700 mpa grade high-strength seismic-resistant rebar and production method therefor, and method for connecting sleeve for mechanical connection of rebar to rebar

Abstract

The present application provides a 700 MPa grade high-strength seismic-resistant rebar and a production method therefor, and a method for connecting a sleeve for mechanical connection of rebars to a rebar. In the rebar, C: 0.24-0.28%, Si: 0.50-0.80%, Mn: 1.20-1.60%, V: 0.165-0.200%, Nb: 0.025-0.050%, N: 0.026-0.040%, [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, with the balance being iron and impurities. In the production method, the soaking zone temperature is 1255-1300℃; water cooling is not employed during the rolling process; after rolling is completed, cooling to room temperature, the temperature at which the rebar is placed on the cooling bed being ≥1125℃, and the cooling rate during natural cooling being <1℃ / s.

Description

700MPa grade high-strength earthquake-resistant steel bars and their production methods, and methods for connecting steel bars to sleeves used for mechanical connections.

[0001] This application claims priority to Chinese Patent Application No. 202510139206.0, filed on February 8, 2025, entitled "700MPa Grade High-Strength Seismic Reinforcing Steel Bar and its Production Method Thereof," and also claims priority to Chinese Patent Application No. 202510177296.2, filed on February 18, 2025, entitled "Production Method of Round Steel Bar for Preparing Sleeves, Sleeves Prepared from the Round Steel Bar, and Method for Connecting Sleeves and Reinforcing Steel Bars," the contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of iron and steel smelting technology, and in particular to a 700MPa grade high-strength earthquake-resistant steel bar and its production method, as well as a method for connecting the steel bar with a sleeve for mechanical connection. Background Technology

[0003] Building structures are developing towards larger spans, heavier loads, super high-rise buildings, and higher safety (seismic resistance), placing stringent demands on building materials, especially reinforcing steel, in terms of strength, ductility, seismic resistance, and weldability. Existing reinforcing steel products have low strength levels, with yield strength below 600 MPa, making it difficult to meet usage requirements. While strong water penetration processes can improve strength, they negatively impact ductility, seismic resistance, and weldability. Obtaining reinforcing steel with ultrafine grains can improve strength and ductility, but it is detrimental to the strength-to-yield ratio / seismic resistance. High alloying can improve strength, ductility, and seismic resistance, but it affects weldability and is costly, failing to simultaneously improve the strength, ductility, seismic resistance, and weldability of reinforcing steel. Summary of the Invention

[0004] The purpose of this application is to provide a 700MPa grade high-strength earthquake-resistant steel bar and its production method.

[0005] To achieve one of the aforementioned objectives, this application provides a method for producing 700MPa grade high-strength earthquake-resistant steel bars. The steel bars, by weight percentage, contain the following components: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, satisfying [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, with the remainder being iron and unavoidable impurities; wherein [Nb], [V], [N], and [C] are the weight percentages of the corresponding elements.

[0006] The production method of reinforcing bars includes heating, rolling and cooling steel billets with the chemical composition as described above to obtain reinforcing bars, wherein:

[0007] During the heating process, the temperature of the steel billet in the soaking zone of the heating furnace is 1255~1300℃;

[0008] In the rolling process, the steel billet is placed in the rolling mill and rolled into steel bars, and no water cooling is used throughout the rolling process;

[0009] In the cooling process, after rolling, the steel bars are directly sent to the cooling bed to cool naturally to room temperature in the air. The temperature of the steel bars on the cooling bed is ≥1125℃, and the cooling rate during natural cooling is <1℃ / s.

[0010] In one embodiment of this application, the process includes smelting, LF refining, and continuous casting processes performed sequentially before the heating process. In the LF refining process, vanadium-nitrogen alloy and niobium-iron alloy are added to the ladle upon arrival. The amount of vanadium-nitrogen alloy added is 2-3 kg / t, the vanadium content in the vanadium-nitrogen alloy is 75-80%, the nitrogen content is 5-10%, and the remainder is iron and other impurity elements. The amount of niobium-iron alloy added is 0.6-1.2 kg / t, the niobium content in the niobium-iron alloy is 40-45%, and the remainder is iron and other impurity elements.

[0011] In one embodiment of this application, in the LF refining process, nitrogen blowing begins after the ladle is energized. During nitrogen blowing, the nitrogen flow rate is controlled at 850~1000 L / min, the pressure is controlled at 0.4~0.8 MPa, and the nitrogen blowing time t satisfies the following formula:

[0012] t = λ × (XAB) / 20, where λ is a coefficient related to the temperature of molten steel, and its value is 4~6;

[0013] X represents the target nitrogen content in the steel reinforcement;

[0014] A represents the increase in nitrogen content resulting from the addition of vanadium-nitrogen alloy to molten steel;

[0015] B represents the nitrogen content of the molten steel, ranging from 30 to 60.

[0016] X, A, and B are all in ppm, and t is in min.

[0017] In one embodiment of this application, during the smelting process, the deoxidation alloying is carried out by adding alloys and slag in the following order: ferrosilicon alloy, silicon manganese alloy, and lime. The amount of ferrosilicon alloy added is 20-30 kg / t, the manganese content in the ferrosilicon alloy is 65-70%, the silicon content is 17-20%, and the remainder is iron and other impurity elements. The amount of ferrosilicon alloy added is 9-14 kg / t, the silicon content in the ferrosilicon alloy is 70-75%, and the remainder is iron and other impurity elements.

[0018] In one embodiment of this application, during the smelting process, the tapping temperature is controlled at 1595~1635℃, and the argon gas blowing control valve at the bottom of the ladle is opened before tapping. Argon gas is blown throughout the tapping process. The argon gas pressure is controlled at 0.4~0.5MPa in the first 1 / 3 of the tapping process and at 0.3~0.4MPa in the last 2 / 3 of the tapping process.

[0019] In one embodiment of this application, in the continuous casting process, the casting speed is controlled at 3.0~3.6m / min, and the crystallizer water flow rate is 1900~2000L / min.

[0020] One embodiment of this application also provides a reinforcing steel bar, comprising the following components by weight percentage: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, and satisfying [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, with the balance being iron and unavoidable impurities; wherein, [Nb], [V], [N], and [C] are the weight percentages of the corresponding elements.

[0021] In one embodiment of this application, the microstructure of the reinforcing steel includes pearlite, ferrite and bainite, with pearlite accounting for 50-70% of the volume, the average size of the pearlite clusters being 10-20 μm, and the pearlite lamellar spacing being 200-300 nm; the average grain size of the ferrite being 4-8 μm; and the volume percentage of bainite being 5-10%.

[0022] In one embodiment of this application, the microstructure of the reinforcing steel includes pearlite, ferrite and bainite, with pearlite accounting for 55-65% of the volume, the average size of the pearlite clusters being 11-16 μm, and the pearlite lamellar spacing being 230-270 nm; the average grain size of the ferrite being 5-6 μm; and the volume percentage of bainite being 6-8%.

[0023] In one embodiment of this application, the reinforcing steel bar includes a carbonitride precipitate phase with a size ≤50nm and a volume content ≥3×10⁻⁶. 6 pcs / mm 3 .

[0024] In one embodiment of this application, the reinforcing steel bar includes a carbonitride precipitate phase with a size ≤40 nm and a volume content ≥5 × 10⁻⁶. 6 pcs / mm 3 .

[0025] In one embodiment of this application, the steel bar has a yield strength ≥720MPa, a tensile strength ≥910MPa, an elongation after fracture ≥15%, a maximum force elongation ≥10%, and a strength-to-yield ratio >1.26.

[0026] An embodiment of this application also provides a method for connecting a sleeve for mechanical connection of reinforcing bars to reinforcing bars, comprising the following steps:

[0027] Rebar processing: External threads are machined at the ends of the rebars using upset straight threads. When the rebar diameter d is 10~25mm, the upset pressure is 10~23MPa, the upset base circle diameter is d+4~d+6mm, and the upset length is d+2mm; when the rebar diameter d is 26~50mm, the upset pressure is 30~55MPa, the upset base circle diameter is d+6.5mm, and the upset length is d+3mm; the rebars used are those described above.

[0028] Sleeve machining: The internal thread of the sleeve matches the external thread of the reinforcing bar; the tensile strength of the sleeve is ≥900MPa, and the elongation after fracture is ≥20%.

[0029] Mechanical connection between reinforcing bar and sleeve: Tighten the reinforcing bar and sleeve with threads, with no more than 1.0 thread of the reinforcing bar not screwed into the sleeve, and the torque is 450~550 N·m.

[0030] In one embodiment of this application, the external thread of the reinforcing bar has a triangular tooth profile with a tooth profile angle of 60~70°; when the nominal diameter of the reinforcing bar is <25mm, the thread pitch P is 2.5mm, and when the nominal diameter of the reinforcing bar is ≥25mm, the thread pitch P is 3.0mm.

[0031] One or more technical solutions provided in this application have at least the following technical effects or advantages:

[0032] The production method for 700MPa grade high-strength seismic-resistant steel bars provided in this application adopts a low-to-medium carbon content + vanadium-nitrogen microalloying design. Through fine-grain strengthening and precipitation strengthening, it solves the problem of balancing strength and plasticity in the steel bars. Based on this, niobium is introduced, and through phase transformation strengthening and precipitation strengthening, the tensile strength of the steel bars is improved, solving the technical problem of insufficient strength-to-yield ratio in high-strength steel bars. Through the coordinated design of carbon, nitrogen, niobium, and vanadium, combined with the development of rolling technology, the strengthening effects of niobium and vanadium are fully utilized, overcoming the limitations of further increasing the strength of high-strength steel bars and the technical problem of insufficient strength-to-yield ratio. Through the development of a controlled cooling process (high-temperature heating + slow cooling after rolling), the regulation of the "ferrite + pearlite + bainite" multiphase structure is achieved, and the steel bars also contain carbonitride precipitates, significantly improving the strength and toughness of the steel. In other words, under the synergistic effect of carbon, nitrogen, niobium, and vanadium, the problem of balancing strength, plasticity, seismic resistance, and weldability in steel bars is solved, realizing the development of high-strength seismic-resistant steel bars with a yield strength of over 700MPa. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] This application provides a method for producing reinforcing steel bars. The reinforcing steel bars, by weight percentage, contain the following components: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, and satisfying [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, and 0.67≤([Nb]) / [N] ≤0.23%. +[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, the balance is iron and unavoidable impurities; where, carbon equivalent Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15, [Nb], [V], [N], [C], [Mn], [Cr], [Mo], [Cu], [Ni] are the weight percentages of the corresponding elements. When the steel bar does not contain any of these elements, the weight percentage of that element is taken as 0.

[0035] The production method of reinforcing bars includes heating, rolling and cooling steel billets with the chemical composition as described above to obtain reinforcing bars, wherein:

[0036] During the heating process, the temperature of the steel billet in the soaking zone of the heating furnace is 1255~1300℃;

[0037] In the rolling process, the steel billet is placed in the rolling mill and rolled into steel bars, and no water cooling is used throughout the rolling process;

[0038] In the cooling process, after rolling, the steel bars are directly sent to the cooling bed to cool naturally to room temperature in the air. The temperature of the steel bars on the cooling bed is ≥1125℃, and the cooling rate during natural cooling is <1℃ / s.

[0039] A vanadium-nitrogen microalloying design was adopted, addressing the issue of balancing strength and plasticity in reinforcing steel through grain refinement and precipitation strengthening. Building upon this, niobium was introduced, and its phase transformation and precipitation strengthening further enhanced the tensile strength of the steel, resolving the technical challenge of insufficient strength-to-yield ratio in high-strength reinforcing steel. Through the coordinated design of carbon, nitrogen, niobium, and vanadium, combined with the development of rolling processes, the strengthening effects of niobium and vanadium were fully utilized, overcoming the limitations of further strength enhancement and the technical challenge of insufficient strength-to-yield ratio in high-strength reinforcing steel. The development of controlled rolling and controlled cooling processes enabled the regulation of the "ferrite + pearlite + bainite" multiphase microstructure, and the steel also contained carbonitride precipitates, significantly improving the strength and toughness of the steel. In short, through the synergistic effect of carbon, nitrogen, niobium, and vanadium, combined with high-temperature heating and slow cooling after rolling, the challenge of balancing strength, plasticity, seismic resistance, and weldability in reinforcing steel was solved, enabling the development of high-strength seismic-resistant reinforcing steel with a yield strength exceeding 700 MPa.

[0040] The functions of each chemical component have the following characteristics:

[0041] Carbon (C): A strengthening element that significantly improves the strength of reinforcing bars; however, excessive carbon content is detrimental to the plasticity, toughness, and weldability of steel. To ensure good overall performance of the steel, the carbon content is controlled at 0.24~0.28%.

[0042] Silicon (Si): A strengthening element that, through solid solution strengthening, can simultaneously improve the yield strength and tensile strength of reinforcing steel. Silicon is also a ferrite-forming element, which is beneficial for controlling microstructure and optimizing mechanical properties. However, excessive silicon content is detrimental to welding. Taking all factors into consideration, the silicon content should be controlled at 0.50~0.80%.

[0043] Manganese (Mn): Manganese is a commonly used solid solution strengthening element that can significantly improve the strength and toughness of steel. The manganese content in earthquake-resistant steel bars should be designed to be as high as possible; however, excessive manganese content increases hardenability and easily forms MnS inclusions, reducing the plasticity of the steel. Taking all factors into consideration, the manganese content should be controlled at 1.20~1.60%.

[0044] Vanadium (V): Vanadium is a strong carbonitride forming element. Through the precipitation of carbonitrides, it can significantly improve the strength and toughness of steel. However, the strengthening effect of vanadium is affected by nitrogen content. When the vanadium content is too high, the effect of further strength improvement is not obvious. Taking all factors into consideration, the vanadium content should be controlled at 0.165~0.200%.

[0045] Niobium (Nb): Niobium enhances the strength and toughness of steel through precipitation strengthening and grain refinement. Niobium-containing steel, through controlled rolling processes, can promote the formation of hard phase structures (lowering the ferrite transformation temperature and increasing the pearlite and bainite transformation temperatures), which is beneficial for adjusting the strength-ductility and seismic resistance (strength-yield ratio) of reinforcing bars. However, excessively high niobium content can easily lead to cracking in billets during continuous casting. Considering all factors, the niobium content should be controlled at 0.025~0.050%.

[0046] Nitrogen (N): A solid solution strengthening and precipitation strengthening element. It combines with alloying elements such as niobium and vanadium to form carbonitrides, thereby increasing strength. However, excessive content is detrimental to plasticity; excessive content increases smelting difficulty, and the strengthening effect is not fully realized. Taking all factors into consideration, the nitrogen content is controlled at 0.026~0.040%.

[0047] A low-carbon equivalent alloy with niobium, vanadium, and nitrogen composite reinforcement was designed. The low-carbon equivalent ensures the weldability of the reinforcing steel. Vanadium, through precipitation, solid solution, and grain refinement, significantly improves the strength and plasticity of the reinforcing steel, but negatively impacts the strength-to-yield ratio, affecting seismic performance. Higher strength grades of reinforcing steel are more difficult to meet seismic performance requirements. To mitigate this impact, the rolling process was designed to promote the formation of hard phase structures (lowering the ferrite transformation temperature and increasing the pearlite and bainite transformation temperatures), thereby increasing the tensile strength and improving the strength-to-yield ratio of the reinforcing steel, thus enhancing its seismic performance. When niobium and vanadium are added together, niobium's affinity for carbon and nitrogen is higher than that for vanadium. To ensure the full utilization of the effects of niobium and vanadium, the carbon and nitrogen content must be controlled. Simultaneously, the solid solution strengthening effect of carbon and nitrogen also affects the strength, plasticity, and seismic resistance of the reinforcing steel. Therefore, only the synergistic effect of niobium, vanadium, carbon, and nitrogen can guarantee a harmonious balance of strength, plasticity, seismic resistance, and weldability of the reinforcing steel. Taking all factors into account, the synergistic reinforcement effect is optimal when [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, and 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77.

[0048] In one embodiment of this application, the process includes smelting, LF refining, and continuous casting steps performed sequentially before the heating step. Specifically, in the LF refining step, a vanadium-nitrogen alloy and a niobium-iron alloy are added to the ladle upon arrival. The vanadium-nitrogen alloy is added at a rate of 2-3 kg / t, containing 75-80% vanadium and 5-10% nitrogen, with the remainder being iron and other impurities. The niobium-iron alloy is added at a rate of 0.6-1.2 kg / t, containing 40-45% niobium, with the remainder being iron and other impurities. Adding the vanadium-nitrogen alloy in the LF refining step increases the nitrogen content while meeting the vanadium requirement, reducing the amount of nitrogen required for the nitrogen blowing process described below, thereby reducing the nitrogen blowing time and shortening the overall production process.

[0049] In one embodiment of this application, in the LF refining process, nitrogen blowing begins after the ladle is energized. During nitrogen blowing, the nitrogen flow rate is controlled at 850~1000 L / min, the pressure is controlled at 0.4~0.8 MPa, and the nitrogen blowing time t satisfies the following formula:

[0050] t = λ × (XAB) / 20, where λ is a coefficient related to the temperature of molten steel, and its value is 4~6;

[0051] X represents the target nitrogen content in the steel reinforcement;

[0052] A represents the increase in nitrogen content resulting from the addition of vanadium-nitrogen alloy to molten steel;

[0053] B represents the nitrogen content of the molten steel, ranging from 30 to 60.

[0054] X, A, and B are all in ppm, and t is in min.

[0055] In this application, the nitrogen content of the reinforcing steel is too high. Nitrogen is added by blowing nitrogen gas into the molten steel to increase the nitrogen content. Compared to existing nitrogen-addition processes, nitrogen blowing is simpler and more economical. Simultaneously, nitrogen blowing also acts as a gentle stir, making the alloying elements in the molten steel more uniform. Using a higher flow rate of nitrogen gas accelerates the nitrogen addition process and prevents nitrogen from failing to dissolve in the molten steel and instead entering the air as nitrogen gas due to excessive blowing time. Calculating the blowing time based on the target nitrogen content using the aforementioned formula allows for more precise control of the nitrogen content. During nitrogen blowing, the argon gas is turned off. After nitrogen blowing is completed, the argon gas is turned on for stirring for ≥5 minutes. The LF refining temperature is 1545~1565℃.

[0056] In one embodiment of this application, during the smelting process, the deoxidation alloying process involves adding alloys and slag in the following order: ferrosilicon alloy, silicon-manganese alloy, and lime. The amount of ferrosilicon alloy added is 20-30 kg / t, with a manganese content of 65-70%, a silicon content of 17-20%, and the remainder being iron and other impurities. The amount of ferrosilicon alloy added is 9-14 kg / t, with a silicon content of 70-75%, and the remainder being iron and other impurities. In the smelting process, the inexpensive ferrosilicon alloy is added first for deoxidation alloying. The silicon in the ferrosilicon alloy is used for deoxidation. After adding the ferrosilicon alloy, the manganese content reaches the target manganese content for steel reinforcement. Then, the ferrosilicon alloy is added to supplement the insufficient silicon content, thus reducing the alloy cost.

[0057] In one embodiment of this application, during the smelting process, the tapping temperature is controlled at 1595~1635℃, and the argon gas blowing control valve at the bottom of the ladle is opened before tapping. Argon gas is blown throughout the tapping process. The argon gas pressure is controlled at 0.4~0.5MPa in the first 1 / 3 of the tapping stage and at 0.3~0.4MPa in the last 2 / 3 of the tapping stage. The higher argon gas pressure in the early stage provides stronger soft stirring, allowing the added silicon-manganese alloy and ferrosilicon alloy to be better dispersed in the molten steel, thus homogenizing the molten steel. The later stage is relatively gentle, which is conducive to tapping.

[0058] In one embodiment of this application, during the continuous casting process, the casting speed is controlled at 3.0~3.6 m / min, and the crystallizer water flow rate is 1900~2000 L / min. During continuous casting, a long nozzle and sealing gasket, an immersion nozzle, and a covering agent in the tundish are used for full protective casting, and argon is blown through the long nozzle. Furthermore, the electromagnetic stirring current in the crystallizer is 350 A, the frequency is 3~5 Hz, and the secondary cooling water volume is automatically distributed according to the parameters (L / m) in Table 1 below:

[0059] Table 1. Parameters corresponding to secondary cooling water ratio.

[0060]

[0061] Where m, c, and co are correction coefficients related to steel grade, used to adjust the secondary cooling water ratio. The set value of the secondary cooling water ratio = drawing speed × m; c and co are the range of values ​​for m, where co is the upper limit and c is the lower limit.

[0062] Using a faster casting speed and reducing the cooling rate during continuous casting can prevent niobium from causing cracks on the surface of the steel billet.

[0063] This application embodiment also provides a steel bar prepared according to the aforementioned steel bar production method. The steel bar contains the following components by weight percentage: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, and satisfies [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, and the balance is iron and unavoidable impurities; wherein, [Nb], [V], [N], and [C] are the weight percentages of the corresponding elements.

[0064] In one embodiment of this application, the microstructure of the reinforcing steel includes pearlite, ferrite and bainite, with pearlite accounting for 50-70% of the volume, the average size of the pearlite clusters being 10-20 μm, and the pearlite lamellar spacing being 200-300 nm; the average grain size of the ferrite being 4-8 μm; and the volume percentage of bainite being 5-10%.

[0065] In one embodiment of this application, the microstructure of the reinforcing steel includes pearlite, ferrite and bainite, with pearlite accounting for 55-65% of the volume, the average size of the pearlite clusters being 11-16 μm, and the pearlite lamellar spacing being 230-270 nm; the average grain size of the ferrite being 5-6 μm; and the volume percentage of bainite being 6-8%.

[0066] In one embodiment of this application, the reinforcing steel bar includes a carbonitride precipitate phase, the carbonitride precipitate phase size being ≤50nm and the volume content being ≥3×10 6 pcs / mm 3 .

[0067] In one embodiment of this application, the reinforcing steel bar includes a carbonitride precipitate phase with a size ≤40 nm and a volume content ≥5 × 10⁻⁶. 6 pcs / mm 3 .

[0068] In one embodiment of this application, the steel bar has a yield strength ≥720MPa, a tensile strength ≥910MPa, an elongation after fracture ≥15%, a total elongation at maximum force ≥10%, and a strength-to-yield ratio >1.26.

[0069] This application also provides a method for connecting a sleeve for mechanical connection of reinforcing bars to reinforcing bars, including the following steps:

[0070] Rebar processing: External threads are machined at the ends of the rebars using upset straight threads. When the rebar diameter d is 10~25mm, the upset pressure is 10~23MPa, the upset base circle diameter is d+4~d+6mm, and the upset length is d+2mm; when the rebar diameter d is 26~50mm, the upset pressure is 30~55MPa, the upset base circle diameter is d+6.5mm, and the upset length is d+3mm; the rebar is the aforementioned type.

[0071] Sleeve machining: The internal thread of the sleeve matches the external thread of the reinforcing bar; the tensile strength of the sleeve is ≥900MPa, and the elongation after fracture is ≥20%.

[0072] Mechanical connection between reinforcing bar and sleeve: Tighten the reinforcing bar and sleeve with threads, with no more than 1.0 thread of the reinforcing bar not screwed into the sleeve, and the torque is 450~550 N·m.

[0073] The aforementioned reinforcing bars have a tensile strength ≥910MPa, and when paired with sleeves having a tensile strength ≥900MPa, they meet industry standards for strength matching between reinforcing bars and sleeves. The ends of the reinforcing bars are machined with upset straight threads, resulting in an end diameter larger than the middle diameter, enabling them to withstand greater tensile stress and avoiding the risk of brittle fracture at the threads during tension. The torque is carefully controlled to prevent the reinforcing bars from failing to screw into the sleeve or damaging the threaded connection between the sleeve and the reinforcing bar.

[0074] The chemical composition of the sleeve, by mass percentage, includes: C: 0.50~0.60%, Si: 0.50~1.00%, Mn: 0.50~1.00%, Cr: 0.30~0.50%, N: 0.01~0.02%, V, Nb, Ti at least one, and the mass percentages of V, Nb, and Ti satisfy V: 0.10~0.30%, Nb: 0.01~0.05%, Ti: 0.01~0.05%, with the remainder being Fe and unavoidable impurities.

[0075] The microstructure of the sleeve consists of ferrite and pearlite, with pearlite accounting for ≥90% of the volume, a pearlite lamellar spacing of 150~200nm, and an average pearlite cluster size of 5~10μm.

[0076] Furthermore, the external thread of the reinforcing bar has a triangular tooth profile with a tooth angle of 60~70°; when the nominal diameter of the reinforcing bar is <25mm, the pitch P is 2.5mm, and when the nominal diameter of the reinforcing bar is ≥25mm, the pitch P is 3.0mm.

[0077] Furthermore, the outer diameter of the sleeve is 28~76.5mm, the length is 60~120mm, and the thickness is 6~10mm.

[0078] The technical solution of this application will be further described below with reference to some specific embodiments.

[0079] This application provides steel bars of Examples 1-5 and Comparative Examples 1-3 as follows. The steel bars are produced by converter, LF refining, continuous casting, heating, rolling and cooling processes. Table 2 shows the weight percentage of chemical composition of the steel bars of Examples 1-5 and Comparative Examples 1-3. Table 3 shows the process parameters of the steelmaking process of Examples 1-5 and Comparative Examples 1-3. Table 4 shows the process parameters of the LF refining process of Examples 1-5 and Comparative Examples 1-3. Table 5 shows the process parameters of the continuous casting process of Examples 1-5 and Comparative Examples 1-3. Table 6 shows the process parameters of the heating, rolling and cooling processes of Examples 1-5 and Comparative Examples 1-3.

[0080] The microstructure and properties of the steel bars in Examples 1-5 and Comparative Examples 1-3 obtained by referring to the standards GB / T13298 "Metallic materials - Test method for microstructure", GB / T28900 "Steel for reinforced concrete" and GB / T228.1 "Metallic materials - Tensile testing - Part 1 - Test method at room temperature" are shown in Tables 7 and 8.

[0081] The sleeves with the properties in Table 9 and the connection process of the steel bars and sleeves in Table 10 were used for connection. The performance of the steel bar and sleeve connection parts of Examples 1-5 and Comparative Examples 1-3 obtained by referring to the standard JGJ107 "Technical Specification for Mechanical Connection of Steel Bars" is shown in Tables 11 and 12.

[0082]

[0083] Note: In Table 2, V+Nb, C+N, V / N, (V+Nb) / (C+N) are actually [Nb]+[V], [C]+[N], [V] / [N], [Nb]+[V]) / ([C]+[N], just with the parentheses omitted in this table.

[0084]

[0085]

[0086]

[0087]

[0088]

[0089] Note: In the microstructure column of Table 7, F represents ferrite, P represents pearlite, and B represents bainite.

[0090]

[0091]

[0092]

[0093]

[0094]

[0095] As shown in Tables 2-8, the high-strength earthquake-resistant steel bars provided by the embodiments of this application have a yield strength ≥720MPa, tensile strength ≥910MPa, elongation after fracture ≥15%, total elongation at maximum force ≥10%, and strength-to-yield ratio >1.26. Their comprehensive performance is significantly better than that of comparative examples 1-3.

[0096] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0097] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for producing reinforcing steel bars, characterized in that, The steel reinforcement contains the following components by weight percentage: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, and satisfies [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, with the remainder being iron and unavoidable impurities; where [Nb], [V], [N], and [C] are the weight percentages of the corresponding elements. The production method of reinforcing bars includes heating, rolling, and cooling steel billets with the chemical composition as described above to obtain reinforcing bars, wherein: During the heating process, the temperature of the steel billet in the soaking zone of the heating furnace is 1255~1300℃; In the rolling process, the steel billet is placed in the rolling mill and rolled into steel bars, and no water cooling is used throughout the rolling process; In the cooling process, after rolling, the steel bars are directly sent to the cooling bed to cool naturally to room temperature in the air. The temperature of the steel bars on the cooling bed is ≥1125℃, and the cooling rate during natural cooling is <1℃ / s.

2. The method for producing reinforcing bars according to claim 1, characterized in that, Before the heating process, there are also smelting, LF refining and continuous casting processes carried out in sequence. In the LF refining process, vanadium-nitrogen alloy and niobium-iron alloy are added to the ladle. The amount of vanadium-nitrogen alloy added is 2~3 kg / t. The vanadium content in the vanadium-nitrogen alloy is 75~80%, the nitrogen content is 5~10%, and the remainder is iron and other impurity elements. The amount of niobium-iron alloy added is 0.6~1.2 kg / t. The niobium content in the niobium-iron alloy is 40~45%, and the remainder is iron and other impurity elements.

3. The method for producing reinforcing bars according to claim 2, characterized in that, In the LF refining process, nitrogen blowing begins after the ladle is energized. During nitrogen blowing, the nitrogen flow rate is controlled at 850~1000 L / min, the pressure is controlled at 0.4~0.8 MPa, and the nitrogen blowing time t satisfies the following formula: t = λ × (XAB) / 20, where λ is a coefficient related to the temperature of molten steel, and its value is 4~6; X represents the target nitrogen content in the steel reinforcement; A represents the increase in nitrogen content resulting from the addition of vanadium-nitrogen alloy to molten steel; B represents the nitrogen content of the molten steel, ranging from 30 to 60. X, A, and B are all in ppm, and t is in min.

4. The method for producing reinforcing bars according to claim 2, characterized in that, In the smelting process, the deoxidation alloying is carried out by adding the alloy and slag in the following order: silicon-manganese alloy, ferrosilicon alloy, and lime. The amount of silicon-manganese alloy added is 20-30 kg / t, the manganese content in the silicon-manganese alloy is 65-70%, the silicon content is 17-20%, and the remainder is iron and other impurity elements. The amount of ferrosilicon alloy added is 9-14 kg / t, the silicon content in the ferrosilicon alloy is 70-75%, and the remainder is iron and other impurity elements.

5. The method for producing reinforcing bars according to claim 4, characterized in that, During the smelting process, the tapping temperature is controlled at 1595~1635℃, and the argon gas control valve at the bottom of the ladle is opened before tapping. Argon gas is blown throughout the tapping process. The argon gas pressure is controlled at 0.4~0.5MPa in the first 1 / 3 of the tapping process and at 0.3~0.4MPa in the last 2 / 3 of the tapping process.

6. The method for producing reinforcing bars according to claim 2, characterized in that, In the continuous casting process, the casting speed is controlled at 3.0~3.6m / min, and the water flow rate in the crystallizer is 1900~2000L / min.

7. A steel reinforcement, comprising, by weight percentage, the following components: C: 0.24~0.28%, Si: 0.50~0.80%, Mn: 1.20~1.60%, V: 0.165~0.200%, Nb: 0.025~0.050%, N: 0.026~0.040%, and satisfying [Nb]+[V]≤0.23%, [V] / [N]≤6.35, 0.28%≤[C]+[N]≤0.31%, 0.67≤([Nb]+[V]) / ([C]+[N])≤0.77, carbon equivalent Ceq≤0.57%, with the balance being iron and unavoidable impurities; wherein, [Nb], [V], [N], and [C] represent the weight percentage of the corresponding elements.

8. The reinforcing bar according to claim 7, characterized in that, The microstructure of steel bars includes pearlite, ferrite and bainite. Pearlite accounts for 50-70% of the volume, with an average pearlite cluster size of 10-20 μm and a pearlite lamellar spacing of 200-300 nm. Ferrite has an average grain size of 4-8 μm, and bainite accounts for 5-10% of the volume.

9. The reinforcing bar according to claim 8, characterized in that, The microstructure of steel bars includes pearlite, ferrite and bainite. Pearlite accounts for 55-65% of the volume, with an average pearlite cluster size of 11-16 μm and a pearlite lamellar spacing of 230-270 nm. Ferrite has an average grain size of 5-6 μm, and bainite accounts for 6-8% of the volume.

10. The reinforcing bar according to claim 7, characterized in that, The reinforcing steel bars contain carbonitride precipitates with a size ≤50nm and a volume content ≥3×10⁻⁶. 6 pcs / mm 3 .

11. The reinforcing bar according to claim 10, characterized in that, The reinforcing steel bars contain carbonitride precipitates with a size ≤40nm and a volume content ≥5×10⁻⁶. 6 pcs / mm 3 .

12. The reinforcing bar according to claim 7, characterized in that, The steel bars have a yield strength ≥720MPa, tensile strength ≥910MPa, elongation after fracture ≥15%, total elongation at maximum force ≥10%, and a strength-to-yield ratio >1.

26.

13. A method for connecting a sleeve to a reinforcing bar in a mechanical connection, characterized in that, Includes the following steps: Rebar processing: External threads are machined at the ends of the rebars using an upset straight threading method. When the rebar diameter d is 10~25mm, the upset pressure is 10~23MPa, the upset base circle diameter is d+4~d+6mm, and the upset length is d+2mm; when the rebar diameter d is 26~50mm, the upset pressure is 30~55MPa, the upset base circle diameter is d+6.5mm, and the upset length is d+3mm; the rebar used is the rebar described in claim 7. Sleeve machining: The internal thread of the sleeve matches the external thread of the reinforcing bar; the tensile strength of the sleeve is ≥900MPa, and the elongation after fracture is ≥20%. Mechanical connection between reinforcing bar and sleeve: Tighten the reinforcing bar and sleeve with threads, with no more than 1.0 thread of the reinforcing bar not screwed into the sleeve, and the torque is 450~550 N·m.

14. The method for connecting a sleeve for mechanical connection of reinforcing bars to reinforcing bars according to claim 13, characterized in that, The external thread of the reinforcing bar has a triangular tooth profile with a tooth profile angle of 60~70°; when the nominal diameter of the reinforcing bar is <25mm, the thread pitch P is 2.5mm, and when the nominal diameter of the reinforcing bar is ≥25mm, the thread pitch P is 3.0mm.

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