Wire rod for fastener, ultrahigh-strength fastener with low yield ratio and manufacturing method of ultrahigh-strength fastener

Abstract

The invention discloses a wire rod for a fastener, which comprises Fe, inevitable impurities, and the following chemical elements in percentage by mass: 0.30 to 0.50 percent of C (carbon), 0.30 to 1.50 percent of Mn (manganese), 0.40 to 0.80 percent of Ni (nickel), 0.80 to 1.40 percent of Cr (chromium), 0.80 to 1.40 percent of Mo (molybdenum), 0.05 to 0.20 percent of Ti (titanium), 0.50 to 1.30 percent of Cu (copper), 0.50 to 1.50 percent of Co (cobalt) and 0.002 to 0.010 percent of Ca (calcium). Correspondingly, the invention further discloses a manufacturing method of the wire rod, a fastener manufactured through the wire rod and a manufacturing method of the fastener. By adopting the method, the ultrahigh-strength fastener with low yield ratio can be obtained.

Description

Technical Field

[0001] The invention relates to a steel material and a preparation method thereof, and in particular to a fastener wire rod and a preparation method thereof. Background Art

[0002] Ultra-high strength fasteners can reduce weight and increase installation space by reducing their own size under the same clamping force, so the function and volume of the connected parts can be optimized, so that the equipment can achieve the goal of overall weight reduction and performance optimization. Therefore, ultra-high strength fastener steel is an important research direction in the current steel field.

[0003] For example, the Chinese patent document with publication number CN1900343A, publication date January 24, 2007, and titled "Method for manufacturing steel with excellent delayed fracture resistance and tensile strength of 1600 MPa or more and its molded products", discloses a method for manufacturing steel with excellent delayed fracture resistance and tensile strength of 1600 MPa or more and its molded products, which adds 3.0-10.0% Mo to precipitate carbides to produce significant secondary hardening, thereby improving the strength of the steel. The tensile strength of the steel produced by this method reaches 1600 MPa, but the Mo content in the steel is relatively high.

[0004] For another example, the Chinese patent document with publication number CN1900344A, publication date January 24, 2007, and titled “High-strength bolts with excellent delayed fracture resistance and manufacturing method thereof” discloses high-strength bolts with excellent delayed fracture resistance and manufacturing method thereof. The bolt head and shaft are formed from steel, and then heated to 900-1100°C and quenched, and then tempered at 580°C or above. The maximum strength can reach 1864MPa. However, the patent does not make any requirements on the yield strength ratio of the material.

[0005] For example, the Chinese patent document with the publication number CN112981277A and the publication date of June 18, 2021, entitled "A method for preparing ultra-high strength medium-carbon nano-bainite steel", discloses a method for preparing ultra-high strength medium-carbon nano-bainite steel, which is to roll the medium-carbon nano-bainite steel after full austenitization and low-temperature bainite phase transformation at room temperature, wherein the single-pass reduction should be controlled at more than 4%, the cumulative reduction is at least 15%, and then medium-temperature tempering is performed. The patent does not make any requirements on the yield strength ratio of the material.

[0006] It can be seen that the existing technology mainly focuses on the tensile strength of the material, but not enough attention is paid to the yield strength ratio, which makes it difficult for the invented material to meet the tightening process requirements of high-end bolts. Summary of the invention

[0007] One of the purposes of the present invention is to provide a wire rod for fasteners, which can obtain hot-rolled wire rod for fasteners with excellent mechanical properties through reasonable chemical composition design and manufacturing process, and after adopting isothermal heat treatment process, ultra-high strength fasteners with low yield strength ratio can be obtained.

[0008] In order to achieve the above object, the present invention provides a wire rod for fasteners, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0009] C: 0.30~0.50wt.%, Mn: 0.30~1.50wt.%, Ni: 0.40~0.80wt.%, Cr: 0.80~1.40wt.%, Mo: 0.80~ 1.40wt.%, Ti: 0.05~0.20wt.%, Cu: 0.50~1.30wt%, Co: 0.50~1.50wt%, Ca: 0.002~0.010wt%.

[0010] Accordingly, the present invention also provides a wire rod for fasteners, wherein the mass percentage of each chemical element is:

[0011] C: 0.30~0.50wt.%, Mn: 0.30~1.50wt.%, Ni: 0.40~0.80wt.%, Cr: 0.80~1.40wt.%, Mo: 0.80~1.40wt.%, Ti: 0.05~0.20wt.%, Cu: 0.50~1.30wt%, Co: 0.50~1.50wt%, Ca: 0.002~0.010wt%; the balance is Fe and unavoidable impurities.

[0012] In the fastener wire rod of the present invention, the design principles of the chemical elements are as follows:

[0013] C: In the wire rod for fasteners described in the present invention, C is an effective element for obtaining strength. Adding an appropriate amount of C element in steel is beneficial to the strength of the steel. However, it should be noted that the C element content in the steel should not be too high. When the C element content in the steel is too high, it will have an adverse effect on the cold heading property, toughness and delayed fracture resistance of the steel. Based on this, considering the strength and cold heading property, toughness, delayed fracture resistance and other comprehensive considerations of the material, in the present invention, the mass percentage of C element can be controlled at 0.30-0.50wt.%.

[0014] Mn: In the wire rod for fasteners described in the present invention, Mn is an important element for stabilizing the austenite phase, and on the other hand, it fixes S in the steel in the form of MnS, thereby preventing hot brittleness. If the Mn content is too high, it is easy to cause segregation at the grain boundary and reduce the grain boundary strength. Based on this, in the present invention, the mass percentage of the Mn element is controlled to be 0.30-1.50wt.%.

[0015] Ni: In the wire rod for fasteners described in the present invention, Ni has the functions of stabilizing austenite, increasing hardenability, improving low-temperature toughness, etc. In addition, the Ni element can improve the structure of the iron oxide layer, increase density, improve the corrosion resistance of steel, and inhibit the adsorption of hydrogen, which has a positive effect on the delayed fracture resistance of steel. Therefore, considering the cost factor comprehensively, in the present invention, the mass percentage of Ni can be controlled to 0.40-0.80wt.%.

[0016] Cr: In the wire rod for fasteners described in the present invention, Cr has the effect of improving the hardenability and corrosion resistance of steel, and plays an effective role in the strength and delayed fracture resistance of steel. However, it should be noted that the Cr content in the steel should not be too high. When the Cr content in the steel is too high, the cold working performance of the steel will be reduced and the production cost will increase. Based on this, in the present invention, the mass percentage of the Cr element is controlled to 0.80-1.40wt.%.

[0017] Mo: In the wire rod for fasteners described in the present invention, Mo mainly precipitates Mo-containing carbides during the heat treatment stage, thereby producing significant secondary strengthening. However, it should be noted that excessive Mo should not be added to the steel, as adding too much Mo to the steel will increase the material cost. Therefore, considering the cost factor comprehensively, in the present invention, the mass percentage of the Mo element can be controlled within 0.80 to 1.40 wt.%.

[0018] Ti: In the wire rod for fasteners described in the present invention, Ti forms precipitates with C and N in the steel, which can refine the austenite grains during the heating stage of the steel, thereby improving the strength of the steel plate. However, too much Ti will form coarse inclusions, which is detrimental to the performance of the steel. Therefore, in the present invention, the mass percentage of the Ti element is controlled to be 0.05-0.20wt.%.

[0019] Cu: In the wire rod for fasteners described in the present invention, Cu can improve corrosion resistance and inhibit hydrogen intrusion, thereby further improving the delayed fracture resistance of steel. When the content of Cu element is lower than 0.5%, the above effect cannot be achieved. However, if the Cu content is too high, the high-temperature plasticity of the steel will be reduced, and cracks will be easily generated during hot working. Therefore, the Cu content is controlled at 0.50-1.30wt%.

[0020] Co: In the wire rod for fasteners of the present invention, Co as a core element can increase the phase transition free energy difference, promote bainite phase transition, and shift the phase transition kinetic curve to the left, thereby achieving the purpose of shortening the bainite phase transition time. Based on this, the present invention controls the Co content to 0.50-1.50wt%.

[0021] Ca: In the wire rod for fasteners of the present invention, the Ca element improves the fluidity of molten steel, changes the composition, quantity and form of non-metallic inclusions, improves the purity of steel, and also plays a role in desulfurization. Based on this, the present invention controls the Ca content to 0.002-0.010wt%.

[0022] Furthermore, in a preferred embodiment of the present invention, the wire rod for fasteners further contains 0<Si≤0.20wt.%.

[0023] In the preferred technical solution of the present invention, Si is used to inhibit the precipitation of cementite in bainite steel and can also be dissolved in ferrite, thereby playing a role of solid solution strengthening, thereby improving the strength and hardness of the steel plate; but if the Si content is too high, the plasticity and toughness of the steel will be significantly reduced. Based on this, in the preferred embodiment of the present invention, the Si element is added and its upper limit is controlled to 0.20wt.%.

[0024] Furthermore, among the inevitable impurities in the wire rod for fasteners described in the present invention: P≤0.015wt.%, S≤0.015wt.%.

[0025] The impurity elements in the wire rod for fasteners of the present invention are mainly P and S. Under the condition that technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in the steel should be reduced as much as possible. Among them, the impurity elements P and S are easy to segregate at the grain boundaries, reduce the toughness of the steel, and have a great influence on the cold working performance of the steel. Therefore, in some embodiments, P≤0.015wt.% and S≤0.015wt.% can be controlled.

[0026] Furthermore, the microstructure of the wire rod for fasteners described in the present invention is martensite+bainite+pearlite.

[0027] Furthermore, the volume phase ratio of martensite is 5-30%.

[0028] Another object of the present invention is to provide an ultra-high strength fastener with a low yield ratio, which is made by using the wire rod for fasteners described above and undergoing an isothermal heat treatment process. It has a low yield ratio and ultra-high strength, and can be effectively used in bolts and related products in the fields of vehicles and ships, construction engineering, energy, bridges and transportation, and has very good promotion prospects and application value.

[0029] Based on the above purpose, the present invention provides an ultra-high strength fastener with a low yield ratio, which is made by isothermal heat treatment of the wire rod for fasteners described above.

[0030] Furthermore, the microstructure of the ultra-high strength fastener with low yield ratio described in the present invention is martensite+bainite.

[0031] Furthermore, the ultra-high strength fastener with low yield strength ratio described in the present invention has a tensile strength of ≥1800MPa and a yield strength ratio of 0.8 to 0.9.

[0032] Another object of the present invention is to provide a method for manufacturing wire rod for fasteners.

[0033] Based on the above object, the present invention also provides a method for manufacturing the wire rod for fasteners as described above, which comprises the steps of:

[0034] smelting and casting;

[0035] Rolled into wire rod;

[0036] Use the Stelmore air cooling line for slow cooling with a cooling rate of ≤5℃ / s.

[0037] In the manufacturing method of the wire rod for fasteners of the present invention, after being rolled into the wire rod, it is slowly cooled through the Stelmor air cooling line because the wire rod has a high hardenability and a faster cooling rate will easily produce more abnormal martensitic structures.

[0038] Furthermore, in the method for manufacturing wire rod for fasteners described in the present invention, in the step of rolling into wire rod, the starting rolling temperature is controlled to be 1090-1180°C, the inlet temperature of the finishing mill is 980-1030°C, the inlet temperature of the sizing mill is 900-950°C, and the spinning temperature is 840-880°C.

[0039] Another object of the present invention is to provide a method for manufacturing ultra-high strength fasteners with a low yield ratio. The manufacturing method is simple to operate and can obtain ultra-high strength fasteners with a low yield ratio, which can be effectively applied to bolts and related products in the fields of vehicles and ships, construction engineering, energy, bridges and transportation.

[0040] Based on the above purpose, the present invention also provides a method for manufacturing an ultra-high strength fastener with a low yield ratio as described above, which comprises the steps of:

[0041] smelting and casting;

[0042] Rolled into wire rod;

[0043] Use Stelmore air cooling line for slow cooling with a cooling rate of ≤5℃ / s;

[0044] Isothermal spheroidizing annealing;

[0045] Parts shape processing;

[0046] Isothermal heat treatment: heat to austenitizing temperature of 860-940°C, keep warm for 30-90 minutes, then cool to 250-400°C at a rate of 15-30°C / s, and keep warm for 60-180 minutes.

[0047] In the manufacturing method of the fastener described in the present invention, based on the prepared wire rod, the present invention uses isothermal spheroidizing annealing treatment to optimize the organizational properties of the wire rod to improve the plasticity of the material and prevent quality problems such as cracking and surface roughness during the part processing and forming process. After the part shape processing is completed, a specially designed isothermal heat treatment is also used to obtain a fastener with a tensile strength of ≥1800MPa and a yield strength ratio of 0.8 to 0.9.

[0048] Furthermore, in the manufacturing method of the fastener described in the present invention, in the step of rolling into wire rod, the starting rolling temperature is controlled to be 1090-1180°C, the inlet temperature of the finishing mill is 980-1030°C, the inlet temperature of the sizing mill is 900-950°C, and the spinning temperature is 840-880°C.

[0049] In the isothermal heat treatment step, the isothermal medium may be a molten salt solution. The quenching cooling medium may also be a molten salt solution.

[0050] The wire rod for fasteners and the fasteners of the present invention have the following advantages and beneficial effects:

[0051] The present invention can obtain an ultra-high strength fastener with a low yield ratio by reasonably designing the chemical composition, especially accurately controlling the contents of C, Si, Mn, Ni, Cr, Mo, Cu, Co and Ca, and cooperating with an optimized isothermal heat treatment process.

[0052] In some embodiments, the tensile strength of the fastener of the present invention is ≥1800 MPa, and the yield strength ratio is 0.8-0.9.

[0053] The fastener described in the present invention has a low alloy cost and excellent strength and plasticity. Under the premise of tensile strength ≥1800MPa, it can achieve a yield strength ratio of 0.8 to 0.9 and has a wider plastic deformation zone, which is beneficial to controlling the tightening force during bolt assembly. Therefore, it can be effectively applied to bolts and related products in the fields of vehicles and ships, construction engineering, energy, bridges and transportation, and has very good promotion prospects and application value. DETAILED DESCRIPTION

[0054] The wire rod for fasteners, the fasteners and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0055] Examples 1-6

[0056] The fastener wire rods of Examples 1-6 are all prepared by the following steps:

[0057] (1) Smelting, secondary refining and casting to obtain a cast billet. Table 1 lists the mass percentage ratios of the chemical elements in various embodiments of the present invention.

[0058] (2) Rolling into wire rod: Control the starting rolling temperature to 1090-1180°C, the finishing mill inlet temperature to 980-1030°C, the sizing mill inlet temperature to 900-950°C, the spinning temperature to 840-880°C, and the wire rod specification to φ5.5-20mm.

[0059] (3) After rolling, the wire rod is slowly cooled by a Stelmor air cooling line at a cooling rate of ≤5°C / s to cool to room temperature.

[0060] In some embodiments, when the wire rod passes through the Stelmore air cooling line, all fans can be turned off and a heat preservation cover can be put on to achieve slow cooling.

[0061] The fasteners of Examples 1-6 are prepared based on the wire rods obtained in each Example by continuing to adopt the following steps:

[0062] (4) Isothermal spheroidizing annealing.

[0063] (5) The annealed wire rod is subjected to the processes of drawing, cold heading, turning and milling to perform part shape processing.

[0064] (6) Isothermal heat treatment: first heat to the austenitizing temperature of 860-940°C, keep warm for 30-90 minutes, and then cool to 250-400°C at a rate of 15-30°C / s, and keep warm for 60-180 minutes. In the above cooling and isothermal operations, the isothermal and cooling medium uses molten salt liquid to ensure rapid cooling of the parts and precise temperature control.

[0065] Table 1 lists the mass percentages of the chemical elements in Examples 1-6.

[0066] Table 1. (wt%, the balance is Fe and other inevitable impurities except P and S)

[0067]

[0068] Table 2 lists the specific process parameters for manufacturing wire rods of Examples 1-6.

[0069] Table 2.

[0070]

[0071] Table 3 lists the specific process parameters of the isothermal heat treatment for manufacturing the fasteners of Examples 1-6.

[0072] Table 3.

[0073]

[0074] The wire rods of Examples 1-6 and the final fasteners were sampled to observe and analyze their microstructures. The relevant observation and analysis results are listed in Table 4 below.

[0075] Table 4 lists the microstructural characteristics of the wire rods and fasteners of Examples 1-6.

[0076] Table 4.

[0077]

[0078] In addition, in order to verify the mechanical properties of the fasteners of each embodiment, the inventors sampled the fasteners of embodiments 1-6 obtained through the above process steps again, and tested the mechanical properties of the fasteners of each embodiment, and the obtained mechanical properties test results are listed in Table 5. The specific testing method is as follows:

[0079] Tensile test: The test is carried out in accordance with the national standard GB / T 228.1-2010 "Metallic material tensile test Part 1: Room temperature test method", and the test specimen is processed into ASTM American M14 thread tensile specimen, and the test environment is room temperature 10 ~ 35 ℃.

[0080] Table 5 lists the performance test results of the fasteners of Examples 1-6.

[0081] Table 5.

[0082] serial number Yield strength(MPa) Tensile strength(MPa) Yield-to-strength ratio Example 1 1529 1820 0.84 Example 2 1606 1846 0.87 Example 3 1588 1868 0.85 Example 4 1554 1872 0.83 Example 5 1631 1896 0.86 Example 6 1580 1859 0.85

[0083] It can be seen from Table 5 that the tensile strength of the fasteners of Examples 1-6 of the present invention is all above 1800 MPa, the yield strength is all above 1520 MPa, and the yield strength ratio is between 0.8 and 0.9. Therefore, it is beneficial to control the tightening force during bolt assembly, and can be applied to bolts and related products in the fields of vehicles, ships, construction engineering, energy, bridges, transportation, etc.

[0084] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0085] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A wire rod for fasteners, comprising Fe and unavoidable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: C: 0.30~0.50wt.%, Mn: 0.30~1.50wt.%, Ni: 0.40~0.80wt.%, Cr: 0.80~1.40wt.%, Mo: 0.80~ 1.40wt.%, Ti: 0.05~0.20wt.%, Cu: 0.50~1.30wt%, Co: 0.50~1.50wt%, Ca: 0.002~0.010wt%.

2. The wire rod for fasteners according to claim 1, characterized in that: The mass percentage of each chemical element is: C: 0.30~0.50wt.%, Mn: 0.30~1.50wt.%, Ni: 0.40~0.80wt.%, Cr: 0.80~1.40wt.%, Mo: 0.80~1.40wt.%, Ti: 0.05~0.20wt.%, Cu: 0.50~1.30wt%, Co: 0.50~1.50wt%, Ca: 0.002~0.010wt%; the balance is Fe and unavoidable impurities.

3. The wire rod for fasteners according to claim 1 or 2, characterized in that: It also contains 0<Si≤0.20 wt.%.

4. The wire rod for fasteners according to claim 1 or 2, characterized in that: Among the inevitable impurities: P ≤ 0.015 wt.%, S ≤ 0.015 wt.%.

5. The wire rod for fasteners according to claim 1 or 2, characterized in that: Its microstructure is martensite + bainite + pearlite.

6. The wire rod for fasteners according to claim 5, characterized in that: The volume phase ratio of martensite is 5 to 30%.

7. An ultra-high strength fastener with a low yield-to-strength ratio, characterized in that: The wire rod is prepared by isothermal heat treatment according to any one of claims 1 to 6.

8. The ultra-high strength fastener with low yield ratio as claimed in claim 7, characterized in that: Its microstructure is martensite + bainite.

9. The ultra-high strength fastener with low yield ratio according to claim 7, characterized in that: Its tensile strength is ≥1800MPa and its yield strength ratio is 0.8-0.

9.

10. The method for manufacturing a wire rod for a fastener according to any one of claims 1 to 6, characterized in that: It includes the steps of: smelting and casting; Rolled into wire rod; Use the Stelmore air cooling line for slow cooling with a cooling rate of ≤5℃ / s.

11. The manufacturing method according to claim 10, characterized in that: In the step of rolling into wire rod, the starting rolling temperature is controlled to be 1090-1180°C, the inlet temperature of the finishing mill is 980-1030°C, the inlet temperature of the sizing mill is 900-950°C, and the spinning temperature is 840-880°C.

12. The method for manufacturing a low yield ratio ultra-high strength fastener according to any one of claims 7 to 9, characterized in that: Includes steps: smelting and casting; Rolled into wire rod; Use Stelmore air cooling line for slow cooling with a cooling rate of ≤5℃ / s; Isothermal spheroidizing annealing; Parts shape processing; Isothermal heat treatment: heat to the austenitizing temperature of 860-940°C, keep warm for 30-90 minutes, then cool to 250-400°C at a rate of 15-30°C / s, and keep warm for 60-180 minutes.

13. The manufacturing method according to claim 12, characterized in that: In the step of rolling into wire rod, the starting rolling temperature is controlled to be 1090-1180°C, the inlet temperature of the finishing mill is 980-1030°C, the inlet temperature of the sizing mill is 900-950°C, and the spinning temperature is 840-880°C.

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