Thick-walled high-toughness low-yield-ratio steel for petroleum casing pipe, petroleum casingpipe, and manufacturing method for petroleum casing pipe

Abstract

The present invention relates to a thick-walled high-toughness low-yield-ratio steel for a petroleum casing pipe, a petroleum casing pipe, and a manufacturing method for the petroleum casing pipe. In addition to Fe and inevitable impurities, the steel further comprises the following chemical components in percentage by mass: C: 0.30-0.40%, Si: 0.10-0.25%, Mn: 1.0-1.5%, Ti: 0.01-0.10%, Cr: 0.1-1.0%, Al: 0.015-0.035%, Ni≤0.2%, Nb: 0.010-0.040%, P≤0.015%, S≤0.003%, and N≤0.005%, satisfying the following relational expressions: C+Mn+5Cr-2Si+Ti+Nb≥1.8%, preferably 1.800-6.288%, and C+Mn / 6+Cr / 10+Ni / 15≤0.65%, wherein element symbols in the expressions are substituted with the mass percentage contents of the corresponding elements. The steel of the present invention has a yield strength of 379-552 MPa, a tensile strength of greater than or equal to 655 MPa, a yield ratio of less than or equal to 0.700, and a 0°C transverse Charpy impact energy of more than 50 J. The steel of the present invention can be used for producing a petroleum casing pipe having stable performance and a wall thickness of greater than or equal to 14 mm, wherein the petroleum casing pipe has a yield ratio of less than or equal to 0.750.

Description

Thick-wall high-toughness low-yield-to-tensile-ratio oil casing steel, oil casing and manufacturing method thereof TECHNICAL FIELD

[0001] The present application belongs to the technical field of oil casing, and particularly relates to a thick-wall high-toughness low-yield-to-tensile-ratio oil casing steel, an oil casing and a manufacturing method thereof. BACKGROUND

[0002] Oil casing is the lifeline of maintaining the operation of an oil well. Due to different geological conditions, the stress state under the well is complex, and tensile, compressive, bending and torsional stresses are comprehensively applied to the pipe body of the oil casing, which puts forward higher requirements on the performance of the oil casing. K55 is a casing according to the API SPEC 5CT standard, and the yield strength requirement is 379-552 MPa, the tensile strength requirement is greater than 655 MPa, and the upper limit of the yield-to-tensile ratio is 0.84. The yield-to-tensile ratio of the steel pipe material used for the oil casing is reduced, plastic deformation is easily generated under external force, but the steel pipe will not easily break, and the plastic deformation can be used as a sign of fracture. Therefore, for some special geological conditions and working conditions, the lower the yield-to-tensile ratio of the steel pipe, the higher the safety and the better the reliability.

[0003] According to the API standard, K55 steel grade oil casing has two types of seamless pipes and welded pipes, and the welded pipe has the advantages of high production efficiency, good size accuracy, wide specification range and low cost. However, for K55 steel grade high-frequency resistance welding (HFW) casing with low yield-to-tensile ratio, especially large specification thick-wall casing, the production difficulty is very high, and only a few manufacturers can produce it.

[0004] Chinese patent 1 (publication number CN102828121A) discloses a K55 grade high-frequency resistance welding oil casing steel and a manufacturing method thereof, and the oil casing steel has the following chemical components: C: 0.055-0.095%, Si: 0.15-0.30%, Mn: 1.50-1.85%, P≤0.020%, S≤0.008%, Ti: 0.010-0.050%, Als: 0.02-0.06%, Nb: 0.04-0.07%, Cr: 0.23-0.31%, V: 0.01-0.04%, N≤0.008%, and the rest is Fe and inevitable impurities, and the microstructure is a mixed structure of ferrite, bainite and M / A.

[0005] Chinese patent 2 (publication number CN101845583A) discloses a kind of oil casing steel and its manufacturing method, the oil casing steel has the following chemical components: C: 0.31~0.35%, Si: 0.1~0.45%, Mn: 0.8~1.5%, P≤0.02%, S≤0.01%, Al≤0.024%, V≤0.1%, Mo≤0.35%, the rest is Fe and inevitable impurities, its yield ratio is controlled at 0.56~0.80, meets the performance requirement of K55 oil casing to hot-rolled steel coil.

[0006] US patent 1 (publication number US10738371B2) discloses a kind of K55 resistance welded oil well pipe and hot-rolled steel plate, the oil well pipe has the following chemical components: C: 0.30~0.50%, Si: 0.05~0.40%, Mn: 0.50~1.20%, P≤0.030%, S≤0.020%, Al: 0.002~0.080%, N≤0.0080%, Cu≤0.30%, Ni≤0.30%, Cr≤0.30%, Mo≤0.10%, V≤0.10%, Nb≤0.050%, Ti≤0.030%, Ca≤0.0100%, the rest is Fe and inevitable impurities, its organization is ferrite+pearlite. SUMMARY

[0007] The purpose of the present application is to provide a kind of oil casing steel with high toughness and low yield ratio of wall thickness, oil casing and its manufacturing method.The yield strength of the steel is 379~552MPa, tensile strength ≥655MPa, yield ratio ≤0.700, has excellent welding performance and impact toughness, 0 ℃ transverse charpy impact energy >50J.The steel can be used to produce and manufacture the stable performance of K55 oil casing of wall thickness ≥14mm, the yield ratio of the K55 oil casing is ≤0.750.

[0008] To achieve the above object, the first aspect of the present application provides a kind of oil casing steel with high toughness and low yield ratio of wall thickness, in addition to containing Fe and inevitable impurities, the steel also contains the following chemical components with mass percentage content as follows: C:0.30~0.40%, Si:0.10~0.25%, Mn:1.0~1.5%, Ti:0.01~0.10%, Cr:0.1~1.0%, Al:0.015~0.035%, Ni≤0.20%, preferably 0.1~0.2%, Nb:0.010~0.040%, P≤0.015%, S≤0.003%, N≤0.005%, and satisfy the following relationship: C+Mn+5Cr-2Si+Ti+Nb≥1.800%, preferably 1.800~6.288%, C+Mn / 6+Cr / 10+Ni / 15≤0.650%, wherein each element symbol is substituted into the mass percentage content of each element.

[0009] In the present disclosure, by controlling the contents of C and Mn, the carbon equivalent is controlled to ensure good welding performance and avoid the damage to toughness caused by too high contents of C and Mn; meanwhile, Ti and Nb micro-alloying is used, on the one hand, the Ti and Nb carbonitrides formed at high temperature can inhibit the growth of austenite, thereby refining the grains, improving the toughness and increasing the impact energy, on the other hand, the Ti and Nb carbonitrides can act as the nucleation sites of proeutectoid ferrite, promoting the precipitation of soft ferrite, thereby reducing the yield ratio. In order to better reduce the yield ratio, the present disclosure also reduces the content of Si to reduce the strength of soft ferrite. The present disclosure adds a certain amount of Cr element, Cr is a typical ferrite-forming element and has a certain grain coarsening effect, which can coarsen ferrite, realize ferrite modification, promote the precipitation of polygonal ferrite and inhibit acicular ferrite, thereby increasing the volume fraction of soft ferrite. In addition, the present application can preferably add a certain amount of Ni to reduce the brittle-ductile transition temperature and improve the low-temperature impact toughness.

[0010] Preferably, the steel contains the following chemical components with mass percentage content as follows: C:0.30~0.40%, Si:0.10~0.25%, Mn:1.0~1.5%, Ti:0.01~0.10%, Cr:0.1~1.0%, Al:0.015~0.035%, Ni≤0.2%, preferably 0.1~0.2%, Nb:0.010~0.040%, P≤0.015%, S≤0.003%, N≤0.005%, and the balance is Fe and inevitable impurities.

[0011] Preferably, the steel has a chemical composition with mass percentage content satisfying at least one of the following: C: 0.32-0.38%, and / or, Si: 0.10-0.20%, and / or, Mn: 1.0-1.3%, and / or, Ti: 0.01-0.04%, and / or, Cr: 0.2-0.5%, and / or, Al: 0.02-0.03%; P≤0.012%, and / or, S≤0.002%.

[0012] Preferably, the steel according to the present application has a metallographic structure at room temperature of ferrite + pearlite, wherein the volume fraction of ferrite is >10%, preferably 12.0-23.5%; preferably, the type of ferrite is polygonal ferrite + acicular ferrite, wherein the proportion of polygonal ferrite in the total volume of ferrite is ≥85%, and the rest is acicular ferrite.

[0013] Preferably, the steel according to the present application has a yield strength of 379-552 MPa, a tensile strength ≥655 MPa, a yield strength ratio ≤0.700, and a 0℃ transverse Charpy impact energy >50 J.

[0014] In the chemical composition design of the steel for oil casing according to the present application:

[0015] C: C is the main solid solution strengthening element, which can improve the strength of the steel, but too high C content will reduce the toughness and significantly worsen the weldability of the material. Therefore, the C content is controlled to be 0.30-0.40%, preferably C: 0.32-0.38%.

[0016] Si: Si is a commonly used deoxidizer, which has a strong solid solution strengthening effect on ferrite, but it should not be too high. Therefore, the Si content is controlled to be 0.10-0.25%, preferably Si: 0.10-0.20%.

[0017] Mn: Mn is one of the important alloying elements, which strongly improves the hardenability of the steel. A certain amount of Mn content is needed to ensure the pearlite content and strength, but too high Mn content has the disadvantage of promoting the segregation of the structure, which is not conducive to the toughness and weldability. Therefore, the Mn content is controlled to be 1.0-1.5%, preferably Mn: 1.0-1.3%.

[0018] Ti: Ti is the strongest carbide-forming element, which also has a strong affinity for nitrogen. High-temperature carbonitride can significantly refine the austenite grains, promote the precipitation of proeutectoid ferrite, reduce the yield ratio and improve the impact energy. In addition, due to the effects of grain refinement and carbon fixation, it is also beneficial to the weldability. However, too high Ti content is easy to form coarse precipitates, which will damage the performance. Therefore, the Ti content is controlled to be 0.01-0.10%, preferably Ti: 0.01-0.04%.

[0019] Nb: Nb is a strong carbonitride forming element, which can refine grains, improve strength and impact toughness, and the carbonitride can be used as a proeutectoid ferrite nucleation site. Therefore, the content of Nb is controlled to be 0.010-0.040% in the application.

[0020] Cr: Cr increases hardenability and has a certain grain coarsening effect, can modify ferrite, promote the generation of polygonal ferrite, increase the volume fraction of ferrite, and reduce the yield ratio. Therefore, the content of Cr is controlled to be 0.1-1.0% in the application, preferably, Cr: 0.2-0.5%.

[0021] Ni: Ni can reduce the ductile-brittle transition temperature, improve the plasticity and toughness of the steel, especially the low-temperature toughness. Therefore, the content of Ni is controlled to be ≤0.2% in the application, preferably 0.1-0.2%.

[0022] Al: Al is a commonly used deoxidizing element, which can refine grains. Therefore, the content of Al is controlled to be 0.015-0.035% in the application, preferably, Al: 0.02-0.03%.

[0023] P and S: P and S are conventional harmful impurity elements in steel, which are seriously segregated in steel and are not conducive to plasticity and toughness. When the content of S is too high, MnS is generated, which damages impact toughness, and SO2 generated during welding forms bubbles and porosity. Therefore, the content of P and S should be reduced as much as possible, especially the content of S. In the application, P≤0.015%, S≤0.003%, preferably, P≤0.012%, S≤0.002%.

[0024] N: N has no significant strengthening effect, and the formation of nitrides such as coarse TiN will damage the strengthening effect of micro-alloy elements and reduce the performance of the material. Therefore, the content of N is controlled to be ≤0.005% in the application.

[0025] Preferably, the chemical composition design of the steel must satisfy the following relationships at the same time: C+Mn+5Cr-2Si+Ti+Nb≥1.800% (preferably 1.800-6.288%) (1), C+Mn / 6+Cr / 10+Ni / 15≤0.65% (2).

[0026] C and Mn are important strengthening elements, which can improve the tensile strength and thus reduce the yield ratio; Cr promotes the generation of polygonal ferrite and thus reduces the yield ratio; Si is a ferrite strengthening element, which increases the strength of soft ferrite and is not conducive to the reduction of the yield ratio; Ti and Nb are proeutectoid ferrite nucleation sites, which are conducive to the generation of ferrite and the reduction of the yield ratio. Therefore, the content of C+Mn+5Cr-2Si+Ti+Nb is controlled to be ≥1.800%, preferably 1.800-6.288% in the application to ensure a lower yield ratio.

[0027] Meanwhile, since a welding process is required to manufacture the large-size welded pipe, the elements of C, Mn, Cr, Ni and the like are not conducive to the welding stability. Therefore, in order to avoid abnormal conditions such as cracking during pipe manufacturing, the present application should control C+Mn / 6+Cr / 10+Ni / 15≤0.65%.

[0028] The second aspect of the present application provides a high-toughness and low-yield-strength-ratio oil casing made of the above-mentioned steel.

[0029] Preferably, the wall thickness of the oil casing of the present application is ≥14 mm, preferably 14-20 mm, the yield strength is 379-552 MPa, the tensile strength is ≥655 MPa, the yield strength ratio is ≤0.750, and the 0℃ transverse Charpy impact energy is >50 J.

[0030] The third aspect of the present application provides a method for manufacturing a high-toughness and low-yield-strength-ratio oil casing, which comprises the following steps in sequence:

[0031] 1) Smelting and casting:

[0032] Smelting and casting are performed based on the chemical composition of the above-mentioned steel, wherein the casting is performed by using a continuous casting process, the overheat degree of the molten steel is controlled to be ≤30℃, and a continuous casting billet is obtained.

[0033] 2) Rolling:

[0034] The continuous casting billet is heated to 1180-1230℃ and then hot-rolled to obtain a hot-rolled plate strip, wherein the finish rolling temperature is controlled to be 815-880℃, preferably 820-880℃.

[0035] 3) Cooling and coiling:

[0036] The hot-rolled plate strip is cooled to 560-620℃ at a cooling speed of 10-15℃ / s and then coiled to obtain a coil, wherein the coiling temperature satisfies the following relationship: |T 卷取 -590|+4D≤90, preferably 70-90; wherein T 卷取 is the coiling temperature, unit: ℃; and D is the wall thickness of the oil casing, unit: mm.

[0037] 4) Butt welding of the coil:

[0038] The coil is butt welded to obtain a butt-welded coil, wherein the wire feeding speed of the butt welding is 7-11 m / min, the welding speed of the butt welding is 45-50 cm / min, and the wire extension of the butt welding is 15-25 mm.

[0039] 5) the butt-welded plate coil is subjected to pipe-making by high-frequency resistance welding to obtain a welded pipe, wherein the pipe-making is performed under the following conditions: a welding temperature of 1250-1350 DEG C, a welding power of 850±25 kW, and a pipe-making welding speed of 16±3 m / min;

[0040] 6) heat treatment:

[0041] the welded pipe or the welded pipe weld is subjected to on-line normalizing heat treatment to obtain the oil casing, wherein the on-line normalizing heat treatment is performed at a temperature of 900-950 DEG C. Preferably, in step 1), converter steelmaking, secondary refining, and vacuum degassing are used for smelting.

[0042] The manufacturing process of the oil casing described in the present application strictly controls rolling, cooling, and coiling to ensure the performance of the steel plate. Reducing the finish rolling temperature can avoid excessively coarse grains and damage the impact toughness. Controlling a certain cooling speed and coiling temperature can ensure the occurrence of ferrite+pearlite transformation. Cooling+reducing the coiling temperature can promote the refinement of pearlite and improve the tensile strength to reduce the yield strength ratio. However, excessively large cooling speed, excessively low temperature, excessively large difference between the finish rolling temperature and the coiling temperature can easily lead to excessively high strength, and even abnormal bainite structure, and excessively high coiling temperature can easily lead to low strength. Therefore, using a coiling temperature of 560-620 DEG C can ensure a certain supercooling degree to improve the tensile strength, and can promote the diffusion of carbon elements to form ferrite to reduce the yield strength. Meanwhile, the coiling temperature needs to be controlled according to the wall thickness, |T 卷取 -590|+4D≤90, preferably 70-90, to avoid the formation of uneven abnormal structure and damage the performance when the steel plate is excessively thick.

[0043] The present application first butt-welds the plate coil, then uses the high-frequency resistance welding process to make the pipe, and performs normalizing heat treatment on the whole welded pipe to ensure the stability of the welded pipe and avoid abnormal weld performance and cracking.

[0044] In the prior art, the oil casing is designed by adjusting C, Si, Mn+NbVTi micro-alloying, the strength and yield strength ratio are controlled in the performance design, and there is no special design for the large-wall-thickness specification casing. In the present application, the ferrite is modified by using Cr elements, the structure is controlled to be ferrite+pearlite, the manufactured oil casing has a wall thickness of ≥14 mm, preferably 14-20 mm, a yield strength ratio of ≤0.750, and a 0 DEG C transverse Charpy impact energy of >50 J.

[0045] Compared with the prior art, the present application has the following beneficial effects:

[0046] (1) The present application realizes ferrite modification by optimizing component design and adding Cr element, forms polygonal ferrite, and reduces yield ratio; adopts fine-grain solid carbon design of alloy elements to obtain excellent weldability and impact toughness; adopts low-cost alloy elements such as Ti, and has excellent economy and market competitiveness.

[0047] (2) The present application can produce large-size welded sleeves by reducing heating temperature, strictly controlling rolling, cooling and coiling processes, reducing yield ratio while improving toughness, and cooperating with welding process.

[0048] (3) The present application can produce oil casing steel with high toughness and low yield ratio, wherein the volume fraction of ferrite is > 10%; the yield strength of the steel is 379-552 MPa, the tensile strength is ≥ 655 MPa, the yield ratio is ≤ 0.700, and the 0℃ transverse Charpy impact energy is > 50J; meanwhile, oil casing with wall thickness ≥ 14 mm, preferably 14-20 mm, yield strength 379-552 MPa, tensile strength ≥ 655 MPa, yield ratio ≤ 0.750, and 0℃ transverse Charpy impact energy > 50J can also be manufactured. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a microstructure photograph of the oil casing of Example 1 under an optical microscope.

[0050] Figure 2 is a microstructure photograph of the oil casing of Comparative Example 1 under an optical microscope. DETAILED DESCRIPTION

[0051] The present application will be further explained and described below in conjunction with examples and the accompanying drawings. It should be clear that the following examples are only used to describe the specific embodiments of the present application and do not constitute any limitation on the scope of protection of the present application.

[0052] Examples 1-10 and Comparative Examples 1-5.

[0053] The method for manufacturing oil casing in Examples 1-10 and Comparative Examples 1-5 comprises the following steps in sequence:

[0054] 1) Smelting and casting:

[0055] Smelting and casting are carried out based on the chemical composition of the steel described in Table 1, wherein the casting adopts continuous casting process, and the superheat of the molten steel is controlled to be ≤ 30℃, to obtain a continuous casting billet;

[0056] 2) Rolling:

[0057] The continuous casting billet is heated and then hot-rolled to obtain a hot-rolled plate strip;

[0058] 3) Cooling + coiling:

[0059] cooling the hot-rolled strip, and then coiling to obtain a coil;

[0060] 4) butt welding of the coil:

[0061] butt welding the coil to obtain a butt-welded coil;

[0062] 5) pipe-making by high-frequency resistance welding of the butt-welded coil to obtain a welded pipe;

[0063] 6) heat treatment:

[0064] carrying out on-line normalizing heat treatment on the welded pipe weld to obtain the oil casing.

[0065] Table 1 shows the chemical composition of the steel of Examples 1-10 and Comparative Examples 1-5 of the present application, with the balance being Fe and unavoidable impurities.

[0066] Table 2 shows the specific manufacturing process parameters of the oil casing of Examples 1-10 and Comparative Examples 1-5 of the present application.

[0067] The yield strength and tensile strength of the steel and the oil casing were sampled in accordance with the standard GB / T 228.1-2021 “Metallic Materials-Tensile Testing-Part 1: Method of Tensile Test at Room Temperature” for room temperature tensile property testing.

[0068] The steel and the oil casing were processed into V-shaped impact specimens in accordance with the standard GB / T 229-2020 “Metallic Materials Charpy Pendulum Impact Test Method”, and then 0℃ transverse impact property testing was carried out.

[0069] The microstructure of the steel and the oil casing was prepared according to the standard GB / T 13298-2015 “Metallic Materials Microstructure Test Method”, and different microstructures were distinguished and measured using image analysis software. Specifically, after the metallographic preparation was completed, different microstructure types were determined manually, and different metallographic structures were color-coded using image software. Finally, the area ratio was obtained by the ratio of the number of pixels of different metallographic structures to the number of pixels of the entire metallographic photograph. The volume fraction of ferrite means the area ratio of ferrite in different microstructures under the metallographic microscope. The determination of the proportion of polygonal ferrite (equiaxed polygonal or massive) and acicular ferrite (fine and long needle-like or lath-like) in ferrite also refers to the above process.

[0070] The wall thickness of the oil casing was tested in accordance with the standard API Spec 5CT.

[0071] Table 3 lists the mechanical properties and microstructure composition of the steel plates of the examples and comparative examples of the present application.

[0072] Table 4 lists the mechanical properties and wall thickness of the oil casings made from the examples and comparative examples of the present application.

[0073] As can be seen from the data in the table, the thick-walled K55 oil casing with high toughness and low yield ratio can be produced by the embodiment of the present application. The microstructure of the steel plate for the oil casing produced by the embodiment of the present application is ferrite + pearlite, the yield strength is between 431-530 MPa, the tensile strength is ≥663 MPa, the yield ratio is <0.68, the 0℃ impact energy is between 54-85 J, the ferrite volume fraction is 12.0-23.5%, wherein the proportion of polygonal ferrite in the total volume of ferrite is ≥85%, and the rest is acicular ferrite. The yield strength of the oil casing produced by the embodiment of the present application is between 460-549 MPa, the tensile strength is ≥671 MPa, the yield ratio is <0.71, the 0℃ impact energy is between 52-83 J, and the wall thickness is 14-20 mm.

[0074] Compared with the embodiments 1-10, the comprehensive performance of the steel and the oil casing obtained in the comparative example 1-5 is poor.

[0075] The manufacturing process of the steel and the oil casing in the comparative example 1 is unreasonable, wherein the coiling temperature is too low, and the strength and impact performance of the obtained steel and oil casing do not meet the requirements.

[0076] The steel and the oil casing in the comparative example 2 are free of Ti, high in Nb, and high in C content, the ferrite volume fraction is too low, and the yield strength is higher than the performance requirement value.

[0077] The steel and the oil casing in the comparative example 3 are free of Cr, and the yield ratio is too high.

[0078] The steel and the oil casing in the comparative example 4 are free of C+Mn / 6+Cr / 10+Ni / 15, which reaches 0.787, resulting in reduced toughness, difficult welding, and the obtained oil casing is fractured during the weld cold bending test.

[0079] The steel and the oil casing in the comparative example 5 are low in carbon content, good in toughness, but low in strength, which does not meet the requirements.

[0080] FIG. 1 is a microstructure photograph of the oil casing in the embodiment 1 under an optical microscope. As can be seen from the figure, the microstructure of the oil casing in the embodiment 1 at room temperature is ferrite (mainly polygonal ferrite, the proportion of which in the total volume of ferrite is 95%) + pearlite.

[0081] FIG. 2 is a microstructure photograph of the oil casing in the comparative example 1 under an optical microscope. As can be seen from the figure, the microstructure of the oil casing in the comparative example 1 is multi-acicular ferrite + bainite.

[0082] In summary, the steel for petroleum casing pipe with high toughness and low yield ratio can be produced by the present application, and the steel produced by the present application has excellent welding performance, and the K55 casing pipe with large size and thick wall can be manufactured by the steel, and has excellent market competitiveness.

[0083] Table 4 is the mechanical property parameters and wall thickness of the petroleum casing pipe:

Claims

1. A steel, characterized in that, The steel comprises, in addition to Fe and inevitable impurities, the following chemical components with mass percentage content: C: 0.30-0.40%, Si: 0.10-0.25%, Mn: 1.0-1.5%, Ti: 0.01-0.10%, Cr: 0.1-1.0%, Al: 0.015-0.035%, Ni≤0.2%, preferably 0.1-0.2%, Nb: 0.010-0.040%, P≤0.015%, S≤0.003%, N≤0.005%, and satisfies the following relations: C+Mn+5Cr-2Si+Ti+Nb≥1.800%, preferably 1.800-6.288%, C+Mn / 6+Cr / 10+Ni / 15≤0.650%, wherein each element symbol is substituted for the mass percentage content of each element.

2. Steel according to claim 1, characterized in that The steel comprises the following chemical components with mass percentage content: C: 0.30-0.40%, Si: 0.10-0.25%, Mn: 1.0-1.5%, Ti: 0.01-0.10%, Cr: 0.1-1.0%, Al: 0.015-0.035%, Ni≤0.2%, preferably 0.1-0.2%, Nb: 0.010-0.040%, P≤0.015%, S≤0.003%, N≤0.005%, and the balance is Fe and inevitable impurities.

3. Steel according to claim 1 or 2, characterized in that The mass percentage content of the chemical components of the steel satisfies at least one of the following: C: 0.32-0.38%, and / or, Si: 0.10-0.20%, and / or, Mn: 1.0-1.3%, and / or, Ti: 0.01-0.04%, and / or, Cr: 0.2-0.5%, and / or, Al: 0.020-0.030%; P≤0.012%, and / or, S≤0.002%.

4. Steel according to any one of claims 1 - 3, characterized in that, The steel has a metallographic structure at room temperature of ferrite+pearlite, wherein the volume fraction of ferrite is >10%, preferably 12.0-23.5%; preferably, the type of ferrite is polygonal ferrite+acicular ferrite, wherein the proportion of polygonal ferrite in the total volume of ferrite is ≥85%, and the rest is acicular ferrite.

5. Steel according to any one of claims 1 - 4, characterized in that, The steel has a yield strength of 379-552 MPa, a tensile strength ≥655 MPa, a yield strength ratio ≤0.700, and a 0℃ transverse Charpy impact energy >50 J.

6. A petroleum casing characterized by, The oil casing is made of the steel according to any one of claims 1-5.

7. The oil casing of claim 6, wherein, The oil casing has a wall thickness ≥14 mm, a yield strength of 379-552 MPa, a tensile strength ≥655 MPa, a yield strength ratio ≤0.750, and a 0℃ transverse Charpy impact energy >50 J.

8. A method of manufacturing the oil casing according to claim 6 or 7, characterized in that, The method comprises the following steps performed in sequence: 1) smelting and casting: The smelting and casting are performed based on the chemical composition of the steel according to any one of claims 1-5, wherein the casting is performed by a continuous casting process, the superheat of the molten steel is controlled to be ≤30℃, and a continuous casting billet is obtained; 2) rolling: The continuous casting billet is heated to 1180-1230 °C and hot-rolled to obtain a hot-rolled strip, wherein the finishing temperature is controlled to be 815-880 °C, preferably 820-880 °C; 3) cooling + coiling: The hot-rolled strip is cooled to 560-620°C at a cooling rate of 10-15°C / s and then coiled to obtain a coil, wherein the coiling temperature satisfies the following relationship: |T 卷取 - 590 + 4D < 90, preferably 70-90; wherein T 卷取 is the coiling temperature in °C; and D is the wall thickness of the oil casing in mm. 4) butt welding of the coil: The coil is butt welded to obtain a butt-welded coil, wherein the wire feeding speed of the butt welding is 7-11 m / min, the welding speed of the butt welding is 45-50 cm / min, and the wire extension of the butt welding is 15-25 mm; 5) pipe-making of the butt-welded coil by high-frequency resistance welding to obtain a welded pipe, wherein the pipe-making is performed under the following conditions: welding temperature is 1250-1350 °C, welding power is 850±25 kW, and the welding speed of the pipe-making is 16±3 m / min; 6) heat treatment: The welded pipe or the welded seam of the welded pipe is subjected to on-line normalizing heat treatment to obtain the oil casing, wherein the temperature of the on-line normalizing heat treatment is 900-950 °C.

9. The method of claim 8, wherein, In step 1), the smelting is performed by converter steelmaking, secondary refining, and vacuum degassing.

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