A 13mm gauge high-performance prestressed steel wire and steel strand rod for improving the delayed fracture resistance and a preparation method thereof

Abstract

This invention belongs to the field of metallurgy, and specifically relates to a 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand, and its preparation method. The chemical composition of the steel, by weight percentage, is: C: 0.81%–0.84%, Si: 0.25%–0.35%, Mn: 0.72%–0.82%, P≤0.015%, S≤0.015%, Cr: 0.25%–0.30%, V: 0.025%–0.040%, O≤0.0035%, N≤0.0050%, H≤0.0002%; the remainder is Fe and unavoidable impurities. This invention provides a new and more effective combination of composition and process for prestressed steel wire rod, significantly improving the delayed fracture resistance of prestressed steel wire rod and its prestressed materials while ensuring high strength and high performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy, and specifically relates to a 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand, and its preparation method. Background Technology

[0002] As infrastructure develops towards longer spans, the strength requirements for prestressed steel are becoming increasingly stringent (above 2000 MPa). However, the higher the strength of the steel, the more susceptible it becomes to hydrogen embrittlement and delayed fracture. For prestressed members subjected to long-term, high tensile stress, delayed fracture is catastrophic, with its failure being sudden and without warning.

[0003] To address the problem of delayed fracture in wire rods used for prestressed steel wires and strands, the industry has explored various methods: 1) Extremely low S content (≤0.001%), with 0.1% to 0.5% Mo added.

[0004] 2) Focus on rare earth treatment: A type of bridge cable steel, by adding trace amounts of rare earth elements (Ce, La), spheroidizes the elongated MnS inclusions, which significantly reduces the anisotropy of transverse impact toughness and the sensitivity to hydrogen-induced cracking.

[0005] In summary, although various technologies have been developed to improve the delayed fracture resistance of high-strength steel, existing technologies often suffer from problems such as high cost, narrow process window, limited effectiveness, or lack of stability when facing higher strength requirements of 2000MPa and above. Summary of the Invention

[0006] The purpose of this invention is to provide a 13mm specification wire rod with improved resistance to delayed fracture of high-performance prestressed steel wire and strand, and its preparation method. It provides a new and more effective combination of components and processes for prestressed steel wire rod, which significantly improves the resistance to delayed fracture of prestressed steel wire rod and prestressed materials while ensuring high strength and high performance.

[0007] To achieve the above objectives, the present invention employs the following technical solution: A 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand. The chemical composition of the wire rod steel, by weight percentage, is as follows: C: 0.81%~0.84%, Si: 0.25%~0.35%, Mn: 0.72%~0.82%, P≤0.015%, S≤0.015%, Cr: 0.25%~0.30%, V: 0.025%~0.040%, O≤0.0035%, N≤0.0050%, H≤0.0002%; the remainder is Fe and unavoidable impurities.

[0008] The wire rod has a tensile strength ≥1200MPa and a reduction of area ≥30% after aging (artificial or natural). Non-metallic inclusions: Class A and B ≤1.5 grade; Class C and D ≤1.0 grade; DS ≤1.0 grade; the sorbitization rate of the wire rod reaches 85%~90%.

[0009] The role of key chemical elements in this invention: Carbon: The higher the carbon content of prestressed steel, the more severe the central carbon segregation, which easily leads to the formation of central martensite and network cementite in the wire rod, thus deteriorating the drawing performance. When the carbon content is >0.85%, controlling central segregation in continuously cast billets is difficult. This invention controls the C content to 0.81%–0.84%.

[0010] Silicon: Silicon ensures the "stabilization treatment" effect in prestressed steel wires and strands, achieving high strength and high relaxation resistance, suppressing aging brittleness, and helping to improve hardenability. This invention controls Si content to be 0.25%–0.35%.

[0011] Manganese: In prestressed steel, manganese plays a role in solid solution strengthening, improving hardenability, refining grains, and improving hot working properties. It works synergistically with other alloying elements (such as C, Si, and Cr) to further optimize strength, toughness, and delayed fracture resistance. It improves the strength and hardness of the steel while maintaining a certain level of ductility and toughness. This invention controls the Mn content to be 0.72%–0.82%.

[0012] Chromium: Chromium has the characteristics of refining grains, improving hardenability, enhancing tempering resistance, and improving corrosion resistance. Adding a small amount of Cr can increase the sorbite ratio, improve drawability, and increase strength while maintaining a certain degree of ductility; it also reduces aging sensitivity, as Cr can reduce the influence of free nitrogen and decrease the tendency for strain aging. Higher chromium levels make steel more sensitive to changes in cooling rate during rolling. Cr is an element prone to segregation; excessive content can easily lead to central segregation. Cr content is usually controlled to below 0.35%. When Mn and Cr work together, the total amount must be controlled to avoid excessive residual austenite or bainite, which can lead to uneven microstructure. This invention controls Cr at 0.25%–0.30%.

[0013] Vanadium: Carbonitrides precipitate during cooling or aging, significantly improving yield strength and tensile strength. Vanadium can solidify free nitrogen in steel, reduce embrittlement tendency during long-term use, improve durability, and enhance stress relaxation resistance; it also provides good drawing properties and a favorable microstructure, reducing the risk of wire breakage during drawing; and it suppresses aging brittleness. In prestressed steel, vanadium enhances material properties through grain refinement and precipitation strengthening. Grain refinement results in a uniform and fine sorbite microstructure, improving strength and toughness. This invention controls V content to 0.025%–0.040%.

[0014] Phosphorus: Phosphorus can increase the strength and hardness of steel, increase temper brittleness, and reduce the plasticity and toughness of steel, especially at low temperatures. The P content should be controlled below 0.015%.

[0015] Sulfur: Sulfur is generally a harmful impurity in steel, reducing its plasticity, increasing its brittleness, and deteriorating its quality. Steel with high sulfur content is prone to brittle fracture during high-temperature pressure processing. The sulfur content should be controlled below 0.015% as much as possible.

[0016] Nitrogen (N): Generally a harmful residual element, it induces strain-aging brittleness, making materials prone to fracture during drawing and affecting fatigue performance and resistance to strain corrosion. Excessive nitrogen content promotes bubble or nitride inclusions, severely impairing material continuity and fatigue life. However, when combined with strong nitride-forming elements (V) and strictly controlled, nitrogen can be transformed into a beneficial factor.

[0017] The preparation method of wire rod with improved delayed fracture resistance of high-performance prestressed steel wire and strand includes hot metal pretreatment, top and bottom blowing converter smelting, argon station blowing, LF+RH refining, billet continuous casting, billet slow cooling pit slow cooling, walking beam furnace heating, high pressure water descaling, controlled rolling, controlled cooling (first fast cooling with large air volume fan + slow cooling with insulation cover), vertical core rack coiling, transportation, slow cooling in PF line insulation channel, sampling, inspection, packaging, and warehousing.

[0018] Specifically, it includes: 1) The KR method is used for desulfurization in the pretreatment of molten iron. After desulfurization, S≤0.030 and the slag is removed cleanly.

[0019] 2) Top-and-bottom blowing converter smelting: final temperature 1580~1650℃, final [P]: ≤0.010%, smelting time: 35~70min, final carbon: 0.45~0.55%, final oxygen: 0.0140~0.0350%; argon station: static argon blowing time ≥5 minutes; post-treatment temperature: 1500~1520℃; deoxidation and alloying after tapping; no deoxidizer added, aluminum deoxidation is prohibited, and the use of substances containing aluminum, titanium, nickel, copper, etc. is prohibited; nitrogen and hydrogen are controlled throughout the process, and the use of damp materials is strictly prohibited; there is no slag or residual steel at the tapping port to prevent steel spillage. Smelting adopts a steel retention operation, with a steel retention amount ≥15 tons.

[0020] 3) LF Furnace Refining: White slag is produced, with refining time ≥15 min; ferrosilicon powder is used for deoxidation of the slag, and aluminum-containing materials are prohibited; a slight positive pressure is maintained inside the furnace, and a low-nitrogen carburizing agent is used; static blowing time ≥15 min; temperature at the LF station: 1475~1550℃; actual outlet temperature: 1475~1535℃; number of energizations: 2~3 times; total energization time: 14~29 min; no calcium treatment is performed. LF cycle: 50~60 min for ladle change, 40~45 min for continuous casting. Refining time should not exceed 120 min.

[0021] 4) RH vacuum refining: Deep processing: cycle ≤ 35 min, vacuum degree ≤ 100 Pa; deep vacuum: 15-18 min.

[0022] Bottom-blown argon: After repressurization (vacuum breaking), the stirring time for "soft argon blowing" is ≥25min and does not exceed 35min.

[0023] 5) Full-process protective casting is adopted. An integral nozzle is used in the tundish, with an insertion depth of 80–120 mm. The temperature of the molten steel arriving at the casting station is 1515–1535℃, and the molten steel's settling time in the continuous casting machine is ≥4 min. The tundish temperature is ≥1479℃, with an initial superheat of 25–35℃ and a continuous superheat of 15–25℃. High-carbon protective slag is used in the crystallizer. Electromagnetic stirring is used in the crystallizer at 320A current and 3Hz frequency, continuously; electromagnetic stirring is used at the solidification end at 400A current and 8Hz frequency, alternating. The constant casting speed for billet continuous casting (160mm × 160mm cross-section) is controlled at 1.8–1.9 m / s. A steel retention operation is adopted in the ladle, with a ladle residue control of ≥7 tons of molten steel + slag. Slag removal control is implemented in the ladle, and it is strictly prohibited for slag from the ladle to enter the tundish and contaminate the molten steel.

[0024] 6) The continuous casting billet is slowly cooled in the heat preservation pit for more than 72 hours, and the temperature when it comes out of the pit is below 150℃.

[0025] 7) Walking beam furnace heating: Heating stage 1 temperature: 800~900℃; Heating stage 2 temperature: 1100~1150℃; Soaking stage temperature: 1080~1130℃; Furnace time: 1.5~2.5 / h; Furnace air-fuel ratio: 0.5~0.6.

[0026] 8) High-pressure water descaling: High-pressure water pressure ≥20MPa.

[0027] 9) Controlled rolling: Initial rolling temperature: 980~1040℃, temperature at the sizing mill: 870~930℃, wire drawing temperature: 820~880℃.

[0028] 10) Controlled cooling: The speed of the No. 1 air-cooled roller conveyor is 1.00~1.10m / s, and the speed of other sections increases by 2%~5%; after cooling by the fans, the rollers enter the insulation cover for slow cooling; the fan cooling devices are all set at 25%~35% to distribute the air volume, so that the coil connection points and sparse parts are cooled evenly; the air volume of No. 1 to No. 6 variable frequency fans is turned on at 200,000~205,000 m³ / s. 3 The cooling rate is 15-22℃ / s; the air volume of the 7-15# variable frequency fans is 150,000-185,000 m³ / h. 3The cooling rate is 12.0~14℃ / s; other fans are turned off; vertical core racks are used for winding and transportation. An insulated passageway is provided outside the PF transport line. There are 22 insulation covers; the insulation cover above the fan opens when it is turned on, and then closes. The air-cooled transport line has a total of 25 fans (fan spacing is 3m), and the number of fans or airflow can be increased or decreased according to mechanical performance and seasonal ambient temperature.

[0029] Compared with the prior art, the beneficial effects of the present invention are: 1) The 13mm large-diameter high-performance prestressed steel wire and strand wire rod of the present invention has excellent mechanical properties: tensile strength ≥1200MPa, reduction of area after aging (artificial aging or natural aging) ≥30%; non-metallic inclusions: Class A and B ≤1.5 grade; Class C and D ≤1.0 grade; DS ≤1.0 grade; sorbitization rate of wire rod reaches 85%~90%.

[0030] 2) The 13mm large-diameter high-performance prestressed steel wire and strand of this invention exhibits high resistance to delayed fracture. The prestressed steel composition of this invention does not use the addition of precious metal Mo, reducing alloy costs; instead, appropriate amounts of microalloying element V are added according to tensile strength requirements and different product specifications. This increases strength while combining with nitrogen to form carbonitrides, mitigating the impact of nitrogen on delayed fracture. This invention strictly controls hydrogen to avoid hydrogen-induced delayed fracture in prestressed materials; high sorbitization rate is used to improve resistance to delayed fracture. This invention strictly controls the content of harmful elements S and P through hot metal pretreatment and converter smelting; LF+RH refining ensures steel purity; non-metallic inclusions in the steel are controlled as follows: Class A and B ≤ 1.5 grade; Class C and D ≤ 1.0 grade; DS ≤ 1.0 grade. Control of harmful elements and high purity in the steel improves the resistance to delayed fracture of prestressed materials; reducing defects in the cast billet reduces hydrogen-induced crack initiation sites.

[0031] 3) After coiling, a high-volume variable frequency fan is used for rapid cooling. The cooling rate before phase transformation is 15℃ / s to 22℃ / s, and the cooling rate during and after phase transformation is 12℃ / s to 14℃ / s. The air volume is distributed through a Jialing device to improve the uniformity of the coil structure and mechanical properties, ensuring a high sorbitization rate ≥85% and avoiding the formation of a network cementite structure. The number and air volume of fans are controlled according to the wire rod specifications; the air volume is appropriately adjusted by frequency conversion according to the ambient temperature in different seasons. By controlling the rolling process and the cooling through the high-volume fan, an ideal high sorbitization rate microstructure is obtained, with a sorbitization rate of 85% (grade 2.0) to 90% (grade 1.5).

[0032] 4) Hydrogen control process "using online residual heat slow cooling instead of offline heat treatment": This involves integrating a special controlled cooling-insulation process into the rolling process. After being cooled by the fan on the air conveyor line, the coiled wire enters an insulation hood for slow cooling; a vertical core rack is used for coiling and transportation, while simultaneously utilizing the residual heat of the wire rod for slow cooling; finally, it enters the insulation channel of the PF line equipped with a heating device for slow cooling. This online slow cooling process allows sufficient time for hydrogen and stress to diffuse and release, while also promoting the precipitation of microalloying elements in a favorable form. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto. Experimental methods for which specific conditions are not specified in the embodiments are generally determined according to national / industry standards; if there is no corresponding national / industry standard, then they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0034] The present invention relates to a 13mm large-size delayed fracture resistant prestressed steel wire rod using a continuous casting billet one-fire forming process: "Blast furnace molten iron → molten iron pretreatment (desulfurization, slag removal) → top and bottom combined blowing converter smelting → argon blowing at argon station → refining LF+RH → continuous casting of billet (integral nozzle, argon seal protection, full-process protective casting, electromagnetic stirring in the crystallizer, electromagnetic stirring at the end of solidification, and liquid level control in the tundish and crystallizer) → slow cooling in the billet slow cooling pit → heating in the walking beam furnace → high-pressure water descaling → controlled rolling and controlled cooling (first fast cooling with a large air volume fan + slow cooling with an insulation cover) → coiling and transportation on a vertical core rack → slow cooling in the PF line insulation channel → sampling, inspection, and judgment → packaging and delivery to the warehouse."

[0035] The following are embodiments of the present invention.

[0036] The chemical composition of the wire rod steel melting furnace is shown in Table 1.

[0037] Table 1 Chemical composition of wire rod steel in the examples: The process parameters for converter smelting and post-furnace argon blowing are shown in Table 2.

[0038] Table 2. Process parameters for converter smelting and post-furnace argon blowing in the examples: The process parameters for LF furnace + RH refining are shown in Table 3.

[0039] Table 3. LF furnace + RH refining process parameters (I): Table 3. LF furnace + RH refining process parameters (II): The continuous casting process parameters are shown in Table 4.

[0040] Table 4 Continuous casting process parameters for the example: The temperature control of the intermediate package is shown in Table 5.

[0041] Table 5 shows the packaging temperature control in the embodiments: The continuous casting speed control and the slow cooling time of the billet slow cooling pit are shown in Table 6.

[0042] Table 6. Casting speed and slow cooling time of billet in the examples: The results of the low-magnification test are shown in Table 7. The cross-section of the billet is 160mm x 160mm.

[0043] Table 7. Low-magnification inspection results of continuously cast square billets in the examples: Steel rolling process parameters: 1) The heating temperature of the walking beam furnace is shown in Table 8.

[0044] Table 8 Heating temperatures in the examples: 2) High-pressure water descaling pressure: 20MPa. After descaling, the iron oxide scale on the surface of the steel billet is completely removed.

[0045] 3) Rolling temperature control is shown in Table 9.

[0046] Table 9. Rolling temperature control in the examples: 4) The process parameters of the air-cooled transport line controlled cooling process are shown in Table 10.

[0047] Table 10. Cooling process settings for the air-cooled transport line in the embodiment: 13mm diameter: Cooling speed of fans 1~6 is about 15℃ / s~20℃ / s, and cooling speed of fans 7~15 is about 12~14℃ / s.

[0048] 5) Collection Station: Vertical core racks are used for winding and transporting, and the rewinding machine rotates the cores 90° to unload them onto the C-hook of the PF line.

[0049] 6) Slow cooling in the insulated passageway of the PF transport line: The wire is slowly cooled by its own residual heat within the insulation channel, releasing hydrogen and internal stress.

[0050] 7) Finishing and packaging: The ends of the wire rod that have not been pierced and any defective parts are cleanly removed.

[0051] 8) The test results of the mechanical properties of wire rod are shown in Table 11.

[0052] Table 11 Mechanical properties of wire rod in the examples: 9) Inspection of non-metallic inclusions in wire rods is shown in Table 12.

[0053] Table 12. Inspection results of non-metallic inclusions in wire rods from the examples: 10) The results of the inspection of wire rod grain size, sorbite grade, microstructure, and decarburized layer are shown in Table 13.

[0054] Table 13. Inspection results of wire rod grain size, sorbite grade, microstructure, and decarburized layer in the examples: Metallographic examination results of the coil: The microstructure is S+F+C. （少量） The sorbite grade is 2.0 to 1.5, which is equivalent to a sorbitization rate of 85% to 90%; there is no network cementite or martensite abnormal structure; the total decarburized layer depth is less than 1.0D% (D is the nominal diameter), the grain size is grade 8, the non-metallic inclusions are all below grade 1.5, and all tests meet the standards.

[0055] Delayed fracture resistance: The wire rods showed no delayed fracture after being drawn into wire and naturally aged; the prestressed products passed the user's stringent constant load delayed fracture test; there were no quality complaints regarding delayed fracture.

[0056] Applications of wire rod: 13mm diameter wire rod is used for 1860MPa high-strength prestressed galvanized steel wire and 1960~2000MPa prestressed steel wire and steel strand.

[0057] Obviously, the above embodiments are merely illustrative examples and not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand, characterized in that, The chemical composition of wire rod steel, by weight percentage, is as follows: C: 0.81%~0.84%, Si: 0.25%~0.35%, Mn: 0.72%~0.82%, P≤0.015%, S≤0.015%, Cr: 0.25%~0.30%, V: 0.025%~0.040%, O≤0.0035%, N≤0.0050%, H≤0.0002%; the remainder is Fe and unavoidable impurities.

2. The 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 1, characterized in that, The wire rod has a tensile strength ≥1200MPa and a reduction of area ≥30% after aging.

3. The 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 1, characterized in that, The sorbitization rate of the wire rod is 85%–90%, and the decarburization layer depth is less than 1.0D.

4. A method for preparing 13mm diameter wire rod with improved delayed fracture resistance of high-performance prestressed steel wire and strand as described in claim 1, comprising steelmaking, continuous casting, furnace heating, controlled rolling, controlled cooling, and slow cooling in a PF wire insulation channel, characterized in that, The cooling rate before phase change is controlled at 15–22℃ / s; the cooling rate during and after phase change is 12℃ / s–14℃ / s; the controlled cooling and slow cooling of the PF line insulation channel include: the speed of the No. 1 air-cooled roller conveyor is 1.00–1.10 m / s, and the speed of other sections is increased by 2%–5%; after cooling by the fan, the rollers enter the insulation cover for slow cooling; the fan cooling devices are all set at 25%–35%; the air volume of the No. 1–6 variable frequency fans is 200,000–205,000 m³ / s. 3 / h, the air volume of variable frequency fans #7 to #15 is 150,000 to 185,000 m³ / h. 3 / h, the remaining fans are shut down; vertical core frame is used for core collection and transportation; an insulated channel is set up outside the PF transport line to utilize the residual heat of the wire rod for slow cooling.

5. The method for preparing the 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 4, characterized in that, The steelmaking process includes: 1) Top and bottom blowing converter smelting: final temperature 1580~1650℃, final [P]: ≤0.010%, smelting time: 35~70min, final carbon: 0.45%~0.55%, final oxygen: 0.0140%~0.0350%; argon station: static argon blowing time ≥5 minutes; post-treatment temperature: 1500~1520℃; 2) LF furnace refining: white slag is produced, and the white slag refining time is ≥15min; ferrosilicon powder is used for deoxidation in the slag; static blowing time is ≥15min; temperature at LF station: 1475~1550℃; actual outlet temperature: 1475~1535℃; number of energizations: 2~3 times; total energization time: 14~29min; 3) RH vacuum refining: Deep treatment: cycle ≤ 35 min, vacuum degree ≤ 100 Pa; deep vacuum: 15~18 min, soft blowing argon not exceeding 35 min.

6. The method for preparing the 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 4, characterized in that, The continuous casting includes: molten steel temperature at the station: 1515~1535℃; molten steel settling time in the continuous casting machine ≥4min; tundish temperature ≥1479℃; continuous casting superheat 15~25℃; constant casting speed control of billet continuous casting 1.8~1.9m / s; and slow cooling of continuous casting billet in the heat preservation pit for more than 72 hours.

7. The method for preparing the 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 4, characterized in that, The heating process in the furnace includes: heating stage 1 temperature: 800-900℃; heating stage 2 temperature: 1100-1150℃; soaking zone temperature: 1080-1130℃; furnace time: 1.5-2.5 / h; and furnace air-fuel ratio: 0.5-0.

6.

8. The method for preparing the 13mm diameter wire rod for improving the delayed fracture resistance of high-performance prestressed steel wire and strand according to claim 4, characterized in that, The controlled rolling process includes: initial rolling temperature: 980~1040℃, inlet sizing and reducing mill temperature: 870~930℃, and wire drawing temperature: 820~880℃.

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