Wire rod for fastener and production method for wire rod, and fastener and production method therefor

Abstract

The present application discloses a wire rod for fastener and a production method thereof, and a production method for fastener. The chemical composition of the wire rod and fastener comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

Description

[0001] The present application is based on and claims priority to Chinese Patent Application No. 202311084987.5 filed on August 28, 2023, the entire contents of which are incorporated herein by reference.TECHNICAL FIELD

[0002] The present application belongs to the field of steel smelting technology, relates to a wire rod for fastener and a production method thereof, and also relates to a fastener and a production method of the fastener.BACKGROUND

[0003] Fasteners, as the source of industry, are widely used in the fields of automotive, mechanical manufacturing, energy, aerospace and the like. In recent years, with the development of economy and the advancement of technology, industrial applications have proposed requirements of high design stress and lightweight for steel for fastener. To meet the requirements, the priority is to improve the strength of fasteners, so that under the same clamping force, the weight can be reduced and installation space can be increased by reducing their own size, thereby optimizing the function and volume of connected components to achieve the purpose of overall weight reduction and performance optimization. Further, improving the strength of fasteners can also improve the fuel efficiency of automobiles, reduce fuel consumption, and achieve ultra-low carbon emissions.

[0004] However, with the improvement of steel strength, especially when the tensile strength exceeds 1200MPa, the delayed fracture problem of steel becomes very prominent. High-strength fasteners have very high notch sensitivity and are prone to delayed fracture at notch stress concentration parts such as the transition between the shank and head or at the thread root, causing major safety accidents and economic losses. Therefore, how to improve the delayed fracture resistance of fasteners while improving their strength has been a hot topic in domestic and foreign research work in recent years.SUMMARY

[0005] In order to solve at least one of the above technical problems, an object of the present application is to provide a wire rod for fastener and a production method thereof, and also provides a fastener and a production method of the fastener.

[0006] To achieve one of the above objects, an embodiment of the present application provides a wire rod for fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0007] As a further improvement of an embodiment of the present application, the chemical composition of the wire rod for fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0008] As a further improvement of an embodiment of the present application, the chemical composition of the wire rod for fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0009] To achieve one of the above objects, an embodiment of the present application also provides a production method for the wire rod for fastener as described above, comprising sequentially performing molten steel smelting process, continuous casting process, slow cooling process, heating process, high-speed wire rolling process, and controlled cooling process; wherein, in the continuous casting process, the temperature of the tundish is controlled at 1510~1530°C; in the slow cooling process, the continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C; the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min; in the high-speed wire rolling process, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C; in the controlled cooling process, the laying head temperature is 890~910°C; the heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed, the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off, the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s, and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C.

[0010] As a further improvement of an embodiment of the present application, the molten steel smelting process comprises sequentially performing converter steelmaking step and Ladle Furnace refining step; in the converter steelmaking step, the molten steel at the converter steelmaking end point contains C 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying, argon gas is blown from the bottom of the ladle throughout the tapping process, and the flow rate of argon gas blown from the bottom of the ladle is 400~800L / min; in the Ladle Furnace refining step, after the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5-10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.

[0011] As a further improvement of an embodiment of the present application, in the continuous casting process, the continuous casting adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, and the mold flux adopts medium carbon steel mold flux; the superheat of molten steel in the tundish is controlled at 25~35°C, the mold level height is 80~87% of the mold height, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.

[0012] To achieve one of the above objects, an embodiment of the present application also provides a fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0013] As a further improvement of an embodiment of the present application, the chemical composition of the fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0014] As a further improvement of an embodiment of the present application, the chemical composition of the fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0015] As a further improvement of an embodiment of the present application, the tensile strength of the fastener is ≥2000MPa, the yield strength is ≥1750MPa, and the delayed fracture strength ratio is ≥0.85.

[0016] To achieve one of the above objects, an embodiment of the present application also provides a production method for the fastener as described above, wherein the fastener is prepared by using wire rod for fastener and sequentially performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process; wherein, the annealing process comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace; in the warm heading forming process, the fine wire obtained from the drawing process is heated to 530~570°C and then headed to form; the quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, and the holding time is 30~60min; the heating temperature of the tempering step is 550~580°C, and the holding time is 60~90min, after tempering, air cooling to room temperature.

[0017] As a further improvement of an embodiment of the present application, the quenching medium of the quenching step is oil.

[0018] Compared with the prior art, the beneficial effects of the present application are: (1) In the chemical composition design of the present application, under comprehensive consideration of the effects of different elements on the strength and toughness, delayed fracture resistance and forming performance of fasteners, the addition and content of each element are precisely selected and controlled, under the condition of greatly reducing the contents of C, Si, Mn elements, through the limitation of the content relationship of Cr, Ni, Mo, Ti elements, Cr and Ni form interstitial solid solution in steel and form clusters to significantly improve the strength, corrosion resistance and oxidation resistance of steel, inhibit hydrogen invasion, and Ni and Ti combine to generate nano precipitated phases with finer size than carbides, which strengthen the matrix while forming stronger hydrogen traps, further improving the delayed fracture resistance of steel. In addition, Mo segregates at the interface between precipitated phases and matrix, which can promote and accelerate the precipitation process, so that the finally prepared fastener has tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, while also reducing the dependence of high-strength fasteners on precious elements such as Mo, V, Co, greatly saving production costs and facilitating industrial production. (2) On the basis of the aforementioned chemical composition design, through the regulation of a series of process means, the wire rod for fastener has good microstructure and mechanical properties, the wire rod microstructure is fine equiaxed ferrite and martensite composite microstructure, the tensile strength is 1250~1350MPa, and the wire rod surface quality is excellent. (3) Further combined with a series of forming processes including acid pickling and phosphating, drawing, warm heading forming, thread rolling, quenching and tempering, and surface polishing, not only can cracking of fasteners during the heading forming process be avoided, but also the finally prepared fastener has a microstructure of tempered sorbite, tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications. In addition, compared to the CNC lathe forming process, the production efficiency can be significantly improved, which is suitable for industrial mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is a schematic diagram of specimen notch when performing slow strain rate tensile test on fasteners in the present application.DETAILED DESCRIPTION

[0020] The technical solution of the present application will be further introduced below in combination with specific embodiments, but the scope of protection is not limited to the description made.

[0021] An embodiment of the present application provides a wire rod for ultra-high strength fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0022] Wherein, [B] represents the percent by mass of B, [Nb] represents the percent by mass of Nb, [V] represents the percent by mass of V, [Cr] represents the percent by mass of Cr, [Ni] represents the percent by mass of Ni, [Mo] represents the percent by mass of Mo, and [Ti] represents the percent by mass of Ti.

[0023] In the chemical composition design of the present application, under comprehensive consideration of the effects of different elements on the strength and toughness, delayed fracture resistance and forming performance of fasteners, the addition and content of each element are precisely selected and controlled, under the condition of greatly reducing the contents of C, Si, Mn elements, through the limitation of the content relationship of Cr, Ni, Mo, Ti elements, Cr and Ni form interstitial solid solution in steel and form clusters to significantly improve the strength, corrosion resistance and oxidation resistance of steel, inhibit hydrogen invasion, and Ni and Ti combine to generate nano precipitated phases with finer size than carbides, which strengthen the matrix while forming stronger hydrogen traps, further improving the delayed fracture resistance of steel. In addition, Mo segregates at the interface between precipitated phases and matrix, which can promote and accelerate the precipitation process, so that the finally prepared fastener has tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, while also reducing the dependence of high-strength fasteners on precious elements such as Mo, V, Co, greatly saving production costs and facilitating industrial production.

[0024] Preferably, the chemical composition of the wire rod for ultra-high strength fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0025] More preferably, the chemical composition of the wire rod for ultra-high strength fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0026] An embodiment of the present application also provides an ultra-high strength fastener, which is prepared by using the above wire rod for ultra-high strength fastener as base material, and has the same chemical composition as the wire rod for ultra-high strength fastener.

[0027] An embodiment of the present application also provides a preferred production method for the wire rod for ultra-high strength fastener, wherein the production method comprises sequentially performing molten steel smelting process, continuous casting process, slow cooling process, heating process, high-speed wire rolling process, and controlled cooling process to prepare the wire rod for ultra-high strength fastener. The chemical composition of the wire rod is as described above and will not be repeated here.

[0028] The production method will be described in detail below according to the production sequence.(1) Molten steel smelting process

[0029] It comprises sequentially performing converter steelmaking step and Ladle Furnace refining step.a. Converter steelmaking step

[0030] Scrap steel and blast furnace hot metal are sent to the converter for steelmaking, the molten steel at the converter steelmaking end point contains C with a percent of 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying to reduce oxidation burning loss and improve the utilization efficiency of alloys.

[0031] Preferably, argon gas is blown from the bottom of the ladle throughout the tapping process, and the flow rate of argon gas blown from the bottom of the ladle is 400~800L / min.b. Ladle Furnace refining step

[0032] After the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5~10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.(2) Continuous casting process

[0033] The molten steel from the Ladle Furnace refining process is continuously cast into continuous cast billet, the temperature of the tundish is controlled at 1510~1530°C to improve the fluidity of molten steel, improve the poor fluidity of molten steel caused by the high content of Cr, Ni, Mo elements in the steel, and improve the uniformity of alloy elements.

[0034] Preferably, the continuous casting process adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, and the mold flux adopts medium carbon steel mold flux.

[0035] Preferably, during the continuous casting process the superheat of molten steel in the tundish is controlled at 25~35°C, the mold level height is 80~87% of the mold height, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.(3) Slow cooling process

[0036] The continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C.(4) Heating process

[0037] The continuous cast billet after the slow cooling process is sent to a heating furnace for heating, the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min.

[0038] Through segmented control of heating temperature, heating rate and heating time, the temperature of the continuous cast billet can be uniform, and alloy elements can be fully dissolved in austenite, thereby avoiding cracks occurred in the continuous cast billet due to excessive temperature stress during subsequent rolling process, while improving rolling production rhythm and production efficiency.(5) High-speed wire rolling process

[0039] The continuous cast billet after the heating process is rolled into wire rod and coiled into coils, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C. This can avoid the brittle temperature range of the steel of the present application, so that the rolled wire rod has good microstructure and mechanical properties, and improves the surface quality of the wire rod.

[0040] Wherein, the diameter of the wire rod is 5~26mm.(6) Controlled cooling process

[0041] The obtained coil is sent to the Stelmor cooling line for cooling, and the laying head temperature is 890~910°C.

[0042] The heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed; the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off, the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s, and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C, that is, the temperature of the wire rod when entering the first closed heat retaining cover is controlled at 750~780°C. Through the above process and parameter settings, the microstructure uniformity of the obtained wire rod can be improved, so that fine equiaxed ferrite and martensite microstructure are formed in the wire rod, which is beneficial to subsequent annealing, drawing and forming processing when further preparing fasteners.

[0043] Thus, the production method of the present embodiment, on the basis of the aforementioned chemical composition design, through the regulation of a series of process means, enables the finally prepared wire rod for fastener to have good microstructure and mechanical properties, the microstructure of the wire rod is fine equiaxed ferrite and martensite composite microstructure, the tensile strength is 1250~1350MPa, and the wire rod surface quality is excellent.

[0044] Further, the above wire rod for ultra-high strength fastener is further processed by sequentially performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process to prepare ultra-high strength fastener. The specific processes are as follows:(7) Annealing process

[0045] The annealing process comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace. This way, the wire rod can have spheroidal pearlite microstructure, allowing carbides to precipitate uniformly and distribute in the ferrite matrix, thereby reducing the hardness of the wire rod to facilitate subsequent drawing and forming processing.(8) Acid pickling and phosphating process

[0046] The annealed wire rod is subjected to acid pickling and phosphating treatment to remove the oxide scale on the surface of the wire rod and form a phosphate film on the surface of the wire rod to improve its lubrication performance and reduce damage to the die during subsequent drawing and heading forming.(9) Drawing process

[0047] The wire rod after acid pickling and phosphating process is drawn to fine wire of set size, and the diameter reduction amount during drawing is controlled according to the finished product requirements of the fastener, wherein the so-called diameter reduction amount during drawing is the difference between the diameter of the wire rod and the diameter of the fine wire.

[0048] Preferably, the diameter reduction amount during drawing is controlled at 0.5~1.5mm.(10) Warm heading forming process

[0049] The fine wire obtained from the drawing process is heated to 530~570°C and then headed to form. Through warm heading forming, the large resistance and work hardening generated during cold heading forming can be avoided, so as to facilitate heading of fasteners with complex shapes or large deformation.(11) Thread rolling process

[0050] Thread rolling processing is performed on the fastener after warm heading forming to form threads.(12) Quenching and tempering process

[0051] The quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, and the holding time is 30~60min, so that the microstructure of the fastener is completely transformed into austenite, then cooled with a cooling rate of 80~90°C / s, so that the supercooled austenite is transformed into martensite; the heating temperature of the tempering step is 550~580°C, and the holding time is 60~90min, so that the microstructure of the fastener is transformed into tempered sorbite, after tempering, air cooling to room temperature, thereby both improving the strength of the fastener and improving plasticity and toughness, so that the fastener has good comprehensive mechanical properties.

[0052] The quenching medium of the quenching step is oil.(13) Surface polishing process

[0053] Surface polishing treatment is performed on the fastener after the quenching and tempering process.

[0054] Thus, the fastener further prepared by drawing in the present embodiment, on the basis of the aforementioned chemical composition design, through a series of forming processes including acid pickling and phosphating, drawing, warm heading forming, thread rolling, quenching and tempering, and surface polishing, not only can cracking during the heading forming process be avoided, but also the finally prepared fastener has a microstructure of tempered sorbite, tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications. In addition, compared to the CNC lathe forming process, the production efficiency can be significantly improved, which is suitable for industrial mass production.

[0055] The specific embodiments of the present application are further introduced below through 10 examples. Of course, these 10 examples are only part of the numerous variant embodiments contained in the present embodiment, not all. Other embodiments made on the basis of the aforementioned embodiments do not depart from the technical spirit of the present application.Examples

[0056] First, Examples 1~10 all provide a wire rod for ultra-high strength fastener, and an ultra-high strength fastener prepared from the wire rod by further performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process, the chemical compositions of the wire rod and fastener are shown in Table 1, with the balance being Fe and unavoidable impurities. Table 1Example12345678910C0.310.350.380.420.270.340.330.360.280.41Si0.270.180.230.170.30.470.320.430.250.38Mn0.580.720.620.530.680.730.650.780.630.52P0.0090.0070.010.0120.0060.0050.0080.010.0060.007S0.0020.0030.0040.0050.0030.0030.0040.0020.0050.004Cr8.946.627.516.2610.457.635.349.6811.178.25Chemical composition in percent by mass %Ni2.642.132.682.513.582.682.493.464.32.58Cu0.020.030.010.020.020.030.010.020.030.02Ti0.0850.0720.0930.0680.0820.0710.0740.0530.0860.067Mo0.370.310.380.390.440.340.390.270.380.28Re0.0760.0250.0510.0840.0620.0730.0470.0150.0920.038Al0.0430.0370.0280.0190.0240.0450.0360.0330.0380.032O0.00070.00080.00090.00070.00090.00060.00070.00050.00060.0008N0.00340.00370.00420.00350.00440.00330.00410.00490.00380.0032B-0.00210.0022--0.002-0.0017-0.0018Nb0.0370.054-0.0520.0690.063-0.0540.061-V-0.079-0.063-0.0750.056-0.0580.057

[0057] The production method is as follows:(1) Molten steel smelting processa. Converter steelmaking step

[0058] Scrap steel and blast furnace hot metal are sent to the converter for steelmaking, the molten steel at the converter steelmaking end point contains C with a percent of 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, argon gas is blown from the bottom of the ladle throughout the tapping process, the argon gas flow rate blown from the bottom of the ladle is 400~800L / min, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying.b. Ladle Furnace refining step

[0059] After the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5~10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.(2) Continuous casting process

[0060] The molten steel from the Ladle Furnace refining process is continuously cast into continuous cast billet, the temperature of the tundish is controlled at 1510~1530°C, the superheat of molten steel in the tundish is 25~35°C, the mold level height is 80~87% of the mold height, the continuous casting process adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, the mold flux adopts medium carbon steel mold flux, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.(3) Slow cooling process

[0061] The continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C.(4) Heating process

[0062] The continuous cast billet after the slow cooling process is sent to a heating furnace for heating, the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min.(5) High-speed wire rolling process

[0063] The continuous cast billet after the heating process is rolled into wire rod with a diameter of 5-26mm and coiled into coils, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C.(6) Controlled cooling process

[0064] The obtained coil is sent to the Stelmor cooling line for cooling, and the laying head temperature is 890~910°C. The heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed; the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off; the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s; and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C.(7) Annealing process

[0065] Comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace.(8) Acid pickling and phosphating process

[0066] The annealed wire rod is subjected to acid pickling and phosphating treatment.(9) Drawing process

[0067] The wire rod after acid pickling and phosphating process is drawn to fine wire of set size, and the diameter reduction amount during drawing is controlled according to the finished product requirements of the fastener.(10) Warm heading forming process

[0068] The fine wire obtained from the drawing process is heated to 530~570°C and then headed to form.(11) Thread rolling process

[0069] Thread rolling processing is performed on the fastener after warm heading forming to form threads.(12) Quenching and tempering process

[0070] The quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, the holding time is 30~60min, then cooled, the quenching medium is oil, the cooling rate is 80~90°C / s, the heating temperature of the tempering step is 550~580°C, the holding time is 60~90min, after tempering, air cooling to room temperature.(13) Surface polishing process

[0071] Surface polishing treatment is performed on the fastener after the quenching and tempering process.

[0072] The fasteners of Examples 1~10 did not crack during the heading forming process, that is, the cracking rate was 0; during the processing, the average processing time for a single fastener was 0.5~1.5s, and the yield rate was >95%.

[0073] Samples were taken from the fasteners of 10 examples respectively for mechanical property testing and delayed fracture resistance testing.

[0074] In terms of mechanical properties, referring to ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products, a tensile testing machine was used to test the yield strength, tensile strength, and elongation after fracture of the fasteners.

[0075] In terms of delayed fracture resistance, the slow strain rate tensile test method was adopted, and the test process was as follows: Step 1, two groups of fastener specimens 10 were taken, referring to Figure 1, notches 20 were made in the middle of the two groups of fastener specimens 10 along the circumferential direction, the notch angle α=60°, one group of specimens was used as the hydrogen-charged group specimens, and the other group of specimens was used as the control group specimens. Step 2, the hydrogen-charged group specimens were placed in 4g / L NaOH solution for electrochemical hydrogen charging, the hydrogen charging time was 48h, the current density was 15A / m 2< , after hydrogen charging was completed, the specimens were cleaned and dried.

[0076] The control group specimens did not undergo the operation of Step 2.

[0077] Step 3, both groups of specimens were subjected to tensile test under room temperature using a slow strain rate tensile testing machine, the tensile strain rate was 5×10 -6< / s, and the tensile strengths of the two groups of specimens were measured respectively; Step 4, calculate the delayed fracture strength ratio, delayed fracture strength ratio = tensile strength of hydrogen-charged group specimens / tensile strength of control group specimens. Wherein, the larger the delayed fracture strength ratio, the better the delayed fracture resistance of the specimen, generally a delayed fracture strength ratio above 0.7 is judged as qualified, and a delayed fracture strength ratio above 0.8 is judged as excellent.

[0078] The mechanical property test results and delayed fracture strength ratio are shown in Table 2. Table 2ExampleTensile strength / MPaYield strength / MPaElongation after fracture / %Delayed fracture strength ratioExample 12174193612.60.88Example 22136187213.10.89Example 32142184714.20.86Example 42093180315.50.90Example 52291197214.90.87Example 62179189113.70.88Example 72085179614.20.89Example 82253201412.40.86Example 92348210311.80.86Example 102167192513.30.87

[0079] In summary, the fasteners of the examples of the present application, on the basis of chemical composition design, combined with the control of the entire production process of wire rod and fasteners, avoid cracking of fasteners during the heading forming process, and the prepared fasteners have tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with ultra-high tensile strength, excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, and can significantly improve production efficiency, which is suitable for industrial mass production.

[0080] It should be understood that although this specification is described in terms of embodiments, not every embodiment contains only one independent technical solution. This narrative manner of the specification is merely for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in various embodiments may also be appropriately combined to form other embodiments understandable to those skilled in the art.

[0081] The series of detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present application and are not intended to limit the protection scope of the present application. Any equivalent embodiments or modifications made without departing from the technical spirit of the present application should be included within the protection scope of the present application.

Examples

examples

Examples

[0056]First, Examples 1~10 all provide a wire rod for ultra-high strength fastener, and an ultra-high strength fastener prepared from the wire rod by further performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process, the chemical compositions of the wire rod and fastener are shown in Table 1, with the balance being Fe and unavoidable impurities.

Table 1

Example12345678910

C0.310.350.380.420.270.340.330.360.280.41

Si0.270.180.230.170.30.470.320.430.250.38

Mn0.580.720.620.530.680.730.650.780.630.52

P0.0090.0070.010.0120.0060.0050.0080.010.0060.007

S0.0020.0030.0040.0050.0030.0030.0040.0020.0050.004

Cr8.946.627.516.2610.457.635.349.6811.178.25

Chemical composition in percent by mass %Ni2.642.132.682.513.582.682.493.464.32.58

Cu0.020.030.010.020.020.030.010.020.030.02

Ti0.0850.0720.0930.0680.0820.0710.0740.0530.0860.067

Mo0.370.310.380.390.440.340...

[0001] The present application is based on and claims priority to Chinese Patent Application No. 202311084987.5 filed on August 28, 2023, the entire contents of which are incorporated herein by reference.TECHNICAL FIELD

[0002] The present application belongs to the field of steel smelting technology, relates to a wire rod for fastener and a production method thereof, and also relates to a fastener and a production method of the fastener.BACKGROUND

[0003] Fasteners, as the source of industry, are widely used in the fields of automotive, mechanical manufacturing, energy, aerospace and the like. In recent years, with the development of economy and the advancement of technology, industrial applications have proposed requirements of high design stress and lightweight for steel for fastener. To meet the requirements, the priority is to improve the strength of fasteners, so that under the same clamping force, the weight can be reduced and installation space can be increased by reducing their own size, thereby optimizing the function and volume of connected components to achieve the purpose of overall weight reduction and performance optimization. Further, improving the strength of fasteners can also improve the fuel efficiency of automobiles, reduce fuel consumption, and achieve ultra-low carbon emissions.

[0004] However, with the improvement of steel strength, especially when the tensile strength exceeds 1200MPa, the delayed fracture problem of steel becomes very prominent. High-strength fasteners have very high notch sensitivity and are prone to delayed fracture at notch stress concentration parts such as the transition between the shank and head or at the thread root, causing major safety accidents and economic losses. Therefore, how to improve the delayed fracture resistance of fasteners while improving their strength has been a hot topic in domestic and foreign research work in recent years.SUMMARY

[0005] In order to solve at least one of the above technical problems, an object of the present application is to provide a wire rod for fastener and a production method thereof, and also provides a fastener and a production method of the fastener.

[0006] To achieve one of the above objects, an embodiment of the present application provides a wire rod for fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0007] As a further improvement of an embodiment of the present application, the chemical composition of the wire rod for fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0008] As a further improvement of an embodiment of the present application, the chemical composition of the wire rod for fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0009] To achieve one of the above objects, an embodiment of the present application also provides a production method for the wire rod for fastener as described above, comprising sequentially performing molten steel smelting process, continuous casting process, slow cooling process, heating process, high-speed wire rolling process, and controlled cooling process; wherein, in the continuous casting process, the temperature of the tundish is controlled at 1510~1530°C; in the slow cooling process, the continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C; the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min; in the high-speed wire rolling process, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C; in the controlled cooling process, the laying head temperature is 890~910°C; the heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed, the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off, the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s, and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C.

[0010] As a further improvement of an embodiment of the present application, the molten steel smelting process comprises sequentially performing converter steelmaking step and Ladle Furnace refining step; in the converter steelmaking step, the molten steel at the converter steelmaking end point contains C 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying, argon gas is blown from the bottom of the ladle throughout the tapping process, and the flow rate of argon gas blown from the bottom of the ladle is 400~800L / min; in the Ladle Furnace refining step, after the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5-10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.

[0011] As a further improvement of an embodiment of the present application, in the continuous casting process, the continuous casting adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, and the mold flux adopts medium carbon steel mold flux; the superheat of molten steel in the tundish is controlled at 25~35°C, the mold level height is 80~87% of the mold height, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.

[0012] To achieve one of the above objects, an embodiment of the present application also provides a fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0013] As a further improvement of an embodiment of the present application, the chemical composition of the fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0014] As a further improvement of an embodiment of the present application, the chemical composition of the fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0015] As a further improvement of an embodiment of the present application, the tensile strength of the fastener is ≥2000MPa, the yield strength is ≥1750MPa, and the delayed fracture strength ratio is ≥0.85.

[0016] To achieve one of the above objects, an embodiment of the present application also provides a production method for the fastener as described above, wherein the fastener is prepared by using wire rod for fastener and sequentially performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process; wherein, the annealing process comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace; in the warm heading forming process, the fine wire obtained from the drawing process is heated to 530~570°C and then headed to form; the quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, and the holding time is 30~60min; the heating temperature of the tempering step is 550~580°C, and the holding time is 60~90min, after tempering, air cooling to room temperature.

[0017] As a further improvement of an embodiment of the present application, the quenching medium of the quenching step is oil.

[0018] Compared with the prior art, the beneficial effects of the present application are: (1) In the chemical composition design of the present application, under comprehensive consideration of the effects of different elements on the strength and toughness, delayed fracture resistance and forming performance of fasteners, the addition and content of each element are precisely selected and controlled, under the condition of greatly reducing the contents of C, Si, Mn elements, through the limitation of the content relationship of Cr, Ni, Mo, Ti elements, Cr and Ni form interstitial solid solution in steel and form clusters to significantly improve the strength, corrosion resistance and oxidation resistance of steel, inhibit hydrogen invasion, and Ni and Ti combine to generate nano precipitated phases with finer size than carbides, which strengthen the matrix while forming stronger hydrogen traps, further improving the delayed fracture resistance of steel. In addition, Mo segregates at the interface between precipitated phases and matrix, which can promote and accelerate the precipitation process, so that the finally prepared fastener has tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, while also reducing the dependence of high-strength fasteners on precious elements such as Mo, V, Co, greatly saving production costs and facilitating industrial production. (2) On the basis of the aforementioned chemical composition design, through the regulation of a series of process means, the wire rod for fastener has good microstructure and mechanical properties, the wire rod microstructure is fine equiaxed ferrite and martensite composite microstructure, the tensile strength is 1250~1350MPa, and the wire rod surface quality is excellent. (3) Further combined with a series of forming processes including acid pickling and phosphating, drawing, warm heading forming, thread rolling, quenching and tempering, and surface polishing, not only can cracking of fasteners during the heading forming process be avoided, but also the finally prepared fastener has a microstructure of tempered sorbite, tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications. In addition, compared to the CNC lathe forming process, the production efficiency can be significantly improved, which is suitable for industrial mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is a schematic diagram of specimen notch when performing slow strain rate tensile test on fasteners in the present application.DETAILED DESCRIPTION

[0020] The technical solution of the present application will be further introduced below in combination with specific embodiments, but the scope of protection is not limited to the description made.

[0021] An embodiment of the present application provides a wire rod for ultra-high strength fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0022] Wherein, [B] represents the percent by mass of B, [Nb] represents the percent by mass of Nb, [V] represents the percent by mass of V, [Cr] represents the percent by mass of Cr, [Ni] represents the percent by mass of Ni, [Mo] represents the percent by mass of Mo, and [Ti] represents the percent by mass of Ti.

[0023] In the chemical composition design of the present application, under comprehensive consideration of the effects of different elements on the strength and toughness, delayed fracture resistance and forming performance of fasteners, the addition and content of each element are precisely selected and controlled, under the condition of greatly reducing the contents of C, Si, Mn elements, through the limitation of the content relationship of Cr, Ni, Mo, Ti elements, Cr and Ni form interstitial solid solution in steel and form clusters to significantly improve the strength, corrosion resistance and oxidation resistance of steel, inhibit hydrogen invasion, and Ni and Ti combine to generate nano precipitated phases with finer size than carbides, which strengthen the matrix while forming stronger hydrogen traps, further improving the delayed fracture resistance of steel. In addition, Mo segregates at the interface between precipitated phases and matrix, which can promote and accelerate the precipitation process, so that the finally prepared fastener has tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, while also reducing the dependence of high-strength fasteners on precious elements such as Mo, V, Co, greatly saving production costs and facilitating industrial production.

[0024] Preferably, the chemical composition of the wire rod for ultra-high strength fastener comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0025] More preferably, the chemical composition of the wire rod for ultra-high strength fastener comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

[0026] An embodiment of the present application also provides an ultra-high strength fastener, which is prepared by using the above wire rod for ultra-high strength fastener as base material, and has the same chemical composition as the wire rod for ultra-high strength fastener.

[0027] An embodiment of the present application also provides a preferred production method for the wire rod for ultra-high strength fastener, wherein the production method comprises sequentially performing molten steel smelting process, continuous casting process, slow cooling process, heating process, high-speed wire rolling process, and controlled cooling process to prepare the wire rod for ultra-high strength fastener. The chemical composition of the wire rod is as described above and will not be repeated here.

[0028] The production method will be described in detail below according to the production sequence.(1) Molten steel smelting process

[0029] It comprises sequentially performing converter steelmaking step and Ladle Furnace refining step.a. Converter steelmaking step

[0030] Scrap steel and blast furnace hot metal are sent to the converter for steelmaking, the molten steel at the converter steelmaking end point contains C with a percent of 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying to reduce oxidation burning loss and improve the utilization efficiency of alloys.

[0031] Preferably, argon gas is blown from the bottom of the ladle throughout the tapping process, and the flow rate of argon gas blown from the bottom of the ladle is 400~800L / min.b. Ladle Furnace refining step

[0032] After the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5~10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.(2) Continuous casting process

[0033] The molten steel from the Ladle Furnace refining process is continuously cast into continuous cast billet, the temperature of the tundish is controlled at 1510~1530°C to improve the fluidity of molten steel, improve the poor fluidity of molten steel caused by the high content of Cr, Ni, Mo elements in the steel, and improve the uniformity of alloy elements.

[0034] Preferably, the continuous casting process adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, and the mold flux adopts medium carbon steel mold flux.

[0035] Preferably, during the continuous casting process the superheat of molten steel in the tundish is controlled at 25~35°C, the mold level height is 80~87% of the mold height, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.(3) Slow cooling process

[0036] The continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C.(4) Heating process

[0037] The continuous cast billet after the slow cooling process is sent to a heating furnace for heating, the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min.

[0038] Through segmented control of heating temperature, heating rate and heating time, the temperature of the continuous cast billet can be uniform, and alloy elements can be fully dissolved in austenite, thereby avoiding cracks occurred in the continuous cast billet due to excessive temperature stress during subsequent rolling process, while improving rolling production rhythm and production efficiency.(5) High-speed wire rolling process

[0039] The continuous cast billet after the heating process is rolled into wire rod and coiled into coils, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C. This can avoid the brittle temperature range of the steel of the present application, so that the rolled wire rod has good microstructure and mechanical properties, and improves the surface quality of the wire rod.

[0040] Wherein, the diameter of the wire rod is 5~26mm.(6) Controlled cooling process

[0041] The obtained coil is sent to the Stelmor cooling line for cooling, and the laying head temperature is 890~910°C.

[0042] The heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed; the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off, the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s, and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C, that is, the temperature of the wire rod when entering the first closed heat retaining cover is controlled at 750~780°C. Through the above process and parameter settings, the microstructure uniformity of the obtained wire rod can be improved, so that fine equiaxed ferrite and martensite microstructure are formed in the wire rod, which is beneficial to subsequent annealing, drawing and forming processing when further preparing fasteners.

[0043] Thus, the production method of the present embodiment, on the basis of the aforementioned chemical composition design, through the regulation of a series of process means, enables the finally prepared wire rod for fastener to have good microstructure and mechanical properties, the microstructure of the wire rod is fine equiaxed ferrite and martensite composite microstructure, the tensile strength is 1250~1350MPa, and the wire rod surface quality is excellent.

[0044] Further, the above wire rod for ultra-high strength fastener is further processed by sequentially performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process to prepare ultra-high strength fastener. The specific processes are as follows:(7) Annealing process

[0045] The annealing process comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace. This way, the wire rod can have spheroidal pearlite microstructure, allowing carbides to precipitate uniformly and distribute in the ferrite matrix, thereby reducing the hardness of the wire rod to facilitate subsequent drawing and forming processing.(8) Acid pickling and phosphating process

[0046] The annealed wire rod is subjected to acid pickling and phosphating treatment to remove the oxide scale on the surface of the wire rod and form a phosphate film on the surface of the wire rod to improve its lubrication performance and reduce damage to the die during subsequent drawing and heading forming.(9) Drawing process

[0047] The wire rod after acid pickling and phosphating process is drawn to fine wire of set size, and the diameter reduction amount during drawing is controlled according to the finished product requirements of the fastener, wherein the so-called diameter reduction amount during drawing is the difference between the diameter of the wire rod and the diameter of the fine wire.

[0048] Preferably, the diameter reduction amount during drawing is controlled at 0.5~1.5mm.(10) Warm heading forming process

[0049] The fine wire obtained from the drawing process is heated to 530~570°C and then headed to form. Through warm heading forming, the large resistance and work hardening generated during cold heading forming can be avoided, so as to facilitate heading of fasteners with complex shapes or large deformation.(11) Thread rolling process

[0050] Thread rolling processing is performed on the fastener after warm heading forming to form threads.(12) Quenching and tempering process

[0051] The quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, and the holding time is 30~60min, so that the microstructure of the fastener is completely transformed into austenite, then cooled with a cooling rate of 80~90°C / s, so that the supercooled austenite is transformed into martensite; the heating temperature of the tempering step is 550~580°C, and the holding time is 60~90min, so that the microstructure of the fastener is transformed into tempered sorbite, after tempering, air cooling to room temperature, thereby both improving the strength of the fastener and improving plasticity and toughness, so that the fastener has good comprehensive mechanical properties.

[0052] The quenching medium of the quenching step is oil.(13) Surface polishing process

[0053] Surface polishing treatment is performed on the fastener after the quenching and tempering process.

[0054] Thus, the fastener further prepared by drawing in the present embodiment, on the basis of the aforementioned chemical composition design, through a series of forming processes including acid pickling and phosphating, drawing, warm heading forming, thread rolling, quenching and tempering, and surface polishing, not only can cracking during the heading forming process be avoided, but also the finally prepared fastener has a microstructure of tempered sorbite, tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications. In addition, compared to the CNC lathe forming process, the production efficiency can be significantly improved, which is suitable for industrial mass production.

[0055] The specific embodiments of the present application are further introduced below through 10 examples. Of course, these 10 examples are only part of the numerous variant embodiments contained in the present embodiment, not all. Other embodiments made on the basis of the aforementioned embodiments do not depart from the technical spirit of the present application.Examples

[0056] First, Examples 1~10 all provide a wire rod for ultra-high strength fastener, and an ultra-high strength fastener prepared from the wire rod by further performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process, the chemical compositions of the wire rod and fastener are shown in Table 1, with the balance being Fe and unavoidable impurities. Table 1Example12345678910C0.310.350.380.420.270.340.330.360.280.41Si0.270.180.230.170.30.470.320.430.250.38Mn0.580.720.620.530.680.730.650.780.630.52P0.0090.0070.010.0120.0060.0050.0080.010.0060.007S0.0020.0030.0040.0050.0030.0030.0040.0020.0050.004Cr8.946.627.516.2610.457.635.349.6811.178.25Chemical composition in percent by mass %Ni2.642.132.682.513.582.682.493.464.32.58Cu0.020.030.010.020.020.030.010.020.030.02Ti0.0850.0720.0930.0680.0820.0710.0740.0530.0860.067Mo0.370.310.380.390.440.340.390.270.380.28Re0.0760.0250.0510.0840.0620.0730.0470.0150.0920.038Al0.0430.0370.0280.0190.0240.0450.0360.0330.0380.032O0.00070.00080.00090.00070.00090.00060.00070.00050.00060.0008N0.00340.00370.00420.00350.00440.00330.00410.00490.00380.0032B-0.00210.0022--0.002-0.0017-0.0018Nb0.0370.054-0.0520.0690.063-0.0540.061-V-0.079-0.063-0.0750.056-0.0580.057

[0057] The production method is as follows:(1) Molten steel smelting processa. Converter steelmaking step

[0058] Scrap steel and blast furnace hot metal are sent to the converter for steelmaking, the molten steel at the converter steelmaking end point contains C with a percent of 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, argon gas is blown from the bottom of the ladle throughout the tapping process, the argon gas flow rate blown from the bottom of the ladle is 400~800L / min, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying.b. Ladle Furnace refining step

[0059] After the molten steel from the converter steelmaking step is poured into the ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5~10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.(2) Continuous casting process

[0060] The molten steel from the Ladle Furnace refining process is continuously cast into continuous cast billet, the temperature of the tundish is controlled at 1510~1530°C, the superheat of molten steel in the tundish is 25~35°C, the mold level height is 80~87% of the mold height, the continuous casting process adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, the mold flux adopts medium carbon steel mold flux, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.(3) Slow cooling process

[0061] The continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C.(4) Heating process

[0062] The continuous cast billet after the slow cooling process is sent to a heating furnace for heating, the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min.(5) High-speed wire rolling process

[0063] The continuous cast billet after the heating process is rolled into wire rod with a diameter of 5-26mm and coiled into coils, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C.(6) Controlled cooling process

[0064] The obtained coil is sent to the Stelmor cooling line for cooling, and the laying head temperature is 890~910°C. The heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed; the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off; the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s; and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C.(7) Annealing process

[0065] Comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace.(8) Acid pickling and phosphating process

[0066] The annealed wire rod is subjected to acid pickling and phosphating treatment.(9) Drawing process

[0067] The wire rod after acid pickling and phosphating process is drawn to fine wire of set size, and the diameter reduction amount during drawing is controlled according to the finished product requirements of the fastener.(10) Warm heading forming process

[0068] The fine wire obtained from the drawing process is heated to 530~570°C and then headed to form.(11) Thread rolling process

[0069] Thread rolling processing is performed on the fastener after warm heading forming to form threads.(12) Quenching and tempering process

[0070] The quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, the holding time is 30~60min, then cooled, the quenching medium is oil, the cooling rate is 80~90°C / s, the heating temperature of the tempering step is 550~580°C, the holding time is 60~90min, after tempering, air cooling to room temperature.(13) Surface polishing process

[0071] Surface polishing treatment is performed on the fastener after the quenching and tempering process.

[0072] The fasteners of Examples 1~10 did not crack during the heading forming process, that is, the cracking rate was 0; during the processing, the average processing time for a single fastener was 0.5~1.5s, and the yield rate was >95%.

[0073] Samples were taken from the fasteners of 10 examples respectively for mechanical property testing and delayed fracture resistance testing.

[0074] In terms of mechanical properties, referring to ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products, a tensile testing machine was used to test the yield strength, tensile strength, and elongation after fracture of the fasteners.

[0075] In terms of delayed fracture resistance, the slow strain rate tensile test method was adopted, and the test process was as follows: Step 1, two groups of fastener specimens 10 were taken, referring to Figure 1, notches 20 were made in the middle of the two groups of fastener specimens 10 along the circumferential direction, the notch angle α=60°, one group of specimens was used as the hydrogen-charged group specimens, and the other group of specimens was used as the control group specimens. Step 2, the hydrogen-charged group specimens were placed in 4g / L NaOH solution for electrochemical hydrogen charging, the hydrogen charging time was 48h, the current density was 15A / m 2< , after hydrogen charging was completed, the specimens were cleaned and dried.

[0076] The control group specimens did not undergo the operation of Step 2.

[0077] Step 3, both groups of specimens were subjected to tensile test under room temperature using a slow strain rate tensile testing machine, the tensile strain rate was 5×10 -6< / s, and the tensile strengths of the two groups of specimens were measured respectively; Step 4, calculate the delayed fracture strength ratio, delayed fracture strength ratio = tensile strength of hydrogen-charged group specimens / tensile strength of control group specimens. Wherein, the larger the delayed fracture strength ratio, the better the delayed fracture resistance of the specimen, generally a delayed fracture strength ratio above 0.7 is judged as qualified, and a delayed fracture strength ratio above 0.8 is judged as excellent.

[0078] The mechanical property test results and delayed fracture strength ratio are shown in Table 2. Table 2ExampleTensile strength / MPaYield strength / MPaElongation after fracture / %Delayed fracture strength ratioExample 12174193612.60.88Example 22136187213.10.89Example 32142184714.20.86Example 42093180315.50.90Example 52291197214.90.87Example 62179189113.70.88Example 72085179614.20.89Example 82253201412.40.86Example 92348210311.80.86Example 102167192513.30.87

[0079] In summary, the fasteners of the examples of the present application, on the basis of chemical composition design, combined with the control of the entire production process of wire rod and fasteners, avoid cracking of fasteners during the heading forming process, and the prepared fasteners have tensile strength ≥2000MPa, yield strength ≥1750MPa, and delayed fracture strength ratio ≥0.85, with ultra-high tensile strength, excellent toughness, forming performance and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, and can significantly improve production efficiency, which is suitable for industrial mass production.

[0080] It should be understood that although this specification is described in terms of embodiments, not every embodiment contains only one independent technical solution. This narrative manner of the specification is merely for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in various embodiments may also be appropriately combined to form other embodiments understandable to those skilled in the art.

[0081] The series of detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present application and are not intended to limit the protection scope of the present application. Any equivalent embodiments or modifications made without departing from the technical spirit of the present application should be included within the protection scope of the present application.

Examples

examples

Examples

[0056]First, Examples 1~10 all provide a wire rod for ultra-high strength fastener, and an ultra-high strength fastener prepared from the wire rod by further performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process, the chemical compositions of the wire rod and fastener are shown in Table 1, with the balance being Fe and unavoidable impurities.

Table 1

Example12345678910

C0.310.350.380.420.270.340.330.360.280.41

Si0.270.180.230.170.30.470.320.430.250.38

Mn0.580.720.620.530.680.730.650.780.630.52

P0.0090.0070.010.0120.0060.0050.0080.010.0060.007

S0.0020.0030.0040.0050.0030.0030.0040.0020.0050.004

Cr8.946.627.516.2610.457.635.349.6811.178.25

Chemical composition in percent by mass %Ni2.642.132.682.513.582.682.493.464.32.58

Cu0.020.030.010.020.020.030.010.020.030.02

Ti0.0850.0720.0930.0680.0820.0710.0740.0530.0860.067

Mo0.370.310.380.390.440.340...

Claims

1. A wire rod, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

2. The wire rod according to claim 1, wherein its chemical composition comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

3. The wire rod according to claim 1, wherein its chemical composition comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

4. A production method for the wire rod according to claim 1, wherein the method comprises sequentially performing molten steel smelting process, continuous casting process, slow cooling process, heating process, high-speed wire rolling process, and controlled cooling process; wherein, in the continuous casting process, the temperature of the tundish is controlled at 1510~1530°C; in the slow cooling process, the continuous cast billet obtained from the continuous casting process is air cooled to 650~700°C and then sent to a soaking pit for slow cooling, the cooling rate of slow cooling is 5.5~7°C / h, and the billet is taken out of the soaking pit after cooling to below 200°C; the heating process comprises a heating stage and a soaking stage, the temperature of the heating stage is 1100~1140°C, the heating rate is 0.3~0.4°C / s, and the heating time is 20~30min; the temperature of the soaking stage is 1150~1190°C, and the soaking time is 15~25min; in the high-speed wire rolling process, the rolling start temperature is 1050~1090°C, and the rolling finish temperature is 910~940°C; in the controlled cooling process, the laying head temperature is 890~910°C; the heat retaining covers of the 1st to 2nd sections of the Stelmor cooling line roller table are opened, the heat retaining covers of the remaining roller tables are all closed, the 1st to 3rd fans are turned on with a fan air volume of 60~90%, the remaining fans are turned off, the roller table speed of the 1st to 2nd sections of roller table is 0.25~0.4m / s, the roller table speed of the remaining roller tables is 0.15~0.3m / s, and the temperature of the wire rod when reaching the 3rd section of roller table is controlled at 750~780°C.

5. The production method for the wire rod according to claim 4, wherein the molten steel smelting process comprises sequentially performing converter steelmaking step and Ladle Furnace refining step; in the converter steelmaking step, the molten steel at the converter steelmaking end point contains C 0.05~0.15%, P≤0.012%, the tapping temperature is 1620~1660°C, after 25~30% of the molten steel has been tapped out, aluminum ingot, ferrosilicon, ferromanganese, ferrochromium, ferronickel, and ferromolybdenum are sequentially added into the molten steel for deoxidation alloying, argon gas is blown from the bottom of the ladle throughout the tapping process, and the flow rate of argon gas blown from the bottom of the ladle is 400~800L / min; in the Ladle Furnace refining step, after the molten steel from the converter steelmaking step is poured into the Ladle furnace, lime and fluorite are sequentially added for white slag conditioning, after white slag refining for 5~10min, ferrotitanium and rare earth ferroalloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying, during alloying the argon gas flow rate blown from the bottom of the ladle is 300~450L / min; thereafter, electric heating is conducted and sampling is performed for chemical composition detection, alloy is supplemented for chemical composition fine-tuning according to the detection results, during chemical composition fine-tuning the argon gas flow rate blown from the bottom of the ladle is 150~300L / min; thereafter, soft stirring is performed with a soft stirring time of 25~35min; the tapping temperature at the end point of Ladle Furnace refining is 1530~1560°C.

6. The production method for the wire rod according to claim 4, wherein in the continuous casting process, the continuous casting adopts argon sealing for ladle shroud, alkaline tundish covering agent, and submerged entry nozzle for fully protected casting, and the mold flux adopts medium carbon steel mold flux; the superheat of molten steel in the tundish is controlled at 25~35°C, the mold level height is 80~87% of the mold height, the mold level fluctuation range is within ±3%, the electromagnetic stirring current of the mold is 700~750A, the stirring frequency is 3~5Hz, the secondary cooling specific water flow is controlled at 0.25~0.30L / Kg, and the casting speed is 2.3~2.5m / min.

7. A fastener, wherein its chemical composition comprises in percent by mass: C 0.25~0.45%, Si 0.15~0.50%, Mn 0.5~0.8%, P≤0.015%, S≤0.005%, Cr 5~12%, Ni 2.0~4.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

8. The fastener according to claim 7, wherein its chemical composition comprises in percent by mass: C 0.30~0.45%, Si 0.20~0.40%, Mn 0.60~0.75%, P≤0.015%, S≤0.005%, Cr 6~11%, Ni 2.5~4.0%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

9. The fastener according to claim 7, wherein its chemical composition comprises in percent by mass: C 0.30~0.40%, Si 0.20~0.35%, Mn 0.60~0.70%, P≤0.015%, S≤0.005%, Cr 6.5~10%, Ni 2.5~3.5%, Cu≤0.1%, Ti 0.05~0.10%, Mo 0.25~0.50%, Re 0.01~0.10%, Al 0.015~0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, V, with the balance being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0~3.5, [Mo] / [Ti] is 4~6.

10. The fastener according to claim 7, wherein its tensile strength is ≥2000MPa, yield strength is ≥1750MPa, and delayed fracture strength ratio is ≥0.85.

11. A production method for the fastener according to claim 7, wherein the fastener is prepared by using wire rod as base material and sequentially performing annealing process, acid pickling and phosphating process, drawing process, warm heading forming process, thread rolling process, quenching and tempering process, and surface polishing process; wherein, the annealing process comprises a heating stage, a first cooling stage and a second cooling stage, the heating rate of the heating stage is 4~6°C / min, after heating to a temperature of 760~800°C, holding for 3~5h; the cooling rate of the first cooling stage is 1~1.5°C / min, after cooling to a temperature of 680~720°C, holding for 2~4h; in the second cooling stage, the wire rod is naturally cooled to room temperature in the heating furnace; in the warm heading forming process, the fine wire obtained from the drawing process is heated to 530~570°C and then headed to form; the quenching and tempering process comprises quenching and tempering steps, the heating temperature of the quenching step is 850~880°C, and the holding time is 30~60min; the heating temperature of the tempering step is 550~580°C, and the holding time is 60~90min, after tempering, air cooling to room temperature.

12. The production method for the fastener according to claim 11, wherein the quenching medium of the quenching step is oil.

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