Ultrahigh-strength hot-rolled wire rod having 1600 mpa tensile strength grade and manufacturing method therefor

Abstract

The present invention relates to an ultrahigh-strength hot-rolled wire rod having 1600 MPa tensile strength grade and a manufacturing method therefor. A C-Si-Mn-Al-Cr-V-Mo composition design is used for the hot-rolled wire rod. The manufacturing method comprises performing an online molten salt cooling treatment on a wire rod resulting from high-speed wire rolling and wire laying, wherein the temperature of a molten salt in the online molten salt cooling treatment is 480-540ºC, the isothermal holding time is 100-190 s, and the cooling rate of the wire rod is greater than or equal to 42ºC / s. The wire rod obtains a mixed microstructure composed of predominantly fine and uniform sorbite and containing a small amount of ferrite and dispersively distributed carbide precipitate phase. The wire rod can achieve a net-shaped carbide grade of 0, a sorbite microstructure interlamellar spacing of 60-80 nm, a tensile strength of greater than or equal to 1610 MPa, and a section shrinkage rate of greater than or equal to 30%. The wire rod is used in application fields such as manufacturing ultrahigh-strength bridge cables and steel strands, meets ultrahigh strengthening and lightweight requirements, and has good market application prospects.

Description

Ultra-high strength hot-rolled wire rod with tensile strength of 1600 MPa and manufacturing method thereof TECHNICAL FIELD

[0001] The present application belongs to the technical field of alloy materials, and particularly relates to an ultra-high strength hot-rolled wire rod with tensile strength of 1600 MPa and a manufacturing method thereof. BACKGROUND

[0002] For the application field of ultra-high strength bridge cable steel wire, the hot-rolled wire rod is generally a high-carbon steel grade. Molten iron is smelted and refined by an electric furnace to form molten steel, and then the molten steel is continuously cast, rolled and drawn to form a wire rod. The wire rod after drawing is usually subjected to final microstructure and performance control by a Stelmor air cooling line. However, the cooling control ability of the Stelmor air cooling line is insufficient. For example, the patent CN102352469B discloses an ultra-high strength vanadium-titanium composite micro-alloyed high-carbon steel wire rod and a manufacturing method thereof. The steel composition is C-Mn-Si-V-Ti, the drawing temperature is medium-high, and the cooling process is controlled by a Stelmor air cooling line. Even if fast cooling is used before the pearlite phase change of the air cooling line, the cooling rate can only reach 14℃ / s. The tensile strength Rm of the product is 1330-1410 MPa, and the reduction of area Z is ≥32%. The reason for the insufficient strength and plasticity is that, on the one hand, increasing the carbon content can increase the strength of the steel, but high carbon content increases the risk of precipitation of network carbides. The time of the wire rod in the network carbide precipitation sensitive zone during the cooling process is still relatively long, which leads to the precipitation of network carbides, causing a loss of strength, especially aggravating the deterioration of the plasticity of the wire rod. On the other hand, the grains in the as-cast structure are relatively coarse. Although the addition of V, Ti and other alloying elements can inhibit the growth of austenite grains during hot rolling, the VC particle coarsening caused by improper cooling control weakens the precipitation strengthening effect. The more and coarser the VC particles precipitate, the more the plasticity of the wire rod will be significantly reduced. The reduction of plasticity will lead to the fracture of the wire rod during the drawing process. In addition, the small amount of sorbite structure and the increase of the interlamellar spacing caused by improper cooling control will further reduce the strength of the wire rod. Furthermore, the insufficient cooling control ability of the air cooling line and the uneven cooling rate of the windward and leeward surfaces of the wire rod will increase the performance fluctuation of the wire rod.

[0003] Although there is a method of replacing air cooling with online water bath cooling after drawing in the prior art, for example, the patent CN114369760B discloses a wire rod for stress corrosion resistant ultra-high strength steel strand and a manufacturing method thereof. The steel composition is C-Mn-Si-Cr-V-B, the drawing temperature is high, and the cooling process is controlled by online water bath. The cooling rate can reach more than 15℃ / s. The tensile strength Rm of the product is 1360-1450 MPa, and the reduction of area Z is 20-25%. However, the cooling rate is still insufficient, which leads to limited improvement of the strength and plasticity. In addition, the bubbles formed on the surface of the wire rod by the water bath heating will also cause uneven cooling of the wire rod, increasing the mechanical property fluctuation of the wire rod.

[0004] And although for example the 2500MPa grade steel wire rod and its manufacturing method disclosed in patent CN112176258A, adopts C-Mn-Si-Cr-V-Ti composition, Stelmor cooling, coiling and salt bath cooling process design, uses salt bath cooling to reduce solidification segregation and improve material uniformity, so that the tensile strength Rm of the wire rod is 1610-1660MPa, the reduction of area Z is ≥28%, but after coiling, it needs to be spread, heated and offline salt bath heat treated and coiled again, the process is more, the energy consumption is larger.

[0005] SUMMARY

[0006] The present application aims to at least solve one of the above technical problems to a certain extent, and provides a tensile strength 1600MPa grade ultra-high strength hot rolled wire rod and its manufacturing method, which realizes the development of hot rolled wire rod to ultra-high strength and light weight, and has good market application prospect.

[0007] The technical scheme adopted by the present application to solve its technical problems is:

[0008] A manufacturing method of a tensile strength 1600MPa grade ultra-high strength hot rolled wire rod, the chemical composition and mass percentage of the hot rolled wire rod include: C: 0.90%-1.10%, Si: 0.60%-1.10%, Mn: 0.50%-0.90%, Al: 0.10%-0.30%, P≤0.020%, S≤0.008%, Cr: 0.10%-0.50%, V: 0.030%-0.070%, Mo: 0.10%-0.40%, the rest is Fe and inevitable impurities; the manufacturing method includes: the wire rod after high wire rolling and wire drawing is treated by on-line molten salt cooling, the molten salt temperature of on-line molten salt cooling treatment is 480-540℃, the isothermal time is 100-190s, and the cooling speed of the wire rod is ≥42℃ / s.

[0009] The chemical composition and mass percentage design of the above hot rolled wire rod includes:

[0010] (1) Carbon: C element is the most basic and economical strengthening element in steel, which is used to form enough carbide strengthening phase to significantly enhance the strength of the material to ensure the strength level of the steel, which is beneficial to improve the strength grade of the wire rod after drawing in the field of bridge cable, wire and the like, but too high C element will increase the continuous casting segregation and reduce the plasticity of the wire rod, resulting in fracture during drawing or twisting, therefore the mass percentage of C is controlled to be 0.90%-1.10%.

[0011] (2) Silicon and aluminum: Si and Al elements have similar effects in steel, which can expand the ferrite formation range, and enrichment at the ferrite / cementite interface is conducive to inhibiting the formation of cementite, improving the thermal stability during processing, and thus improving the plasticity of the wire rod. However, excessive Si will cause decarburization and reduce the surface quality of the wire rod, and excessive Al will cause nozzle clogging during continuous casting and accelerate decarburization, so the mass percentage of Si is controlled to be 0.60% to 1.10%, and the mass percentage of Al is controlled to be 0.10% to 0.30%.

[0012] (3) Manganese: Mn element can expand the austenite phase region, which is conducive to the refinement and regulation of the phase transformation structure and the content of the matrix structure. At the same time, the solid-solution Mn element can improve the strength of the steel to ensure the strength level of the steel. However, excessive Mn can easily promote the segregation of residual elements, increase the overheating sensitivity of the steel, and increase the difficulty of controlling the cooling structure, so the mass percentage of Mn is controlled to be 0.50% to 0.90%.

[0013] (4) Chromium: Cr element is a medium-strong carbide-forming element that can significantly improve the hardenability of the steel and strengthen the matrix. However, excessive Cr will produce low-temperature structures, which is not conducive to structure control and reduces plasticity and toughness, so the mass percentage of Cr is controlled to be 0.10% to 0.50%.

[0014] (5) Vanadium: V micro-alloying element mainly functions to form dispersed precipitation strengthening phase at medium temperature, strengthen the matrix, and reduce the hydrogen-induced cracking sensitivity of the wire rod, thereby making the wire rod have excellent hydrogen embrittlement resistance. However, excessive V will lead to high material cost and reduced plasticity, so the mass percentage of V is controlled to be 0.030% to 0.070%.

[0015] (6) Molybdenum: Mo element can improve the thermal stability of micro-alloy carbides, inhibit the precipitation and coarsening of Cr and the coarsening tendency of VC, which is conducive to the role of Cr and V and improves the material strength. However, excessive Mo content will increase the risk of decarburization, ferrite, and brittle phase, leading to reduced plasticity, so the mass percentage of Mo is controlled to be 0.10% to 0.40%.

[0016] (6) Phosphorus and sulfur: P will cause grain coarsening, significantly reducing the plasticity and toughness of the steel, and S will increase harmful impurities in the steel, causing thermal embrittlement, so the mass percentage of P is limited to ≤0.020%, and the mass percentage of S is limited to ≤0.008%.

[0017] The above manufacturing method is based on the design of V, Al, and Mo components, the addition of appropriate Cr and Si alloying elements, and the process of directly performing online molten salt cooling treatment after spinning:

[0018] On the one hand, the cooling speed of the wire rod is obviously increased compared with the Stelmor air cooling line and the on-line water bath treatment. Under the effect of ultra-fast cooling, the wire rod with high carbon content can quickly pass through the harmful net-shaped carbide precipitation temperature range, avoid the secondary cementite from precipitating from austenite and growing along the grain boundary to form a net-shaped distribution before the phase transformation of austenite, and quickly enter the sorbite phase region to precipitate fine and uniform sorbite, thereby improving the strength and especially the plasticity of the wire rod.

[0019] On the other hand, the high-temperature wire rod after spinning is rapidly cooled to the molten salt temperature through on-line molten salt cooling, the temperature drives the phase transformation of the wire rod, and the dispersed precipitation strengthening phase is formed at the medium temperature section. The higher the molten salt temperature, the longer the isothermal time, and the lower the cooling speed, the greater the tendency of VC coarsening when precipitating, the lower the content of austenite transformed into sorbite, the higher the content of ferrite, and the larger the sorbite lamellar spacing, which reduces the strength of the wire rod. However, if the molten salt temperature is too high and the isothermal time is too long, the continuous coarsening of VC will reduce the strength and especially the plasticity of the wire rod. On the contrary, the lower the molten salt temperature, the shorter the isothermal time, and the higher the cooling speed, the finer and more uniform the VC precipitates, the higher the content of austenite transformed into sorbite, the lower the content of ferrite, and the lower the sorbite lamellar spacing, which increases the strength and reduces the plasticity of the wire rod. However, if the molten salt temperature is too low, abnormal bainite structure will appear, which will significantly reduce the plasticity of the wire rod. If the isothermal time is too low, the structure will not be fully transformed, which will reduce the strength of the wire rod. Furthermore, on the basis of Mo inhibiting the coarsening of Cr and VC, by controlling the molten salt temperature and the isothermal time, the wire rod can obtain fine and uniform sorbite structure and dispersed distribution of carbides, so as to enhance the strength of the hot-rolled wire rod, realize excellent hydrogen brittleness resistance and strength-plasticity matching.

[0020] Preferably, the manufacturing method comprises high-speed wire rolling using a continuous casting billet obtained from molten steel through a continuous casting process, the C segregation index of the continuous casting billet is ≤1.04, the superheat degree of the continuous casting tundish of the continuous casting process is controlled to be 10-30°C, the carbon segregation of high-carbon steel is improved by reducing the undercooling degree, the mold molten steel surface fluctuation is controlled to be within ±3 mm, the pouring is stable, the first-end electromagnetic stirring current is 200-400 A, and the frequency is 1-5 Hz; the end electromagnetic stirring current is 350-550 A, and the frequency is 9-12 Hz. The use of electromagnetic stirring makes the impurities float to obtain good internal quality of the casting billet, promotes the formation of equiaxed crystal solidification structure, and avoids the uneven structure caused by segregation to affect the product quality.

[0021] Preferably, the high-speed wire rolling uses high-pressure descaling water to remove the oxide scale on the surface of the continuous casting billet before rough rolling, the pressure of the high-pressure descaling water is ≥16 MPa, and the oxide scale is prevented from being pressed into the surface of the wire rod during rolling to affect the surface quality.

[0022] Preferably, the high linear rolling adopts a heavy pre-precision rolling unit, a heavy precision rolling unit and a heavy reducing and sizing unit to carry out low-temperature rolling at a temperature of ≤860℃, and the final rolling reduction is 7-11%, so that the wire rod is refined by low-temperature heavy rolling with large reduction, the grain size of the wire rod is ≤18μm, the strain energy accumulation in the grain and the small original austenite grain provide a good phase change basic structure for the on-line molten salt cooling treatment, which is beneficial to further refining the interlamellar spacing, improving the sorbite rate and the material strength.

[0023] Preferably, the wire drawing adopts low-temperature wire drawing at a temperature of ≤860℃, compared with the conventional Stelmor air cooling, high-temperature wire drawing is needed to avoid the precipitation of net carbide during wire drawing or to reduce the net carbide precipitated in the air cooling section after wire drawing, since the on-line molten salt cooling treatment is directly carried out after wire drawing to improve the net carbide problem, low-temperature wire drawing can be adopted to further reduce the tendency of austenite grain growth, so that fine and numerous austenite grains are obtained, which is beneficial to the decomposition of the subsequent small austenite grains into fine sorbite structure, improving the sorbite rate and the material strength.

[0024] Preferably, the molten salt circulation speed of the on-line molten salt cooling treatment is 230-280t / h, the higher the molten salt circulation speed, the better the temperature precision of the molten salt, but the energy consumption cost increases, the temperature precision of the molten salt is controlled to be within ±3℃, so that the transformation temperature of the wire rod during the phase change process is ensured after the rapid heat exchange between the wire rod and the molten salt, and the cooling speed of the wire rod is preferably ≤50℃ / s, so that the wire rod obtains sufficient phase change, further improving the strength and plasticity of the wire rod.

[0025] A hot-rolled wire rod with a tensile strength of 1600MPa, which is obtained by the manufacturing method of any one of the above-mentioned hot-rolled wire rods with a tensile strength of 1600MPa.

[0026] Preferably, the hot-rolled wire rod has a sorbite structure with a volume percentage of ≥96%, and the rest is a mixed structure composed of ferrite and dispersed precipitated phase, the sorbite structure has high strength and good plasticity, and the sorbite rate is high, there is no residual austenite phase and bainite abnormal structure, and the wire rod has higher strength and better plasticity due to the fine and uniform VC particle dispersion, and the wire rod is suitable for being drawn into high-strength steel wire.

[0027] Preferably, the sorbite structure has a finer interlamellar spacing, and the interlamellar spacing of the sorbite structure is 60-80nm, so that the strength of the wire rod is higher.

[0028] Preferably, the hot-rolled wire rod has a net carbide content of <0.5 level, which can reach 0 level.

[0029] Preferably, the diameter of the hot-rolled wire rod is 12.5-15.0 mm, the tensile strength is ≥1610 MPa, and the reduction of area is ≥30%, so that the large-diameter hot-rolled wire rod has excellent strength and plasticity matching, the strength and plasticity have a level comparable to that of offline salt bath treatment, but the process steps are significantly less, the energy consumption is lower, and good industrial production adaptability is achieved.

[0030] Preferably, since the wire rod can be quickly and uniformly heat-exchanged when passing through the molten salt, there is no cold face and back cold face compared to the Stelmor air cooling line, and there is no bubble interference compared to online water bath cooling, so the cooling and organizational change are more uniform, and the mechanical fluctuation of the same circle of the hot-rolled wire rod is ≤24 MPa.

[0031] The application of the above-mentioned tensile strength 1600 MPa grade ultra-high strength hot-rolled wire rod includes the manufacture of bridge cables and steel strands, including bridge cables and steel strands of 2100 MPa grade and above.

[0032] Compared with the prior art, the beneficial effects of the present application are:

[0033] (1) In view of the problem that the strength grade of the existing hot-rolled wire rod is insufficient and cannot realize the development of hot-rolled wire rod in the direction of ultra-high strength and light weight and meet the market use requirements, the manufacturing method of the present application adopts C-Si-Mn-Al-Cr-V-Mo composition design and direct online rapid salt bath treatment process design after wire drawing, quickly passes through the harmful net-shaped carbide precipitation temperature interval through ultra-fast cooling, can achieve net-shaped carbide 0 grade, on the basis of Mo inhibiting the precipitation and coarsening of Cr and VC, by controlling the molten salt temperature and isothermal time, the isothermal phase change in the medium temperature section and the precipitation of the VC strengthening phase are simultaneously inhibited, the VC particle coarsening is inhibited, the sorbite content is increased, the sorbite lamella spacing is reduced, the bainite abnormal organization is avoided, a mixed organization mainly composed of fine and uniform sorbite organization and containing a small amount of ferrite and dispersedly distributed carbide precipitated phase is obtained, the sorbite organization lamella spacing is 60-80 nm, the excellent hydrogen brittleness resistance and strength and plasticity matching are realized, and further combined with the chemical composition characteristics of the steel + low temperature heavy load + low temperature wire drawing + online rapid cooling technology, the wire rod strength is further improved by further refining the fine primary austenite grains, and good industrial production adaptability is achieved.

[0034] (2) The present application successfully develops a tensile strength 1600 MPa grade hot-rolled wire rod, the tensile strength is ≥1610 MPa, the reduction of area is ≥30%, and the mechanical fluctuation of the same circle is ≤24 MPa, which is used in the application fields of ultra-high strength bridge cables, steel strands and the like, meets the demand for ultra-high strengthening and light weight, and has good market application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0035] The above and / or additional aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the following drawings of which:

[0036] Figure 1 is a metallographic structure diagram of the present application embodiment 1;

[0037] Figure 2 is a metallographic structure diagram of the present application comparative example 1;

[0038] Figure 3 is a metallographic structure diagram of the present application embodiment 2;

[0039] Figure 4 is a metallographic structure diagram of the present application comparative example 2;

[0040] Figure 5 is a metallographic structure diagram of the present application embodiment 3;

[0041] Figure 6 is a metallographic structure diagram of the present application comparative example 3;

[0042] Figure 7 is a metallographic structure diagram of the present application embodiment 4. DETAILED DESCRIPTION

[0043] The embodiments described below with reference to the accompanying drawings are exemplary, merely for illustration and do not limit the description of the features and characteristics of the present application, to propose the best mode of carrying out the present application, are intended to explain the present application, and are sufficient to enable a person skilled in the art to carry out the present application, and cannot be understood as any limitation on the scope of the present application, the scope of the present application is limited by the appended claims only.

[0044] Embodiment 1:

[0045] A preferred embodiment of the manufacturing method of the tensile strength 1600MPa grade ultra-high strength hot-rolled rod described in the present application, the chemical composition and mass percentage of the hot-rolled rod includes: C: 0.9%, Si: 1.1%, Mn: 0.78%, Al: 0.26%, P: 0.02%, S: 0.008%, Cr: 0.25%, V: 0.05%, Mo: 0.4%, the rest is Fe and unavoidable impurities; the manufacturing method is produced according to the process flow of continuous casting→heating furnace→high-speed wire rolling→wire drawing→on-line molten salt cooling treatment, specifically:

[0046] The continuous casting process is used to make the high-temperature molten steel obtained by molten iron pretreatment, converter smelting, LF furnace refining and VD vacuum degassing into continuous casting billets through a continuous casting machine, the superheat of the continuous casting tundish is controlled at 10-15℃, the crystallizer steel liquid surface fluctuation is controlled within ±3mm to stabilize pouring, electromagnetic stirring is used, the first end electromagnetic stirring current is 300A, the frequency is 2Hz; the end electromagnetic stirring current is 500A, the frequency is 10Hz, so that the impurities float to obtain good internal quality of the continuous casting billet, the C segregation index of the continuous casting billet is 1.03, and the specification is 160mm×160mm square billet.

[0047] The heating furnace process is used to heat the continuous casting billet to high temperature red steel with rolling plasticity through a heating furnace; the high wire rolling process is used to roll the high temperature red steel into wire through a rolling line hot rolling; the red steel discharged from the heating furnace is oxidized on the surface to form a scale, and the scale on the surface of the continuous casting billet is removed by high pressure descaling water before rough rolling; the pressure of the high pressure descaling water is 17 MPa; the rolling line sequentially uses a heavy load pre-finishing rolling mill group at 860℃, a heavy load finishing rolling mill group at 852℃ and a heavy load reducing mill group at 840℃ to perform low temperature rolling; the final rolling reduction is 8.5%; the wire grain size is ≤17μm; the strain energy accumulation in the grain and the small primary austenite grains provide a good phase change basic structure for the on-line molten salt cooling treatment.

[0048] The wire drawing process is used to draw the wire rolled off the rolling line into a coil through a wire drawing machine; low temperature wire drawing is performed at a wire drawing temperature of 851℃ to further reduce the austenite grain growth tendency; the coil specification is a diameter of 12.5mm.

[0049] The on-line molten salt cooling treatment uses a salt bath tank with molten salt; when the coil after wire drawing passes through the salt bath tank, the coil exchanges heat with the molten salt to rapidly drop to the molten salt temperature; the high temperature molten salt heated by the coil in the salt bath tank continuously circulates with the relatively low temperature molten salt outside the salt bath tank to control the molten salt temperature rise in the salt bath tank; the molten salt circulation speed is 230-250t / h; the molten salt temperature is controlled to be 508℃; the molten salt temperature precision control is within ±3℃; the isothermal time is 135s; the coil cooling speed is 45℃ / s; the coil obtains fine and uniform sorbite structure and dispersed distribution of carbide; VC particles are precipitated fine and uniformly dispersed to enhance the strength of the hot rolled coil and realize the matching of strength and plasticity; the coil after the salt bath tank is cleaned, transported along a track, air cooled, coiled, transported along a line, finished, packaged, stored in a warehouse to obtain the hot rolled coil product; the hot rolled coil metallographic structure is shown in Fig. 1.

[0050] Comparative Example 1

[0051] A hot rolled coil manufacturing method, which is different from Example 1, is produced according to the process flow of continuous casting→heating furnace→high wire rolling→wire drawing→Stelmor air cooling; the rolling line of the high wire rolling sequentially uses a heavy load pre-finishing rolling mill group at 965℃, a heavy load finishing rolling mill group at 944℃ and a heavy load reducing mill group at 936℃; the wire drawing temperature is 952℃; the Stelmor air cooling is that the coil after wire drawing passes through a Stelmor air cooling line; the Stelmor line heat preservation cover is fully opened; the roller way speed is 1.1m / s; the first fan No.1-13 is fully opened at 100%; the total air volume of each fan is 260,000m 3 t / h; the average cooling speed of the coil is 13℃ / s; the hot rolled coil is obtained; the hot rolled coil metallographic structure is shown in Fig. 2.

[0052] Example 2

[0053] A preferred embodiment of the method for manufacturing the hot-rolled wire rod with tensile strength of 1600 MPa, the chemical composition and mass percentage of the hot-rolled wire rod comprising: C: 0.97%, Si: 0.6%, Mn: 0.9%, Al: 0.22%, P: 0.018%, S: 0.007%, Cr: 0.32%, V: 0.046%, Mo: 0.1%, and the rest being Fe and inevitable impurities; the manufacturing method is produced according to the process flow of continuous casting→heating furnace→high-speed wire rolling→wire rod forming→on-line molten salt cooling treatment, specifically:

[0054] The continuous casting process is used to make the high-temperature molten steel obtained through molten iron pretreatment, converter smelting, LF furnace refining, and VD vacuum degassing into a continuous casting billet, the overheat degree of the continuous casting tundish is controlled within 12-20℃, the crystallizer molten steel surface fluctuation is controlled within ±3mm to stabilize pouring, electromagnetic stirring is used, the first-end electromagnetic stirring current is 250A, and the frequency is 4Hz; the end electromagnetic stirring current is 400A, and the frequency is 10Hz, so that impurities float to obtain good internal quality of the continuous casting billet, the C segregation index of the continuous casting billet is 1.01, and the specification is 160mm×160mm square billet.

[0055] The heating furnace process is used to heat the continuous casting billet through the heating furnace to reach the high-temperature red steel with rolling plasticity; the high-speed wire rolling process is used to roll the high-temperature red steel into a wire rod through the rolling line hot rolling, the red steel discharged from the heating furnace is oxidized on the surface to form an oxide skin, the oxide skin on the surface of the continuous casting billet is removed by using high-pressure descaling water before rough rolling, the pressure of the high-pressure descaling water is 18MPa, the rolling line sequentially adopts a heavy-load pre-precision rolling mill group at 850℃, a heavy-load precision rolling mill group at 842℃, and a heavy-load reducing mill group at 829℃ to perform low-temperature rolling, the final rolling reduction is 9%, the wire rod grain size is ≤16μm, and the strain energy accumulation in the grain and the small primary austenite grains provide a good phase change basic structure for the on-line molten salt cooling treatment.

[0056] The wire rod forming process is used to make the wire rod discharged from the rolling line into a wire rod through the wire rod forming machine, low-temperature wire rod forming is performed at a wire rod forming temperature of 840℃ to further reduce the austenite grain growth tendency, and the wire rod specification is a diameter of 13mm.

[0057] The online molten salt cooling treatment adopts a salt bath tank with molten salt, and the wire rod after wire drawing is rapidly cooled to the temperature of the molten salt by heat exchange with the molten salt when passing through the salt bath tank. The high-temperature molten salt heated by the wire rod in the salt bath tank and the relatively low-temperature molten salt outside the salt bath tank continuously circulate to control the temperature rise of the molten salt in the salt bath tank. The circulation speed of the molten salt is 250-270 t / h, the temperature of the molten salt is controlled to be 536 ℃, the precision of the temperature of the molten salt is controlled to be within ±3 ℃, the isothermal time is 182 s, and the cooling speed of the wire rod is 43 ℃ / s. The wire rod obtains fine and uniform sorbite structure and dispersed carbide, VC particles are precipitated fine and uniformly dispersed, the strength of the hot-rolled wire rod is enhanced, and the strength and plasticity are matched, the wire rod obtained after passing through the salt bath tank is cleaned, transported along the track, air-cooled, coiled, transported along the line, and packaged, stored, and obtained as the finished product of the hot-rolled wire rod coil. The metallographic structure of the hot-rolled wire rod is shown in Fig. 3, and Pa1 in Fig. 3 is the measured distance of 10 sorbite lamellas.

[0058] Comparative Example 2

[0059] A manufacturing method of a hot-rolled wire rod, which is different from that of Example 2 in that the temperature of the molten salt in the online molten salt cooling treatment is 576 ℃, the isothermal time is 234 s, and the cooling speed of the wire rod is 39 ℃ / s. The hot-rolled wire rod is obtained, and the metallographic structure of the hot-rolled wire rod is shown in Fig. 4. Pa1 in Fig. 4 is the measured distance of 10 sorbite lamellas.

[0060] Example 3

[0061] A preferred embodiment of the manufacturing method of the anti-tensile-strength 1600 MPa grade ultra-high-strength hot-rolled wire rod, the chemical composition and mass percentage of the hot-rolled wire rod include: C: 1.1%, Si: 0.88%, Mn: 0.5%, Al: 0.3%, P: 0.018%, S: 0.008%, Cr: 0.47%, V: 0.033%, Mo: 0.33%, and the rest is Fe and inevitable impurities; the manufacturing method is produced according to the process flow of continuous casting→heating furnace→high-speed wire rolling→wire drawing→online molten salt cooling treatment, specifically:

[0062] The continuous casting process is used to make high-temperature molten steel obtained by molten iron pretreatment, converter smelting, LF furnace refining, and VD vacuum degassing into continuous casting billets. The superheat of the continuous casting tundish is controlled to be 14-22 ℃, the steel liquid level fluctuation in the crystallizer is controlled to be within ±3 mm to stabilize pouring, electromagnetic stirring is used, the first-end electromagnetic stirring current is 400 A, and the frequency is 2 Hz; the end electromagnetic stirring current is 500 A, and the frequency is 9 Hz, so that the impurities float to obtain good internal quality of the continuous casting billet. The C segregation index of the continuous casting billet is 0.98, and the specification is 160 mm x 160 mm square billet.

[0063] The heating furnace process is used to heat the continuous casting billet to a high-temperature red steel with rolling plasticity through a heating furnace; the high-speed wire rolling process is used to roll the high-temperature red steel into a wire through a rolling line hot rolling; the red steel discharged from the heating furnace is oxidized on the surface to form a scale; the scale on the surface of the continuous casting billet is removed by high-pressure descaling water before rough rolling; the pressure of the high-pressure descaling water is 18 MPa; the rolling line sequentially uses a heavy-load pre-precision rolling mill group at 822 ℃, a heavy-load precision rolling mill group at 814 ℃, and a heavy-load reducing mill group at 807 ℃ to perform low-temperature rolling; the final rolling reduction is 11%; the grain size of the wire is ≤16 μm; the strain energy accumulation in the grain and the small primary austenite grains provide a good phase transformation basic structure for the on-line molten salt cooling treatment.

[0064] The wire drawing process is used to draw the wire rolled off the rolling line into a coil through a wire drawing machine; low-temperature wire drawing is performed at a wire drawing temperature of 824 ℃ to further reduce the austenite grain growth tendency; the coil specification is a diameter of 14 mm.

[0065] The on-line molten salt cooling treatment uses a salt bath tank with molten salt; when the coil after wire drawing passes through the salt bath tank, the coil exchanges heat with the molten salt to rapidly drop to the molten salt temperature; the high-temperature molten salt heated by the coil in the salt bath tank continuously circulates with the relatively low-temperature molten salt outside the salt bath tank to control the molten salt temperature rise in the salt bath tank; the molten salt circulation speed is 260-280 t / h; the molten salt temperature is controlled to be 486 ℃; the molten salt temperature precision control is within ±3 ℃; the isothermal time is 119 s; the coil cooling speed is 47 ℃ / s; the coil obtains fine and uniform sorbite structure and dispersed carbides; VC particles are precipitated fine and uniformly dispersed to enhance the strength of the hot-rolled coil and realize the strength and plasticity matching; the coil after the salt bath tank is cleaned, air-cooled, and collected and wound, and then transported along a line to be finished, packaged, and stored to obtain a hot-rolled coil product; the hot-rolled coil metallographic structure is shown in FIG. 5; and in FIG. 5, Pa1 is the measured distance of 10 sorbite lamellas.

[0066] Comparative Example 3:

[0067] A hot-rolled coil manufacturing method, which is different from Example 3 in that the molten salt temperature of the on-line molten salt cooling treatment is 435 ℃, the isothermal time is 95 s, and the coil cooling speed is 53 ℃ / s to obtain a hot-rolled coil; and the hot-rolled coil metallographic structure is shown in FIG. 6.

[0068] Example 4:

[0069] The chemical composition and mass percentage of the hot-rolled wire rod according to the preferred embodiment of the application include: C: 1.04%, Si: 0.93%, Mn: 0.83%, Al: 0.1%, P: 0.016%, S: 0.006%, Cr: 0.14%, V: 0.062%, Mo: 0.26%, and the rest is Fe and inevitable impurities; the manufacturing method is produced according to the process flow of continuous casting-heating furnace-high-speed wire rolling-wire bar forming-on-line molten salt cooling treatment, and specifically:

[0070] The continuous casting process is used to make the high-temperature molten steel obtained through molten iron pretreatment, converter smelting, LF furnace refining and VD vacuum degassing into a continuous casting billet, the overheat degree of the continuous casting tundish is controlled within 20-30℃, the crystallizer steel liquid surface fluctuation is controlled within ±3mm to stabilize pouring, electromagnetic stirring is used, the first-end electromagnetic stirring current is 400A, and the frequency is 4Hz; the end electromagnetic stirring current is 350A, and the frequency is 12Hz, so that impurities float to obtain good internal quality of the continuous casting billet, the C segregation index of the continuous casting billet is 1.03, and the specification is 160mm*160mm square billet.

[0071] The heating furnace process is used to heat the continuous casting billet through the heating furnace to obtain high-temperature red steel with plasticity; the high-speed wire rolling process is used to roll the high-temperature red steel into a wire rod through a rolling line, the red steel discharged from the heating furnace is oxidized on the surface to form an oxide skin, the oxide skin on the surface of the continuous casting billet is removed by using high-pressure descaling water before rough rolling, the pressure of the high-pressure descaling water is 18MPa, the rolling line sequentially adopts a heavy-load pre-precision rolling mill group at 830℃, a heavy-load precision rolling mill group at 821℃ and a heavy-load reducing mill group at 819℃ to perform low-temperature rolling, the final rolling reduction is 8%, the grain size of the wire rod is ≤18μm, and the strain energy accumulation in the grain and the small primary austenite grains provide a good phase change basic structure for the on-line molten salt cooling treatment.

[0072] The wire bar forming process is used to form the wire rod discharged from the rolling line into a wire rod through a wire bar machine, low-temperature wire bar forming is performed at a wire bar temperature of 840℃ to further reduce the growth tendency of austenite grains, and the wire rod specification is a diameter of 14mm.

[0073] The online molten salt cooling treatment adopts a salt bath tank with molten salt inside. The wire rod after wire drawing is cooled rapidly to the temperature of molten salt by heat exchange with the molten salt when passing through the salt bath tank. The high-temperature molten salt heated by the wire rod in the salt bath tank and the relatively low-temperature molten salt outside the salt bath tank continuously circulate to control the temperature rise of the molten salt in the salt bath tank. The circulation speed of the molten salt is 245-275 t / h, the temperature of the molten salt is controlled at 522℃, the temperature precision of the molten salt is controlled within ±3℃, the isothermal time is 174 s, and the cooling speed of the wire rod is 44℃ / s. The wire rod obtains fine and uniform sorbite structure and dispersed carbides, and the VC particles are fine and uniformly dispersed, so as to enhance the strength of the hot-rolled wire rod and realize the matching of strength and plasticity, and the wire rod obtained after passing through the salt bath tank is cleaned, transported along the track, air-cooled, coiled, and then transported along the line for finishing, packaging and storage, to obtain the coiled product of the hot-rolled wire rod. The metallographic structure of the hot-rolled wire rod is shown in Fig. 7, and Pa1 in Fig. 7 is the measured distance of 10 sorbite lamellas.

[0074] Comparative Example 4

[0075] A method for manufacturing a hot-rolled wire rod, which is different from Example 4 in that the overheat degree of the continuous casting tundish in the continuous casting process is controlled at 25-35℃, the crystallizer molten steel surface fluctuation is controlled within ±5 mm to stabilize pouring, electromagnetic stirring is used, the terminal electromagnetic stirring current is 600 A, the frequency is 4 H, and the C segregation index of the continuous casting billet is 1.16, to obtain the hot-rolled wire rod.

[0076] Comparative Example 5

[0077] A method for manufacturing a hot-rolled wire rod, which is different from Example 4 in that the rolling line in the high-speed wire rolling process adopts a heavy-load pre-finishing rolling mill group at 920℃, a heavy-load finishing rolling mill group at 945℃ and a heavy-load reducing mill group at 904℃ in sequence to perform low-temperature rolling, and the final rolling reduction is 3%, and the wire rod grain size is ≤28 μm, and the wire drawing process adopts wire drawing at 925℃ to obtain the hot-rolled wire rod.

[0078] The hot-rolled wire rods obtained in the above examples and comparative examples are subjected to structure and performance detection. The same circle mechanical fluctuation range test method is as follows: 2 circles of wire rod are taken at 5 m from the coiled end, the lap area position is taken as the base point, each circle of wire rod is equally divided into 8 segments, and 1 tensile sample is taken on each segment. The strength range of the tensile sample after tensile test is the same circle difference of the wire rod. The tensile test is performed according to GB-T228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method to obtain the tensile strength and the reduction of area. The structure detection is performed according to the metal microstructure detection method of GB / T13298 standard to obtain the comparison results shown in Table 1.

[0079] Table 1. Comparison results of wire rod structure and performance of different hot-rolled wire rods and manufacturing methods

[0080] In view of the problems that the existing hot-rolled wire rod is insufficient in strength grade, cannot realize the development of the hot-rolled wire rod to the direction of super-high strengthening and light weight, and cannot meet the use requirements of the market, it can be seen from the comparison results of examples 1-4 and comparative example 1 that, on the basis of the composition design containing V, Al and Mo, and the addition of suitable Cr and Si alloy elements, the process of directly carrying out online molten salt cooling treatment after wire drawing is adopted, compared with the conventional Stelmor air cooling line, under the action of ultra-fast cooling, the harmful net-shaped carbide precipitation temperature interval can be quickly bypassed, and the net-carbon precipitation problem can be effectively improved.

[0081] Meanwhile, by controlling the molten salt temperature and isothermal time, the wire rod can obtain fine and uniform sorbite organization and dispersed distribution of carbides, the microstructure is ≥96% of sorbite organization, the rest is a mixed organization composed of ferrite and dispersed precipitates, the sorbite organization lamella spacing reaches 60-80 nm, compared with the conventional Stelmor air cooling line hot-rolled wire rod, the sorbite rate is higher, the precipitated phase and sorbite organization lamella spacing are finer, the excellent hydrogen brittleness resistance and strength-plasticity matching are realized, the wire rod strength is effectively improved, the diameter of the hot-rolled wire rod is 12.5-15.0 mm, the tensile strength is ≥1610 MPa, and the reduction of area is ≥30%, which can reach the level of offline salt bath treatment, but the process and energy consumption are smaller, and because the wire rod exchanges heat with the molten salt more uniformly, the mechanical properties of the wire rod are effectively controlled, the mechanical fluctuation of the same circle is ≤24 MPa, which can be used for manufacturing super-high strength bridge cable, steel strand and other application fields, meets the demand of super-high strengthening and light weight, and has good market application prospect.

[0082] It can be seen from the comparison results of example 2 and comparative example 2 that, if the molten salt temperature is too high and the isothermal time is too long, the strength loss will be caused by the decrease of the sorbite rate and the increase of the sorbite lamella spacing, the wire rod strength will be further reduced due to the continuous coarsening of VC, and the plasticity will be significantly reduced; it can be seen from the comparison results of example 3 and comparative example 3 that, when the molten salt temperature is too low, the bainite abnormal organization will appear, causing the significant reduction of the plasticity of the wire rod, and when the isothermal time is too low, the organization is not fully transformed, causing the reduction of the sorbite rate and the significant reduction of the plasticity, which is manifested as the sharp reduction of the reduction of area, indicating that the hot-rolled wire rod is easy to be broken, even if the strength is high, but it is not suitable for drawing application, therefore, the temperature is used to drive the phase change of the wire rod organization, and the dispersed precipitation strengthening phase is formed at the medium temperature, which can avoid the appearance of bainite, residual austenite and other abnormal organizations, and further realize the strength-plasticity matching of the wire rod.

[0083] From the comparison results of Example 4 and Comparative Example 4, it can be seen that segregation can cause uneven structure and affect product quality; from the comparison results of Example 4 and Comparative Example 5, it can be seen that the present application combines the chemical composition characteristics of steel + low temperature heavy load + low temperature wire drawing + online rapid cooling technology, and can further refine the grains with small original austenite grains, improve the strength of the rod, and has good industrial production adaptability.

[0084] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present application, and are not used to limit the protection scope of the present application. The molten steel of the continuous casting billet can be smelted by top and bottom combined blowing, the steel liquid fluidity is improved, and the impurity elements such as P and S in the steel liquid are reduced. Any equivalent embodiment or change made without departing from the spirit of the present application shall be included in the protection scope of the present application.

Claims

1. A method for manufacturing ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa, characterized in that: The chemical composition and mass percentage of the hot-rolled wire rod include: C: 0.90% to 1.10%, Si: 0.60% to 1.10%, Mn: 0.50% to 0.90%, Al: 0.10% to 0.30%, P≤0.020%, S≤0.008%, Cr: 0.10% to 0.50%, V: 0.030% to 0.070%, Mo: 0.10% to 0.40%, and the remainder is Fe and unavoidable impurities. The manufacturing method includes: high-speed rolling using a continuous casting billet, the continuous casting billet is obtained by continuous casting of molten steel, and the C segregation index of the continuous casting billet is ≤1.

04. The final rolling reduction of the high-speed wire rolling is 7-11%, and the wire grain size is ≤18μm; the wire rod after high-speed wire rolling and spinning is subjected to online molten salt cooling treatment, the molten salt temperature of the online molten salt cooling treatment is 480-540℃, the isothermal time is 100-190s, and the wire rod cooling rate is ≥42℃ / s; the hot-rolled wire rod structure is a sorbite structure with a volume percentage of ≥96%, and the rest is a mixed structure composed of ferrite and dispersed precipitate phases. The interlamellar spacing of the sorbite structure is 60-80nm, the cross-sectional shrinkage rate is ≥30%, and the network carbide of the hot-rolled wire rod is <0.5 level.

2. The method for manufacturing the ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 1, characterized in that: The superheat of the continuous casting tundish in the continuous casting process is controlled at 10-30°C, the fluctuation of the steel liquid level in the crystallizer is controlled within ±3mm, the electromagnetic stirring current at the head end is 200-400A, and the frequency is 1-5Hz; the electromagnetic stirring current at the end end is 350-550A, and the frequency is 9-12Hz.

3. The method for manufacturing the ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 1, characterized in that: The high-speed wire rolling adopts a heavy-load pre-finishing mill, a heavy-load finishing mill and a heavy-load reducing and sizing mill to perform low-temperature rolling at a temperature of ≤860°C.

4. The method for manufacturing ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 1, characterized in that: The silk spinning is carried out at a low temperature of 860°C or less.

5. The method for manufacturing ultra-high strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 1, characterized in that: The molten salt circulation rate of the online molten salt cooling treatment is 230-280 t / h, and the molten salt temperature accuracy is controlled within ±3°C.

6. An ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa, characterized in that: The hot-rolled wire rod is obtained by the method for manufacturing ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa according to any one of claims 1 to 5.

7. The ultra-high-strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 6, characterized in that: The hot-rolled wire rod has a diameter of 12.5 to 15.0 mm, a tensile strength of ≥1610 MPa, and a mechanical fluctuation within the same coil of ≤24 MPa.

8. The use of the ultra-high strength hot-rolled wire rod with a tensile strength of 1600 MPa according to claim 6, characterized in that: Including use in the manufacture of bridge cables and steel strands.

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