Steel pipe for oil cylinder and method for manufacturing the same

Abstract

A steel pipe for oil cylinders and a manufacturing method thereof are disclosed. The steel pipe for oil cylinders contains, in addition to 90% by weight or more of Fe and unavoidable impurities, the following chemical elements by weight: C: 0.16-0.3%, Si: 0.15-0.5%, Mn: 1.2-1.8%, Nb: 0.02-0.04%, Mo: 0.1-0.2%, and optionally Ti: 0.015-0.03% and B: 0.0015-0.0035%. According to the present invention, after tension drawing and quenching of the steel pipe, different stepwise cooling processes are adopted to increase the rigidity and straightness of the steel pipe, thereby controlling the distribution of phase change and thermal stress in the entire wall thickness of the steel pipe for oil cylinders, controlling the distribution of ferrite in the microstructure of the steel pipe for oil cylinders, effectively reducing the residual stress of the steel pipe for oil cylinders, and preventing the occurrence of cracks in the inner wall. As a result, a steel pipe for oil cylinders with high strength and low residual stress is obtained. The steel pipe for oil cylinders has a yield strength of 600 MPa or more, a tensile strength of 730 MPa or more, and a residual stress of 50 MPa or less.

Description

Technical Field

[0001] The present invention relates to the field of materials. In particular, it relates to a steel pipe for an oil cylinder and a method for manufacturing the same.

Background Art

[0002] Steel pipes for oil cylinders are widely used in oil cylinders for construction machinery and air cylinder barrels, and can withstand loads such as impact fatigue and friction during use. Residual stress is an important factor that affects the fatigue life of seamless pipes, extrusion resistance, internal pressure resistance, and deformation due to machining. By reducing or removing the residual stress of the steel pipe for an oil cylinder, the service life of the steel pipe for an oil cylinder can be significantly improved. This is one of the important goals in the subsequent production management of steel pipes for oil cylinders.

[0003] Currently, conventional methods for reducing or removing residual stress include high-temperature stress-relieving annealing and mechanical / physical methods. However, these processes are costly and increase the production process.

[0004] Chinese Patent Application Publication No. 201810365440.5 discloses "a method for removing residual stress of quenched seamless steel pipes and a two-way chain-type cooling bed adopted", and by controlling the straightness of the steel pipe after rolling and before quenching and the two-way chain of the cooling bed after quenching to remove residual stress, the quenching and stress-relieving annealing processes are no longer required, achieving the goal of cost reduction.

[0005] Chinese Patent Application Publication No. 201420805596.8 discloses "an asymmetric steel pipe correction roller", and a special correction roller for removing the residual stress and oxide film of the steel pipe is designed by controlling the force applied to the steel pipe in the correction process.

[0006] Chinese Patent Application Publication No. 200910210718.2 discloses a "method for controlling the residual stress level of a conveyor steel pipe," which involves deriving a formula and comparing that formula with the measured elastic recovery amount of the steel pipe to obtain the residual stress level of the steel pipe, and is a method for measuring and characterizing the residual stress level of a steel pipe. [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to provide a steel pipe for oil cylinders and a method for manufacturing the same. Compared with conventional steel pipe products for oil cylinders, the present invention significantly reduces residual stress in the steel pipe for oil cylinders, preventing cracks from forming in the inner wall while achieving higher strength. According to the present invention, the yield strength of the steel pipe for oil cylinders is 600 MPa or more, the tensile strength is 730 MPa or more, and the residual stress is 50 MPa or less. [Means for solving the problem]

[0008] In one embodiment, the present invention provides a steel pipe for an oil cylinder that, in addition to 90% by weight or more of Fe and unavoidable impurities, further contains, by weight percent (W%), the following chemical elements: C: 0.16-0.3%, Si: 0.15-0.5%, Mn: 1.2-1.8%, Nb: 0.02-0.04%, Mo: 0.1-0.2%, and optionally Ti: 0.015-0.03%, and B: 0.0015-0.0035%.

[0009] Preferably, the steel pipe for the oil cylinder contains, by weight percent, 0.16-0.3% C, 0.15-0.5% Si, 1.2-1.8% Mn, 0.02-0.04% Nb, 0.1-0.2% Mo, optionally 0.015-0.03% Ti and 0.0015-0.0035% B, with the remainder being Fe and unavoidable impurities.

[0010] Preferably, among the above unavoidable impurities, P ≤ 0.01% and S ≤ 0.001%.

[0011] In some embodiments, the wall thickness of the steel pipe for the oil cylinder is 20 mm or more, and the steel pipe for the oil cylinder contains Ti: 0.015 to 0.03% and B: 0.0015 to 0.0035%. In some embodiments, the wall thickness of the steel pipe for the oil cylinder is less than 20 mm, and the steel pipe for the oil cylinder may or may not contain Ti: 0.015 to 0.03% and B: 0.0015 to 0.0035%.

[0012] Preferably, the wall thickness of the steel pipe for the oil cylinder is 9 mm or more.

[0013] Preferably, in the wall thickness direction of the steel pipe for the oil cylinder, the microstructure from the outer wall to point t / 2 is tempered sorbite, and the microstructure from point t / 2 to the inner wall is tempered sorbite + ferrite, with the ferrite distributed in a gradient, the ferrite content increasing as the distance to the inner wall decreases, the ferrite content in the microstructure at point t / 2 is 3% or more, and the ferrite content in the microstructure at the inner wall (i.e., on the surface of the inner wall) is 5% or more.

[0014] Here, unless otherwise specified, t represents the wall thickness (mm) of the steel pipe for the oil cylinder.

[0015] Preferably, the steel pipe for the oil cylinder has a yield strength of 600 MPa or more, a tensile strength of 730 MPa or more, and a residual stress of 50 MPa or less.

[0016] Preferably, the residual stress of the steel pipe for the oil cylinder is 40 MPa or less.

[0017] Preferably, the steel pipe for the oil cylinder has a yield ratio (i.e., the ratio of yield strength to tensile strength) of 0.92 or less.

[0018] Preferably, the ferrite content in the microstructure of the steel pipe for the oil cylinder of the present invention at point t / 2 is 0.5t to 1.0t%.

[0019] Preferably, the ferrite content in the microstructure on the inner wall of the steel pipe for the oil cylinder of the present invention is 1.5t to 2.0t%.

[0020] Here, the ferrite content is defined as the ratio of ferrite in the microstructure expressed in area%, and is measured by a metallographic method.

[0021] The elements in the steel pipe for the oil cylinder of the present invention are designed based on the following principles.

[0022] C: C is an interstitial solid solution strengthening element and has a great influence on hardenability. If the C content is less than 0.16%, the strength becomes too low. If the C content exceeds 0.3%, cracks will occur on the inner wall after step cooling. Therefore, in the present invention, the C content is controlled to be in the range of 0.16 to 0.3%.

[0023] Si: Si is a commonly used deoxidizer and a powerful element for precipitating iron, and it improves the hardenability to a certain extent. If the Si content is less than 0.15%, the above effects cannot be fully exerted. If the Si content exceeds 0.5%, Si will cause problems with the surface quality. Therefore, in the present invention, the Si content is controlled to be in the range of 0.15 to 0.5%.

[0024] Mn: Mn is a solid solution strengthening element and is also a powerful element for improving hardenability. If the Mn content is less than 1.2%, the hardenability is insufficient and the strength is low. If the Mn content exceeds 1.8%, the hardenability becomes too high, so the precipitation of ferrite content from the t / 2 point to the inner wall after step cooling is reduced, and both the phase change and thermal stress on the inner wall become significant tensile stresses, resulting in cracks on the inner wall. Therefore, in the present invention, the Mn content is controlled to be in the range of 1.2 to 1.8%.

[0025] Nb: Nb is a carbide precipitation strengthening element, which refines austenite grains, functions as a nucleation site, and promotes the precipitation of ferrite during step cooling.

[0026] Mo: Mo significantly improves hardenability and enhances the balance between strength and toughness as well as tempering stability. When the Mo content is in the range of 0.1% - 0.2%, due to the effect of Mo on hardenability, the ferrite from the t / 2 point to the inner wall is controlled to be distributed with a gradient, ensuring that cracks do not occur on the inner wall during the step-cooling process while ensuring strength.

[0027] Ti and B: The combined addition of Ti and B significantly improves hardenability. For pipes with a wall thickness of 20 mm or more, it is necessary to improve the hardenability of steel pipes for oil cylinders, and it is also necessary to avoid a significant decrease in strength due to a large increase in the ferrite content from the t / 2 point to the inner wall. On the other hand, Ti precipitates as carbonitride and can function as a nucleation point for ferrite in the step-cooling process, effectively controlling the precipitation rate of ferrite. If the Ti content is less than 0.015% or the B content is less than 0.0015%, the above effects cannot be fully exerted. If the Ti content exceeds 0.03% or the B content exceeds 0.0035%, there is no significant improvement effect on improving hardenability. Therefore, in the present invention, the Ti content is controlled to be in the range of 0.015 - 0.03%, and the B content is controlled to be in the range of 0.0015 - 0.0035%.

[0028] Inevitable impurities include P and S, and it should be noted that both are harmful elements in steel. If the mass ratio of P is too high, bias occurs at the grain boundaries, making the grain boundaries brittle and potentially significantly reducing toughness. If the mass ratio of S is too high, the amount of inclusions in the steel increases, adversely affecting low-temperature toughness. Therefore, the contents of P and S in the steel should be minimized.

[0029] The present invention controls the distribution of ferrite in a steel pipe for an oil cylinder by controlling the content of elements such as Nb, Mo, and Ti. In the wall thickness direction of the steel pipe for an oil cylinder, the microstructure from the outer wall to point t / 2 is tempered sorbite, and the microstructure from point t / 2 to the inner wall is tempered sorbite + ferrite, and the ferrite is distributed with a gradient, with the ferrite content increasing as the distance to the inner wall decreases. The ferrite content in the microstructure at point t / 2 is 3% or more, and the ferrite content in the microstructure at the inner wall is 5% or more. Here, t is the wall thickness (mm) of the steel pipe for the oil cylinder. The microstructure from the outer wall to point t / 2 is tempered sorbite, and the strength and toughness levels of the tempered sorbite are good, ensuring sufficient rigidity of the outer layer of the steel pipe for the oil cylinder. The microstructure from point t / 2 to the inner wall is tempered sorbite + ferrite, ensuring good toughness and a low yield ratio of the steel pipe for the oil cylinder.

[0030] Furthermore, the ferrite precipitate is distributed with a gradient, and the ferrite content increases with distance from the inner wall. Because ferrite has good ductility and toughness, residual stress in the inner wall of the oil cylinder steel pipe can be well controlled during the cooling process, and cracks can be prevented from forming in the inner wall during the water quenching process. In addition, high strength can be ensured in the oil cylinder steel pipe while significantly reducing residual stress.

[0031] As a result of diligent research, the inventors have found that the amount of ferrite precipitated in steel pipes for oil cylinders and the wall thickness of the steel pipes for oil cylinders are directly related in a specific way. The ferrite content at the t / 2 point of the wall thickness of the steel pipes for oil cylinders is in the range of 0.5t to 1.0t%, and the ferrite content in the microstructure of the inner wall is in the range of 1.5t to 2.0t%. If the ferrite content is too low, the yield ratio becomes too high, the residual stress becomes too large, the safety of use decreases, and the risk of cracks occurring in the inner wall during water quenching increases. If the ferrite content is too high, the strength of the steel pipes for oil cylinders becomes too low, and they cannot meet the requirements for use. Furthermore, if the wall thickness is large, and sufficient ferrite does not precipitate, the residual stress of the steel pipes for oil cylinders increases, and there is a strong tendency for cracks to occur in the inner wall.

[0032] In another embodiment, the present invention relates to a method for manufacturing a steel pipe for an oil cylinder as described above, (1) Smelting and casting process: A process of smelting and casting molten steel having the elemental composition described above to obtain a cast billet; (2) The process of heating the cast billet; (3) The process of drilling holes in the heated cast billet; (4) A process of obtaining a steel pipe by continuously rolling a perforated cast billet; (5) A process of forcibly cooling and reheating the steel pipe; (6) Process of tension reduction and cooling of the reheated steel pipe: After tension reduction (i.e., diameter reduction by stretch reducing) is performed on the reheated steel pipe, only the outer wall of the steel pipe is water-cooled (i.e., the inner wall of the steel pipe is not water-cooled in this process), the starting temperature of the cooling of the steel pipe is controlled to be ≥Ar3, and the ending temperature of the cooling of the steel pipe is controlled to be ≥B f and ≤ B s A process of controlling the temperature to -100°C and controlling the cooling rate to be within the range of 25-35°C / s; (7) The process of straightening the cooled steel pipe; (8) Process of quenching the straightened steel pipe: The straightened steel pipe is heated to Ac3+30 to Ac3+60°C (i.e., the quenching temperature is controlled to be Ac3+30 ≤ and ≤ Ac3+60°C), and after quenching, the steel pipe is rotated and cooled in stages by water cooling. Water cooling is first performed by external water spraying, and when the temperature drops below Ar3-70°C ≤ inner wall temperature ≤ Ar3-30°C, water is injected into the steel pipe from one end until the inner cavity of the steel pipe is filled with cooling water, and the steel pipe is cooled to room temperature; (9) The process of tempering the hardened steel pipe; and (10) A process of straightening steel pipes after they have been discharged from the furnace to obtain steel pipes for oil cylinders. This provides a method that includes this.

[0033] Preferably, in step (2), heating is carried out at a temperature of 1250 to 1280°C for 3 to 4 hours.

[0034] Preferably, in step (3), the perforation is carried out at a temperature of 1100 to 1230°C.

[0035] Preferably, in step (4), the finish rolling is carried out at a temperature of 900 to 1000°C.

[0036] Preferably, in step (5), the steel pipe is forcibly air-cooled to Ar3-50°C or lower, and then reheated to 950-980°C.

[0037] Preferably, in step (6), the tension drawing is performed at a temperature of 850 to 900°C.

[0038] Preferably, in step (7), the straightened steel pipe is allowed to cool naturally to room temperature.

[0039] Preferably, in step (9), tempering is carried out at a temperature of (550-2×t)°C.

[0040] Preferably, in step (10), the straightening is performed at a temperature of ≥400°C.

[0041] This invention reduces residual stress in oil cylinder steel pipes and improves their performance by designing the composition of the oil cylinder steel pipes and controlling the cooling process during water quenching, without adding any further manufacturing processes.

[0042] After tension drawing of the steel pipe at 850-900°C, only the outer wall of the steel pipe is water-cooled, the cooling start temperature is controlled to be ≥Ar3, and the cooling end temperature is controlled to be ≥B f and ≤ B s Controlled to -100℃ (where B f is the temperature at the end of the phase change of bainite during the cooling process, and B s is the temperature at which the phase change of bainite begins during the cooling process. The cooling rate is controlled to be 25-35°C / s during the cooling process. The main purpose of this process is to rapidly cool and harden the steel pipe through uniform cooling. This makes it possible to reduce the straightness of the steel pipe to 2 mm / m or less, preferably less than 1.5 mm / m, and at the same time refines the microstructure in the rolled state, laying the foundation for obtaining a good balance of performance after subsequent tempering. Furthermore, this cooling reduces the residual stress level in the rolled state.

[0043] The steel pipes are rapidly cooled to the finish cooling temperature and then immediately straightened. Temperature-based straightening helps ensure straightness and simultaneously reduces the residual stress level in the rolled state, resulting in a straightness of 2 mm / m or less, preferably less than 1.5 mm / m, of the straightened steel pipes. After that, the steel pipes are moved onto a cooling bed and allowed to cool naturally to room temperature.

[0044] The steel pipe of the present invention is cooled in stages by water cooling after quenching, and rotated during the cooling process. During cooling, the outer wall of the steel pipe is first cooled by external water spraying, and when Ar3-70℃ ≤ inner wall temperature ≤ Ar3-30℃, internal water spraying is turned on to inject water into the steel pipe, and the inner wall of the steel pipe is cooled until the inner cavity of the steel pipe is filled with cooling water, and the steel pipe is cooled to room temperature.

[0045] This invention provides cooling for a steel pipe for an oil cylinder through a stepwise cooling process based on the following principle.

[0046] 1) Because the entire length of the steel pipe is cooled simultaneously by external water spraying, the uniformity of the cooling is excellent. When cooling the inner wall of the steel pipe, one end of the pipe is cooled first, followed by the other end, which increases the rigidity of the steel pipe. The excellent uniformity of the cooling ensures a better level of straightness in the steel pipe, and large residual stresses caused by corrective deformation due to subsequent bending of the pipe are avoided.

[0047] 2) The residual stress in steel pipes is closely related to phase change and thermal stress during the cooling process. Stepwise cooling effectively controls the distribution of phase change and thermal stress across the entire wall thickness of the steel pipe, enabling mutual cancellation of the martensite phase change stress and thermal stress, and effectively reducing the residual stress level of the steel pipe. In the technical solution of the present invention, by using a stepwise cooling method in which external watering is followed by internal watering, the phase change stress is tensile stress and thermal stress is compressive stress in the outer wall of the steel pipe, and the phase change stress is compressive stress and thermal stress is tensile stress in the central part of the steel pipe, and the two can cancel each other out.

[0048] 3) Due to the cooling transformation structure of the inner wall, a ferrite structure is partially precipitated rather than a complete martensitic structure, which effectively reduces the residual stress of the inner wall. In the technical solution of the present invention, both the phase change stress and thermal stress of the inner wall are tensile stresses, but the inner wall is the surface that is cooled later. Cooling of the inner wall begins when, after cooling by external water spraying, Ar3-70°C ≤ inner wall temperature ≤ Ar3-30°C. At this time, a ferrite structure is precipitated on the inner wall, the transformation rate of the martensitic structure decreases, and the phase change stress in the inner wall is reduced. Therefore, the residual stress level of the inner wall is effectively reduced, and cracks do not occur in the inner wall.

[0049] In this invention, rapid cooling is employed in the cooling process after tension drawing to improve the hardness and uniformity of the steel pipe for oil cylinders and to reduce the residual stress level of the rolled steel pipe for oil cylinders. Furthermore, by performing stepwise cooling after quenching, a cooling rate gradient is formed in the wall thickness direction. In the cooling process, the outer wall is cooled first, and then the center and inner wall are cooled. The cooling rate of the center and inner wall is slower than that of the outer wall. A transformation of the ferrite structure occurs first, followed by a transformation of martensite. As one approaches the inner wall, the cooling rate gradually slows down, and the amount of ferrite precipitate increases. Also, as the wall thickness of the steel pipe for oil cylinders increases, the cooling rate of the center and inner wall decreases further, promoting ferrite precipitate, and the amount of ferrite precipitate increases accordingly.

[0050] When Ar3-70℃ ≤ inner wall temperature ≤ Ar3-30℃, water is injected into the inner wall to cool it. At this time, a ferrite structure precipitates on the inner wall, and the rate of transformation to a martensitic structure decreases. Therefore, the residual stress level within 1 mm of the inner wall is effectively reduced.

[0051] This invention controls the precipitation of ferrite in the microstructure of the steel pipe for oil cylinders through a stepwise cooling process, with the ferrite content increasing closer to the inner wall. Because ferrite has good ductility and toughness, residual stress in the inner wall of the steel pipe for oil cylinders can be well controlled during cooling, preventing cracks from forming in the inner wall during the water quenching process.

[0052] This invention exhibits the following beneficial effects.

[0053] In the composition design of the oil cylinder steel pipe of the present invention, the hardenability of the oil cylinder steel pipe is improved by controlling the content of C, Si, and Mn elements, thereby preventing cracks from forming on the inner wall of the oil cylinder steel pipe during the subsequent stepwise cooling process. On the other hand, the distribution of ferrite in the oil cylinder steel pipe is controlled by controlling the content of Nb and Mo elements, thereby further preventing cracks from forming on the inner wall during the stepwise cooling process.

[0054] Furthermore, depending on the wall thickness of the steel pipe for the oil cylinder, Ti or B may be added to improve hardenability. This avoids a significant increase in ferrite content from point t / 2 to the inner wall, which would reduce strength, when the wall thickness of the steel pipe for the oil cylinder is 20 mm or more.

[0055] According to the present invention, after tension drawing and quenching of the steel pipe based on the composition design, different stepwise cooling processes are employed. On the one hand, by increasing the rigidity and straightness of the steel pipe, large residual stresses caused by subsequent bending deformation of the pipe are avoided. On the other hand, by controlling the distribution of phase change and thermal stress throughout the entire wall thickness of the steel pipe for oil cylinders, mutual elimination of martensite phase change stress and thermal stress can be achieved, effectively reducing the residual stress level of the steel pipe. Finally, by controlling the distribution of ferrite in the microstructure of the steel pipe for oil cylinders, phase change stress in the inner wall is reduced, the residual stress level of the inner wall is effectively reduced, and cracks do not occur in the inner wall. As a result, a steel pipe for oil cylinders with high strength and low residual stress is obtained. The yield strength of the steel pipe for oil cylinders is 600 MPa or more, the tensile strength is 730 MPa or more, and the residual stress is between 0 MPa and 50 MPa. [Brief explanation of the drawing]

[0056] [Figure 1] This is a photograph showing the measurement of residual stress in a steel pipe for an oil cylinder manufactured in Comparative Example 1 of the present invention, using the slit method. [Figure 2]This is a photograph showing the measurement of residual stress in a steel pipe for an oil cylinder manufactured in Example 1 of the present invention, using the slit method. [Figure 3] This is a photograph of the metallic structure of the outer wall surface of a steel pipe for an oil cylinder according to Embodiment 1 of the present invention. [Figure 4] This is a photograph of the metallographic structure at a point where the wall thickness of the steel pipe for the oil cylinder of Embodiment 1 of the present invention is half. [Figure 5] This is a photograph of the metallic structure of the inner wall surface of a steel pipe for an oil cylinder according to Embodiment 1 of the present invention. [Modes for carrying out the invention]

[0057] The present invention will be further described below with reference to examples.

[0058] The steel pipes for oil cylinders in Examples 1 to 8 of the present invention were manufactured according to the method described above. The steel pipes for oil cylinders in Comparative Examples 1 to 8 were manufactured in substantially the same manner as in Examples 1 to 8, however, one or more of the elemental compositions and / or manufacturing process parameters of Comparative Examples 1 to 8 were not included within the scope of protection of the present invention.

[0059] Table 1 shows the chemical compositions of Examples 1-8 and Comparative Examples 1-8 of the present invention.

[0060] Table 2 shows the specific manufacturing process parameters for Examples 1 to 8 and Comparative Examples 1 to 8 of the present invention. In step (1), molten steel was smelted and cast according to the elemental composition shown in Table 1 to obtain a cast billet. In step (7), the processed steel pipe obtained in step (6) was straightened, and the straightened steel pipe was allowed to cool naturally to room temperature.

[0061] Table 3 shows the performance parameters of the steel pipes for oil cylinders obtained in Examples 1-8 and Comparative Examples 1-8 of the present invention.

[0062] In this application, yield strength and tensile strength are measured according to GB / T228, and residual stress is measured according to ISO / TR10400.

[0063] Figures 1 and 2 are photographs showing the measurement of residual stress in steel pipes for oil cylinders manufactured in Comparative Example 1 and Example 1 of the present invention, respectively, using the slit method. As can be seen from the figures, the residual stress of the steel pipes for oil cylinders of the present invention is significantly smaller than that of conventional steel pipes for oil cylinders, the slits in the seamless pipes obtained by the method of the present invention are smaller than those in the seamless pipes manufactured by conventional processes, and the residual stress of the seamless pipes obtained by the present invention (e.g., Example 1) is significantly smaller than that of the seamless pipes manufactured by conventional processes (e.g., Comparative Example 1).

[0064] Figures 3 to 5 are photographs of the metallographic structure of various parts of the steel pipe for the oil cylinder of the present invention. As can be seen from the photographs, the metallographic structure from the outer wall to point t / 2 is tempered sorbite, and at point t / 2, a ferrite structure of t% precipitated. The ferrite from point t / 2 to the inner wall was distributed with a gradient. The ferrite content increased with increasing distance from the inner wall. The ferrite content in the metallographic structure of the inner wall reached 2t%.

[0065] As can be seen from Table 3, the inner wall of the steel pipe for oil cylinder obtained in this invention is free of cracks, the residual stress is less than 50 MPa, and furthermore, the residual stress can be reduced to 0.

[0066] Because one or more of the elemental composition and / or manufacturing process parameters were not within the scope of protection of the present invention, the steel pipes for oil cylinders obtained in Comparative Examples 1 to 8 exhibited higher residual stress and were susceptible to cracking of the inner wall. When the wall thickness exceeded 20 mm, in Comparative Examples 4 and 5, although the residual stress of the steel pipes for oil cylinders was low due to the absence of Ti and B elements, the strength of the steel pipes for oil cylinders did not meet the requirements of the present invention.

[0067] In summary, the present invention provides a steel pipe for oil cylinders with superior overall performance compared to the prior art by designing the chemical elemental composition of the steel pipe in combination with a specific manufacturing process.

[0068] Table 1

[0069] Table 2

[0070] Table 3

Claims

1. The material contains, by weight percent, C: 0.16-0.3%, Si: 0.15-0.5%, Mn: 1.2-1.8%, Nb: 0.02-0.04%, Mo: 0.1-0.2%, optionally Ti: 0.015-0.03%, and B: 0.0015-0.0035%, with the remainder being Fe and unavoidable impurities. In the wall thickness direction of the steel pipe for the oil cylinder described above, the microstructure from the outer wall to point t / 2 is tempered sorbite, and the microstructure from point t / 2 to the inner wall is tempered sorbite + ferrite, with the ferrite distributed in a gradient, and the ferrite content increasing with increasing distance to the inner wall, the ferrite content in the microstructure at point t / 2 being 3% or more, and the ferrite content in the microstructure at the inner wall being 5% or more, where t is the wall thickness (mm) of the steel pipe for the oil cylinder. The above-mentioned steel pipe for oil cylinders has a yield strength of 600 MPa or more, a tensile strength of 730 MPa or more, and a residual stress of 50 MPa or less.

2. The steel pipe for an oil cylinder according to Claim 1, further comprising, by weight, Ti: 0.015 to 0.03% and B: 0.0015 to 0.0035%.

3. The steel pipe for the oil cylinder according to Claim 1, wherein the steel pipe for the oil cylinder has a residual stress of 40 MPa or less.

4. The steel pipe for the oil cylinder according to Claim 1, wherein the yield ratio of the steel pipe for the oil cylinder is 0.92 or less.

5. The steel pipe for the oil cylinder described in claim 1, wherein the wall thickness of the steel pipe for the oil cylinder described above is 20 mm or more, and the steel pipe for the oil cylinder contains Ti: 0.015 to 0.03% and B: 0.0015 to 0.0035%.

6. The steel pipe for an oil cylinder according to claim 1, wherein the ferrite content in the microstructure at point t / 2 of the steel pipe for the oil cylinder is 0.5t to 1.0t%.

7. The steel pipe for an oil cylinder according to claim 1, wherein the ferrite content in the microstructure of the inner wall of the steel pipe for the oil cylinder is 1.5t to 2.0t%.

8. The steel pipe for an oil cylinder according to claim 1, wherein the wall thickness of the steel pipe for the oil cylinder is 9 mm or more.

9. A method for manufacturing a steel pipe for an oil cylinder according to any one of claims 1 to 8, (1) Smelting and casting process: A process of smelting and casting molten steel having the elemental composition described in claim 1 to obtain a cast billet; (2) The step of heating the above-mentioned cast billet; (3) The process of drilling holes in the heated cast billet; (4) A step of continuously rolling the perforated cast billet to obtain a steel pipe; (5) A step of forcibly cooling the steel pipe and then reheating it; (6) Steps to perform tension reduction and cooling on the reheated steel pipe: After the tension reduction, the outer wall of the steel pipe is cooled with water, and the cooling start temperature of the steel pipe is set to ≥ Ar ３ The cooling end temperature of the steel pipe is controlled to be ≥ B ｆ and ≤ B ｓ A process of controlling the temperature to -100°C and controlling the cooling rate to be within the range of 25 to 35°C / s; (7) The process of straightening the cooled steel pipe; (8) Process of quenching the straightened steel pipe: Set the quenching temperature to Ac ３ +30 ≤ and ≤Ac ３ The temperature is controlled to +60°C, and after the above quenching, the steel pipe is cooled in stages by water cooling while rotating it. The water cooling is first performed by external water spraying, Ar ３ -70℃≦Inner wall temperature≦Ar ３ When the temperature drops below -30°C, a step is to inject water into the steel pipe from one end until the inner cavity of the steel pipe is filled with cooling water, thereby cooling the steel pipe to room temperature; (9) The process of tempering the quenched steel pipe; and (10) A process of straightening the steel pipe after it has been discharged from the furnace to obtain the steel pipe for the oil cylinder. A method that includes this.

10. The following conditions: In step (2), the heating is carried out at a temperature of 1250 to 1280°C for 3 to 4 hours; In step (3), the perforation is carried out at a temperature of 1100 to 1230°C; In step (4), the finish rolling is carried out at a temperature of 900 to 1000°C; In step (5), the steel pipe is Ar ３ Forced air cooling to below -50°C, followed by reheating to 950-980°C; In step (6), the tension drawing is performed at a temperature of 850 to 900°C; In step (7), the straightened steel pipe is allowed to cool naturally to room temperature; In step (9), the tempering is carried out at a temperature of (550 - 2 × t) °C; and In step (10), the straightening is performed at a temperature of ≥400°C. The method according to claim 9, which satisfies one or more of the following.

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