) is controlled within the range of 0.001–0.015%.

[0044] Preferably, the total mass fraction of Ce and La satisfies: [Ce] + [La] ≥ 0.01%.

[0045] It should be noted that when [Ce]+[La]≥0.01%, the corrosion resistance and low-temperature toughness of the reinforcing steel are significantly better than those with lower contents. In corrosion tests conducted in simulated frozen soil environments, reinforcing steel meeting the [Ce]+[La]≥0.01% requirement exhibits a significantly lower relative corrosion rate than ordinary reinforcing steel. Furthermore, in low-temperature impact tests at -45℃, the impact energy KV2 of reinforcing steel meeting this content requirement is significantly higher than that of ordinary reinforcing steel.

[0046] Vanadium (V) and titanium (Ti) are strong carbonitride producing elements that can improve the strength of steel through grain refinement, precipitation strengthening, and solid solution strengthening. Adding V can lower the ductile-brittle transition temperature of steel; the addition of V and Ti precipitates can effectively pin austenite grain boundaries, refining the grains and improving the low-temperature toughness of the weld heat-affected zone.

[0047] Preferably, the total mass fraction of V and Ti satisfies: [V] + [Ti] ≥ 0.03%.

[0048] More preferably, the mass fraction of V is ≤0.025%.

[0049] It should be noted that when [V]+[Ti]≥0.03%, the steel bars exhibit significantly higher impact energy KV2 in the -45℃ low-temperature impact test, and the low-temperature toughness is significantly improved; at the same time, it can also improve the yield strength R of the steel bars. eL and tensile strength R m All meet the design requirements (R) el ≥400MPa, R m ≥540MPa), while maintaining a low yield strength ratio (R el / R m <0.8).

[0050] Preferably, the grain size of the 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering is grade 9 to 12.

[0051] Preferably, the relative corrosion rate of the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering is <60%.

[0052] Compared with existing technologies, this invention optimizes the chemical composition design by adding elements such as nickel (Ni), chromium (Cr), and copper (Cu) to form a dense oxide film on the surface of the reinforcing steel, thereby improving its corrosion resistance. By adding rare earth elements (such as cerium (Ce) and lanthanum (La), the purification effect of rare earth elements is utilized to reduce inclusions in the steel, while simultaneously improving the corrosion resistance and oxidation resistance of the reinforcing steel. Furthermore, by adding vanadium (V) and titanium (Ti), their grain-refining and precipitation-strengthening effects are utilized to improve the strength and low-temperature toughness of the reinforcing steel. This invention solves the problem of insufficient corrosion resistance of ordinary reinforcing steel in permafrost regions due to chloride and sulfate ion corrosion, and addresses the issue of brittle fracture of existing reinforcing steel under extreme low-temperature conditions. As a result, the relative corrosion rate of the reinforcing steel is significantly reduced, its corrosion resistance is greatly improved, its low-temperature toughness is significantly enhanced, and its brittle transition temperature is lowered.

[0053] On the other hand, this invention discloses a method for preparing low-temperature corrosion-resistant steel bars for 400MPa grade frozen soil engineering, comprising: S1. Smelting: Steel is obtained by smelting raw materials in a converter or electric furnace. S2, Refining: The molten steel is refined using an LF furnace, and rare earth elements are added to the molten steel after refining. S3, Continuous Casting: The refined molten steel is continuously cast to form steel billets; S4. Heating: The steel billet obtained by continuous casting is heated to 1000℃~1150℃; S5. Controlled rolling: The heated steel billet is subjected to two stages of rolling: rough rolling and finish rolling. The finish rolling is carried out at low temperature in the non-recrystallization zone. S6. Cooling Control: The rolled steel billet is subjected to controlled cooling at a rate of 0.2℃ / s to 30℃ / s.

[0054] It should be noted that the non-recrystallization zone refers to the area where the material's temperature is below the recrystallization temperature during rolling. Within this temperature range, dynamic recrystallization does not occur during deformation; instead, cold deformation (or work hardening) dominates. The recrystallization temperature refers to the temperature at which the grains within the material recrystallize after cold deformation and heating, forming new, undistorted grains. The recrystallization temperature typically depends on the material's composition, the degree of deformation, and the heating rate. For most carbon steels and low-alloy steels, the recrystallization temperature is generally between 600℃ and 900℃.

[0055] During implementation, low-temperature rolling in the non-recrystallization zone is adopted in the finishing rolling stage. When producing low-temperature corrosion-resistant steel bars, the grains can be effectively refined, and the low-temperature toughness and strength of the steel bars can be significantly improved. The cooling rate is precisely controlled within a range of 0.2 to 30°C / s, which allows for better low-temperature toughness and strength. Adding rare earth elements Ce and / or La after refining in the LF furnace at this unique timing minimizes the oxidation loss of rare earth elements, increases their solid solution content in the steel, and thus significantly enhances the corrosion resistance of the reinforcing steel.

[0056] Compared with existing technologies, the present invention performs low-temperature rolling in the non-recrystallization zone, followed by controlled cooling at a rate of 0.2–30°C / s to refine the grain structure. This results in steel bars with a grain size of 9–12, a microstructure of fine-grained ferrite and pearlite, a ferrite content ≥60%, a yield strength ≥400 MPa, a tensile strength ≥540 MPa, a yield-to-tensile ratio <0.8, an elongation ≥9%, and significantly improved low-temperature impact toughness, achieving a low-temperature impact strength of -45°C. ≥27J, which solves the problem that existing steel bars are difficult to balance in terms of strength and toughness in low-temperature environments and the problem that ordinary steel bars have insufficient toughness due to coarse grains in low-temperature environments.

[0057] Compared with the prior art, the present invention adds rare earth elements (Ce or La) to molten steel after LF refining, which reduces the oxidation loss of rare earth elements, increases their yield (>50%), increases the solid solubility of rare earth elements in steel, enhances their corrosion resistance, reduces the formation of rare earth oxide inclusions, and further purifies the steel. This solves the problems of easy oxidation and low yield of rare earth elements during the addition process, as well as the problem that rare earth elements cannot fully exert their purification and corrosion resistance effects in the prior art.

[0058] Preferably, in step S1, the final composition of the smelting process is controlled as follows: C ≤ 0.05% and P ≤ 0.010%.

[0059] It should be noted that while carbon is the primary strengthening element in steel, excessively high carbon content significantly increases the brittle transition temperature and reduces its low-temperature toughness. By controlling the carbon content to a low level (≤0.05%), the brittle transition temperature of reinforcing bars can be effectively reduced, allowing them to maintain good toughness even in extreme low-temperature environments (such as -45℃) and preventing brittle fracture. Lower carbon content also reduces oxidation and decarburization on the steel surface, thereby improving the surface quality and dimensional accuracy of the reinforcing bars. Furthermore, low-carbon steel is easier to control during subsequent refining and rolling processes, enabling better achievement of fine-grained microstructure and high performance. For example, in LF furnace refining and controlled rolling processes, low carbon content helps reduce inclusion formation and improve steel purity.

[0060] It should be noted that phosphorus is a harmful element in steel, easily segregating at grain boundaries, leading to grain boundary embrittlement and significantly reducing the steel's low-temperature toughness. Controlling the phosphorus content to extremely low levels (≤0.010%) can effectively reduce phosphorus segregation at grain boundaries, improving the toughness of reinforcing steel at low temperatures and preventing brittle fracture. Low phosphorus content also helps improve the purity of the steel, reducing the formation of inclusions. Phosphorus in steel usually exists in the form of phosphides, which reduce the steel's plasticity and toughness. Strictly controlling the phosphorus content can reduce the formation of these harmful inclusions, thereby improving the overall performance of the reinforcing steel. Steel with low phosphorus content exhibits better plasticity and toughness during hot working (such as rolling and forging), reducing the formation of cracks and defects. Simultaneously, reducing phosphorus content can indirectly improve the corrosion resistance of the reinforcing steel. Phosphates easily become the starting point of corrosion in corrosive media; reducing phosphorus content can reduce the corrosion rate of reinforcing steel in corrosive environments, extending its service life.

[0061] Compared with existing technologies, this invention reduces the impact of impurity elements on low-temperature toughness and corrosion resistance by strictly controlling the P and S content: P≤0.015%, S≤0.01%, and oxygen content≤30ppm. It also reduces the negative impact of oxide inclusions on performance, resulting in a significant improvement in the low-temperature toughness of the steel bars, a lower brittle transition temperature, further enhanced corrosion resistance, and a reduced corrosion rate. This solves the problems of insufficient low-temperature toughness caused by the enrichment of impurity elements in existing steel bars and the increased inclusions and performance degradation caused by high oxygen content in the steel.

[0062] Preferably, the refined O content in step S2 is ≤30ppm.

[0063] It should be noted that oxygen in steel mainly exists in the form of oxides, such as silicon dioxide and aluminum oxide. These oxide inclusions reduce the toughness and ductility of steel, especially at low temperatures. By controlling the oxygen content to ≤30ppm, the number and size of oxide inclusions can be significantly reduced, thereby improving the low-temperature toughness of the reinforcing steel. Oxide inclusions in steel often become the initiation point for cracks, especially during welding and cold deformation. Reducing the oxygen content reduces the presence of these inclusions, thereby reducing the risk of crack formation and improving the crack resistance of the reinforcing steel. Lowering the oxygen content can significantly improve the purity of the steel and reduce the formation of inclusions. Pure steel not only helps improve the mechanical properties of the reinforcing steel but also enhances its machinability and weldability.

[0064] Low oxygen content helps form a more uniform microstructure, reduces component segregation and microstructure inhomogeneity, thereby improving the overall performance consistency of the reinforcing steel. When the oxygen content after refining is controlled at ≤30ppm, the comprehensive performance of the reinforcing steel is significantly better than that of steel with higher content. In the low-temperature impact test at -45℃, the steel bars with an oxygen content ≤30ppm exhibited a higher impact energy KV2, which was significantly higher than that of steel bars with high oxygen content. In the corrosion test in a simulated frozen soil environment, the relative corrosion rate of steel bars with an oxygen content ≤30ppm was significantly lower than that of steel bars with high oxygen content, which can effectively extend the service life of reinforced concrete structures.

[0065] Preferably, in step S3, the continuously cast billet is cold-charged into the furnace or hot-charged.

[0066] Preferably, the heating temperature of the furnace in step S4 is controlled at 1000℃~1150℃.

[0067] Within this temperature range, the plasticity of steel before rolling can be ensured, oxidation and decarburization can be reduced, heating efficiency can be improved, microstructure can be optimized, production efficiency can be increased, and environmental impact can be reduced.

[0068] Preferably, the initial rolling temperature of the rough rolling in step S5 is 950℃~1100℃.

[0069] In practice, the initial rolling temperature range ensures that the steel billet possesses good plasticity, enabling it to withstand significant deformation during rough rolling without cracking or other defects. This temperature range is higher than the recrystallization temperature of steel, allowing the material to recover its plasticity through a dynamic recrystallization mechanism during deformation, thus achieving rolling with large deformation. The higher initial rolling temperature also helps refine the grains, as recrystallization occurs more readily at higher temperatures, resulting in finer grain sizes. This fine-grained structure not only improves the strength of the reinforcing steel but also enhances its low-temperature toughness, which is particularly important for reinforcing steel used in extreme low-temperature environments.

[0070] The applicant's research found that when the roughing rolling temperature is within the range of 950℃ to 1100℃, the final properties of the reinforcing steel (including strength, toughness, and low-temperature impact toughness) are significantly better than those of reinforcing steel at temperatures below or above this range; in a low-temperature impact test at -45℃, reinforcing steel with an initial rolling temperature within the range of 950℃ to 1100℃ exhibits a higher impact energy KV2; the yield strength R of the reinforcing steel... el and tensile strength R m All meet the design requirements (R) el ≥400MPa, R m ≥540MPa), while maintaining a high elongation after fracture (A gt ≥9%).

[0071] Preferably, the finishing rolling temperature in step S5 is 800℃~930℃.

[0072] The applicant's research found that a final rolling temperature of 800℃ to 930℃ ensures that the final microstructure of the reinforcing steel is fine-grained ferrite and pearlite, with the content of fine-grained ferrite ≥60%. This microstructure not only improves the strength and toughness of the reinforcing steel but also enhances its resistance to large deformations. The comprehensive performance of reinforcing steel within this final rolling temperature range is significantly better than that of steel within lower or higher temperature ranges. In a low-temperature impact test at -45℃, reinforcing steel with a final rolling temperature in the 800℃ to 930℃ range exhibits a higher impact energy KV2, significantly higher than that of steel in other temperature ranges. The yield strength R of the reinforcing steel is also significantly higher. el and tensile strength R m All meet the design requirements (R) el ≥400MPa, R m ≥540MPa), while maintaining a high elongation after fracture (A gt ≥9%).

[0073] Preferably, the rough rolling adopts a large deformation process with a rolling ratio of 30% to 70%, which is beneficial to increase the strain in the core of the steel bar, improve the uniformity of the microstructure, and improve the mechanical properties of the steel.

[0074] Preferably, the cumulative deformation of the finishing rolling passes is greater than 60%, which can effectively refine the grain structure and improve the low-temperature toughness of the steel.

[0075] Preferably, the diameter range of the low-temperature corrosion-resistant steel bars used in 400MPa grade frozen soil engineering is 6mm to 50mm.

[0076] Specifically, step S6 can be 0.2℃ / s, 0.7℃ / s, 1.2℃ / s, 1.7℃ / s, 2.2℃ / s, 2.7℃ / s, 3.2℃ / s, 3.7℃ / s, 4.2℃ / s, 4.7℃ / s, 5.2℃ / s, 5.7℃ / s, 6.2℃ / s, 6.7℃ / s, 7.2℃ / s, 7.7℃ / s, 8.2℃ / s, 8.7℃ / s, 9.2℃ / s, 9.7℃ / s, 10.2℃ / s, 10.7℃ / s, 11.2℃ / s, 11.7℃ / s, 12.2℃ / s, 12.7℃ / s, 13.2℃ / s, 13.7℃ / s, 14.2℃ / s, 14.7℃ / s, or 15.2℃ / s. , 15.7℃ / s, 16.2℃ / s, 16.7℃ / s, 17.2℃ / s, 17.7℃ / s, 18.2℃ / s, 18.7℃ / s, 19 .2℃ / s, 19.7℃ / s, 20.2℃ / s, 20.7℃ / s, 21.2℃ / s, 21.7℃ / s, 22.2℃ / s, 22.7℃ / s, 23.2℃ / s, 23.7℃ / s, 24.2℃ / s, 24.7℃ / s, 25.2℃ / s, 25.7℃ / s, 26.2℃ / s, 26 .7℃ / s, 27.2℃ / s, 27.7℃ / s, 28.2℃ / s, 28.7℃ / s, 29.2℃ / s, 29.7℃ / s or 30℃ / s.

[0077] Compared with existing technologies, this invention optimizes the entire process of steel reinforcement through full-process control, from converter (or electric furnace) smelting, LF furnace refining, continuous casting of steel billets, heating of continuously cast billets to controlled rolling, controlled cooling, slow cooling on a cooling bed, and finishing of steel bars. It further refines the process parameters, with rough rolling start temperature of 950-1100℃, finishing rolling finish temperature of 800-930℃, and heating temperature of 1000-1150℃. This enables the industrial continuous production of steel bars, improves production efficiency, ensures the uniformity of steel bar structure and the consistency of performance, and solves the problems of difficulty in achieving industrial continuous production in existing preparation processes and the problem of uneven steel bar structure and performance in existing processes.

[0078] To better illustrate the present invention, the following embodiments and comparative examples are provided:

[0079] Example 1 This embodiment provides a method for preparing low-temperature corrosion-resistant steel bars for frozen soil engineering, including: The chemical composition of the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering in this embodiment is shown in Table 1.

[0080] The preparation method of the low-temperature corrosion-resistant steel bars for 400MPa grade frozen soil engineering includes: converter smelting → LF furnace refining → continuous casting of steel billets → heating of continuously cast billets → controlled rolling → controlled cooling → finished product, specifically including: (1) Converter smelting, control the final composition of smelting: C≤0.05%, P≤0.010%, slag blocking and tapping steel, after deoxidation and alloying, enter the argon station for bottom blowing argon, and add alloying components such as Mn, V, and Ti.

[0081] (2) Rare earth Ce+La is added to molten steel after refining in the LF furnace. The yield is controlled at >50%, and the O content in the later stage of refining is controlled at ≤30ppm.

[0082] (3) The slow-cooled continuous casting billet is cold-charged into the furnace, and the heating temperature of the furnace is controlled at 1050℃.

[0083] (4) The roughing process adopts a large deformation process with a rolling ratio of 70% and a roughing start temperature of 1000℃; the finishing stage adopts low-temperature rolling in the non-recrystallization zone with a rolling ratio of 30%, a cumulative deformation of more than 60%, and a final rolling temperature of 850℃.

[0084] (5) The post-rolling controlled cooling process is adopted, and the cooling rate of the cooling section is controlled at 3℃ / s.

[0085] The low-temperature corrosion-resistant steel bars used in this 400MPa grade frozen soil engineering project have a diameter of 20mm, and the hot-rolled microstructure consists of >60% fine-grained ferrite (content 71%) and pearlite, with a grain size of 9.2. Figure 1 Here is a metallographic image of the low-temperature corrosion-resistant steel. Figure 2 This is a picture of the actual product after the salt spray test of the low-temperature corrosion-resistant steel. The mechanical properties and low-temperature toughness of the low-temperature corrosion-resistant steel are shown in Table 2, and the corrosion resistance is shown in Table 3.

[0086] Yield strength (R) el ) and tensile strength (R m Test standard: GB / T 228.1-2010 Metallic materials, tensile testing - Part 1: Test method at room temperature.

[0087] Elongation (A) gt Test standard: GB / T 228.1-2010.

[0088] Yield ratio (R) el / R m Test standard: in accordance with GB / T 228.1-2010.

[0089] Low-temperature impact energy (KV2) test standard: GB / T 229-2007 "Charpy impact test method for metallic materials".

[0090] Average corrosion rate test standard: GB / T 10125-2012 "Artificial Atmosphere Corrosion Test - Salt Spray Test".

[0091] Relative corrosion rate test standard: based on ISO 10289:2003 or GB / T 10125-2012, and then compared with the corrosion rate of a reference material (such as Q355 steel).

[0092] Grain size testing standard: GB / T 6394-2010 "Method for determination of average grain size of metals".

[0093] Example 2 This embodiment provides a method for preparing low-temperature corrosion-resistant steel bars for frozen soil engineering, including: The chemical composition of the 400MPa grade low-temperature corrosion-resistant steel for frozen soil engineering in this embodiment is shown in Table 1, and the preparation method is the same as in Example 1.

[0094] The diameter of the low-temperature corrosion-resistant steel bar used in this 400MPa grade frozen soil engineering project is 16mm. The microstructure in the hot-rolled state is fine-grained ferrite and pearlite, with a grain size of 9.8. The mechanical properties and low-temperature toughness of this low-temperature corrosion-resistant steel are shown in Table 2, and the corrosion resistance is shown in Table 3.

[0095] Example 3 This embodiment provides a method for preparing low-temperature corrosion-resistant steel bars for frozen soil engineering, including: The chemical composition of the 400MPa grade low-temperature corrosion-resistant steel for frozen soil engineering in this embodiment is the same as that in Example 2, as shown in Table 1. The final rolling temperature is 800℃, and the rest of the preparation methods are the same as those in Example 2.

[0096] The diameter of the low-temperature corrosion-resistant steel bar used in this 400MPa grade frozen soil engineering project is 16mm. The hot-rolled microstructure consists of fine-grained ferrite and pearlite, with a grain size of 10.5. The mechanical properties and low-temperature toughness of this low-temperature corrosion-resistant steel are shown in Table 2, and its corrosion resistance is shown in Table 3.

[0097] Example 4 This embodiment provides a method for preparing low-temperature corrosion-resistant steel bars for frozen soil engineering, including: The chemical composition of the 400MPa grade low-temperature corrosion-resistant steel for frozen soil engineering in this embodiment is shown in Table 1, with increased content of alloying elements to improve corrosion resistance. The preparation method is the same as in Example 1.

[0098] The diameter of the low-temperature corrosion-resistant steel bar used in this 400MPa grade frozen soil engineering project is 16mm. The microstructure in the hot-rolled state is fine-grained ferrite and pearlite, with a grain size of grade 10. The mechanical properties and low-temperature toughness of this low-temperature corrosion-resistant steel are shown in Table 2, and the corrosion resistance is shown in Table 3.

[0099] Example 5 This embodiment provides a method for preparing low-temperature corrosion-resistant steel bars for frozen soil engineering, including: The chemical composition of the 400MPa grade low-temperature corrosion-resistant steel for frozen soil engineering in this embodiment is shown in Table 1. The content of alloying elements to improve strength and corrosion resistance has been increased. The preparation method is the same as in Example 1.

[0100] The diameter of the low-temperature corrosion-resistant steel bar used in this 400MPa grade frozen soil engineering project is 16mm. The hot-rolled microstructure consists of fine-grained ferrite and pearlite, with a grain size of 10.5. The mechanical properties and low-temperature toughness of this low-temperature corrosion-resistant steel are shown in Table 2, and its corrosion resistance is shown in Table 3.

[0101] Comparative Example 1 Comparative Example 1 provides an existing 400MPa strength grade threaded steel bar, HRB400aE, with the same initial rolling temperature as Example 1 and a final rolling temperature of 1000℃. Its chemical composition is shown in Table 1, which differs from that of Example 1, but is otherwise the same as Example 1. Figure 3 This is a picture of the actual product after the salt spray test of the low-temperature corrosion-resistant steel.

[0102] Comparative Example 2 Comparative Example 1 provides an existing 400MPa strength grade threaded steel bar HRB400E, with the same initial rolling temperature as Example 1 and the final rolling temperature of 1000℃. Its chemical composition is shown in Table 1, which is different from that of Example 1, but otherwise the same as Example 1.

[0103] Comparative Example 3 Comparative Example 1 provides an existing 400MPa strength grade threaded steel bar HRB400E, with the same initial rolling temperature as Example 1 and the final rolling temperature of 840℃. Its chemical composition is the same as that of Comparative Example 2, as shown in Table 1, and the rest is the same as that of Example 1.

[0104] Table 1 Chemical composition (wt%) of the examples and comparative examples

[0105] Table 2 Mechanical properties of the examples and comparative examples

[0106] Table 3 Corrosion resistance of the examples and comparative examples

[0107] As can be seen from the performance data in Tables 2 and 3, the 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering of this invention simultaneously possess good low-temperature toughness, corrosion resistance, and mechanical properties; the yield strength is between 450MPa and 495MPa, the tensile strength is between 590MPa and 620MPa, and the yield strength ratio is between 0.763 and 0.798, showing a good balance between strength and plasticity. The average impact energy at -45℃ is between 87J and 114J, significantly higher than the comparative example, demonstrating superior low-temperature toughness; the average corrosion rate is between 1.63g / m²·h and 1.76g / m²·h, compared to the corrosion rate of ordinary Q400E steel bars which is between 39.00% and 42.08%, <50%, and far lower than the comparative example.

[0108] Comparing Example 1 and Comparative Examples 1-3, it can be seen that the elemental composition scheme that meets the requirements of this invention is far superior to the existing technical solutions in terms of corrosion resistance and low-temperature toughness.

[0109] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-temperature corrosion-resistant steel bar for frozen soil engineering with a pressure rating of 400MPa, characterized in that, Chemical composition by weight percentage: C: 0.03–0.25%, Si: 0.1–0.5%, Mn: 0.2–1.5%, P: ≤0.015%, S: ≤0.015%, Ni: 0.01–0.30%, Cr: 0.01–0.60%, Cu: 0.01–0.30%, Ce+La: 0.001–0.015%, V: 0.001–0.050%, Ti: 0.001–0.050%, with the balance being Fe and unavoidable trace inclusions.

2. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 1, characterized in that, The content of sulfur (S) is ≤0.011%.

3. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 2, characterized in that, The Ni content is controlled at 0.018–0.29%, the Cr content at 0.01–0.38%, and the Cu content at 0.01–0.30%.

4. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 3, characterized in that, The total mass fraction of Ni, Cr and Cu is ≥0.4%.

5. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 1, characterized in that, The total mass fraction of Ce and La is ≥0.01%.

6. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 1, characterized in that, The total mass fraction of V and Ti is ≥0.03%.

7. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 1, characterized in that, The grain size of the 400MPa grade low-temperature corrosion-resistant steel bars used in frozen soil engineering is grade 9 to 12.

8. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to claim 1, characterized in that, The 400MPa grade low-temperature corrosion-resistant steel bars for frozen soil engineering exhibit low-temperature impact strength at -45℃. ≥27J.

9. The 400MPa grade low-temperature corrosion-resistant steel bar for frozen soil engineering according to any one of claims 1-8, characterized in that, The average corrosion rate of the 400MPa grade low-temperature corrosion-resistant steel bars used in frozen soil engineering is between 1.63g / m²·h and 1.76g / m²·h.

10. A method for preparing low-temperature corrosion-resistant steel bars for 400MPa grade frozen soil engineering, characterized in that, The steel reinforcement for preparing the 400MPa grade low-temperature corrosion resistant steel for frozen soil engineering as described in any one of claims 1-9 comprises: S1. Smelting: Steel is obtained by smelting raw materials in a converter or electric furnace. S2, Refining: The molten steel is refined using an LF furnace, and rare earth elements are added to the molten steel after refining. S3, Continuous Casting: The refined molten steel is continuously cast to form steel billets; S4. Heating: The steel billet obtained by continuous casting is heated to 1000℃~1150℃; S5. Controlled rolling: The heated steel billet is subjected to two stages of rolling: rough rolling and finish rolling. The finish rolling is carried out at low temperature in the non-recrystallization zone. S6. Cooling Control: The rolled steel billet is subjected to controlled cooling at a rate of 0.2℃ / s to 30℃ / s.

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