A large-sized non-quenched and tempered cold-heading steel, its production method, fasteners prepared therefrom, and their preparation methods

Abstract

The present invention discloses a large-sized non-quenched cold heading steel, its production method, fasteners prepared therefrom, and their preparation methods. The cold heading steel contains C, Si, Mn, Al, Nb, Cr, Mg, and Pr, and its metallographic structure is an ultrafine-grained ferrite + pearlite structure. The large coil of the cold heading steel has excellent drawing and cold heading properties, low notch sensitivity, and high fatigue performance. It does not need to undergo spheroidizing annealing treatment and quenching and tempering heat treatment, and can be directly drawn and cold headed and then prepared into fasteners with large deformation amounts through stabilization treatment. It has excellent drawing and cold heading properties and can be used to manufacture fasteners with large deformation amounts of grade 9.8.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading steel, and in particular relates to large-size non-quenched and tempered cold heading steel and a production method thereof, and a fastener prepared therefrom and a preparation method thereof. Background Art

[0002] Fasteners are one of the three basic mechanical parts. The raw material of fasteners is mainly cold heading steel. Through cold heading forming, the workpiece often undergoes a total deformation of 70-80% during the manufacturing process. Therefore, the raw material is required to have low hardness and good plasticity before processing, and high toughness after processing. Therefore, cold heading steel wire is usually subjected to a "two-drawing and one-retracting" process before cold heading. That is, it must first undergo several processes such as pickling, drawing, spheroidizing annealing (or soft annealing) and drawing, and then undergo tempering treatment after cold heading. Spheroidizing annealing and tempering treatment consume energy and time, not only increasing the production cost of fasteners, but also polluting the environment. If the process is not properly controlled, it will also lead to problems such as surface decarburization and workpiece deformation, affecting the quality of the fasteners.

[0003] For example, Chinese patent CN 114231703 A discloses a method for producing high-strength simplified annealed cold heading steel, which includes subjecting raw materials to electric furnace smelting, LF refining, RH vacuum degassing, billet continuous casting, billet rolling, and high-speed wire rolling in sequence according to elemental ratios to obtain hot-rolled wire rods, which are then annealed to obtain cold-forged steel. The elemental ratio of the raw materials includes at least 96.2 wt% Fe, 0.01 wt% to 0.1 wt% V, 0.01 wt% to 0.1 wt% Nb, and 0.002 wt% to 0.01 wt% N; (V+Nb) / N=7 to 15. The high-speed wire rolling includes rolling the wire rod in roughing and intermediate mills at a temperature above 950°C, then controlling the temperature of the wire rod inlet to a wire sizing unit to 790°C to 860°C, and rapidly cooling the wire rod in a water tank to a wire-spinning temperature of 770°C to 800°C. This patent requires spheroidizing annealing of the cold heading steel, which is a complicated process.

[0004] Energy-saving cold heading steel is one of the future development directions, including steel for annealed fasteners and non-tempered fasteners, and the market demand is urgent. Non-tempered cold heading steel is a new type of structural steel for the production of fasteners. The use of non-tempered steel to manufacture high-strength fastener products can eliminate the annealing treatment of steel before cold drawing and the tempering treatment after bolt forming, which greatly simplifies the production process, shortens the production cycle, reduces energy consumption, and avoids problems such as surface oxidation, decarburization, and workpiece deformation and cracking caused by heat treatment. However, the non-tempered cold heading steel currently developed at home and abroad is mainly concentrated in specifications of 16 and below, and there is little research on large-size non-tempered cold heading steel. However, with the increasing tension of energy and resources, there is an urgent need to develop large-size non-tempered cold heading steel products. Summary of the Invention

[0005] The object of the present invention is to provide a large-sized non-quenched and tempered cold heading steel and its production method. The specification of the large-sized non-quenched and tempered cold heading steel coil is Φ20 - 50 mm, and its metallographic structure is ultrafine ferrite + pearlite structure. The ferrite grain size is above grade 12.5, with excellent drawing and cold heading properties, low notch sensitivity, high fatigue performance and low-temperature impact toughness. It does not need to go through spheroidizing annealing treatment and quenching and tempering heat treatment, and can be directly drawn and cold headed and then prepared into fasteners with large deformation after stabilization treatment. It has excellent drawing and cold heading properties and can be used to make fasteners with large deformation of grade 9.8.

[0006] The object of the present invention is also to provide a fastener and its preparation method, which is obtained by drawing the large-sized non-quenched and tempered cold heading steel coil of the present invention, then cold heading or cold bending, and finally stabilization treatment. The mechanical properties of the fastener can reach grade 9.8, and the axial fatigue cycle times under 120 KN load > 2.5 million times.

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

[0008] A large-sized non-quenched and tempered cold heading steel, the cold heading steel contains by weight percentage: C 0.10% - 0.30%, Si 0.90% - 1.20%, Mn 0.30% - 0.50%, Al 0.040% - 0.060%, Nb 0.030% - 0.050%, Cr 0.30% - 0.50%, Mg 0.0010% - 0.0030%, Pr 0.0040% - 0.0070%, P ≤ 0.015%, S ≤ 0.015%, O ≤ 0.0015%, and the rest is Fe and other inevitable impurities;

[0009] Among them, in order to ensure that the steel has excellent comprehensive strength and plasticity, it is necessary to control the grain size level above grade 12.5, and it is necessary to control the G value ≥ 0.15, G = 1.9*(%Nb) + 1.3*(%Al) + 25.9*(%Nb)*(%Al) - 0.03*(%Mn);

[0010] In addition, due to the high Si content and Al content, in order to ensure that the steel has a sufficiently high purity, it is necessary to control the A value ≥ 0.047, A = (11.9*(%Mg) + 7.3*(%Pr)) / (0.9*(%Si) + 5.6*(%Al));

[0011] In addition, in order to ensure that the recovery rate of Pr and Mg alloys reaches more than 80%, it is also necessary to control the D value ≤ 0.07, D = 10.0*(%S) + 19.9*(%O).

[0012] The specification of the large-sized non-quenched cold heading steel is Φ20 - 50 mm; its metallographic structure is ultra-fine grained ferrite + pearlite structure, and the ferrite grain size is above grade 12.5.

[0013] The R of the large-sized non-quenched cold heading steel hot-rolled coil m is 740 - 780 MPa, Z ≥ 67%, notch sensitivity NSR value ≥ 1.60, axial tensile-compressive fatigue limit ≥ 450 MPa, and KV2 impact toughness value at -40°C ≥ 240 J; all inclusions in the cold heading steel are plastic inclusions, and the inclusion size is below 6.5 μm.

[0014] The present invention also provides a production method of the large-sized non-quenched cold heading steel, and the production method includes the following steps: hot metal pretreatment → converter smelting → LF furnace refining → continuous casting → billet heating → KOCKS rolling of coil → coiling → controlled cooling → finished product of hot-rolled coil.

[0015] 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 ≤ 100 mm, and the tapping time ≥ 5 min.

[0016] [[ID=??]]

[0017] In the continuous casting step, Pr wire is added to the center of the mold, and the wire feeding speed is controlled at 15 - 30 m / min. In addition, the ratio of the wire feeding speed to the casting speed is maintained at 8 - 15. If it is lower than this ratio, the Pr recovery rate is relatively low, and the wire feeding amount is insufficient, resulting in a relatively low Pr content. If it is higher than this ratio, the effect of Pr on modifying inclusions in the steel is insufficient, and the full plasticization effect cannot be achieved.

[0018] In the billet heating step: the soaking temperature is controlled at 1120 - 1200°C. If it is lower than this temperature, alloying elements such as Nb cannot be fully dissolved in austenite. If it is higher than this temperature, severe decarburization will occur. In addition, due to the relatively high Si content, the activity of C is accelerated, and the steel has a relatively high decarburization sensitivity. Therefore, an "S"-type heating curve is adopted during heating. In the preheating section and heating section of the heating furnace, the billet temperature is limited to be lower than the full decarburization sensitive zone. The preheating section temperature is controlled at 550 - 600°C, and the heating section is controlled at 750 - 820°C. The billet temperature is quickly heated to above the full decarburization sensitive zone temperature in the high-temperature section of the soaking section.

[0019] It should be noted that there seems to be a missing number in the ID for the English translation of the step description in ID=14. It should be something like ID=14 in the English translation as well for better consistency.In the KOCKS rolling step: control the temperature of the steel entering the KOCKS to be 790 - 810 °C, and the temperature of the steel exiting the KOCKS to be 730 - 760 °C. In this temperature range, proeutectoid ferrite phase transformation occurs, forming a large number of ferrite nucleation sites, increasing the ferrite content. At the same time, combined with deformation, the deformation-induced ferrite phase transformation refines the microstructure. Meanwhile, the pinning effect of the precipitation of carbonitrides of Nb and Al at the grain boundaries is used to further refine the grains, enabling the steel to obtain excellent plasticity and toughness. Temperatures higher or lower than this range cannot achieve this effect.

[0020] In the controlled cooling step: after the steel exits the KOCKS rolling mill, the cooling rate in the cooling section is controlled to be 1.5 °C / s - 2.2 °C / s. Cooling rates lower than this value will result in coarse grains, and cooling rates higher than this value cannot complete the phase transformation in the cooling section.

[0021] The present invention also provides a fastener, which is obtained by subjecting the large-sized non-quenched and tempered cold heading steel described in the present invention to drawing, cold heading or cold bending, and then to stabilization treatment.

[0022] The reduction ratio of the drawing is controlled to be 18% - 24%. If the reduction ratio is lower than this value, the bolt strength grade is less than 9.8, and if the reduction ratio is higher than this value, it will cause greater wear of the drawing die.

[0023] The conditions for the stabilization treatment are to keep the temperature at 150 °C - 350 °C for 1 h - 1.5 h. Temperatures lower than this range or too short treatment time cannot completely eliminate the residual stress, while temperatures higher than this range or too long treatment time will increase the cost and cause a decrease in strength.

[0024] The mechanical properties of the fastener reach grade 9.8, and the number of axial fatigue cycles under a load of 1.2 million N > 2.5 million times.

[0025] The present invention also provides a preparation method for the fastener described above, including the following steps: drawing the large-sized coil of the non-quenched and tempered cold heading steel described in the present invention, then performing cold heading or cold bending, and finally performing stabilization treatment.

[0026] The functions and controls of the components in the large-sized non-quenched and tempered cold heading steel provided by the present invention are as follows:

[0027] C: C is the most basic and effective strengthening element in steel. In addition, in the steel of the present invention, it forms carbonitrides with elements such as Nb and Al, playing a role in fine grain strengthening and dispersion strengthening. However, as its content increases, the ductility decreases. The C content is controlled to be 0.10% - 0.30%.

[0028] Si: Si can accelerate the precipitation of proeutectoid ferrite during steel rolling, which promotes the refinement of grain size. In addition, in the steel of the present invention, due to the large specifications, it is necessary to particularly ensure high yield strength and yield ratio. Si can increase the elastic limit of the steel, thereby increasing the yield strength and yield ratio. However, the increase of Si element will increase the diffusion of carbon in the steel and exacerbate the decarburization of the steel. The Si content is controlled at 0.90% - 1.20%.

[0029] Mn, Al, Nb: Mn forms a solid solution with Fe, increasing the hardness and strength of ferrite and austenite in the steel. However, at the same time, Mn has a strong nitrogen fixation effect, which will lead to insufficient precipitation of carbonitrides of Nb and Al, thus reducing the fine grain effect; Al is a strong deoxidizing element and can also improve the oxidation resistance of the steel. In addition, the carbide particles formed by Al and C, N can refine the grain size, thereby ensuring that the steel has high comprehensive strength and plasticity. However, with the increase of Al content, the amount of coarse carbonitride inclusions increases; Nb forms carbonitrides with carbon and nitrogen in the steel, having the effects of fine grain strengthening and dispersion strengthening. After Nb exceeds 0.03%, this effect no longer increases significantly. The Mn content is controlled at 0.30% - 0.50%, the Al content is controlled at 0.040% - 0.060%, and the Nb content is controlled at 0.030% - 0.050%. In the steel of the present invention, in order to achieve the fine grain structure effect of large - specification cold - heading steel, it is necessary to control the ratio relationship of Mn, Al, Nb, and it is necessary to control G value ≥ 0.15, G = 1.9*(%Nb)+1.3*(%Al)+25.9*(%Nb)*(%Al)-0.03*(%Mn). At the same time, combined with subsequent soaking temperature control and controlled rolling and controlled cooling control, finally, a grain size level above 12.5 is obtained, achieving high strength and toughness, thus meeting the requirements of non - quenched and tempered steel.

[0030] Cr: The Cr element significantly improves the strength and toughness in the steel. It precipitates in the form of carbides, increasing the hydrogen trapping points and improving the resistance to delayed fracture. However, excessive Cr increases the brittleness tendency of the steel. The Cr content is controlled at 0.30% - 0.50%.

[0031] Mg, Pr: Mg and Pr form a composite modifier in steel, which is more effective than single addition. First of all, Mg not only has excellent affinity with oxygen and sulfur, but also has extremely strong control ability over the morphology and size of inclusions. Mg can transform Al2O3 inclusions in steel into high-melting-point MgO·Al2O3. Since it exists in a solid state in molten steel and there is no process of aggregation and growth, 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. Adding an appropriate amount of Pr element to the steel 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. Since the Si and Al contents added in the present invention are relatively high, in order to ensure that the steel has a sufficiently high purity, it is necessary to control the value of A≥0.047, A = (11.9*(%Mg)+7.3*(%Pr)) / (0.9*(%Si)+5.6*(%Al)), so that all inclusions are plastically modified and the inclusion size is controlled below 6.5μm, so that both the produced steel and the processed fasteners have high fatigue life.

[0032] P: P is an element with a strong segregation tendency, which can cause cold brittleness and lead to cold heading cracking. Control P≤0.015%.

[0033] S, O: S is likely to form MnS inclusions with manganese in 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 and Pr are prone to react with S and O to produce slag, in order to ensure that the recovery rate of Pr and Mg alloys reaches more than 80%, it is also necessary to control the value of D≤0.07, D = 10.0*(%S)+19.9*(%O).

[0034] The structure of the large coil of cold heading steel produced according to the composition of the present invention is ultra-fine-grained ferrite + pearlite, the ferrite grain size is above grade 12.5, all inclusions in the steel are plastic inclusions, the inclusion size is below 6.5μm, and the mechanical properties of the steel are: R m 740 - 780 MPa, Z≥67%, one-sixth cold upsetting is qualified, it has excellent drawing and cold heading properties, low notch sensitivity and high fatigue performance, the notch sensitivity NSR value≥1.60, and has an axial tensile-compressive fatigue limit≥450 MPa.

[0035] The cold heading steel coil provided by the present invention is pickled and phosphated or shot blasted and then drawn. After being drawn into fine wire, it is directly cold headed or cold bent into fasteners. The mechanical properties of the fasteners can reach grade 9.8 (Rm≥900MPa, Rp0.2≥720MPa, A≥10%, Z≥48%, yield ratio≥0.8), the KV2 impact toughness value at -40°C≥240J, and the axial fatigue cycle times under a load of 120KN>2.5 million times.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. By controlling the ratio relationship of Mn, Al, and Nb to satisfy 1.9*(%Nb)+1.3*(%Al)+25.9*(%Nb)*(%Al)-0.03*(%Mn)≥0.15, the present invention improves the fine grain structure effect of large - sized cold heading steel.

[0038] 2. By adding Mg and Pr to form a composite modifier for inclusions in the steel, the present invention improves the mechanical properties and fatigue life of the steel; and in order to ensure that the recovery rate of Pr and Mg alloys reaches more than 80%, control D value≤0.07, D = 10.0*(%S)+19.9*(%O).

[0039] 3. By controlling A value≥0.047, A=(11.9*(%Mg)+7.3*(%Pr)) / (0.9*(%Si)+5.6*(%Al)), the present invention makes the inclusions all achieve metamorphic plasticity, controls the inclusion size below 6.5μm, ensures that the steel has a sufficiently high purity, and enables both the produced steel and the processed fasteners to have a high fatigue life.

[0040] 4. The structure of the cold heading steel coil provided by the present invention is ultrafine - grained ferrite + pearlite, the ferrite grain size is above grade 12.5, all the inclusions in the steel are plastic inclusions, the inclusion size is below 6.5μm, and the mechanical properties of the steel are: R m 740 - 780MPa, Z≥67%, notch sensitivity NSR value≥1.60, axial tension - compression fatigue limit≥450MPa, the KV2 impact toughness value at -40°C≥240J, and one - sixth cold upsetting is qualified. It has excellent drawing and cold heading properties, low notch sensitivity and high fatigue performance.

[0041] 5. The method for processing the cold heading steel provided by the present invention into fasteners has a low manufacturing cost, does not require spheroidizing annealing treatment and quenching and tempering heat treatment, can be directly drawn and cold headed into fasteners with a large deformation amount, and can prepare grade 9.8 fasteners after low - temperature stabilization treatment. Its axial fatigue cycle times under a load of 120KN>2.5 million times. Description of the Drawings

[0042] Figure 1 It is the metallographic structure diagram of the cold heading steel in Example 1, and the metallographic structure is ultrafine-grained ferrite + pearlite structure;

[0043] Figure 2 It is the metallographic structure diagram of the cold heading steel in Comparative Example 1, and the metallographic structure is martensite structure;

[0044] Figure 3 It is the metallographic structure diagram of the cold heading steel in Comparative Example 6, and the metallographic structure is pearlite + ferrite. Detailed implementation manners

[0045] A large-sized non-quenched and tempered cold heading steel provided by the present invention, the large-sized non-quenched and tempered cold heading steel contains by weight percentage: C 0.10% - 0.30%, Si 0.90% - 1.20%, Mn 0.30% - 0.50%, Al 0.040% - 0.060%, Nb 0.030% - 0.050%, Cr 0.30% - 0.50%, Mg 0.0010% - 0.0030%, Pr 0.0040% - 0.0070%, P ≤ 0.015%, S ≤ 0.015%, O ≤ 0.0015%, and the balance is Fe and other inevitable impurities;

[0046] Among them, the G value ≥ 0.15, G = 1.9*(%Nb) + 1.3*(%Al) + 25.9*(%Nb)*(%Al) - 0.03*(%Mn);

[0047] The A value ≥ 0.047, P = (11.9*(%Mg) + 7.3*(%Pr)) / (0.9*(%Si) + 5.6*(%Al));

[0048] The D value ≤ 0.07, D = 10.0*(%S) + 19.9*(%O).

[0049] The production method of the large-sized non-quenched and tempered cold heading steel includes the following steps: hot metal pretreatment → converter smelting → LF furnace refining → continuous casting → billet heating → large coil KOCKS rolling → coiling → controlled cooling → hot rolled large coil finished product.

[0050] In the converter smelting step: the converter end point C ≤ 0.10%, P ≤ 0.008%; slag blocking tapping, strictly controlling the slag volume, the slag layer thickness of the ladle ≤ 100mm, and the tapping time ≥ 5min.

[0051] In the LF furnace refining step, the LF refining time ≥ 30min, and the white slag holding time ≥ 20min; feeding Mg wire after alloying, and controlling the wire feeding speed at 30m / min - 40m / min.

[0052] In the continuous casting step, Pr wire is added to the center of the mold, and the wire feeding speed is controlled at 15 m / min to 30 m / min. In addition, the ratio of the wire feeding speed to the casting speed is maintained at 8 to 15.

[0053] In the slab heating step: the soaking temperature is controlled at 1120 to 1200 °C, and the temperature of the steel slab is restricted to be lower than the fully decarburized sensitive zone in the preheating section and heating section of the heating furnace. The temperature of the preheating section is controlled at 550 to 600 °C, and the heating section is controlled at 750 to 820 °C. The temperature of the steel slab is quickly heated above the fully decarburized sensitive zone temperature in the high-temperature section of the soaking section.

[0054] In the KOCKS rolling step: control the temperature entering KOCKS at 790 to 810 °C and the temperature leaving KOCKS at 730 to 760 °C.

[0055] In the controlled cooling step: after the steel exits the KOCKS rolling mill, the cooling rate in the cooling section is controlled at 1.5 °C / s to 2.2 °C / s.

[0056] After being drawn, cold upset or cold bent, and then subjected to stabilization treatment, the large-size non-quenched and tempered cold heading steel described in the present invention can be used to prepare 9.8-grade fasteners.

[0057] The reduction ratio of the drawing is controlled at 18% to 24%.

[0058] The conditions for the stabilization treatment are to keep warm at 150 °C to 350 °C for 1 h to 1.5 h.

[0059] The present invention will be described in detail below in conjunction with embodiments.

[0060] The composition and weight percentage 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 inevitable impurities.

[0061] Table 1

[0062] Steel grade C Si Mn Alt Nb Cr Mg Pr P S O G value A value D value A 0.10 1.2 0.4 0.040 0.050 0.4 0.001 0.0070 0.010 0.004 0.0009 0.187 0.048 0.058 B 0.30 0.95 0.34 0.045 0.041 0.35 0.003 0.0040 0.009 0.002 0.0010 0.174 0.059 0.040 C 0.22 1.1 0.50 0.056 0.044 0.30 0.002 0.0055 0.007 0.002 0.0007 0.205 0.049 0.034 D 0.15 1.15 0.30 0.060 0.030 0.50 0.0015 0.0064 0.012 0.002 0.0009 0.173 0.047 0.038 E 0.26 0.90 0.36 0.052 0.035 0.46 0.0024 0.0048 0.008 0.003 0.0012 0.170 0.058 0.054 F 0.19 0.97 0.43 0.041 0.038 0.34 0.0013 0.0059 0.009 0.001 0.0009 0.153 0.053 0.028 G 0.20 0.95 0.47 0.044 0.046 0.37 0.0023 0.0065 0.008 0.002 0.0011 0.183 0.068 0.042 H 0.13 1.03 0.39 0.055 0.033 0.44 0.0027 0.0044 0.005 0.004 0.0008 0.170 0.052 0.056 I 0.27 1.11 0.42 0.041 0.032 0.36 0.0023 0.0056 0.013 0.005 0.0009 0.135 0.056 0.068 J 0.17 1.20 0.39 0.060 0.039 0.42 0.0011 0.0042 0.011 0.003 0.0010 0.201 0.031 0.050 K 0.40 0.19 0.83 0.035 / 1.07 / / 0.011 0.004 0.0012 / / /

[0063] The production process parameters of the cold heading steel coil in each embodiment and comparative example are shown in Table 2.

[0064] Table 2 Production process parameters

[0065]

[0066]

[0067] The coils of cold heading steel in the above-mentioned embodiments and comparative examples are drawn, then cold upset or cold bent, and finally subjected to stabilization treatment to prepare fasteners. The processing process parameters of the fasteners are shown in Table 3.

[0068] Table 3

[0069]

[0070]

[0071] The performance detection method of hot-rolled heavy coil is as follows:

[0072] 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, and conduct microstructure evaluation and grain size level determination according to GB / T 13298 "Test Methods for Metallographic Microstructure of Metals" and GB / T 6394 "Determination Methods for Average Grain Size of Metals".

[0073] Measurement of inclusion size: Measure and statistically calculate the inclusion size on the longitudinal surface of the steel by ASPEX scanning electron microscope, and the detection area is 160 mm 2 .

[0074] Tensile properties: Take a 400-mm-long specimen from the wire rod, conduct a tensile test, and test the R m , A, and Z values.

[0075] Cold upsetting: The wire rod shall be cold upset according to the following requirements: X = h1 / h = 1 / 6; (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 upset test, no visible cracks, fissures, cracks, and hairline defects shall appear on the surface of the specimen. For a group of 10 specimens, all being qualified is regarded as qualified, otherwise calculate the qualification rate.

[0076] -40°C KV2 impact test: Machine a V-notch impact specimen of 10 mm × 10 mm × 55 mm, conduct a -40°C impact test according to GB / T 229 "Metallic Materials - Charpy Pendulum Impact Test Method" to obtain the impact toughness value, with three specimens in a group, and calculate the average value.

[0077] Notch sensitivity: Conduct a notch sensitivity test according to HB 5214-1996 "Test Methods for Notch Tensile Test of Metals at Room Temperature", and measure the notch sensitivity by the ratio of the tensile strength of the notched specimen to the tensile strength of the unnotched specimen, that is, the NSR value. The larger the NSR value, the lower the notch sensitivity. Since both this embodiment and the comparative example are plastic specimens, a notch strengthening effect is generated, so the NSR values are all > 1. The NSR value of the embodiment is ≥ 1.60, which is significantly better than the comparative sample, has good notch sensitivity, and is suitable for making bolts.

[0078] Axial tension-compression fatigue test: High-cycle tension-compression fatigue test was carried out in accordance with the standard of GB / T 3075 "Metallic materials - Fatigue testing - Axial force-controlled method". Axial stress control was adopted, the strain cycle ratio R was -1, the frequency was 101 - 118 Hz, the room temperature was 20 °C, the fatigue test loading waveform was sine wave, and the test end criterion was 10 7 times or specimen failure. 20 bolts were taken as a group to determine the axial tension-compression fatigue limit.

[0079] The detection methods for the properties of fasteners are as follows:

[0080] Mechanical properties: Mechanical property detection was carried out in accordance with GB-T 3098.1 "Mechanical properties of fasteners - Bolts, screws and studs", and the R m , Rp0.2, A, and Z values were measured and the yield ratio was calculated.

[0081] Fatigue test: Axial fatigue test was carried out on an MTS fatigue testing machine. Axial stress control was adopted, the strain cycle characteristic was R = -1, the control load was 120 KN, the frequency was 10 Hz, 20 bolts were taken as a group, and the test was considered successful if the number of cycles of 80% of the specimens exceeded 2.5 million times.

[0082] The properties of the large coil of cold heading steel in each example and comparative example are shown in Table 4, and the properties of the fasteners prepared therefrom are shown in Table 5.

[0083] Table 4

[0084]

[0085]

[0086] Table 5

[0087]

[0088] The chemical composition, production method and fastener processing method of the steel in Examples 1 to 8 were properly controlled, suitable for the production of large-sized hot-rolled large coils, obtaining an ultrafine grain structure, having high comprehensive strength and plasticity, high purity, excellent cold heading performance, drawing performance and fatigue performance, and good notch sensitivity, suitable for making various types of fasteners; the strength grade of the fasteners directly made from the hot-rolled large coils without annealing and quenching-tempering treatment reached 9.8 levels and had a high fatigue life.

[0089] Although the chemical compositions of Comparative Example 1 and Comparative Example 2 are within the required range, the production process control is improper. The feeding speeds of the Mg wire and the Pr wire in Comparative Example 1 are both too fast, the inclusions are not fully plasticized, the inclusion size is large, and the fatigue properties of the steel and the fasteners are poor. The KOCKS temperature in Comparative Example 2 is controlled too high, resulting in abnormal martensite structure in the steel, deteriorating the mechanical properties and fatigue properties of the steel, and the reduction of area rate of the processed fasteners is also low. In Comparative Example 3, when processing fasteners, the area reduction rate of the fine wire drawing is insufficient, the strength level of the fine wire does not reach 9.8 grade, and the fatigue life is low under the control load of 120 KN. The control of the fine grain coefficient G value in Comparative Example 4 is improper, resulting in the grain size of the steel not reaching above 12.5 grade, and the mechanical properties, fatigue properties and notch sensitivity are all insufficient. In Comparative Example 5, the purity coefficient D value does not meet the requirements, the inclusion control of the steel does not achieve the effect of the present invention, and the fatigue properties are poor. Comparative Example 6 is the market general steel ML40Cr, which is not suitable for the production of non-quenched and tempered fasteners. The reduction of area rate is low after drawing without annealing, and the fatigue life is also insufficient.

[0090] The detailed description of a large-size non-quenched and tempered cold heading steel, its production method, and the fasteners prepared therefrom and their production methods with reference to the embodiments above 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 large-sized non-quenched and tempered cold-heading steel, characterized in that, The cold heading steel contains by weight percentage: C 0.10% - 0.30%, Si 0.90% - 1.20%, Mn 0.30% - 0.50%, Al 0.040% - 0.060%, Nb 0.030% - 0.050%, Cr 0.30% - 0.50%, Mg 0.0010% - 0.0030%, Pr 0.0040% - 0.0070%, P ≤ 0.015%, S ≤ 0.015%, O ≤ 0.0015%, and the balance is Fe and other inevitable impurities; Among them, the G value ≥ 0.15, G = 1.9*(%Nb) + 1.3*(%Al) + 25.9*(%Nb)*(%Al) - 0.03*(%Mn); The A value ≥ 0.047, A = (11.9*(%Mg) + 7.3*(%Pr)) / (0.9*(%Si) + 5.6*(%Al)); The D value ≤ 0.07, D = 10.0*(%S) + 19.9*(%O).

2. The large-size non-quenched cold-heading steel according to claim 1, characterized in that The large - sized non - quenched and tempered cold heading steel coil has a specification of Φ20 - 50mm; its metallographic structure is ultrafine - grained ferrite + pearlite structure, and the ferrite grain size is above grade 12.

5.

3. The large-sized non-quenched cold-heading steel according to claim 1, characterized in that, The R of the large-sized non-quenched and tempered cold-heading steel hot-rolled coil m is 740~780 MPa, Z≥67%, notch sensitivity NSR value≥1.60, axial tension-compression fatigue limit≥450 MPa, and KV2 impact toughness value≥240 J at -40°C; all inclusions in the cold-heading steel are plastic inclusions with inclusion size below 6.5 μm.

4. The production method of large-size non-quenched cold-heading steel according to any one of claims 1-3, characterized in that, The production method includes the following steps: hot metal pretreatment → converter smelting → LF furnace refining → continuous casting → billet heating → KOCKS rolling of large coils → coiling → controlled cooling → finished hot - rolled large coils.

5. The production method according to claim 4, 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.

6. The production method according to claim 4, characterized in that, In the LF furnace refining step, the LF refining time ≥ 30min, and the white slag holding time ≥ 20min; after alloying, feed Mg wire, and the wire - feeding speed is controlled at 30m / min - 40m / min.

7. The production method according to claim 4, characterized in that, In the continuous casting step, add Pr wire to the center of the mold, the wire - feeding speed is controlled at 15m / min - 30m / min, and in addition, the ratio of the wire - feeding speed to the casting speed is maintained at 8 - 15.

8. The production method according to claim 4, characterized in that, In the billet heating step: the temperature of the preheating section is controlled at 550 - 600°C, the heating section is controlled at 750 - 820°C, and the soaking temperature is controlled at 1120 - 1200°C.

9. The production method according to claim 4, characterized in that, In the KOCKS rolling step: control the temperature entering KOCKS at 790 - 810°C and the temperature leaving KOCKS at 730 - 760°C.

10. The production method according to claim 4, characterized in that, In the controlled cooling step, the cooling rate is controlled at 1.5°C / s - 2.2°C / s.

11. A fastener, characterized in that, It is obtained by stabilization treatment after drawing, cold heading or cold bending of the large - sized non - quenched and tempered cold heading steel described in any one of claims 1 - 3.

12. The fastener according to claim 11, characterized in that, The reduction of area rate of the drawing is controlled at 18% - 24%.

13. The fastener according to claim 11, wherein The conditions of the stabilization treatment are heat preservation at 150°C - 350°C for 1h - 1.5h.

14. The fastener according to claim 11, wherein The mechanical properties of the fastener reach grade 9.8, and the number of axial fatigue cycles under a 120KN load > 2.5 million times.

15. The preparation method of the fastener according to any one of claims 11-14, characterized in that, The preparation method includes the following steps: drawing the large - sized non - quenched and tempered cold heading steel coil described in any one of claims 1 - 3, then cold heading or cold bending, and finally stabilization treatment.

Images