890 mpa-grade cylinder pipe and manufacturing method therefor

Abstract

The present disclosure provides a cylinder tube, comprising, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%. The present disclosure further provides a method for manufacturing the aforementioned cylinder tube. Through a rational design of chemical composition and, preferably, in combination with specific processes, the cylinder tube exhibits high strength, and possesses low residual stress and a low yield ratio, while exhibiting good weldability.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel and manufacturing method therefor, particularly to a cylinder tube and manufacturing method therefor.BACKGROUND

[0002] Currently, the demand for lightweighting in engineering machinery is increasing, imposing higher and higher strength requirements on cylinder tubes. However, as strength increases, the carbon equivalent of the tube material rises, leading to a decline in welding performance. At the same time, higher strength results in increased residual stress in the cylinder tube, causing dimensional changes of the tube material after machining and oil leakage from the cylinder.

[0003] The Chinese patent application with publication number CN116162849A, published on May 26, 2023, titled "A cylinder tube and manufacturing method therefor" discloses a cylinder tube. Its composition in percentage by mass is: C: 0.16-0.3%, Si: 0.15-0.5%, Mn: 1.2-1.8%, P ≤ 0.01%, S ≤ 0.001%, Nb: 0.02-0.04%, Mo: 0.1-0.2%; and when the cylinder tube wall thickness is ≥ 20 mm, Ti and B are added: Ti: 0.015-0.03%, B: 0.0015-0.0035%, with the balance of Fe and other inevitable impurities. After the steel tube undergoes stretch reducing and quenching, different stepwise cooling processes are applied. By increasing the rigidity and straightness of the steel tube and controlling the distribution of phase transformation and thermal stress across the entire wall thickness of the cylinder tube, as well as regulating the distribution of ferrite in the cylinder tube's microstructure, a cylinder tube with relatively high strength and low residual stress is obtained. This cylinder tube has a yield strength ≥ 600 MPa, tensile strength ≥ 730 MPa, and residual stress ≤ 50 MPa.

[0004] However, the strength of the steel material in the above patent document remains insufficiently high.SUMMARY

[0005] One of the objectives of the present disclosure is to provide a cylinder tube. Through a rational design of chemical composition and, preferably, further combined with process design, the cylinder tube exhibits high strength, and possesses low residual stress and a low yield ratio, while exhibiting good weldability.

[0006] To achieve the above objective, the present disclosure provides a cylinder tube, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%.

[0007] In one preferred embodiment, the cylinder tube of the present disclosure comprises the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%, and the balance of Fe and inevitable impurities.

[0008] In one preferred embodiment, the mass percentage contents of the chemical elements in the cylinder tube of the present disclosure further satisfy at least one of the following: Si: 0.15-0.35%; Mn: 1.1-1.5%; Cr: 0.5-1%.

[0009] In one preferred embodiment, a carbon equivalent of the cylinder tube of the present disclosure is 0.72 or less, preferably 0.45-0.65, more preferably 0.50-0.65, the carbon equivalent being calculated by the following formula: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the % sign of the mass percentage content of corresponding chemical element.

[0010] In one preferred embodiment, a microstructure of the outer wall and the center of the cylinder tube of the present disclosure is tempered sorbite + bainite, and a microstructure of the inner wall is tempered sorbite + bainite + ferrite.

[0011] In one preferred embodiment, a total volume fraction of bainite and ferrite in the microstructure of the inner wall is less than 20%, and a volume fraction of bainite in the microstructure of the outer wall and the center is less than 10%.

[0012] In one preferred embodiment, the properties of the cylinder tube of the present disclosure satisfy: a yield strength ≥ 890 MPa, a tensile strength ranging from 960 to 1100 MPa, a yield-to-tensile ratio ≤ 0.93, and / or an impact toughness KV 8 at -40 °C ≥ 45 J, preferably KV 8 ≥ 70 J.

[0013] In one preferred embodiment, a residual stress of the cylinder tube of the present disclosure is -100 to 48 MPa, preferably -30 to 40 MPa.

[0014] The steel material of the present disclosure adopts a composition design of a low-carbon, micro-alloyed steel grade containing W, Cr, and Mo, with the carbon equivalent controlled to ≤ 0.72, thereby ensuring good weldability.

[0015] In another aspect, the present disclosure further provides a method for manufacturing the cylinder tube, comprising the following steps: (1) Smelting and continuous casting to obtain a tube blank; (2) Heating, piercing, rolling, and sizing; (3) Online controlled cooling: wherein a starting cooling temperature is 800 °C or more, cooling is performed by spraying water onto outer wall with an average cooling rate of 20-40 °C / s, and a finishing cooling temperature is 600-680 °C; then air cooling to room temperature; (4) Quenching: wherein a heating temperature is 820-880 °C and a holding time is 20-40 min; after heating, cooling is performed by water cooling, and a steel tube is rotated when cooling, wherein water spray cooling is performed to the outer wall of the steel tube by external water pouring; and after cooling for 35-40 s, water spray cooling is performed to inner wall of the steel tube; and (5) Tempering.

[0016] In one preferred embodiment, in step (4) of quenching, for water spray cooling the outer wall of the steel tube by external water pouring, a flow rate density is 3000-3500 m 3< / (h*mm 2< ), where "h" represents hours.

[0017] In one preferred embodiment, in step (5) of tempering, a tempering temperature is 500-600 °C, and a holding time is 20-30 min.

[0018] In the manufacturing method of the present disclosure, the online controlled cooling process utilizes the residual heat of the tube after rolling to perform rapid cooling, refining the rolled microstructure, and thereby further refines the microstructure after quenching and tempering heat treatment, ensuring a good balance of strength and toughness.

[0019] Furthermore, in the manufacturing method of the present disclosure, during the quenching and tempering heat treatment process, a technique of first cooling the outer wall and then delaying water spray cooling of the inner wall is employed. The seamless tube manufactured through this process exhibits characteristics such as easy welding, low yield-to-tensile ratio, and low residual stress.DETAILED DESCRIPTION

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which the present disclosure pertains.

[0021] As used herein, the term "and / or" relates to and encompasses any and all possible combinations of one or more of the listed items.

[0022] Herein, Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the % sign of the mass percentage content of corresponding chemical element.

[0023] Herein, the volume fraction of the microstructure is determined through metallographic structure identification and phase proportion analysis methods.

[0024] Herein, the yield strength and tensile strength are determined in accordance with the GB / T 228.1-2000 standard.

[0025] Herein, the impact toughness KV 8 at -40 °C is determined in accordance with the GB / T 229-2007 standard, "Metallic materials-Charpy pendulum impact test method."

[0026] Herein, the residual stress is determined in accordance with the ISO / TR 10400 standard.

[0027] In the cylinder tube of the present disclosure, the design principles for each chemical element are as follows: C: In the cylinder tube of the present disclosure, C is a fundamental element for ensuring strength and hardenability. At the same time, the C content affects the carbon equivalent and weldability. If the C content in the steel is lower than 0.14%, the hardenability of the steel is poor, and its strength cannot meet the requirements. If the C content in the steel exceeds 0.20%, the excessive C content will lead to poor weldability of the steel and deteriorates its plasticity and toughness. Therefore, the present disclosure controls the mass percentage of C element to 0.14-0.20%. Si: In the cylinder tube of the present disclosure, Si element serves as an important deoxidizer in the steel. The Si element can be solid-solutioned in ferrite to improve the yield strength of the steel. However, it should be noted that the content of Si element in the steel should not be excessively high. When the content of Si element in the steel exceeds 0.55%, it will deteriorate the processibility and toughness of the steel material. When the content of Si element in the steel is too low (below 0.15%), it makes the steel material more prone to oxidation. Therefore, the present disclosure controls the mass percentage of Si element to 0.15-0.55%. Mn: In the cylinder tube of the present disclosure, Mn element serves as a deoxidizing and desulfurizing agent, which significantly affects the hardenability as well as the strength and toughness of the steel. When the content of Mn element in the steel is 0.5% or more, it can produce favorable effects. However, excessive Mn increases the hardenability of the steel excessively, reduces the toughness of the weld heat-affected zone of the steel, and may lead to center segregation during continuous casting, thereby impairing the impact toughness of the base material. Therefore, the present disclosure controls the mass percentage of Mn element to 0.5-1.5%. Cr: In the cylinder tube of the present disclosure, Cr element serves to enhance the strength and hardenability of the steel, and its effect is further improved when combined with Mo element. However, it should be noted that an excessively high Cr content in the steel significantly increases the carbon equivalent, thereby raising the susceptibility to weld cracking and reducing the toughness of the weld heat-affected zone. Therefore, the present disclosure controls the mass percentage of Cr element to 0.5-1.5%. Mo: In the cylinder tube of the present disclosure, Mo is one of the mainly added elements, which enhances the hardenability of the steel. The combined effect of Mo and Cr further improves the hardenability. Additionally, Mo element also contributes to better precipitation strengthening and solid solution strengthening effect. Mo interacts well with microalloying elements, effectively refining precipitates, increasing their stability and volume fraction, and improving the strength and toughness of the weld heat-affected zone. However, it should be noted that Mo is an expensive element. Adding excessively high amount of Mo to the steel not only leads to an excessively high carbon equivalent but also significantly increases the alloy cost. Therefore, the present disclosure controls the mass percentage of Mo element to 0.25-0.65%. W: W element serves as one of the mainly added elements in this steel grade. It effectively enhances strength through solution strengthening without increasing the carbon equivalent, has no significant adverse effect on weldability, and contributes to maintaining strength after welding. However, excessive W content may lead to surface defects in rolled tubes and significantly increase costs. Therefore, the present disclosure controls the mass percentage of W element to 0.3-0.8%. V: In the cylinder tube of the present disclosure, V element can refine the grains in the steel, and the carbides formed with its participation can significantly enhance the strength of the steel. However, when the addition amount of V element in the steel exceeds a certain level, its strengthening effect becomes less pronounced. Moreover, as V is a relatively expensive alloying element, it should not be added in excess. Therefore, the present disclosure controls the mass percentage of V element to 0.05-0.1%. Nb: The primary function of Nb element is to refine the phase transformation and grains through the precipitation of carbides. In the present disclosure, adding a small amount of Nb element refines the microstructure, improves the strength-toughness balance, and contributes to mitigating post-weld softening. However, an excessively high Nb content increases costs without providing additional refinement effects. Therefore, the present disclosure controls the mass percentage of Nb element to 0.02-0.06%. Al: In the cylinder tube of the present disclosure, Al serves as an effective deoxidizing element. However, excessive addition of Al to the steel can easily lead to the formation of alumina inclusions. To maximize the proportion of acid-soluble aluminum relative to total aluminum, an appropriate amount of Al wire is fed after vacuum degassing. Therefore, the present disclosure controls the mass percentage of Al element to 0.01-0.05%. Ca: In the cylinder tube of the present disclosure, Ca can purify the molten steel, promote the spheroidization of MnS, and improve the impact toughness of the material. However, it should be noted that the content of Ca element in the steel should not be excessively high. When the content of Ca element in the steel is too high, coarse nonmetallic inclusions tend to form, which adversely affect the performance of the steel. Therefore, the present disclosure controls the mass percentage of Ca element to 0.0005-0.005%.

[0028] Furthermore, it should be noted that in the cylinder tube of the present disclosure, there are also some inevitable impurity elements, primarily S and P. To obtain a cylinder tube with better performance and higher quality, the content of these impurity elements should be minimized as much as technically feasible.

[0029] Through the design of composition and process, the present disclosure obtains a steel material with the outer wall and central microstructure being tempered sorbite + a small amount of bainite, while the inner wall microstructure being tempered sorbite + a small amount of bainite + a small amount of ferrite. This is because, when only the outer wall is cooled, the temperature of the inner wall drops to Ar3 or less, causing partial ferrite precipitation. At this point, after water spray cooling is applied to the inner wall, the remaining austenite transforms into martensite and bainite, which then converts into tempered sorbite + a small amount of bainite + a small amount of ferrite during the tempering process.

[0030] In the manufacturing method of the present disclosure, in order to further refine the rolled microstructure and thereby utilize the microstructural inheritance characteristics to refine the final quenched and tempered microstructure, a rapid cooling technique is adopted after sizing, which effectively utilizes the residual heat of the rolled tube. The hot deformation during the sizing process results in numerous dislocations in the tube body, and these dislocations are effectively retained after rapid cooling to a certain temperature. These dislocations can serve as nucleation sites for phase transformation and precipitate formation. Additionally, the rapid cooling increases the supercooling for phase transformation, thereby enhancing the phase transformation driving force. These two aspects lead to a significantly refined rolled microstructure. Based on this, the specific parameters for the online controlled cooling adopted in the present disclosure are as follows: a starting cooling temperature ≥ 800 °C, cooling is performed by spraying water onto outer wall with an average cooling rate of 20-40 °C / s, and a finishing cooling temperature is 600-680 °C; then air cooling to room temperature, followed by subsequent quenching and tempering heat treatment. In the present disclosure, the online controlled cooling step, by outer wall water spray cooling, forms a hardened layer on the outer wall. This increases the deformation resistance of the outer wall, ensuring that the tube body is less prone to bending deformation during subsequent cooling process, thereby maintaining the straightness of the tube body. Simultaneously, it reduces the overall residual stress level of the tube. As the surface is a hardened layer, compressive stress is generated, enhancing the tube's tolerance to surface-like defects.

[0031] In the manufacturing method of the present disclosure, after the heating step of quenching, cooling is performed by water cooling, and a steel tube is rotated when cooling, wherein water spray cooling is performed to the outer wall of the steel tube by external water pouring, and after cooling for 35-40 s, water spray cooling is performed to the inner wall to ensure that areas with insufficient cooling effect during outer wall cooling achieve adequate cooling through the inner wall cooling. Since the entire length of the steel tube is cooled simultaneously by external water pouring and the tube is rotated during cooling, good cooling uniformity is ensured, and the stiffness of the steel tube is increased. The better cooling uniformity ensures that the steel tube achieves good straightness, avoiding significant residual stress caused by subsequent bending and straightening deformation of the tube. Compared to the method of simultaneous cooling with external water pouring on the outer wall + axial flow cooling on the inner wall, the aforementioned cooling method of the present disclosure can achieve better straightness.

[0032] Additionally, the residual stress in the steel tube is closely related to phase transformation and thermal stress during the cooling process. Through the aforementioned cooling method of the present disclosure, a gradient distribution of phase transformation and thermal stress is achieved along the wall thickness direction. After water cooling, the outer wall generates compressive stress due to thermal expansion and contraction, and the phase transformation stress is tensile stress, with the sum of the two ranging from 0 to 200 MPa. At the center of the wall thickness, the thermal stress is tensile stress, and the phase transformation stress is compressive stress, with the sum of the two being negative. For the inner wall, both the thermal stress and phase transformation stress are positive values, thereby posing a certain risk of cracking of the inner wall. Tempering treatment should be performed as soon as possible after cooling.

[0033] After tempering, the residual stress of the cylinder tube of the present disclosure along the wall thickness direction of the tube body is reduced, but the trend of the gradient distribution does not change. The overall residual stress level of the tube body after tempering is -100 to 48 MPa.

[0034] The cylinder tube and manufacturing method therefor of the present disclosure have the following advantages and beneficial effects: The cylinder tube of the present disclosure adopts a composition design of a low-carbon, microalloyed steel grade containing W, Cr, and Mo, with the carbon equivalent of the steel grade controlled to ≤ 0.72, thereby providing it with good weldability.

[0035] The online controlled cooling process of the present disclosure utilizes the residual heat of the rolled tube to perform rapid cooling, which refines the rolled microstructure, and thereby further refines the microstructure after quenching and tempering heat treatment, ensuring a good balance of strength and toughness. Additionally, during the quenching and tempering heat treatment process, a process of first cooling the outer wall, and then delaying water spray cooling of the inner wall is adopted. The seamless tube manufactured through this process exhibits characteristics such as easy welding, low yield-to-tensile ratio, and low residual stress.

[0036] The cylinder tube and manufacturing method therefor of the present disclosure will be further explained and illustrated below with reference to specific examples. However, such explanation and illustration shall not be construed as an improper limitation of the technical solution of the present disclosure.Examples 1-8 and Comparative Examples 1-8

[0037] The cylinder tubes of Examples 1-8 were manufactured by the following steps: (1) Smelting and continuous casting were performed according to conventional processes to obtain a tube blank; (2) The tube blank was heated, pierced, rolled, and sized according to conventional processes to obtain a tube body; (3) Utilizing the residual heat of the rolled tube, online controlled cooling was carried out after sizing: wherein a starting cooling temperature was ≥ 800 °C, cooling was performed by spraying water onto the outer wall with an average cooling rate of 20-40 °C / s, and a finishing cooling temperature was 600-680 °C; followed by air cooling to room temperature; (4) Quenching: a heating temperature was 820-880 °C and a holding time was 20-40 min; after heating, cooling was performed by water cooling, and the steel tube was rotated when cooling, wherein water spray cooling was performed to the outer wall of the steel tube by external water pouring; and after cooling for 35-40 s, water spray cooling was performed to the inner wall of the steel tube, until the tube body was cooled to 100 °C or less; optionally, for water spray cooling the outer wall of the steel tube by external water pouring, a flow rate density was 3000-3500 m 3< / (h*mm 2< ). The water spray pressure could be 0.5 MPa. (5) Tempering: the tempering temperature was controlled to 500-600 °C, with a holding time of 20-30 min.

[0038] The comparative cylinder tubes of Comparative Examples 1-8 were manufactured using the same steps described above, except for the compositional contents and specific process parameters.

[0039] Table 1 lists the contents of the chemical elements in percentage by mass for the cylinder tubes of Examples 1-8 and the comparative cylinder tubes of Comparative Examples 1-8. Table 1. (wt%, the balance of Fe and inevitable impurities)CSiMnCrMoWVNbAlCaCarbon equivalentExample 10.140.161.50.50.30.30.060.0250.010.00050.56Example 20.150.2510.60.250.350.070.0350.0250.0020.50Example 30.190.350.61.50.280.40.050.020.0350.00150.66Example 40.20.550.51.10.30.80.10.060.030.0030.58Example 50.170.450.71.10.60.550.10.0450.0250.0050.65Example 60.160.30.81.20.650.60.090.0450.0450.0030.68Example 70.150.280.91.30.40.70.080.0480.0250.00090.66Example 80.160.3211.40.50.550.070.0350.050.0010.72Comparative Example 10.23 0.250.60.80.50.50.0750.0250.030.0020.61Comparative Example 20.12 0.240.90.90.550.60.0850.030.020.0010.58Comparative Example 30.21 0.311.51.50.650.750.10.040.0350.00150.91Comparative Example 40.160.270.90.90.40.60.0950.060.0250.00250.59Comparative Example 50.170.30.60.60.50.450.0950.040.0250.0020.51Comparative Example 60.180.350.70.80.350.520.090.050.0250.0020.54Comparative Example 70.190.3210.80.50.430.080.0250.030.00150.63Comparative Example 80.160.231.10.80.550.70.070.020.040.0020.63Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. As Examples 1-8 and Comparative Examples 1-8 contained neither Cu nor Ni element, the numerical values for these two elements were substituted with 0 for calculation.

[0040] Table 2 lists the specific process parameters for the online controlled cooling and quenching and tempering treatment steps (i.e., the quenching and tempering steps) for the cylinder tubes of Examples 1-8 and the comparative cylinder tubes of Comparative Examples 1-8. Table 2.Online controlled coolingQuenchingTemperingStarting cooling temperature (°C)Average cooling rate (°C / s)Finishing cooling temperature (°C)Heating temperature (°C)Holding time (min)Delay time before starting inner spray (s)Water spray flow rate density (m 3< / (h *< mm 2< ))Temperature (°C)Holding time (min)Example 1800256008602535340051020Example 2810206808204036350050025Example 3820306308603537330051022Example 4805356208503235300051030Example 5850406008552038340051028Example 6820306108802540320060027Example 7830356008602839310053026Example 8800256458503038350054023Comparative Example 1810306308503836340052525Comparative Example 2810306208504037320051525Comparative Example 3820356408402538350055024Comparative Example 4 / / / 8303638330052025Comparative Example 583022670920 3436340057025Comparative Example 684035660800 2535340050026Comparative Example 7800256108402820 340052027Comparative Example 88103060088029 / 330053028Note: The three columns of online controlled cooling data for Comparative Example 4 are marked as " / ", indicating that the online controlled cooling process was not applied in this comparative example; the delay time before starting inner spray for Comparative Example 8 is marked as " / ", indicating that inner water spray was not performed in this comparative example.

[0041] Samples were taken from the cylinder tubes of Examples 1-8 and the comparative tubes of Comparative Examples 1-8 respectively, and subjected to testing. The obtained test results are listed in Table 3. The testing procedures for the relevant properties are described below: (1) Microstructure: A cross-section of the tube was taken for microstructural observation. The sample was ground and polished. The metallographic sample was etched using 4% nitric acid + alcohol. After etching, the microstructure was observed using a metallographic microscope. (2) Tensile properties: The room-temperature tensile properties were measured in accordance with the GB / T 228.1-2000 standard. (3) Impact toughness: The impact energy at -40 °C was measured in accordance with the GB / T 229-2007 standard, "Metallic materials-Charpy pendulum impact test method." (4) Weldability: Weldability evaluation was conducted in accordance with NB / T 47014-2011, "Welding procedure qualification for pressure equipment." (5) Residual stress: Residual stress was measured in accordance with the ISO / TR 10400 standard.

[0042] Table 3 lists the test results of the cylinder tubes from Examples 1-8 and the comparative cylinder tubes from Comparative Examples 1-8. Table 3Bainite volume fraction in the outer wall and central microstructure (%)Total volume fraction of ferrite and bainite in the inner wall microstructure (%)Yield strength (MPa)Tensile strength (MPa)Yield-to-tensile ratioImpact toughness KV 8 at -40 °C (J)Residual stress (MPa)WeldabilityExample 151093010300.908030GoodExample 261193510250.918020GoodExample 34894010500.907025GoodExample 471095510550.9190-5GoodExample 53893010500.8910010GoodExample 65898010850.9090-15GoodExample 74996010800.8980-30GoodExample 831590010300.878540GoodComparative Example 121494010700.886035Poor Comparative Example 2415860 10200.8420 -35GoodComparative Example 331496010950.8840 0Poor Comparative Example 42993010400.8940 80 GoodComparative Example 511099010550.94 9030GoodComparative Example 6820 850 9600.883025GoodComparative Example 75896010200.94 95250 GoodComparative Example 8412880 9900.897010Good

[0043] As can be seen from Table 3, all of the cylinder tubes of Examples 1-8 exhibited yield strength ≥ 900 MPa, tensile strength ≥ 1025 MPa, impact energy at -40 °C ≥ 70 J, yield-to-tensile ratio ≤ 0.91, residual stress ranging from -30 to 40 MPa, and additionally had good weldability. In contrast, Comparative Examples 1-3 failed to achieve a good balance between high strength, low yield-to-tensile ratio, good low-temperature toughness, and excellent weldability.

[0044] All publications, patent applications, patents, and other references mentioned in the present disclosure are hereby incorporated by reference in their entirety.

[0045] While the present disclosure has been illustrated and described with reference to certain preferred embodiments herein, it should be understood by those skilled in the art that the above description is provided in further detail in conjunction with specific embodiments and should not be construed as limiting the specific implementations of the present disclosure to these descriptions. Those skilled in the art may make various changes in form and detail, including several simple deductions or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

1. A cylinder tube, comprising, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%.

2. The cylinder tube as claimed in claim 1, wherein the cylinder tube comprises the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%, and the balance of Fe and inevitable impurities.

3. The cylinder tube as claimed in claim 1 or 2, wherein the mass percentage contents of the chemical elements in the cylinder tube further satisfy at least one of the following: Si: 0.15-0.35%; Mn: 1.1-1.5%; Cr: 0.5-1%.

4. The cylinder tube as claimed in any one of claims 1-3, wherein a carbon equivalent of the cylinder tube is 0.72 or less, preferably 0.45-0.65, more preferably 0.50-0.65, the carbon equivalent being calculated by the following formula: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where each chemical element symbol is substituted with the numerical value before the % sign of the mass percentage content of corresponding chemical element.

5. The cylinder tube as claimed in any one of claims 1-4, wherein a microstructure of an outer wall and a center of the cylinder tube is tempered sorbite + bainite, and a microstructure of an inner wall is tempered sorbite + bainite + ferrite.

6. The cylinder tube as claimed in claim 5, wherein a total volume fraction of bainite and ferrite in the microstructure of the inner wall is less than 20%, and a volume fraction of bainite in the microstructure of the outer wall and the center is less than 10%.

7. The cylinder tube as claimed in any one of claims 1-6, wherein the properties of the cylinder tube satisfy: a yield strength ≥ 890 MPa, a tensile strength ranging from 960 to 1100 MPa, a yield-to-tensile ratio ≤ 0.93, and / or an impact toughness KV8 at -40 °C ≥ 45 J, preferably KV8 ≥ 70 J.

8. The cylinder tube as claimed in any one of claims 1-7, wherein a residual stress of the cylinder tube is -100 to 48 MPa, preferably -30 to 40 MPa.

9. A method for manufacturing the cylinder tube as claimed in any one of claims 1-8, comprising the following steps: (1) Smelting and continuous casting to obtain a tube blank; (2) Heating, piercing, rolling, and sizing to obtain a tube body; (3) Online controlled cooling: wherein a starting cooling temperature is 800 °C or more, cooling is performed by spraying water onto outer wall with an average cooling rate of 20-40 °C / s, and a finishing cooling temperature is 600-680 °C; then air cooling to room temperature; (4) Quenching: wherein a heating temperature is 820-880 °C and a holding time is 20-40 min; after heating, cooling is performed by water cooling, and a steel tube is rotated when cooling wherein water spray cooling is performed to the outer wall of the steel tube by external water pouring; and after cooling for 35-40 s, water spray cooling is performed to inner wall of the steel tube; and (5) Tempering.

10. The method as claimed in claim 9, wherein in step (4) of quenching, for water spray cooling the outer wall of the steel tube by external water pouring, a flow rate density is 3000-3500 m3 / (h*mm2).

11. The method as claimed in claim 9 or 10, wherein in step (5) of tempering, a tempering temperature is 500-600 °C, and a holding time is 20-30 min.

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