High-strength steel pipe for a scooter and method of manufacturing the same

Abstract

The application discloses a high-strength steel pipe for a scooter, which contains Fe and inevitable impurities, and the following chemical elements in percentage by mass: C: 0.25-0.35%; Si: 0.05-0.35%; Mn: 0.7-1.2%; Mo: 0.1-0.6%; Al: 0.015-0.060%; Cr: 0.5-1.2%; V: 0.03-0.1%; Nb: 0.02-0.01%; La: 0.001-0.006%; and the high-strength steel pipe further satisfies 1.3%<=Mo+10V+10Nb<=2.1%. The application further discloses a manufacturing method of the high-strength steel pipe, which comprises the following steps: preparing a pipe blank; heating and then homogenizing the pipe blank, naturally cooling the pipe blank after reducing the diameter and the wall thickness of the pipe blank; pickling, phosphorizing and saponifying after intermediate annealing heat treatment; cold working to the size of a finished product; and induction quenching and tempering.

Description

Technical Field

[0001] This invention relates to a steel pipe and its manufacturing method, and more particularly to a high-strength steel pipe and its manufacturing method. Background Technology

[0002] Extreme sports scooters, as a tool for extreme sports, not only provide physical exercise but also promote mental and physical well-being. They represent a new product form in skateboarding following traditional skateboards. Scooters can reach speeds of up to 20 km / h and are very popular among modern young people.

[0003] Typically, extreme sports scooters consist of handlebars and stems, wheels mounted on the forks, and a skateboard connected to the forks. When an athlete uses the scooter, the parts of the skateboard that bear the greatest stress are the handlebars and the connection between the handlebars and stems. When an athlete falls from a height, the handlebars often break due to insufficient strength, and the connection between them also frequently fails.

[0004] The patent, with publication number CN 215883948 U and publication date September 24, 2021, entitled "A Front Fork Head for an Extreme Sports Scooter," relates to a front fork head for an extreme sports scooter, and more particularly to the technical field of extreme sports scooter accessories. It includes a front fork body and a moving block, with a connecting rod provided on the moving block. One end of the connecting rod is connected to the moving block, and a detachable directional control lever is provided at the end of the connecting rod away from the moving block. The patent mainly introduces the design and assembly mode of the tubular parts of the extreme sports scooter.

[0005] The aforementioned patent documents mainly concern the design and assembly mode of the tubular parts of extreme sports scooters, but do not provide solutions for the breakage of the handlebars due to insufficient strength. Summary of the Invention

[0006] One of the objectives of this invention is to provide a high-strength steel pipe for scooters. This high-strength steel pipe for scooters, through reasonable chemical element composition design and reasonable manufacturing process, can achieve high strength, high impact resistance and high impact fatigue resistance. In addition, the material has good weldability, and the welded position does not break under impact service conditions.

[0007] To achieve the above objectives, the present invention provides a high-strength steel tube for scooters, which contains Fe and other unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0008] C: 0.25–0.35%; Si: 0.05–0.35%; Mn: 0.7–1.2%; Mo: 0.1–0.6%; Al: 0.015–0.060%; Cr: 0.5–1.2%; V: 0.03–0.1%; Nb: 0.02–0.1%; La: 0.001–0.006%; and the mass percentage of each chemical element also meets the following condition: 1.3% ≤ Mo + 10V + 10Nb ≤ 2.1%.

[0009] Furthermore, the present invention provides a high-strength steel pipe for scooters, wherein the mass percentage of each chemical element is as follows: C: 0.25-0.35%; Si: 0.05-0.35%; Mn: 0.7-1.2%; Mo: 0.1-0.6%; Al: 0.015-0.060%; Cr: 0.5-1.2%; V: 0.03-0.1%; Nb: 0.02-0.1%; La: 0.001-0.006%; the balance being Fe and unavoidable impurities; and the mass percentage content of each chemical element also satisfies: 1.3% ≤ Mo + 10V + 10Nb ≤ 2.1%.

[0010] The high-strength steel pipe for scooters described in this invention employs the addition of Cr and Mo elements to enhance the steel pipe's strength during composition design, and the addition of V and Nb elements to refine the grain size. Furthermore, by controlling the synergistic relationship among Mo, V, and Nb elements, the overall strength and toughness of the steel are optimized. The addition of the rare earth element La ensures the steel's purity and improves its impact toughness. Simultaneously, while ensuring strength and impact toughness, the synergistic addition of alloying elements is controlled to prevent fracture at the welded locations of the steel pipe under impact service conditions.

[0011] Specifically, the design principles of the chemical elements in the high-strength steel pipe for scooters described in this invention are as follows:

[0012] C: In the high-strength steel pipe for scooters described in this invention, carbon (C) is one of the main elements for improving the strength of steel. It is primarily used to address the strength issue of high-strength steel pipes for scooters. Through the formation of carbides, it can effectively increase the strength of the steel, and its addition cost is low. High finished product strength can be achieved with relatively few alloying elements. It should be noted that excessively high C content will significantly increase the C equivalent, affecting weldability; conversely, excessively low C content will affect the strength of the steel. Therefore, to maximize the role of C, the mass percentage of C in the high-strength steel pipe for scooters described in this invention is controlled between 0.25% and 0.35%.

[0013] Si: In the high-strength steel pipe for scooters described in this invention, Si is added as a reducing agent and deoxidizer during the steelmaking process. Si does not form carbides in steel, and its solid solubility in steel is relatively high, which can better strengthen the ferrite in the steel and improve the strength of the steel. Therefore, appropriately increasing the Si content in this invention can improve the strength of the steel in the quenched state. It should be noted that when the Si content is less than 0.05%, the strength of the steel will be insufficient; at the same time, when the Si content exceeds 0.35%, the toughness of the steel pipe will decrease. Therefore, in the high-strength steel pipe for scooters described in this invention, the mass percentage of Si is controlled between 0.05% and 0.35%. Further, the mass percentage of Si can be controlled between 0.1% and 0.3%.

[0014] Mn: In the high-strength steel pipe for scooters described in this invention, Mn is an important alloying element and a weak carbide-forming element. Mn primarily enhances the strength of steel through solid solution strengthening, and its addition effectively increases the hardenability of the steel, ensuring the high strength of the finished steel pipe. Therefore, in this invention, a Mn content of 0.7% significantly improves the strength of the steel. It should be noted that excessively high Mn content significantly reduces the plasticity and impact toughness of the steel, significantly impacts weldability, and also affects the rotational fatigue performance of the finished steel pipe. Therefore, in the high-strength steel pipe for scooters described in this invention, the mass percentage of Mn is controlled between 0.7% and 1.2%.

[0015] Mo: In the high-strength steel pipe for scooters described in this invention, Mo plays a role in solid solution strengthening and improving the hardenability of the steel. Since this steel grade needs to achieve high tensile strength and toughness after quenching, a significant solid solution strengthening and improved hardenability effect is achieved only when the Mo content reaches 0.1%. It should be noted that when the Mo content exceeds a certain range, it will affect the toughness and weldability of the steel pipe. Cost factors must also be considered. Therefore, in the high-strength steel pipe for scooters described in this invention, the mass percentage of Mo is controlled between 0.1% and 0.6%.

[0016] Al: In the high-strength steel pipe for scooters described in this invention, Al has a deoxidizing effect and helps improve the toughness and workability of the steel. When the Al content reaches 0.015% or more, its effect on improving the toughness and workability of the steel is more significant. However, it should be noted that when the Al content exceeds 0.060%, the sulfide inclusions in the steel increase, which will have a significant impact on the toughness and fatigue performance of the steel pipe. Therefore, in order to achieve the best effect of Al, the mass percentage of Al in the high-strength steel pipe for scooters described in this invention is controlled between 0.015% and 0.060%.

[0017] Cr: In the high-strength steel pipe for scooters described in this invention, Cr is a medium-strength carbide-forming element. Part of the Cr in the steel replaces iron to form alloy cementite, improving the steel's stability; the other part dissolves in ferrite, playing a solid solution strengthening role and improving the strength and hardness of the ferrite. Simultaneously, Cr is also a major element in improving the hardenability of steel. When the Cr content reaches 0.5%, the effect on improving the strength and hardenability of the steel is quite significant. However, it should be noted that when the Cr content exceeds 1.2%, the toughness of the steel pipe matrix and the weld seam will be significantly affected. Therefore, in the high-strength steel pipe for scooters described in this invention, the mass percentage of Cr is controlled between 0.5% and 1.2%.

[0018] V: In the high-strength steel pipe for scooters described in this invention, element V has a strong affinity for carbon and nitrogen, thus forming corresponding stable compounds with them. V mainly exists in steel in the form of carbides, and its main function is to refine the steel's microstructure and grain size, forming dispersed VC and VN particles with C and N elements. Simultaneously, VC and VN particles can improve the strength and toughness of the steel. However, it should be noted that adding too much V will form a large lamellar structure, significantly affecting the steel's impact toughness. Therefore, to better utilize its beneficial effects, the mass percentage of V in the high-strength steel pipe for scooters described in this invention is controlled between 0.03% and 0.1%.

[0019] Nitrogen (Nb): In the high-strength steel pipe for scooters described in this invention, nitrogen (Nb) is added to the steel as a microalloying element. However, Nb does not change the structure of iron; instead, it combines with carbon, nitrogen, and sulfur in the steel to alter the microstructure. The strengthening effect of Nb on steel is mainly fine-grain strengthening and dispersion strengthening. Nb can not only form stable carbides and carbonitrides with carbon and nitrogen in the steel, but also disperse carbides to form steel with fine grains. Furthermore, Nb can induce precipitation and control the cooling rate to achieve dispersed distribution of precipitates, adjusting the toughness level of the steel over a wide range. Therefore, when the Nb content reaches 0.02%, it has a significant effect on improving the strength, toughness, high-temperature oxidation resistance, and corrosion resistance of the steel; it can also lower the brittle transition temperature of the steel, resulting in good weldability and formability. However, it should be noted that when the Nb content reaches 0.1%, the above effects are saturated and the cost is high. Therefore, taking into account the requirements of this invention and cost control issues, the mass percentage of Nb element in the high-strength steel pipe for scooters described in this invention is controlled between 0.02% and 0.1%.

[0020] La: In the high-strength steel pipe for scooters described in this invention, La (La) has a strong deoxidizing effect. Adding an appropriate amount of La to the steel can not only significantly reduce harmful impurities but also significantly improve the purity of the steel. Simultaneously, the impact toughness and weldability of the steel can also be significantly improved. It should be noted that when the La content is too high, it will actually reduce the strength of the steel. Therefore, considering the requirements and cost factors of this invention, the mass percentage of La in the high-strength steel pipe for scooters described in this invention is controlled between 0.001% and 0.006%.

[0021] Furthermore, in the unavoidable impurities of the high-strength steel pipe for scooters described in this invention, S≤0.006%, P≤0.015%, and O≤0.003%.

[0022] The unavoidable impurities in this invention are mainly S, P, and O elements, and their contents should be as low as possible when conditions permit. In particular, an increase in S content has a significant impact on the sulfide content of the steel; therefore, it is desirable to control the S mass percentage below 0.006% and the P mass percentage below 0.015%. Meanwhile, the O content in the steel has a significant impact on oxide inclusions; therefore, it is desirable to control the O mass percentage below 0.003%.

[0023] Furthermore, the microstructure of the high-strength steel pipe for scooters described in this invention consists of columnar bainite and dispersed granular carbides, and the inner and outer surfaces of the high-strength steel pipe for scooters are free of decarburization.

[0024] Furthermore, the particle size of the granular carbide in the high-strength steel pipe for scooters described in this invention is no greater than 50 μm, and the granular carbide includes VC and NbC.

[0025] Furthermore, the wall thickness of the high-strength steel pipe for scooters described in this invention is 0.8 to 4.5 mm.

[0026] Furthermore, the outer diameter of the high-strength steel pipe for scooters described in this invention is 15-50 mm.

[0027] Furthermore, the high-strength steel pipe for scooters described in this invention meets the following performance requirements: yield strength ≥ 1300 MPa, tensile strength ≥ 1600 MPa, impact absorption energy at -60℃ ≥ 40 J, elongation ≥ 20%, lateral deformation force > 1200 kg without fracture, and fatigue cycles of up and down bending > 100,000 times.

[0028] Accordingly, another objective of the present invention is to provide a method for manufacturing a high-strength steel pipe for scooters. The high-strength steel pipe for scooters produced by this method has high strength, high impact resistance and high impact fatigue resistance. In addition, the material has good weldability and the welded position does not break under impact service conditions, which are excellent properties that make it suitable for extreme sports.

[0029] To achieve the above objectives, the present invention provides a method for manufacturing the high-strength steel pipe for scooters, comprising the following steps:

[0030] (1) Obtaining a tube blank;

[0031] (2) Heat the tube blank until it is homogenized, then reduce the diameter and wall thickness of the tube blank and let it cool naturally.

[0032] (3) After intermediate annealing heat treatment, pickling, phosphating and saponification are performed;

[0033] (4) Cold process to finished dimensions;

[0034] (5) Induction hardening + tempering: The quenching temperature is controlled at 880~920℃ and the holding time is 15~30min; the tempering temperature is 200~550℃ and the holding time is 30~75min.

[0035] In the manufacturing method described in this invention, the quenching and tempering process is used to ensure the strength and toughness of the steel pipe, while ensuring that there is no decarburization on the surface of the steel pipe. The straightness of the steel pipe can also be ensured by adjusting the torque of the clamping rollers during the quenching process after heating with induction coil.

[0036] Furthermore, in the manufacturing method of the high-strength steel pipe for scooters described in this invention, in step (1), the pipe blank is heated to 1200℃~1250℃ and homogenized for 15~35min.

[0037] In this embodiment, the tube blank is heated to 1200℃~1250℃ and homogenized for 15~35 minutes because the tube blank has the best thermoplasticity in this temperature range, ensuring that the steel pipe does not produce defects such as folding during the hot piercing forming process, while ensuring the production efficiency of the steel pipe.

[0038] Furthermore, in the manufacturing method of the high-strength steel pipe for scooters described in this invention, in step (4), cold working is carried out by cold drawing or cold rolling.

[0039] Furthermore, in the method for manufacturing high-strength steel pipes for scooters according to the present invention, in step (4), the elongation coefficient of each cold working pass is ≤1.5.

[0040] In this embodiment, controlling the elongation coefficient of each cold working pass to ≤1.5 is to ensure the production efficiency of the steel pipe while avoiding defects such as cracking after cold working.

[0041] Furthermore, in the manufacturing method of the high-strength steel pipe for scooters described in this invention, in step (5), the frequency of the quenching induction coil is 5000-8000HZ, and the speed at which the steel pipe passes through is 2-10m / min, so as to ensure that the steel pipe is completely quenched after quenching.

[0042] The high-strength steel pipe for scooters and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0043] This invention improves the strength of high-strength steel pipes for scooters by controlling the addition of Cr and Mo elements from the perspective of composition design. At the same time, it refines the grains by adding V and Nb elements, and achieves the optimal ratio of comprehensive strength and toughness of steel by controlling the synergistic relationship of Mo, V and Nb elements. In addition, the addition of rare earth element La ensures the purity of steel and improves the impact toughness of steel.

[0044] Accordingly, from a manufacturing process perspective, this invention employs specific quenching and tempering processes, in conjunction with the composition design of this invention, to obtain the microstructure morphology that this invention aims to achieve, ultimately realizing a good balance between strength and toughness.

[0045] In some embodiments, the high-strength steel pipe for scooters described in this invention has a yield strength ≥1300MPa, tensile strength ≥1600MPa, impact absorption energy at -60℃ ≥40J, elongation ≥20%, lateral deformation force >1200kg without fracture, and fatigue cycles of up and down bending >100,000 times.

[0046] The high-strength steel pipe for scooters and its manufacturing method described in this invention can be effectively applied to extreme sports scooters, and it has good prospects for promotion and application value. Detailed Implementation

[0047] The high-strength steel pipe for scooters and its manufacturing method described in this invention will be further explained and illustrated below with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0048] Examples A1-A10 and Comparative Examples B1-B5

[0049] The high-strength steel pipes for scooters in Examples A1-A10 and Comparative Examples B1-B5 were all prepared using the following steps:

[0050] (1) Smelting and producing tube blanks, the chemical element mass percentage of the tube blanks is shown in Table 1;

[0051] (2) The tube blank is heated to 1200℃~1250℃ in a ring heating furnace and heated for 15~35min; then hot piercing is carried out by a vertical conical hot piercing machine, and the tube blank is reduced in diameter and wall thickness by a three-roll tension reducing machine and then cooled naturally.

[0052] (3) After intermediate annealing heat treatment, pickling, phosphating and saponification are performed;

[0053] (4) The steel pipe is cold-drawn or cold-rolled to the finished size. In some embodiments, the elongation coefficient of each cold working is controlled to be ≤1.5. The wall thickness of the finished steel pipe after cold working is 0.8 to 4.5 mm and the outer diameter is 15 to 50 mm.

[0054] (5) Induction hardening + tempering heat treatment: The quenching temperature is controlled at 880-920℃, the holding time is 15-30 min, the tempering temperature is controlled at 200-550℃, and the holding time is 30-75 min. In some embodiments, the frequency of the quenching induction coil is controlled at 5000-8000Hz, and the speed at which the steel pipe passes through is 2-10 m / min.

[0055] It should be noted that the high-strength steel pipes for scooters in Examples A1-A10 of this invention are all prepared using the above steps, and their chemical composition and related process parameters meet the design specifications and control requirements of this invention. While the steel pipes for scooters in Comparative Examples B1-B5 are also prepared using the above process, the chemical composition and / or related process parameters of the steel pipes for scooters in Comparative Examples B1-B5 do not meet the design requirements of this invention.

[0056] Table 1 lists the mass percentage of chemical elements in Examples A1-A10 and Comparative Examples B1-B5 of this case.

[0057] Table 1. (wt.%, balance Fe and other unavoidable impurities besides P, S and O)

[0058]

[0059]

[0060] Table 2 lists the process parameters for each step in Examples A1 to A10 and Comparative Examples B1 to B5.

[0061] Table 2.

[0062]

[0063]

[0064] Samples were taken from the high-strength steel pipes for scooters of Examples A1-A10 and the comparative steel pipes of Comparative Examples B1-B5. After etching with an etchant, the microstructure was observed under an optical microscope, and the test results are listed in Table 3.

[0065] Table 3.

[0066]

[0067] As can be seen from Table 3, some comparative examples have blocky bainitic structures, and the precipitates in some comparative examples are larger than the precipitates in the examples, or are not VC and NbC precipitates, all of which affect the properties of the steel.

[0068] In addition, samples were taken from the high-strength steel pipes for scooters of Examples A1-A10 and the comparative steel pipes of Comparative Examples B1-B5, and their various properties were tested. The results are listed in Table 3.

[0069] in:

[0070] (1) Tensile strength, yield strength and elongation were measured according to the methods specified in national standard GB / T 228.

[0071] (2) The impact absorption energy at -60℃ was calculated based on the full-thickness notched sample and converted to the national standard half-size sample in GB / T 229.

[0072] (3) The lateral deformation force is measured based on the load borne by the entire pipe. If the lateral deformation force is greater than 600 kg and the pipe does not break, it is considered qualified; otherwise, it is considered unqualified.

[0073] (4) The bending fatigue test is conducted on the entire pipe. A load of >200Kg is applied to the middle of the transverse position of the steel pipe and the test is performed. The number of bending fatigue cycles is >100,000 times and is considered qualified; otherwise, it is considered unqualified.

[0074] Table 4 lists the comprehensive mechanical property parameters of the steel pipes in Examples A1-A10 and Comparative Examples B1-B5.

[0075] Table 4.

[0076]

[0077] As can be seen from Tables 1, 2, 3, and 4, the steel pipes in Examples A1-A10 all achieved excellent mechanical properties because they adopted the chemical element mass percentages of this invention and were all manufactured according to the manufacturing method provided by this invention. Examples A1-A10 all exhibit tensile strength ≥1730MPa, yield strength ≥1320MPa, elongation ≥20%, impact absorption energy at -60℃ ≥45J, transverse deformation force >1200kg without fracture, and fatigue cycles after vertical bending >100,000 times.

[0078] Unlike the embodiments, the mass percentage of chemical elements in each steel pipe in Comparative Examples B1-B5 does not conform to the range defined by the present invention, and therefore their comprehensive mechanical properties are significantly inferior to those of the high-strength steel pipes for scooters in Examples A1-A10.

[0079] It should be noted that the prior art portion of the protection scope of this invention is not limited to the embodiments given in this application. All prior art that does not contradict the solution of this invention, including but not limited to prior patent documents, prior publications, prior public uses, etc., can be included in the protection scope of this invention.

[0080] 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.

[0081] It should also be noted that the above examples are merely specific embodiments of the present invention, and the present invention is obviously not limited to the above embodiments, with many similar variations. All modifications that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should fall within the protection scope of this invention.

Claims

1. A high-strength steel pipe for scooters, characterized in that, Its mass percentage of each chemical element is as follows: C: 0.25~0.35%; Si: 0.05~0.35%; Mn: 0.7~1.2%; Mo: 0.1~0.6%; Al：0.015～0.060％； Cr：0.5～1.2％； V：0.03～0.1％； Nb ：0.02～0.1％； La: 0.001–0.006%; balance Fe and unavoidable impurities; Its mass percentage content of each chemical element also meets the following requirement: 1.3% ≤ Mo + 10V + 10Nb ≤ 2.1%; Its performance meets the following requirements: yield strength ≥1300MPa, tensile strength ≥1600MPa, impact absorption energy at -60℃ ≥40J, elongation ≥20%, transverse deformation force >1200kg without fracture, and fatigue cycles of upper and lower bending >100,000 times.

2. The high-strength steel pipe for scooters as described in claim 1, characterized in that, The mass percentage of Si further satisfies the following requirement: Si: 0.1–0.3%.

3. The high-strength steel pipe for scooters as described in claim 1, characterized in that, In unavoidable impurities, S ≤ 0.006%, P ≤ 0.015%, and O ≤ 0.003%.

4. The high-strength steel pipe for scooters as described in claim 1, characterized in that, Its microstructure includes columnar bainite and diffusely distributed granular carbides, and the inner and outer surfaces of the high-strength steel pipe for the scooter are free of decarburization.

5. The high-strength steel pipe for scooters as described in claim 4, characterized in that, The particle size of the granular carbide is no greater than 50 μm, and the granular carbide includes VC and NbC.

6. The high-strength steel pipe for scooters as described in claim 1, characterized in that, Its wall thickness is 0.8 to 4.5 mm.

7. The high-strength steel pipe for scooters as described in claim 1, characterized in that, Its outer diameter is 15-50 mm.

8. A method for manufacturing a high-strength steel pipe for scooters as described in any one of claims 1-7, characterized in that, Including the following steps: (1) Obtaining a tube blank; (2) Heat the tube blank until it is homogenized, then reduce the diameter and wall thickness of the tube blank and let it cool naturally. (3) After intermediate annealing heat treatment, pickling, phosphating and saponification are performed; (4) Cold work to the finished steel pipe dimensions; (5) Induction hardening + tempering: The quenching temperature is controlled at 880~920℃ and the holding time is 15~30min; the tempering temperature is 200~550℃ and the holding time is 30~75min.

9. The manufacturing method as described in claim 8, characterized in that, In step (2), the tube blank is heated to 1200℃~1250℃, and the heating time is 15-35min.

10. The manufacturing method as described in claim 8, characterized in that, In step (4), cold working is performed by cold drawing or cold rolling.

11. The manufacturing method as described in claim 8, characterized in that, In step (4), the elongation coefficient of each cold working pass is ≤1.

5.

12. The manufacturing method as described in claim 8, characterized in that, In the induction hardening step (5), the frequency of the hardening induction coil is controlled to be 5000-8000Hz, and the speed at which the steel pipe passes through is 2-10m / min.

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