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

Abstract

Disclosed are a wire rod for a fastener and a production method for the wire rod, and a fastener and a production method therefor. The wire rod and the fastener comprise the following chemical components in percentage 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%; at least one of B, Nb, and V; and the balance being Fe and inevitable impurities, wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

Description

Wire rod for fastener and production method thereof, fastener and production method thereof

[0001] This application is based on the Chinese patent application with application number 202311084987.5 and application date of August 28, 2023, and claims the priority of the Chinese patent application. The entire content of the above patent application is hereby introduced into this application as a reference. Technical Field

[0002] The present invention belongs to the technical field of steel smelting, and relates to a wire rod for a fastener and a production method thereof, and also relates to a fastener and a production method thereof. Background Art

[0003] Fasteners, the foundation of industry, are widely used in the automotive, machinery, energy, aerospace, and other fields. In recent years, with economic development and technological advancements, industrial applications have placed higher design stress and lightweight requirements on fastener steel. To meet these requirements, the primary focus is to increase fastener strength. This reduces weight and increases installation space by reducing fastener size while maintaining the same clamping force. This, in turn, optimizes the function and volume of connected components, achieving overall weight reduction and performance optimization. Furthermore, improving fastener strength can improve vehicle fuel efficiency, reduce fuel consumption, and achieve ultra-low carbon emissions.

[0004] However, as steel strength increases, especially when tensile strength exceeds 1200 MPa, delayed fracture becomes a prominent issue. High-strength fasteners are highly notch-sensitive, making delayed fracture prone to occur at locations where notch stress is concentrated, such as the transition between the shank and the head or at the root of the thread, causing significant safety incidents and economic losses. Therefore, improving fasteners' delayed fracture resistance while simultaneously increasing their strength has been a hot topic of research both domestically and internationally in recent years.

[0005] Summary of the Invention

[0006] In order to solve at least one of the above technical problems, the purpose of the present invention is to provide a wire rod for a fastener and a production method thereof, and also provide a fastener and a production method of the fastener.

[0007] To achieve one of the above objectives, one embodiment of the present invention provides a wire rod for fasteners, the chemical composition of which, in percentage by mass, includes: 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0008] As a further improvement of one embodiment of the present invention, the chemical composition of the wire rod for fasteners includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0009] As a further improvement of one embodiment of the present invention, the chemical composition of the wire rod for fasteners includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0010] To achieve one of the above objectives, an embodiment of the present invention further provides a method for producing a wire rod for fasteners as described above, comprising the steps of molten steel smelting, continuous casting, slow cooling, heating, high-speed wire rolling, and controlled cooling in sequence; wherein,

[0011] During the continuous casting process, the temperature of the tundish is controlled to be 1510-1530°C;

[0012] In the slow cooling process, the continuous casting billet obtained in the continuous casting process is air-cooled to 650-700°C, and then sent to a holding pit for slow cooling at a cooling rate of 5.5-7°C / h. The billet is cooled to below 200°C before exiting the holding pit.

[0013] The heating process includes a heating section and a soaking section. The temperature of the heating section is 1100-1140°C, the heating rate is 0.3-0.4°C / s, and the heating time is 20-30 minutes. The temperature of the soaking section is 1150-1190°C, and the soaking time is 15-25 minutes.

[0014] In the high-speed wire rolling process, the starting rolling temperature is 1050-1090°C, and the finishing rolling temperature is 910-940°C;

[0015] In the controlled cooling process, the spinning temperature is 890-910°C; the insulation covers of the 1st to 2nd roller sections of the Stelmor cooling line are opened, and the insulation covers of the other roller sections are all closed, the 1st to 3rd fans are turned on, the fan air volume is 60-90%, and the other fans are turned off, the roller speed of the 1st to 2nd roller sections is 0.25-0.4m / s, and the roller speed of the other roller sections is 0.15-0.3m / s, and the temperature of the wire rod when it reaches the 3rd roller section is controlled to be 750-780°C.

[0016] As a further improvement of one embodiment of the present invention, the molten steel smelting process includes converter smelting and LF refining steps performed sequentially;

[0017] In the converter smelting step, the molten steel at the end of the converter smelting has a C content of 0.05-0.15% and a P content of ≤0.012%, and the tapping temperature is 1620-1660° C. After 25-30% of the steel has been tapped, aluminum ingots, ferrosilicon, ferromanganese, ferrochrome, ferronickel, and ferromolybdenum are sequentially added for deoxidation and alloying. During the tapping process, argon is blown from the bottom of the ladle at a flow rate of 400-800 L / min.

[0018] In the LF refining step, after the molten steel produced by the converter smelting step is injected into the LF furnace, lime and fluorite are sequentially added to adjust the white slag. After the white slag is refined for 5 to 10 minutes, ferrotitanium and rare earth iron alloy are sequentially added, and then at least one of ferroboron, ferrovanadium, and ferroniobium is added for alloying. During the alloying, the argon flow rate of the ladle bottom is 300 to 450 L / min; then, the power is turned on to increase the temperature and samples are taken for chemical composition detection. According to the detection results, alloy is added to fine-tune the chemical composition. During the chemical composition fine-tuning, the argon flow rate of the ladle bottom is 150 to 300 L / min; then, soft stirring is performed for 25 to 35 minutes; and the tapping temperature at the end point of LF refining is 1530 to 1560°C.

[0019] As a further improvement of one embodiment of the present invention, in the continuous casting process, the continuous casting process adopts argon sealing of the long water nozzle of the large ladle, alkaline tundish covering agent, and submerged water nozzle for full protection casting, and the protective slag adopts medium carbon steel protective slag; the superheat of the molten steel in the tundish is controlled to be 25-35°C, the liquid level height of the crystallizer is 80-87% of the crystallizer height, the liquid level fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 700-750A, the stirring frequency is 3-5Hz, the secondary cooling water ratio is controlled to be 0.25-0.30L / Kg, and the pulling speed is 2.3-2.5m / min.

[0020] To achieve one of the above objectives, an embodiment of the present invention further provides a fastener, the chemical composition of which, in percentage by mass, includes: 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0021] As a further improvement of one embodiment of the present invention, the chemical composition of the fastener includes, by mass percentage, the following: 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0022] As a further improvement of one embodiment of the present invention, the chemical composition of the fastener includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0023] As a further improvement of an embodiment of the present invention, the tensile strength of the fastener is ≥2000 MPa, the yield strength is ≥1750 MPa, and the delayed fracture strength ratio is ≥0.85.

[0024] To achieve one of the above objectives, an embodiment of the present invention further provides a method for producing the fastener as described above, wherein the fastener is prepared by sequentially subjecting a wire rod for fasteners to annealing, pickling and phosphating, drawing, warm upsetting, thread rolling, tempering treatment, and surface polishing; wherein,

[0025] The annealing process includes a heating section, a first cooling section, and a second cooling section. The heating section has a heating rate of 4 to 6°C / min, and the temperature is raised to 760 to 800°C and then kept at this temperature for 3 to 5 hours. The first cooling section has a cooling rate of 1 to 1.5°C / min, and the temperature is cooled to 680 to 720°C and then kept at this temperature for 2 to 4 hours. In the second cooling section, the wire rod is naturally cooled to room temperature in the heating furnace.

[0026] In the warm upsetting process, the refined wire obtained in the drawing process is heated to 530-570° C. and then upsetting is performed;

[0027] The tempering treatment process includes quenching and tempering steps. The heating temperature of the quenching step is 850-880°C and the holding time is 30-60 minutes; the heating temperature of the tempering step is 550-580°C and the holding time is 60-90 minutes. After tempering, the steel is air-cooled to room temperature.

[0028] As a further improvement of one embodiment of the present invention, the quenching medium in the quenching step is oil.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) In the chemical composition design of the present invention, the effects of different elements on the toughness, delayed fracture resistance and formability of steel as well as the formability of fasteners are comprehensively considered, and the addition and content of each element are accurately selected and controlled. Under the condition of greatly reducing the content of C, Si and Mn elements, by limiting the relationship between the content of Cr, Ni, Mo and Ti elements, Cr and Ni form interstitial solid solutions in the steel, forming clusters to significantly improve the strength, corrosion resistance and oxidation resistance of the steel, inhibit the intrusion of hydrogen, and Ni and Ti will combine to form nano-precipitates that are finer than carbides. While precipitating the matrix, strong hydrogen traps are formed, further improving the delayed fracture resistance of the steel. In addition, Mo is segregated at the interface between the precipitated phase and the matrix, which can promote and accelerate the precipitation process, so that the tensile strength of the final prepared fasteners is ≥2000MPa, the yield strength is ≥1750MPa, and the delayed fracture strength ratio is ≥0.85. It has excellent toughness, formability and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications, and reducing the dependence of high-strength fasteners on precious elements such as Mo, V, and Co, greatly saving production costs and facilitating industrial production.

[0031] (2) Based on the aforementioned chemical composition design, a series of process control measures are adopted to make the wire rod for fasteners have good microstructure and mechanical properties. The wire rod microstructure is a composite microstructure of fine equiaxed ferrite and martensite, with a tensile strength of 1250-1350 MPa and excellent surface quality.

[0032] (3) Further combining a series of forming processes including pickling and phosphating, drawing, warm upsetting, thread rolling, tempering, and surface polishing, not only can the fasteners avoid cracking during the upsetting process, but the microstructure of the final fasteners is tempered troostite, with a tensile strength of ≥2000MPa, a yield strength of ≥1750MPa, and a delayed fracture strength ratio of ≥0.85. It has excellent toughness, forming properties, and delayed fracture resistance, greatly improving the safety of fasteners in industrial applications. In addition, compared with the CNC lathe forming process, it can greatly improve production efficiency and is suitable for industrial mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG1 is a schematic diagram of a specimen notch when a slow strain rate tensile test is performed on a fastener in the present invention. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further described below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0035] One embodiment of the present invention provides a wire rod for ultra-high strength fasteners, the chemical composition of which includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0036] Here, [B] represents the mass percentage of B, [Nb] represents the mass percentage of Nb, [V] represents the mass percentage of V, [Cr] represents the mass percentage of Cr, [Ni] represents the mass percentage of Ni, [Mo] represents the mass percentage of Mo, and [Ti] represents the mass percentage of Ti.

[0037] In the chemical composition design of the present invention, the effects of different elements on the toughness, delayed fracture resistance and formability of the steel as well as the formability of the fastener are comprehensively considered, and the addition and content of each element are accurately selected and controlled. Under the condition of greatly reducing the content of C, Si and Mn elements, by limiting the relationship between the content of Cr, Ni, Mo and Ti elements, Cr and Ni form interstitial solid solutions in the steel, forming clusters to significantly improve the strength, corrosion resistance and oxidation resistance of the steel, inhibit the intrusion of hydrogen, and Ni and Ti will combine to form nano-precipitates that are finer than carbides, which will strengthen the steel. At the same time, strong hydrogen traps are formed in the matrix, further improving the delayed fracture resistance of the steel. In addition, Mo is segregated at the interface between the precipitated phase and the matrix, which can promote and accelerate the precipitation process, so that the tensile strength of the final prepared fasteners is ≥2000MPa, the yield strength is ≥1750MPa, and the delayed fracture strength ratio is ≥0.85. It has excellent toughness, forming properties and delayed fracture resistance, which greatly improves the safety of fasteners in industrial applications and reduces the dependence of high-strength fasteners on precious elements such as Mo, V, and Co, greatly saving production costs and facilitating industrial production.

[0038] Preferably, the chemical composition of the wire rod for ultra-high strength fasteners includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0039] More preferably, the chemical composition of the wire rod for ultra-high strength fasteners includes, by mass percentage, 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, and V, with the remainder being Fe and unavoidable impurities; wherein [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

[0040] An embodiment of the present invention further provides an ultra-high strength fastener, which is prepared using the above-mentioned wire rod for ultra-high strength fasteners as a base material and has the same chemical composition as the wire rod for ultra-high strength fasteners.

[0041] One embodiment of the present invention further provides a preferred method for producing the ultra-high-strength fastener wire rod, comprising sequentially performing molten steel smelting, continuous casting, slow cooling, heating, high-speed wire rolling, and controlled cooling to produce the ultra-high-strength fastener wire rod. The chemical composition of the wire rod is as previously described and will not be further elaborated here.

[0042] The production method is described in detail below according to the production sequence.

[0043] (1) Molten steel smelting process

[0044] It includes the steps of converter smelting and LF refining carried out in sequence.

[0045] a. Converter smelting steps

[0046] Scrap steel and blast furnace molten iron are fed into a converter for smelting. The C content of the molten steel at the end of the converter smelting is 0.05-0.15%, P≤0.012%, and the tapping temperature is 1620-1660°C. After 25-30% of the steel is tapped, aluminum ingots, ferrosilicon, ferromanganese, ferrochrome, ferronickel, and ferromolybdenum are added in sequence for deoxidation and alloying to reduce oxidation and burning loss and improve the utilization efficiency of the alloy.

[0047] Preferably, argon is blown from the bottom of the ladle throughout the steel tapping process, and the flow rate of the argon blown from the bottom of the ladle is 400 to 800 L / min.

[0048] b. LF refining step

[0049] After the molten steel produced by the converter smelting step is injected into the LF furnace, lime and fluorite are added in sequence to adjust the white slag. After the white slag is refined for 5 to 10 minutes, titanium iron and rare earth iron alloy are added in sequence, and then at least one of ferroboron, ferrovanadium and ferroniobium is added for alloying. During the alloying period, the argon flow rate of the ladle bottom is 300 to 450 L / min; then the power is turned on to increase the temperature and samples are taken for chemical composition detection. According to the detection results, alloy is added to fine-tune the chemical composition. During the chemical composition fine-tuning period, the argon flow rate of the ladle bottom is 150 to 300 L / min; then soft stirring is performed for 25 to 35 minutes; the steel tapping temperature at the end point of LF refining is 1530 to 1560°C.

[0050] (2) Continuous casting process

[0051] The molten steel produced in the LF refining process is continuously cast into continuous casting billets, and the temperature of the tundish is controlled at 1510-1530℃ to improve the fluidity of the molten steel, improve the poor fluidity of the molten steel caused by the high content of Cr, Ni and Mo elements in the steel, and improve the uniformity of the alloy elements.

[0052] Preferably, the continuous casting process adopts argon sealing of large ladle long nozzle, alkaline tundish covering agent, and submerged nozzle for full protection casting, and the protective slag adopts medium carbon steel protective slag.

[0053] Preferably, during the continuous casting process, the superheat of the molten steel in the tundish is controlled to be 25-35°C, the liquid level in the crystallizer is 80-87% of the crystallizer height, the liquid level fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 700-750A, the stirring frequency is 3-5Hz, the secondary cooling water volume is controlled to be 0.25-0.30L / Kg, and the pulling speed is 2.3-2.5m / min.

[0054] (3) Slow cooling process

[0055] The continuous casting billet obtained in the continuous casting process is air-cooled to 650-700°C and then sent to a holding pit for slow cooling at a cooling rate of 5.5-7°C / h. It is cooled to below 200°C and then taken out of the holding pit.

[0056] (4) Heating process

[0057] The continuous casting billet after the slow cooling process is sent into the heating furnace for heating. The heating process includes a heating section and a soaking section. The temperature of the heating section is 1100-1140°C, the heating rate is 0.3-0.4°C / s, and the heating time is 20-30 minutes; the temperature of the soaking section is 1150-1190°C, and the soaking time is 15-25 minutes.

[0058] By controlling the heating temperature, heating speed and heating time in sections, the temperature of the continuous casting billet can be made uniform, and the alloy elements can be fully dissolved in the austenite, thereby avoiding cracks caused by excessive temperature stress in the subsequent rolling process, while improving the rolling production rhythm and production efficiency.

[0059] (5) High-speed wire rolling process

[0060] The continuous casting slab after the heating process is rolled into wire rod and then coiled into a coil. The starting rolling temperature is 1050-1090°C and the finishing rolling temperature is 910-940°C. This avoids the brittle temperature range of the steel of the present invention, ensures that the rolled wire rod has good microstructure and mechanical properties, and improves the surface quality of the wire rod.

[0061] The diameter of the wire rod is 5 to 26 mm.

[0062] (6) Control cooling process

[0063] The obtained coils are sent to the Stelmor cooling line for cooling, and the spinning temperature is 890-910℃.

[0064] The insulation covers of the first and second roller sections of the Stelmor cooling line are open, while the insulation covers of the remaining roller sections are all closed. The first to third fans are turned on with an air volume of 60-90%, and the remaining fans are closed. The roller speed of the first and second roller sections is 0.25-0.4 m / s, and the roller speed of the remaining roller sections is 0.15-0.3 m / s. The temperature of the wire rod is controlled to be 750-780°C when it reaches the third roller section, that is, the temperature of the wire rod when it enters the first closed insulation cover is controlled to be 750-780°C. Through the above process and parameter settings, the microstructural uniformity of the resulting wire rod can be improved, and fine equiaxed ferrite and martensite structures can be formed in the wire rod, which is beneficial for subsequent annealing, drawing, and forming in the further preparation of fasteners.

[0065] Thus, the production method of this embodiment, based on the aforementioned chemical composition design, is regulated by a series of process means, so that the wire rod for fasteners finally prepared has good structure and mechanical properties. The wire rod structure is a composite structure of fine equiaxed ferrite and martensite, the tensile strength is 1250~1350MPa, and the surface quality of the wire rod is excellent.

[0066] Furthermore, the ultra-high strength fastener wire rod is further processed through annealing, pickling and phosphating, drawing, warm upsetting, thread rolling, tempering treatment, and surface polishing to produce an ultra-high strength fastener. The specific process is as follows:

[0067] (7) Annealing process

[0068] The process includes a heating section, a first cooling section, and a second cooling section. The heating section heats up at a rate of 4-6°C / min, reaching a temperature of 760-800°C and then holding the temperature for 3-5 hours. The first cooling section cools at a rate of 1-1.5°C / min, cooling to a temperature of 680-720°C and then holding the temperature for 2-4 hours. In the second cooling section, the wire rod is naturally cooled to room temperature in the heating furnace. This allows the wire rod to have a spherical pearlite structure, allowing carbides to precipitate uniformly and be distributed within the ferrite matrix, thereby reducing the wire rod's hardness and facilitating subsequent drawing and forming processes.

[0069] (8) Pickling and phosphating process

[0070] The annealed wire rod is pickled and phosphated 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 mold during subsequent drawing and upsetting.

[0071] (9) Drawing process

[0072] The pickled and phosphated wire rod is drawn into a set size of fine wire, and the drawing reduction is controlled according to the finished product requirements of the fastener. The so-called drawing reduction is the difference between the diameter of the wire rod and the diameter of the fine wire.

[0073] Preferably, the drawing reduction is controlled to be 0.5 to 1.5 mm.

[0074] (10) Warm upsetting process

[0075] The refined wire obtained in the drawing process is heated to 530-570° C. and then upsetting. Warm upsetting can avoid the high resistance and cold work hardening generated during cold upsetting, making it easier to upset fasteners with complex shapes or large deformation.

[0076] (11) Thread rolling process

[0077] The fasteners formed by warm heading are subjected to thread rolling to form threads.

[0078] (12) Tempering process

[0079] The invention comprises quenching and tempering steps. The heating temperature of the quenching step is 850-880°C and the holding time is 30-60 minutes, so that the structure of the fastener is completely transformed into austenite. Then the fastener is cooled at a cooling rate of 80-90°C / s to transform the supercooled austenite into martensite. The heating temperature of the tempering step is 550-580°C and the holding time is 60-90 minutes, so that the structure of the fastener is transformed into tempered bainite. After tempering, the fastener is air-cooled to room temperature, thereby improving the strength, plasticity and toughness of the fastener and making the fastener have good comprehensive mechanical properties.

[0080] The quenching medium in the quenching step is oil.

[0081] (13) Surface polishing process

[0082] Polishing is performed on the surface of fasteners after the quenching and tempering process.

[0083] Thus, the fasteners produced by further drawing in this embodiment, based on the aforementioned chemical composition design, undergo a series of forming processes including pickling and phosphating, drawing, warm upsetting, thread rolling, tempering, and surface polishing. This not only avoids cracking during the upsetting process, but also exhibits a tempered bainite microstructure, a tensile strength of ≥2000 MPa, a yield strength of ≥1750 MPa, and a delayed fracture strength ratio of ≥0.85. These fasteners exhibit excellent toughness, formability, and delayed fracture resistance, significantly enhancing the safety of the fasteners in industrial applications. Furthermore, compared to CNC lathe forming processes, this significantly improves production efficiency, making it suitable for industrial mass production.

[0084] The following 10 embodiments further illustrate the specific embodiments of the present invention. Of course, these 10 embodiments are only a portion of the many variations of this embodiment, and are not exhaustive. Other embodiments based on the aforementioned embodiments do not depart from the technical purpose of the present invention.

[0085] Example

[0086] First, Examples 1 to 10 all provide a wire rod for an ultra-high-strength fastener, and an ultra-high-strength fastener prepared from the wire rod by further annealing, pickling and phosphating, drawing, warm upsetting, thread rolling, tempering, and surface polishing. The chemical composition of the wire rod and the fastener is shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0087] Table 1

[0088] The production method is as follows:

[0089] (1) Molten steel smelting process

[0090] a. Converter smelting steps

[0091] Scrap steel and blast furnace molten iron are fed into a converter for smelting. The molten steel at the end of the converter smelting contains C0.05-0.15%, P≤0.012%, and the tapping temperature is 1620-1660°C. Argon is blown from the bottom of the ladle throughout the tapping process, and the argon flow rate of the ladle bottom blowing is 400-800L / min. After 25-30% of the steel is tapped, aluminum ingots, ferrosilicon, ferromanganese, ferrochrome, ferronickel, and ferromolybdenum are added in sequence for deoxidation and alloying.

[0092] b. LF refining step

[0093] After the molten steel produced by the converter smelting step is injected into the LF furnace, lime and fluorite are added in sequence to adjust the white slag. After the white slag is refined for 5 to 10 minutes, titanium iron and rare earth iron alloy are added in sequence, and then at least one of ferroboron, ferrovanadium and ferroniobium is added for alloying. During the alloying period, the argon flow rate of the ladle bottom is 300 to 450 L / min; then the power is turned on to increase the temperature and samples are taken for chemical composition detection. According to the detection results, alloy is added to fine-tune the chemical composition. During the chemical composition fine-tuning period, the argon flow rate of the ladle bottom is 150 to 300 L / min; then soft stirring is performed for 25 to 35 minutes; the steel tapping temperature at the end point of LF refining is 1530 to 1560°C.

[0094] (2) Continuous casting process

[0095] The molten steel produced by the LF refining process is continuously cast into continuous casting billets, the temperature of the ladle is controlled to be 1510-1530°C, the superheat of the molten steel in the ladle is 25-35°C, the liquid level height of the crystallizer is 80-87% of the crystallizer height, and the continuous casting process adopts argon sealing of the long water nozzle of the large ladle, alkaline ladle covering agent, and immersed water nozzle for full protection casting. The protective slag uses medium carbon steel protective slag, the liquid level fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 700-750A, the stirring frequency is 3-5Hz, the secondary cooling water ratio is controlled to be 0.25-0.30L / Kg, and the casting speed is 2.3-2.5m / min.

[0096] (3) Slow cooling process

[0097] The continuous casting billet obtained in the continuous casting process is air-cooled to 650-700°C and then sent to a holding pit for slow cooling at a cooling rate of 5.5-7°C / h. It is cooled to below 200°C and then taken out of the holding pit.

[0098] (4) Heating process

[0099] The continuous casting billet after the slow cooling process is sent into the heating furnace for heating. The heating process includes a heating section and a soaking section. The temperature of the heating section is 1100-1140°C, the heating rate is 0.3-0.4°C / s, and the heating time is 20-30 minutes; the temperature of the soaking section is 1150-1190°C, and the soaking time is 15-25 minutes.

[0100] (5) High-speed wire rolling process

[0101] The continuous casting billet after the heating process is rolled into a wire rod with a diameter of 5 to 26 mm and then coiled into a coil. The starting rolling temperature is 1050 to 1090°C and the finishing rolling temperature is 910 to 940°C.

[0102] (6) Control cooling process

[0103] The resulting coil is fed into the Stelmor cooling line for cooling, with a spinning temperature of 890-910°C. The insulation covers of the first and second rollers of the Stelmor cooling line are opened, while the insulation covers of the remaining rollers are all closed; the first to third fans are turned on with a fan volume of 60-90%, and the remaining fans are turned off; the roller speed of the first and second rollers is 0.25-0.4 m / s, and the roller speed of the remaining rollers is 0.15-0.3 m / s; and the temperature of the wire rod when it reaches the third roller is controlled to be 750-780°C.

[0104] (7) Annealing process

[0105] It includes a heating section, a first cooling section and a second cooling section. The heating section has a heating rate of 4 to 6°C / min, and the temperature is heated to 760 to 800°C and then kept warm for 3 to 5 hours. The first cooling section has a cooling rate of 1 to 1.5°C / min, and the temperature is cooled to 680 to 720°C and then kept warm for 2 to 4 hours. In the second cooling section, the wire rod is naturally cooled to room temperature in the heating furnace.

[0106] (8) Pickling and phosphating process

[0107] The annealed wire rod is pickled and phosphating treated.

[0108] (9) Drawing process

[0109] The pickled and phosphated wire rod is drawn to the set size of the fine wire, and the drawing reduction is controlled according to the finished product requirements of the fastener.

[0110] (10) Warm upsetting process

[0111] The fine wire obtained in the drawing process is heated to 530-570° C. and then upsetting is performed.

[0112] (11) Thread rolling process

[0113] The fasteners formed by warm heading are subjected to thread rolling to form threads.

[0114] (12) Tempering process

[0115] The method comprises quenching and tempering steps. The heating temperature of the quenching step is 850-880°C, the holding time is 30-60 minutes, and then cooling is performed. The quenching medium is oil and the cooling rate is 80-90°C / s. The heating temperature of the tempering step is 550-580°C, the holding time is 60-90 minutes, and the steel is air-cooled to room temperature after tempering.

[0116] (13) Surface polishing process

[0117] Polishing is performed on the surface of fasteners after the quenching and tempering process.

[0118] The fasteners of Examples 1 to 10 did not crack during the upsetting process, that is, the cracking rate was 0; during the processing, the average processing time of a single fastener was 0.5 to 1.5 seconds, and the yield rate was >95%.

[0119] The fasteners of the ten embodiments were sampled and subjected to mechanical property tests and delayed fracture resistance tests.

[0120] In terms of mechanical properties, referring to ASTM A370 standard test methods and definitions for mechanical properties tests of steel products, a tensile testing machine is used to test the yield strength, tensile strength and elongation after fracture of fasteners.

[0121] In terms of delayed fracture resistance, the slow strain rate tensile test method is adopted, and the test process is as follows:

[0122] Step 1: Take two groups of fastener samples 10, refer to Figure 1, and make a notch 20 along the circumferential direction in the middle of the two groups of fastener samples 10, with a notch angle α = 60°. Use one group of samples as hydrogen charging group samples and the other group of samples as control group samples.

[0123] Step 2: Place the hydrogen-charged sample in a 4g / L NaOH solution for electrochemical hydrogenation. The hydrogenation time is 48h and the current density is 15A / m 2 After hydrogen charging is completed, the sample is cleaned and dried.

[0124] The control group samples did not undergo step 2.

[0125] Step 3: Both groups of samples were subjected to room temperature tensile tests using a slow strain rate tensile testing machine. The tensile strain rate was 5×10 -6 / s, and the tensile strength of the two groups of samples were measured respectively;

[0126] Step 4. Calculate the delayed fracture strength ratio: Delayed fracture strength ratio = tensile strength of the hydrogen-charged sample / tensile strength of the control sample. A larger delayed fracture strength ratio indicates better delayed fracture resistance. Generally, a delayed fracture strength ratio of 0.7 or higher is considered acceptable, while a delayed fracture strength ratio of 0.8 or higher is considered excellent.

[0127] The mechanical properties test results and delayed fracture strength ratio are shown in Table 2.

[0128] Table 2

[0129] In summary, the fasteners of the embodiments of the present application, based on the chemical composition design, combined with the control of the wire rod and the entire production process of the fasteners, avoid the occurrence of cracking of the fasteners during the upsetting process, and the prepared fasteners have a tensile strength ≥2000MPa, a yield strength ≥1750MPa, and a delayed fracture strength ratio ≥0.85. They have ultra-high tensile strength, excellent toughness, forming performance and delayed fracture resistance, which greatly improves the safety of the fasteners in industrial applications, and can greatly improve production efficiency, making them suitable for industrial mass production.

[0130] It should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each implementation method can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0131] The detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A wire rod, characterized in that: Its chemical composition includes, by mass percentage, 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, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

2. The wire rod according to claim 1, characterized in that Its chemical composition includes, by mass percentage, 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, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

3. The wire rod according to claim 1, characterized in that Its chemical composition includes, by mass percentage, 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, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

4. A method for producing a wire rod as claimed in claim 1, characterized in that: It includes the following steps: molten steel smelting, continuous casting, slow cooling, heating, high-speed wire rolling, and controlled cooling; In the continuous casting process, the temperature of the tundish is controlled to be 1510-1530°C; In the slow cooling process, the continuous casting billet obtained in the continuous casting process is air-cooled to 650-700°C, and then sent to a holding pit for slow cooling at a cooling rate of 5.5-7°C / h. The billet is cooled to below 200°C and then taken out of the holding pit. The heating process includes a heating section and a soaking section. The temperature of the heating section is 1100-1140°C, the heating rate is 0.3-0.4°C / s, and the heating time is 20-30 minutes. The temperature of the soaking section is 1150-1190°C, and the soaking time is 15-25 minutes. In the high-speed wire rolling process, the starting rolling temperature is 1050-1090°C, and the final rolling temperature is 910-940°C; In the controlled cooling process, the spinning temperature is 890-910°C; the insulation covers of the 1st to 2nd rollers of the Stelmor cooling line are opened, and the insulation covers of the other rollers are all closed, the 1st to 3rd fans are turned on, the fan air volume is 60-90%, and the other fans are closed, the roller speed of the 1st to 2nd rollers is 0.25-0.4m / s, and the roller speed of the other rollers is 0.15-0.3m / s, and the temperature of the wire rod when it reaches the 3rd roller is controlled to be 750-780°C.

5. The method for producing wire rod according to claim 4, characterized in that: The molten steel smelting process includes converter smelting and LF refining steps performed in sequence; In the converter smelting step, C in the molten steel at the end of the converter smelting is 0.05-0.15%, P is ≤0.012%, the tapping temperature is 1620-1660° C., and after 25-30% of the steel is tapped, aluminum ingots, ferrosilicon, ferromanganese, ferrochrome, ferronickel, and ferromolybdenum are sequentially added for deoxidation and alloying. During the tapping process, argon is blown from the bottom of the ladle at a flow rate of 400-800 L / min. In the LF refining step, after the molten steel produced in the converter smelting step is injected into the LF furnace, lime and fluorite are added in sequence to adjust the white slag. After the white slag is refined for 5 to 10 minutes, titanium iron and rare earth iron alloy are added in sequence, and then at least one of ferroboron, ferrovanadium and ferroniobium is added for alloying. During the alloying, the argon flow rate of the bottom of the ladle is 300 to 450 L / min; then the power is turned on to increase the temperature and samples are taken for chemical composition detection, and the alloy is added according to the detection results to fine-tune the chemical composition. During the chemical composition fine-tuning, the argon flow rate of the bottom of the ladle is 150 to 300 L / min; then soft stirring is performed, and the soft stirring time is 25 to 35 minutes; steel tapping at the end point of LF refining The temperature is 1530~1560℃.

6. The method for producing wire rod according to claim 4, characterized in that: In the continuous casting process, the continuous casting process adopts argon sealing of the long water inlet of the large ladle, alkaline tundish covering agent, and immersed water inlet for full protection casting, and the protective slag adopts medium carbon steel protective slag; the superheat of the molten steel in the tundish is controlled to be 25-35°C, the liquid level height of the crystallizer is 80-87% of the crystallizer height, the liquid level fluctuation range is within ±3%, the electromagnetic stirring current of the crystallizer is 700-750A, the stirring frequency is 3-5Hz, the secondary cooling water volume is controlled to be 0.25-0.30L / Kg, and the pulling speed is 2.3-2.5m / min.

7. A fastener, characterized in that: Its chemical composition includes, by mass percentage, 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, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

8. The fastener according to claim 7, characterized in that Its chemical composition includes, by mass percentage, 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%, Re0.01-0.10%, Al 0.015-0.050%, O≤0.001%, N≤0.005%, and at least one of B, Nb, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

9. The fastener according to claim 7, characterized in that Its chemical composition includes, by mass percentage, 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, and V, and the rest is Fe and unavoidable impurities; wherein, [B]+[Nb]+[V]≤0.15%, [Cr] / [Ni] is 2.0-3.5, and [Mo] / [Ti] is 4-6.

10. The fastener according to claim 7, characterized in that Its tensile strength is ≥2000MPa, yield strength is ≥1750MPa, and delayed fracture strength ratio is ≥0.

85.

11. A method for producing a fastener according to claim 7, characterized in that: The fastener is made of wire rod as the base material through annealing, pickling and phosphating, drawing, warm upsetting, thread rolling, tempering treatment and surface polishing. The annealing process includes a heating section, a first cooling section and a second cooling section. The heating section has a heating rate of 4 to 6°C / min, and the temperature is heated to 760 to 800°C and then kept at this temperature for 3 to 5 hours. The first cooling section has a cooling rate of 1 to 1.5°C / min, and the temperature is cooled to 680 to 720°C and then kept at this temperature for 2 to 4 hours. In the second cooling section, the wire rod is naturally cooled to room temperature in the heating furnace. In the warm upsetting process, the fine wire obtained in the drawing process is heated to 530-570° C. and then upset and formed; 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 30-60 minutes. The heating temperature of the tempering step is 550-580°C and the holding time is 60-90 minutes. After tempering, the quenching step is air-cooled to room temperature.

12. The method for producing a fastener according to claim 11, characterized in that: The quenching medium in the quenching step is oil.

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