A production method for realizing on-line spheroidizing annealing of CrMo cold heading steel

Abstract

The application discloses a kind of production methods for realizing CrMo cold upsetting steel online spheroidizing annealing, belong to steel smelting field, solve the problems such as long traditional offline annealing cycle, high cost, spheroidizing quality is not good etc..The method is through converter smelting, LF+RH refining, continuous casting and other multiple steps, accurately control temperature and time in each link, isothermal treatment is formed in salt bath tank within 10s after wire drawing to form upper bainite structure, and the molten salt wrapping rod realizes STC furnace annealing under the protection of atmosphere, and finally the spheroidizing grade of rod is greater than or equal to 5.0, without full decarburization, which can be directly light drawing + cold upsetting forming.The process shortens the processing flow by more than 70%, reduces the overall energy consumption by more than 80%, realizes efficient green cost reduction and carbon reduction, and adapts to the production needs of high-strength fasteners.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, specifically to a production method for realizing online spheroidizing annealing of CrMo cold heading steel. Background Technology

[0002] Fasteners are core components in machinery manufacturing, automotive, shipbuilding, and railway industries, and upgrading their performance and reducing production costs are key demands for industry development. High-strength fasteners of grades 10.9-12.9, due to their harsh service environments, place extremely high demands on the strength, toughness, plasticity, and processing stability of the raw material, cold heading steel. CrMo cold heading steel, with its excellent hardenability and comprehensive mechanical properties, has become the core raw material for this grade of fasteners. However, traditional CrMo cold heading steel production and processing technologies suffer from numerous technical bottlenecks, making it difficult to meet the current industry's needs for efficient, green, and low-cost development.

[0003] The core problems in the production and subsequent processing of existing CrMo cold heading steel wire rods are concentrated in the following aspects: First, the microstructure and properties are poorly compatible, and the cold working risk is high. The microstructure of CrMo cold heading steel wire rods produced by conventional processes is mostly a mixture of pearlite + ferrite or martensite + bainite. Among them, the pearlite lamellae are coarse, and the bainite and martensite have poor plasticity, resulting in high tensile strength of wire rods (generally 900~1100MPa) and low section reduction of area (mostly below 30%). In the direct cold heading process, cracking and excessive die wear are likely to occur. Secondly, the annealing process is cumbersome, resulting in high energy consumption and long cycle time. To improve processing performance, downstream enterprises need to adopt the traditional "two-drawing, two-balling" process (pickling-softening annealing → pickling and phosphating saponification → rough drawing → balling annealing → pickling and phosphating saponification → fine drawing). This process requires multiple pickling, heating, and cooling cycles, which not only consumes a lot of energy (the energy consumption of a single offline annealing cycle accounts for more than 30% of the entire process), but also has a production cycle of tens of hours. The material flow is complex, significantly increasing production costs. Thirdly, balling annealing has low efficiency and insufficient quality stability. In CrMo cold heading steel, strong carbide-forming elements such as Cr and Mo reduce the carbon atom diffusion coefficient, increasing the difficulty of balling. Existing offline annealing processes require heating the wire rod to 750-800℃ and holding it for 10-20 hours to achieve a balling grade of 5.0 or higher. Although some simplified processes attempt to shorten the annealing time, they are prone to carbide segregation and uneven balling, leading to product defects due to performance fluctuations during cold heading.

[0004] To address the aforementioned issues, various improvement solutions have been proposed within the industry, but all suffer from significant technical shortcomings, making it difficult to achieve full-process optimization. For example, patent CN115852106B, a method for spheroidizing annealing of medium-low carbon alloy cold heading steel, improves carbide distribution uniformity by optimizing the spheroidizing annealing temperature regime (700-720℃, 6h holding time, segmented cooling). However, it remains an offline process, requiring additional energy to heat the cold wire rod, and it doesn't solve the problem of coarse initial microstructure in the rolled wire rod. The spheroidizing cycle still requires 8-10 hours, resulting in limited production efficiency improvement and failing to reduce the complexity of the "two-drawing, two-spheroidizing" process. Patent CN117802291A, a simplified manufacturing process for annealed 12.9 grade fastener wire rod, optimizes billet heating, multi-pass rolling, and Stellmore air-cooling linear controlled cooling processes, reducing the tensile strength of 12.9 grade fastener wire rod to below 800MPa and achieving a reduction in area ≥40%, enabling direct drawing processing. However, this technology is only applicable to low- and medium-carbon chromium-molybdenum alloy steels and relies on precise control of rolling deformation and cooling rate (e.g., cooling rate after pre-finishing rolling is 0.20~1.0℃ / s), requiring extremely high equipment precision. Furthermore, it does not address the optimization of the spheroidizing annealing process, failing to meet the spheroidized microstructure (≥5.0 grade) requirements for large deformation cold heading of CrMo cold heading steel. Patents CN119243018A (a production method for online softening of high-strength cold heading steel wire rod) and CN121514268A (a new process for improving cold heading performance of SCM435 series cold heading steel through online softening) achieve online softening through processes such as low-temperature rolling (inlet / reduction sizing temperature 780-830℃) and slow cooling in a heat-insulating corridor (heat-insulating time ≥90min), omitting the softening annealing process before rough drawing. However, the core technology focuses on reducing strength and increasing plasticity, without designing a process specifically for the spheroidization requirements of CrMo cold heading steel. The wire rod microstructure is F+P, which cannot meet the stringent requirement of 1 / 6 cold heading without cracking, resulting in low product adaptability and issues such as surface decarburization and reduced surface hardness, making industrial application difficult. Patent CN119843151A describes a simplified method for manufacturing fully bainitic medium-carbon chromium cold heading steel using precision wire machining. This method uses salt bath cooling instead of traditional Stellmore air cooling to obtain a fully bainitic microstructure, achieving a spheroidization grade of 6.0 under the "one ball, one draw" process. However, this method still requires two pickling processes and one spheroidizing annealing process for the wire rod, resulting in insufficient optimization of overall energy consumption and process flow, and inadequate economic viability for large-scale production.

[0005] In summary, existing technologies either focus on optimizing offline annealing processes, failing to address energy consumption and cycle time issues; or simplify some processes through controlled rolling and cooling, but cannot meet the core requirements of spheroidization grade and large deformation cold heading for CrMo cold heading steel, increasing the cost and complexity of industrial applications. As European companies adopt alloy substitution and process simplification strategies to seize market share, the domestic fastener industry urgently needs to develop a highly efficient online spheroidization annealing technology suitable for CrMo cold heading steel. By integrating the entire process of smelting, rolling, cooling, and annealing, while ensuring a spheroidization grade ≥5.0 and no complete decarburization (total decarburization depth ≤1%D), the offline annealing process can be omitted, shortening the processing flow, reducing energy consumption and production costs, achieving benchmarking against international advanced processes, and promoting technological upgrading and green development in the high-strength fastener industry. Summary of the Invention

[0006] The purpose of this invention is to provide a production method for online spheroidizing annealing of CrMo cold heading steel, so as to overcome the defects of existing technologies such as poor microstructure adaptability, high annealing energy consumption, difficulty in decarburization control, and low spheroidizing efficiency.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A production method for online spheroidizing annealing of CrMo cold heading steel is disclosed. The method includes the following steps in sequence: converter smelting, LF+RH refining, continuous casting, billet heating, rolling, and wire drawing. After the wire drawing process, the method further includes continuous online isothermal treatment, hot coiling, online STC furnace annealing, and water washing. After the wire drawing is completed, the wire rod is controlled to enter an isothermal salt bath at a temperature of 560-600℃ within 10 seconds for online isothermal treatment, and the isothermal heat treatment time is more than 240 seconds. The hot coiling temperature is above 530℃. The online STC furnace spheroidizing annealing temperature is 700-720℃, the holding time is 4-6 hours, and no hydrogen purging or RX cracking gas protection is required. After the wire rod exits the online STC furnace, it is transported via a PF line, maintaining an overall temperature above 200℃, and then enters a water washing tank for surface molten salt cleaning.

[0008] Preferably, the temperature for heating the billet is 950-1020℃.

[0009] Preferably, the rolling temperature at the finishing mill and the sizing temperature is 780-830℃.

[0010] Preferably, the spinning temperature is 750-790℃.

[0011] Preferably, after the hot coiling, the overall temperature of the wire rod entering the online STC furnace is ≥500℃.

[0012] Preferably, after the wire rod is kept at a constant temperature in the online STC furnace for 4-6 hours, it is cooled to 350°C in a slow cooling zone for 3-4 hours before exiting the online STC furnace.

[0013] Preferably, after the molten salt on the surface of the wire rod is cleaned, the wire rod is cooled by water, and the temperature is rapidly reduced from 200°C to room temperature within 10 seconds.

[0014] Preferably, the surface of the wire rod is not completely decarburized, and the total decarburization depth is ≤1%D, where D is the diameter of the wire rod.

[0015] Preferably, the spheroidization grade of the wire rod is ≥5.0, which enables 1 / 6 cold heading without cracking, and allows for direct cold heading after light drawing in the next process, thus achieving anneal-free cold heading fasteners.

[0016] Compared with the prior art, the beneficial effects of the present invention are: After the billet is peeled and finished, it is heated at a low temperature of 950-1020℃, rolled into the finishing mill and sizing temperature of 780-830℃, and wire drawing temperature of 750-790℃, laying the foundation for wire rod without complete decarburization and with a total decarburization depth ≤1%D. (2) After the wire is drawn, it is quickly placed in an isothermal salt bath at 560-600℃ within 10s and held for more than 240s to make the austenite obtain a great degree of supercooling, avoid the ferrite and pearlite transformation zone, and isothermally transform into upper bainite structure in the upper bainite region of this steel grade, so as to achieve the overall uniform distribution of cementite in the wire rod, which is conducive to the occurrence of subsequent spheroidizing process. (3) During the process of the wire rod exiting the salt tank, a uniform layer of molten salt adheres to the surface, which can form a protective film to prevent the wire rod from coming into contact with oxygen in the air in the heating and heat preservation zone of the STC furnace and causing surface decarburization. This allows the STC furnace to be protected without hydrogen or RX cracking gas. (4) The wire rod is hot-rolled and quickly transferred into the online STC furnace. The overall temperature of the wire rod is ≥500℃, and it can be raised from 500℃ to 700-720℃. Compared with the traditional spheroidizing annealing process (heating starts at room temperature), it can achieve relatively low heating energy consumption and transfer cycle. (5) The wire rod is treated with isothermal salt bath to obtain upper bainite structure and carbides are evenly distributed. This can reduce the spheroidizing annealing temperature. Compared with traditional offline spheroidizing annealing, the temperature is reduced by about 60°C and the overall spheroidizing annealing time is reduced by about 20 hours.

[0017] (6) The wire rod is rapidly cooled by low-temperature wire drawing and isothermal salt bath treatment. The overall thickness of the iron oxide scale on the surface of the wire rod is controlled below 6μm. The surface is then coated with molten salt to isolate it from the air, so that the thickness of the iron oxide scale does not increase after annealing and the structure does not change. The wire rod is also rapidly cooled by online water washing, so that a gap is formed between the iron oxide scale and the substrate, which is beneficial for mechanical dephosphorization or pickling treatment in the downstream wire drawing process.

[0018] (7) The spheroidization grade of the wire rod is ≥5.0, which meets the 1 / 6 cold heading qualification. Customers can directly carry out light drawing + cold heading processing to achieve direct fastener forming. Compared with the traditional two-ball and two-drawing process, it saves one large drawing (reduction rate ≥30%), one pickling and phosphating treatment and two offline annealing. The overall processing flow is shortened by more than 70%, and the annealing energy consumption is reduced by more than 80%, which realizes green carbon reduction and improves the competitiveness of the entire industrial chain products. Attached Figure Description

[0019] Figure 1 Metallographic structure diagram of wire rod in Example 1; Figure 2 Metallographic structure diagram of wire rod in Example 2; Figure 3 Metallographic structure diagram of wire rod in Example 3; Figure 4 Here is a metallographic image of the wire rod used in Comparative Example 1; Figure 5 The image shows the metallographic structure of two wire rods for comparison. Detailed Implementation

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below through examples.

[0021] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0022] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments. Example 1

[0023] This embodiment provides a production method for online spheroidizing annealing of CrMo cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire drawing, online isothermal treatment, hot coiling, online STC furnace annealing, and water washing.

[0024] The specific steps are as follows: (1) A 100-ton converter was used for smelting, a 100-ton LF furnace was used for refining, RH vacuum degassing was used, and a 6-strand continuous casting machine was used for casting to obtain a 180mm×240mm billet. After the billet was fully peeled and inspected, it was heated. The soaking temperature was 960-1000℃, the total heating time was 120-140min, the initial rolling temperature was 890-930℃, the temperature of entering the finishing mill and the sizing temperature was controlled at 780-800℃, the wire drawing temperature was 750-770℃, and the wire rod diameter was φ14.0mm.

[0025] (2) After the wire rod is spun out, the main speed of the roller conveyor is set to 0.8 m / s, so that the wire rod can quickly enter the isothermal salt bath at 590±5℃ within 10s, and then slowly undergo isothermal transformation at the main speed of the roller conveyor at 0.08 m / s, so as to ensure that the wire rod is in the salt bath for ≥240s.

[0026] (3) After the wire rod completes the isothermal transformation of upper bainite in the isothermal salt bath, it is rapidly hot-rolled in the range of 560-580℃, while ensuring that the surface layer is uniformly coated with molten salt. Then it is quickly put into the online STC furnace through the vertical core frame.

[0027] (4) The wire rod is heated in the heating zone of the STC furnace. It is rapidly heated to 710±5℃ in about 30 minutes and kept at that temperature for 5 hours. Then it enters the slow cooling zone and is cooled to about 350℃ in 3.5 hours before exiting the online STC furnace. It is then transported to the PF line and transferred to the online cleaning tank.

[0028] (5) The temperature of the wire rod entering the online cleaning tank is controlled at 240±20℃. After entering the online cleaning tank, the temperature of the coil is reduced to room temperature within 10 seconds, and the surface is cleaned by molten salt.

[0029] (6) The chemical composition of the obtained wire rod is shown in Table 1. The remainder is Fe and unavoidable impurities.

[0030] (7) The metallographic structure of the obtained product is as follows: Figure 1 As shown, the spheroidization level of the wire rod after online annealing is level 6. Example 2

[0031] This embodiment provides a production method for achieving online spheroidizing annealing of CrMo cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire drawing, online isothermal treatment, hot coiling, online STC furnace annealing, and water washing.

[0032] The specific operation steps are as follows: (1) 100-ton converter smelting, 100-ton LF furnace refining, RH vacuum degassing, 6-machine 6-strand continuous casting machine casting to obtain 180mm×240mm billet. After the billet is fully peeled and inspected, the billet is heated. The soaking temperature is 950-980℃, the total heating time is 120-140min, the rolling temperature is 880-910℃, the temperature of entering the finishing mill and reducing the sizing temperature is 790-810℃, the wire drawing temperature is 760-780℃, and the wire rod diameter is φ18.0mm.

[0033] (2) After the wire rod is spun out, the main speed of the roller conveyor is set to 0.7 m / s, so that the wire rod can quickly enter the isothermal salt bath at 560±5℃ within 10s, and then slowly undergo isothermal transformation at the main speed of the roller conveyor at 0.06 m / s, so as to ensure that the wire rod is in the salt bath for ≥240s.

[0034] (3) After the wire rod completes the isothermal transformation of upper bainite in a salt bath at 560±5℃, it is rapidly hot-rolled in the range of 530-550℃, while ensuring that the surface layer is uniformly coated with molten salt. Then it is quickly fed into the online STC furnace through a vertical core rack.

[0035] (4) The wire rod is heated in the heating zone of the STC furnace. It is rapidly heated to 710±5℃ in about 30 minutes and kept at that temperature for 5 hours. Then it enters the slow cooling zone and is cooled to about 350℃ in 3.5 hours before exiting the STC furnace. It is then transported to the PF line and transferred to the online cleaning tank.

[0036] (5) The temperature of the wire rod entering the online cleaning tank is controlled at 230±20℃. After entering the online cleaning tank, the temperature of the coil is reduced to room temperature within 10 seconds, and the surface is cleaned by molten salt.

[0037] (6) The chemical composition of the obtained wire rod is shown in Table 1. The remainder is Fe and unavoidable impurities.

[0038] (7) The metallographic structure of the obtained product is as follows: Figure 2 As shown, the spheroidization level of the wire rod after online annealing is level 6. Example 3

[0039] Please see Figure 3 This embodiment provides a production method for achieving online spheroidizing annealing of CrMo cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire drawing, online isothermal treatment, hot coiling, online STC furnace annealing, and water washing.

[0040] The specific steps are as follows: (1) The 100-ton converter was used for smelting, the 100-ton LF furnace was used for refining, the RH vacuum degassing was used, and the 6-machine 6-strand continuous casting machine was used for casting to obtain a 180mm×240mm billet. After the billet was fully peeled and inspected, the billet was heated. The soaking temperature was 980-1020℃, the total heating time was 120-140min, the initial rolling temperature was 900-930℃, the temperature of entering the finishing mill and the sizing temperature was controlled at 810-830℃, the wire drawing temperature was 770-790℃, and the wire rod diameter was φ22.0mm.

[0041] (2) After the wire rod is spun out, the main speed of the roller conveyor is set to 0.6 m / s, so that the wire rod can quickly enter the isothermal salt bath at 590±5℃ within 10s, and slowly undergo isothermal transformation at the main speed of the roller conveyor at 0.05 m / s, so as to ensure that the wire rod spends ≥240 minutes in the salt bath.

[0042] (3) After the wire rod completes the isothermal transformation of upper bainite in the isothermal salt bath, it is rapidly hot-rolled in the range of 550-570℃, while ensuring that the surface layer is uniformly coated with molten salt. Then it is quickly put into the online STC furnace through the vertical core frame.

[0043] (4) The wire rod is heated in the heating zone of the STC furnace. It is rapidly heated to 715±5℃ in about 30 minutes and kept at that temperature for 6 hours. Then it enters the slow cooling zone and is cooled to about 350℃ in 3.5 hours before exiting the STC furnace. It is then transported to the PF line and transferred to the online cleaning tank.

[0044] (5) The temperature of the wire rod entering the online cleaning tank is controlled at 230±20℃. After entering the online cleaning tank, the temperature of the coil is reduced to room temperature within 10 seconds, and the surface is cleaned by molten salt.

[0045] (6) The chemical composition of the obtained wire rod is shown in Table 1. The remainder is Fe and unavoidable impurities.

[0046] (7) The metallographic structure of the obtained product is as follows: Figure 3 As shown, the spheroidization level of the wire rod after online annealing is level 5.

[0047] Comparative Example 1 Please see Figure 4 This embodiment provides a production method for achieving online spheroidizing annealing of CrMo cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire drawing, online isothermal treatment, hot coiling, online STC furnace annealing, and water washing.

[0048] The specific steps are as follows: (1) The 100-ton converter was used for smelting, the 100-ton LF furnace was used for refining, the RH vacuum degassing was used, and the 6-machine 6-strand continuous casting machine was used for casting to obtain a 180mm×240mm billet. After the billet was fully peeled and inspected, the billet was heated. The soaking temperature was 1100-1140℃, the total heating time was 140-160min, the initial rolling temperature was 1040℃, the temperature of entering the finishing mill and the sizing temperature was controlled at 880-910℃, the wire drawing temperature was 840-860℃, and the wire rod diameter was φ22.0mm.

[0049] (2) After the wire rod is spun out, the main speed of the roller conveyor is set to 0.6 m / s, so that the wire rod can quickly enter the isothermal salt bath at 590±5℃ within 10s, and the isothermal transformation is carried out slowly at the main speed of the roller conveyor at 0.30 m / s, so as to ensure that the wire rod is in the salt bath for 40s.

[0050] (3) After the wire rod completes the isothermal transformation of upper bainite in the salt bath, it is rapidly hot-rolled in the range of 600-620℃, while ensuring that the surface layer is uniformly coated with molten salt. Then it is quickly put into the online STC furnace through the vertical core frame.

[0051] (4) The wire rod is heated in the heating zone of STC. It is rapidly heated to 715±5℃ in about 30 minutes and kept at that temperature for 3 hours before entering the slow cooling zone. It is cooled to about 350℃ in 3.5 hours before exiting the STC furnace and being transported to the PF line and transferred to the online cleaning tank.

[0052] (5) The temperature of the wire rod entering the online cleaning tank is controlled at 230±20℃. After entering the online cleaning tank, the temperature of the coil is reduced to room temperature within 10 seconds, and the surface is cleaned by molten salt.

[0053] (6) The chemical composition of the obtained wire rod is shown in Table 1. The remainder is Fe and unavoidable impurities.

[0054] (7) The metallographic structure of the obtained product is as follows: Figure 4 As shown, the spheroidization level of the wire rod after online annealing is below level 4.

[0055] Comparative Example 2 Please see Figure 5 This embodiment provides a production method for achieving online spheroidizing annealing of CrMo cold heading steel. The specific implementation steps include converter smelting, LF+RH refining, continuous casting, billet peeling, billet heating, rolling, wire drawing, and offline STC furnace annealing.

[0056] The specific steps are as follows: (1) The 100-ton converter was used for smelting, the 100-ton LF furnace was used for refining, the RH vacuum degassing was used, and the 6-machine 6-strand continuous casting machine was used for casting to obtain a 180mm×240mm billet. After the billet was fully peeled and inspected, the billet was heated. The soaking temperature was 980-1020℃, the total heating time was 120-140min, the initial rolling temperature was 900-930℃, the temperature of entering the finishing mill and the sizing temperature was controlled at 810-830℃, the wire drawing temperature was 770-790℃, and the wire rod diameter was φ22.0mm.

[0057] (2) After the wire rod is spun out, it is cooled to room temperature by setting the main speed of the roller table to 0.4 m / s, and then the wire rod is slowly cooled and wound up in the Steyrmo roller table.

[0058] (3) The wire rod is heated and spheroidized in the heating zone of the STC furnace. The spheroidizing annealing temperature is 765±5℃ and held for 8 hours. After cooling down to 740±5℃ and holding for 6 hours, it is cooled with the furnace (about 5 hours) to about 300℃. It is then removed from the STC furnace and air-cooled before being repackaged.

[0059] (4) The chemical composition of the obtained wire rod is shown in Table 1. The remainder is Fe and unavoidable impurities.

[0060] (5) The metallographic structure diagram of the obtained product is as follows: Figure 5 As shown, the spheroidization level of the wire rod after online annealing is below level 4.

[0061] Table 1. Chemical composition and its mass percentage (%)

[0062] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A production method for achieving online spheroidizing annealing of CrMo cold heading steel, the method comprising the steps of converter smelting, LF+RH refining, continuous casting, billet heating, rolling, and wire drawing, characterized in that, The coiling process is followed by continuous online isothermal treatment, hot coiling, online STC furnace annealing, and water washing. After the coiling is completed, the coil is controlled to enter an isothermal salt bath at a temperature of 560-600℃ within 10 seconds for online isothermal treatment, and the isothermal heat treatment time is more than 240 seconds. The hot coiling temperature is above 530℃. The online STC furnace spheroidizing annealing temperature is 700-720℃, and the holding time is 4-6 hours, without the need for hydrogen charging or RX cracking gas protection. After the coil exits the online STC furnace, it is transported via a PF line, maintaining an overall temperature above 200℃, and then enters a water washing tank for surface molten salt cleaning.

2. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, The temperature for heating the billet is 950-1020℃.

3. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, The rolling temperature for the finishing mill and the sizing temperature is 780-830℃.

4. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, The spinning temperature is 750-790℃.

5. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, After the hot coiling, the overall temperature of the wire rod entering the online STC furnace is ≥500℃.

6. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, After being held at the online STC furnace for 4-6 hours, the wire rod is cooled to 350°C in a slow cooling zone for 3-4 hours before exiting the online STC furnace.

7. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to claim 1, characterized in that, After the molten salt on the surface of the wire rod is cleaned, the wire rod is cooled by water, and the temperature drops rapidly from 200°C to room temperature within 10 seconds.

8. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to any one of claims 1-7, characterized in that, The surface of the wire rod is not completely decarburized, and the total decarburization depth is ≤1%D, where D is the diameter of the wire rod.

9. The production method for realizing online spheroidizing annealing of CrMo cold heading steel according to any one of claims 1-7, characterized in that, The spheroidization grade of the wire rod is ≥5.0, which enables 1 / 6 cold heading without cracking and allows for direct cold heading after light drawing in the next process, thus achieving anneal-free cold heading fasteners.

Images