Ultra-high strength oil casing resistant to hydrogen sulfide corrosion and method for manufacturing the same

Abstract

The application discloses an ultra-high strength oil casing with H2S corrosion resistance, which contains Fe and inevitable impurities, and additionally contains the following chemical elements: C: 0.15-0.28%, Si: 0.1-0.5%, Mn: 0.2%-0.5%, Cr: 0.4-0.8%, Mo: 0.8-1.5%, V: 0.15-0.3%, Nb: 0.05-0.15%, W: 0.2-0.6, Al: 0.01-0.05%, Ce: 0.002-0.006%; 0O≤0.002%; wherein the mass percentage of Ce and O satisfies Ce / O: 1-3.5. Correspondingly, the application also discloses a manufacturing method of the above-mentioned ultra-high strength oil casing with H2S corrosion resistance. Through reasonable chemical element component design, the oil casing with ultra-high strength and excellent H2S stress corrosion resistance can be obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of steel and its manufacturing method, especially relates to a kind of oil casing and its manufacturing method. BACKGROUND

[0002] At present, most of the developed oil and gas reservoirs contain H2S, and the H2S-containing oil and gas is buried deep, mostly at 6000-9000 m. With the increasing development of sulfur-containing oil and gas wells and the continuous increase of oil and gas drilling depth, the demand for 110-130 ksi steel grade sulfur-resistant pipes to meet the requirements of tensile strength and internal pressure strength of casing string for deep high-acid oil and gas field development is increasing.

[0003] Therefore, in the prior art disclosed seamless pipe, different technical means are adopted to improve the hydrogen sulfide stress corrosion resistance of steel.

[0004] For example, Japanese Patent Document No. JP2004332059A, published on November 25, 2004, entitled "Low Alloy Steel", proposes to improve the hydrogen sulfide stress corrosion resistance by reducing amorphous inclusions.

[0005] For example, Japanese Patent Document No. JP2005350754A, published on December 22, 2005, entitled "LOW ALLOY STEEL FOR OIL WELL TUBE HAVING EXCELLENT SULFIDE STRESS CRACKING RESISTANCE", proposes a technique to control dislocation density and hydrogen diffusion coefficient to improve the hydrogen sulfide stress corrosion resistance of 125 ksi steel grade sulfur-resistant pipes. SUMMARY

[0006] One of the purposes of the present application is to provide a super-high strength oil casing resistant to H2S corrosion, which can obtain an oil casing with super-high strength and excellent H2S stress corrosion resistance by reasonable chemical element composition design.

[0007] In order to achieve the above-mentioned purpose, the present application provides a super-high strength oil casing resistant to H2S corrosion, which contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows:

[0008] C: 0.15-0.28%, Si: 0.1-0.5%, Mn: 0.2%-0.5%, Cr: 0.4-0.8%, Mo: 0.8-1.5%, V: 0.15-0.3%, Nb: 0.05-0.15%, W: 0.2-0.6%, Al: 0.01-0.05%, Ce: 0.002-0.006%; 0

[0009] The mass percentages of Ce and O also satisfy the Ce / O ratio of 1 to 3.5.

[0010] Accordingly, the present invention also provides an ultra-high strength oil casing resistant to H2S corrosion, wherein the mass percentage of each element is as follows:

[0011] C: 0.15–0.28%, Si: 0.1–0.5%, Mn: 0.2%–0.5%, Cr: 0.4–0.8%, Mo: 0.8–1.5%, V: 0.15–0.3%, Nb: 0.05–0.15%, W: 0.2–0.6%, Al: 0.01–0.05%, Ce: 0.002–0.006%; 0 < O ≤ 0.002%; balance is Fe and other unavoidable impurities.

[0012] The mass percentages of Ce and O also satisfy the Ce / O ratio of 1 to 3.5.

[0013] Many factors influence the resistance of materials to hydrogen sulfide stress corrosion cracking. Among them, carbides formed by alloying elements can act as precipitation strengthening agents and hydrogen traps, reducing hydrogen diffusion in the steel and thus improving resistance to hydrogen sulfide stress corrosion cracking. The interface between the precipitate and the matrix is ​​the main hydrogen trapping point; therefore, the smaller the precipitate size, the more interfaces per unit volume, allowing for the capture and dispersion of more hydrogen. Larger carbides, such as Cr... 23 C6 and other compounds readily precipitate at grain boundaries and lath boundaries, becoming diffusion channels for hydrogen and accelerating its diffusion within the steel, thus causing crack-sensitive points to accumulate. Additionally, hard Al2O3 inclusions in the steel are also sensitive points for hydrogen damage, and crack initiations easily form near these inclusions.

[0014] Based on this, in the technical solution described in this invention, on the one hand, the strength of the material is improved by adding W alloy through its precipitation strengthening and solid solution strengthening effects. The resulting W2C-type carbides are dispersed within the grains, which can increase the amount of hydrogen captured and reduce the susceptibility to sulfide stress corrosion cracking. On the other hand, reducing the Cr content suppresses larger carbides such as Cr 23 C6 and other elements readily precipitate at grain boundaries and lath boundaries, reducing the number of hydrogen-damaged sensitive points. Furthermore, this invention incorporates Ce, which reacts with large Al2O3 inclusions in the steel, altering their shape and size. After VD treatment, all large inclusions float to the surface and are removed, forming smaller and more regularly shaped CeAlO3 and its composite inclusions in the steel, thereby improving resistance to sulfide stress corrosion cracking.

[0015] Specifically, in the ultra-high strength oil casing resistant to H2S corrosion described in this invention, the design principles of each chemical element are as follows:

[0016] C: In the anti-H2S corrosion ultra-high strength oil casing described in the present application, C is an important element to ensure strength and hardenability. When the content of C element in the steel is lower than 0.15%, the strength is difficult to reach 125 ksi steel grade. When the content of C element in the steel is too high, quenching cracks are prone to occur, and the coarse grain boundary carbide M 23 C6 is prone to precipitate, thereby affecting the sulfur resistance performance. Therefore, in the present application, the mass percentage of C element is controlled between 0.15-0.28%.

[0017] Si: In the anti-H2S corrosion ultra-high strength oil casing described in the present application, Si is an element brought into the steel by deoxidizer. When the content of Si is higher than 0.5%, the cold brittleness tendency of the steel is significantly increased. When the content of Si is lower than 0.1%, the deoxidation effect is reduced. Therefore, in the present application, the mass percentage of Si element is controlled between 0.1-0.5%.

[0018] Mn: Mn has beneficial effects such as expanding the austenite phase region, increasing hardenability, and refining grains. However, when the content of Mn is higher than 0.5%, segregation is prone to occur during solidification, thereby leading to composition segregation in the steel and affecting the sulfur resistance performance of the steel. In addition, in order to ensure the deoxidation and increase the hardenability effect, it is necessary to maintain the content of Mn element in the steel above 0.2%. Based on this, in the anti-H2S corrosion ultra-high strength oil casing described in the present application, the mass percentage of Mn element is controlled between 0.2%-0.5%.

[0019] Cr: In the anti-H2S corrosion ultra-high strength oil casing described in the present application, Cr is an element to improve strength and hardenability, which can effectively improve the corrosion resistance of the steel. However, it needs to be noted that the content of Cr element in the steel should not be too high. Too high Cr content will lead to the precipitation of coarse Cr 23 C6 carbide at the grain boundary during tempering, which is not conducive to the hydrogen sulfide stress corrosion resistance performance. Based on this, in the present application, the mass percentage of Cr element is controlled between 0.50-0.80%.

[0020] Mo: Mo is an element to improve strength, tempering stability, and hardenability, and can also improve the corrosion resistance of the material. When the content of Mo is higher than 1.5%, coarse carbide is prone to precipitate, which is not conducive to the hydrogen sulfide stress corrosion resistance. When the content of Mo is lower than 0.8%, it is difficult to ensure that the strength reaches 125 ksi steel grade during high-temperature tempering. Based on this, in the anti-H2S corrosion ultra-high strength oil casing described in the present application, the mass percentage of Mo element is controlled between 0.8-1.5%.

[0021] V: V is an effective grain refining and precipitation strengthening element, which can improve high temperature tempering resistance, can ensure that the steel reduces dislocation density when high temperature tempering, and the fine VC precipitated phase is a good hydrogen trap, which can improve the resistance to hydrogen sulfide stress corrosion effect. However, it should be noted that the content of V element in the steel should not be too high, and too high V will lead to temper brittleness and reduce the stress corrosion resistance of the steel. Therefore, in the super high strength oil casing for resisting H2S corrosion according to the present application, the mass percentage of V element is controlled between 0.15-0.3%.

[0022] Nb: Nb is an effective grain refining element, and the Nb precipitated phase formed in the austenite region can refine the grain, and has a positive effect on the strength and toughness of the steel and the sulfur resistance. In the oil casing for resisting H2S corrosion according to the present application, the mass percentage of Nb element is controlled between 0.05-0.15%.

[0023] W: In the super high strength oil casing for resisting H2S corrosion according to the present application, W is one of the core elements, which exists in the steel in the form of solid solution and precipitation, and the precipitated phase can be uniformly distributed in the steel, has good precipitation strengthening effect, can improve the high temperature tempering resistance and the strength of the material. However, when the content of W is too high, the precipitated phase is easy to coarsen, forming a hydrogen-induced crack sensitive point. When the content of W is too low, the strengthening effect is insufficient to reach the strength level of 125 ksi. Therefore, the mass percentage of W element is controlled between 0.2-0.6%.

[0024] Al: Al is an essential element for steel deoxidization, so it is impossible to completely avoid the introduction of Al element in the steel. However, the content of Al element in the steel should not be too high, because when the content of Al element in the steel exceeds 0.05%, it will have adverse effects on the casting process. Therefore, the mass percentage of Al element is controlled between 0.01-0.05% in the present application.

[0025] Ce: In the super high strength oil casing for resisting H2S corrosion according to the present application, Ce plays a role in purifying the steel liquid, modifying inclusions and precipitation strengthening. Ce is easy to combine with O, S and other elements at high temperature to form high melting point oxide, sulfide and sulfide particles. In addition, Ce also has good modification and modification effect on hard inclusions such as Al2O3. When the content of Ce in the steel liquid is more than 20 ppm, it can play a modification role to generate small size CeAlO3+CaS composite inclusions. After VD treatment, almost all large size inclusions are removed by floating, the size of inclusions in the steel is significantly reduced, which is beneficial to improve the strength and toughness of the casing steel and the resistance to SSC performance. However, when there is too much Ce, brittle and hard iron-rare earth intermetallic compounds will be formed, which will reduce the resistance to SSC performance of the material. In addition, too much Ce will also cause nozzle clogging, leading to casting difficulties. Therefore, the mass percentage of Ce is controlled between 0.002-0.006% in the present application.

[0026] In addition, in the anti-H2S corrosion ultra-high strength oil casing according to the present application, the mass percentage of Ce / O should be controlled to be 1-3.5. This is because that the O element in the steel can form Al2O3 inclusions with Al element, and these large-size inclusions are strong hydrogen traps in the hydrogen sulfide environment, which can easily lead to hydrogen sulfide stress corrosion cracking. The inventors have found that when the ratio of the added Ce content and O content is greater than or equal to 1, Ce can form granular CeAlO3 inclusions, thereby refining the inclusions and avoiding the harmful effects of large-size Al2O3 inclusions. However, when the ratio of the added Ce content and O content is greater than or equal to 3.5, it will be difficult to cast. Therefore, the mass percentage of Ce and O should satisfy the relationship: 1≤Ce / O≤3.5.

[0027] Further, in the anti-H2S corrosion ultra-high strength oil casing according to the present application, the mass percentage of each chemical element can further satisfy at least one of the following conditions:

[0028] C: 0.18-0.25%;

[0029] Cr: 0.4-0.7%;

[0030] Mo: 0.9-1.2%;

[0031] V: 0.18-0.25%;

[0032] Nb: 0.05-0.1%;

[0033] W: 0.2-0.5%;

[0034] Al: 0.015-0.04%;

[0035] Ce: 0.002-0.005%.

[0036] Further, in the anti-H2S corrosion ultra-high strength oil casing according to the present application, the unavoidable impurities should satisfy the following conditions: S≤0.002%, P≤0.01%, N≤0.008%.

[0037] Further, the S, P and N should be controlled to be: S≤0.0015%, P≤0.008%, N≤0.007%.

[0038] In the above technical solution of the present application, P, S, N and O are all harmful impurity elements. In order to obtain steel with better performance and quality, the content of P, S, N and O should be reduced as much as possible when the technology permits.

[0039] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the microstructure matrix is tempered sorbite.

[0040] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the microstructure matrix is tempered sorbite.

[0041] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the microstructure matrix is tempered sorbite. 2 .

[0042] As described above, the alloy precipitates with large size, such as Cr 23 C6, are easy to precipitate at grain boundaries and lath boundaries, become the diffusion channel of hydrogen, accelerate the diffusion of hydrogen in the steel, and make the crack sensitive points gather, therefore, the application controls the number of alloy precipitates with diameter above 200nm to be ≤5 / 100μm 2 .

[0043] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the microstructure matrix is tempered sorbite.

[0044] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the microstructure matrix is tempered sorbite.

[0045] Further, in the anti-H2S corrosion ultra-high strength oil casing provided by the application, the performance satisfies: yield strength ≥862MPa, impact energy at 0℃ ≥120J, and the anti-H2S stress corrosion performance satisfies K 1SCC value ≥25MPa*m 1 / 2 .

[0046] Correspondingly, another object of the application is to provide the manufacturing method of the anti-H2S corrosion ultra-high strength oil casing, which has simple process and low production cost.

[0047] In order to achieve the above object, the application provides the manufacturing method of the anti-H2S corrosion ultra-high strength oil casing, which comprises the following steps:

[0048] (1) smelting and continuous casting to obtain a pipe blank;

[0049] (2) heating, piercing, rolling, sizing;

[0050] (3) quenching: the austenitizing temperature is 880-920℃, and the quenching is performed after holding for 40-60min;

[0051] (4) tempering: the tempering temperature is 690-720℃, the holding time is 50-80min, and then air cooling.

[0052] Further, in the step (1) of the manufacturing method, the casting process controls the superheat of the molten steel at 40-50℃, and the continuous casting speed is controlled at 1.5-1.8m / min.

[0053] After adding Ce, the viscosity of the molten steel increases, and the flowability becomes poor. Therefore, in order to ensure the flowability of the molten steel and prevent the nozzle from being blocked due to nozzle clogging during the casting process, it is preferred to maintain a high superheat and a low continuous casting speed. Therefore, the preferred embodiment of the present application controls the superheat of the molten steel at 40-50℃, and at the same time, the continuous casting speed is reduced to 1.5-1.8m / min.

[0054] Further, in the step (2) of the manufacturing method, the heating is to the soaking temperature of 1200-1280℃.

[0055] Further, in the step (2) of the manufacturing method, the piercing temperature is controlled at 1150-1250℃.

[0056] Further, in the step (2) of the manufacturing method, the finish rolling temperature is controlled at 880-940℃.

[0057] The anti-H2S corrosion ultra-high strength oil casing and the manufacturing method thereof have the following advantages and beneficial effects:

[0058] The anti-H2S corrosion ultra-high strength oil casing controls the number of large-size alloy precipitated phases by controlling the reasonable proportioning of alloy elements, and modifies the inclusions so that the size of the inclusions is small, thereby obtaining an oil well pipe with both ultra-high strength and anti-H2S stress corrosion performance.

[0059] The anti-H2S corrosion ultra-high strength oil casing reaches the 125ksi steel grade, the yield strength is ≥862MPa, the impact energy at 0℃ is ≥120J, and the anti-H2S stress corrosion performance meets the K 1SCC The value is ≥25MPa*m 1 / 2 and does not break for 720h under the loading of 80% nominal yield strength D solution of NACE TM0177 standard A method.

[0060] The anti-H2S corrosion ultra-high strength oil casing can be used for the exploitation of oil and natural gas containing hydrogen sulfide in ultra-deep wells of more than 9000 meters, and has very important practical significance. DETAILED DESCRIPTION

[0061] The anti-H2S corrosion ultra-high strength oil casing and the manufacturing method thereof will be further explained and described in combination with specific examples below, however, the explanation and description does not constitute undue limitation on the technical scheme of the present application.

[0062] Examples 1-6 and Comparative Examples 1-8

[0063] The anti-H2S corrosion ultra-high strength oil casing of Examples 1-6 and the comparative oil casing of Comparative Examples 1-8 are both prepared by the following steps:

[0064] (1) Smelting, LF furnace refining, VD furnace vacuum treatment and continuous casting are performed according to the chemical composition shown in Table 1 to obtain a pipe blank: wherein pure Ce or Ce alloy is added to the molten steel in the LF furnace and VD furnace processes, argon blowing is performed for more than 5 minutes before continuous casting is started; during the casting process, the superheat of the molten steel is controlled to be 40-50℃, and the continuous casting speed is 1.5-1.8 m / min.

[0065] (2) Heating: heated to a soaking temperature of 1200-1280℃.

[0066] (3) Piercing: the piercing temperature is controlled to be 1150-1250℃.

[0067] (4) Rolling: the finish rolling temperature is controlled to be 880-940℃.

[0068] (5) Sizing: the sizing temperature is controlled to be 840-910℃, and the pipe is obtained. (7) Quenching: the austenitizing temperature is 880-920℃, and the pipe is quenched after holding for 40-60 minutes;

[0069] (8) Tempering: the tempering temperature is 690-720℃, and the holding time is 50-80 minutes, and then air cooling.

[0070] It should be noted that the chemical element composition and the related process design of the anti-H2S corrosion oil casing of Examples 1-6 meet the design specification requirements of the present application. The chemical element content of Comparative Examples 1-8 does not meet the design of the present application.

[0071] Table 1 lists the mass percentage of each chemical element of the anti-H2S corrosion oil casing of Examples 1-6 and the comparative oil casing of Comparative Examples 1-8.

[0072] Table 1. (wt%, the balance is Fe and other unavoidable impurities except P, O, N and S)

[0073]

[0074]

[0075] ​

[0076] Table 2 lists the specific process parameters of the anti-H2S corrosion ultra-high strength oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-8 in the above process steps.

[0077] Table 2.

[0078]

[0079]

[0080] The anti-H2S corrosion ultra-high strength oil casings of Examples 1-6 and the comparative oil casings of Comparative Examples 1-8 were sampled respectively, and were detected, and the obtained test results are listed in Table 3. The relevant test processes are as follows:

[0081] (1) Microstructure observation: After polishing, the inclusions of the sample were observed by optical microscope, and the microstructure of the test sample was analyzed by EVO MA25 scanning electron microscope and JEM 2100F transmission electron microscope.

[0082] (2) Tensile test: The tensile property test at room temperature was carried out according to the standard of GB / T 228.1-2000.

[0083] (3) Impact test: The 0℃ impact energy test was carried out according to the standard of GB / T 229-2007 "Metallic materials Charpy pendulum impact test method".

[0084] (4) Sulfur resistance test: The sulfur resistance test was carried out according to the D method and A method in the standard of NACE TM0177-2016.

[0085] Table 3 lists the test results of Examples 1-6 and Comparative Examples 1-8.

[0086] Table 3.

[0087]

[0088]

[0089] As can be seen from Table 3, the yield strength of the oil casings of Examples 1-6 is ≥870MPa, the impact energy at 0℃ is ≥140J, and the anti-H2S stress corrosion performance meets K 1SCC The value is ≥26.5MPa*m 1 / 2 and the NACE TM0177 standard A method D solution loading 80% nominal yield strength is not broken for 720h.

[0090] The microstructure of the anti-H2S corrosion oil casing prepared in Examples 1-6 is tempered sorbite, and the size of CeAlO3 inclusions and CeAlO3+CaS composite inclusions is less than 6 μm. In addition, the number of alloy precipitates with a diameter of more than 200 nm is less than or equal to 4 per 100 μm 2 .

[0091] Comparative Examples 1-8 are chemical element compositions that do not meet the design specifications of the present application, but their processes are basically consistent with the requirements of the present application. Among them:

[0092] Comparative Example 1 has a C content lower than the requirements of the present application, which results in a strength that does not meet the requirements of the 125 ksi steel grade, and the performance detected by the NACE TM0177 standard A method is unqualified.

[0093] Comparative Example 2 has a C content higher than the requirements of the present application, which results in a large number of precipitates with a diameter of more than 200 nm, reducing the sulfur resistance.

[0094] Comparative Example 3 has Mo and V contents lower than the requirements of the present application, which results in a strength that does not meet the requirements of the 125 ksi steel grade, and the performance detected by the NACE TM0177 standard A method is unqualified.

[0095] Comparative Example 4 has Mo and V contents higher than the requirements of the present application, which results in a large number of precipitates with a diameter of more than 200 nm, reducing the sulfur resistance and impact performance.

[0096] Comparative Example 5 does not contain W and has a Cr content higher than the requirements of the present application, which results in a large number of precipitates with a diameter of more than 200 nm, and the sulfur resistance is unqualified.

[0097] Comparative Example 6 has a W content higher than the requirements of the present application, which results in a large number of precipitates with a diameter of more than 200 nm, and the sulfur resistance and impact performance are unqualified.

[0098] Comparative Example 7 does not contain Ce, which results in large inclusion sizes and unqualified sulfur resistance.

[0099] Comparative Example 8 has a Ce / O content higher than the requirements of the present application, which results in a large number of precipitates with a diameter of more than 200 nm, large inclusion sizes, and unqualified sulfur resistance.

[0100] Therefore, the technical solution of the present application can obtain an oil casing with both ultra-high strength and H2S stress corrosion resistance.

[0101] It should be noted that the combination of the technical features in the case is not limited to the combination of the claims in the case or the combination of the embodiments in the case. All the technical features disclosed in the case can be freely combined or combined in any way, unless contradictory.

[0102] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily conceived by those skilled in the art, and should all fall within the scope of protection of the present application.

Claims

1. An ultra-high strength oil casing resistant to H2S corrosion, characterized in that, The mass percentage of each element is: C: 0.15-0.28%, Si: 0.1-0.5%, Mn: 0.2-0.5%, Cr: 0.4-0.8%, Mo: 0.8-1.5%, V: 0.15-0.3%, Nb: 0.05-0.15%, W: 0.2-0.6%, Al: 0.01-0.05%, Ce: 0.002-0.006%, 0 The mass percentage of Ce and O also satisfies Ce / O: 1-3.

5. The microstructure matrix of the ultra-high strength oil casing is tempered sorbite, and has W2C type carbide dispersedly distributed in the grain, and the number of alloy precipitated phases with a diameter of 200 nm or more in the microstructure is ≤5 / 100 μm 2 , having CeAlO3 inclusions and CeAlO3+CaS composite inclusions, the size of the CeAlO3 inclusions and the CeAlO3+CaS composite inclusions is ≤6 μm; The performance of the super-high strength oil casing satisfies: yield strength ≥ 862 MPa, impact energy at 0 ℃ ≥ 120 J, H2S stress corrosion resistance performance satisfies K 1SCC ≥ 25 MPa*m 1 / 2 .

2. The ultra-high strength oil casing resistant to H2S corrosion according to claim 1, characterized in that, The mass percentage of each chemical element further satisfies at least one of the following: C：0.18～0.25％； Cr：0.4～0.7％； Mo: 0.9-1.2%; V：0.18～0.25％； Nb: 0.05-0.1%; W：0.2～0.5％； Al：0.015～-0.04％； Ce: 0.002-0.005%.

3. The anti-H2S corrosion ultra-high strength oil casing according to claim 1, characterized in that, Among the inevitable impurities: S≤0.002%, P≤0.01%, N≤0.008%.

4. The method of producing an ultra-high strength oil casing resistant to H2S corrosion according to any one of claims 1 to 3, characterized by, It includes the steps of: (1) smelting and continuous casting to obtain a pipe blank; (2) heating, piercing, rolling, sizing; (3) quenching: austenitizing temperature is 880-920℃, holding for 40-60min, then quenching; (4) tempering: tempering temperature is 690-720℃, holding time is 50-80min, then air cooling.

5. The production method according to claim 4, wherein In step (1), the overheat degree of molten steel during casting is controlled at 40-50℃, and the continuous casting speed is controlled at 1.5-1.8m / min.

6. The production method according to claim 4, wherein In step (2), heating to soaking temperature 1200-1280℃.

7. The production method according to claim 4, wherein In step (2), the piercing temperature is controlled at 1150-1250℃.

8. The production method according to claim 4, wherein In step (2), the finish rolling temperature is controlled at 880-940℃.

9. The production method according to claim 4, wherein In step (2), the sizing temperature is controlled at 840-910℃.

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