A non-annealed cold-heading steel, its manufacturing method, and fasteners obtained therefrom

Abstract

The present invention discloses a non-annealed cold-heading steel, a manufacturing method thereof, and fasteners obtained therefrom. The cold-heading steel contains C, Si, Mn, Al, Cr, Mo, B, and Mg, and its metallographic structure is ferrite + degenerated pearlite. The hot-rolled wire rod of the cold-heading steel has low strength, high plasticity, and high cold-heading performance, and does not require spheroidizing annealing treatment. It can be directly drawn and cold-headed into fasteners with large deformation amounts, has excellent drawing and cold-heading performance, and can manufacture fasteners with large deformation amounts of 10.9 grade to 12.9 grade. In addition, the cold-heading steel has high purity, and both the steel and the fasteners have high fatigue performance and long service life.

Description

Technical Field

[0001] The present invention belongs to the technical field of cold heading steel, and particularly relates to an annealing-free cold heading steel, a manufacturing method thereof, and fasteners obtained therefrom. Background Art

[0002] Fasteners are mainly formed by cold heading. During the manufacturing process of workpieces, they often have to withstand a total deformation of 70-80%. Therefore, in addition to having good internal metallurgical quality and excellent surface quality, the raw materials are particularly required to have low hardness and good plasticity before cold working, so as to obtain low deformation resistance and high deformation ability. Therefore, cold heading steel wire rods usually need to go through several processes such as pickling, drawing, spheroidizing (softening) annealing, and secondary drawing before cold heading. Some steels also need to go through multiple spheroidizing annealings, and then tempering treatment after cold heading.

[0003] For example, Chinese Patent CN 114231703 A discloses a production method of high-strength simplified annealing cold heading steel, which includes successively performing electric furnace smelting, LF refining, RH vacuum degassing, bloom continuous casting, bloom rolling, and high-speed wire rod rolling on raw materials according to element ratios to obtain hot-rolled wire rods, and annealing the hot-rolled wire rods to obtain cold forging steel; the element ratios of the raw materials include at least 96.2 wt% of Fe, 0.01 wt% - 0.1 wt% of V, 0.01 wt - 0.1 wt% of Nb, and 0.002 wt% - 0.01 wt% of N; (V + Nb) / N = 7 - 15; the high-speed wire rod rolling includes rolling the wire rod on rough and medium rolling mills at a temperature above 950°C, then controlling the temperature of the wire rod entering the reducing sizing mill to be 790°C - 860°C, and rapidly cooling it to the laying temperature of 770°C - 800°C through a water tank. In this patent, the cold heading steel needs to be spheroidized annealed, and the process is complex.

[0004] Spheroidizing annealing and tempering treatment are the most energy-consuming and time-consuming processes in the fastener industry. They not only increase production costs, but also increase emissions and pollute the environment. If the process control is improper, it will also lead to unstable fastener quality. Annealing-free cold heading steel is an important development direction in cold heading. Summary of the Invention

[0005] The purpose of the present invention is to provide an annealing-free cold heading steel and a manufacturing method thereof. The hot-rolled wire rods of the cold heading steel have excellent drawing, cold heading properties, stress corrosion cracking resistance, and notch sensitivity resistance. They do not require spheroidizing annealing treatment and can be directly drawn and cold headed into large deformation fasteners of grades 10.9 - 12.9. In addition, the cold heading steel has high purity, and both the steel and the fasteners have high fatigue performance and long service life.

[0006] The object of the present invention is to provide a fastener, which is obtained by drawing or cold heading the non-annealing cold heading steel described in the present invention and then performing quenching and tempering heat treatment. Its strength level reaches 10.9 to 12.9 levels, and the axial fatigue cycle times under a load of 1.5 million KN > 3 million times.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A non-annealing cold heading steel, the cold heading steel contains by weight percentage: C 0.33% - 0.37%, Si 0.15% - 0.25%, Mn 0.60% - 0.70%, Al 0.040% - 0.050%, Cr 0.90% - 1.10%, Mo 0.15% - 0.25%, B 0.0010% - 0.0020%, Mg 0.0010% - 0.0030%, P ≤ 0.015%, S ≤ 0.003%, O ≤ 0.0010%, N ≤ 0.0045%, H ≤ 0.00015%, and the rest is Fe and other inevitable impurities;

[0009] Among them, Al / N ≥ 10.0, so as to give full play to the effective role of B in the steel;

[0010] The W value ≥ 3.35,

[0011] W = 29.3*(% B) + 11.7*(% Al) + 0.86*(% Mn) + 1.22*(% C) + 1.15*(% Cr) + 4.30*(% Mo), so as to ensure the hardenability of the steel;

[0012] The D value ≤ 0.04, D = 10.0*(% S) + 19.9*(% O), so as to ensure that the recovery rate of the Mg alloy reaches more than 90%.

[0013] The metallographic structure of the non-annealing cold heading steel is ferrite + degenerated pearlite, where the volume percentage of the ferrite structure is 60% - 70%, and the volume percentage of the degenerated pearlite structure is 30% - 40%. The average ferrite grain size is 10μm - 13μm; the cold heading steel with such a structure has excellent drawing, cold heading performance, resistance to delayed fracture performance and notch sensitivity resistance, and does not require spheroidizing annealing treatment, and can be directly drawn and cold headed into large deformation fasteners of 10.9 to 12.9 levels.

[0014] The Rm of the hot-rolled wire rod of the non-annealing cold heading steel ≤ 650 MPa, the elongation A ≥ 25%, the reduction of area ≥ 53%, and the hardness ≤ 88 HRB; the inclusions in the cold heading steel are all plastic inclusions, and the inclusion size is below 5.0μm.

[0015] The strength level of the non-annealed cold-heading steel reaches grade 10.9 to 12.9 after quenching and tempering heat treatment, and the axial fatigue cycle times under a 150 KN load are > 3 million times.

[0016] The present invention also provides a manufacturing method for the non-annealed cold-heading steel, and the manufacturing method includes the following steps: hot metal pretreatment → converter / electric furnace smelting → LF furnace refining → RH vacuum bloom / round billet continuous casting → bloom / round billet heating → high-speed wire rod rolling → cooling by Stelmor cooling line → slow cooling in the insulation channel → finished hot-rolled wire rod.

[0017] The specifications of the finished hot-rolled wire rod are Φ6mm to Φ30mm.

[0018] In the converter smelting step: the converter end point C ≤ 0.10%, P ≤ 0.008%; slagging-off during tapping, strictly controlling the slag volume, the slag layer thickness of the ladle ≤ 100mm, and the tapping time ≥ 5min.

[0019] In the LF furnace refining step, the LF refining time ≥ 30min, and the white slag holding time ≥ 20min to ensure sufficient deoxidation, desulfurization and dephosphorization.

[0020] In the RH vacuum step, the vacuum degree ≤ 100 Pa, the vacuum holding time ≥ 15min, and the determined hydrogen ≤ 0.00015%; feed Mg wire 8 minutes before breaking the vacuum, and the wire feeding speed is controlled at 30m / min to 40m / min. Higher than this speed cannot fully realize the plasticization effect of inclusions, and lower than this speed will affect the efficiency of the RH process.

[0021] In the bloom / round billet continuous casting step, argon sealing is used for the whole process of protective casting, the mold uses a stopper rod for automatic control, an integral submerged nozzle is used, and the casting speed of the continuous casting billet is controlled at 1.8m / min to 2.7m / min.

[0022] In the bloom / round billet heating step: the soaking temperature of the bloom / round billet is controlled at 1050 - 1150 °C. Lower than this temperature, alloying elements cannot be fully dissolved in austenite, and higher than this temperature will cause serious decarburization.

[0023] In the high-speed wire rod rolling step: the finish rolling temperature is controlled at 800 °C - 850 °C, and the reducing and sizing temperature is controlled at 780 °C - 820 °C. Rolling is carried out in the non-recrystallization zone of austenite, and it will not cause excessive refinement of ferrite grain size.

[0024] In the cooling step by the Stelmor cooling line: the laying temperature is controlled at 770 °C - 800 °C, the roller table reference speed is controlled at 0.05m / s - 0.10m / s, and the insulation hood is fully closed to obtain a degenerated pearlite structure.

[0025] In the slow cooling step of the heat preservation channel: after exiting the Stelmor cooling line, it enters the heat preservation channel for slow cooling, further softening online, and staying in the heat preservation channel for 30 min to 40 min. If the time is less than this, the hardness in the hot-rolled state is too high, and if it is more than this time, the softening effect will no longer improve.

[0026] The functions and controls of the components in the cold heading steel without annealing provided by the present invention are as follows:

[0027] C: C is the most basic and effective strengthening element in steel. However, as its content increases, the ductility decreases. In order to balance the high cold heading performance, high plasticity of the steel of the present invention and ensure the final strength level of the fasteners, the C content is controlled at 0.33% to 0.37%.

[0028] Si: Most of Si will dissolve in ferrite, strengthening ferrite, thereby improving the strength and hardness of the cold heading steel. However, Si will form brittle inclusions SiO2 in the steel, reducing plasticity and cold heading performance. In order to ensure the low strength, high plasticity and high purity of the steel of the present invention in the hot-rolled state, the Si content is controlled at 0.15% to 0.25%.

[0029] Mn: Mn forms a solid solution with Fe, improving the hardness and strength of ferrite and austenite in the steel and enhancing the hardenability of the steel. However, excessive Mn will reduce the plasticity of the steel, and the Mn content is controlled at 0.60% to 0.70%.

[0030] Al, N: Part of Al in the steel acts as a nitrogen-fixing element to promote the solution of B, and the other part dissolves in the steel together with B, inhibiting the transformation of pearlite and ferrite, increasing the ability of the steel to obtain martensite at a lower cooling rate, and enhancing the hardenability of the steel. However, excessive Al will form too many AlN precipitation phases with N, resulting in excessive grain refinement by pinning at the grain boundaries, leading to too high strength in the hot-rolled state. In addition, excessive Al will form coarse inclusions, reducing the purity of the steel. The Al content is controlled at 0.040% to 0.050%. The steel of the present invention adopts RH vacuum smelting, and the gas content is controlled at a low level. The N content is controlled below 0.0045%. However, in order to ensure the recovery rate of effective boron and the hardenability of the steel, it is necessary to control w(Al) / w(N)≥10.0. If the value is lower than this, sufficient hardenability cannot be guaranteed.

[0031] B: The B content is one of the main elements to ensure hardenability. By adding B, the contents of C, Mn, Cr, and Mo can be reduced, thereby significantly reducing the strength and hardness of the steel. In addition, during the rolling process of the hot-rolled wire rod, due to the strong grain boundary segregation phenomenon of boron, the concentration of solid solution boron is high at the α / γ interface. Using the slow cooling phase transformation process after thermo-mechanical rolling, an appropriate amount of boron phase [M3(C,B)+M 23(C, B)6] becomes the deposition core of excess carbon, accelerating carbon deposition, thereby promoting the moderate growth of the α phase, enabling the hot-rolled wire rod to obtain degenerated pearlite and the ferrite not being overly refined, forming the best microstructure beneficial to cold heading properties. However, B and nitrogen have a strong affinity and often form inclusions such as boron nitride during the steelmaking process, reducing the recovery rate of effective boron and resulting in insufficient hardenability of the steel. Therefore, Al element is added for nitrogen fixation. Since the steel of the present invention contains elements such as Cr and Mo that effectively improve hardenability, B element is not added excessively, and the B content is controlled at 0.0010% - 0.0020%. In addition, to ensure that the steel has sufficient hardenability, it is also necessary to ensure that the W value ≥ 3.35, where W = 29.3*(%B) + 11.7*(%Al) + 0.86*(%Mn) + 1.22*(%C) + 1.15*(%Cr) + 4.30*(%Mo), and the coefficients in the formula are determined according to the contribution of each alloy element to hardenability.

[0032] Mg: Mg not only has an excellent affinity with oxygen and sulfur but also has a strong ability to control the morphology and size of inclusions. Mg can transform inclusions such as SiO2 and A12O3 into spherical rare-earth composite oxysulfides, thereby improving the mechanical properties and fatigue life of the steel. The size of its oxide inclusions is very small and is dispersed in the steel, having basically no negative impact on the mechanical properties of the steel. Control Mg at 0.0010% - 0.0030%.

[0033] P: P is an element with a strong segregation tendency, which will cause cold brittleness and lead to cold heading cracking. Control P ≤ 0.015%.

[0034] S, O: S easily forms MnS inclusions in the steel, which is harmful to the processing performance of the steel; O forms oxide inclusions in the steel, damaging the processing performance and service life of the steel. Control S ≤ 0.015% and O ≤ 0.0015%. In addition, since Mg easily reacts with S and O to produce slag, to ensure that the recovery rate of the Mg alloy reaches more than 90%, it is also necessary to control the D value ≤ 0.04, where D = 10.0*(%S) + 19.9*(%O).

[0035] H: As the strength level of the steel increases, the internal stress increases, and the material is more likely to absorb hydrogen, resulting in an increase in the hydrogen-induced delayed fracture sensitivity. To improve the resistance to delayed fracture of the steel, the hydrogen content in the steel must first be reduced. Control the H content ≤ 0.00015%.

[0036] The cross-sectional structure of the hot-rolled wire rod of cold heading steel manufactured according to the above components of the present invention is ferrite + degenerated pearlite structure, where the volume percentage of the ferrite structure is 60% - 70%, the volume percentage of the degenerated pearlite structure is 30% - 40%, the average ferrite grain size is 10μm - 13μm, the inclusions in the steel are all plastic inclusions, the inclusion size is below 5.0μm, the mechanical properties of the hot-rolled wire rod are Rm ≤ 650MPa, elongation A ≥ 25%, reduction of area ≥ 53%, hardness ≤ 88HRB, and it has excellent drawing, cold heading properties, resistance to delayed fracture and notch sensitivity. It does not require spheroidizing annealing treatment and can be directly drawn and cold headed into 10.9 - 12.9 grade high-deformation fasteners.

[0037] Due to the low strength, high plasticity and high cold heading performance of the hot-rolled wire rod of cold heading steel provided by the present invention, it does not require spheroidizing annealing treatment and can be directly drawn and cold headed into high-deformation fasteners such as flange bolts and sleeves. After being made into fasteners and subjected to conventional quenching and tempering heat treatment, among them, the quenching temperature is 870 - 900°C, the tempering temperature is 500 - 600°C, and the strength level reaches 10.9 - 12.9 grades (10.9 grade: R m ≥ 1040MPa, R p0.2 ≥ 940MPa, A ≥ 9%, Z ≥ 48%, yield ratio ≥ 0.9, HRC hardness 32 - 39; 12.9 grade: R m ≥ 1220MPa, R p0.2 ≥ 110MPa, A ≥ 8%, Z ≥ 44%, yield ratio ≥ 0.9, HRC hardness 39 - 44), and the axial fatigue cycle times under 150KN load > 3 million times.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. By controlling Al / N ≥ 10.0, the present invention ensures the solid solution of effective boron and the hardenability of the steel;

[0040] 2. By restricting the elements B, Al, Mn, C, Cr, Mo that have a significant impact on hardenability in the W value formula, and stipulating that W = 29.3*(%B) + 11.7*(%Al) + 0.86*(%Mn) + 1.22*(%C) + 1.15*(%Cr) + 4.30*(%Mo) ≥ 3.35, the present invention further ensures that the steel has sufficient hardenability;

[0041] 3. By adding Mg, inclusions such as SiO2 and A12O3 are transformed into spherical rare earth composite oxysulfides, thereby improving the mechanical properties and fatigue life of the steel; and in order to ensure that the recovery rate of the Mg alloy reaches more than 90%, control D value ≤ 0.04, D = 10.0*(%S) + 19.9*(%O).

[0042] 4. The manufacturing method of the cold-heading steel provided by the present invention has low manufacturing cost, does not require spheroidizing annealing treatment, can be directly drawn and cold-headed into fasteners with large deformation, and after quenching and tempering heat treatment, the strength level of the fasteners reaches 10.9 to 12.9 levels, and the axial fatigue life under 100 KN load > 3 million times. Description of the Drawings

[0043] Figure 1 It is the metallographic structure diagram of the cold-heading steel in Example 1, which is 65% ferrite + 35% degenerated pearlite structure;

[0044] Figure 2 It is the metallographic structure diagram of the cold-heading steel in Comparative Example 1, which is 60% ferrite + 20% bainite and martensite + 10% pearlite structure. Detailed Description of the Invention

[0045] An annealing-free cold-heading steel provided by the present invention, the cold-heading steel contains by weight percentage: C 0.33% - 0.37%, Si 0.15% - 0.25%, Mn 0.60% - 0.70%, Al 0.040% - 0.050%, Cr 0.90% - 1.10%, Mo 0.15% - 0.25%, B 0.0010% - 0.0020%, Mg 0.0010% - 0.0030%, P ≤ 0.015%, S ≤ 0.003%, O ≤ 0.0010%, N ≤ 0.0045%, H ≤ 0.00015%, and the rest is Fe and other inevitable impurities;

[0046] Among them, Al / N ≥ 10.0;

[0047] The W value ≥ 3.35,

[0048] W = 29.3*(%B) + 11.7*(%Al) + 0.86*(%Mn) + 1.22*(%C) + 1.15*(%Cr) + 4.30*(%Mo);

[0049] The D value ≤ 0.04, D = 10.0*(%S) + 19.9*(%O).

[0050] The manufacturing method of the annealing-free cold-heading steel includes the following steps: hot metal pretreatment → converter / electric furnace smelting → LF furnace refining → RH vacuum → bloom / round billet continuous casting → bloom / round billet heating → high-speed wire rod rolling → Stelmor cooling line cooling → heat preservation channel slow cooling → hot-rolled wire rod finished product.

[0051] The specifications of the hot-rolled wire rod finished product are Φ6mm - Φ30mm.

[0052] In the converter smelting step: at the end of the converter, C ≤ 0.10% and P ≤ 0.008%; slagging-off tapping is carried out, strictly controlling the slag volume entering the ladle, and the thickness of the slag layer in the ladle ≤ 100 mm, and the tapping time ≥ 5 min.

[0053] In the LF furnace refining step, the LF refining time ≥ 30 min, and the white slag holding time ≥ 20 min to ensure sufficient deoxidation, desulfurization and dephosphorization.

[0054] In the RH vacuum step, the vacuum degree ≤ 100 Pa, the vacuum holding time ≥ 15 min, and the hydrogen content ≤ 0.00015%; Mg wire is fed 8 min before breaking the vacuum, and the wire feeding speed is controlled at 30 m / min - 40 m / min.

[0055] In the billet / circular billet continuous casting step, argon sealing is used for the whole process of protective casting, the mold is automatically controlled by a stopper rod, an integral submerged nozzle is used, and the drawing speed of the continuous casting billet is controlled at 1.8 m / min - 2.7 m / min.

[0056] In the billet / circular billet heating step: the soaking temperature of the billet / circular billet is controlled at 1050 - 1150 °C.

[0057] In the high-speed wire rod rolling step: the finishing rolling temperature is controlled at 800 °C - 850 °C, and the sizing temperature is controlled at 780 °C - 820 °C.

[0058] In the Stelmor cooling line cooling step: the laying temperature is controlled at 770 °C - 800 °C, the reference speed of the roller table is controlled at 0.05 m / s - 0.10 m / s, and the heat preservation cover is fully closed.

[0059] In the heat preservation channel slow cooling step: after coming out of the Stelmor cooling line, it enters the heat preservation channel for slow cooling, and further softening is carried out online, and it stays in the heat preservation channel for 30 min - 40 min.

[0060] The present invention will be described in detail below with reference to the embodiments.

[0061] The components and weight percentages of the steel grades used for the cold heading steel in each embodiment and comparative example are shown in Table 1, and the balance is iron and unavoidable impurities.

[0062] Table 1

[0063]

[0064]

[0065] The production process parameters of the cold heading steel in each embodiment and comparative example are shown in Table 2.

[0066] Table 2 Production process parameters

[0067]

[0068]

[0069] The performance detection method of hot-rolled wire rods is as follows:

[0070] Microstructure and ferrite grain size: Take a 15-mm-long specimen from the wire rod, polish the cross-section, etch it with 4% nitric acid alcohol, conduct microstructure evaluation according to GB / T 13298 "Test Methods for Metallographic Microstructure of Metals", and measure the average grain size according to GB / T 6394 "Determination Method for Average Grain Size of Metals".

[0071] Measurement of inclusion size: Measure and statistically calculate the inclusion size on the longitudinal surface of the steel through an ASPEX scanning electron microscope, and the detection area is 160 mm 2 .

[0072] Tensile properties in the hot-rolled state: Take a 400-mm-long specimen from the wire rod, conduct a tensile test, and test the R m , A, and Z values.

[0073] Hardness: Conduct 3-point hardness tests on the cross-section according to GB / T 230.1 "Metallic Materials - Rockwell Hardness Test - Part 1: Test Method", and calculate the average value.

[0074] Cold upsetting: The wire rod shall be cold upset according to the following requirements: X = h1 / h = 1 / 7; (where: h is the height of the specimen before cold upsetting (twice the wire rod diameter); h1 is the height of the specimen after cold upsetting.) After the cold upsetting test, no visible cracks, fissures, cracks, and hairline defects shall appear on the surface of the specimen. For a group of 10 samples, if all are qualified, it is considered qualified; otherwise, calculate the qualified rate.

[0075] Resistance to delayed fracture test: Take samples from the intermediate billet to process delayed fracture specimens, immerse the specimens in an acidic aqueous solution of 15% HCl for 30 minutes, wash with water and dry, apply a certain load, and compare the loads that do not fracture after more than 100 hours. At this time, the value obtained by dividing the load that does not fracture after more than 100 hours after acid immersion by the maximum load during the tensile test without acid immersion is defined as the delayed fracture strength ratio. If the delayed fracture strength ratio is 0.70 or more, it is judged as qualified; if it is 0.80 or more, it is judged as excellent.

[0076] Notch sensitivity test: The notch sensitivity test was carried out in accordance with HB 5214-1996 "Metallic materials - Tensile testing at ambient temperature - Test method for notched specimens". The notch sensitivity was measured by the ratio of the tensile strength of the notched specimen to that of the unnotched specimen, i.e., the NSR value. The larger the NSR value, the lower the notch sensitivity. Since both the examples and the comparative examples in this embodiment are plastic specimens, a notch strengthening effect is generated, so the NSR values are all > 1. The NSR value of the example is ≥ 1.6, which is significantly better than the comparative sample, has good notch sensitivity, and is suitable for making bolts.

[0077] The performance detection methods for fasteners are as follows:

[0078] Tensile properties and hardness of fasteners in the heat-treated state: The tensile properties and HRC hardness of fasteners were tested in accordance with GB / T 3098.1 "Mechanical properties of fasteners - Bolts, screws and studs".

[0079] Fatigue test of fasteners: Axial fatigue tests were carried out on an MTS fatigue testing machine. Axial stress control was adopted, the strain cycle characteristic was R = -1, the control load was 150 KN, the frequency was 10 Hz, and 20 bolts were taken as a group. The test was considered successful if the number of cycles of 80% of the specimens exceeded 3 million times.

[0080] The properties of the cold-heading steel hot-rolled wire rods prepared in each example and comparative example are shown in Table 3, and the quenching and tempering heat treatment processes and properties of the fasteners are shown in Table 4.

[0081] List of performance detection of hot-rolled wire rods in Table 3

[0082]

[0083]

[0084]

[0085] List of performance detection of examples and comparative examples of the present invention in Table 4

[0086]

[0087] The chemical compositions and manufacturing methods of the steels in Examples 1 to 8 are properly controlled, which are suitable for the production of hot-rolled wire rods for high-strength fasteners without annealing. Their microstructure is ferrite + degenerated pearlite structure, with low strength and hardness, good plasticity, excellent drawing and cold-heading properties, and sufficient hardenability and high purity. Large-deformation fasteners that can be directly drawn and cold-headed without spheroidizing annealing treatment can be produced. After conventional quenching and tempering treatment, the strength level of the fasteners is 10.9 to 12.9, and they have high resistance to delayed fracture, low notch sensitivity, and excellent fatigue performance.

[0088] Although the chemical composition of Comparative Example 1 is within the required range, both the reduction-sizing temperature and the wire-drawing temperature are relatively high, and both the cooling rate and the residence time in the holding channel are inappropriate, resulting in the generation of abnormal martensite-austenite constituent, and the mechanical properties do not meet the requirements of the present invention. It is impossible to directly draw and cold upset without annealing, and qualified bolts cannot be obtained; Comparative Example 2 is the commonly used 10.9-grade to 12.9-grade cold-heading steel SCM435 in the market. Due to the relatively high alloying elements such as Mn, Cr, and Mo, although controlled according to the production process of the present invention, there is still martensite-austenite constituent in the hot-rolled structure, and it is also impossible to directly draw and cold upset without annealing, and qualified bolts cannot be obtained; In Comparative Example 3, the feeding speed of the Mg wire is not properly controlled, resulting in relatively low purity of the steel and relatively low fatigue life of the fastener; In Comparative Example 4, both the Al / N value and the hardenability index W value are not properly controlled, and the hardenability of the steel is insufficient. The fasteners made cannot meet the requirements of 10.9 grade and above. Moreover, due to the insufficient strength of the fastener finished product, the axial fatigue life is relatively low under a load of 150 KN; In Comparative Example 5, the D value is not properly controlled, resulting in a relatively low recovery rate of the Mg alloy and the purity of the steel not meeting the requirements, and the fatigue life of the fasteners made is relatively low.

[0089] The above detailed description of a non-annealing cold-heading steel, its manufacturing method, and the fasteners obtained therefrom with reference to the embodiments is illustrative rather than restrictive. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications within the general concept of the present invention should fall within the protection scope of the present invention.

Claims

1. A non-annealed cold-heading steel, characterized in that, The cold heading steel contains by weight percentage: C 0.33% - 0.37%, Si 0.15% - 0.25%, Mn 0.60% - 0.70%, Al 0.040% - 0.050%, Cr 0.90% - 1.10%, Mo 0.15% - 0.25%, B 0.0010% - 0.0020%, Mg 0.0010% - 0.0030%, P ≤ 0.015%, S ≤ 0.003%, O ≤ 0.0010%, N ≤ 0.0045%, H ≤ 0.00015%, and the balance is Fe and other inevitable impurities; wherein, Al / N ≥ 10.0; The W value ≥ 3.35, W = 29.3*(%B) + 11.7*(%Al) + 0.86*(%Mn) + 1.22*(%C) + 1.15*(%Cr) + 4.30*(%Mo); The D value ≤ 0.04, D = 10.0*(%S) + 19.9*(%O).

2. The cold heading steel without annealing according to claim 1, characterized in that The metallographic structure of the non - annealing cold heading steel is ferrite + degenerated pearlite, wherein the volume percentage of the ferrite structure is 60% - 70%, the volume percentage of the degenerated pearlite structure is 30% - 40%, and the average ferrite grain size is 10μm - 13μm.

3. The cold heading steel without annealing according to claim 1, characterized in that, The hot - rolled wire rod of the non - annealing cold heading steel has Rm ≤ 650MPa, elongation A ≥ 25%, reduction of area Z ≥ 53%, and hardness ≤ 88HRB; the inclusions in the cold heading steel are all plastic inclusions with inclusion size below 5.0μm.

4. The cold heading steel without annealing according to claim 1, characterized in that, The non - annealing cold heading steel reaches strength levels of 10.9 - 12.9 grades after quenching and tempering heat treatment, and the axial fatigue cycle times under 150KN load > 3 million times.

5. The manufacturing method of the non-annealed cold-heading steel according to any one of claims 1-4, characterized in that, The manufacturing method includes the following steps: hot metal pretreatment → converter / electric furnace smelting → LF furnace refining → RH vacuum → bloom / round billet continuous casting → bloom / round billet heating → high - speed wire rod rolling → cooling on the Stelmor cooling line → slow cooling in the insulation channel → finished hot - rolled wire rod.

6. The manufacturing method according to claim 5, characterized in that, In the converter smelting step: the converter end point C ≤ 0.10%, P ≤ 0.008%; slag - stopping tapping, the slag layer thickness of the ladle ≤ 100mm, and the tapping time ≥ 5min.

7. The manufacturing method according to claim 5, characterized in that In the LF furnace refining step, the LF refining time ≥ 30min, and the white slag holding time ≥ 20min.

8. The manufacturing method according to claim 5, characterized in that, In the RH vacuum step, the vacuum degree ≤ 100 Pa, the vacuum holding time ≥ 15min, and the hydrogen content determination ≤ 0.00015%; feed Mg wire 8min before breaking the vacuum, and the wire - feeding speed is controlled at 30m / min - 40m / min.

9. The manufacturing method according to claim 5, characterized in that, In the bloom / round billet continuous casting step, argon - sealed full - process protection casting is adopted, the crystallizer is automatically controlled by a stopper rod, an integral submerged nozzle is used, and the casting speed of the continuous casting billet is controlled at 1.8m / min - 2.7m / min.

10. The manufacturing method according to claim 5, characterized in that, In the bloom / round billet heating step: the soaking temperature of the bloom / round billet is controlled at 1050 - 1150°C.

11. The manufacturing method according to claim 5, characterized in that, In the high - speed wire rod rolling step: the finish rolling temperature is controlled at 800°C - 850°C, and the sizing - reducing temperature is controlled at 780°C - 820°C.

12. The manufacturing method according to claim 5, characterized in that, In the cooling step of the Stelmor cooling line: the laying temperature is controlled at 770°C to 800°C, the reference speed of the roller table is controlled at 0.05 m / s to 0.10 m / s, and the heat preservation cover is fully closed.

13. The manufacturing method according to claim 5, wherein In the slow cooling step of the heat preservation channel: stay in the heat preservation channel for 30 min to 40 min.

14. A fastener, characterized in that, It is obtained by tempering heat treatment after drawing or cold heading of the non-annealed cold heading steel described in any one of claims 1-4.

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