Production method of low-temperature-resistant reinforcing steel bar for frozen soil environment

Abstract

The invention discloses a production method of a low-temperature-resistant reinforcing steel bar for a frozen soil environment, and belongs to the technical field of low-temperature high-strength reinforcing steel bar manufacturing. According to the method, deep purification and grain refinement of molten steel are achieved by optimizing the design of a low-carbon Cu-Ni-Cr-Sb-rare earth composite alloy system and combining a converter-LF-VD smelting purification process, a full-protection continuous casting process and a precise controlled rolling and controlled cooling process; a three-section accelerated cooling process and surface diffusion tempering treatment are adopted, so that Cu, Ni and Sb elements form an enrichment layer on the surface layer of the steel bar; and then a Cu2O-Cr2O3-Sb2O5-rare earth oxide composite film layer is formed through rare earth composite passivation treatment, so that the corrosion resistance and low-temperature toughness of the reinforcing steel bar are remarkably improved. The method is stable in process, low in cost and high in adaptability, the obtained reinforcing steel bar still keeps excellent strength and impact toughness in the environment of 60 DEG C, and the method is suitable for production of reinforcing steel bars for bridges, railways, tunnels and ocean structures in frozen soil regions.

Description

Technical Field

[0001] This invention relates to the field of steel metallurgy and steel materials for construction, and in particular to a method for producing low-temperature resistant steel bars for use in frozen soil environments. Background Technology

[0002] Existing steel reinforcement used in cold-region components (such as HRB and Q-series steel) often faces problems such as low-temperature embrittlement, chloride corrosion, and early failure caused by freeze-thaw cycles. Traditional methods improve corrosion resistance through surface plating or coating, but these methods suffer from drawbacks such as peeling, difficulty in repair, and limited lifespan. Simply alloying to improve corrosion resistance often sacrifices low-temperature toughness or significantly increases costs. This invention proposes an industrialized and scalable overall solution by rationally designing the matrix composition (low carbon + appropriate micro-alloying), implementing steel purification during the refining stage (rare earth micro-addition), employing controlled rolling and cooling with online heat treatment, and forming a dense composite passivation film on the finished product. Summary of the Invention

[0003] The purpose of this invention is to provide a method for producing low-temperature resistant steel bars for frozen soil environments. By coordinating the design of alloy composition, optimizing smelting and refining, strictly controlling continuous casting and controlled rolling parameters, adopting segmented accelerated cooling and online tempering, and combining surface diffusion enrichment and rare earth composite passivation treatment, a steel bar product with high strength, high and low temperature toughness and excellent corrosion resistance in low-temperature environments can be obtained.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] This invention discloses a method for producing low-temperature resistant steel bars for use in frozen soil environments, comprising:

[0006] Alloy system: Designed using a low-carbon Cu-Ni-Cr-Sb-rare earth composite alloy system; the chemical composition by mass percentage is: C 0.07~0.15%, Si 0.20~0.35%, Mn 1.15~1.60%, Cr 0.25~0.60%, Ni 0.30~0.70%, Cu 0.20~0.50%, Nb 0.02~0.05%, V 0.02~0.07%, Ti 0.010~0.025%, Ce 0.002~0.015%, Sb 0.05~0.10%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities;

[0007] Smelting and refining: The process involves converter → LF refining → VD vacuum degassing. Ce-based rare earth alloying agent is added at the end of the LF process. The basicity of the LF slag is 3.0 to 3.5, the VD vacuum degree is ≤70Pa, and the vacuum time is 15 to 20 minutes. This ensures that the oxygen content of the molten steel is ≤20ppm and the hydrogen content is ≤2.5ppm, which significantly purifies and refines inclusions.

[0008] Continuous casting and heating: full protective casting, superheat 25-35℃, casting speed 0.75-1.05m / min; heating soaking temperature 1180-1220℃, holding temperature 90-150min, and tapping temperature controlled at 1170-1200℃;

[0009] Controlled rolling and deformation: Two-stage or three-stage controlled rolling is adopted: roughing rolling final temperature 960~980℃, finishing rolling final temperature 840~860℃; the cumulative reduction rate in roughing rolling is ≥60%, and the cumulative deformation in finishing rolling stage is ≥60% to ensure rolling of non-recrystallized or partially non-recrystallized areas to refine the matrix structure and promote precipitation strengthening;

[0010] Segmented accelerated cooling and online tempering: After rolling, the product enters the ACC segmented cooling system: First segment: final rolling → 650℃, cooling rate 30~35℃ / s; Second segment: 650→400℃, cooling rate 12~18℃ / s; Third segment: 400→200℃, cooling rate 5~8℃ / s; Subsequently, online tempering is carried out using residual heat or a short furnace at 470~490℃ for 20~40min to promote the stabilization of precipitated phases and release residual stress.

[0011] Furthermore, it also includes: surface diffusion and composite passivation: surface diffusion / enrichment treatment is performed after rolling or on the finished product line at 660-690℃ for 30-45 min to enrich Cu, Ni, Cr and Sb on the surface; acid washing, followed by treatment in a neutral phosphate-silicate passivation solution for 15-25 min, and then rare earth activation is performed using a CeO2 salt solution for 8-15 min. After drying, a Cu2O·Cr2O3·rare earth oxide composite film with a thickness of 0.3-1.0 μm is formed. The film is dense and has a self-healing tendency.

[0012] Furthermore, the passivation solution of the neutral phosphate-silicate system has a pH of 6.5–7.5.

[0013] Furthermore, the performance meets the following requirements: impact absorption energy ≥90J at −40℃, and still ≥80J at −60℃; yield strength 640~680MPa, tensile strength 780~820MPa; mass loss from salt spray corrosion ≤0.045 g / m²·h after 1000h, with stable performance and good repeatability.

[0014] Furthermore, the chemical composition by weight percentage is as follows: C 0.09%; Si 0.28%; Mn 1.30%; Cr 0.30%; Ni 0.40%; Cu 0.30%; Nb 0.03%; V 0.04%; Ti 0.015%; Ce 0.006%; P 0.010%; S 0.006%; Sb 0.06%; balance Fe and unavoidable impurities.

[0015] Furthermore, the chemical composition by mass percentage is as follows: C 0.10%; Si 0.27%; Mn 1.35%; Cr 0.40%; Ni 0.55%; Cu 0.45%; Nb 0.035%; V 0.05%; Ti 0.018%; Ce 0.008%; P 0.010%; S 0.005%; Sb 0.07%; balance Fe and unavoidable impurities.

[0016] Furthermore, the chemical composition by mass percentage is as follows: C 0.08%; Si 0.26%; Mn 1.28%; Cr 0.45%; Ni 0.60%; Cu 0.35%; Nb 0.04%; V 0.06%; Ti 0.020%; Ce 0.010%; La 0.002%; P 0.009%; S 0.004%; Sb 0.08%; balance Fe and unavoidable impurities.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0018] Product performance indicators: Yield strength 640~680 MPa, tensile strength 780~820 MPa, elongation ≥18%, impact absorption energy ≥90 J at −40℃ (actual impact energy ≥125 J at −60℃); mass loss due to salt spray corrosion ≤0.045 g·m⁻¹ after 1000 h. -2 Grain size ≥ 10.

[0019] (1) It takes into account both high strength and excellent low temperature toughness, significantly reducing the risk of brittle fracture under frozen soil service; (2) A dense composite passivation film is formed on the surface, which significantly reduces the chloride ion erosion rate and has self-repair capability; (3) Steel liquid purification and micro-alloying work together to significantly refine the microstructure and improve the inclusion morphology, thereby enhancing the overall durability. Detailed Implementation

[0020] The following is a detailed description of a method for producing low-temperature resistant steel bars for frozen soil environments according to the present invention.

[0021] 1. Composition Design and Raw Material Preparation: Based on the requirements of frozen soil environment for the low-temperature toughness and corrosion resistance of reinforcing steel, a low-carbon Cu-Ni-Cr-Sb-rare earth composite alloy system is adopted. The carbon content is strictly controlled at 0.10-0.16% to ensure good weldability and anti-embrittlement ability. By adding 0.4-0.6% Cr, 0.3-0.5% Ni, and 0.05-0.10% Sb, a stable corrosion-resistant element system is formed. At the same time, 0.02-0.05% rare earth element Ce is added to purify the molten steel, refine the grains, and improve the morphology of inclusions. The raw materials are low-phosphorus, low-sulfur high-quality scrap steel and molten iron, with a carbon deviation of ≤0.02% entering the furnace.

[0022] 2. Smelting and Refining Purification: A 120t converter is used for smelting, controlling the final carbon content to 0.08-0.10% and the temperature to 1650-1670℃. After deep deoxidation with aluminum wire, the slag is transferred to LF refining. The refining slag adopts a CaO-Al2O3-CaF2 system, with the basicity controlled at 3.0-3.5 to promote the adsorption of oxide inclusions. At the end of LF, 3-5 kg / t of Ce-Si-Fe composite rare earth alloy is added to ensure that rare earth elements are evenly distributed in the molten steel, forming dispersed oxides. After VD vacuum treatment for 15-20 min, with a vacuum degree ≤70Pa, the molten steel is guaranteed to have [H] ≤2ppm, [O] ≤0.002%, and [S] ≤0.005%, resulting in ultra-pure molten steel.

[0023] 3. Continuous Casting Process Control: A fully protected continuous casting process is adopted, with superheat controlled at 25–35℃, cooling water flow in the secondary cooling zone at 0.35–0.45 L / kg, and casting speed controlled at 0.8–0.9 m / min. Electromagnetic stirring and soft reduction technology are used to prevent center segregation and shrinkage cavities, and to improve the density of the cast billet. After the billet is cut, shot peening is performed to ensure that the surface is free of cracks and slag inclusions.

[0024] 4. Heating and Homogenization: The billet is heated to 1180–1220℃ in a homogenizing furnace and held for 90–120 minutes to ensure uniform internal temperature. Before exiting the furnace, the surface temperature difference is checked and found to be ≤20℃ to prevent overheating or underheating. This process helps the carbides to dissolve fully, providing a basis for uniform microstructure for subsequent controlled rolling.

[0025] 5. Controlled rolling process: During the rough rolling stage, the temperature is controlled at 950–980℃, and the reduction rate is ≥60% to fully break down the as-cast microstructure; during the finish rolling stage, the final rolling temperature is controlled at 830–860℃, and the cumulative deformation is ≥70% to prevent grain growth. Throughout the rolling process, a zoned temperature control system ensures that the temperature difference in the thickness direction is ≤25℃, significantly improving the uniformity of the microstructure and low-temperature toughness.

[0026] 6. Accelerated Cooling in Three Stages (ACC): A three-stage accelerated cooling system is employed. The first stage (860–650℃) has a cooling rate of 30–35℃ / s to control austenite grains and pre-formed bainite nucleation. The second stage (650–400℃) has a cooling rate of 12–18℃ / s to form fine ferrite and dispersed carbides. The third stage (400–200℃) has a cooling rate of 5–8℃ / s to further refine the microstructure and stabilize phase distribution. The final cooling temperature is controlled at around 200℃ to prevent deformation caused by internal and external temperature differences. Subsequently, residual heat or online tempering in a short furnace at 470–490℃ for 20–40 minutes is used to promote the stabilization of precipitated phases and release residual stress.

[0027] 7. Surface Diffusion Enrichment Treatment: After rolling, the steel bars undergo surface diffusion heat treatment in a continuous furnace at a temperature of 660–690℃ for 30–45 minutes. This allows corrosion-resistant elements such as Cu, Ni, and Sb to migrate along the grain boundaries to the surface, forming an enrichment layer with a thickness of 0.3–0.8 μm. This layer can rapidly form a stable protective film in corrosive media, significantly improving resistance to chloride ion penetration.

[0028] Tempering stabilizes the microstructure: After surface diffusion, a low-temperature tempering treatment is performed at 450–480℃ for 2 hours to eliminate residual rolling stress, stabilize the bainite-ferrite composite microstructure, and prevent stress corrosion cracking. This process also promotes the formation of stable oxide film nuclei by Cr, Ni, and Cu, which is beneficial for the subsequent passivation layer bonding.

[0029] Surface passivation and densification: After pickling with 10% HNO3 + 0.5% HF solution for 15 min to remove oxide scale, the surface of the reinforcing steel is treated in a neutral phosphate-silicate passivation solution for 15–25 min, followed by a secondary passivation in a rare earth activation solution containing 0.1% CeO2 for 10 min, forming a Cu2O-Cr2O3-Sb2O5-rare earth oxide composite film. This film is dense and has strong adhesion, increasing electrochemical impedance by more than 1.5 times.

[0030] Inspection and Performance Evaluation: The finished steel bars were verified by chemical composition analysis, metallographic structure testing, low-temperature impact test, and salt spray corrosion test. The impact absorption energy at -40℃ is ≥90J, and it still maintains ≥80J at -60℃; the yield strength is 640~680MPa, and the tensile strength is 780~820MPa; the mass loss from 1000h salt spray corrosion is ≤0.045 g / m², and the performance is stable and has good repeatability.

[0031] Example 1:

[0032] Chemical composition (mass fraction wt%): C 0.09; Si 0.28; Mn 1.30; Cr 0.30; Ni 0.40; Cu 0.30; Nb 0.03; V 0.04; Ti 0.015; Ce 0.006; P 0.010; S 0.006; Sb 0.06; balance Fe.

[0033] Smelting and refining: Converter tapping temperature 1650℃ → LF refining (slag basicity 3.2), add Ce-Si-Fe alloy 0.8 kg / t at the end of LF (target total Ce content 0.006%); VD vacuum 18min (vacuum degree 60Pa). Maintain O ≤20ppm, H ≤2.5ppm.

[0034] Continuous casting and heating: Superheating at 30℃ and casting speed at 0.9 m / min; homogenization at 1190℃ and holding at 110 min.

[0035] Rolling: Roughing final temperature 970℃; Finishing final temperature 840℃; Total cumulative reduction (roughing + finishing) ≥ 65%.

[0036] Cooling and tempering: ACC segmented cooling: 840→650℃, 32℃ / s; 650→400℃, 15℃ / s; 400→200℃, 6℃ / s. Online tempering: 470℃ × 30 min.

[0037] Surface diffusion and passivation: Surface diffusion at 670℃ for 35 min (to promote the enrichment of Cu and Ni on the surface); pickling (10% HNO3, room temperature for 5 min); neutral passivation for 20 min; rare earth activation for 10 min.

[0038] Performance tests: Yield strength 642 MPa; Tensile strength 785 MPa; Elongation 19.5%; Impact energy at −40℃ 125 J (preferably up to 115 J for −60℃ samples); Mass loss after 1000h salt spray corrosion 0.038 g·m -2 Grain size grade 10.

[0039] Example 2:

[0040] Chemical composition (mass fraction wt%): C 0.10; Si 0.27; Mn 1.35; Cr 0.40; Ni 0.55; Cu 0.45; Nb 0.035; V 0.05; Ti 0.018; Ce 0.008; P 0.010; S 0.005; Sb

[0041] 0.07; balance Fe.

[0042] Smelting: Ce rare earth alloy was added at the end of LF to make Ce ≈0.008%, and VD was performed under vacuum for 16 min (vacuum degree ≤60 Pa).

[0043] Continuous casting and heating: superheat 28℃, casting speed 0.85 m / min; heating homogenization 1200℃, holding for 120 min.

[0044] Rolling: Roughing final temperature 970℃; Finishing final temperature 850℃; Cumulative reduction rate ≥70%.

[0045] Cooling and tempering: ACC: 850→650℃ 34℃ / s; 650→400℃ 17℃ / s; 400→200℃ 6℃ / s. Online tempering 480℃ × 35 min.

[0046] Surface diffusion and passivation: Surface diffusion at 680℃ for 35 min; pickling → neutral passivation → rare earth secondary activation (containing CeO2) → drying.

[0047] Test results: Yield strength 655 MPa; Tensile strength 800 MPa; Elongation 20%; Impact energy at −40℃ 140 J (−60℃ 130 J); Mass loss due to salt spray corrosion after 1000 h 0.025 g·m -2 Grain size grade 11.

[0048] Example 3:

[0049] Chemical composition (mass fraction wt%): C 0.08; Si 0.26; Mn 1.28; Cr 0.45; Ni 0.60; Cu 0.35; Nb 0.04; V 0.06; Ti 0.020; Ce 0.010; La 0.002; P 0.009; S 0.004; Sb 0.08; balance Fe.

[0050] Smelting: Rare earth (Ce 0.010%) is added at the end of LF, and VD vacuum is applied for 15 min (vacuum degree ≤55 Pa) to ensure ultra-clean molten steel.

[0051] Continuous casting and heating: superheat 26℃, casting speed 0.8 m / min; heating 1210℃, holding for 130 min.

[0052] Rolling: Roughing final temperature 960℃; Finishing final temperature 855℃; Cumulative reduction rate ≥75%, finishing rolling adopts strong deformation in the non-recrystallization zone to promote refinement.

[0053] Cooling and tempering: ACC: 855→650℃ 35℃ / s; 650→400℃ 18℃ / s; 400→200℃ 7℃ / s; online tempering 490℃×30 min.

[0054] Surface diffusion and passivation: diffusion at 685℃ for 40-45 min, followed by acid washing → neutral passivation → rare earth activation (containing CeO2) → thermal drying.

[0055] Test results: Yield strength 668 MPa; Tensile strength 815 MPa; Elongation 21%; Impact energy at −40℃ 160 J (−60℃ 150 J); Mass loss due to salt spray corrosion after 1000 h 0.018 g·m -2 Grain size grade 12.

[0056] Comparative Example 1:

[0057] Chemical composition (mass fraction %): C 0.12, Si 0.35, Mn 1.45, Cr 0.15, Ni 0.10, Cu 0.10, Nb 0.02, V 0.03, Ti 0.010, P 0.018, S 0.012, balance Fe.

[0058] Process: The traditional converter → LF refining → continuous casting production flow is adopted. No rare earth elements are added during the LF refining stage, and the oxygen content of the molten steel is controlled at a relatively high level (O≈40 ppm). The superheat during continuous casting is 40℃, and the casting speed is 1.2 m / min. The heating temperature is 1180℃, and the holding time is 80 min. During rolling, the roughing rolling final temperature is approximately 980℃, and the finishing rolling final temperature is 880℃; no refined temperature control of the deformation zone is performed. After rolling, natural air cooling is used, without segmented ACC or online tempering processes. No diffusion enrichment or passivation treatment is performed on the surface.

[0059] Microstructure characteristics: The microstructure consists of ferrite and pearlite, with a grain size of only grade 8, and contains numerous oxide inclusions and coarse MnS bands. Due to the lack of controlled cooling, the microstructure is uneven, with thick and coarse pearlite lamellae on the surface.

[0060] Performance results: Yield strength 620 MPa, tensile strength 760 MPa, elongation 16%, impact energy at -40℃ only 70 J, impact energy at -60℃ 65 J. Mass loss after 1000 hours of salt spray corrosion is 0.075 g·m. -2 After 72 hours, obvious rust spots and peeling appeared on the surface.

[0061] Comparative Example 2:

[0062] Chemical composition (mass fraction %): C 0.11, Si 0.25, Mn 1.35, Cr 0.70, Ni 0.90, Cu 0.40, Nb 0.03, V 0.04, Ti 0.015, P 0.012, S 0.008, balance Fe.

[0063] Process: The smelting flow is converter → LF refining → VD degassing. A relatively high proportion of Cr and Ni was added to improve corrosion resistance, but rare earth purification was not performed. The oxygen content of the molten steel was approximately 30 ppm. Continuous casting superheat was 35℃, and the casting speed was 0.9 m / min. Heating was performed at 1200℃, held for 100 min. A general controlled rolling process was used during rolling, with a roughing rolling final temperature of 970℃ and a finishing rolling final temperature of 860℃, resulting in a cumulative reduction of approximately 60%. Post-rolling cooling employed ACC cooling at a rate of 25℃ / s (single-stage cooling), followed by air cooling; no online tempering was performed. Surface diffusion enrichment and rare earth passivation were not performed.

[0064] Microstructure characteristics: The microstructure is a refined mixture of ferrite and bainite, but the grains are still relatively coarse in some areas. High alloying elements cause some carbide precipitation and agglomeration, and the inclusions are mostly Al2O3-CaO complexes, which are unevenly distributed.

[0065] Performance results: Yield strength 655 MPa, tensile strength 785 MPa, elongation 17%, impact energy at -40℃ 100 J, impact energy at -60℃ 95 J. Mass loss after 1000 hours of salt spray corrosion: 0.045 g·m. -2 .

[0066] Comparative Example 3:

[0067] Chemical composition (mass fraction %): C 0.09, Si 0.27, Mn 1.30, Cr 0.35, Ni 0.50, Cu 0.30, Nb 0.03, V 0.04, Ti 0.015, Ce 0.05, P 0.010, S 0.006, balance Fe.

[0068] Process: The smelting process is the same as in Example 1 of this invention, but 6 kg / t of Ce-Si-Fe alloy was mistakenly added at the end of LF refining, resulting in a Ce content as high as 0.05%. Vacuum treatment for 20 min, vacuum degree 60 Pa. Continuous casting superheat 28℃, casting speed 0.85 m / min. Heating temperature 1190℃, holding for 100 min. Controlled rolling final temperature 845℃, ACC cooling rate 30℃ / s, tempering 470℃×30 min. Surface passivation is the same as in Example 1.

[0069] Microstructural characteristics: Excessive rare earth content leads to the formation of a large number of rare earth oxide and sulfide complex inclusions, with a diameter of 5-10 μm. These inclusions are irregularly aggregated, becoming crack initiation sources. Although the microstructure is refined, local grain boundary enrichment exists.

[0070] Performance results: Yield strength 640 MPa, tensile strength 770 MPa, elongation 18%, impact energy at −40℃ 90 J, impact energy at −60℃ 85 J. Mass loss after 1000 hours of salt spray corrosion: 0.055 g·m³. -2 .

[0071] Table 1 shows the chemical composition of three embodiments and three comparative examples of the present invention. Tables 2 and 3 further illustrate the present invention.

[0072] Table 1 Chemical composition (mass fraction %) of the examples and comparative examples

[0073] Example C Si Mn Cr Ni Cu Nb V Ti Ce Sb Example 1 0.09 0.28 1.30 0.30 0.40 0.30 0.03 0.04 0.015 0.006 0.06 Example 2 0.10 0.27 1.35 0.40 0.55 0.45 0.035 0.05 0.018 0.008 0.07 Example 3 0.08 0.26 1.28 0.45 0.60 0.35 0.04 0.06 0.020 0.010 0.08 Comparative Example 1 0.12 0.35 1.40 — — — — — — — 0.06 Comparative Example 2 0.10 0.30 1.35 0.40 — — 0.035 0.05 0.018 — 0.07 Comparative Example 3 0.13 0.30 1.30 0.30 0.40 0.30 0.03 0.04 0.015 0.05 0.08

[0074] Table 2 Key process parameters for the examples and comparative examples

[0075] Example Heating temperature (°C) Final rolling temperature (°C) ACC Cooling speed: First / Second / Third (°C / s) Online tempering (°C × min) Diffusion (°C × min) Example 1 1190 840 32 / 15 / 6 470×30 670×35 Example 2 1200 850 34 / 17 / 6 480×35 680×35 Example 3 1210 855 35 / 18 / 7 490×30 685×40 Comparative Example 1 1180 900 — / — / — none none Comparative Example 2 1200 850 30 / 15 / 6 470×30 none Comparative Example 3 1190 840 air cooling none none

[0076] Table 3 Mechanical and corrosion resistance properties of the examples and comparative examples

[0077] Example Yield strength (MPa) Tensile strength (MPa) Impact energy (J) at -40℃ Impact energy at -60℃ (J) <![CDATA[1000h Salt spray corrosion mass loss (g·m -2 )]]> Example 1 642 785 135 125 0.038 Example 2 655 800 140 130 0.025 Example 3 668 815 160 150 0.018 Comparative Example 1 620 760 70 65 0.075 Comparative Example 2 648 780 100 95 0.045 Comparative Example 3 640 770 90 85 0.055

[0078] As can be seen from the examples and comparative examples, the present invention has the following advantages: (1) Under similar strength levels, the low-temperature impact energy (−40℃ / −60℃) of the present invention examples is significantly higher than that of the comparative examples, indicating that the controlled rolling and cooling combined with rare earth + surface composite passivation scheme can achieve both high strength and high toughness. (2) The addition of trace amounts of rare earth (0.002~0.012%) can effectively purify the molten steel and refine the inclusions, avoiding the detrimental effect of excessive rare earth leading to coarsening of inclusions (comparative example 3); an appropriate amount of rare earth is the key to improving the film bonding force and re-passivation ability. (3) The Cu2O·Cr2O3·rare earth oxide composite film formed by surface diffusion + rare earth activation secondary passivation shows a lower corrosion rate and stronger scratch self-repair ability in the salt spray test (examples 2 and 3). The above-described examples are only preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, all modifications and improvements made by those skilled in the art to the technical solutions of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for producing low-temperature resistant steel bars for frozen soil environments, characterized in that: include: Alloy system: Designed using a low-carbon Cu-Ni-Cr-Sb-rare earth composite alloy system; The chemical composition by mass percentage is as follows: C 0.07–0.15%, Si 0.20–0.35%, Mn 1.15–1.60%, Cr 0.25–0.60%, Ni 0.30–0.70%, Cu 0.20–0.50%, Nb 0.02–0.05%, V 0.02–0.07%, Ti 0.010–0.025%, Ce 0.002–0.015%, Sb 0.05–0.10%, P ≤ 0.015%, S ≤ 0.010%, with the balance being Fe and unavoidable impurities. Smelting and refining: The process involves converter → LF refining → VD vacuum degassing. Ce-based rare earth alloying agent is added at the end of the LF process. The basicity of the LF slag is 3.0 to 3.5, the VD vacuum degree is ≤70Pa, and the vacuum time is 15 to 20 minutes. This ensures that the oxygen content of the molten steel is ≤20ppm and the hydrogen content is ≤2.5ppm, which significantly purifies and refines inclusions. Continuous casting and heating: full protective casting, superheat 25-35℃, casting speed 0.75-1.05m / min; heating soaking temperature 1180-1220℃, holding temperature 90-150min, and tapping temperature controlled at 1170-1200℃; Controlled rolling and deformation: Two-stage or three-stage controlled rolling is adopted: roughing rolling final temperature 960~980℃, finishing rolling final temperature 840~860℃; the cumulative reduction rate in roughing rolling is ≥60%, and the cumulative deformation in finishing rolling stage is ≥60% to ensure rolling of non-recrystallized or partially non-recrystallized areas to refine the matrix structure and promote precipitation strengthening; Segmented accelerated cooling and online tempering: After rolling, the product enters the ACC segmented cooling system: First segment: final rolling → 650℃, cooling rate 30~35℃ / s; Second segment: 650→400℃, cooling rate 12~18℃ / s; Third segment: 400→200℃, cooling rate 5~8℃ / s; Subsequently, online tempering is carried out using residual heat or a short furnace at 470~490℃ for 20~40min to promote the stabilization of precipitated phases and release residual stress.

2. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 1, characterized in that: Also includes: Surface diffusion and composite passivation: Surface diffusion / enrichment treatment is performed after rolling or on the finished product line at 660-690℃ for 30-45 min to enrich Cu, Ni, Cr and Sb on the surface; pickling, followed by treatment in a neutral phosphate-silicate passivation solution for 15-25 min, and then rare earth activation with a CeO2 salt solution for 8-15 min. After drying, a Cu2O·Cr2O3·rare earth oxide composite film with a thickness of 0.3-1.0 μm is formed. The film is dense and has a self-healing tendency.

3. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 2, characterized in that: The passivation solution for the neutral phosphate-silicate system has a pH of 6.5–7.

5.

4. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 1, characterized in that: Performance meets the following requirements: impact absorption energy ≥90J at −40℃, and still ≥80J at −60℃; yield strength 640~680MPa, tensile strength 780~820MPa; mass loss from 1000h salt spray corrosion ≤0.045 g / m², with stable performance and good repeatability.

5. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 1, characterized in that: The chemical composition by weight percentage is as follows: C 0.09%; Si 0.28%; Mn 1.30%; Cr 0.30%; Ni 0.40%; Cu 0.30%; Nb 0.03%; V 0.04%; Ti 0.015%; Ce 0.006%; P 0.010%; S 0.006%; Sb 0.06%; balance Fe and unavoidable impurities.

6. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 1, characterized in that: The chemical composition by mass percentage is as follows: C 0.10%; Si 0.27%; Mn 1.35%; Cr 0.40%; Ni 0.55%; Cu 0.45%; Nb 0.035%; V 0.05%; Ti 0.018%; Ce 0.008%; P 0.010%; S 0.005%; Sb 0.07%; balance Fe and unavoidable impurities.

7. The method for producing low-temperature resistant steel bars for frozen soil environments according to claim 1, characterized in that: The chemical composition by mass percentage is as follows: C 0.08%; Si 0.26%; Mn 1.28%; Cr 0.45%; Ni 0.60%; Cu 0.35%; Nb 0.04%; V 0.06%; Ti 0.020%; Ce 0.010%; La 0.002%; P 0.009%; S 0.004%; Sb 0.08%; balance Fe and unavoidable impurities.