Method for improving low-temperature toughness and corrosion resistance of reinforcing steel bar used in frozen soil environment

Abstract

The invention discloses a method for improving low-temperature toughness and corrosion resistance of a steel bar for a frozen soil environment. The method comprises the following steps: heating and soaking; a rough rolling stage; a finish rolling stage; a controlled cooling stage; a self-tempering stage; straightening and slowly cooling; the steel bar comprises the following chemical components in percentage by mass: 0.07 to 0.09 percent of C, 0.25 to 0.35 percent of Si, 1.35 to 1.55 percent of Mn, 0.50 to 0.70 percent of Ni, 0.30 to 0.45 percent of Cr, 0.30 to 0.45 percent of Cu, 0.10 to 0.20 percent of Mo, 0.03 to 0.05 percent of Nb, 0.03 to 0.06 percent of V, 0.015 to 0.025 percent of Ti, 0.02 to 0.04 percent of Al, less than or equal to 0.015 percent of P, less than or equal to 0.008 percent of S and the balance of Fe. The invention aims to optimize the microstructure and interface state of the steel bar and realize the balance of strength, toughness and corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of steel manufacturing technology for construction and engineering, and in particular to a method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments. Background Technology

[0002] In permafrost regions, ambient temperatures can drop to -40°C or even -60°C, with dramatic temperature differences between winter and summer, high humidity, and strong saline infiltration. Conventional HRB400 and HRB500 steel bars commonly exhibit the following problems under these conditions: high low-temperature brittleness, with a sharp decrease in impact toughness below -40°C, leading to brittle fracture; poor corrosion resistance under atmospheric and salt corrosion conditions, with rust causing a decrease in yield strength; and the traditional Cr-Cu weathering system exhibits unstable microstructure at low temperatures, with coarsening of precipitates, making it difficult to balance toughness and corrosion resistance. Therefore, there is an urgent need for a steel bar production method that can maintain high strength, high toughness, and excellent corrosion resistance under freeze-thaw cycles and strong corrosive conditions. Existing research mainly focuses on low alloying or surface coatings, but has failed to achieve simultaneous improvement in toughness and corrosion resistance. Summary of the Invention

[0003] The purpose of this invention is to provide a method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments. By combining composite microalloying design (Nb–Ti–V–Ni system) with precise controlled rolling and cooling technology, the microstructure and interface state of the steel bars are optimized to achieve a balance of strength, toughness and corrosion resistance.

[0004] Key innovations include: a Ni–Cr–Cu–Mo composite corrosion-resistant system that maintains excellent surface passivation performance at −60℃; an Nb–V–Ti multi-element microalloying strengthening mechanism that effectively refines grains and improves low-temperature toughness; a two-stage controlled rolling and three-stage controlled cooling strategy that allows for precise control of the ferrite / bainite microstructure ratio and precipitate size; and a stabilization mechanism that refines the oxide film and the Cr–Cu–Ni synergistic passivation layer, significantly improving corrosion potential and polarization resistance.

[0005] Compared with traditional processes, the present invention increases the impact energy by more than 40% and reduces the corrosion rate by more than 50% at −60℃, demonstrating significant technical advantages.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments, comprising the following steps:

[0008] (1) Heating and homogenization: The steel billet is heated to 1180-1220℃ and held for 2.0-2.5 hours. The temperature difference in the furnace is ≤10℃ to achieve carbide dissolution and composition homogenization.

[0009] (2) Rough rolling stage: Multiple rolling passes are performed in the recrystallization zone. The initial rolling temperature is 1100-1150℃, the final rolling temperature is above 950℃, the cumulative reduction rate is 70-75%, and the interval between passes is 6-8 seconds to obtain uniform austenite grains.

[0010] (3) Finishing rolling stage: Deformation control rolling is carried out in the non-recrystallization zone. The initial rolling temperature is 850-880℃, the final rolling temperature is 780-800℃, the cumulative deformation is 60-65%, and the deformation induces the precipitation of microalloyed carbides and refines the grains.

[0011] (4) Controlled cooling stage: Three-stage segmented controlled cooling is adopted. Stage I: 785℃ to 650℃, cooling rate 25~30℃ / s; Stage II: 650~400℃, cooling rate 15~18℃ / s; Stage III: slow cooling to 200℃ and then air cooling to room temperature to form a fine ferrite + bainite composite structure.

[0012] (5) Self-tempering stage: Self-tempering treatment is carried out using residual heat after rolling at a temperature of 450-500℃ and held for 30-40 minutes to promote the precipitation of (Nb,V)C and Cu2O and release residual stress.

[0013] (6) Straightening and slow cooling: After tension straightening, the steel bars are slow cooled to below 100℃ to obtain high-toughness and corrosion-resistant steel bars with uniform structure and stable performance;

[0014] The chemical composition of the steel reinforcement by weight percentage is as follows: C 0.07-0.09%, Si 0.25-0.35%, Mn 1.35-1.55%, Ni 0.50-0.70%, Cr 0.30-0.45%, Cu 0.30-0.45%, Mo 0.10-0.20%, Nb 0.03-0.05%, V 0.03-0.06%, Ti 0.015-0.025%, Al 0.02-0.04%, P≤0.015%, S≤0.008%, with the balance being Fe and unavoidable impurities.

[0015] Furthermore, the cumulative deformation in the controlled rolling zone is controlled to be above 60%, so that the austenite grain size reaches level 9-10 and the average grain size is ≤4.5μm.

[0016] Furthermore, the spray pressure of the cooling device is controlled at 0.45–0.55 MPa, and the cooling uniformity deviation is ≤ ±5℃, in order to prevent differences in layered structure and stress concentration.

[0017] Furthermore, during the tempering stage, nanoscale precipitates mainly composed of (Nb,V,Ti)C and Cu2O·Cr2O3 are formed, with particle sizes controlled at 20–30 nm.

[0018] Furthermore, the final microstructure consists of 60–70% ferrite, 25–35% lower bainite, and 3–5% retained austenite.

[0019] Furthermore, the resulting steel bars have an impact absorption energy of ≥260J at −60℃, a yield strength of ≥600MPa, a tensile strength of ≥680MPa, and an elongation of ≥20%.

[0020] Furthermore, the corrosion rate of the reinforcing steel in a salt spray environment at −60℃ is ≤0.025g / m²·h, and the polarization resistance is ≥25kΩ·cm².

[0021] Furthermore, a two-zone walking beam furnace is used for heating, with the front zone preheating to 1100℃ and the rear zone heating to 1220℃ to suppress the central segregation zone.

[0022] Furthermore, the controlled rolling mill stand adopts a real-time temperature-load feedback system to automatically adjust the reduction rate and speed, thereby achieving precise temperature control.

[0023] Furthermore, the cooling system employs a combination of multi-stage spray and air cooling, with a cooling water temperature difference of ≤10℃, ensuring organizational continuity.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0025] This process enables the steel reinforcement to ultimately form a refined quasi-polygonal ferrite + bainite + dispersed carbide composite structure, with an average grain size ≤4.5μm and a carbide grain size ≤30nm.

[0026] The resulting steel bars have an impact absorption energy of ≥260J at −60℃, a yield strength of ≥600MPa, a tensile strength of ≥680MPa, and an elongation of ≥20%.

[0027] The corrosion rate of the reinforcing steel in a salt spray environment at −60℃ is ≤0.025 g / m²·h¹, and the polarization resistance is ≥25kΩ·cm². Detailed Implementation

[0028] A composite process for improving the low-temperature toughness and corrosion resistance of high-strength steel bars suitable for permafrost environments. This method achieves dual optimization of high toughness and corrosion resistance of steel bars below −60℃ without significantly increasing production costs through compositional microalloying design, controlled rolling and cooling technology, and a self-tempering strengthening mechanism.

[0029] The overall process flow is as follows:

[0030] Material composition design:

[0031] The chemical composition of the steel by mass fraction is as follows: C: 0.07-0.09%, Si: 0.25-0.35%, Mn: 1.35-1.55%, Ni: 0.50-0.70%, Cr: 0.30-0.45%, Cu: 0.30-0.45%, Mo: 0.10-0.20%, Nb: 0.03-0.05%, V: 0.03-0.06%, Ti: 0.015-0.025%, Al: 0.02-0.04%, with the balance being Fe and impurities (P≤0.015%, S≤0.008%).

[0032] The design balances the purification of low carbon, low phosphorus, and low sulfur with the stability of the Cu-Ni-Cr corrosion-resistant system, and achieves fine grain strengthening through Nb-V-Ti microalloying.

[0033] Heating and heat equalization:

[0034] The steel billet is heated in a walking beam furnace in two stages: a preheating zone at 1100℃ and a heating zone at 1220℃, with a heating rate of 8℃ / min and a holding time of 2.5h to ensure complete dissolution of carbides. The furnace exit temperature is controlled at 1210±10℃, and the oxide scale thickness is ≤0.2mm.

[0035] Rough rolling stage:

[0036] The initial rolling temperature is 1100–1150℃, the final rolling temperature is 950–970℃, the cumulative reduction rate is ≥70%, and the reduction rate per pass is about 20%. The rolling rhythm is controlled at 5–8 seconds per pass to ensure austenite grain refinement.

[0037] Finishing rolling stage:

[0038] The first three stands of the finishing mill underwent a total deformation of 60%, with the temperature decreasing from 880℃ to 790℃. The last two stands completed the final deformation in the non-recrystallization zone. The final rolling temperature was controlled at 780±10℃, and the cumulative strain was ensured to be ≥2.5 to achieve dynamic recrystallization and deformation-induced precipitation.

[0039] Segmented cooling:

[0040] A three-stage water-cooling + air-cooling composite cooling system is adopted:

[0041] Zone 1: Final rolling to 650℃, cooling rate 30℃ / s;

[0042] Second zone: Cooling rate of 15℃ / s in the 650~400℃ range;

[0043] Zone 3: Slow cooling from 400 to 200℃, cooling rate 3℃ / s;

[0044] This controlled cooling system suppresses pearlite formation, resulting in a fine bainite + ferrite composite structure.

[0045] Self-tempering enhancement:

[0046] The residual heat is used to self-temper at 450-500℃ for 35 minutes to relax the stress, disperse the carbide precipitation, and form a uniform nanoscale precipitated strengthening phase (NbC, VC, Cu2O).

[0047] Cooling and straightening

[0048] After tempering, slowly cool to below 100℃ and perform tension straightening, controlling the strain to ≤1.5% to prevent microcracks from forming.

[0049] Inspection and performance control:

[0050] The final product's microstructure consists of refined ferrite (approximately 4 μm in diameter) + lower bainite + dispersed carbides. Impact energy at -60℃ ≥ 260 J, tensile strength ≥ 680 MPa, yield strength ≥ 600 MPa, elongation ≥ 20%, and salt spray corrosion rate ≤ 0.025 g·m⁻²·h⁻¹.

[0051] Example 1:

[0052] The selected chemical composition by mass percentage is as follows: C 0.08%, Si 0.30%, Mn 1.45%, Ni 0.60%, Cr 0.35%, Cu 0.40%, Mo 0.15%, Nb 0.04%, V 0.05%, Ti 0.020%, Al 0.03%, P 0.010%, S 0.006%, with the balance being Fe and impurities.

[0053] Process:

[0054] Heating and heat preservation:

[0055] The steel billet is heated in a walking beam furnace in sections. The front section is preheated to 1100℃, and the rear section is heated to 1210℃ at a heating rate of 8℃ / min, and held for 2.5 hours. A weak oxidizing atmosphere is used to control the furnace gas ratio in order to reduce the thickness of the surface oxide scale and prevent SiO2 enrichment from causing a decarburized layer.

[0056] Rough rolling stage:

[0057] A six-stand continuous rolling mill was used, with an initial rolling temperature of 1140℃, a final rolling temperature of 960℃, a pass interval of 6 seconds, and a total reduction rate of 72%, with uniform reduction in each pass. Deformation-induced recrystallization was controlled in this stage to refine the austenite grains to below 10μm.

[0058] Finishing rolling stage:

[0059] The initial rolling temperature is 870℃, the final rolling temperature is 785℃, the cumulative deformation reaches 65%, and the deformation per stand is controlled at 10% to 12%. During finishing rolling, a medium tension is applied to improve the uniformity of deformation and prevent grain elongation.

[0060] Cooling control phase:

[0061] A three-stage spray cooling system is adopted:

[0062] The first zone has a cooling rate of 30℃ / s, reducing the temperature from 785℃ to 650℃.

[0063] The second zone has a cooling rate of 15℃ / s, reducing the temperature to 400℃;

[0064] The third zone will cool slowly to 200°C.

[0065] Cooling water pressure 0.5 MPa, uniformity deviation ≤ ±5℃. After cooling, the microstructure transforms into fine ferrite + lower bainite, with an internal retained austenite content of approximately 3%.

[0066] Self-tempering enhancement:

[0067] The residual heat after rolling is utilized and held at 470℃ for 40 min to promote the precipitation of dispersed (Nb,V)C and Cu2O, with a phase particle size of about 20 nm, increasing the number density by 40% and reducing internal stress.

[0068] Performance results:

[0069] Grain size reaches grade 9, impact energy at −60℃ is 265J, yield strength is 605MPa, tensile strength is 685MPa, elongation is 21%, and salt spray corrosion rate is 0.023 g·m. -2 ·h -1 The microstructure consists of approximately 65% ​​equiaxed ferrite, 30% lower bainite, and 5% retained austenite. The grains are fine with clear boundaries, and the precipitates are evenly distributed.

[0070] Example 2:

[0071] The selected chemical composition by mass percentage is as follows: C 0.09%, Mn 1.50%, Ni 0.65%, Cu 0.42%, Cr 0.38%, Mo 0.15%, Nb 0.05%, V 0.05%, Ti 0.022%, Al 0.035%, with the remainder being Fe and impurities.

[0072] Process steps:

[0073] Heat to 1220℃ and hold for 2.0 hours; oven temperature is 1215℃.

[0074] The roughing rolling temperature is 1150℃, the final rolling temperature is 970℃, the pass interval is 5s, and the total reduction rate is 75%.

[0075] The initial rolling temperature in the finishing rolling zone is 880℃, and the final rolling temperature is 790℃, with a cumulative deformation of 65%, which improves deformation-induced precipitation.

[0076] The controlled cooling process is divided into three stages: cooling rate of 35℃ / s (final rolling to 650℃), cooling rate of 18℃ / s (650 to 400℃), and slow cooling to 180℃.

[0077] Self-tempering temperature 480℃, hold for 35 minutes.

[0078] The microstructure is a refined ferrite + bainite composite structure with a grain size of approximately 4 μm.

[0079] Performance results: Impact energy at −60℃: 275 J; Yield strength: 610 MPa; Tensile strength: 690 MPa; Elongation: 21%; Corrosion rate: 0.022 g·m⁻² -2 ·h -1 The polarization resistance has been increased to 27 kΩ·cm 2 .

[0080] Example 3:

[0081] Chemical composition by mass percentage: C 0.08%, Si 0.28%, Mn 1.40%, Ni 0.70%, Cu 0.45%, Cr 0.40%, Mo 0.18%, Nb 0.04%, V 0.04%, Ti 0.018%, Al 0.025%, with the remainder being Fe and impurities.

[0082] Process parameters:

[0083] Heating temperature 1205℃, hold for 2.5 hours;

[0084] The roughing rolling temperature is 1120℃, and the final rolling temperature is 955℃.

[0085] Finishing rolling temperature: 800℃;

[0086] The cooling rate is divided into zones of 30℃ / s, 17℃ / s, and 4℃ / s;

[0087] Self-tempering temperature 490℃, hold for 30 minutes.

[0088] Performance results: Impact energy 285 J (−60℃), yield strength 620 MPa, elongation 22%, corrosion rate reduced to 0.020 g·m -2 ·h -1 .

[0089] Comparative Example 1:

[0090] The chemical composition is the same as in Example 1.

[0091] Process: Final rolling temperature 900℃, natural air cooling to room temperature, no self-tempering.

[0092] Results: Grain size grade 6, impact energy at −60℃ 180 J, yield strength 580 MPa, corrosion rate 0.045 g·m -2 ·h -1 The microstructure contains large bainite and pearlite aggregates, resulting in low toughness.

[0093] Comparative Example 2:

[0094] The chemical composition is C 0.08%, Mn 1.45%, Cr 0.35%, Mo 0.15%, Nb 0.04%, V 0.05%, with no Ni or Cu added.

[0095] The controlled rolling and controlled cooling parameters are the same as in Example 1.

[0096] Results: Impact energy at −60℃: 195 J; Corrosion rate: 0.040 g·m⁻¹ -2 ·h -1 This indicates that Ni and Cu significantly improve low-temperature toughness and corrosion resistance.

[0097] Comparative Example 3:

[0098] The chemical composition is the same as in Example 2, and the controlled rolling and cooling are completed directly by air cooling.

[0099] Results: High residual stress in the microstructure, microcracks appearing along the rolling direction, impact energy at −60℃ decreasing to 170J, and corrosion rate increasing to 0.050 g·m-2·h-1.

[0100] Table 1. Comparison of chemical composition between the examples and comparative examples.

[0101] serial number C Si Mn Ni Cr Cu Mo Nb V Ti Al P S Example 1 0.08 0.30 1.45 0.60 0.35 0.40 0.15 0.04 0.05 0.020 0.03 0.010 0.006 Example 2 0.09 0.28 1.50 0.65 0.38 0.42 0.15 0.05 0.05 0.022 0.035 0.010 0.007 Example 3 0.08 0.28 1.40 0.70 0.40 0.45 0.18 0.04 0.04 0.018 0.025 0.011 0.006 Comparative Example 1 Same as Example 1 — — — — — — — — — — — — Comparative Example 2 0.08 0.30 1.45 — 0.35 — 0.15 0.04 0.05 — 0.03 0.010 0.007 Comparative Example 3 Same as Example 2 — — — — — — — — — — — —

[0102] Table 2 Comparison of process parameters between the examples and comparative examples

[0103] serial number Experiment type Heating regime Final rolling temperature (°C) Cooling rate (°C / s) zone Self-tempering temperature / time Microstructure characteristics Example 1 Controlled rolling + segmented controlled cooling + self-tempering Insulate at 1210℃ for 2.5 hours 785 30 / 15 / 3 470℃×40min Ferrite + Lower Bainite + Dispersed Carbides Example 2 Strong deformation + rapid cooling Insulate at 1220℃ for 2.0 hours 790 35 / 18 / 3 480℃×35min Refined ferrite + bainite, grain size approximately 4μm Example 3 High Ni-Cu corrosion resistant system Insulate at 1205℃ for 2.5 hours 800 30 / 17 / 4 490℃×30min Ferrite + bainite + retained austenite (approximately 5%) Comparative Example 1 Conventional rolling + air cooling Insulate at 1210℃ for 2.5 hours 900 air cooling none Coarse bainite + pearlite Comparative Example 2 Ni- and Cu-free systems Insulate at 1210℃ for 2.5 hours 785 30 / 15 / 3 470℃×40min Coarse ferrite and bainite aggregate, resulting in uneven precipitates. Comparative Example 3 Omitted self-tempering Insulate at 1220℃ for 2.0 hours 790 35 / 18 / 3 none High residual stress, microcracks along the rolling direction

[0104] Table 3 Performance Comparison of Examples and Comparative Examples

[0105] serial number Impact energy at -60℃ / J Yield strength / MPa Tensile strength / MPa Elongation / % Grain size level <![CDATA[Salt spray corrosion rate (g·m -2 ·h -1 )]]> Polarization resistance (kΩ·cm²) Main performance characteristics Example 1 265 605 685 21 9 0.023 25 The microstructure is uniform and fine, and the low-temperature impact performance is excellent. Example 2 275 610 690 21 10 0.022 27 High dislocation density, significant strengthening, and improved corrosion resistance. Example 3 285 620 700 22 10 0.020 29 Abundant composite precipitates, exhibiting optimal corrosion resistance Comparative Example 1 180 580 660 18 6 0.045 15 Large grains and low toughness Comparative Example 2 195 585 665 19 7 0.040 17 The lack of Ni and Cu results in a non-dense corrosion film. Comparative Example 3 170 575 655 17 6 0.050 14 High internal stress, significant microcracks, and poor corrosion resistance

[0106] As can be seen from the examples and comparative examples, the present invention has the following significant advantages: (1) Significant grain refinement effect: Through the combination of segmented controlled rolling and zoned controlled cooling, the dynamic recrystallization of austenite is fully carried out, and the ferrite grain size is refined from 10-12 μm in the comparative example to 4-5 μm, and the grain size is improved to grade 9-10, which significantly improves the low-temperature impact toughness and crack propagation resistance of the material. (2) Significantly improved low-temperature toughness: The impact absorption energy of the steel bars in the examples at −60℃ reaches 265-285J, which is 40-60% higher than that of the comparative example. It still maintains excellent plasticity, toughness and fracture stability in frozen soil and extremely cold environments, which fully meets the requirements of bridge, subway and port engineering in cold regions. (3) Excellent corrosion resistance: The Cu and Ni composite corrosion resistant system and Cr and Mo elements work together to form a stable Cu2O·Cr2O3 passivation film, and the corrosion rate is reduced from 0.045 g·m -2 ·h -1 Reduced to 0.020 g·m -2 ·h -1 The polarization resistance is nearly doubled, significantly extending the service life. (4) Precipitation strengthening and residual stress coordination. The precipitation of dispersed (Nb,V)C and Cu2O is promoted during the tempering stage. The phase particle size is 20-30nm, and the distribution is uniform. It can significantly eliminate rolling residual stress and avoid the generation of microcracks.

[0107] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments, characterized in that: Includes the following steps: (1) Heating and homogenization: The steel billet is heated to 1180-1220℃ and held for 2.0-2.5 hours. The temperature difference in the furnace is ≤10℃ to achieve carbide dissolution and composition homogenization. (2) Rough rolling stage: Multiple rolling passes are performed in the recrystallization zone. The initial rolling temperature is 1100-1150℃, the final rolling temperature is above 950℃, the cumulative reduction rate is 70-75%, and the interval between passes is 6-8 seconds to obtain uniform austenite grains. (3) Finishing rolling stage: Deformation control rolling is carried out in the non-recrystallization zone. The initial rolling temperature is 850-880℃, the final rolling temperature is 780-800℃, the cumulative deformation is 60-65%, and the deformation induces the precipitation of microalloyed carbides and refines the grains. (4) Cooling control stage: Three-stage segmented cooling is adopted. Stage I: 785℃ to 650℃, cooling rate 25~30℃ / s; Stage II: 650~400℃, cooling rate 15~18℃ / s; Stage III: Slowly cool to 200℃ and then air cool to room temperature to form a fine ferrite + bainite composite structure; (5) Self-tempering stage: Self-tempering treatment is carried out using residual heat after rolling at a temperature of 450-500℃ and held for 30-40 minutes to promote the precipitation of (Nb,V)C and Cu2O and release residual stress. (6) Straightening and slow cooling: After tension straightening, the steel bars are slow cooled to below 100℃ to obtain high-toughness and corrosion-resistant steel bars with uniform structure and stable performance; The chemical composition of the steel reinforcement by weight percentage is as follows: C 0.07-0.09%, Si 0.25-0.35%, Mn 1.35-1.55%, Ni 0.50-0.70%, Cr 0.30-0.45%, Cu 0.30-0.45%, Mo 0.10-0.20%, Nb 0.03-0.05%, V 0.03-0.06%, Ti 0.015-0.025%, Al 0.02-0.04%, P≤0.015%, S≤0.008%, with the balance being Fe and unavoidable impurities.

2. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The cumulative deformation in the controlled rolling zone is controlled to be above 60%, so that the austenite grain size reaches level 9-10 and the average grain size is ≤4.5μm.

3. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The spray pressure of the cooling device is controlled at 0.45 to 0.55 MPa, and the cooling uniformity deviation is ≤ ±5℃, in order to prevent differences in layered structure and stress concentration.

4. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The self-tempering stage forms nanoscale precipitates mainly composed of (Nb,V,Ti)C and Cu2O·Cr2O3, with particle sizes controlled at 20–30 nm.

5. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The final microstructure consists of 60–70% ferrite, 25–35% lower bainite, and 3–5% retained austenite.

6. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The resulting steel bars have an impact absorption energy of ≥260J at −60℃, a yield strength of ≥600MPa, a tensile strength of ≥680MPa, and an elongation of ≥20%.

7. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The corrosion rate of steel bars in a salt spray environment at −60℃ is ≤0.025g / m²·h, and the polarization resistance is ≥25kΩ·cm².

8. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: Heating is achieved using a two-zone walking beam furnace, with the front zone preheating to 1100℃ and the rear zone heating to 1220℃ to suppress the central segregation zone.

9. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The controlled rolling mill stand uses a real-time temperature-load feedback system to automatically adjust the reduction rate and speed, achieving precise temperature control.

10. The method for improving the low-temperature toughness and corrosion resistance of steel bars used in frozen soil environments according to claim 1, characterized in that: The cooling system uses a combination of multi-stage spray and air cooling, with a cooling water temperature difference of ≤10℃, ensuring organizational continuity.