Non-quenched and tempered cold heading steel with excellent corrosion resistance and method for producing the same

Abstract

The application provides a non-quenched and tempered cold heading steel with excellent corrosion resistance and a production method thereof, and the composition of the steel is as follows: C 0.21%-0.30%, Si 0.02%-0.1%, Mn 1.8%-2.5%, V 0.05%-0.20%, B 0.0005%-0.0030%, Ti 0.02%-0.04%, Cr 0.1%-0.3%, Ni 0.1%-0.3%, Cu 0.1%-0.3%, Al 0.015%-0.035%, P≤0.010%, S≤0.010%, T.O≤0.0020%, N≤0.0065%, and the rest is Fe and other inevitable impurities; compared with the prior art, the application optimizes the composition and the process, the austenite grain size of the product is greater than or equal to 11.0, the product has good strength and plasticity and toughness, and has good atmospheric corrosion resistance, and the spheroidizing annealing and the quenching and tempering treatment can be omitted.

Description

Technical Field

[0001] This invention belongs to the technical field of non-quenched and tempered cold heading steel, and particularly relates to a non-quenched and tempered cold heading steel with excellent corrosion resistance and its production method. Background Technology

[0002] Fasteners are a crucial and widely used basic component, found in industries such as machinery manufacturing, engineering structures, railways, automobiles and tractors, and construction. Approximately 70% of connected parts and assemblies are linked by fasteners. Fasteners are primarily formed by cold heading, and the material must withstand 70-80% of the total deformation during manufacturing. This requires the material to have good plasticity and low hardness before cold heading. Therefore, cold-heading steel wire produced by traditional processes requires two time-consuming and energy-intensive heat treatment processes: spheroidizing annealing and quenching and tempering, before cold heading and drawing.

[0003] In recent years, under pressure to save energy and reduce costs, fastener manufacturers have urgently requested steel mills to develop new energy-saving cold heading steel wire rods that can eliminate the need for tempering and pre-drawing spheroidizing annealing. These wire rods can replace tempered steel, saving not only energy (statistically, up to nearly 2500 kWh of electricity per ton of steel), but also avoiding problems such as quenching cracks, workpiece deformation, and surface oxidation and decarburization caused by heat treatment. This simplifies the process, improves production efficiency, and is of particular importance and necessity for energy conservation, consumption reduction, and environmental pollution reduction, resulting in significant economic and social benefits. Therefore, it has been widely used both domestically and internationally.

[0004] Currently, both domestically and internationally, the main methods to achieve equivalent quenched and tempered steel levels are through microalloying, controlled rolling and cooling, and cold work hardening. Internationally, research and development of cold-work-strengthened non-quenched and tempered steel began in the 1980s, and China has also successfully developed non-quenched and tempered cold heading steel, which can be used to produce fasteners with small deformation, such as screws and threaded rods, mainly for the machinery and construction industries.

[0005] However, with the rapid development of the construction and building industry, the environment in which fasteners are used is becoming increasingly complex. This requires fasteners to have good mechanical properties and corrosion resistance to meet the requirements of use in complex environments.

[0006] The patent published on March 8, 2017, with publication number CN 106480376 A, discloses a non-quenched and tempered cold heading steel wire rod for 10.9 grade fasteners and a production method thereof. Its composition and weight percentages are: C 0.10–0.15%; Si 0.50–0.80%; Mn 1.60–2.10%; P ≤0.015%; S ≤0.010%; Cr 0.30–0.50%; B 0.002–0.005%; Ti 0.03–0.05%; unavoidable impurities not exceeding 0.1%; the remainder being iron. Through precise alloy composition design, combined smelting, continuous casting and rolling, and controlled rolling and cooling processes for wire rods, accurate control of the wire rod's microstructure and properties is achieved. The resulting 10.9 grade fastener non-quenched and tempered cold heading steel wire rod has a uniform composition, a granular bainitic microstructure, sufficient strength, and good cold drawability, meeting the requirements for producing 10.9 grade high-strength fasteners using non-quenched and tempered processes. However, fasteners produced by this method do not have good corrosion resistance.

[0007] Patent CN112359275A, published on February 12, 2021, discloses a non-quenched and tempered cold heading steel wire rod for high-strength fasteners and its preparation method. The composition of the non-quenched and tempered cold heading steel wire rod is: C: 0.16–0.18%, Si: ≤0.20%, Mn: 1.40–1.50%, P: ≤0.008%, S: ≤0.008%, Ti: 0.05–0.06%, V: 0.10–0.13%, Al: ≤0.01%, N: 60–90 ppm, with the remainder being Fe and unavoidable impurities. The preparation method includes converter smelting, LF refining, billet continuous casting, and wire rod rolling. By optimizing the composition of elements in the components, employing innovative smelting processes, and combining advanced controlled rolling and cooling technologies, this method effectively improves the strength and plasticity of steel, enabling the production of 10.9-grade high-strength fasteners without annealing or tempering, significantly reducing processing costs for downstream industries. However, fasteners produced by this method do not possess good corrosion resistance.

[0008] Therefore, it is essential to provide economical, high-strength, and corrosion-resistant non-quenched and tempered cold heading steel products. Summary of the Invention

[0009] The purpose of this invention is to provide a non-quenched and tempered cold heading steel with excellent corrosion resistance and its production method. Through composition design and process optimization, the produced non-quenched and tempered cold heading steel has an austenitic grain size of ≥11.0 grade, good strength and toughness, and good atmospheric corrosion resistance. It can be used to make high-strength fasteners with tensile strength of 1000MPa or more, eliminating the need for spheroidizing annealing and quenching and tempering treatments, and the alloy is simple and the cost is low.

[0010] The specific technical solution of this invention is as follows:

[0011] This invention provides a non-quenched and tempered cold heading steel with excellent corrosion resistance, comprising the following components by weight percentage:

[0012] C 0.21%–0.30%, Si 0.02%–0.1%, Mn 1.8%–2.5%, V 0.05%–0.20%, B 0.0005%–0.0030%, Ti 0.02%–0.04%, Cr 0.1%–0.3%, Ni 0.1%–0.3%, Cu 0.1%–0.3%, Alt 0.015%–0.035%, P≤0.010%, S≤0.010%, TO≤0.0020%, N≤0.0065%, with the remainder being Fe and other unavoidable impurities.

[0013] The composition of the non-quenched and tempered cold heading steel with excellent corrosion resistance also satisfies: 4.5≤15×Cr+10×(Ni+Cu)+20×Ti≤10.0.

[0014] The non-quenched and tempered cold heading steel with excellent corrosion resistance is used to make high-strength fasteners with tensile strength of over 1000 MPa, eliminating the need for spheroidizing annealing and quenching and tempering treatments.

[0015] The hot-rolled microstructure of the non-quenched and tempered cold-heading steel with excellent corrosion resistance is a granular bainite + ferrite dual-phase structure, wherein the granular bainite area content is 85% to 95%, the grain size is ≥11.0 grade, and the grain size is 11-14 μm; the tensile strength is 800 MPa ≤ R m ≤860MPa; ensures strength meets the requirements for use of 10.9 grade high-strength bolts.

[0016] The non-quenched and tempered cold heading steel with excellent corrosion resistance, after drawing and stabilization treatment, has an austenite grain size ≥11.0 grade; tensile strength R m ≥1020MPa, yield strength ratio R P0.2 / R m With a strength ≥0.9, elongation after fracture A ≥15%, reduction of area Z ≥52%, and room temperature impact energy KV2 ≥52J, this steel exhibits good strength and toughness, along with excellent resistance to atmospheric corrosion, with a corrosion rate (72h) ≤1.25g / m³. 2 ·h.

[0017] The present invention provides a method for producing non-quenched and tempered cold heading steel with excellent corrosion resistance, comprising the following process flow: batching according to the composition ratio → electric furnace smelting → LF furnace refining + RH vacuum refining → continuous casting of large square billets → rolling of square billets → finishing and peeling → heating → rolling of large coiled wire rods → controlled cooling in the Steyrmo cooling line → finished large coils → packaging and warehousing.

[0018] The electric furnace smelting process is as follows: the final carbon content (C) of the electric furnace is controlled at 0.06-0.20%, and the phosphorus content (P) is ≤0.005%. The process involves slag blocking during tapping. When 1 / 5-1 / 4 of the molten steel is tapped, refining slag and lime are added. When 1 / 4-1 / 3 of the molten steel is tapped, deoxidizer and alloying agents are added in the following order: aluminum ferrophosphate → silicon manganese → medium-carbon manganese → high-carbon ferrochrome → carbon raiser. After tapping, an appropriate amount of aluminum particles is evenly sprinkled onto the slag surface according to the amount of slag discharged.

[0019] The LF furnace refining process involves bottom blowing argon into the ladle throughout the process, with the argon flow rate determined by preventing molten steel from splashing out of the ladle. Pre-melted refining slag and lime are added to form slag, with an alkalinity of R3-5 and a white slag time of ≥20 minutes. Based on the composition analysis results before entering the LF furnace, alloys are added before and during the refining process to adjust the Si, Mn, and Cr content.

[0020] The RH vacuum degassing process is as follows: In the early stage of vacuum degassing, if the vacuum level is ≤100 Pa, the vacuum holding time should be ≥10 minutes; if 100 Pa < vacuum level ≤200 Pa, the vacuum holding time should be ≥15 minutes. In the later stage of vacuum degassing, the holding time should be ≥10 minutes. Based on the composition analysis results from the early stage of vacuum degassing, if composition adjustment is required in the middle stage, a vacuum holding time of at least 5 minutes must be maintained after adjustment. After vacuum breaking, calcium wire feeding is performed. Before leaving the station, soft blowing argon treatment is performed, with a soft blowing time ≥15 minutes. Based on the composition analysis results at the RH endpoint, alloys are added to adjust the V, Ti, B, and Ni contents.

[0021] The large billet continuous casting uses 380mm×450mm large billets. To ensure the uniformity of the wire rod microstructure, this invention utilizes large billets, which effectively reduces material segregation and guarantees the uniformity of the core and edge microstructure. Furthermore, it provides full-process protective casting. A protective sleeve and argon seal are used between the ladle and tundish. The tundish is protected with molten steel covering agent and argon blowing. An immersion nozzle is used between the tundish and the crystallizer. The primary cooling water flow rate is 100-130 m³ / h. 3 / h, secondary cooling water ratio 1.0~1.4l / kg, stable liquid level, casting speed and superheat during casting process, superheat controlled at 15~30℃, casting speed 1.9-2.1mm / min.

[0022] The rolled square billet is a 150mm square billet. The temperature of the billet in the soaking zone of the heating furnace is controlled at 1250–1350℃, and the total heating time is controlled at 250–350 minutes to ensure that alloying elements, especially Ti, are fully dissolved. The initial rolling temperature is controlled at 1100±50℃, and the billet is cooled after rolling at a temperature ≥400℃. After rolling, the 150mm square billet undergoes surface and end grinding treatment to provide good surface quality for subsequent high-speed wire rod rolling and reduce the sensitivity of the wire rod surface to decarburization.

[0023] The large-coil wire rod rolling process: Qualified raw material steel billets can be obtained through the above steps, enabling the rolling of large-coil wire rods with diameters ranging from φ16-30mm. High-speed wire rod rolling employs low-temperature, high-deformation rolling, with a deformation rate ≥50%. To ensure sufficient solid solution of alloying elements, the initial rolling temperature is 970–1030℃. Rolling is completed in the austenite recrystallization zone to achieve recrystallization refinement. The final rolling temperature is 760–800℃. Subsequently, the coils are cooled under controlled conditions on the LCC roller table. To obtain a granular bainite + ferrite dual-phase structure, a rapid cooling followed by slow cooling method is adopted. The first four insulation covers are opened, and the fans are turned on to 100% for rapid cooling at a rate of 4–7℃ / s to cool to 450–500℃, avoiding the formation of pearlite structure. The subsequent five to eleven fans are all turned off, and all insulation covers are closed, with a cooling rate of 0.7–1.0℃ / s to avoid the formation of martensite structure. The coils then exit the insulation tunnel and are coiled and hooked when the temperature reaches 400–440℃. They are then air-cooled to room temperature, packaged, and weighed.

[0024] The hot-rolled microstructure of the non-quenched and tempered cold heading steel with excellent corrosion resistance produced by the above method is: a dual-phase structure of granular bainite + ferrite, with a tensile strength of 860 MPa ≥ R. m ≥800MPa.

[0025] The above method produces non-quenched and tempered cold heading steel with excellent corrosion resistance through the following process: cold drawing → cold heading → thread machining → low-temperature stabilization treatment → surface treatment for fasteners. The low-temperature stabilization process involves heating to 350℃±10℃, holding for 30-40 minutes, and then air cooling. This can be combined with surface treatment processes such as galvanizing and Dacromet coating.

[0026] After the above drawing and stabilization treatments, the tensile strength R of the product is... m ≥1020MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥15%, reduction of area Z≥52%, room temperature impact energy KV2≥52J, austenitic grain size of steel ≥11.0 grade, with good strength and toughness, and good resistance to atmospheric corrosion.

[0027] The design concept of this invention is as follows:

[0028] Carbon (C): Carbon is essential for obtaining high strength and hardness. To obtain non-quenched and tempered steel with bainite as the main component, the C content must be above 0.15%. However, while excessively high C content is beneficial to the strength of the steel, it is extremely detrimental to the cold heading performance, plasticity, and toughness, and worsens the fatigue resistance and machinability of the steel. The C content should be controlled between 0.21% and 0.30%.

[0029] Si: Si is the main deoxidizing element in steel and, as a solid solution hardening element, contributes to increased strength and significantly improves the yield strength ratio. However, excessive Si content will reduce the plasticity and toughness of steel, increase the activity of carbon, promote decarburization and graphitization during rolling heating, make smelting difficult and prone to inclusion formation, and deteriorate the fatigue resistance of steel. Therefore, the Si content should be controlled at 0.02%–0.10%.

[0030] Mn: Mn is an effective element for deoxidation and desulfurization, and it can also promote bainitic phase transformation. When the content is less than 2.0%, it is difficult to achieve these effects. However, excessive Mn content leads to an excessively high residual austenite content after phase transformation, an excessively low bainitic phase transformation temperature, resulting in excessively low yield strength and yield ratio of the steel, excessive internal stress, and deteriorated fatigue performance. Therefore, the Mn content should be controlled between 1.8% and 2.5%.

[0031] Vanadium (V) is an excellent deoxidizer for steel. Adding vanadium to steel can refine the grain structure and improve strength and toughness. V forms V(C,N) precipitates with nitrogen (N) and carbon (C) in steel, exhibiting strong precipitation strengthening. However, due to the low bainitic transformation temperature, V diffusion is suppressed during the transformation process, resulting in a large amount of V dissolved in the steel. Nevertheless, because V is a strong carbide-forming element, the dissolved V can significantly inhibit C diffusion during the bainitic transformation, thus refining the bainitic ferrite and ensuring a high yield strength ratio. Excessive V content leads to higher costs; therefore, the V content should be controlled between 0.05% and 0.20%.

[0032] B: B can significantly delay the precipitation line of ferrite and increase the possibility of obtaining air-cooled bainite. However, excessive B content can easily cause hot brittleness and affect hot working performance. Therefore, B is controlled at 0.0005% to 0.0030%.

[0033] Ti: Ti reacts with N and C elements in steel to form Ti(C,N) precipitates, which inhibit grain growth during heating. Excessive Ti content can easily lead to the formation of large TiN inclusions through liquid precipitation, reducing the fatigue performance of the steel and increasing the likelihood of forging cracks. Ti can also improve corrosion resistance by inhibiting the absorption and formation of hydrogen in corrosive environments. Therefore, the Ti content should be controlled between 0.02% and 0.04%.

[0034] Cr: Cr can effectively delay the bainitic phase transformation to obtain the required high strength, and can also significantly improve the hardness of bainite through solid solution strengthening; at the same time, Cr can improve corrosion resistance by forming a passivation film on the steel surface, and when added in combination with Cu, it can significantly improve the weather resistance of the steel. However, excessive content will deteriorate the toughness and cold workability of the steel, so the Cr content is controlled at 0.10% to 0.30%.

[0035] Ni: Ni stabilizes austenite, enhances the hardenability of steel, improves low-temperature toughness, and reduces the notch sensitivity of fasteners. The addition of Ni also improves the rust layer structure, increases density and adhesion to the steel surface, enhances corrosion resistance, and inhibits hydrogen adsorption, thus benefiting the improvement of delayed fracture resistance. The Ni content should be controlled between 0.10% and 0.30%.

[0036] Cu: Cu significantly improves the corrosion resistance of steel. The cathodic contact between the steel and the secondary Cu deposits on the surface promotes anodizing and forms a well-protective rust layer. Cu also alters the hygroscopicity of the rust layer, thus increasing the critical humidity. However, excessive Cu content reduces the high-temperature plasticity of steel, making it prone to cracking during hot working. Therefore, the Cu content is controlled between 0.10% and 0.30%.

[0037] Alt (Al): Alt is a strong deoxidizing element that improves the oxidation resistance of steel. Alt also refines austenite grains, improving resistance to delayed fracture. Furthermore, this invention adds a higher proportion of Alt, which combines with nitrogen to form AlN, reducing dislocation pinning and significantly decreasing the tendency for blue brittleness, while also improving impact toughness. However, excessive Alt content can lead to the formation of coarse carbonitrides, increasing inclusion content and reducing resistance to delayed fracture. The Alt content is controlled between 0.015% and 0.035%.

[0038] S and P: Impurity elements such as S and P segregate at grain boundaries, which greatly reduces the resistance to delayed fracture. P can form micro-segregation during the solidification of molten steel, and then segregate at the grain boundaries during heating at the austenitizing temperature, significantly increasing the brittleness of the steel and thus increasing its susceptibility to delayed fracture. S forms MnS inclusions and segregates at grain boundaries, thus increasing the susceptibility of the steel to delayed fracture. Therefore, the P and S contents should be controlled at P ≤ 0.010% and S ≤ 0.010%.

[0039] TO and N: Oxygen forms various oxide inclusions in steel. Under stress, stress concentration easily occurs at these oxide inclusions, leading to the initiation of microcracks and thus deteriorating the mechanical properties of the steel, especially its toughness and fatigue resistance. Therefore, in metallurgical production, measures must be taken to minimize its content, controlling TO ≤ 0.0020%. N precipitates Fe4N in steel, with a slow diffusion rate, causing aging of the steel. N also reduces the cold workability of the steel; N should be controlled ≤ 0.0065%.

[0040] Compared with existing technologies, this invention, to ensure high strength of the wire rod, appropriately increases the C content and selects Cr, V, and Ti as fine-tuning alloying elements to improve the steel's ductility and toughness, reduce the content of granular bainite, and introduce ferrite into the microstructure, forming a duplex steel structure. To improve atmospheric corrosion resistance, a certain amount of Ni and Cu are added. Simultaneously, the carbides formed by Ti can inhibit the absorption and formation of hydrogen in corrosive environments, improving corrosion resistance. The chemical composition must meet the following requirement: 4.5 ≤ 15 × Cr + 10 × (Ni + Cu) + 20 × Ti ≤ 10.0. Fasteners produced using this invention can eliminate the need for quenching and tempering treatment. After holding at 350℃ ± 10℃ for 30-40 minutes, the tensile strength R... m ≥1020MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥15%, reduction of area Z≥52%, room temperature impact energy KV2≥52J, austenitic grain size of steel greater than or equal to 11.0 grade, with good strength and toughness, and excellent atmospheric corrosion resistance, which is 1.5 times that of conventional fasteners. Attached Figure Description

[0041] Figure 1 The microstructure of the hot-rolled non-quenched and tempered cold heading steel with excellent corrosion resistance, as described in this invention, consists of granular bainite and ferrite. Detailed Implementation

[0042] The present invention will be further illustrated by the following embodiments and comparative examples.

[0043] Examples 1-5

[0044] A non-quenched and tempered cold heading steel with excellent corrosion resistance comprises the following composition by mass percentage as shown in Table 1. The balance not shown in Table 1 is Fe and other unavoidable impurities.

[0045] Comparative Examples 1-3

[0046] A non-quenched and tempered cold heading steel comprises the following composition by weight percentage as shown in Table 1, where the balance not shown in Table 1 is Fe and other unavoidable impurities.

[0047] Table 1. Chemical composition (wt%) of each embodiment and comparative example of the present invention.

[0048]

[0049] The production methods of the above embodiments and comparative examples of non-quenched and tempered cold heading steel are carried out according to the following process flow:

[0050] Raw material preparation according to the ingredient ratio → electric furnace smelting → LF furnace refining + RH vacuum refining → continuous casting of large billets → rolling of billets → finishing and peeling → heating → rolling of large coil wire rods → controlled cooling in the Steyrmo cooling line → finished large coils → packaging and warehousing.

[0051] The process parameters are as follows:

[0052] The electric arc furnace smelting process is as follows: Smelting is carried out using an electric arc furnace. Oxygen is determined before tapping, and slag discharge during the tapping process is strictly controlled. The final carbon content (C) of the electric furnace is controlled at 0.06-0.20%, and phosphorus (P) is ≤0.005%. Slag is blocked during tapping. Refining slag and lime are added when 1 / 5-1 / 4 of the molten steel has been tapped. Deoxidizers and alloys are added when 1 / 4-1 / 3 of the molten steel has been tapped. The order of addition is: ferroaluminum → ferrosilicon → medium carbon manganese → high carbon ferrochrome → carbon raiser. After tapping, an appropriate amount of aluminum particles is evenly sprinkled onto the slag surface according to the amount of slag discharged.

[0053] The LF furnace refining process involves bottom blowing argon into the ladle throughout the process, with the argon flow rate determined by preventing molten steel from splashing out of the ladle. Pre-melted refining slag and lime are added to form slag, with an alkalinity of R3-5 and a white slag time of ≥20 minutes. Based on the composition analysis results before entering the LF furnace, alloys are added before and during the refining process to adjust the Si, Mn, and Cr content.

[0054] The RH vacuum degassing process is as follows: In the early stage of vacuum degassing, if the vacuum level is ≤100 Pa, the vacuum holding time should be ≥10 minutes; if the vacuum level is ≤200 Pa, the vacuum holding time should be ≥15 minutes. In the later stage of vacuum degassing, the holding time should be ≥10 minutes. Based on the composition analysis results in the early stage of vacuum degassing, if composition adjustment is required in the middle stage, a vacuum holding time of at least 5 minutes must be ensured after adjustment. After vacuum breaking, calcium wire feeding is performed. Before leaving the station, soft blowing argon treatment is performed, with a soft blowing time ≥15 minutes. The pure degassing time is ≥15 minutes, ensuring that the [H] content is ≤1.5 ppm after vacuum treatment. Based on the composition analysis results at the RH endpoint, alloys are added to adjust the V, Ti, B, and Ni contents.

[0055] In continuous casting, the target temperature of the molten steel in the tundish is controlled at 10–40°C above the liquidus temperature. For continuous casting of Φ380mm large round billets, full-process protective casting is used. A protective sleeve and argon seal are employed between the ladle and the tundish. The tundish is protected with a molten steel covering agent and argon blowing. An immersion nozzle is used between the tundish and the crystallizer, with a primary cooling water flow rate of 100–130 m³ / h. 3 / h, secondary cooling water ratio 1.0~1.4l / kg, stable liquid level, casting speed and superheat during casting process, superheat controlled at 15~30℃, casting speed 1.9-2.1mm / min.

[0056] The rolled square billet is a 150mm square billet. The temperature of the billet in the soaking zone of the heating furnace is controlled at 1250–1350℃, and the total heating time is controlled at 250–350 minutes to ensure that alloying elements, especially Ti, are fully dissolved. The initial rolling temperature is controlled at 1100±50℃, and the billet is cooled after rolling at a temperature ≥400℃. After rolling, the 150mm square billet undergoes surface and end grinding treatment to provide good surface quality for subsequent high-speed wire rod rolling and reduce the sensitivity of the wire rod surface to decarburization.

[0057] The large-coil wire rod rolling process: Qualified raw material steel billets can be obtained through the above steps, enabling the rolling of 16-30mm large-coil wire rods. High-speed wire rod rolling employs low-temperature, high-deformation rolling, with a deformation amount ≥50%. To ensure sufficient solid solution of alloying elements, the initial rolling temperature is 970–1030℃. Rolling is completed in the austenite recrystallization zone to achieve recrystallization refinement. The final rolling temperature is 760–800℃. Subsequently, the coils are cooled under controlled conditions on the LCC roller table. To obtain a granular bainite + ferrite dual-phase structure, a rapid cooling followed by slow cooling method is adopted. The first four insulation covers are opened, and the fans are turned on to 100% for rapid cooling at a rate of 4–7℃ / s to cool to 450–500℃, avoiding the formation of pearlite structure. The subsequent five to eleven fans are all turned off, and all insulation covers are closed, with a cooling rate of 0.7–1.0℃ / s to avoid the formation of martensite structure. The coils then exit the insulation tunnel and are coiled and hooked when the temperature reaches 400–440℃. They are then air-cooled to room temperature, packaged, and weighed.

[0058] The wire rolling processes of the various embodiments and comparative examples of the present invention are shown in Table 2.

[0059] Table 2 Wire rolling processes of embodiments and comparative examples of the present invention

[0060]

[0061] The non-quenched and tempered cold heading steels produced according to the above methods in the embodiments and comparative examples do not require quenching and tempering. The process involves: cold drawing → cold heading → thread machining → low-temperature stabilization treatment → surface treatment for fasteners. The low-temperature stabilization treatment process involves heating to 350℃±10℃, holding for 30-40 minutes, and then air cooling. The mechanical properties are shown in Table 3, with tensile strength R... m ≥1020MPa, yield strength ratio R P0.2 / R m The following parameters indicate that the embodiment has good strength and toughness, as well as good atmospheric corrosion resistance: ≥0.9, elongation after fracture A≥15%, reduction of area Z≥52%, room temperature impact energy KV2≥52J, and austenitic grain size of steel≥11.0 grade.

[0062] Mechanical property testing methods GB / T 228.1 Metallic materials, tensile testing—Part 1: Test at room temperature;

[0063] The corrosion performance was tested using the TB / T 2375 method for cyclic immersion corrosion testing of weathering steel for railways.

[0064] Table 3 Mechanical properties of the present invention after quenching and tempering heat treatment in the embodiments of the present invention

[0065]

[0066] The underlined data above does not meet the requirements of this application.

[0067] In Table 3, the performance of Comparative Example 1 is the performance after quenching and tempering. The specific quenching and tempering process is as follows: quenching at 890℃ for 90 min, tempering at 500℃ for 100 min. Comparative Example 1 uses the chemical composition of Example 1 and obtains a ferrite + pearlite + bainite + martensite structure using a conventional slow cooling process, which cannot achieve the purpose of omitting the quenching and tempering process.

[0068] Comparative Example 2 did not contain Cr, Ni, Cu, V, or Ti elements, resulting in insufficient strength, toughness, and corrosion resistance.

[0069] Comparative Example 3, although its chemical composition meets the requirements, does not meet the requirements of the formula in this invention, and its corrosion resistance is insufficient.

Claims

1. A non-quenched and tempered cold heading steel with excellent corrosion resistance, characterized in that, The non-quenched and tempered cold heading steel with excellent corrosion resistance comprises the following components by weight percentage: C 0.21%~0.30%, Si 0.02%~0.1%, Mn 1.8%~2.5%, V 0.05%~0.20%, B 0.0005%~0.0030%, Ti 0.02~0.04%, Cr 0.1~0.3%, Ni 0.1~0.3%, Cu 0.1~0.3%, Alt 0.015%~0.035%, P ≤0.010%, S ≤0.010%, TO≤0.0020%, N≤0.0065%, with the remainder being Fe and other unavoidable impurities; The composition of the non-quenched and tempered cold heading steel with excellent corrosion resistance also satisfies: 4.5≤15×Cr+10×(Ni+Cu)+20×Ti≤10.0; The hot-rolled microstructure of the non-quenched and tempered cold heading steel with excellent corrosion resistance is a dual-phase structure of granular bainite and ferrite, with a tensile strength of 860 MPa ≥ R. m ≥800MPa; The non-quenched and tempered cold heading steel with excellent corrosion resistance, after drawing and stabilization treatment, has a tensile strength R... m ≥1020MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥15%, reduction of area Z≥52%, room temperature impact energy KV2≥52J, austenitic grain size of steel ≥11.0 grade.

2. A method for producing non-quenched and tempered cold heading steel with excellent corrosion resistance as described in claim 1, characterized in that, The production method includes the following process flow: batching materials according to the component ratio → electric furnace smelting → LF furnace refining + RH vacuum refining → continuous casting of large square billets → rolling of square billets → finishing and peeling → heating → rolling of large coiled wire rods → controlled cooling in the Steyrmo cooling line → finished large coils → packaging and warehousing.

3. The production method according to claim 2, characterized in that, The LF furnace refining process involves bottom blowing argon into the ladle throughout the process, with the argon flow rate determined by ensuring that molten steel does not splash out of the ladle; pre-melted refining slag and lime are added to form slag, with an alkalinity of R3-5 and a white slag time of ≥20 minutes.

4. The production method according to claim 2, characterized in that, The RH vacuum degassing process is as follows: in the early stage of vacuum, if the vacuum degree is ≤100 Pa, the vacuum holding time is ≥10 minutes; if 100 Pa < vacuum degree ≤200 Pa, the vacuum holding time is ≥15 minutes; in the later stage of vacuum, the holding time is ≥10 minutes; before leaving the station, soft blowing argon treatment is performed, and the soft blowing time is ≥15 minutes.

5. The production method according to claim 2, characterized in that, The large billet continuous casting adopts 380mm×450mm large billet continuous casting, with full-process protective casting. A protective sleeve and argon seal are used between the ladle and tundish. The tundish is protected by molten steel covering agent and argon blowing. An immersion nozzle is used between the tundish and the crystallizer. The primary cooling water flow rate is 100-130 m³ / h. 3 / h, secondary cooling water volume 1.0~1.4l / kg, superheat controlled at 15~30℃, drawing speed 1.9-2.1mm / min.

6. The production method according to claim 2, characterized in that, The rolled square billet is rolled using a 150 square billet. The temperature of the square billet in the soaking zone of the heating furnace is controlled at 1250-1350℃, the total heating time is controlled at 250-350min, the initial rolling temperature is controlled at 1100±50℃, and the billet is cooled after rolling with a cooling temperature ≥400℃.

7. The production method according to claim 3, characterized in that, The large coil wire rod is rolled with a rolling deformation of ≥50%, with an initial rolling temperature of 970~1030℃ and a final rolling temperature of 760~800℃. It then enters the LCC roller table for controlled cooling, using a rapid cooling rate of 4~7℃ / s to cool to 450~500℃, followed by slow cooling at a rate of 0.7~1.0℃ / s. The temperature is then maintained in the heat preservation tunnel until it reaches 400~440℃, and then air-cooled to room temperature.

8. The production method according to any one of claims 2-7, characterized in that, The hot-rolled microstructure of the non-quenched and tempered cold heading steel with excellent corrosion resistance produced by the method is a dual-phase structure of granular bainite and ferrite, with a tensile strength of 860 MPa ≥ R. m ≥800MPa.

9. The production method according to any one of claims 2-7, characterized in that, The non-quenched and tempered cold heading steel produced has excellent corrosion resistance. After drawing and stabilization treatment, the tensile strength R of the product is high. m ≥1020MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥15%, reduction of area Z≥52%, room temperature impact energy KV2≥52J, austenitic grain size of steel ≥11.0 grade.

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