A B-containing hot-rolled wire rod for 1770MPa grade stranded wire and a manufacturing method thereof

Abstract

The invention relates to a B-containing hot-rolled wire rod for 1770MPa grade stranded wire and a manufacturing method thereof. After rolling and spinning into a wire rod with C-Si-Mn-B component, the wire rod is subjected to an online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥35°C / s to enter a pearlite phase region to form a structure mainly composed of pearlite, and then isothermal tempering is performed to promote microspheroidization of part of the structure. Finally, the hot-rolled wire rod is subjected to roller slow cooling to form a mixed structure composed of tempered pearlite, tempered ferrite and microspheroidized carbide, wherein the volume percentage of tempered pearlite is ≥93%, the high strength and high plasticity matching of the wire rod can be achieved, the structure uniformity and cold working performance are better, the tensile strength is 1130-1180MPa, and the cross-sectional shrinkage is 48%-53%. The wire rod is used to manufacture 1770MPa grade stranded wire, can meet the requirements of improving the pass compression rate, the wire drawing speed and the area reduction rate, and reduces the risk of wire breakage.

Description

Technical Field

[0001] The invention belongs to the field of stranded wire hot-rolled wire rods, and in particular relates to a B-containing hot-rolled wire rod for 1770MPa-grade stranded wire and a manufacturing method thereof. Background Art

[0002] As cold-drawn steel wire has the characteristics of high strength and even ultra-high strength, prestressed steel strands are widely used as structural connection anchoring mechanisms in metallurgy, machinery, chemical industry, petroleum, road traffic and other fields. 1770MPa grade steel strands are generally made of hot-rolled wire rods with specifications of 13~15mm as raw materials. After pickling, they are drawn by a straight-line wire drawing machine for 9 passes and then twisted into strands. With the process of steel wire processing, the compression rate, wire drawing speed and surface reduction rate are greatly improved. In order to avoid the risk of wire breakage during cold processing and the risk of wire breakage of steel strands, higher requirements are put forward for the organizational properties, uniformity and cold processing properties of the steel strand parent material wire rod.

[0003] At present, the hot-rolled wire rod for 1770MPa grade stranded wire generally adopts 82B material as raw material. In order to further improve the strength and hardenability of the wire rod, elements such as Cr and V are added to the composition, and pearlite steel is obtained by combining the Stelmor controlled cooling line after rolling. For example, the Φ14mm high-strength wire rod for 1770MPa prestressed steel strand disclosed in patent CN104805363A and its production method adopt C-Si-Mn-Cr-V composition design, combined with rolling cooling to obtain a wire rod with an average tensile strength of 1241MPa and a cross-sectional shrinkage rate of 33%. Therefore, the hot-rolled wire rod with matching strength and high plasticity, better uniformity of organization and cold working performance still has the following technical difficulties:

[0004] 1. Cr is a carbide-forming element, which can move the carbon curve to the upper left. It is beneficial to reduce the pearlite transformation temperature on the Stelmor air-cooled roller after rolling, and then increase the troostite content to improve the overall performance, but it is easy to form large carbides and significantly reduce the plasticity of the material. V can refine the grains and form precipitation strengthening, but as a precious metal, it also increases the material cost. Therefore, in order to appropriately reduce the difficulty of improving plasticity and material costs, omitting Cr and V elements will bring about a greater strength loss, resulting in the need for a larger area reduction rate to improve the strength of the steel wire, which will increase the risk of wire breakage during the process.

[0005] Second, due to the high C and Mn contents, and the Cr content will further increase the instability of the center segregation control during continuous casting of the steel billet, and also significantly improve the hardenability of the wire rod. In order to avoid the excessive network cementite formed during the controlled cooling process after rolling due to the wire rod being subjected to the secondary cementite precipitation temperature of 750-820°C for too long, which significantly deteriorates the cold working performance and organizational uniformity of the wire rod, the existing technology will adopt strong air cooling after wire laying. However, on the one hand, due to the certain segregation of the wire rod and the low maximum cooling rate of the wire rod, the organization still has network carbides and a large number of tertiary carbides. Carbon body precipitation and coarse cementite carbides will affect the uniformity of the organization, plasticity and toughness, and fatigue resistance, making the performance of the wire rod uneven and making it difficult to improve plasticity. On the other hand, the uncontrollable air volume and direction and large fluctuations in air temperature during strong air cooling further increase the temperature difference between the windward side and the leeward side of the wire rod and between different positions in the same circle, resulting in a higher risk of the emergence of abnormal organizations such as martensite and bainite. The uneven content of pearlite or troostite will increase the brittleness of the wire rod and the instability of its drawing properties, deteriorate the cold processing properties, and cause the risk of wire breakage during drawing and twisting.

[0006] 3. Even if the air cooling treatment is adopted, firstly, fast and then slow, on the one hand, the minimum cooling rate of the wire rod is limited, resulting in a short time and incubation period for the wire rod to pass through the pearlite phase region, and the organization fails to fully transform into pearlite or troostite, resulting in a decrease in the pearlite fraction in the organization, a high proportion of ferrite and loss of wire rod strength. In the subsequent cooling process, the residual austenite organization is still at risk of transforming into an abnormal organization, and the control of the organization and performance of the wire rod is not stable enough; on the other hand, the cementite in the coarse flake pearlite obtained by diffusion-type phase transformation of austenite through continuous cooling is coarse and more difficult to soften. Moreover, the wire rod is in a low temperature state after the phase transformation and lacks thermal power, which will make the organization stress and dislocation density higher. Finally, the wire rod has insufficient plasticity and cold working performance, and is easy to break when the subsequent compression rate and drawing speed are high. Summary of the invention

[0007] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a B-containing hot-rolled wire rod for 1770MPa grade stranded wire and a manufacturing method thereof, which improves the strengthening effect of the carbon element, replaces the addition of Cr and V with a trace amount of B, achieves high strength and high plasticity matching of the wire rod, and has better organizational uniformity and cold processing performance. It is used to manufacture 1770MPa grade stranded wire, can adapt to the requirements of improving the pass compression rate, wire drawing speed and surface reduction rate, and reduce the risk of wire breakage.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] A method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire, the manufacturing method comprising:

[0010] The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.68%-0.73%, Si: 0.15%-0.3%, Mn: 0.20%-0.40%, B: 0.0010%-0.0020%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; after the wire rod is spun into a wire rod at a spinning temperature of ≥885°C, it is subjected to an online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥35°C / s to enter the pearlite phase region to form a structure mainly composed of pearlite, and then isothermal tempering is performed to promote microspheroidization of some structures, and finally it is slowly cooled by a roller to produce a hot-rolled wire rod with a mixed structure composed of tempered pearlite, tempered ferrite and microspheroidized carbide, and the volume percentage of tempered pearlite in the mixed structure is ≥93%.

[0011] The design basis of the chemical composition and mass percentage of the above hot rolled wire rod includes:

[0012] (1) Carbon: As an economical and effective strengthening element, carbon increases the yield strength and tensile strength of the material with the increase of carbon content. However, too high carbon content will lead to an increased decarburization tendency, increased difficulty in controlling component segregation during alloy solidification, and increased risk of abnormal brittle proeutectoid network carbide organization. Therefore, in order to take into account the high strength performance of 1770MPa grade stranded wire and make it easier to control the wire rod organization, the carbon content is appropriately reduced to reduce component segregation. When the wire rod is subjected to online salt bath strong isothermal treatment after spinning, the risk of network carbide precipitation can be further controlled, the peak precipitation of tertiary cementite can be suppressed, and the cementite carbide can be refined, thereby improving the strengthening effect of carbon elements and the uniformity of the organization, reducing the difficulty of micro-spheroidization of the organization, and regulating the peak precipitation temperature of pearlite to provide favorable conditions for the nucleation of fine lamellar pearlite. Therefore, the mass percentage of carbon is controlled to be 0.68%~0.73%.

[0013] (2) Silicon: Si is often used as a deoxidizer during smelting. At the same time, silicon can improve the solid solution strength of austenite and shift the CCT curve to the right, which is beneficial to increase the stability of cementite and inhibit the coarsening of cementite during the online salt bath strong isothermal treatment, thereby facilitating the acquisition of finer pearlite structure and improving the matrix strength. At the same time, it increases the activity of carbon, accelerates the diffusion of carbon, and reduces the difficulty of micro-spheroidization of the structure, which is beneficial to the rapid regulation of the tempered structure. However, too high silicon will increase the graphitization tendency of C and cause abnormal ferrite at the grain boundary, which is detrimental to the uniformity of the structure and the cold working performance. Therefore, in order to take into account the high strength and plasticity of the steel wire rod for stranding and facilitate the regulation of the wire rod structure, the mass percentage of Si is controlled to be 0.15%~0.3%.

[0014] (3) Manganese: As a deoxidizing element in the steelmaking process, Mn can reduce hot brittleness to ensure the smooth progress of the rolling process. At the same time, it can stabilize austenite and increase the strength, wear resistance and hardenability of wire rod by solid solution strengthening. However, when the Mn content is too high, it will aggravate the segregation during the solidification process of the steel billet, slow down the diffusion rate of C, make the grains coarse during the online salt bath strong isothermal treatment, and lead to an increase in residual austenite, expand the bainite and martensite phase transformation temperature range, make the core more prone to martensite and reduce the drawing performance of the wire rod. Therefore, in order to take into account the high strength and toughness of the steel wire rod for stranding and facilitate the uniformity of the wire rod structure, the Mn content is appropriately reduced, and the mass percentage of Mn is controlled to be 0.20%~0.40%.

[0015] (4) Boron: Boron is an element that improves the hardenability of steel and can reduce the sensitivity to decarburization. Trace amounts of Boron melt into austenite in the form of interstitial atoms and can attach to austenite grain boundaries to reduce grain boundary energy, increase the ductile-brittle transition temperature of steel, and hinder the diffusion channel of carbon atoms to increase material strength. However, the price of Boron is relatively high, and too high a price is not conducive to cost control. It will also cause BN inclusions that affect the purity of molten steel, cause coarse grains and brittleness. Therefore, based on the role of B and material cost considerations, on the basis of reducing the C and Mn contents, a proper amount of Boron is added to replace the addition of Cr and V to adjust the pearlite peak precipitation temperature and compensate for the strength loss. The mass percentage of B is controlled to be 0.0010%~0.0020%.

[0016] (5) Phosphorus and sulfur: P and S are impurity elements. The lower the better. Therefore, P is controlled to be ≤ 0.015% and S is ≤ 0.015%.

[0017] The hot-rolled wire rod adopts C-Si-Mn-B composition design, appropriately reduces the C and Mn contents, and does not add Cr elements to reduce central segregation, which provides favorable conditions for controlling network carbides, refining cementite carbides, and avoiding the precipitation of abnormal low-temperature organizations. In combination with the addition of trace B to replace Cr and V, the hardenability of the wire rod is regulated, the hot rolling performance of the steel is improved, and the pearlite phase transformation temperature range is regulated to be lower with a reasonable ratio of C, Si, and Mn, which provides favorable conditions for improving the diffusion activity of C and promoting the nucleation of fine lamellar pearlite. On this basis, the online salt bath strong isothermal treatment is directly carried out after spinning:

[0018] 1. Compared with the existing Stelmor strong air cooling treatment, the online salt bath strong isothermal treatment can significantly increase the cooling rate of the wire rod, quickly skip the precipitation range of the secondary cementite, and combine with the reduction of segregation to effectively avoid the formation of network carbides. At the same time, it inhibits the peak precipitation of tertiary cementite at 700~800℃, thereby refining the cementite carbides, providing favorable conditions for pearlite nucleation and softening, and improving the overall strength and plasticity of the wire rod.

[0019] 2. Compared with the existing Stelmor air cooling, the cooling rate is uncontrollable during the cooling phase transformation process, which produces abnormal tissue and weakens the strengthening effect. The online salt bath strong isothermal treatment can control the wire rod to quickly enter the pearlite phase region in the high-temperature austenite state. On the one hand, the molten salt has a strong heat exchange capacity and can be wrapped on the surface of the wire rod. Compared with the air cooling treatment, it can effectively reduce the temperature difference of the same circle position of the wire rod and the temperature difference between the surface and the core. The segregation of the wire rod decreases, and the temperature of the pearlite phase region is higher than that of the bainite and martensite phase regions. The rapid and uniform heat exchange between the wire rod and the molten salt can prevent the wire rod from entering the bainite or martensite phase region, and avoid the formation of brittle abnormal tissues such as bainite or martensite during the phase transformation process after spinning. On the other hand, it can promote the wire rod to form a pearlite-based tissue with better internal density and core permeability, and use the full transformation of austenite to pearlite tissue to avoid the residual austenite from generating abnormal tissue during the subsequent roller slow cooling process, thereby effectively regulating the uniformity of the tissue and reducing the fluctuation of mechanical properties.

[0020] 3. Compared with the existing Stelmor cooling with limited minimum cooling capacity, the online salt bath strong isothermal treatment can make the wire rod consistent with the molten salt temperature and perform isothermal treatment in the pearlite phase region for a longer time. On the one hand, the cooling rate of supercooled austenite is relatively large, the driving force of phase transformation is relatively large, and the nucleation rate is much larger than the grain growth rate, so that the spacing between the induced pearlite lamellae is relatively fine, and it can be transformed into a pearlite-dominated organization. At the same time, the incubation period is extended, which increases the proportion of pearlite in the organization and reduces the proportion of ferrite, which is beneficial to strengthening the matrix and reducing the difficulty of micro-spheroidization, and then combined with the strengthening effect of trace B, it makes up for the strength loss caused by reducing the C and Mn content and not adding Cr and V components; on the other hand, compared with continuous cooling with air cooling, the wire rod can be kept in a high temperature state for a longer time. Isothermal treatment is carried out in the state to control the phase transformation of the wire rod in the pearlite phase region. This is also because the molten salt temperature can be higher than the temperature of the troostite phase region. With isothermal tempering, more thermal power can be provided to improve the micro-spheroidization level of some organizations, so that the dispersed micro-spheroidized carbides are dispersed in the tempered pearlite and tempered ferrite plastic matrix, which can improve the uniformity of the organization. At the same time, the organization regulation in the tempered state can significantly reduce the organization stress and micro-distortion density on the basis of not excessively deteriorating the matrix strength, and improve the plasticity and toughness. Finally, after slow cooling on the roller, the high temperature state of the wire rod after leaving the salt bath can be continued. On the premise that the austenite organization has been fully transformed and initially tempered and softened, the tempering softening of the matrix organization and the micro-spheroidization of some organizations are further promoted, thereby adjusting the strength and plasticity matching of the wire rod.

[0021] In a preferred embodiment, before rolling, the initial descaling water pressure after the billet leaves the heating furnace is ≥18MPa, and high-pressure dephosphorization water can be used to remove the oxide scale formed on the surface of the billet after heating, so as to avoid the oxide scale being pressed into the wire during rolling and damaging the surface quality of the wire.

[0022] Since the composition of the wire rod is simplified, segregation is reduced, and trace amounts of B improve the rolling performance, the stability of the continuous rolling process can be controlled by combining appropriate rolling temperature and a large reduction, and the amount of deformed austenite driven to undergo recrystallization is continuously increased by utilizing deformation energy storage. At the final rolling entrance temperature, as the rolling temperature becomes lower and the deformation amount increases, the average grain size of the austenite continues to decrease, which can strengthen the matrix and adapt to the wire-pulling temperature. However, if the rolling temperature is too low, the load and wear on the rolling line will increase. Therefore, in a preferred embodiment, during the rolling, the initial rolling temperature is controlled to be 1040~1075°C, the initial rolling reduction is 25%~35%, the intermediate rolling temperature is 950~1000°C, the intermediate rolling reduction is 31%~36%, the final rolling entrance temperature is 900~930°C, and the final rolling reduction is 20%~25%.

[0023] In a preferred embodiment, the molten salt temperature of the online salt bath intensive isothermal treatment is 495~530℃, and the treatment time is 400~700s. This molten salt is the pearlite phase temperature. The lower the molten salt temperature, as the treatment time is extended, the more pearlite is formed in the organization, the finer the interlamellar spacing, and the ferrite ratio decreases, affecting the ratio of tempered pearlite and tempered ferrite, increasing the matrix strength and decreasing the plasticity. However, if the molten salt temperature is too low, troostite or even bainite organization will be formed, and it will be difficult to provide more thermal power to soften the organization, which will result in significantly insufficient plasticity of the wire rod. On the contrary, the higher the molten salt temperature, the coarser the interlamellar spacing of pearlite formed in the organization, which will result in a decrease in matrix strength. If the molten salt temperature is too high, it will be unfavorable for controlling the organization to form a pearlite-dominated organization, reducing the ferrite ratio and inhibiting the coarsening of cementite, and the tempered pearlite formed will be If the pearlite lamellae are too coarse, the strength of the wire rod will decrease too quickly as time goes by, which is not conducive to the control of the organization. The longer the processing time, the more the wire rod can be fully transformed into a pearlite-dominated organization, which can reduce the risk of abnormal organization and increase the proportion of tempered pearlite. The better the softening effect is, the plasticity of the matrix in the tempered state can be adjusted. However, if the processing time is too long, the wire rod will be over-softened, which is not conducive to controlling the matrix strength. On the contrary, the shorter the processing time, the lower the softening effect and the plasticity of the wire rod. However, if the processing time is too short, the micro-spheroidization level of the organization will decrease, and even the risk of organizational abnormality will increase due to insufficient phase transformation of the wire rod, and the proportion of ferrite will increase, which will cause the loss of uniformity and strength-plasticity of the wire rod. Therefore, the molten salt temperature and processing time can be further controlled to promote the full transformation of pearlite and the micro-spheroidization of some organizations, improve the utilization value of carbon, improve the plasticity of the wire rod, and adjust the strength-plasticity matching of the wire rod.

[0024] Since the temperature difference of the wire rod from the spinning temperature to the molten salt temperature is large, a larger molten salt circulation rate is selected to control the molten salt temperature rise, improve the temperature accuracy of the online salt bath strong isothermal treatment, refine the cementite carbide, and promote uniform micro-spheroidization of the organization. In a preferred embodiment, the molten salt circulation rate of the online salt bath strong isothermal treatment is 500~700t / h, and the molten salt temperature rise is ≤7°C.

[0025] In a preferred embodiment, the roller slow cooling controls the wire rod to continuously soften at a slow cooling rate of 0.1-0.4°C / s. A lower slow cooling rate can be used to control the wire rod to slowly cool down from the molten salt temperature, continue the high temperature state, further promote partial tissue microspheroidization, and improve the softening effect.

[0026] In a preferred embodiment, the roller slow cooling adopts a closed insulation cover. After the wire rod is conveyed into the insulation cover through the roller, high-temperature gas of ≥300°C is blown back into the insulation cover, and the wire rod is conveyed at a roller speed of 0.1~0.3m / s. On the one hand, by using the high-temperature gas backblowing, the molten salt on the surface of the wire rod can be quickly removed by hot air. On the other hand, compared with the delayed cooling after closing the insulation cover, the thermal environment of the insulation cover can be maintained in combination with the high-temperature gas. After the strong isothermal treatment in the online salt bath, the slow cooling rate can be controlled by appropriately reducing the roller speed without staying in the insulation cover, which is beneficial to reducing the slow cooling rate and improving the efficiency of the roller slow cooling treatment. The high-temperature gas can also be directly recycled using the hot air generated during the strong isothermal treatment in the online salt bath to recover heat energy and save production energy consumption.

[0027] A B-containing hot-rolled wire rod for 1770 MPa grade stranded wire, the hot-rolled wire rod is manufactured by any one of the above-mentioned methods for manufacturing the B-containing hot-rolled wire rod for 1770 MPa grade stranded wire.

[0028] The hot rolled wire rod, based on the C-Si-Mn carbon composition, appropriately reduces the C and Mn contents, and replaces the large amount of Cr and V with a trace amount of B, which can appropriately reduce the cost, while reducing the influence of segregation, taking into account the hardenability of the wire rod, regulating the peak precipitation temperature of pearlite, and narrowing the temperature range of the martensite and bainite phase transformation. Combined with the online molten salt strong isothermal and roller slow cooling after spinning, a mixed structure composed of tempered pearlite, tempered ferrite, and micro-spheroidized carbide is obtained, which effectively avoids the abnormal organization of martensite and bainite, and the proportion of tempered pearlite in the organization is relatively large compared with the existing Pearlite steel wire rod for stranded wire has a higher proportion of pearlite. Tempered pearlite retains the characteristics of moderate strength and plasticity, such as relatively coarser pearlite lamellae and higher plasticity than troostite, and higher strength than ferrite. After tempering and adjustment to tempered pearlite, the dislocation density decreases and the plasticity is improved. A small amount of ferrite is transformed into tempered ferrite after tempering, and the toughness is improved. The carbide is transformed into micro-spheroidized carbide matrix with better uniformity and reduced tissue stress, so the strength and plasticity of the matrix can be adjusted to match. It is suitable for deep drawing wire making and strand manufacturing of 1770MPa grade stranded wire.

[0029] The finer the tempered pearlite lamellae spacing, the lower the molten salt temperature, the lower the thermal power and softening effect provided in the same time, the higher the wire rod strength and the lower the plasticity. Conversely, the coarser the tempered pearlite lamellae spacing, the lower the wire rod strength and the higher the plasticity. In a preferred embodiment, the tempered pearlite lamellae spacing is 155~190nm.

[0030] In a preferred embodiment, the network carbide level of the hot-rolled wire rod is level 0, which can avoid the adverse effects of the network carbide on the uniformity and plasticity of the structure and further improve the utilization value of carbon.

[0031] Compared with the 1770MPa grade stranded wire rod obtained by the existing Stelmor air-cooled line, the cementite particles can be refined, the abnormal structure is effectively controlled, and the fluctuation of mechanical properties is greatly reduced. In a preferred embodiment, the mechanical property difference of the hot-rolled wire rod is ≤25MPa.

[0032] The hot-rolled wire rod has a high tensile strength, which avoids excessive strength degradation during the plasticity improvement process and increases the difficulty of cold-drawn steel wire making. At the same time, it has significantly improved plasticity and toughness properties, so that when the pass compression rate, wire drawing speed, and surface reduction rate are increased during the steel wire processing process, the cold processing performance can be effectively improved and the risk of wire breakage can be avoided. In a preferred embodiment, the diameter of the hot-rolled wire rod is 12.5~15.0mm, the tensile strength is 1130~1180MPa, and the cross-sectional shrinkage rate is 48%~53%.

[0033] Compared with the prior art, the beneficial effects of the present invention are at least:

[0034] (1) In view of the current situation that the existing high-carbon alloy composition design combined with the Stelmor air-cooled line process leads to low uniformity and plasticity of the steel strand base material wire rod structure, and a high risk of abnormal structures such as martensite and bainite, the present invention appropriately reduces the C and Mn contents on the basis of carbon chemical composition, and replaces the large amount of Cr and V with trace B, which can appropriately reduce material segregation, and combines online molten salt strong isothermal and roller slow cooling technology to promote the wire rod from the high-temperature austenite state to quickly enter the pearlite phase region to form a pearlite-based structure by molten salt heat exchange, which can inhibit secondary cementite and tertiary cementite. , refine cementite carbides, reduce the proportion of ferrite, improve matrix strength and the strengthening effect of carbon elements, make up for the strength loss caused by reducing C and Mn content and not adding Cr and V, and then carry out isothermal tempering to promote micro-spheroidization of some tissues to improve the plasticity of wire rod, and finally, through roller slow cooling, further promote the tempering softening of matrix tissue and micro-spheroidization of some tissues, and carry out continuous softening, which significantly improves the plasticity and cold working performance of wire rod without excessively deteriorating the strength of wire rod, and can effectively improve the uniformity of tissue, avoid the precipitation of abnormal tissues such as martensite and bainite, and has good industrial adaptability.

[0035] (2) The hot-rolled wire rod of the present invention adopts a C-Si-Mn-B composition design, which has lower C and Mn contents than the existing pearlite steel, and does not add Cr and V, which can appropriately reduce the material cost. The microstructure type is a mixed structure composed mainly of tempered pearlite and a small amount of tempered ferrite and micro-spheroidized carbide, with better structural uniformity and cold working performance, and can achieve high strength and high plasticity matching of the wire rod, reaching a tensile strength of 1130~1180MPa, a cross-sectional shrinkage rate of 48%~53%, and a mechanical property difference of ≤25MPa. It is used in the manufacture of 1770MPa grade stranded wire and other application fields, and can meet the requirements of improving the pass compression rate, wire drawing speed and surface reduction rate, avoiding the risk of wire breakage during cold working and the risk of wire breakage in steel strands. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0037] Figure 1 is a metallographic structure diagram of Example 1 of the present invention;

[0038] Figure 2 is the metallographic structure diagram of Example 2 of the present invention;

[0039] Figure 3 It is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0040] The embodiments described below with reference to the accompanying drawings are exemplary and are only for illustration and do not limit the description of the features and characteristics of the present invention, so as to propose the best way to implement the present invention, and are intended to be used to explain the present invention and are sufficient to enable those skilled in the art to implement the present invention, but cannot be understood as any limitation on the scope of the present invention, which is limited only by the appended claims; the organization and performance testing of the wire rods obtained in the following embodiments and comparative examples include: the tensile test is carried out using "GB-T 228.1-2021 Metallic Material Tensile Test Part 1: Room Temperature Test Method" to obtain the tensile strength and cross-sectional shrinkage rate, and the organization test is carried out according to the metal microstructure detection method of GB / T13298 standard; the mechanical property same circle difference test method: take 2 circles of wire rod 5m away from the end of the coil, take the overlap area position as the base point, divide each circle of wire rod into 8 sections on average, take 1 tensile specimen on each section, and the strength extreme difference of the tensile specimen after the tensile test is the mechanical property same circle difference. Embodiment 1:

[0041] A preferred embodiment of the method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.71%, Si: 0.2%, Mn: 0.32%, B: 0.0014%, P: 0.014%, S: 0.012%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online salt bath strong isothermal → roller slow cooling → coiling, specifically:

[0042] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity. After the steel billet comes out of the heating furnace, high-pressure initial rolling descaling water is used to remove the oxide scale on the surface of the steel billet, and then the steel billet is rolled into a wire with a diameter of 13mm through a rolling line. The low-temperature and large-reduction rolling is used to refine the microstructure grains and adapt to the spinning temperature. Specifically: the initial rolling descaling water pressure after the steel billet comes out of the heating furnace is controlled to be 19MPa, the initial rolling temperature is controlled to be 1055℃, the initial rolling reduction is 31%, the intermediate rolling temperature is 965℃, the intermediate rolling reduction is 34%, the final rolling entrance temperature is 920℃, and the final rolling reduction is 23%; the spinning process is used to convert the wire coming out of the rolling line into a wire rod through a spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate spinning temperature, favorable conditions are created for the pearlite phase deformation nucleus. Specifically: the spinning temperature is controlled to be 890℃.

[0043] The online salt bath strong isothermal process adopts a salt bath tank with molten salt inside. The wire rod after spinning is conveyed through the salt bath tank by rollers, and is quickly cooled to the molten salt temperature for online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of 37°C / s until it enters the pearlite phase region, avoiding the precipitation of secondary cementite, inhibiting the peak precipitation of tertiary cementite, refining cementite carbides, promoting the transformation of high-temperature austenite structure to form a structure dominated by pearlite, and then performing long-term isothermal tempering to promote micro-spheroidization of some structures, and adjusting the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 495°C, the processing time is 700s, the molten salt circulation volume is 700t / h, and the molten salt temperature rise is ≤7°C.

[0044] The roller slow cooling process adopts a heat preservation cover to close, and the wire rod coming out of the salt bath tank is conveyed by rollers into the heat preservation cover for slow cooling treatment. By blowing back high-temperature gas ≥300°C, the molten salt on the surface of the wire rod is quickly removed, and at the same time, the roller speed is reduced for continuous softening, further promoting the tempering softening of the matrix structure and the micro-spheroidization of some structures. Specifically: after the wire rod is conveyed into the heat preservation cover by rollers, the high-temperature gas above the salt bath tank is blown back into the heat preservation cover, the high-temperature gas is aimed at the wire rod, and the wire rod is conveyed at a roller speed of 0.1m / s, and the wire rod is controlled to be continuously softened at a slow cooling rate of 0.1°C / s until it is coiled; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the coiling temperature is 295°C. After packaging and storage, the hot-rolled wire rod product is obtained, and its metallographic structure diagram is as follows Figure 1 As shown, the microstructure is 96% tempered pearlite by volume + 1% tempered ferrite by volume + 3% micro-spheroidized carbide by volume, the interlamellar spacing of tempered pearlite is 155nm, the grade of network carbide is 0, the difference in mechanical properties is 23MPa, the tensile strength is 1172MPa, and the section reduction rate is 50%.

[0045] Comparative Example 1:

[0046] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 1 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → Stelmor strong air cooling → roller slow cooling, specifically: the initial rolling temperature is controlled to be 1010°C, the intermediate rolling temperature is 935°C, the final rolling inlet temperature is 865°C, and the spinning temperature is 845°C; the Stelmor strong air cooling adopts an air volume of 200,000 m per fan 3 / h, 1~6# fans are opened at 85%, and the wire rod is controlled to be cooled to 600℃ at 8.8℃ / s. The roller slow cooling adopts closing the insulation cover and the fan, and the wire rod is conveyed at a roller speed of 0.15m / s. The wire rod is controlled to be cooled to 295℃ at a slow cooling rate of 0.8℃ / s. The finished wire rod is obtained after off-line, and the microstructure is 85% pearlite + 15% ferrite by volume, the pearlite lamellar spacing is 232nm, the network carbide grade is 1, the mechanical property same circle difference is 58MPa, the tensile strength is 1110MPa, and the cross-sectional shrinkage rate is 34%.

[0047] From the comparison results of Example 1 and Comparative Example 1, it can be seen that compared with the production by the Stelmor air-cooled line process, the proportion of pearlite is not high and the lamellae are coarse, the dislocation density is large, and the risk of abnormal tissues such as network carbide, martensite, and bainite is high, resulting in low uniformity and plasticity of the steel strand base material wire rod tissue, and poor mechanical properties. The present invention adopts C-Si-Mn-B component design combined with online molten salt strong isothermal and roller slow cooling technology to inhibit secondary cementite and tertiary cementite, refine cementite carbide, and form a mixed tissue composed mainly of tempered pearlite and a small amount of tempered ferrite and micro-spheroidized carbide, reduce the proportion of ferrite, and significantly improve the plasticity and cold working performance of the wire rod without excessively deteriorating the strength of the wire rod, and can avoid the precipitation of abnormal tissues such as martensite and bainite, effectively improve the uniformity of the tissue, and is more suitable for the manufacture of 1770MPa grade stranded wire and other application fields, and can reduce the risk of wire breakage.

[0048] Comparative Example 2:

[0049] A method for manufacturing a hot-rolled wire rod, the manufacturing method differs from that of Example 1 in that: the initial rolling temperature is controlled to be 1025°C, the intermediate rolling temperature is controlled to be 945°C, the final rolling entrance temperature is controlled to be 875°C, the spinning temperature is controlled to be 855°C, the online salt bath strong isothermal process cools the wire rod at a cooling rate of 34°C / s until the wire rod enters the pearlite phase region, and a finished wire rod is obtained after the wire rod is off the line. Embodiment 2:

[0050] A preferred embodiment of the method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.7%, Si: 0.15%, Mn: 0.4%, B: 0.001%, P: 0.013%, S: 0.013%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online salt bath strong isothermal → roller slow cooling → coiling, specifically:

[0051] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity. After the steel billet comes out of the heating furnace, high-pressure initial rolling descaling water is used to remove the oxide scale on the surface of the steel billet, and then the steel billet is rolled into a wire with a diameter of 14mm through a rolling line. The low-temperature and large-reduction rolling is used to refine the microstructure grains and adapt to the spinning temperature. Specifically: the initial rolling descaling water pressure after the steel billet comes out of the heating furnace is controlled to be 19MPa, the initial rolling temperature is controlled to be 1060℃, the initial rolling reduction is 27%, the intermediate rolling temperature is 980℃, the intermediate rolling reduction is 32%, the final rolling entrance temperature is 910℃, and the final rolling reduction is 21%; the spinning process is used to convert the wire coming out of the rolling line into a wire rod through a spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate spinning temperature, favorable conditions are created for the pearlite phase deformation nucleus. Specifically: the spinning temperature is controlled to be 895℃.

[0052] The online salt bath strong isothermal process adopts a salt bath tank with molten salt inside. The wire rod after spinning is conveyed through the salt bath tank by rollers, and is quickly cooled to the molten salt temperature for online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of 35°C / s until it enters the pearlite phase region, avoiding the precipitation of secondary cementite, inhibiting the peak precipitation of tertiary cementite, refining cementite carbides, promoting the transformation of high-temperature austenite structure to form a structure dominated by pearlite, and then performing long-term isothermal tempering to promote micro-spheroidization of some structures, and adjusting the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 530°C, the processing time is 400s, the molten salt circulation volume is 500t / h, and the molten salt temperature rise is ≤7°C.

[0053] The roller slow cooling process adopts a heat preservation cover to close, and the wire rod coming out of the salt bath tank is conveyed by rollers into the heat preservation cover for slow cooling treatment. By blowing back high-temperature gas ≥300°C, the molten salt on the surface of the wire rod is quickly removed, and at the same time, the roller speed is reduced for continuous softening, further promoting the tempering softening of the matrix structure and the micro-spheroidization of some structures. Specifically: after the wire rod is conveyed into the heat preservation cover by rollers, the high-temperature gas above the salt bath tank is blown back into the heat preservation cover, the high-temperature gas is aimed at the wire rod, and the wire rod is conveyed at a roller speed of 0.3m / s, and the wire rod is controlled to be continuously softened at a slow cooling rate of 0.4°C / s until it is coiled; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the coiling temperature is 263°C. After packaging and storage, the hot-rolled wire rod product is obtained, and its metallographic structure diagram is as follows Figure 2 As shown, the microstructure is 93% by volume of tempered pearlite + 5% by volume of tempered ferrite + 2% by volume of micro-spheroidized carbide, the interlamellar spacing of tempered pearlite is 190nm, the grade of network carbide is 0, the difference in mechanical properties is 24MPa, the tensile strength is 1146MPa, and the section shrinkage is 51%.

[0054] Comparative Example 3:

[0055] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 2 in that: the online salt bath strong isothermal process cools the wire rod at a cooling rate of 39°C / s until the wire rod enters the pearlite phase region, the molten salt temperature is 475°C, and a finished wire rod is obtained after it is offline, wherein the finished wire rod has a microstructure of 20% by volume of tempered pearlite + 73% by volume of tempered pearlite + 6% by volume of tempered ferrite + 1% by volume of micro-spheroidized carbide, the lamellar spacing of the tempered bainite is 133nm, the lamellar spacing of the tempered pearlite is 152nm, the network carbide grade is 0, the mechanical property same circle difference is 34MPa, the tensile strength is 1182MPa, and the cross-sectional shrinkage rate is 40%.

[0056] It can be seen from the results of Example 1 and Example 2 that the lower the molten salt temperature, as the treatment time is extended, the proportion of pearlite formed in the organization is greater, the lamellar spacing is finer, the proportion of ferrite decreases, affecting the proportion of tempered pearlite and tempered ferrite, the matrix strength increases, and the plasticity decreases. It can be seen from the results of Example 2 and Comparative Example 3 that when the molten salt temperature is too low, the cooling rate of the wire rod increases accordingly, the degree of supercooling increases, and some troostite structures with finer lamellar spacing are formed in the organization, which then transforms into tempered troostite structures with long-term isothermal treatment. However, due to the relatively low molten salt temperature, it is difficult to provide more thermal power to soften the organization, which will result in significantly insufficient plasticity of the wire rod.

[0057] Comparative Example 4:

[0058] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which differs from that of Example 2 in that: the online salt bath strong isothermal process cools the wire rod at a cooling rate of 32°C / s until the wire rod enters the pearlite phase region, the molten salt temperature is 565°C, and the finished wire rod is obtained after the wire rod is off the line. Embodiment 3:

[0059] A preferred embodiment of the method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.68%, Si: 0.29%, Mn: 0.2%, B: 0.0016%, P: 0.015%, S: 0.012%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online salt bath strong isothermal → roller slow cooling → coiling, specifically:

[0060] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity. After the steel billet comes out of the heating furnace, high-pressure initial rolling descaling water is used to remove the oxide scale on the surface of the steel billet. Then, the steel billet is rolled into a wire with a diameter of 12.5mm through a rolling line. The low-temperature and large-reduction rolling is used to refine the microstructure grains and adapt to the spinning temperature. Specifically: the initial rolling descaling water pressure after the steel billet comes out of the heating furnace is controlled to be 18MPa, the initial rolling temperature is controlled to be 1040℃, the initial rolling reduction is 35%, the intermediate rolling temperature is 950℃, the intermediate rolling reduction is 36%, the final rolling entrance temperature is 900℃, and the final rolling reduction is 25%; the spinning process is used to convert the wire coming out of the rolling line into a wire rod through a spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate spinning temperature, favorable conditions are created for the pearlite phase deformation nucleus. Specifically: the spinning temperature is controlled to be 885℃.

[0061] The online salt bath strong isothermal process adopts a salt bath tank with molten salt inside. The wire rod after spinning is conveyed through the salt bath tank by rollers, and is quickly cooled to the molten salt temperature for online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of 36°C / s until it enters the pearlite phase region, avoiding the precipitation of secondary cementite, inhibiting the peak precipitation of tertiary cementite, refining cementite carbides, promoting the transformation of high-temperature austenite structure to form a structure dominated by pearlite, and then performing long-term isothermal tempering to promote micro-spheroidization of some structures, and adjusting the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 503°C, the processing time is 607s, the molten salt circulation volume is 550t / h, and the molten salt temperature rise is ≤7°C.

[0062] The roller slow cooling process adopts a heat preservation cover to close, and the wire rod coming out of the salt bath tank is conveyed by rollers into the heat preservation cover for slow cooling treatment. By blowing back high-temperature gas ≥300°C, the molten salt on the surface of the wire rod is quickly removed, and at the same time, the roller speed is reduced for continuous softening, further promoting the tempering softening of the matrix structure and the micro-spheroidization of some structures. Specifically: after the wire rod is conveyed into the heat preservation cover by rollers, the high-temperature gas above the salt bath tank is blown back into the heat preservation cover, the high-temperature gas is aimed at the wire rod, and the wire rod is conveyed at a roller speed of 0.2m / s, and the wire rod is controlled to be continuously softened at a slow cooling rate of 0.2℃ / s until it is coiled; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the coiling temperature is 303℃. After packaging and warehousing, the hot-rolled wire rod product is obtained, and its metallographic structure diagram is as follows Figure 3 shown.

[0063] Comparative Example 5:

[0064] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the online salt bath strong isothermal treatment time is 350s, and the finished wire rod is obtained after going offline.

[0065] Comparative Example 6:

[0066] A method for manufacturing a hot-rolled wire rod, the manufacturing method of which is different from that of Example 3 in that the online salt bath strong isothermal treatment time is 780s, and the finished wire rod is obtained after going offline. Embodiment 4:

[0067] A preferred embodiment of the method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.73%, Si: 0.3%, Mn: 0.26%, B: 0.002%, P: 0.014%, S: 0.015%, and the rest are Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online salt bath strong isothermal → roller slow cooling → coiling, specifically:

[0068] The rolling process is used to heat a steel billet with a specification of 220mm×220mm through a heating furnace to obtain a high-temperature steel billet with rollable plasticity. After the steel billet comes out of the heating furnace, high-pressure initial rolling descaling water is used to remove the oxide scale on the surface of the steel billet. Then, the steel billet is rolled into a wire with a diameter of 15mm through a rolling line. The low-temperature and large-reduction rolling is used to refine the microstructure grains and adapt to the spinning temperature. Specifically: the initial rolling descaling water pressure after the steel billet comes out of the heating furnace is controlled to be 20MPa, the initial rolling temperature is controlled to be 1075℃, the initial rolling reduction is 25%, the intermediate rolling temperature is 1000℃, the intermediate rolling reduction is 31%, the final rolling entrance temperature is 930℃, and the final rolling reduction is 20%; the spinning process is used to convert the wire coming out of the rolling line into a wire rod through a spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate spinning temperature, favorable conditions are created for the pearlite phase deformation nucleus. Specifically: the spinning temperature is controlled to be 905℃.

[0069] The online salt bath strong isothermal process adopts a salt bath tank with molten salt inside. The wire rod after spinning is conveyed through the salt bath tank by rollers, and is quickly cooled to the molten salt temperature for online salt bath strong isothermal treatment, so that the wire rod is cooled at a cooling rate of 36°C / s until it enters the pearlite phase region, avoiding the precipitation of secondary cementite, inhibiting the peak precipitation of tertiary cementite, refining cementite carbides, promoting the transformation of high-temperature austenite structure to form a structure dominated by pearlite, and then performing long-term isothermal tempering to promote micro-spheroidization of some structures, and adjusting the strength and plasticity matching of the wire rod. Specifically: the molten salt temperature is 518°C, the processing time is 510s, the molten salt circulation volume is 600t / h, and the molten salt temperature rise is ≤7°C.

[0070] The roller slow cooling process is closed with an insulation cover, and the wire rod coming out of the salt bath tank is conveyed by rollers into the insulation cover for slow cooling treatment. The molten salt on the surface of the wire rod is quickly removed by blowing back high-temperature gas ≥300°C, and the roller speed is reduced for continuous softening, further promoting the tempering softening of the matrix structure and the micro-spheroidization of some structures. Specifically: after the wire rod is conveyed into the insulation cover by rollers, the high-temperature gas above the salt bath tank is blown back into the insulation cover, the high-temperature gas is aimed at the wire rod, and the wire rod is conveyed at a roller speed of 0.25m / s, and the wire rod is controlled to be continuously softened at a slow cooling rate of 0.3°C / s until it is coiled; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the coiling temperature is 278°C. After packaging and warehousing, the hot-rolled wire rod finished product is obtained.

[0071] Comparative Example 7:

[0072] A method for manufacturing hot-rolled wire rod, which differs from Example 4 in that: the roller slow cooling process adopts a heat preservation cover to close, the wire rod is transported at a roller speed of 0.5m / s, the wire rod is controlled to be cooled at a slow cooling rate of 0.75℃ / s, and the finished wire rod is obtained after it comes off the line.

[0073] Comparative Example 8:

[0074] A method for manufacturing a hot-rolled wire rod, which differs from Example 4 in that: the method is manufactured according to the process flow of rolling → spinning → online salt bath strong isothermal → air cooling → coiling, and the air cooling process adopts the method of opening the heat preservation cover, conveying the wire rod at a roller speed of 0.6m / s, controlling the wire rod to be air-cooled at a rate of 1.8℃ / s, and obtaining the finished wire rod after it comes off the line.

[0075] The structure and performance of the wire rods obtained in the above-mentioned Examples 1 to 4 and Comparative Examples 2, 4 to 8 were tested, and the comparative results obtained are shown in Table 1 below:

[0076] Table 1. Comparison of the microstructure and properties of different wire rod compositions and manufacturing methods

[0077]

[0078] It can be seen from the results of Examples 1 to 4 that the present invention can achieve a match between high strength and high plasticity of the wire rod, reaching a tensile strength of 1130 to 1180 MPa, a cross-sectional shrinkage rate of 48% to 53%, and a mechanical property difference of ≤25 MPa. It can adapt to the requirements for improving the pass compression rate, drawing speed and surface reduction rate in the manufacturing of 1770 MPa-grade stranded wire, and avoid the risk of wire breakage during cold processing and the risk of wire breakage in the steel strand.

[0079] From the comparison results of Example 1 and Comparative Example 2, it can be seen that after the spinning temperature continues to be reduced, the rolling temperature and the cooling rate of the wire rod subjected to the online salt bath strong isothermal treatment are correspondingly reduced, which is not conducive to the refinement of cementite carbides. After the spinning temperature is appropriately increased, the wire rod can be controlled to quickly enter the pearlite phase region in the high-temperature austenite state, and the phase transformation driving force is relatively large, providing favorable conditions for pearlite nucleation and refinement.

[0080] It can be seen from the results of Examples 1 to 4 that the higher the molten salt temperature of the online salt bath strong isothermal treatment, the coarser the spacing between the pearlite lamellae formed in the organization, which will lead to a decrease in matrix strength. It can be seen from the comparison results of Example 2 and Comparative Example 4 that the molten salt temperature is too high, the tempered pearlite lamellae formed are too coarse, and as time goes on, the wire rod strength decreases too quickly, which is not conducive to organization control.

[0081] It can be seen from the results of Examples 1 to 4 that the shorter the treatment time of the online salt bath intensive isothermal treatment, the lower the softening effect. It can be seen from the comparison results of Example 3 and Comparative Example 5 that if the treatment time is too short, the micro-spheroidization level of the organization will decrease, and even the risk of organizational abnormality will increase due to insufficient phase transformation of the wire rod, and the proportion of tempered ferrite will increase, which will lead to the loss of uniformity and strong plasticity of the wire rod.

[0082] It can be seen from the results of Examples 1 to 4 that the longer the treatment time of the online salt bath strong isothermal treatment is, the more fully the wire rod is transformed into a pearlite-based structure, the lower the risk of abnormal structure, the higher the proportion of tempered pearlite, and the better the softening effect. It can be seen from the comparison results of Example 3 and Comparative Example 6 that if the treatment time is too long, the wire rod is overly softened, which is not conducive to controlling the strength of the matrix and will also lead to an unnecessary increase in production energy consumption.

[0083] From the comparison results of Example 4 and Comparative Example 7, it can be seen that compared with only closing the insulation cover, closing the insulation cover and blowing back high-temperature gas of ≥300°C into the cover can quickly remove the molten salt on the surface of the wire rod, and control the wire rod to continue to soften at a slow cooling rate of 0.1~0.4°C / s, and the softening effect is better; from the comparison results of Example 4 and Comparative Example 8, it can be seen that compared with air cooling that causes plasticity degradation, slow cooling and continuous softening can further promote the tempering softening of the matrix structure and the micro-spheroidization of some structures, and control the fluctuation of mechanical properties to be smaller.

[0084] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention, and they are not intended to limit the scope of protection of the present invention. For example, steel billets can be obtained by converter smelting, refining, and continuous casting processes. All equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for manufacturing a B-containing hot-rolled wire rod for 1770MPa grade stranded wire, characterized in that: The manufacturing method thereof comprises: The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, wherein the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.68%~0.73%, Si: 0.15%~0.3%, Mn: 0.20%~0.40%, B: 0.0010%~0.0020%, P≤0.015%, S≤0.015%, and the rest are Fe and unavoidable impurities; the wire rod is spun into wire rod at a spinning temperature of ≥885°C, and then subjected to an online salt bath strong isothermal treatment, The wire rod is cooled at a cooling rate of ≥35°C / s to enter the pearlite phase region to form a structure dominated by pearlite, and then isothermal tempering is performed to promote microspheroidization of some structures. Finally, it is slowly cooled through a roller to produce a hot-rolled wire rod with a microstructure including a mixed structure composed of tempered pearlite, tempered ferrite and microspheroidized carbides, wherein the volume percentage of tempered pearlite in the mixed structure is ≥93%; the molten salt temperature of the online salt bath intensive isothermal treatment is 495~530°C, and the treatment time is 400~700s.

2. The method for manufacturing B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 1, characterized in that: Before the rolling, the initial rolling descaling water pressure of the steel billet after it leaves the heating furnace is ≥18MPa.

3. The method for manufacturing B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 1, characterized in that: During the rolling, the initial rolling temperature is controlled to be 1040-1075°C, the initial rolling reduction is 25%-35%, the intermediate rolling temperature is 950-1000°C, the intermediate rolling reduction is 31%-36%, the final rolling entrance temperature is 900-930°C, and the final rolling reduction is 20%-25%.

4. The method for manufacturing B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 1, characterized in that: The molten salt circulation rate of the online salt bath strong isothermal treatment is 500-700 t / h, and the molten salt temperature rise is ≤7°C.

5. The method for manufacturing B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 1, characterized in that: The roller slow cooling controls the wire rod to continuously soften at a slow cooling rate of 0.1-0.4°C / s.

6. The method for manufacturing B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 5, characterized in that: The roller slow cooling adopts a closed insulation cover. After the wire rod is conveyed into the insulation cover through the roller, high-temperature gas of ≥300°C is blown back into the insulation cover, and the wire rod is conveyed at a roller speed of 0.1~0.3m / s.

7. A B-containing hot-rolled wire rod for 1770MPa grade stranded wire, characterized in that: The hot-rolled wire rod is manufactured by the method for manufacturing a B-containing hot-rolled wire rod for 1770 MPa grade stranded wire as described in any one of claims 1 to 6.

8. The B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 7, characterized in that: The interlamellar spacing of the tempered pearlite is 155-190 nm, the network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property difference is ≤25 MPa.

9. The B-containing hot-rolled wire rod for 1770 MPa grade stranded wire according to claim 7, characterized in that: The hot-rolled wire rod has a diameter of 12.5-15.0 mm, a tensile strength of 1130-1180 MPa, and a cross-sectional shrinkage of 48%-53%.

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