Steel wire rod for 2200MPa-grade bridge cable and preparation method of steel wire rod

Abstract

The invention discloses a steel wire rod for a 2200MPa-grade bridge cable and a preparation method of the steel wire rod. The steel wire rod is prepared from the following chemical components: 0.90 to 0.94 percent of C, 1.00 to 1.15 percent of Si, 0.80 to 0.99 percent of Mn, 0.20 to 0.29 percent of Cr, 0.01 to 0.03 percent of Ti, 0.007 to 0.009 percent of N, 0.03 to 0.05 percent of Nb, 0.0005 to 0.0015 percent of B and the balance of iron and inevitable impurities. The ratio of Nb to Ti is 3-3.5. According to the steel wire rod and the preparation method thereof, the low C content and the high Si content are adopted, the control over the mass ratio of Nb to Ti and the synergistic effect of Nb and Ti are combined, under the condition that precipitation strengthening elements such as Al and V are not added, fine grain strengthening is achieved, a foundation is laid for high strength, high uniformity and high purity of the steel wire rod, and the steel wire rod can be used for preparing the 2200 MPa-grade bridge cable steel wire.

Description

Technical Field

[0001] The present application belongs to the technical field of material preparation, and relates to a 2200MPa-grade wire rod for bridge cables and a preparation method thereof. Background Art

[0002] Bridge cables are the core load-bearing components of long-span bridges, such as cable-stayed and suspension bridges. With the increasing demand for longer spans and heavier loads, the strength of bridge cable wire is also evolving. However, high-strength bridge cable wire has become a material bottleneck in the construction of extra-large bridges.

[0003] Using higher-strength steel wire not only improves the safety factor of bridge cables but also reduces material usage, increases bridge live loads, and lowers construction costs. Over the past decade, the strength of bridge cable steel wire has rapidly increased from 1770 MPa to 2100 MPa. However, as the strength level increases, the challenge of achieving this increase also increases, placing correspondingly higher demands on wire rod. Wire rod must maintain high strength while also possessing excellent properties such as high purity, high uniformity, high surface quality, and high plasticity.

[0004] Existing wire rods usually improve their performance by adding a large amount of alloys such as V and Al, but the alloy cost is high, which is not conducive to mass production. Summary of the Invention

[0005] The purpose of this application is to provide a 2200MPa grade wire rod for bridge cables and a preparation method thereof.

[0006] To achieve the above-mentioned application purpose, one embodiment of the present application provides a 2200MPa-level wire rod for bridge cables, the chemical composition of the wire rod comprising, by mass percentage: C 0.90-0.94%, Si 1.00-1.15%, Mn 0.80-0.99%, Cr 0.20-0.29%, Ti 0.01-0.03%, N 0.007-0.009%, Nb 0.03-0.05%, B 0.0005-0.0015%, and the remainder being iron and unavoidable impurities; wherein, Nb / Ti = 3-3.5.

[0007] As a further improvement of an embodiment of the present application, some impurities are calculated as follows in percentage by mass: S≤0.005%, P≤0.01%, Cu≤0.05%, Ni≤0.05%, O≤0.0015%, and H≤0.0001%.

[0008] As a further improvement of an embodiment of the present application, the diameter of the wire rod is 12.5-13.5 mm, the tensile strength Rm is ≥1620 MPa, and the cross-sectional shrinkage Z is ≥30%.

[0009] As a further improvement of one embodiment of the present application, the wire rod can be used to prepare galvanized steel wire or galvanized aluminum steel wire with a diameter of 5-6 mm, a tensile strength ≥2200 MPa, a number of twisting turns ≥25 turns, and an elongation after break ≥6%.

[0010] To achieve the above application objectives, one embodiment further provides a method for preparing 2200 MPa grade steel wire rod, which comprises the following steps: molten steel smelting, bloom continuous casting, bloom grinding, high-speed wire rolling, Stelmor cooling, and salt bath heat treatment.

[0011] In the high-speed wire rolling process, the intermediate billet after blanking and grinding is heated and then sequentially subjected to rough rolling, finishing rolling, and reducing and sizing rolling to form wire rods. The soaking section temperature during heating is 1180-1200° C., the air-fuel ratio in the soaking section is less than 0.55, the starting rolling temperature of rough rolling is 1035-1055° C., the inlet temperature of finishing rolling is 890-920° C., the temperature of reducing and sizing rolling is 820-860° C., and the spinning temperature is 840-860° C.

[0012] In the Stelmor cooling process, before the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≥10°C / s. After the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≤5°C / s. When the temperature of the wire rod drops to 500°C, the wire rod is coiled.

[0013] In the salt bath heat treatment process, after the wire rod is paid off, offline heating and salt bath are carried out in sequence. The offline heating includes heating zone 1, heating zone 2, and heating zone 3. The heating temperature of heating zone 1 is 930-940°C, the heating temperature of heating zone 2 is 920-930°C, the heating temperature of heating zone 3 is 910-920°C, the salt bath temperature is 535-545°C, and the salt bath time is 8-10 minutes.

[0014] As a further improvement of one embodiment of the present application, in the salt bath heat treatment process, the molten salt and the wire rod are controlled to flow in opposite directions during the salt bath, and the flow rate of the molten salt relative to the wire rod is ≥4m / s, and the surface temperature rise of the wire rod during the pearlite phase transformation is controlled to be ≤15°C.

[0015] As a further improvement of an embodiment of the present application, in the salt bath heat treatment process, the temperature fluctuation of the molten salt pool during the salt bath is controlled to be ≤±1°C.

[0016] As a further improvement of an embodiment of the present application, in the salt bath heat treatment process, the total time of the offline heating is 10 to 15 minutes, and the heating time of one heating zone is ≤3 minutes.

[0017] As a further improvement of one embodiment of the present application, in the salt bath heat treatment process, the wire rod is sent to a heating furnace for heating during offline heating, the atmosphere in the heating furnace is a nitrogen-methanol atmosphere, and the carbon content in the atmosphere is 0.88-0.92%.

[0018] As a further improvement of one embodiment of the present application, the molten steel smelting process includes the following steps performed in sequence:

[0019] (1) Desulfurization of molten iron

[0020] Using KR desulfurization technology, adding desulfurizer CaO to remove sulfur from molten iron, the sulfur content of the molten iron after desulfurization is less than 0.005%;

[0021] (2) Converter smelting

[0022] The desulfurized molten iron and scrap steel are added to the converter for oxygen blowing smelting. The tapping temperature is 1580-1620℃. The carbon content in the molten steel is 0.10-0.35% and the phosphorus content is ≤80ppm. Deoxidizer is added when 1 / 3 of the steel is tapped to avoid slag.

[0023] (3) LF refining

[0024] The steel produced in the converter is transferred to the LF furnace for refining, heated and alloyed according to the chemical composition design plan. During the refining process, slag formation is controlled to ensure that the binary basicity of the slag is 2.5-2.7. After the refining is completed, argon soft stirring is performed for 20 minutes, and carbonized rice husk insulation is added.

[0025] (4) RH vacuum treatment

[0026] The molten steel obtained by LF refining was transferred to the RH furnace for vacuum degassing with a degassing time of 20 min.

[0027] Compared with the prior art, the advantages of this application include:

[0028] (1) The chemical composition design scheme of the present application adopts a lower C content and a higher Si content, and combines the control of the mass ratio of Nb to Ti and the synergistic effect of the two. Without adding precipitation strengthening elements such as Al and V, it achieves fine grain strengthening, and can reduce the size of the precipitate phase to 5-10nm. At the same time, the re-dissolution temperature of the precipitate phase is increased from 950℃ to 1020℃, which can alleviate the segregation problem of the continuous casting billet during the continuous casting process and no longer need to go through a high-temperature diffusion process, laying the foundation for the wire rod to have high strength, high uniformity and high purity. It can be used to prepare 2200MPa-level bridge cable wire, and effectively controls the amount of alloy addition, greatly reduces the cost, and is conducive to mass production.

[0029] (2) Based on the chemical composition design scheme, the present application combines the control of rolling, cooling and salt bath heat treatment processes, promotes the dynamic precipitation of niobium carbonitride and inhibits the growth of austenite grains through low-temperature large deformation rolling, and then inhibits the dissolution of niobium carbonitride through low-temperature offline heating in the salt bath heat treatment, thereby improving the fine grain strengthening effect of the wire rod. Then, through the temperature and time control of the salt bath, the wire rod completes the pearlite phase transformation within a lower and narrower temperature range, thereby achieving higher strength and plasticity at a lower alloy addition amount, solving the matching problem of high strength and high plasticity, and improving the uniformity of the structure, laying the foundation for the preparation of 2200MPa-grade bridge cable steel wire that meets the use requirements. DETAILED DESCRIPTION

[0030] The technical solution of the present application is further introduced below in conjunction with specific implementation methods. The technical content described below is only an exemplary implementation method of the present application and is not intended to limit the scope of protection of the present application. The scope of protection of the present application is determined by the attached claims.

[0031] One embodiment of the present application provides a 2200 MPa-grade wire rod for bridge cables and a preparation method thereof.

[0032] The chemical composition of the wire rod includes, by mass percentage, C 0.90-0.94%, Si 1.00-1.15%, Mn 0.80-0.99%, Cr 0.20-0.29%, Ti 0.01-0.03%, N 0.007-0.009%, Nb 0.03-0.05%, B 0.0005-0.0015%, and the balance is iron and unavoidable impurities; wherein Nb / Ti=3-3.5.

[0033] The functions of the various chemical elements of the wire rod described in this embodiment are described in detail below.

[0034] Carbon is the most essential strengthening element in steel. Every 0.01% increase in carbon content increases wire rod strength by approximately 10 MPa. However, excessive carbon can promote the precipitation of proeutectoid cementite at the center segregation site. In severe cases, this can lead to the formation of network cementite, reducing the plasticity of the wire rod and causing wire breakage during drawing. While carbon is the most economical element for increasing strength, excessive carbon content can lead to excessive dissolution into the ferrite during drawing, affecting the wire's torsional properties. Therefore, the carbon content in this application is limited to 0.90-0.94%.

[0035] Si is a ferrite-strengthening element that can increase the strength of ferrite through solid solution strengthening. Si is also an important deoxidizer, helping to reduce the oxygen content in steel and reduce inclusions. Furthermore, Si enrichment at the ferrite / cementite interface helps prevent cementite decomposition during hot-dip galvanizing and stabilization treatments, improving thermal stability during processing. However, excessive Si can cause decarburization, reducing surface quality and wire rod plasticity. Therefore, the Si content in this application is limited to 1.00-1.15%.

[0036] Mn is mainly used in steel to increase the strength of steel. It can also increase the stability of austenite and reduce the phase transition temperature. Mn can also change the sulfide composition and reduce the harmful effects of S. Mn has little effect on the torsional performance of steel wire. Therefore, the Mn content in this application is limited to 0.80-0.99%.

[0037] Cr is a carbide-forming element. In steel, it primarily exists within cementite lamellae and forms alloy cementite through substitution. The addition of Cr improves austenite stability and, at the same cooling rate, refines the spacing between pearlite lamellae. However, excessive Cr content significantly reduces the torsional properties of the finished steel wire. Taking into account both product performance and cost factors, the Cr content in this application is limited to 0.20-0.29%.

[0038] The main purpose of adding Ti is to combine with the free nitrogen in the steel to form dispersed and fine TiN precipitation, which refines the grains. To ensure that the amount and size of TiN precipitation are not too large, the Ti content in this application is controlled to 0.01-0.03%, and the N content is controlled to 0.007-0.009%. In addition, the introduction of TiN can also form a strong trapping effect for hydrogen, which can significantly improve the stress corrosion resistance of the steel wire.

[0039] However, precipitation strengthening elements such as Al and V will form weak traps for H, which will reduce the stress corrosion resistance of the steel wire. Therefore, the use of precipitation strengthening elements such as Al and V is prohibited in this application.

[0040] The addition of Nb is mainly to work synergistically with Ti, utilizing the formed carbonitride precipitation phase to further enhance the effect of grain refinement. The amount of Nb added needs to be controlled at 0.03-0.05%. Excessive Nb will form coarse carbides, which will reduce the refinement effect. Specifically, TiN is usually formed preferentially in the high-temperature austenite region (i.e., above 1200°C), while NbC and Nb(C,N) precipitate in the γ→α phase transformation or low-temperature ferrite; when Nb / Ti>3, that is, the mass ratio of Nb to Ti>3, excess Nb will inhibit the coarsening of TiN and form a finer composite precipitate phase (Ti,Nb)(C,N).

[0041] In high carbon steel, B is mainly concentrated on the original austenite grain boundaries, which plays a role in inhibiting the formation of grain boundary ferrite. In this application, the B content is limited to 0.0005-0.0015%.

[0042] In summary, the chemical composition design scheme of the present application adopts a lower C content and a higher Si content, and combines the control of the mass ratio of Nb to Ti and the synergistic effect of the two. Without adding precipitation strengthening elements such as Al and V, it achieves fine grain strengthening, and can reduce the size of the precipitate phase to 5-10nm. At the same time, the re-dissolution temperature of the precipitate phase is increased from 950℃ to 1020℃, which can alleviate the segregation problem of the continuous casting billet during the continuous casting process, and no high-temperature diffusion process is required, laying the foundation for the wire rod to have high strength, high uniformity and high purity. It can be used to prepare 2200MPa-grade bridge cable steel wire, and effectively controls the amount of alloy addition, greatly reduces the cost, and is conducive to mass production.

[0043] Preferably, in the wire rod, some impurities are as follows, calculated by mass percentage: S≤0.005%, P≤0.01%, Cu≤0.05%, Ni≤0.05%, O≤0.0015%, and H≤0.0001%.

[0044] Impurity elements can deteriorate the toughness and ductility of steel and should be controlled as much as possible within the constraints of process and cost. Especially H, which can significantly reduce the stress corrosion resistance of steel wire, must be controlled within H ≤ 0.0001%.

[0045] Next, the preparation method of the above-mentioned wire rod is introduced.

[0046] The preparation method comprises the following steps: molten steel smelting, bloom continuous casting, bloom grinding, high-speed wire rolling, Stelmor cooling and salt bath heat treatment.

[0047] Among them, in the high-speed wire rolling process, the intermediate billet after blanking and grinding is heated, and then rolled into wire rod through rough rolling, finishing rolling and sizing rolling in sequence. The temperature of the soaking section during heating is 1180-1200°C, the air-fuel ratio of the soaking section is <0.55, the starting rolling temperature of rough rolling is 1035-1055°C, the inlet temperature of finishing rolling is 890-920°C, the temperature of sizing rolling is 820-860°C, and the spinning temperature is 840-860°C.

[0048] In the Stelmor cooling process, before the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≥10°C / s. After the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≤5°C / s. When the temperature of the wire rod drops to 500°C, it is coiled.

[0049] In the salt bath heat treatment process, after the wire rod is paid off, offline heating and salt bath are carried out in sequence. The offline heating includes heating zone 1, heating zone 2, and heating zone 3. The heating temperature of heating zone 1 is 930-940°C, the heating temperature of heating zone 2 is 920-930°C, the heating temperature of heating zone 3 is 910-920°C, the salt bath temperature is 535-545°C, and the salt bath time is 8-10 minutes.

[0050] By heating the wire rod in sections offline before the salt bath and controlling the heating temperature of each section in heating zone 1, heating zone 2, and heating zone 3, on the one hand, the heating temperature does not exceed 940°C, which can effectively control the dissolution of niobium carbonitride and inhibit grain growth. On the other hand, through segmented heating with gradient cooling, the wire rod can quickly pass through the decarburization temperature range in the early stage, reducing the thickness of the decarburization layer. At the same time, the heating time of the wire rod can be extended, which is more conducive to the homogenization of the wire rod structure.

[0051] Based on the chemical composition design scheme, this application combines the control of rolling, cooling and salt bath heat treatment processes to promote the dynamic precipitation of niobium carbonitride and inhibit the growth of austenite grains through low-temperature and large deformation rolling. Furthermore, through low-temperature offline heating in the salt bath heat treatment, the dissolution of niobium carbonitride is inhibited, and the fine grain strengthening effect of the wire rod is improved. Furthermore, through the temperature and time control of the salt bath, the wire rod completes the pearlite phase transformation within a lower and narrower temperature range, thereby achieving higher strength and plasticity with a lower alloy addition amount, solving the problem of matching high strength and high plasticity, and improving the uniformity of the structure, laying the foundation for the preparation of 2200MPa-grade bridge cable steel wire that meets the use requirements.

[0052] Preferably, in the salt bath heat treatment process, the molten salt and the wire rod are controlled to flow in opposite directions during the salt bath, and the flow rate of the molten salt relative to the wire rod is ≥4 m / s, and the surface temperature rise of the wire rod during the pearlite phase transformation is controlled to be ≤15°C.

[0053] In this way, since the salt bath temperature is 535-545℃, the wire rod will undergo a phase transformation from austenite to pearlite within this temperature range. During this process, the wire rod will release heat and generate latent heat of phase transformation. By controlling the flow direction and flow rate of the molten salt relative to the wire rod, the latent heat of phase transformation of the wire rod can be quickly conducted away, so that the surface temperature of the wire rod does not rise higher than 15℃.

[0054] Preferably, during the salt bath heat treatment process, the temperature fluctuation of the molten salt pool during the salt bath is controlled to be ≤±1° C. Thus, by controlling the temperature fluctuation of the molten salt pool, the uniformity of the wire rod cooling can be improved, thereby increasing the number of twists of the final prepared steel wire.

[0055] Specifically, the temperature fluctuations of the molten salt pool are automatically controlled by the equipment, which improves the degree of automation of the entire production process.

[0056] Preferably, in the salt bath heat treatment process, the total offline heating time is 10 to 15 minutes, and the heating time of one heating zone is ≤ 3 minutes. This can avoid the re-dissolution of niobium carbonitride caused by long-term high-temperature heating.

[0057] Preferably, during the salt bath heat treatment process, the wire rod is fed into a heating furnace for heating during offline heating, and the atmosphere in the heating furnace is a nitrogen-methanol atmosphere with a carbon content of 0.88-0.92%. Nitrogen-methanol is a protective gas formed by mixing nitrogen and methanol in a certain proportion. Through the synergistic effect of the inert gas (nitrogen) and the reactive gas (methanol), and by controlling the carbon content of the atmosphere in the heating furnace, the reaction environment is controlled to prevent decarburization and carburization of the wire rod substrate surface.

[0058] The molten steel smelting process includes the following steps performed in sequence:

[0059] (1) Desulfurization of molten iron

[0060] KR desulfurization technology is used to add desulfurizer CaO to remove sulfur from molten iron. After desulfurization, the S content of the molten iron is less than 0.005%.

[0061] (2) Converter smelting

[0062] The desulfurized molten iron and scrap steel are added into the converter for oxygen blowing smelting. The tapping temperature is 1580-1620℃. The C content in the molten steel is 0.10-0.35% and P≤80ppm. Deoxidizer is added when 1 / 3 of the steel is tapped to avoid slag during tapping.

[0063] (3) LF refining

[0064] The steel produced in the converter is transferred to the LF furnace for refining, heated and alloyed according to the above-mentioned chemical composition design scheme. The slag formation is controlled during the refining process so that the binary basicity of the slag is 2.5-2.7. After the refining is completed, argon soft stirring is carried out for 20 minutes, and carbonized rice husk insulation is added.

[0065] (4) RH vacuum treatment

[0066] The molten steel obtained by LF refining was transferred to the RH furnace for vacuum degassing with a degassing time of 20 min.

[0067] Through the above-mentioned molten steel smelting process, the chemical composition of the molten steel obtained by smelting can be roughly consistent with the chemical composition design scheme of the final wire rod, and the control requirements of impurity elements and inclusions can be achieved, so that the final molten steel has a higher purity, laying the foundation for the final prepared wire rod to have high purity, uniform structure, excellent surface quality, high strength and plasticity.

[0068] In the bloom continuous casting process, the molten steel obtained in the molten steel smelting process is continuously cast into a continuous casting bloom with a cross-sectional size of 300mm×360mm to 300mm×400mm, preferably 300mm×390mm.

[0069] Preferably, during the continuous casting process, pouring is started with a superheat of no more than 20°C and a constant casting speed of 0.5m / min, mist cooling is adopted in the secondary cooling zone, the water content is controlled to be 0.18L / kg, and the reduction is ≥20mm, so as to minimize the macro segregation of the continuous casting billet.

[0070] In the blanking and grinding process, the continuous casting blank is blanked to obtain an intermediate blank, which is then fully ground to eliminate surface defects of the continuous casting blank. Preferably, the single-side grinding depth is ≥1.0mm and the corner grinding depth is ≥1.5mm.

[0071] After testing, the diameter of the wire rod is 12.5-13.5 mm; in the wire rod structure, the average original austenite grain size is 9-11 μm, and the sorbite content is ≥95%; in the mechanical properties, the tensile strength Rm is ≥1620 MPa, and the cross-sectional shrinkage rate Z is ≥30%.

[0072] Furthermore, the wire rod is sequentially subjected to pickling, phosphating, drawing, hot-dip galvanizing or hot-dip galvanized aluminum processes to obtain galvanized steel wire or galvanized aluminum steel wire.

[0073] After testing, the obtained galvanized steel wire and galvanized aluminum steel wire have a diameter of 5 to 6 mm, a tensile strength of ≥2200 MPa, a number of twisting circles of ≥25 circles, and an elongation after fracture of ≥6%.

[0074] To sum up, the wire rod of the present application has the advantages of high purity, uniform structure, high surface quality, high strength, good plasticity, etc., and low production cost. It can be used to produce galvanized steel wire or galvanized aluminum steel wire with a diameter of 5-6 mm, tensile strength ≥2200 MPa, number of torsion circles ≥25 circles, and elongation after break ≥6%. The strength level reaches 2200 MPa, which meets the increasingly high quality requirements of wire rods for large-span bridge cables.

[0075] To make the technical solutions and advantages of an embodiment of the present application more clear, the embodiment will be further described below in conjunction with Examples 1 to 3 according to an embodiment of the present application. Obviously, the described Examples 1 to 3 are only part of the embodiments of the present application, not all of the embodiments.

[0076] Specifically, Examples 1 to 3 all provide a wire rod, the chemical composition of which is shown in Table 1 in terms of mass percentage, with the remainder being iron and unavoidable impurities.

[0077] Table 1

[0078]

[0079] The preparation methods of the wire rods of Examples 1 to 3 are as follows:

[0080] (1) Desulfurization of molten iron

[0081] KR desulfurization technology is used to add desulfurizer CaO to remove sulfur from molten iron. After desulfurization, the S content of the molten iron is less than 0.005%.

[0082] (2) Converter smelting

[0083] The desulfurized molten iron and scrap steel are added to the converter for oxygen blowing smelting. The tapping temperature is 1580-1620℃. The C content in the molten steel is 0.10-0.35% and P is ≤80ppm. Deoxidizer is added when 1 / 3 of the steel is tapped to avoid slag.

[0084] (3) LF refining

[0085] The steel produced in the converter was transferred to the LF furnace for refining, heated, and alloyed according to the chemical composition design scheme in Table 1. During the refining process, slag formation was controlled so that the binary basicity of the slag was 2.5-2.7. After the refining was completed, argon soft stirring was performed for 20 minutes, and carbonized rice husk insulation was added.

[0086] (4) RH vacuum treatment

[0087] The molten steel obtained by LF refining was transferred to the RH furnace for vacuum degassing with a degassing time of 20 min.

[0088] (5) Bloom continuous casting

[0089] The molten steel obtained in the molten steel smelting process is continuously cast into continuous casting billets with a cross-sectional size of 300 mm×390 mm.

[0090] During the continuous casting process, pouring was started at a constant casting speed of 0.5 m / min. The superheat was shown in Table 2. Mist cooling was used in the secondary cooling zone. The specific water content was controlled at 0.18 L / kg. The reduction was shown in Table 2.

[0091] (6) Opening and grinding

[0092] After the continuous casting billet was opened, an intermediate billet with a cross-sectional size of 140 mm × 140 mm was obtained, and then the entire billet was ground. The single-side grinding depth and the corner grinding depth were shown in Table 2.

[0093] Table 2

[0094] Superheat / ℃ Reduction amount / mm Single side grinding depth / mm Corner grinding depth / mm Example 1 12 20 1.2 1.8 Example 2 15 25 1.3 1.8 Example 3 16 22 1.2 1.8

[0095] (7) High-speed wire rolling

[0096] After the blanking and grinding, the intermediate billet is heated and then rolled into wire rod through roughing, finishing, and reducing and sizing rolling. The soaking zone temperature during heating is 1180-1200°C, the air-fuel ratio in the soaking zone is <0.55, the starting temperature of roughing is 1035-1055°C, the inlet temperature of finishing is 890-920°C, the temperature of reducing and sizing is 820-860°C, and the laying temperature is 840-860°C.

[0097] (8) Stelmor cooling

[0098] The wire rod produced by the high-speed wire rolling process is cooled on the Stelmor cooling line. Before the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled as shown in Table 3. After the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≤5°C / s. When the temperature of the wire rod drops to 500°C, it is coiled.

[0099] (9) Salt bath heat treatment

[0100] After the wire rod is paid out, it is sequentially subjected to offline heating and salt bath, and then coiled again to obtain the finished wire rod.

[0101] During offline heating, the wire rod is sent into a heating furnace for heating. The atmosphere in the heating furnace is a nitrogen-methanol atmosphere, and the carbon content in the atmosphere is 0.88-0.92%.

[0102] Offline heating includes heating zone 1, heating zone 2, and heating zone 3. The heating temperatures and heating times of heating zone 1, heating zone 2, and heating zone 3 are shown in Table 3, respectively. The salt bath temperature and salt bath time are shown in Table 3, respectively.

[0103] During the salt bath, the molten salt and the wire rod are controlled to flow in opposite directions, and the flow rate of the molten salt relative to the wire rod is ≥4m / s. The surface temperature rise of the wire rod during the pearlite phase transformation is controlled to be ≤15°C, and the temperature fluctuation of the molten salt pool during the salt bath is controlled to be ≤±1°C.

[0104] Table 3

[0105]

[0106] The microstructure and mechanical properties of the wire rods of Examples 1 to 3 were tested respectively. The results are shown in Table 4, which specifically include the diameter of the wire rod, the average prior austenite grain size, the sorbite content, the tensile strength, and the cross-sectional reduction rate Z.

[0107] Table 4

[0108]

[0109] Furthermore, the wire rods of Examples 1 to 3 were subjected to pickling, phosphating, drawing, hot-dip galvanizing or hot-dip galvanized aluminum processes to obtain galvanized steel wire or galvanized aluminum steel wire. The diameter and mechanical properties of the finished steel wire are shown in Table 5.

[0110] Table 5

[0111] Diameter / mm Tensile strength / MPa Twist / turn Elongation after break / % Example 1 5.6 2231 26 6.0 Example 2 5.6 2250 25 6.5 Example 3 5.6 2226 28 6.8

[0112] In summary, it can be seen that the wire rods of Examples 1 to 3 have the advantages of high purity, uniform structure, high surface quality, high strength, good plasticity, etc., and low production cost. They can be used to produce galvanized steel wire or galvanized aluminum steel wire with a diameter of 5 to 6 mm, tensile strength ≥2200 MPa, number of torsion circles ≥25 circles, and elongation after fracture ≥6%. The strength level reaches 2200 MPa, which meets the increasingly high quality requirements of wire rods for large-span bridge cables.

[0113] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of the present application. They are not intended to limit the scope of protection of the present application. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present application should be included in the scope of protection of the present application.

Claims

1. A 2200MPa grade wire rod for bridge cables, characterized in that: The chemical composition of the wire rod includes, by mass percentage, C 0.90-0.94%, Si 1.00-1.15%, Mn 0.80-0.99%, Cr 0.20-0.29%, Ti 0.01-0.03%, N 0.007-0.009%, Nb 0.03-0.05%, B 0.0005-0.0015%, and the balance is iron and unavoidable impurities; wherein Nb / Ti=3-3.

5.

2. The 2200MPa grade wire rod for bridge cables according to claim 1, characterized in that: Some impurities are calculated in mass percentage as follows: S≤0.005%, P≤0.01%, Cu≤0.05%, Ni≤0.05%, O≤0.0015%, and H≤0.0001%.

3. The 2200MPa grade wire rod for bridge cables according to claim 1, characterized in that: The diameter of the wire rod is 12.5-13.5 mm, the tensile strength Rm is greater than or equal to 1620 MPa, and the cross-sectional shrinkage Z is greater than or equal to 30%.

4. The 2200MPa grade wire rod for bridge cables according to claim 1, characterized in that: The wire rod can be used to prepare galvanized steel wire or galvanized aluminum steel wire with a diameter of 5-6 mm, a tensile strength of ≥2200 MPa, a number of twisting turns of ≥25 turns, and an elongation after fracture of ≥6%.

5. A method for preparing a 2200 MPa grade wire rod for bridge cables according to any one of claims 1 to 4, characterized in that: The process includes molten steel smelting, bloom continuous casting, bloom grinding, high-speed wire rolling, Stelmor cooling and salt bath heat treatment. In the high-speed wire rolling process, the intermediate billet after blanking and grinding is heated and then sequentially subjected to rough rolling, finishing rolling, and reducing and sizing rolling to form wire rods. The soaking section temperature during heating is 1180-1200° C., the air-fuel ratio in the soaking section is less than 0.55, the starting rolling temperature of rough rolling is 1035-1055° C., the inlet temperature of finishing rolling is 890-920° C., the temperature of reducing and sizing rolling is 820-860° C., and the spinning temperature is 840-860° C. In the Stelmor cooling process, before the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≥10°C / s. After the temperature of the wire rod drops to 670°C, the cooling rate of the wire rod is controlled to be ≤5°C / s. When the temperature of the wire rod drops to 500°C, the wire rod is coiled. In the salt bath heat treatment process, after the wire rod is paid off, offline heating and salt bath are carried out in sequence. The offline heating includes heating zone 1, heating zone 2, and heating zone 3. The heating temperature of heating zone 1 is 930-940°C, the heating temperature of heating zone 2 is 920-930°C, the heating temperature of heating zone 3 is 910-920°C, the salt bath temperature is 535-545°C, and the salt bath time is 8-10 minutes.

6. The preparation method according to claim 5, characterized in that In the salt bath heat treatment process, the molten salt and the wire rod are controlled to flow in opposite directions during the salt bath, and the flow rate of the molten salt relative to the wire rod is ≥4 m / s, and the surface temperature rise of the wire rod during the pearlite phase transformation is controlled to be ≤15°C.

7. The preparation method according to claim 5, characterized in that In the salt bath heat treatment process, the temperature fluctuation of the molten salt pool during the salt bath is controlled to be ≤±1°C.

8. The preparation method according to claim 5, characterized in that In the salt bath heat treatment process, the total off-line heating time is 10 to 15 minutes, and the heating time of the heating zone is ≤3 minutes.

9. The preparation method according to claim 5, characterized in that In the salt bath heat treatment process, during offline heating, the wire rod is sent to a heating furnace for heating. The atmosphere in the heating furnace is a nitrogen-methanol atmosphere, and the carbon content in the atmosphere is 0.88-0.92%.

10. The preparation method according to claim 5, characterized in that The molten steel smelting process includes the following steps performed in sequence: (1) Desulfurization of molten iron Using KR desulfurization technology, adding desulfurizer CaO to remove sulfur from molten iron, the sulfur content of the molten iron after desulfurization is less than 0.005%; (2) Converter smelting The desulfurized molten iron and scrap steel are added to the converter for oxygen blowing smelting. The tapping temperature is 1580-1620℃. The carbon content in the molten steel is 0.10-0.35% and the phosphorus content is ≤80ppm. Deoxidizer is added when 1 / 3 of the steel is tapped to avoid slag. (3) LF refining The steel produced in the converter is transferred to the LF furnace for refining, heated and alloyed according to the chemical composition design plan. During the refining process, slag formation is controlled to ensure that the binary basicity of the slag is 2.5-2.

7. After the refining is completed, argon soft stirring is performed for 20 minutes, and carbonized rice husk insulation is added. (4) RH vacuum treatment The molten steel obtained by LF refining was transferred to the RH furnace for vacuum degassing with a degassing time of 20 min.

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