Cold heading steel wire rod and preparation method and application thereof

Abstract

The application belongs to the technical field of cold heading steel production, and particularly relates to a cold heading steel wire rod and a preparation method and application thereof. The mass percentage of each chemical component of the cold heading steel wire rod is shown in the specification. The cold heading steel wire rod provided by the application has good corrosion resistance, a weathering index I greater than or equal to 7, and good formability without cold heading cracking, and is suitable for the design and material selection of steel structures with strict requirements on the resistance to delayed fracture. The bolt prepared by the application has high strength, and the strength grade can be higher than or equal to 12.9.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cold heading steel production, in particular to a cold heading steel wire rod and a preparation method and application thereof. BACKGROUND

[0002] Cold heading steel is mainly used for producing fasteners such as bolts and nuts, and is widely used in automobile, construction, light industry and other industries. In order to save energy and reduce cost, it is necessary to require lightweight parts, and improving material strength is the most important measure. For example, the strength grade of fasteners for some automobile engines and construction machinery is required to be 12.9 or above. High-strength fasteners have become an inevitable trend of industry development and are increasingly widely used.

[0003] Corrosion of steel in the atmosphere is inevitable and becomes an important factor affecting the service life of steel. Rusting of fasteners can cause serious harm to structural safety. In order to solve the corrosion failure, anti-corrosion coating such as hot plating, electroplating, mechanical plating is usually used. However, the pre-tightening torque of high-strength fasteners is large, the plating layer itself cannot resist the large friction between threads, and the coating will be damaged during the pre-tightening process of the fastener, thereby causing the plating layer to be thinned, and in severe cases, even causing the base material of the fastener to be exposed to the service environment, thereby reducing the corrosion resistance of the fastener. Moreover, the use of anti-corrosion coating increases the maintenance cost, and the coating process causes health hazards and environmental pollution. Therefore, it is urgent to develop high-strength weather-resistant fastener steel without coating.

[0004] Weather-resistant steel, i.e. on the basis of ordinary carbon steel, adding a certain amount of alloying elements, forming a dense rust layer with protective effect on the surface of steel, thereby preventing oxygen and water in the atmosphere from penetrating into the steel matrix, slowing down the development of rust to the depth of the steel, and improving the atmospheric corrosion resistance of steel materials. The atmospheric corrosion resistance of weather-resistant steel is 2-8 times that of ordinary carbon steel, and the longer the service time, the more outstanding the corrosion resistance.

[0005] Chinese patent application No. 201610833875.9 discloses a 10.9-grade atmospheric corrosion-resistant cold heading steel hot-rolled wire rod, the composition of which is as follows in terms of weight percentage: C 0.25-0.35%, Si 0.15-0.35%, Mn 0.60-0.90%, P≤0.030%, S≤0.030%, Cr 0.60-0.90%, Ni 0.20-0.50%, Cu 0.20-0.50%, and the balance is iron and trace impurities. By adding Cu, Cr, Ni and other elements, the material has the effect of atmospheric corrosion resistance, and has the characteristics of high strength, good corrosion resistance, good forming performance and the like.

[0006] The patent with the Chinese patent application number 201711333475.2 discloses a kind of 1000MPa grade fastener weathering cold heading steel wire rod, its component is according to percentage by weight: C 0.25-0.55%, Si 0.10-0.40%, Mn 0.80-1.20%, P≤0.035%, S≤0.040%, Cr 0.40-0.70%, Ni 0.15-0.45%, Cu 0.20-0.40%, Mo 0.15-0.45%, the rest is Fe and inevitable impurities, the weathering connecting pair made of satisfies 1000MPa grade requirement, the corrosion rate of accelerated corrosion test is less than 1 / 3 of ordinary cold heading steel.

[0007] The patent with the Chinese patent application number 201911247314.0 discloses a kind of 10.9 grade fastener high weathering cold heading steel, its main chemical component composition and mass percentage are: C 0.33-0.43%, Si 0.20-0.50%, Mn 0.35-0.55%, Cr 0.60-1.00%, Ni 0.50-0.80%, Cu 0.20-0.40%, V 0.01-0.10%, Alt 0.015-0.040%, Re 0.01-0.10%, P 0.010-0.030%, O≤0.0015%, N≤0.006%, the rest is Fe and inevitable impurities, by chemical component design, combined with low temperature rolling process, satisfy 10.9 grade fastener strength requirement, and have high weather resistance.But, patent 201610833875.9, 201711333475.2, 201911247314.0 steel strength is 1200MPa below, can not satisfy the high strength requirement of fastener, and the weathering index I value of patent 201610833875.9 and 201711333475.2 is all below 7.0, only has general weathering performance.

[0008] The patent with the Chinese patent application number 201711215120.3 discloses a 1040MPa grade weather-resistant bolt resistant to delayed fracture, the chemical composition of which has the mass percentage of C 0.21-0.32%, Si 0.10-0.50%, Mn 0.60-1.00%, P 0.008-0.020%, S≤0.005%, Cr 0.82-1.20%, Ni 0.25-0.50%, Cu 0.25-0.50%, Mo 0.05-0.20%, Nb 0.015-0.060%, V 0.015-0.090%, Ti 0.008-0.035%, B 0.0008-0.0035%, Al 0.015-0.040%, Ca 0.003-0.007%, Zr 0.015-0.045%, Re 0.010-0.045%, and the balance of Fe and inevitable impurities. The bolt is resistant to delayed fracture and atmospheric corrosion by using high-purity smelting-continuous casting-rolling technology. However, the patent 201711215120.3 adds a large amount of Ti and Zr elements, which forms nitride inclusions and is harmful to the toughness of the material. Meanwhile, the steel has a large amount of added alloy, which is not conducive to the cost control of the steel. Moreover, the weathering index I of the steel in the method is basically below 7.0, only having general weathering performance, and the strength level is below 1100MPa, which cannot meet the high strength requirement of fasteners.

[0009] The patent with the Chinese patent application number 201910360766.3 discloses a high-strength weather-resistant fastener steel, which has the mass percentage of C 0.35-0.45%, Si 0.01-2.0%, Mn 0.3-2.2%, P≤0.012%, S≤0.005%, Cr 0.6-3.2%, Mo 0.1-0.3%, Ni 0.2-2.0%, Cu 0.3-0.6%, Al 0.001-0.1%, Ti 0.01-0.5%, B 0.0005-0.01%, Nb 0.015-0.060%, Re 0.010-0.045%, and the balance of Fe and other inevitable impurities. After smelting, casting, soaking treatment, initial rolling, rolling, and quenching and tempering treatment, the weathering index I of the steel is≥7, and the strength meets the requirement of 10.9 grade.

[0010] The patent with the Chinese patent application number 201911247222.2 discloses a kind of high weathering resistance cold heading steel for 12.9 grade fastener, its main chemical component composition and mass percentage are as follows: C 0.35-0.45%, Si 0.30-0.50%, Mn 0.60-1.00%, Cr 0.60-0.83%, Ni 0.50-0.80%, Cu 0.20-0.40%, Mo 0.05-0.15%, Ti 0.01-0.06%, Alt 0.015-0.040%, Re 0.01-0.10%, P 0.010-0.030%, O≤0.0015%, N≤0.006%, the rest is Fe and inevitable impurities, by chemical composition design, combined with low temperature rolling process, meet the strength requirements of 12.9 grade fastener, and have high weather resistance. However, the weathering steels in patents 201910360766.3 and 201911247222.2 add more C, Re, Mo, Ti and other elements, although the weather resistance is improved, but it is not conducive to the forming of cold heading steel, which can cause the brittleness of steel to rise, affect the delayed fracture resistance of steel, and increase the risk of cold heading cracking of steel. SUMMARY

[0011] Therefore, the technical problem to be solved by the present application is to overcome the lack of a comprehensive cold heading steel that can ensure the strength and corrosion resistance of steel, while ensuring no cracking risk and excellent forming performance, thereby providing a cold heading steel wire rod and a preparation method and application thereof.

[0012] To this end, the present application provides a cold heading steel wire rod, comprising the following mass percentage of chemical components: C 0.25-0.34%, Si 0.25-0.40%, Mn 0.75-0.90%, P≤0.025%, S≤0.020%, Cr 0.90-1.10%, Ni 0.30-0.60%, Cu 0.25-0.40%, Nb 0.02-0.04%, Al 0.010-0.045%, V 0.02-0.08%, N 0.003-0.007%, the rest is Fe and inevitable impurities, and satisfies ([Cr]+2[Ni]) / [Cu] is 5-6.5 and [C]+[Mn] / 6+([Cr]+[Nb]+[V]) / 5+([Ni]+[Cu]) / 20+[Si] / 15 is 0.7%-0.8%, wherein [Cr], [Ni], [Cu], [C], [Mn], [Nb], [V], [Si] represent the mass percentage of Cr, Ni, Cu, C, Mn, Nb, V, Si in the cold heading steel wire rod, respectively.

[0013] Furthermore, the cold heading steel wire rod contains a mixed structure of pearlite, bainite and ferrite; preferably, in the cold heading steel wire rod, the area percentage of pearlite is 45-60%, the area percentage of bainite is 25-40%, and the area percentage of ferrite is 10-25%; preferably, the grain size of the wire rod is level 9-11.

[0014] The present invention also provides a method for preparing the cold heading steel wire rod as described above, comprising the following steps:

[0015] S1, converter smelting;

[0016] S2, LF furnace refining;

[0017] S3, continuous casting;

[0018] S4, cooling of the casting;

[0019] S5, heating;

[0020] S6. Rolling.

[0021] Furthermore, the step S1 satisfies one or more of the following AC:

[0022] A. The percentage of C at the smelting end point is 0.06-0.20%, P≤0.012%;

[0023] B. The temperature of steel tapping from the converter is 1630℃~1670℃;

[0024] C. After 1 / 2-1 / 4 of the steel is tapped, alloy materials containing Al, Si, Mn, Cr, Ni, and Cu are added in sequence for preliminary alloying.

[0025] Furthermore, the S2 step includes the following steps: after the converter molten steel enters the LF station, 2.5-4.5Kg / t of lime and 0.5-2.0Kg / t of fluorite are added in sequence to adjust the white slag, and after the LF white slag is refined for 5-10 minutes, alloy materials containing V and Nb are added for alloying, during which argon is blown at 300-450L / min; then, sampling is taken when the temperature is raised to 1540-1560°C at a heating rate of 2.0-4.0°C / min, during which argon is blown at 150-300mL / min; the temperature of LF steel tapping is 1560°C to 1580°C.

[0026] Furthermore, in step S3, the temperature of the tundish during continuous casting is 1525°C to 1550°C; and / or, step S4 includes the following steps: air-cooling the continuous casting billet, and after cooling to 700°C-750°C, slowly cooling it to a temperature of ≤200°C, and the slow cooling time is 72h-96h.

[0027] Further, in the step S5, the temperature of the soaking section during heating is 1050-1100 DEG C, and the heating time is 60-90 min; in the step S6, the starting rolling temperature is 950-1000 DEG C, the final rolling temperature is 880-910 DEG C, and the temperature of the wire drawing is controlled to be 850-880 DEG C.

[0028] Further, the step S6 comprises the following steps: the front 1-2 sections of the roller table are opened to the heat preservation cover, and 1-3 air blowers are opened, the air volume of each air blower is 50-80%, the remaining heat preservation covers and air blowers are all closed, the temperature entering the heat preservation cover is 720-750 DEG C, the cooling speed before entering the heat preservation cover is 7-10 DEG C / s, and the cooling speed in the heat preservation cover is 0.8-1.2 DEG C / s.

[0029] Further, after the step S6, a step of performing a quenching and tempering treatment on the cold heading steel wire rod is further included; the quenching and tempering treatment is that the wire rod is quenched at 860-900 DEG C and then tempered at 510-550 DEG C.

[0030] The application further provides a fastener prepared from the cold heading steel wire rod, and the strength grade of the fastener is greater than or equal to 12.9.

[0031] The following describes the component design of the cold heading steel for the high-strength weather-resistant fastener:

[0032] C: C plays a solid solution strengthening role in the steel, improves the hardenability and the strength of the steel, but too high C content will reduce the ductility of the steel, increase the risk of delayed fracture of the bolt, also cause the plasticity and the cold heading forming performance to be poor, and affect the atmospheric corrosion resistance of the steel. Considering the strength, corrosion resistance and cold heading forming performance of the fastener, the carbon content range is determined to be 0.25-0.34%.

[0033] Si: Si is a deoxidizing element in the steel, and improves the strength and hardness of the steel through solid solution strengthening, and is enriched on the surface of the steel, which can improve the stability of the rust layer and the corrosion resistance, but the increase of Si content will aggravate the decarburization of the steel, and when the Si content is too high, the solid solution strengthening effect reaches saturation, which is not conducive to the plasticity and toughness. Considering the organization and performance design, the content range is controlled to be 0.25-0.40%.

[0034] Mn: Mn plays a solid solution strengthening and fine grain strengthening role, and inhibits the pearlite and bainite transformation, improves the stability of the austenite organization and the hardenability of the steel. At the same time, the addition of Mn helps to form a rust layer on the surface of the steel, which improves the corrosion resistance of the steel, but too much Mn will cause the corrosion product particles to grow, increase the corrosion rate, and also reduce the plasticity of the steel. Considering comprehensively, the content is controlled to be 0.75-0.90%.

[0035] P, S: both are impurity elements, P can promote the steel to produce amorphous rust layer, improve the stability of rust layer, and increase the weather resistance of steel, but too high P content will reduce the toughness and plasticity of steel, which will lead to cold brittleness; S will deteriorate the atmospheric corrosion resistance of steel, and will lead to hot brittleness of steel, and S will form MnS inclusions with Mn, which will affect the plasticity of steel, and P and S are both impurity elements that are easy to segregate, which is harmful to the uniformity of the structure and performance of the cold heading steel. In order to improve the cold heading performance of the steel and increase its uniform deformation ability, P is limited to ≤0.025%, and S is limited to ≤0.020%.

[0036] Cr: significantly improves the hardenability of the steel, refines the grains, and improves the spheroidizing performance of the cold heading steel. At the same time, Cr element can form a dense oxide film on the surface of the steel to improve the passivation ability of the steel, improve the density and adhesion ability of the corrosion layer on the surface of the steel, and improve the corrosion resistance of the steel. However, excessive Cr will increase the tendency of temper brittleness of the steel. Considering the strength, microstructure control and corrosion resistance, the Cr content is limited to 0.90-1.10%.

[0037] Ni: can enhance the hardenability of the steel, reduce the ductile-brittle transition temperature, significantly improve the low temperature toughness of the steel, improve the high temperature oxidation resistance, stress corrosion resistance, plasticity, weldability and toughness of the steel, and can effectively prevent Cu from being hot brittle. In addition, too high Ni will increase the cost of the material. Considering the low temperature performance requirement and the high cost of Ni element, the Ni content is limited to 0.30-0.60%.

[0038] Cu: significantly improves the atmospheric corrosion resistance of the steel, makes the rust layer on the surface of the steel dense and improves the adhesion, and at the same time can improve the hardenability and reduce the ductile-brittle transition temperature. However, excessive Cu in the steel will produce high crack sensitivity, leading to hot brittleness of the steel. Considering the above, the Cu content is limited to 0.25-0.40%.

[0039] Nb: solid solution in austenite, significantly improves the hardenability of the steel, reduces the overheating sensitivity and temper brittleness of the steel, increases the strength, forms fine Nb(C, N) precipitates in the steel, becomes a beneficial hydrogen trap in the steel, hinders the aggregation of hydrogen atoms to harmful hydrogen traps, and improves the resistance to delayed fracture of the fastener. However, excessive Nb will increase the cost and aggravate the segregation tendency. Considering the above, the Nb content is controlled to 0.02-0.04%.

[0040] Al: is a strong deoxidizing element, which can improve the oxidation resistance of the steel, and at the same time can refine the grains and improve the strength and toughness of the steel. The Al content should be controlled in the range of 0.010-0.045%.

[0041] V: Refine the grain size of the organization, improve the strength and toughness of the steel, can form carbide with C, N, can improve the resistance to hydrogen corrosion, at the same time can increase the hardenability of the steel, improve the uniformity of the organization after quenching and tempering, in order to ensure the safety of high strength fastener in use, the range of V is defined as 0.02-0.08%.

[0042] N: Solid solution strengthening effect, can improve the hardenability, in high Cr steel, appropriate amount of N can improve the strength of the steel without reducing the plasticity, but excessive N will precipitate Fe4N in the steel, resulting in aging brittleness of the steel, reducing the cold working performance of the steel, after comprehensive consideration, N is controlled in 0.003-0.007%.

[0043] ([Cr]+2[Ni]) / [Cu]: Cr, Ni, Cu synergistic effect index, adding enough Cr and Ni can make up for the decrease of the ductility of the steel with the increase of Cu, at the same time promote the formation of passivation film on the surface of the steel, reduce the corrosion induced sensitivity of the steel matrix, improve the corrosion resistance of the steel, but excessive Cr and Ni will cause the adhesion of the oxide skin to increase during the heating process of the steel, thereby increasing the difficulty of pickling and dephosphorization, comprehensive consideration limits the range of ([Cr]+2[Ni]) / [Cu] to 5-6.5.

[0044] [C]+[Mn] / 6+([Cr]+[Nb]+[V]) / 5+([Ni]+[Cu]) / 20+[Si] / 15: The calculation formula of the carbon equivalent of the steel in the application, which is obtained by analyzing the production data of cold heading steel. In order to make the strength of the fastener after quenching and tempering reach 12.9 level or above, and considering the alloy cost, the value of carbon equivalent is controlled in 0.7%-0.8%.

[0045] Weathering index I: The calculation formula of weathering index defined in ASTM G101 standard of American Society for Testing and Materials, which is used for evaluating the corrosion resistance of weathering steel. The calculation formula is I=26.01[Cu]+3.88[Ni]+1.2[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.1[Ni][P]-33.39[Cu] 2 It is generally considered that when I≥7.0, the steel has high weathering performance.

[0046] The technical scheme of the application has the following advantages:

[0047] 1.A cold heading steel wire rod, comprising the following chemical components in mass percentage: C 0.25-0.34%, Si 0.25-0.40%, Mn 0.75-0.90%, P≤0.025%, S≤0.020%, Cr 0.90-1.10%, Ni 0.30-0.60%, Cu 0.25-0.40%, Nb 0.02-0.04%, Al 0.010-0.045%, V 0.02-0.08%, N 0.003-0.007%, the rest being Fe and inevitable impurities, and satisfying ([Cr]+2[Ni]) / [Cu] being 5-6.5 and [C]+[Mn] / 6+([Cr]+[Nb]+[V]) / 5+([Ni]+[Cu]) / 20+[Si] / 15 being 0.7%-0.8%, wherein [Cr], [Ni], [Cu], [C], [Mn], [Nb], [V], [Si] represent the mass percentage of Cr, Ni, Cu, C, Mn, Nb, V, Si in the cold heading steel wire rod respectively. The cold heading steel wire rod has good corrosion resistance, weathering index I≥7, and good formability without cold heading cracking, and is suitable for the design and material selection of steel structure with strict requirement on delayed fracture resistance, and the bolt prepared therefrom has high strength performance with strength grade reaching 12.9 or above.

[0048] 2. The preparation method of the cold heading steel wire rod provided by the present application, in the step S5, the temperature of the soaking section of the heating furnace is controlled to be 1050-1100℃, and the heating time of the heating furnace is 60-90min; in the step S6, the open rolling temperature is controlled to be 950-1000℃, the final rolling temperature is 880-910℃, and the wire laying temperature is 850-880℃. When the temperature of the heating furnace is lower than 1050℃, the elements such as Ni, Cu and Nb cannot be dissolved in the austenite; when the temperature of the heating furnace is higher than 1100℃, coarse austenite grains are formed, and the decarburization of the steel is increased. When the open rolling temperature is lower than 950℃, the load of the rolling mill is too large; when the open rolling temperature is higher than 1000℃, the temperature drop during the rolling process is too large, the wire rod grains are coarse, and abnormal structure is easily formed. When the final rolling temperature is lower than 880℃, the load of the rolling mill is too large, the deformation resistance of the steel is increased, the recovery and recrystallization process is not perfect, the residual stress is increased, the plasticity is reduced, and the defects such as size out-of-tolerance, ears and folding are easily produced; when the final rolling temperature is higher than 910℃, the grains are coarse, the mechanical properties are affected, and the secondary decarburization of the wire rod surface is caused, and the depth of the decarburization layer of the wire rod surface is increased. When the wire laying temperature is lower than 850℃, the water cooling from the finishing mill to the reducing diameter is too large, the abnormal structure and the low surface hardness are easily caused, and the temperature difference between the core and the surface of the wire rod is increased; when the wire laying temperature is higher than 880℃, the grains are coarse, and the entire phase change process cannot be completed on the Stelmor wire, the abnormal structure is easily produced, and the cold heading performance is affected.

[0049] 3. The preparation method of the cold heading steel wire rod provided by the present application, the step S6 comprises the following steps: the front 1-2 section of the roller is opened to the heat preservation cover, and 1-3 air blowers are opened, the air volume of each air blower is 50-80%, the remaining heat preservation covers and air blowers are all closed, the entering temperature of the heat preservation cover is 720-750℃, the cooling speed before entering the heat preservation cover is 7-10℃ / s, and the cooling speed in the heat preservation cover is 0.8-1.2℃ / s. By controlling the temperature of the cold heading steel wire rod before entering the heat preservation cover and other process parameters, the abnormal structure of the cold heading steel wire rod in the heat preservation cover can be avoided, the ferrite structure is more fine and uniform, the performance of the cold heading steel wire rod is improved, and the cracking rate of the cold heading steel wire rod is reduced. DETAILED DESCRIPTION

[0050] The following examples are provided to better further understand the present application, and do not limit the content and protection scope of the present application, and do not constitute a limitation, anyone who is inspired by the present application or combines the present application with other prior art features to obtain any product which is the same as or similar to the present application falls within the protection scope of the present application.

[0051] The specific experimental steps or conditions are not indicated in the examples, and the operation or conditions can be performed according to the conventional experimental steps described in the literature in the art. The reagents or instruments used are not indicated by the manufacturer, and are conventional reagent products that can be obtained on the market.

[0052] Embodiment

[0053] Embodiments 1-10 provide a series of cold heading steel wires, the chemical components and the mass percentages thereof are shown in Table 1, the rest is Fe and inevitable impurities, and the production process is: converter smelting→LF refining→continuous casting→slab slow cooling→heating furnace heating→high-speed wire rolling. The specific process parameters are shown in Tables 2 and 3, and the specific operation is as follows:

[0054] (1) Converter smelting: after the molten iron enters the converter, oxygen blowing is performed, and the end point C and P content is controlled. After 1 / 4 tapping, aluminum ingot, ferrosilicon, silicon manganese, high-carbon chromium iron, nickel plate and copper plate are sequentially added for preliminary alloying according to the target steel grade component content, and the converter tapping temperature is shown in Table 2.

[0055] (2) LF refining: after the converter steel enters the LF station, 3 Kg of lime and 1 Kg of fluorite are sequentially added for white slag adjustment, and after LF white slag refining for 5 min, ferrovanadium and niobium iron are added for alloying, and argon blowing is performed at 400 L / min during the period; then sampling after heating, adding alloy to adjust the composition according to the target steel grade component content, heating at a rate of 4.0 ℃ / min to 1540 ℃, and argon blowing at 300 L / min during the period, and the LF tapping temperature is shown in Table 2.

[0056] (3) Continuous casting: during the continuous casting process, argon sealing with a large ladle long nozzle, alkaline tundish covering agent and submerged entry nozzle are used for full protection pouring, and medium carbon steel protective slag is used to obtain continuous casting billets. Among them, the tundish temperature is shown in Table 2.

[0057] (4) Slab slow cooling: first, the continuous casting billet is air cooled to the target temperature, then enters the pit for slow cooling, and then comes out of the pit. Among them, the air cooling temperature, slow cooling time and out-of-pit temperature are shown in Table 2.

[0058] (5) Heating furnace heating: the soaking section temperature of the heating furnace, the heating time, and the opening rolling temperature are shown in Table 3.

[0059] (6) High-speed wire rolling: the first 1-2 segments of the roller are opened with a heat preservation cover, and 1-3 air blowers are opened, the rest of the heat preservation covers and air blowers are all closed, and after entering the cover, slow cooling is performed to make the wire cool to obtain uniform pearlite, ferrite and bainite mixed structure. Through the organization control of the wire, the annealing process can be simplified to meet the cold heading forming requirements of the steel. The final rolling temperature, wire drawing temperature and cooling process parameters during rolling are shown in Table 3. The microstructure and grain size of the wire after rolling are shown in Table 4. The cold heading steel wire obtained is quenched and then tempered, and the quenching and tempering temperatures are shown in Table 3.

[0060] Table 1 Chemical composition (wt%) of cold heading steel wire rods of Examples 1-10 and Comparative Examples 1-10

[0061]

[0062]

[0063] Table 2 Process parameters for cold heading steel wire rod smelting and slow cooling of ingots in Examples 1-10 and Comparative Examples 1-10

[0064]

[0065]

[0066] Table 3 Process parameters for heating, rolling and controlled cooling of cold heading steel wire rods of Examples 1-10 and Comparative Examples 1-10

[0067]

[0068]

[0069] Table 4 Microstructure and grain size of cold heading steel wire rods of Examples 1-10 and Comparative Examples 1-10

[0070] Pearlite (%) Bainite (%) Ferrite (%) Martensite (%) Grain size (grade) Example 1 50 37 13 0 10 Example 2 57 33 10 0 10 Example 3 45 40 15 0 9 Example 4 60 29 11 0 11 Example 5 48 30 22 0 10 Example 6 52 25 23 0 9 Example 7 47 34 19 0 11 Example 8 53 36 11 0 10 Example 9 50 25 25 0 10 Example 10 55 26 19 0 11 Comparative Example 1 51 25 24 0 11 Comparative Example 2 47 30 23 0 9 Comparative Example 3 50 25 25 0 11 Comparative Example 4 49 32 19 0 10 Comparative Example 5 60 30 10 0 9 Comparative Example 6 50 40 10 0 10 Comparative Example 7 49 40 11 0 9 Comparative Example 8 52 33 15 0 11 Comparative Example 9 55 28 17 0 7 Comparative Example 10 65 10 15 10 9

[0071] Comparative Example

[0072] Comparative Examples 1-10 provide a series of cold heading steel wire rods, the chemical composition and mass percentage thereof are shown in Table 1, and the remainder is Fe and unavoidable impurities. The production process flow of Comparative Examples 1-10 is the same as that of Examples 1-10, and the specific process parameters are shown in Tables 2 and 3.

[0073] The process flow of Comparative Example 9 is basically the same as that of Comparative Example 5, with the only difference being that the heating temperature and rolling temperature during the rolling process do not meet the requirements of the present invention, specifically: the temperature of the soaking section of the heating furnace, the start rolling temperature, the final rolling temperature, and the spinning temperature are all higher than the required range of the present invention.

[0074] The process flow of Comparative Example 10 is basically the same as that of Comparative Example 5, with the only difference being that the cooling control process during the rolling process does not meet the requirements of the present invention, specifically: the insulation cover and the fan are all closed, and the temperature entering the cover does not meet the requirements of the present invention.

[0075] Experimental Example 1

[0076] 1. Weathering index of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 1, 4, the evaluation method of weathering index I is as follows: Weathering index I = 26.01[Cu] + 3.88[Ni] + 1.2[Cr] + 1.49[Si] + 17.28[P] - 7.29[Cu][Ni] - 9.1[Ni][P] - 33.39[Cu] 2 It is generally believed that when I≥7.0, the steel has high weathering resistance. The weathering index of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 1, 4 is shown in Table 5.

[0077] Table 5 Weathering index of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 1, 4

[0078] Weathering Index I Example 1 7.39 Example 2 7.60 Example 3 7.44 Example 4 7.38 Example 5 7.41 Example 6 7.74 Example 7 7.43 Example 8 7.31 Example 9 7.77 Example 10 7.53 Comparative Example 1 6.83 Comparative Example 4 1.98

[0079] 2. Test the cold heading cracking rate of the cold heading steel wire rod prepared from Examples 1-10 and Comparative Examples 5, 6, 7, 9, 10, the test and evaluation method of cold heading cracking rate: the cold heading steel wire rod is spheroidizing annealed, then 1 / 3 cold upset test is carried out according to YB / T 5293-2014, the cold heading cracking rate is the percentage of the number of cold heading cracking samples in the total test samples, and when the cold heading cracking rate is 0, it is the best. The cold heading cracking rate of Examples 1-10 and Comparative Examples 5, 6, 7, 9, 10 is shown in Table 6.

[0080] Table 6 Cold heading cracking rate of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 5, 6, 7, 9, 10

[0081] Cold heading cracking rate Example 1 0 Example 2 0 Example 3 0 Example 4 0 Example 5 0 Example 6 0 Example 7 0 Example 8 0 Example 9 0 Example 10 0 Comparative Example 5 23％ Comparative Example 6 18％ Comparative Example 7 12％ Comparative Example 9 37％ Comparative Example 10 35％

[0082] 3. Test the strength of the cold heading steel wire rod prepared from Examples 1-10 and Comparative Examples 1-3 after quenching and tempering, the test and evaluation method of tensile strength and yield strength: after quenching and tempering of the wire rod, room temperature tensile test is carried out according to GB / T 228.1-2021, and when the tensile strength of the cold heading steel wire rod is ≥1220MPa, it represents that the strength of the prepared fastener is ≥12.9 grade. The tensile test results of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 1, 2, 3 after quenching and tempering are shown in Table 7.

[0083] Table 7 Tensile test results of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 1-3 after quenching and tempering

[0084]

[0085]

[0086] 4. Test results of the delayed fracture resistance of the cold heading steel wire rod prepared from the cold heading steel wire rod of Examples 1-10 and Comparative Examples 6 and 8 after quenching and tempering treatment. The delayed fracture resistance test method and evaluation method are as follows:

[0087] The delayed fracture resistance test was performed by slow strain rate test method, and the test process was as follows: after quenching and tempering treatment of the steel, the notched sample was processed according to the GB / T 39039-2020 standard; the sample was subjected to electrochemical hydrogen charging in 4 g / L NaOH solution, the hydrogen charging time was 48 h, and the current density was 15 A / m 2 ; after hydrogen charging, the sample was cleaned and dried, and then subjected to room temperature tensile test on a slow strain rate tensile testing machine, the tensile strain rate was 5×10 -6 / s, and the tensile strength of the hydrogen-charged notched sample was measured; the same parameters were used to test the notched sample without hydrogen charging, and the tensile strength of the notched sample without hydrogen charging was obtained; the delayed fracture strength ratio was calculated, and the delayed fracture strength ratio was the ratio of the tensile strength of the hydrogen-charged notched sample to the tensile strength of the notched sample without hydrogen charging. The greater the delayed fracture strength ratio, the better the delayed fracture resistance. Generally, it is considered that the delayed fracture strength ratio is 0.7 or more to be qualified, and 0.8 or more to be excellent. The test results of the delayed fracture resistance of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 6 and 8 after quenching and tempering treatment are shown in Table 8.

[0088] Table 8 Test results of the delayed fracture resistance of the cold heading steel wire rod of Examples 1-10 and Comparative Examples 6 and 8 after quenching and tempering treatment

[0089]

[0090]

[0091] The cold heading steel wire rod prepared from Examples 1-10 has a weathering index I≥7.0 on the chemical composition, has excellent delayed fracture resistance in addition to high weathering resistance, has excellent cold heading forming performance without cracking during processing, and the strength grade of the cold heading steel wire rod of Examples 1-10 can reach 12.9 or more after quenching and tempering treatment.

[0092] The mass percentages of the chemical components of the cold heading steel wire rod of Comparative Example 1-8 are not within the protection scope of the present application, as shown in the results of Table 5 and Table 7, the cold heading steel wire rod of Comparative Example 1-3 cannot meet the strength requirement of 12.9 grade after the quenching and tempering treatment, and the cold heading steel wire rod of Comparative Example 1 cannot meet the high weather resistance requirement because the Cr content is low, resulting in the weather resistance index being less than 7.0; in the preparation of the cold heading steel wire rod of Comparative Example 4, Ni and Cu are not added to the steel as alloying elements, resulting in the steel not having weather resistance; the cold heading steel wire rod of Comparative Example 5-7 respectively adds more C, Cr, Mo and Ti, resulting in the cold heading forming performance of the wire rod being poor, the cold heading cracking rate being high, and the cold heading steel wire rod of Comparative Example 6 also having a decrease in the resistance to delayed fracture; the cold heading steel wire rod of Comparative Example 8 has an improper Cr, Ni and Cu ratio, (Cr+2xNi) / Cu being 4.13, which is lower than the requirement range 5-6.5 of the cold heading steel wire rod of the present application, resulting in the steel having poor plasticity and resistance to delayed fracture; the cold heading steel wire rod of Comparative Example 9 has a high heating and rolling temperature in the process, resulting in coarse grains and decarburization in the structure of the wire rod, causing the steel to have poor plasticity and a high cold heading cracking rate; the cold heading steel wire rod of Comparative Example 10 has a high cover temperature and a low cooling rate before entering the cover in the process, resulting in a high percentage of pearlite and a low percentage of bainite in the cold heading steel wire rod, and the wire rod also produces martensite in the cover without being transformed, resulting in a high cold heading cracking rate.

[0093] Obviously, the above embodiments are only examples for clearly illustrating the present application, and are not intended to limit the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments are not required to be exhausted, and the obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A cold heading steel wire rod, characterized in that: The invention comprises the following chemical components in percentage by mass: C 0.25-0.34%, Si 0.25-0.40%, Mn 0.75-0.90%, P≤0.025%, S≤0.020%, Cr 0.90-1.10%, Ni 0.30-0.60%, Cu 0.25-0.40%, Nb 0.02-0.04%, Al 0.010-0.045%, V 0.02-0.08%, N 0.003-0.007%, and the rest are Fe and unavoidable impurities, and satisfy ([Cr]+2[Ni]) / [Cu] is 5-6.5 and [C]+[Mn] / 6+([Cr]+[Nb]+[V]) / 5+([Ni]+[Cu]) / 20+[Si] / 15 is 0.7%-0.8%, where [Cr], [Ni], [Cu], [C], [Mn], [Nb], [V], and [Si] represent the mass percentages of Cr, Ni, Cu, C, Mn, Nb, V, and Si in the cold heading steel wire rod, respectively; The structure of the cold heading steel wire rod is a mixed structure of pearlite, bainite and ferrite.

2. The cold heading steel wire rod according to claim 1, characterized in that: In the cold heading steel wire rod, the area percentage of pearlite is 45-60%, the area percentage of bainite is 25-40%, and the area percentage of ferrite is 10-25%; and the grain size of the wire rod is level 9-11.

3. A method for preparing cold heading steel wire rod according to claim 1 or 2, characterized in that: The steps include: S1, converter smelting; S2, LF furnace refining; S3, continuous casting; S4, cooling of the casting; S5, heating; S6. Rolling.

4. The method for preparing cold heading steel wire rod according to claim 3, characterized in that: The S1 step satisfies one or more of the following AC: A. The percentage of C at the smelting end point is 0.06-0.20%, P≤0.012%; B. The temperature of steel tapping from converter is 1630℃~1670℃; C. After 1 / 2-1 / 4 of the steel is tapped, alloy materials containing Al, Si, Mn, Cr, Ni, and Cu are added in sequence for preliminary alloying.

5. The method for preparing cold heading steel wire rod according to claim 3, characterized in that: The S2 step includes the following steps: after the converter molten steel enters the LF station, 2.5-4.5 kg / t of lime and 0.5-2.0 kg / t of fluorite are added in sequence to adjust the white slag; after the LF white slag is refined for 5-10 minutes, an alloy material containing V and Nb is added for alloying, during which argon is blown at 300-450 L / min; then, the temperature is raised to 1540-1560° C. at a heating rate of 2.0-4.0° C. / min, and sampling is performed, during which argon is blown at 150-300 L / min; the temperature of the LF steel tapping is 1560° C.-1580° C.

6. The method for preparing cold heading steel wire rod according to claim 3, characterized in that: In step S3, the tundish temperature during continuous casting is 1525°C~1550°C; and / or, step S4 includes the following steps: air cooling the continuous casting billet, and after cooling to 700°C-750°C, slowly cooling it to a continuous casting billet temperature ≤200°C, and the slow cooling time is 72h-96h.

7. The method for preparing cold heading steel wire rod according to claim 3, characterized in that: In step S5, the temperature of the soaking section during the heating process is 1050°C-1100°C, and the heating time is 60-90 minutes; in step S6, the starting rolling temperature is 950°C-1000°C, the finishing rolling temperature is 880°C-910°C, and the spinning temperature is 850°C-880°C.

8. The method for preparing cold heading steel wire rod according to claim 3, characterized in that: The step S6 includes the following steps: opening the insulation cover of the first 1-2 roller conveyors and turning on 1-3 fans, with the air volume of each fan being 50-80%, and the remaining insulation covers and fans are all closed, the temperature entering the cover is 720-750°C, the cooling rate before entering the insulation cover is 7-10°C / s, and the cooling rate inside the insulation cover is 0.8-1.2°C / s.

9. The method for preparing cold heading steel wire rod according to any one of claims 3 to 8, characterized in that: After step S6, the method further includes performing a quenching and tempering treatment on the cold heading steel wire rod; the quenching and tempering treatment is to quench the wire rod at 860-900° C. and then temper it at 510-550° C.

10. A fastener, characterized in that: It is prepared from the cold heading steel wire rod according to claim 1 or 2; the strength grade of the fastener is ≥12.9.

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