A wire rod for fasteners, an ultra-high strength fastener with low yield ratio, and a method of manufacturing the same

Abstract

Disclosed is a wire rod for fasteners, which contains Fe and inevitable impurities, and the following chemical elements in mass percentage: C: 0.30-0.50 wt.%, Mn: 0.30-1.50 wt.%, Ni: 0.40-0.80 wt.%, Cr: 0.80-1.40 wt.%, Mo: 0.80-1.40 wt.%, Ti: 0.05-0.20 wt.%, Cu: 0.50-1.30 wt.%, Co: 0.50-1.50 wt.%, and Ca: 0.002-0.010 wt.%. Correspondingly, the application also discloses a manufacturing method of the wire rod, a fastener manufactured by using the wire rod and a manufacturing method of the fastener. The application can obtain an ultra-high strength fastener with low yield ratio.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of steel and its preparation method, and more particularly to a kind of fastener wire rod and its preparation method. BACKGROUND

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

[0003] For example, the Chinese patent document with publication number CN1900343A and publication date January 24, 2007, entitled "Manufacturing method of steel with tensile strength of 1600 MPa or above and its formed product with excellent delayed fracture resistance" discloses a manufacturing method of steel with tensile strength of 1600 MPa or above and its formed product with excellent delayed fracture resistance. By adding 3.0-10.0% Mo to precipitate carbides, significant secondary hardening is produced, thereby improving the strength of the steel. The steel produced by this method has a tensile strength of 1600 MPa, but the Mo content in the steel is too high.

[0004] For another example, the Chinese patent document with publication number CN1900344A and publication date January 24, 2007, entitled "High-strength bolt with excellent delayed fracture resistance and manufacturing method thereof" discloses a high-strength bolt with excellent delayed fracture resistance and a manufacturing method thereof. The steel material is formed into a bolt head and a shaft, then heated to 900-1100°C and quenched, and then tempered at a temperature of 580°C or above, with a maximum strength of 1864 MPa. However, this patent does not require the material to have a yield strength ratio.

[0005] For another example, the Chinese patent document with publication number CN112981277A and publication date June 18, 2021, entitled "Preparation method of ultra-high strength medium carbon nanobainite steel" discloses a preparation method of ultra-high strength medium carbon nanobainite steel. The medium carbon nanobainite steel after complete austenitization and low-temperature bainite transformation is subjected to room temperature rolling deformation, wherein the single pass reduction should be controlled at more than 4%, and the cumulative reduction is at least 15%, and then subjected to medium temperature tempering treatment. This patent does not require the material to have a yield strength ratio.

[0006] It can be seen that the existing technology mainly focuses on the tensile strength of the material, and the yield strength ratio is not emphasized enough, resulting in the difficulty of the invented material to meet the tightening process requirements of high-end bolts. SUMMARY

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

[0008] In order to achieve the above-mentioned purposes, the present application provides a wire rod for fastener, which contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows:

[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] Correspondingly, the present application also provides a wire rod for fastener, which contains the following chemical elements with mass percentage as follows:

[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 inevitable impurities.

[0012] In the wire rod for fastener described in the present application, the design principles of each chemical element are as follows:

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

[0014] Mn: In the fastener wire rod described in this invention, Mn is an important element for stabilizing the austenite phase, and it also helps to prevent hot brittleness by fixing sulfur in the steel through the formation of MnS. If the Mn content is too high, it can easily cause grain boundary segregation and reduce grain boundary strength. Therefore, in this invention, the mass percentage of Mn is controlled between 0.30 and 1.50 wt.%.

[0015] Ni: In the fastener wire rod described in this invention, Ni has the functions of stabilizing austenite, increasing hardenability, and improving low-temperature toughness. In addition, Ni can improve the iron oxide layer structure, increase density, improve the corrosion resistance of steel, and inhibit hydrogen adsorption, thus having a positive effect on the delayed fracture resistance of steel. Therefore, taking into account cost factors, in this invention, the mass percentage of Ni can be controlled at 0.40-0.80 wt.%.

[0016] Cr: In the fastener wire rod described in this invention, Cr improves the hardenability and corrosion resistance of the steel, and plays an effective role in the strength and delayed fracture resistance of the 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, it will lead to a decrease in the cold working performance of the steel and an increase in production costs. Based on this, in this invention, the mass percentage of Cr is controlled between 0.80 and 1.40 wt.%.

[0017] Mo: In the fastener wire rod described in this 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 this will increase material costs. Therefore, considering cost factors, the mass percentage of Mo in this invention can be controlled between 0.80 and 1.40 wt.%.

[0018] Ti: In the fastener wire rod described in this invention, Ti forms precipitates with C and N in the steel. During the heating stage of the steel, Ti can refine the austenite grains, thereby improving the strength of the steel plate. However, excessive Ti will form coarse inclusions, which is detrimental to the performance of the steel. Therefore, in this invention, the mass percentage content of Ti is controlled at 0.05–0.20 wt.%.

[0019] Cu: In the fastener wire rod described in this invention, Cu can improve corrosion resistance and inhibit hydrogen intrusion, thereby further improving the steel's resistance to delayed fracture. When the Cu content is less than 0.5%, the above effect cannot be achieved. However, if the Cu content is too high, it will reduce the high-temperature plasticity of the steel and make it prone to cracking during hot working. Therefore, the Cu content is controlled at 0.50 to 1.30 wt%.

[0020] Co: In the fastener wire rod described in this invention, Co, as a core element, can increase the phase transformation free energy difference, promote bainitic phase transformation, and shift the phase transformation kinetic curve to the left, thereby shortening the bainitic phase transformation time. Based on this, the Co content is controlled at 0.50–1.50 wt%.

[0021] Ca: In the fastener wire rod described in this invention, Ca element improves the fluidity of molten steel, alters the composition, quantity, and morphology of non-metallic inclusions, improves the purity of the steel, and also plays a role in desulfurization. Based on this, the Ca content is controlled at 0.002–0.010 wt%.

[0022] Furthermore, in a preferred embodiment of the present invention, the fastener bar also contains 0 < Si ≤ 0.20 wt.%.

[0023] In a preferred embodiment of the present invention, Si is used to suppress the precipitation of cementite in bainitic steel and can also dissolve in ferrite, thereby playing a role in solid solution strengthening and improving the strength and hardness of the steel plate; however, excessive Si content will significantly reduce the plasticity and toughness of the steel. Based on this, in a preferred embodiment of the present invention, Si is added and its upper limit is controlled to 0.20 wt.%.

[0024] Furthermore, in the unavoidable impurities of the fastener wire rods described in this invention: P ≤ 0.015 wt.%, S ≤ 0.015 wt.%.

[0025] The impurity elements in the fastener wire rods described in this invention are mainly phosphorus (P) and sulfur (S). Where technical conditions permit, the content of impurity elements in the steel should be reduced as much as possible to obtain steel with better performance and higher quality. Both P and S impurity elements tend to segregate at grain boundaries, reducing the toughness of the steel and significantly affecting its cold working properties. Therefore, in some embodiments, P ≤ 0.015 wt.% and S ≤ 0.015 wt.% can be controlled.

[0026] Furthermore, the microstructure of the fastener wire rod described in this invention is martensite + bainite + pearlite.

[0027] Furthermore, the volumetric proportion of martensite is 5-30%.

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

[0029] To achieve the above objectives, the present invention provides an ultra-high strength fastener with a low yield strength ratio, which is made by isothermal heat treatment of the fastener wire rods described above.

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

[0031] Furthermore, the ultra-high strength fastener with low yield strength ratio described in this invention has a tensile strength ≥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 fastener strips.

[0033] To achieve the above objectives, the present invention also provides a method for manufacturing fastener wire rod as described above, comprising the steps of:

[0034] Smelting and casting;

[0035] Rolled into wire rod;

[0036] The Stellmore air-cooled line is used for slow cooling with a cooling rate of ≤5℃ / s.

[0037] In the method for manufacturing fastener wire rods described in this invention, after being rolled into wire rods, they are slowly cooled by a Stellmore air-cooling line because wire rods have high hardenability, and if the cooling rate is too fast, more abnormal martensite structures are likely to be generated.

[0038] Furthermore, in the method for manufacturing fastener wire rods according to the present invention, in the step of rolling into wire rods, the initial 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 reducing mill is 900-950°C, and the wire drawing temperature is 840-880°C.

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

[0040] To achieve the above objectives, the present invention also provides a method for manufacturing an ultra-high strength fastener with a low yield strength ratio as described above, comprising the steps of:

[0041] Smelting and casting;

[0042] Rolled into wire rod;

[0043] Slow cooling with a cooling rate of ≤5℃ / s is achieved using the Stellmore air-cooling line;

[0044] Isothermal spheroidizing annealing;

[0045] Parts external machining;

[0046] Isothermal heat treatment: Heat to the austenitizing temperature of 860-940℃, hold for 30-90 min, then cool to 250-400℃ at a rate of 15-30℃ / s, and hold for 60-180 min.

[0047] In the fastener manufacturing method described in this invention, based on the obtained wire rod, isothermal spheroidizing annealing is employed to optimize the microstructure and properties of the wire rod, thereby improving material plasticity and preventing quality problems such as cracking and surface roughness during part processing. After the part's shape is machined, a specially designed isothermal heat treatment is also used to obtain fasteners with a tensile strength ≥1800MPa and a yield strength ratio of 0.8 to 0.9.

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

[0049] In the isothermal heat treatment step, molten salt solution can be used as the isothermal medium. Molten salt solution can also be used as the quenching cooling medium.

[0050] The fastener wire rod and fastener described in this invention have the following advantages and beneficial effects:

[0051] This invention, through the rational design of chemical composition, especially the precise control of the contents of C, Si, Mn, Ni, Cr, Mo, Cu, Co, and Ca, combined with an optimized isothermal heat treatment process, can obtain an ultra-high strength fastener with a low yield strength ratio.

[0052] In some embodiments, the fasteners of the present invention have a tensile strength ≥1800MPa and a yield strength ratio of 0.8 to 0.9.

[0053] The fasteners described in this invention have low alloy costs and excellent strength and plasticity. Under the premise of tensile strength ≥1800MPa, they can achieve a yield strength ratio of 0.8 to 0.9 and have a wider plastic deformation range, which is beneficial for controlling the tightening force during bolt assembly. Therefore, they can be effectively applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges and transportation, and have very good promotion prospects and application value. Detailed Implementation

[0054] The following will further explain and illustrate the fastener wire rod, fastener and manufacturing method of the present invention with reference to specific embodiments. However, such explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0055] Examples 1-6

[0056] The fastener strips in Examples 1-6 were all prepared using the following steps:

[0057] (1) Smelting, secondary refining and casting to obtain a billet. Table 1 lists the mass percentage of each chemical element in each embodiment of the present invention.

[0058] (2) Rolling into wire rod: control the initial rolling temperature to be 1090~1180℃, the finishing mill inlet temperature to be 980~1030℃, the sizing mill inlet temperature to be 900~950℃, the wire drawing temperature to be 840~880℃, and the wire rod specifications to be φ5.5~20mm.

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

[0060] In some implementations, all fans can be turned off and an insulation cover placed over the wire rod as it passes through the Stellmore air-cooled line to achieve slow cooling.

[0061] The fasteners of Examples 1-6 are further manufactured using the following steps based on the wire rods obtained in each example:

[0062] (4) Isothermal spheroidizing annealing.

[0063] (5) The annealed wire rod is processed by drawing, cold heading, turning and milling to shape the part.

[0064] (6) Isothermal heat treatment: First, heat to the austenitizing temperature of 860-940℃ and hold for 30-90 minutes, then cool to 250-400℃ at a rate of 15-30℃ / s and hold for 60-180 minutes. In the above cooling and isothermal operations, molten salt solution is used as the isothermal and cooling medium to ensure rapid cooling of the parts and precise temperature control.

[0065] Table 1 lists the mass percentage of each chemical element in Examples 1-6.

[0066] Table 1. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0067]

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

[0069] Table 2.

[0070]

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

[0072] Table 3.

[0073]

[0074] Samples of the wire rods and the final fasteners from Examples 1-6 were taken for observation and analysis of their microstructure. The relevant observation and analysis results are listed in Table 4 below.

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

[0076] Table 4.

[0077]

[0078] Furthermore, to verify the mechanical properties of the fasteners in each embodiment, the inventors resampled the fasteners of Embodiments 1-6 obtained through the above-described process steps and conducted mechanical property tests on the fasteners of each embodiment. The results of the mechanical property tests are listed in Table 5. The specific testing methods are as follows:

[0079] Tensile test: The test was conducted according to the national standard GB / T 228.1-2010 "Metallic materials, tensile testing - Part 1: Test at room temperature". The specimens were fabricated into ASTM M14 threaded tensile test specimens, and the test environment was room temperature (10–35°C).

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

[0081] Table 5.

[0082] No. Yield strength (MPa) Tensile strength (MPa) Yield 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] As can be seen from Table 5, the fasteners of Examples 1-6 of the present invention all have tensile strengths above 1800 MPa, yield strengths above 1520 MPa, and yield strength ratios between 0.8 and 0.9. Therefore, they are beneficial for controlling tightening force during bolt assembly and can be applied to bolts and related products in fields such as vehicles, ships, construction engineering, energy, bridges, and transportation.

[0084] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0085] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A fastener strip, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.30~0.50 wt.%, Mn: 0.30~1.50 wt.%, Ni: 0.40~0.80 wt.%, Cr: 0.80~1.40 wt.%, Mo: 0.80~1.40 wt.%, Ti: 0.05~0.20 wt.%, Cu: 0.50~1.30 wt.%, Co: 0.50~1.50 wt.%, Ca: 0.002~0.010 wt.%; the balance is Fe and unavoidable impurities; The microstructure of the fastener wire rod is martensite + bainite + pearlite, wherein the volume ratio of martensite is 5-30%.

2. The fastener strip as described in claim 1, characterized in that, It also contains 0 < Si ≤ 0.20 wt.%.

3. The fastener strip as described in claim 1, characterized in that, In unavoidable impurities: P ≤ 0.015 wt.%, S ≤ 0.015 wt.%.

4. A high-strength fastener with a low yield strength ratio, characterized in that, It is made by isothermal heat treatment of fastener wire rod as described in any one of claims 1-3.

5. The ultra-high strength fastener with low yield strength ratio as described in claim 4, characterized in that, Its microstructure consists of martensite and bainite.

6. The ultra-high strength fastener with low yield strength ratio as described in claim 4, characterized in that, Its tensile strength is ≥1800MPa, and its yield strength ratio is 0.8~0.

9.

7. The method for manufacturing fastener wire rod as described in any one of claims 1-3, characterized in that, It includes the following steps: Smelting and casting; Rolled into wire rod; The Stellmore air-cooled line is used for slow cooling with a cooling rate of ≤5℃ / s.

8. The manufacturing method as described in claim 7, characterized in that, In the rolling process to produce wire rod, the initial rolling temperature is controlled at 1090–1180℃, the finishing mill inlet temperature is controlled at 980–1030℃, the sizing mill inlet temperature is controlled at 900–950℃, and the wire drawing temperature is controlled at 840–880℃.

9. The method for manufacturing ultra-high strength fasteners with low yield strength ratio as described in any one of claims 4-6, characterized in that, Including the following steps: Smelting and casting; Rolled into wire rod; Slow cooling with a cooling rate of ≤5℃ / s is achieved using the Stellmore air-cooling line; Isothermal spheroidizing annealing; Parts external machining; Isothermal heat treatment: Heat to the austenitizing temperature of 860-940℃, hold for 30-90 min, then cool to 250-400℃ at a rate of 15-30℃ / s, and hold for 60-180 min.

10. The manufacturing method as described in claim 9, characterized in that, In the rolling process to produce wire rod, the initial rolling temperature is controlled at 1090–1180℃, the finishing mill inlet temperature is controlled at 980–1030℃, the sizing mill inlet temperature is controlled at 900–950℃, and the wire drawing temperature is controlled at 840–880℃.

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