Low-carbon high-plasticity hot-rolled wire rod for 1860MPa-grade stranded wire and manufacturing method of low-carbon high-plasticity hot-rolled wire rod

Abstract

The invention relates to a low-carbon high-plasticity hot-rolled wire rod for a 1860MPa-grade stranded wire and a manufacturing method of the low-carbon high-plasticity hot-rolled wire rod. C-Si-Mn-Cr components are subjected to hot-rolled spinning to form a wire rod, and then the wire rod is subjected to online molten salt isothermal treatment, so that the wire rod is cooled at a cooling speed of greater than or equal to 34 DEG C / s and enters a sorbite phase region from an austenite state to form a structure taking sorbite as a main part, and isothermal tempering is carried out, so that the low-carbon high-plasticity hot-rolled wire rod is obtained. And finally, slow cooling is conducted through a roller way, the hot-rolled wire rod with the microstructure comprising not less than 89% of tempered sorbite and the balance of tempered ferrite and microspheroidized carbide is prepared, the carbon content can be reduced, the material cost can be further reduced, the export tax refund requirement can be met, the abnormal structure precipitation risk is effectively reduced, and the structure uniformity is improved. According to the method, the strength of the steel wire rod is matched with the high plasticity, the tensile strength is 1190-1240 MPa, the percentage reduction of area is 49%-54%, and the yield and the use performance of the 1860 MPa grade steel strand can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled wire rods for stranded wires, and specifically relates to a low-carbon and high-plasticity hot-rolled wire rod for 1860MPa-grade stranded wires and a manufacturing method thereof. Background Art

[0002] As an important connection and anchoring mechanism, 1860MPa grade steel strand is widely used in construction, machinery, bridges, roads and other systems. Domestic 1860MPa grade steel strand is mainly produced with alloy products with a carbon content of 0.82%, and 82B hot-rolled wire rod is used as raw material. It is made into steel strand after pickling, drawing, stranding and stabilization treatment. However, with the export of steel products, some countries have implemented anti-dumping policies against my country. For example, Thailand imposes a tax of 30% of the transaction amount on steel strand products with a carbon content of more than 0.76%. Therefore, manufacturing wire rods for 1860MPa grade strands with reduced carbon content is of great significance to expanding product exports.

[0003] The existing 1860MPa grade steel strand wire rod is generally manufactured by rolling combined with Stelmor air cooling line. Once the carbon content is reduced to 0.76% or below, the product strength drops sharply. It is necessary to increase the content of hardenability elements such as Cr, Mn, B, etc. to ensure the product strength. For example, the low-carbon, high-strength and high-toughness steel strand wire rod and its production process disclosed in patent CN104789880B, SWRH72B-S wire rod adopts C-Si-Mn-Cr-B composition design, combined with post-rolling segmented fast and then slow cooling to obtain a wire rod structure with a sorbite content ≥90%, achieving a tensile strength Rm ≥1200MPa and a cross-sectional shrinkage Z ≥34%, but there are still the following problems and technical difficulties: 1. The Cr and Mn contents are relatively high, and the addition of the expensive B component will increase the material cost. In order to reduce the material cost, reducing the Cr and Mn contents and eliminating the B element will result in a greater strength loss. Excessive deterioration of the wire rod strength will require a larger cold drawing reduction ratio to make the steel wire reach the corresponding strand strength grade. The greater the cold drawing reduction ratio, the greater the plasticity loss, and the higher the risk of wire breakage during the wire drawing process, affecting the yield rate of the strand.

[0004] 2. With the increase of C, Mn and Cr contents, it will increase the difficulty of segregation control in the early smelting and continuous casting process, especially it is easier to aggregate to form grain boundary network cementite, which will sharply reduce the plastic toughness of the material and affect the fatigue performance of the stranded wire. In order to reduce the adverse effects of segregation and network cementite, strong air cooling treatment will be used in the early stage of phase transformation of the Stelmor air-cooled wire after spinning to increase the cooling rate of the wire rod and reduce the time for the wire rod to pass through the temperature range of secondary cementite and tertiary cementite precipitation to reduce the risk of network carbon. However, on the one hand, the maximum intensity of air cooling is limited, and the influence of network carbon cannot be effectively avoided under the influence of segregation. The coarsening of the hard and brittle tertiary cementite precipitation nuclei in the crystal will also lead to the formation of The uniformity of the weave and the strength and plasticity of the steel wire mesh decrease; on the other hand, with the increase of the air volume of the strong air cooling fan, the uncontrollability of the air volume, wind speed and wind direction will also increase. Affected by the stacking density of the wire rod, the windward side and the leeward side, the temperature difference at different positions on the wire rod surface will further increase. Due to the influence of air temperature on wind temperature, for example, the weather temperature in summer and winter also has a significant fluctuation effect on the cooling effect. At the same time, due to the influence of segregation and high Cr, B and other alloying to increase the hardenability of the material, the C curve of the steel will shift to the right, resulting in a higher risk of abnormal structures such as martensite and bainite, further deteriorating the drawing performance, affecting the uniformity of the wire rod structure and causing large fluctuations in the mechanical properties.

[0005] 3. The cementite phase in the fine pearlite stranded wire steel wire rod structure is the strength phase and the ferrite is the plastic phase. In order to make the wire rod more suitable for later deep drawing processing, the addition of hardenability elements such as Cr and B also allows the wire rod to undergo phase transformation in the low temperature range to obtain a refined pearlite lamellar spacing to improve the matrix strength. However, in addition to the increased risk of precipitation of abnormal low-temperature structures, on the one hand, air cooling after rolling makes it difficult for the wire rod to cool quickly, especially on the same cross-section, the wire rod core cools more slowly, which will result in a coarser troostite lamellar spacing and fewer fine lamellar layers. The troostitization rate from the edge to the center is significantly different, and the structure is not uniform. Although reducing the air cooling intensity is beneficial to the uniformity of the troostitization rate, It will also lose supercooling and matrix strength. After reducing the content of hardenability elements, it will further increase the difficulty of troostite nucleation, increase the amount of pearlite and ferrite, and cause strength loss and fluctuation of mechanical properties. On the other hand, the controlled cooling after rolling is continuous cooling. Even if the air volume is reduced, the wire rod is in a low temperature state after phase transformation, which will cause the austenite to fail to fully transform into troostite. The residual austenite in the core will continue to transform into martensite during subsequent slow cooling or at room temperature. During drawing, brittle fracture will occur from the easy center as the deformation increases. At the same time, insufficient thermal power and high dislocation density of troostite will generate large internal stress, which will affect the increase of surface reduction, and then cause insufficient final plasticity and cause fracture during deep drawing and torsion. Summary of the invention

[0006] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a low-carbon and high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire and a manufacturing method thereof, which can reduce the carbon content and further reduce the material cost, meet the export tax rebate requirements, effectively reduce the risk of abnormal tissue precipitation, improve tissue uniformity, achieve matching of wire rod strength and high plasticity, and is conducive to improving the yield rate and performance of 1860MPa grade steel stranded wire.

[0007] The technical solution adopted by the present invention to solve its technical problem is: A method for manufacturing a low-carbon high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire, the manufacturing method comprising: The wire rod is produced by rolling according to the chemical composition of the hot-rolled wire rod, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.71%~0.75%, Si: 0.25%~0.40%, Mn: 0.42%~0.58%, Cr: 0.16%~0.28%, 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 ≥930°C, it is subjected to online molten salt isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥34°C / s, enters the sorbite phase region from the austenite state, forms a structure mainly composed of sorbite, and isothermal tempering is performed to promote carbide spheroidization, and finally is slowly cooled through a roller to produce a hot-rolled wire rod with a mixed structure composed of tempered sorbite, tempered ferrite and micro-spheroidized carbides, and the volume percentage of the tempered sorbite is ≥89%.

[0008] The design basis of the chemical composition and mass percentage of the above hot rolled wire rod includes: (1) Carbon: C is an element necessary to ensure the strength of wire rod and is cheaper than other alloying elements. It determines the volume fraction of cementite strengthening phase in tempered troostite structure, which is beneficial to the formation of refined troostite lamellar structure, thereby improving the matrix strength and improving the drawing deformation performance. However, too high C content will make it more difficult to control the composition segregation during the solidification process of the alloy, increase the risk of forming network cementite precipitated along the grain boundary and producing abnormal structures such as bainite and martensite, and is not conducive to export supply. Therefore, in order to take into account the high strength of 1860MPa grade stranded wire, reduce the difficulty of controlling the uniformity of wire rod structure and the pressure of export tax, it is selected to appropriately reduce the carbon content. Using low carbon content to reduce composition segregation is beneficial for further improving carbide segregation, avoiding damage to the uniformity of steel, and reducing the risk of low-temperature structure precipitation after the wire rod is subjected to online molten salt isothermal treatment after spinning. Salt bath can also be used to inhibit the peak precipitation of tertiary cementite and refine cementite carbides to improve the strengthening effect and compensate for the strength loss. Therefore, the mass percentage of C is controlled to be 0.71%~0.75%.

[0009] (2) Silicon: Si is often added to molten steel as a deoxidizer during the smelting process. It can reduce inclusions and exist in the form of solid solution, which is beneficial to refining the spacing between troostite lamellae. At the same time, it can inhibit the precipitation of cementite, make the cementite thinner, and accelerate the diffusion of carbon. It is beneficial to reduce the difficulty of the troostite structure to transform into micro-spheroidization to form tempered troostite during isothermal tempering, and reduce the strength loss of wire rod. However, Si is a ferrite strengthening element and can reduce the austenite phase area. Too much silicon will increase the decarburization tendency and reduce the toughness of steel, which is not conducive to the deep cold drawing of stranded wire. Therefore, in order to take into account the high strength of stranded wire rod and facilitate the control of wire rod structure, the mass percentage of Si is controlled to 0.25%~0.40%.

[0010] (3) Manganese: Mn is often added as a deoxidizer in the steelmaking process. As an austenite stabilizing element, it can increase the strength of steel and reduce the phase transformation temperature. It is more convenient to quickly skip the three cementite peak precipitation temperatures during the online molten salt isotherm and enter the troostite phase region to form a troostite structure with fine lamellar spacing, thereby improving the tensile strength of the stranded wire. However, when the Mn content is too high, it will aggravate the tendency of grain coarsening and component segregation, increase the hardenability of the wire rod, increase the risk of abnormal low-temperature structure precipitation in the wire rod, reduce the drawing performance of the wire rod, and increase the difficulty of controlling the uniformity of the structure. Therefore, in order to take into account the high strength and toughness combination of the stranded wire rod, minimize the tempering troostitization rate and the difference in the distribution of micro-spheroidized carbides from the edge to the center of the wire rod, and reduce the material cost, the Mn content is appropriately reduced, and the mass percentage of Mn is controlled to be 0.42%~0.58%.

[0011] (4) Chromium: As a medium-strong carbide-forming element, Cr can improve the austenite stabilization degree of the wire rod, prevent the growth of grains during hot rolling, improve the hardenability of the wire rod during cooling, delay the nucleation and growth of carbides, promote sufficient sorbite phase transformation, and is beneficial to the refinement of the sorbite lamella spacing. However, high Cr content is prone to increase microsegregation and cause the formation of quenching structure during controlled cooling. Therefore, in order to take into account the high strength and toughness of the stranded wire rod, and on the basis of the online molten salt isotherm that can increase the degree of supercooling and refine the cementite lamella to refine the sorbite, the Cr content can be appropriately reduced. Therefore, the mass percentage of Cr is controlled to be 0.16%~0.28%.

[0012] (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%.

[0013] The hot-rolled wire rod adopts C-Si-Mn-Cr composition design, which has lower carbon content than 82B wire rod, lower Mn and Cr content than 72B-S wire rod, and does not contain B and other components, so the material cost is lower, and the central alloy element segregation can be appropriately reduced, which is more conducive to the precipitation control of cementite and low-temperature abnormal structure, and can regulate the rise of the peak precipitation temperature of sorbite, providing more favorable conditions for isothermal tempering softening. The manufacturing method selects a higher spinning temperature to prepare for high-temperature austenitization, increase supercooling, and promote sorbite nucleation. After spinning, the online molten salt isothermal treatment is directly carried out: 1. Compared with Stelmor controlled cooling, which weakens the strengthening effect of carbon due to limited cooling capacity, online molten salt isothermal treatment can utilize the high heat exchange capacity of molten salt to quickly cool the wire rod, and quickly skip the precipitation temperature range of secondary cementite at 750~820℃ and tertiary cementite at 700~800℃ from the high-temperature austenite state. Combined with the decrease in segregation, it can effectively avoid the formation of network carbides and deterioration of organizational uniformity, and avoid the large-scale precipitation of tertiary cementite in the crystal, which can refine the cementite carbide, thereby improving the matrix strength and reducing the difficulty of isothermal tempering.

[0014] 2. Compared with Stelmor controlled cooling, which increases the risk of abnormal organization due to the uncontrollable cooling rate of the wire rod, on the one hand, the molten salt wrapped on the surface of the wire rod can transfer heat evenly, and the high heat exchange capacity of the molten salt can reduce the temperature gradient from the edge to the core of the wire rod on the same cross section, so that the wire rod can quickly enter the troostite phase region from the high-temperature austenite state for phase transformation. Combined with the segregation and hardenability decline of the wire rod, the troostite peak precipitation temperature is relatively high, which can prevent the wire rod from entering the bainite or martensite phase region to produce brittle and hard abnormal organization; on the other hand, the wire rod can be maintained in the troostite state. The temperature of the phase zone undergoes isothermal phase transformation for a longer time, which promotes the full transformation of austenite structure to sorbite structure, compensates for the increased difficulty of full sorbitization caused by reducing the content of hardenability elements, and can avoid residual austenite in the core to form abnormal martensite structure during the subsequent slow cooling process of the roller, further effectively controlling the abnormal structure. At the same time, the molten salt temperature is not affected by the ambient temperature, and the air temperature has a smaller impact on the cooling effect, which can make the production line more stable, effectively avoid a significant decrease in plasticity, improve organizational uniformity, and reduce fluctuations in mechanical properties in the same circle.

[0015] 3. Compared with Stelmor controlled cooling, which is difficult to control the organizational state due to continuous cooling, on the one hand, the online molten salt isothermal treatment can quickly cool the wire rod from the spinning temperature to the sorbite phase temperature, which can significantly improve the supercooling, increase the sorbite nucleation rate, and compensate for the adverse effect of reducing the content of hardenability elements on the refinement of sorbite lamellae, and promote the formation of a sorbite structure with fine lamella spacing at the edge and core of the wire rod. The matrix strength and organizational uniformity are higher, making the proportion of tempered sorbite higher and reducing the proportion of tempered ferrite. It can improve the utilization value of carbon and compensate for the strength loss caused by reducing the content of C, Cr, and Mn and eliminating the B element; on the other hand, the peak precipitation temperature of sorbite is relatively high, and the online molten salt isothermal treatment can maintain the consistency of the wire rod and the molten salt temperature, so that the wire rod is in the sorbite phase region. Longer high-temperature treatment provides more thermal power, allowing the newly formed troostite to transform into spheroidized tempered troostite, ferrite to temper and regulate plasticity, and carbide to transform into spheroidized micro-spheroidized carbide, effectively reducing the internal stress and dislocation density of the organization, exerting the dispersion precipitation strengthening effect of Cr, softening and toughening the matrix, and at the same time further reducing the temperature difference from the edge to the center of the wire rod, which can make the phase change and tempering more uniform, and the tempered troostite and micro-spheroidized carbide are evenly distributed. With further roller slow cooling, the high temperature state of the wire rod after leaving the salt bath can be utilized to continue the softening effect of the later stage of the online molten salt isothermal treatment, so as to promote further toughening of the wire rod organization, effectively improve the area reduction rate, and then regulate the matching of wire rod strength and plasticity, which is suitable for deep drawing manufacturing of 1860MPa stranded wire production.

[0016] Before rolling, since the alloy content of the steel is relatively low, a higher heating furnace temperature can be selected to promote the homogenization of the billet composition and avoid excessive heating temperature and long furnace time leading to decarburization and burning, thereby reducing the surface quality of the wire. In a preferred embodiment, before rolling, the heating furnace heating temperature is controlled to be ≤1140°C and the furnace time is ≤3h.

[0017] Since the spinning temperature is relatively high and the online time can be shortened by rapid cooling through molten salt, there is no need to consider the adverse effects of high-temperature rolling on controlled cooling production efficiency. Therefore, the selection of a higher rolling temperature can reduce the deformation resistance of the rolled piece and the mill load, further improve the rolling speed and efficiency, control the wire size accuracy, select a suitable rolling temperature and deformation amount, promote dynamic recrystallization in the final rolling process, and then refine the grains and strengthen the matrix. In a preferred embodiment, during the rolling, the initial rolling temperature is controlled to be 1090~1130℃, the initial rolling reduction is 26%~32%, the final rolling temperature is ≥950℃, and the final rolling reduction is 31%~38%.

[0018] In a preferred embodiment, the molten salt temperature of the online molten salt isothermal treatment is 575~620℃, and the treatment time is 85~200s. The molten salt temperature is in the troostite phase region. The lower the molten salt temperature is, the finer the spacing between the troostite lamellae formed is, but the thermal power of tempering softening is relatively reduced, the strength of the wire rod increases, and the plasticity decreases. However, if the molten salt temperature is too low, there is a risk of bainite structure appearing, and it is not conducive to rapid tempering softening, which will lead to insufficient plasticity of the wire rod. On the contrary, the higher the molten salt temperature is, the coarser the spacing between the troostite lamellae formed is, which can provide more thermal power for spheroidization and toughening, the strength of the wire rod decreases, and the plasticity increases. However, if the molten salt temperature is too high, there is a risk of Pearlite affects the nucleation of troostite, increases the proportion of tempered ferrite, and increases the risk of rapid loss of strength, resulting in insufficient strength of the wire rod; the longer the processing time, the more complete the transformation of the wire rod to troostite, the lower the proportion of tempered ferrite, and the increase of tempering softening effect, but if the processing time is too long, it will lead to an increase in online time and energy consumption, excessive softening of the wire rod or loss of strength and plasticity due to the precipitation coarsening of Cr and the aggregation and growth of micro-spheroidized carbides; conversely, the shorter the processing time, the lower the softening effect, the higher the strength of the wire rod and the lower the plasticity, but if the processing time is too short, it is not conducive to increasing the proportion of tempered troostite and controlling the precipitation of abnormal tissues, which will lead to loss of strength and plasticity of the wire rod and increased mechanical fluctuations.

[0019] In a preferred embodiment, the online molten salt isothermal treatment is divided into a front-stage treatment and a back-stage treatment. The treatment time of the front-stage treatment is ≥20s, and the molten salt circulation volume of the front-stage treatment is greater than the molten salt circulation volume of the back-stage treatment. Since the temperature difference of the wire rod from the spinning temperature to the troostite phase temperature is large, a larger molten salt circulation volume is selected for the front-stage treatment to enable the wire rod to be cooled quickly, reduce the temperature gradient from the edge to the core of the wire rod, and promote the formation of a structure dominated by troostite with fine lamellar spacing. After rapid cooling, the wire rod is maintained at the molten salt temperature. The molten salt circulation volume can be reduced in the back-stage treatment to reduce energy consumption. Accurate temperature control can further promote the untransformed residual austenite to continue to transform into a troostite structure, while promoting isothermal tempering of the wire rod structure and promoting the transformation of carbides into spheroidized structure.

[0020] In a preferred embodiment, the molten salt circulation rate of the front-stage treatment is 480-680 t / h, and the molten salt temperature rise is ≤10°C.

[0021] In a preferred embodiment, the molten salt circulation rate of the post-processing is 220-320 t / h, and the molten salt temperature rise is ≤3°C.

[0022] In a preferred embodiment, the roller slow cooling controls the wire rod to be cooled to below 300°C at a slow cooling rate of 0.45~0.7°C / s. The roller slow cooling further controls the slow cooling rate. Slow cooling can be used to promote further toughening and spheroidizing tempering of the wire rod structure, improve the softening effect of the wire rod, avoid insufficient cooling and reduce the plasticity of the wire rod, and avoid the slow cooling rate being too slow to affect efficient production.

[0023] Since the temperature of the wire rod is relatively high after leaving the salt bath, in a preferred embodiment, the roller slow cooling adopts a cover insulation cover but does not lock the insulation cover, so that the residual heat of the wire rod itself can be used to increase the temperature inside the cover, and the wire rod is transported through the insulation cover at a roller speed of 0.1~0.4m / s, which can further control the slow cooling speed and avoid the roller speed being too low to increase the online time.

[0024] A low-carbon and high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire, wherein the hot-rolled wire rod is manufactured by any one of the above-mentioned methods for manufacturing the low-carbon and high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire.

[0025] The above-mentioned hot-rolled wire rod adopts a low-carbon and boron-free composition design, and the Cr and Mn contents are relatively lower, which can reduce material costs and segregation effects. Combined with the online molten salt isothermal technology, a high proportion of tempered troostite, a small amount of tempered ferrite and micro-spheroidized carbide structures are obtained, and the cementite carbides are refined. The troostite has a finer interlamellar spacing than pearlite, and has better strength and drawing properties. After the transformation to spheroidization, the stress and dislocation density are reduced and the troostite is transformed into tempered troostite, the tempered troostite has both the strength characteristics and high toughness characteristics of troostite, and the carbide is transformed into spheroidization. It becomes evenly distributed on the plastic matrix, which can maximize the strengthening effect of carbon elements. Compared with the existing 1860MPa stranded wire rod, it can make up for the loss of strength and plasticity caused by reducing the carbon content and hardenability element content. At the same time, the tempered state organization is used to effectively improve the plasticity of the wire rod, improve the overall strength and plasticity of the wire rod, and the tempered troostite content and micro-spheroidized carbide distribution from the edge to the core are more uniform, and there is no abnormal martensite or bainite organization, and the mechanical fluctuation is smaller. It can effectively reduce the risk of wire breakage during drawing and stranding, and improve the fatigue performance of stranded wire steel.

[0026] The finer the lamellar spacing of the tempered troostite, the higher the strength of the tempered troostite. However, when the lamellar spacing is fine, the molten salt temperature decreases, the softening effect is lost, the wire rod strength increases, and the plasticity decreases. In a preferred embodiment, the lamellar spacing of the tempered troostite is 95~130nm.

[0027] In a preferred embodiment, the network carbide level of the hot-rolled wire rod is level 0, which can effectively avoid the deterioration effect of the network carbide on the drawing performance of the wire rod and improve the uniformity of the structure.

[0028] In a preferred embodiment, the mechanical property difference of the hot-rolled wire rod is ≤30 MPa, which has smaller fluctuation in mechanical properties than that of existing hot-rolled wire rods, and can further reduce the risk of crack propagation caused by increased drawing amount.

[0029] In a preferred embodiment, the diameter of the hot-rolled wire rod is 7.0~12.0mm, the tensile strength is 1190~1240MPa, and the cross-sectional shrinkage rate is 49%~54%. The wire rod diameter is relatively smaller and the tensile strength is higher, which is more conducive to rapid drawing to the target steel wire diameter and strength grade. At the same time, the wire rod has a significantly improved cross-sectional shrinkage rate, which can inhibit cracking and wire breakage during plastic deformation, thereby helping to improve the drawing speed, production efficiency and yield rate of stranded steel.

[0030] Compared with the prior art, the beneficial effects of the present invention are at least: (1) Aiming at the current situation that the existing 1860MPa grade stranded wire rod has a high risk of abnormal structures such as martensite and bainite in the wire rod due to high alloying of Cr, B, etc., combined with the Stelmor air-cooled wire process, and it is difficult to improve the strength and plasticity, the present invention adopts C-Si-Mn-Cr chemical composition design combined with online molten salt isothermal technology to control the wire rod to quickly enter the troostite phase region from the high-temperature austenite state, which can inhibit the peak precipitation of tertiary cementite, refine cementite carbides, and form a structure dominated by troostite structure with fine interlamellar spacing, promote the full transformation of troostite structure, and promote the tempering of wire rod structure and spheroidization of carbides. Finally, it is slowly cooled by rollers to improve the softening effect of wire rod, which can effectively reduce the risk of abnormal structure precipitation, improve the uniformity of structure and the proportion of tempered troostite, thereby maximizing the strengthening effect of carbon element, compensating for the strength loss caused by reducing the content of C, Cr, and Mn and eliminating the B element, and at the same time using the tempered state to regulate the significantly toughened structure to achieve the matching of wire rod strength and high plasticity, and has good industrial adaptability.

[0031] (2) In view of the current situation that the existing 1860MPa grade stranded wire 82B and 72B-S wire rods have high material cost, insufficient organizational uniformity and plasticity, the low-carbon and high-plasticity hot-rolled wire rod of the present invention adopts a boron-free component design with reduced carbon content, and the Cr and Mn contents are relatively lower, which can further reduce material cost and segregation effects. The microstructure type is a mixed structure composed of tempered troostite, tempered ferrite, and micro-spheroidized carbides, which can achieve matching of wire rod strength and high plasticity, achieving a tensile strength of 1190~1240MPa and a cross-sectional shrinkage rate of 49%~54%. It is used in the manufacture of 1860MPa grade stranded wire exported to Thailand and other application fields, can reduce the risk of deep drawing and strand cracking, meet the export tax rebate requirements, and has good market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] 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: Figure 1 is a metallographic structure diagram of Example 1 of the present invention; Figure 2 is the metallographic structure diagram of Example 2 of the present invention; Figure 3 It is the metallographic structure diagram of Example 3 of the present invention. DETAILED DESCRIPTION

[0033] 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. They 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. They should not be construed as limiting the scope of the present invention, which is limited only by the appended claims. Embodiment 1:

[0034] A preferred embodiment of the method for manufacturing a low-carbon, high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 74%, Si: 0.33%, Mn: 0.49%, Cr: 0.25%, P: 0.012%, S: 0.01%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity, avoiding the risk of burning and decarburization. After the steel billet comes out of the heating furnace, it is rolled into a wire with a diameter of 10mm through a rolling line to promote dynamic recrystallization during the final rolling process and refine the grains. Specifically: the heating temperature of the heating furnace is controlled to be ≤1120°C, the time in the furnace is 2.5h, the initial rolling temperature is 1100°C, the initial rolling reduction is 29%, the final rolling temperature is 955°C, and the final rolling reduction is 36%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate wire-spinning temperature, favorable conditions are created for refining the troostite layer. Specifically: the wire-spinning temperature is controlled to be 935°C.

[0035] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 35°C / s, and enters the troostite phase region from the austenite state, inhibits the peak precipitation of tertiary cementite, refines the cementite carbide, and partially forms a high-temperature austenite structure with fine interlamellar spacing as the main structure of troostite. Then, it is conveyed by a roller through the second stage salt bath tank for back-end treatment, and the temperature is precisely controlled to promote the untransformed residual austenite. The molten salt temperature of the front-end treatment and the back-end treatment is 604℃, the treatment time of the front-end treatment is 30s, the molten salt circulation volume is 535t / h, and the molten salt temperature rise is ≤10℃; the treatment time of the back-end treatment is 100s, the molten salt circulation volume is 270t / h, and the molten salt temperature rise is ≤3℃, that is, the online molten salt isothermal treatment time is 130s.

[0036] The roller slow cooling process adopts a cover insulation cover but does not lock the insulation cover, and the wire rod coming out of the second salt bath tank is conveyed into the insulation cover by rollers for slow cooling treatment to avoid insufficient cooling and reduce the plasticity of the wire rod. At the same time, slow cooling promotes further toughening of the wire rod structure and further spheroidization of carbides. Specifically: the wire rod is conveyed at a roller speed of 0.3m / s, and the wire rod is controlled to be cooled to 292°C at a slow cooling rate of 0.6℃ / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 1 shown.

[0037] Comparative Example 1: 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 heating temperature of the heating furnace is controlled to be ≤1060°C, the furnace time is 3.5h, the initial rolling temperature is 1030°C, the final rolling temperature is 895°C, and the spinning temperature is 870°C; the Stelmor strong air cooling adopts an air volume of 200,000 m per fan 3 / h, open 1~5# fans at 75%, cool the wire rod to 685℃ at a cooling rate of 7.5℃ / s, and the roller slow cooling adopts closing the fan and covering the heat preservation cover to cool the wire rod to 295℃ at a cooling rate of 0.8℃ / s, and the hot-rolled wire rod is obtained after off-line.

[0038] Comparative Example 2: A method for manufacturing a hot-rolled wire rod, which differs from Example 1 in that: the heating temperature of the heating furnace is controlled to be ≤1080°C, the furnace time is 3h, the initial rolling temperature is 1060°C, the final rolling temperature is 925°C, the spinning temperature is 905°C, the online molten salt isothermal process causes the wire rod to cool at a cooling rate of 31°C / s, and a hot-rolled wire rod finished product is obtained after it comes off the line. Embodiment 2:

[0039] A preferred embodiment of the method for manufacturing a low-carbon, high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 71%, Si: 0.37%, Mn: 0.58%, Cr: 0.16%, P: 0.012%, S: 0.015%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity, avoiding the risk of burning and decarburization. After the steel billet comes out of the heating furnace, it is rolled into a wire with a diameter of 9mm through a rolling line to promote dynamic recrystallization during the final rolling process and refine the grains. Specifically: the heating temperature of the heating furnace is controlled to be ≤1130°C, the time in the furnace is 3h, the initial rolling temperature is 1115°C, the initial rolling reduction is 27%, the final rolling temperature is 960°C, and the final rolling reduction is 33%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate wire-spinning temperature, favorable conditions are created for refining the troostite layer. Specifically: the wire-spinning temperature is controlled to be 940°C.

[0040] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 34°C / s, and enters the troostite phase region from the austenite state, inhibits the peak precipitation of tertiary cementite, refines the cementite carbide, and partially forms a high-temperature austenite structure with fine interlamellar spacing as the main structure of troostite. Then, it is conveyed by a roller through the second stage salt bath tank for back-end treatment, and the temperature is precisely controlled to promote the untransformed residual austenite. The molten salt temperature of the front and rear treatments is 620°C, the treatment time of the front treatment is 20s, the molten salt circulation volume is 480t / h, and the molten salt temperature rise is ≤10°C; the treatment time of the rear treatment is 160s, the molten salt circulation volume is 250t / h, and the molten salt temperature rise is ≤3°C, that is, the online molten salt isothermal treatment time is 180s.

[0041] The roller slow cooling process adopts a cover insulation cover but does not lock the insulation cover, and the wire rod coming out of the second salt bath tank is conveyed into the insulation cover by rollers for slow cooling treatment to avoid insufficient cooling and reduce the plasticity of the wire rod. At the same time, slow cooling promotes further toughening of the wire rod structure and further spheroidization of carbides. Specifically: the wire rod is conveyed at a roller speed of 0.4m / s, and the wire rod is controlled to be cooled to 280℃ at a slow cooling rate of 0.7℃ / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 2 shown.

[0042] Comparative Example 3: A method for manufacturing a hot-rolled wire rod, which differs from Example 2 in that the online molten salt isothermal process cools the wire rod at a cooling rate of 37°C / s, the molten salt temperature of the front-end treatment and the back-end treatment is 565°C, and the hot-rolled wire rod finished product is obtained after going offline.

[0043] Comparative Example 4: A method for manufacturing a hot-rolled wire rod, which differs from Example 2 in that the online molten salt isothermal process cools the wire rod at a cooling rate of 32°C / s, the molten salt temperature of the front-end treatment and the back-end treatment is 635°C, and the hot-rolled wire rod finished product is obtained after going offline. Embodiment 3:

[0044] A preferred embodiment of the method for manufacturing a low-carbon, high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 73%, Si: 0.25%, Mn: 0.55%, Cr: 0.22%, P: 0.015%, S: 0.012%, and the rest is Fe and unavoidable impurities; the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 180mm×180mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity, avoiding the risk of burning and decarburization. After the steel billet comes out of the heating furnace, it is rolled into a wire with a diameter of 7mm through a rolling line to promote dynamic recrystallization during the final rolling process and refine the grains. Specifically: the heating temperature of the heating furnace is controlled to be ≤1115°C, the time in the furnace is 2h, the initial rolling temperature is 1090°C, the initial rolling reduction is 32%, the final rolling temperature is 950°C, and the final rolling reduction is 38%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate wire-spinning temperature, favorable conditions are created for refining the troostite layer. Specifically: the wire-spinning temperature is controlled to be 930°C.

[0045] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 36°C / s, and enters the troostite phase region from the austenite state, inhibits the peak precipitation of tertiary cementite, refines the cementite carbide, and partially forms a high-temperature austenite structure with fine interlamellar spacing as the main structure. Then, it is conveyed by a roller through the second stage salt bath tank for back-end treatment, and the temperature is precisely controlled to promote the untransformed residual austenite. The molten salt temperature of the front and rear treatments is 588°C, the treatment time of the front treatment is 35s, the molten salt circulation volume is 610t / h, and the molten salt temperature rise is ≤10°C; the treatment time of the rear treatment is 165s, the molten salt circulation volume is 220t / h, and the molten salt temperature rise is ≤3°C, that is, the online molten salt isothermal treatment time is 200s.

[0046] The roller slow cooling process adopts a cover insulation cover but does not lock the insulation cover, and the wire rod coming out of the second salt bath tank is conveyed into the insulation cover by rollers for slow cooling treatment to avoid insufficient cooling and reduce the plasticity of the wire rod. At the same time, slow cooling promotes further toughening of the wire rod structure and further spheroidization of carbides. Specifically: the wire rod is conveyed at a roller speed of 0.15m / s, and the wire rod is controlled to be cooled to 285℃ at a slow cooling rate of 0.5℃ / s; the coiling process is used to coil the wire rod into a coil through a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage. Its metallographic structure diagram is as follows Figure 3 shown.

[0047] Comparative Example 5: A method for manufacturing a hot-rolled wire rod, which differs from Example 3 in that the online molten salt isothermal treatment time is 250s, the front-end treatment time is 35s, the back-end treatment time is 215s, and the hot-rolled wire rod finished product is obtained after going offline.

[0048] Comparative Example 6: A method for manufacturing a hot-rolled wire rod, which differs from Example 3 in that the online molten salt isothermal treatment time is 80 s, the front-end treatment time is 35 s, the back-end treatment time is 45 s, and the hot-rolled wire rod finished product is obtained after going offline. Embodiment 4:

[0049] A preferred embodiment of the method for manufacturing a low-carbon, high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to the present invention, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 75%, Si: 0.4%, Mn: 0.42%, Cr: 0.28%, 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 molten salt isothermal → roller slow cooling → coiling, specifically: The rolling process is used to heat the steel billet with a specification of 220mm×220mm through a heating furnace to achieve a high-temperature steel billet that can be rolled into plasticity, avoiding the risk of burning and decarburization. After the steel billet comes out of the heating furnace, it is rolled into a wire with a diameter of 12mm through a rolling line to promote dynamic recrystallization during the final rolling process and refine the grains. Specifically: the heating temperature of the heating furnace is controlled to be ≤1140°C, the time in the furnace is 3h, the initial rolling temperature is 1130°C, the initial rolling reduction is 26%, the final rolling temperature is 970°C, and the final rolling reduction is 31%; the wire-spinning process is used to convert the wire coming out of the rolling line into a wire rod through a wire-spinning mechanism, and the wire rod is spread on the roller and transported along the roller. Through an appropriate wire-spinning temperature, favorable conditions are created for refining the troostite layer. Specifically: the wire-spinning temperature is controlled to be 945°C.

[0050] The online molten salt isothermal process adopts a two-stage salt bath tank with molten salt inside. The wire rod after spinning is conveyed by a roller through the first stage salt bath tank for front-end treatment, so that the wire rod is cooled at a cooling rate of 37°C / s, and enters the troostite phase region from the austenite state, inhibits the peak precipitation of tertiary cementite, refines the cementite carbide, and partially forms a high-temperature austenite structure with fine interlamellar spacing as the main structure of troostite. It is then conveyed by a roller through the second stage salt bath tank for back-end treatment, with precise temperature control to promote the untransformed residual Austenite continues to transform into troostite structure, while promoting isothermal tempering of wire rod structure, promoting carbide spheroidization, and adjusting the strength and plasticity matching of wire rod. Specifically: the molten salt temperature of the front and rear treatments is 575°C, the treatment time of the front treatment is 25s, the molten salt circulation volume is 680t / h, and the molten salt temperature rise is ≤10°C; the treatment time of the rear treatment is 60s, the molten salt circulation volume is 320t / h, and the molten salt temperature rise is ≤3°C, that is, the online molten salt isothermal treatment time is 85s.

[0051] The roller slow cooling process adopts a cover insulation cover but does not lock the insulation cover, and the wire rod coming out of the second salt bath tank is conveyed by rollers into the insulation cover for slow cooling treatment to avoid insufficient cooling and reduce the plasticity of the wire rod. At the same time, slow cooling promotes further toughening of the wire rod structure and further spheroidization of carbides. Specifically: the wire rod is conveyed at a roller speed of 0.1m / s, and the wire rod is controlled to be cooled to 288°C at a slow cooling rate of 0.45°C / s; the coiling process is used to coil the wire rod into a coil by a coiling drum, and the hot-rolled wire rod product is obtained after packaging and storage.

[0052] Comparative Example 7: A method for manufacturing a hot-rolled wire rod, which differs from Example 4 in that: the manufacturing method is manufactured according to the process flow of rolling → spinning → online molten salt isothermal → air cooling, and specifically: the air cooling process adopts opening the insulation cover, conveying the wire rod at a roller speed of 0.3m / s, controlling the wire rod to be air-cooled at a cooling rate of 1.4℃ / s, and obtaining the hot-rolled wire rod finished product after off-line.

[0053] The hot-rolled wire rods obtained in the above embodiments and comparative examples are subjected to organizational and performance testing, including: the tensile test is conducted according to GB-T 228.1-2021 Metallic Material Tensile Test Part 1: Room Temperature Test Method to obtain tensile strength and cross-sectional shrinkage; the organizational test is conducted according to the metal microstructure detection method of GB / T13298 standard; the mechanical property same-circle difference test method is as follows: 2 circles of wire rod are taken 5m away from the end of the coil, and each circle of wire rod is divided into 8 sections on average with the overlap area as the base point, and 1 tensile specimen is taken from each section. The extreme difference in strength of the tensile specimen after the tensile test is the same-circle difference in mechanical properties. The comparison results obtained are shown in Table 1 below: Table 1. Comparison of microstructure and performance of hot rolled wire rods with different compositions and manufacturing methods

[0054] It can be seen from the comparison results of Example 1 and Comparative Example 1 that compared with the existing Stelmor cooling line, the uncontrollable cooling rate and limited cooling capacity during the cooling phase transition weaken the strengthening effect, reduce the Cr and Mn content, and omit the B element, resulting in a large strength loss. At the same time, the risk of the appearance of network carbon and abnormal organization is high, resulting in large fluctuations in the mechanical properties of the wire rod and insufficient final plasticity of the wire rod. The present invention is based on the C-Si-Mn-Cr chemical composition combined with the online molten salt isothermal technology to control the wire rod to quickly enter the troostite phase region from the high-temperature austenite state, which can inhibit the three The peak precipitation of cementite refines cementite carbides to form a structure dominated by troostite with fine lamellar spacing, which makes up for the strength loss. At the same time, the tempered state is used to regulate the significantly toughened structure. From the results of Examples 1 to 4, it can be seen that the hot-rolled wire rod can reach a tensile strength of 1190 to 1240 MPa and a cross-sectional shrinkage rate of 49% to 54%, which can be used to manufacture 1860 MPa grade stranded wire and other application fields. It can not only meet the export tax rebate needs, but also further reduce material costs, take into account strength, and significantly improve plasticity to reduce the risk of wire breakage in stranded wire manufacturing.

[0055] From the comparison results of Example 1 and Comparative Example 2, it can be seen that since the online time can be shortened by rapid cooling of the online molten salt, a higher spinning temperature is adopted, which is beneficial to improving the supercooling degree, increasing the sorbite nucleation rate, and promoting the formation of a structure dominated by sorbite structure with fine lamellar spacing at the edge and core of the wire rod, so as to improve the matrix strength and tissue uniformity, and is also beneficial to increasing the rolling temperature, reducing the wear on the rolling line, and improving the rolling efficiency.

[0056] It can be seen from the results of Examples 1 to 4 that the lower the molten salt temperature of the online molten salt isothermal treatment, the finer the spacing between the formed troostite lamellae, but the thermodynamic force of tempering softening is relatively reduced, the strength of the wire rod increases, and the plasticity decreases. It can be seen from the comparison results of Example 2 and Comparative Example 3 that if the molten salt temperature is too low, there is a risk of the appearance of bainite structure, which is unfavorable for rapid tempering softening, and will lead to insufficient plasticity of the wire rod.

[0057] It can be seen from the results of Examples 1 to 4 that the higher the molten salt temperature of the online molten salt isothermal treatment, the coarser the spacing between the formed troostite lamellae, which can provide more thermal power for spheroidization and toughening, and the strength of the wire rod decreases and the plasticity increases. It can be seen from the comparison results of Example 2 and Comparative Example 4 that if the molten salt temperature is too high, it will affect the nucleation of troostite, and there is a risk of the appearance of tempered pearlite and excessive loss of strength, resulting in insufficient strength of the wire rod.

[0058] It can be seen from the results of Examples 1 to 4 that the longer the treatment time of the online molten salt isothermal treatment, the more complete the transformation of the wire rod into troostite and the better the tempering softening effect. It can be seen from the comparison results of Example 3 and Comparative Example 5 that too long a treatment time will lead to an increase in online time and energy consumption. At the same time, the wire rod will be over-softened or lose its strength and plasticity due to the precipitation and coarsening of Cr and the aggregation and growth of micro-spheroidized carbides.

[0059] It can be seen from the results of Examples 1 to 4 that the shorter the treatment time of the online molten salt isothermal treatment, the lower the softening effect, the smaller the proportion of micro-spheroidized carbides, the higher the strength of the wire rod, and the lower the plasticity. From the comparison results of Example 3 and Comparative Example 6, it can be seen that if the treatment time is too short, even if the front-stage treatment has retained the high molten salt circulation cooling treatment, the treatment time of the rear-stage treatment is too short, which is not conducive to increasing the proportion of tempered troostite and organizational softening, and will lead to plastic loss of the wire rod and increased mechanical fluctuations.

[0060] From the comparison results of Example 4 and Comparative Example 7, it can be seen that slow cooling on a roller can promote further toughening and spheroidizing tempering of the wire rod structure by slow cooling, and can improve the softening effect of the wire rod compared to air cooling, avoiding insufficient cooling and reducing the plasticity of the wire rod.

[0061] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. 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 low-carbon, high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire, It is 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, and the chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.71%~0.75%, Si: 0.25%~0.40%, Mn: 0.42%~0.58%, Cr: 0.16%~0.28%, 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 ≥930°C, it is subjected to online molten salt isothermal treatment, so that the wire rod is cooled at a cooling rate of ≥34°C / s, enters the sorbite phase region from the austenite state, forms a structure mainly composed of sorbite, and isothermal tempering is performed to promote carbide spheroidization, and finally is slowly cooled through a roller to produce a hot-rolled wire rod with a mixed structure composed of tempered sorbite, tempered ferrite and micro-spheroidized carbides, and the volume percentage of the tempered sorbite is ≥89%.

2. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to claim 1, It is characterized in that Before the rolling, the heating temperature of the heating furnace is controlled to be ≤1140°C and the time in the furnace is ≤3h.

3. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860MPa grade stranded wire according to claim 1, It is characterized in that During the rolling, the initial rolling temperature is controlled to be 1090-1130° C., the initial rolling reduction is 26%-32%, the final rolling temperature is ≥950° C., and the final rolling reduction is 31%-38%.

4. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 1, It is characterized in that The molten salt temperature of the online molten salt isothermal treatment is 575-620° C., and the treatment time is 85-200 s.

5. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 4, It is characterized in that The online molten salt isothermal treatment is divided into a front-end treatment and a back-end treatment. The treatment time of the front-end treatment is ≥20s, and the molten salt circulation amount of the front-end treatment is greater than the molten salt circulation amount of the back-end treatment.

6. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 5, It is characterized in that The molten salt circulation volume of the front-end treatment is 480-680 t / h, and the molten salt temperature rise is ≤10°C; the molten salt circulation volume of the back-end treatment is 220-320 t / h, and the molten salt temperature rise is ≤3°C.

7. The method for manufacturing the low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 4, It is characterized in that The roller slow cooling controls the wire rod to be cooled to below 300° C. at a slow cooling rate of 0.45-0.7° C. / s.

8. A low carbon high plasticity hot rolled wire rod for 1860MPa grade stranded wire, It is characterized in that The hot-rolled wire rod is manufactured by the method for manufacturing low-carbon and high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire as described in any one of claims 1 to 7.

9. The low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 8, It is characterized in that The interlamellar spacing of the tempered bainite is 95-130 nm, the network carbide grade of the hot-rolled wire rod is grade 0, and the mechanical property difference is ≤30 MPa.

10. The low-carbon high-plasticity hot-rolled wire rod for 1860 MPa grade stranded wire according to claim 8, It is characterized in that The hot-rolled wire rod has a diameter of 7.0-12.0 mm, a tensile strength of 1190-1240 MPa, and a cross-sectional shrinkage of 49%-54%.

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