Steel pipe, its preparation method and application

Abstract

The present invention provides a steel pipe, a preparation method thereof and an application, relating to the technical field of steel pipes. The preparation method includes: performing piercing hot rolling treatment, solution treatment, relaxation treatment, quenching treatment and tempering treatment on an ingot in sequence to obtain a steel pipe; wherein, the time of the relaxation treatment is 30 to 60 s, the temperature of the pipe after the relaxation treatment is 850 to 890 °C, and the temperature of the cooling medium for the quenching treatment is 15 to 40 °C. Controlling the time of the relaxation treatment and the temperature of the pipe after the relaxation treatment within the above ranges helps to make the tissue structure be austenite and in a state that can be transformed into martensite. Controlling the temperature of the cooling medium for the quenching treatment within the above range helps to form a martensite structure with a low dislocation density. Finally, the pipe is subjected to tempering treatment, which helps to form tempered martensite with a low dislocation density, thereby helping to make the mechanical strength of the steel pipe reach the level of 110 ksi and have excellent SSC resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of steel pipes, and in particular, to a steel pipe, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous consumption of shallow oil and gas resources and the increasing demand for oil and gas resources in industries such as energy and chemical industries, the focus of oil and gas exploration and exploitation has to shift to some deep oil and gas fields with extremely harsh exploitation conditions. Therefore, the oil pipes previously used in shallow oil and gas fields are no longer applicable to deep oil and gas fields in a highly corrosive H2S gas environment. As the most important type of pipe in oil and gas exploration and exploitation, the service performance of oil well pipes in such highly corrosive conditions needs to be improved urgently. Specifically, it is necessary to have good and stable SSC resistance while having relatively high strength. Summary of the Invention

[0003] The main object of the present invention is to provide a steel pipe, a preparation method thereof, and an application thereof, so as to solve the problem that the steel pipe in the prior art has poor impact energy and SSC resistance.

[0004] To achieve the above object, according to one aspect of the present invention, a preparation method of a steel pipe is provided. The preparation method includes: performing piercing hot rolling treatment, solution treatment, relaxation treatment, quenching treatment, and tempering treatment on an ingot in sequence to obtain a steel pipe; wherein, the time of the relaxation treatment is 30 - 60 s, the temperature of the pipe after the relaxation treatment is 850 - 890 °C, the temperature of the cooling medium for the quenching treatment is 15 - 40 °C, the yield strength of the steel pipe at 25 °C is 758 - 828 MPa, the steel pipe does not fracture under a stress of 606 MPa in a saturated H2S environment for at least 720 hours; the microstructure of the steel pipe is tempered martensite.

[0005] Further, the time of the relaxation treatment is 40 - 50 s; and / or, the temperature of the cooling medium for the quenching treatment is 20 - 35 °C.

[0006] Further, the temperature of the solution treatment is 930 - 950 °C; and / or, the holding time of the solution treatment is 35 - 45 min.

[0007] Further, the temperature of the tempering treatment is 680 - 700 °C; and / or, the holding time of the tempering treatment is 55 - 65 min; and / or, the piercing hot rolling treatment includes heating the ingot to 1200 - 1300 °C, holding for 5 - 6 h, and then performing rolling.

[0008] Furthermore, the outer diameter of the pipe after the above-mentioned piercing hot rolling treatment is 270 - 275 mm; and / or, the wall thickness of the pipe after the piercing hot rolling treatment is 13.5 - 14.0 mm; and / or, the temperature of the pipe after the piercing hot rolling treatment is 830 - 890 °C; and / or, the pipe after the piercing hot rolling treatment is cooled down to 450 - 710 °C by on-line water ring spray accelerated cooling and then air-cooled to 20 - 30 °C, and the average rate of accelerated cooling and temperature reduction is 1 - 15 °C / s.

[0009] Furthermore, by mass percentage, the above-mentioned ingot comprises the following elements: the content of C element is 0.24 - 0.27%, the content of Si element ≤ 0.3%, the content of Mn element is 0.4 - 0.6%, the content of Cr element is 0.4 - 0.6%, the content of Mo element is 0.6 - 0.8%, the content of V element is 0.05 - 0.1%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element ≤ 0.005%, the content of P element ≤ 0.01%, the total content of inevitable impurities ≤ 0.15 wt%, the content of a single impurity is less than 0.05 wt%, and the balance is Fe element.

[0010] Furthermore, by mass percentage, the above-mentioned ingot comprises the following elements: the content of C element is 0.26 - 0.27%, the content of Si element is 0.2 - 0.3%, the content of Mn element is 0.46 - 0.52%, the content of Cr element is 0.5 - 0.6%, the content of Mo element is 0.7 - 0.8%, the content of V element is 0.05 - 0.06%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element is 0.003 - 0.005%, the content of P element is 0.008 - 0.01%, the total content of inevitable impurities ≤ 0.15 wt%, the content of a single impurity is less than 0.05 wt%, and the balance is Fe element.

[0011] According to another aspect of the present invention, there is provided a steel pipe prepared by the foregoing preparation method.

[0012] Furthermore, the tensile strength of the above-mentioned steel pipe at 25 °C ≥ 793 MPa, and the impact energy ≥ 170 J.

[0013] According to still another aspect of the present invention, there is provided the application of the foregoing steel pipe as an oil casing on oil exploration equipment.

[0014] Applying the technical solution of the present invention, the beneficial effects of this application are as follows: If the relaxation treatment time in this application is too long and the temperature of the pipe after relaxation treatment is too low, it is easy to cause an increase in the thermal stability of the austenite structure, resulting in a lag in subsequent martensitic transformation, and some austenite may be difficult to transform into martensite; if the relaxation treatment time is too short and the temperature of the pipe after relaxation treatment is too high, it is not conducive to forming sufficient austenite structure; controlling the relaxation treatment time and the temperature of the pipe after relaxation treatment within the above ranges helps to make the tissue structure be austenite and in a state that can be transformed into martensite. Controlling the temperature of the cooling medium for quenching treatment within the above ranges helps to control the cooling rate of the pipe, so that the austenite structure undergoes a rapid cooling stage to form a martensite structure with a low dislocation density. Finally, the pipe undergoes tempering treatment, which helps to form tempered martensite with a low dislocation density, thereby helping to make the mechanical strength of the steel pipe reach the 110 ksi level and having excellent SSC resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The specification drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0016] Figure 1 Shows the microstructure diagram of the steel pipe in Embodiment 1 of this application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0018] As analyzed in the background art of this application, there are problems with poor impact energy and SSC resistance of steel pipes in the prior art. To solve the above problems, this application provides a steel pipe and its preparation method and application.

[0019] In a typical implementation manner of this application, a preparation method of a steel pipe is provided. The preparation method includes: sequentially performing piercing hot rolling treatment, solution treatment, relaxation treatment, quenching treatment, and tempering treatment on an ingot to obtain a steel pipe; wherein, the relaxation treatment time is 30 - 60 s, the temperature of the pipe after relaxation treatment is 850 - 890 °C, the temperature of the cooling medium for quenching treatment is 15 - 40 °C, the yield strength of the steel pipe at 25 °C is 758 - 828 MPa, the steel pipe does not break under a stress of 606 MPa in a saturated H2S environment for at least 720 hours; the tissue structure of the steel pipe is tempered martensite.

[0020] In this application, if the relaxation treatment time is too long and the temperature of the pipe after relaxation treatment is too low, it is easy to cause an increase in the thermal stability of the austenite structure, resulting in a lag in the subsequent martensite transformation. It may cause some austenite to be difficult to transform into martensite. If the relaxation treatment time is too short and the temperature of the pipe after relaxation treatment is too high, it is not conducive to the formation of sufficient austenite structure. Controlling the relaxation treatment time and the temperature of the pipe after relaxation treatment within the above ranges helps to make the organizational structure austenite and in a state that can be transformed into martensite. Controlling the temperature of the cooling medium for quenching treatment within the above ranges helps to control the cooling rate of the pipe, enabling the austenite structure to undergo a rapid cooling stage to form a martensite structure with a low dislocation density. Finally, the pipe undergoes tempering treatment, which helps to form tempered martensite with a low dislocation density, thereby helping to make the mechanical strength of the steel pipe reach the 110 ksi level and have excellent SSC resistance.

[0021] In an embodiment of the present application, the relaxation treatment time is 40 - 50 s, specifically it can be 40 s, 42 s, 44 s, 46 s, 48 s, 50 s, and the range values between any two values; and / or, the temperature of the cooling medium for quenching treatment is 20 - 35 °C, specifically it can be 20 °C, 25 °C, 30 °C, 35 °C, and the range values between any two values.

[0022] Controlling the relaxation treatment time within the above ranges helps to further control the state of the austenite structure, making it easier to transform into martensite structure in the subsequent process, thereby helping to improve the mechanical strength and SSC resistance of the steel pipe. Controlling the temperature of the cooling medium for quenching treatment within the above ranges helps to further reduce the dislocation density of the martensite structure, thereby helping to further improve the SSC resistance of the steel pipe.

[0023] In an embodiment of the present application, the average cooling rate of the relaxation treatment is 1.3 - 1.7 °C / s, specifically it can be 1.3 °C / s, 1.4 °C / s, 1.5 °C / s, 1.6 °C / s, 1.7 °C / s, and the range values between any two values.

[0024] Controlling the average cooling rate of the relaxation treatment within the above ranges helps to promote the formation of the austenite structure and control the austenite structure in a state that is easy to transform into martensite structure, thereby helping to further improve the yield strength, tensile strength, impact energy, and SSC resistance of the steel pipe.

[0025] In an embodiment of the present application, the solution treatment temperature is 930 - 950 °C; and / or, the solution treatment holding time is 35 - 45 min.

[0026] Controlling the solution treatment temperature and time within the above ranges helps to improve the uniformity and stability of the microstructure, thereby contributing to enhancing the corrosion resistance and mechanical properties of the material.

[0027] In one embodiment of the present application, the temperature of the above tempering treatment is 680 - 700 °C; and / or, the holding time of the tempering treatment is 55 - 65 min; and / or, the piercing hot rolling treatment includes heating the ingot to 1200 - 1300 °C, holding for 5 - 6 h and then performing rolling.

[0028] The function of the tempering treatment is to reduce the internal stress generated during the quenching process, improve the toughness and plasticity of the material, and maintain high strength at the same time. Controlling the tempering treatment temperature within the above range helps the carbides in the martensite structure to precipitate and stabilize, forming tempered martensite, which thus helps to improve the toughness of the steel pipe, reduce the risk of brittle fracture, and contribute to improving the SSC resistance performance. Controlling the holding time of the tempering treatment within the above range helps the internal stress of the material to be fully released, and the carbides in the microstructure can precipitate evenly, forming a more stable organizational structure. Controlling the conditions of the piercing hot rolling treatment within the above range helps the alloying elements to dissolve and distribute evenly in the material, control the grain size, reduce the non-uniformity of the microstructure, and provide a more stable basis for the subsequent solution treatment.

[0029] In one embodiment of the present application, the outer diameter of the pipe after the piercing hot rolling treatment is 270 - 275 mm; and / or, the wall thickness of the pipe after the piercing hot rolling treatment is 13.5 - 14.0 mm; and / or, the temperature of the pipe after the piercing hot rolling treatment is 830 - 890 °C; and / or, the pipe after the piercing hot rolling treatment is cooled down to 450 - 710 °C by online water ring spray cooling and then air-cooled to 20 - 30 °C, and the average rate of the accelerated cooling is 1 - 15 °C / s.

[0030] Controlling the temperature of the pipe after the piercing hot rolling treatment within the above range helps to improve the plasticity and fluidity of the material, thereby contributing to forming a more uniform microstructure, and further contributing to enhancing the H2S corrosion resistance performance of the material. Controlling the two-step cooling of the pipe after the piercing hot rolling treatment and controlling the average rate of the accelerated cooling within the above range helps to form a more uniform and stable microstructure, thus contributing to improving the H2S resistance and mechanical properties of the steel pipe.

[0031] In an embodiment of the present application, by mass percentage, the above ingot comprises the following elements: the content of C element is 0.24 - 0.27%, the content of Si element ≤ 0.3%, the content of Mn element is 0.4 - 0.6%, the content of Cr element is 0.4 - 0.6%, the content of Mo element is 0.6 - 0.8%, the content of V element is 0.05 - 0.1%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element ≤ 0.005%, the content of P element ≤ 0.01%, the total content of inevitable impurities ≤ 0.15wt%, the content of a single impurity is less than 0.05wt%, and the balance is Fe element.

[0032] An excessively high carbon content will lead to an increase in the brittleness of the material and reduce its toughness. Controlling the content of C element within the above range helps to maintain good toughness while increasing the strength of the steel pipe. Controlling the content of Si element and Mn element within the above range helps to increase the strength of the steel pipe. Controlling the content of Cr element, Mo element, V element, Nb element and Al element within the above range helps to improve the SSC resistance of the steel pipe. Excessive S element and P element are not conducive to improving the toughness of the material. Controlling the content of S element and P element within the above range helps to improve the mutual synergistic effect between components, thereby helping to improve the yield strength, tensile strength, impact work and SSC resistance of the steel pipe.

[0033] In order to further improve the yield strength, tensile strength, impact work and SSC resistance of the steel pipe, in an embodiment of the present application, by mass percentage, preferably the above ingot comprises the following elements: the content of C element is 0.26 - 0.27%, the content of Si element is 0.2 - 0.3%, the content of Mn element is 0.46 - 0.52%, the content of Cr element is 0.5 - 0.6%, the content of Mo element is 0.7 - 0.8%, the content of V element is 0.05 - 0.06%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element is 0.003 - 0.005%, the content of P element is 0.008 - 0.01%, the total content of inevitable impurities ≤ 0.15wt%, the content of a single impurity is less than 0.05wt%, and the balance is Fe element.

[0034] In an embodiment of the present application, by mass percentage, the above ingot comprises the following elements: the content of C element is 0.26%, the content of Si element is 0.22%, the content of Mn element is 0.46%, the content of Cr element is 0.51%, the content of Mo element is 0.71%, the content of V element is 0.06%, the content of Nb element is 0.02%, the content of Al element is 0.025%, the content of S element is 0.004%, the content of P element is 0.008%, and the balance is Fe element; the time of relaxation treatment is 40 - 50 s, and the temperature of the cooling medium for quenching treatment is 20 - 35 °C.

[0035] In another typical embodiment of the present application, a steel pipe is provided, and the steel pipe is prepared by the aforementioned preparation method.

[0036] Since the above steel pipe is prepared by the preparation method of the present application, therefore, the steel pipe has excellent yield strength, tensile strength, impact energy and anti-SSC performance.

[0037] In an embodiment of the present application, the tensile strength of the above steel pipe at 25 °C is ≥793 MPa, and the impact energy is ≥170 J.

[0038] The steel pipe with the above yield strength, tensile strength, impact energy and anti-SSC performance is more suitable for oil exploration.

[0039] In an embodiment of the present application, the yield strength of the above steel pipe at 25 °C is 759 - 806 MPa, the tensile strength is 845 - 879 MPa, and the impact energy is 181 - 197 J; and / or, the steel pipe does not fracture after being loaded with a stress of 606 MPa in a saturated H2S environment for 734 - 739 hours.

[0040] In yet another typical embodiment of the present application, the application of the aforementioned steel pipe as an oil casing on an oil exploration device is provided.

[0041] Using the steel pipe of the present application as an oil casing on an oil exploration device helps to improve the service life of the oil exploration device and helps the oil exploration device to operate in deep oil and gas fields.

[0042] The beneficial effects of the present application will be further described below in conjunction with embodiments.

[0043] Example 1

[0044] (1)By mass percentage, the ingot includes the following elements: the content of C element is 0.26%, the content of Si element is 0.22%, the content of Mn element is 0.46%, the content of Cr element is 0.51%, the content of Mo element is 0.71%, the content of V element is 0.06%, the content of Nb element is 0.02%, the content of Al element is 0.025%, the content of S element is 0.004%, the content of P element is 0.008%, and the balance is Fe element.

[0045] (2)The above ingot is subjected to piercing hot rolling treatment. Specifically, the ingot is heated in a rotary hearth furnace at a temperature of 1260 °C for a holding time of 5 hours. After that, rolling is carried out, and finally it is rolled into an oil casing with an outer diameter of 273 mm and a wall thickness of 13.8 mm. The final rolling temperature is 870 °C. The pipe after piercing hot rolling is cooled down to 450 °C by on-line water ring spray cooling, and then air-cooled to 25 °C. The average cooling rate during the accelerated cooling is 13 °C / s;

[0046] (3)The cooled pipe is cut into several sample pipes with a length of 300 mm. The sample pipes are first placed in a heat treatment furnace for solution treatment. The solution treatment temperature is 940 °C and the holding time is 40 min.

[0047] (4)The sample pipes after solution treatment are subjected to relaxation treatment. The average cooling rate during the relaxation treatment is 1.5 °C / s, and the relaxation treatment time is 33 s. When the surface temperature of the sample pipe reaches 890 °C, water quenching treatment is carried out, and the water temperature is 15 °C, and it is quenched and cooled to 15 °C; then the quenched pipe is subjected to tempering treatment. The tempering temperature is 690 °C and the holding time is 60 min. After tempering, it is air-cooled to room temperature to obtain a steel pipe.

[0048] Example 2

[0049] The difference from Example 1 is that in step (4), the water temperature is 40 °C, and it is quenched and cooled to 40 °C; then the quenched pipe is subjected to tempering treatment. The tempering temperature is 690 °C and the holding time is 60 min. After tempering, it is air-cooled to room temperature, and finally a steel pipe is obtained.

[0050] Example 3

[0051] The difference from Example 1 is that in step (4), the sample pipes after solution treatment are subjected to relaxation treatment. The average cooling rate during the relaxation treatment is 1.5 °C / s, and the relaxation treatment time is 60 s. When the surface temperature of the sample pipe reaches 850 °C, water quenching treatment is carried out, and the water temperature is 30 °C, and it is quenched and cooled to 30 °C; then the quenched pipe is subjected to tempering treatment. The tempering temperature is 690 °C and the holding time is 60 min. After tempering, it is air-cooled to room temperature, and finally a steel pipe is obtained.

[0052] Example 4

[0053] The difference from Example 1 is that: (1) By mass percentage, the ingot includes the following elements: the content of C element is 0.27%, the content of Si element is 0.21%, the content of Mn element is 0.52%, the content of Cr element is 0.53%, the content of Mo element is 0.75%, the content of V element is 0.05%, the content of Nb element is 0.023%, the content of Al element is 0.028%, the content of S element is 0.003%, the content of P element is 0.009%, and the balance is Fe element.

[0054] (2) The above ingot is subjected to piercing hot rolling treatment. Specifically, the ingot is heated in a ring furnace at a temperature of 1260 °C for a holding time of 5 hours. After that, rolling is carried out, and finally it is rolled into an oil casing with an outer diameter of 273 mm and a wall thickness of 13.8 mm. The final rolling temperature is 870 °C. The tube after piercing hot rolling treatment is cooled down to 500 °C by on-line water ring spray cooling, and then air-cooled to 25 °C. The average cooling rate of the accelerated cooling is 10 °C / s;

[0055] (3) The above cooled tube is cut into several sample tubes with a length of 300 mm. The sample tubes are first placed in a heat treatment furnace for solution treatment. The solution temperature is 940 °C and the holding time is 40 min.

[0056] (4) The sample tubes after solution treatment are subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the relaxation treatment time is 60 s. When the surface temperature of the sample tube reaches 850 °C, water quenching treatment is carried out, and the water temperature is 28 °C, and the quenching is cooled to 28 °C; Subsequently, the quenched tube is subjected to tempering treatment. The tempering temperature is 690 °C and the holding time is 60 min. After tempering, it is air-cooled to room temperature to obtain a steel tube.

[0057] Example 5

[0058] The difference from Example 1 is that in step (4), the sample tubes after solution treatment are subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the relaxation treatment time is 50 s. When the surface temperature of the sample tube reaches 865 °C, water quenching treatment is carried out, and the water temperature is 30 °C, and the quenching is cooled to 30 °C; Subsequently, the quenched tube is subjected to tempering treatment. The tempering temperature is 690 °C and the holding time is 60 min. After tempering, it is air-cooled to room temperature, and finally a steel tube is obtained.

[0059] Example 6

[0060] The difference from Example 1 lies in that in step (4), after solution treatment of the sample tube, relaxation treatment is carried out. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the time of the relaxation treatment is 40 s. When the surface temperature of the sample tube reaches 880 °C, water quenching treatment is carried out, the water temperature is 30 °C, and it is quenched and cooled to 30 °C; then the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0061] Example 7

[0062] The difference from Example 1 lies in that in step (4), the water temperature is 20 °C, and it is quenched and cooled to 20 °C; then the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0063] Example 8

[0064] The difference from Example 1 lies in that in step (4), the water temperature is 35 °C, and it is quenched and cooled to 35 °C; then the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0065] Example 9

[0066] The difference from Example 1 lies in that in step (4), after solution treatment of the sample tube, relaxation treatment is carried out. The average cooling rate of the relaxation treatment is 2 °C / s, and the time of the relaxation treatment is 33 s. When the surface temperature of the sample tube reaches 874 °C, water quenching treatment is carried out, the water temperature is 15 °C, and it is quenched and cooled to 15 °C; then the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0067] Example 10

[0068] The difference from Example 1 lies in that the temperature of the solution treatment is 960 °C and the holding time of the solution treatment is 50 min, and finally a steel pipe is obtained.

[0069] Example 11

[0070] The difference from Example 1 lies in that the temperature of the tempering treatment is 720 °C and the holding time of the tempering treatment is 70 min, and finally a steel pipe is obtained.

[0071] Example 12

[0072] The difference from Example 1 is that in step (2), the ingot is heated in a rotary hearth furnace at a temperature of 1260 °C for a holding time of 5 hours, followed by rolling. Finally, an oil casing pipe with an outer diameter of 273 mm and a wall thickness of 13.8 mm is rolled, and the final rolling temperature is 870 °C. The pipe after piercing and hot rolling is cooled down to 650 °C by online water ring spray accelerated cooling, and then air-cooled to 25 °C. The average rate of accelerated cooling is 5 °C / s, and finally a steel pipe is obtained.

[0073] Example 13

[0074] The difference from Example 1 is that in step (2), the ingot is heated in a rotary hearth furnace at a temperature of 1260 °C for a holding time of 5 hours, followed by rolling. Finally, an oil casing pipe with an outer diameter of 273 mm and a wall thickness of 13.8 mm is rolled, and the final rolling temperature is 870 °C. The pipe after piercing and hot rolling is cooled down to 400 °C by online water ring spray accelerated cooling, and then air-cooled to 25 °C. The average rate of accelerated cooling is 15 °C / s, and finally a steel pipe is obtained.

[0075] Example 14

[0076] The difference from Example 1 is that by mass percentage, the ingot comprises the following elements: the content of C element is 0.27%, the content of Si element is 0.3%, the content of Mn element is 0.6%, the content of Cr element is 0.6%, the content of Mo element is 0.8%, the content of V element is 0.1%, the content of Nb element is 0.03%, the content of Al element is 0.03%, the content of S element is 0.005%, the content of P element is 0.01%, and the balance is Fe element, and finally a steel pipe is obtained.

[0077] Example 15

[0078] The difference from Example 1 is that by mass percentage, the ingot comprises the following elements: the content of C element is 0.3%, the content of Si element is 0.4%, the content of Mn element is 0.7%, the content of Cr element is 0.3%, the content of Mo element is 0.5%, the content of V element is 0.2%, the content of Nb element is 0.05%, the content of Al element is 0.05%, the content of S element is 0.005%, the content of P element is 0.01%, and the balance is Fe element, and finally a steel pipe is obtained.

[0079] Comparative Example 1

[0080] The difference from Example 1 is that the relaxation treatment is cancelled, and the sample tube after solution treatment is immediately quenched with water after being taken out of the furnace, and finally a steel pipe is obtained.

[0081] Comparative Example 2

[0082] The difference from Example 1 is that in step (4), the solution-treated sample tube is subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the time of the relaxation treatment is 20 s. When the surface temperature of the sample tube reaches 910 °C, water quenching treatment is carried out, the water temperature is 30 °C, and it is quenched and cooled to 30 °C; subsequently, the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0083] Comparative Example 3

[0084] The difference from Example 1 is that in step (4), the solution-treated sample tube is subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the time of the relaxation treatment is 33 s. When the surface temperature of the sample tube reaches 890 °C, water quenching treatment is carried out, the water temperature is 50 °C, and it is quenched and cooled to 50 °C; subsequently, the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0085] Comparative Example 4

[0086] The difference from Example 1 is that in step (4), the solution-treated sample tube is subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the time of the relaxation treatment is 33 s. When the surface temperature of the sample tube reaches 890 °C, water quenching treatment is carried out, the water temperature is 10 °C, and it is quenched and cooled to 10 °C; subsequently, the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0087] Comparative Example 5

[0088] The difference from Example 1 is that in step (4), the solution-treated sample tube is subjected to relaxation treatment. The average cooling rate of the relaxation treatment is 1.5 °C / s, and the time of the relaxation treatment is 93 s. When the surface temperature of the sample tube reaches 800 °C, water quenching treatment is carried out, the water temperature is 30 °C, and it is quenched and cooled to 30 °C; subsequently, the quenched pipe is subjected to tempering treatment, the tempering temperature is 690 °C, the holding time is 60 min, and it is air-cooled to room temperature after tempering, and finally a steel pipe is obtained.

[0089] Performance Test

[0090] Three parallel samples are taken from the steel pipes prepared in the examples and comparative examples for yield strength, tensile strength and impact energy tests. The yield strength and tensile strength are tested according to the GB / T228.1 standard, the test temperature is 25 °C, the tensile specimen size is 5 mm in diameter and 25 mm in gauge length; the impact specimen size is 10 mm×10 mm×55 mm, V-notch, the test temperature is 25 °C, and it is tested according to the GB / T229 standard.

[0091] Evaluating the SSC resistance performance: Take 3 parallel samples from the steel pipes prepared in the examples and comparative examples, and according to the NACE TM0177 standard, adopt Method A and Solution A, apply a stress of 606 MPa in a saturated H2S environment, and examine the fracture time.

[0092] The results of yield strength, tensile strength, impact energy and SSC fracture time are shown in Table 1.

[0093] Table 1

[0094]

[0095] Figure 1 This is the microstructure diagram of the steel pipe in Example 1 of this application. From Figure 1 it can be seen that the microstructure is tempered martensite.

[0096] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0097] In this application, if the relaxation treatment time is too long and the temperature of the pipe after relaxation treatment is too low, it is easy to cause an increase in the thermal stability of the austenite structure, resulting in a lag in the subsequent martensite transformation, and some austenite may be difficult to transform into martensite; if the relaxation treatment time is too short and the temperature of the pipe after relaxation treatment is too high, it is not conducive to the formation of sufficient austenite structure; controlling the relaxation treatment time and the temperature of the pipe after relaxation treatment within the above ranges helps to make the tissue structure be austenite and in a state that can be transformed into martensite. Controlling the temperature of the cooling medium for quenching treatment within the above ranges helps to control the cooling rate of the pipe, so that the austenite structure passes through the rapid cooling stage to form a martensite structure with a low dislocation density. Finally, the pipe undergoes tempering treatment, which helps to form tempered martensite with a low dislocation density, thereby helping to make the mechanical strength of the steel pipe reach the 110 ksi level and have excellent SSC resistance performance.

[0098] The above are only the embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a steel pipe, characterized in that, The preparation method includes: sequentially performing piercing hot rolling treatment, solution treatment, relaxation treatment, quenching treatment, and tempering treatment on the ingot to obtain the steel pipe; wherein, the time of the relaxation treatment is 30 - 60 s, the temperature of the pipe after the relaxation treatment is 850 - 890 °C, the temperature of the cooling medium for the quenching treatment is 15 - 40 °C, the yield strength of the steel pipe at 25 °C is 758 - 828 MPa, the steel pipe does not break under a stress of 606 MPa in a saturated H2S environment for at least 720 hours; the microstructure of the steel pipe is tempered martensite; By mass percentage, the ingot includes the following elements: the content of C element is 0.24 - 0.27%, the content of Si element ≤ 0.3%, the content of Mn element is 0.4 - 0.6%, the content of Cr element is 0.4 - 0.6%, the content of Mo element is 0.6 - 0.8%, the content of V element is 0.05 - 0.1%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element ≤ 0.005%, the content of P element ≤ 0.01%, the total content of inevitable impurities ≤ 0.15 wt%, the content of a single impurity is less than 0.05 wt%, and the balance is Fe element.

2. The manufacturing method of the steel pipe according to claim 1, characterized in that, The time of the relaxation treatment is 40 - 50 s; and / or, the temperature of the cooling medium for the quenching treatment is 20 - 35 °C.

3. The method for preparing a steel pipe according to claim 1, wherein, The temperature of the solution treatment is 930 - 950 °C; and / or, the holding time of the solution treatment is 35 - 45 min.

4. The manufacturing method of the steel pipe according to claim 1, characterized in that The temperature of the tempering treatment is 680 - 700 °C; and / or, the holding time of the tempering treatment is 55 - 65 min; and / or, the piercing hot rolling treatment includes heating the ingot to 1200 - 1300 °C and performing rolling after holding for 5 - 6 h.

5. The preparation method of the steel pipe according to claim 1, characterized in that, The outer diameter of the pipe after the piercing hot rolling treatment is 270 - 275 mm; and / or, the wall thickness of the pipe after the piercing hot rolling treatment is 13.5 - 14.0 mm; and / or, the temperature of the pipe after the piercing hot rolling treatment is 830 - 890 °C; and / or, the pipe after the piercing hot rolling treatment is cooled by online water ring spray to 450 - 710 °C at an average rate of 1 - 15 °C / s and then air-cooled to 20 - 30 °C.

6. The manufacturing method of the steel pipe according to claim 1, characterized in that, By mass percentage, the ingot comprises the following elements: the content of C element is 0.26 - 0.27%, the content of Si element is 0.2 - 0.3%, the content of Mn element is 0.46 - 0.52%, the content of Cr element is 0.5 - 0.6%, the content of Mo element is 0.7 - 0.8%, the content of V element is 0.05 - 0.06%, the content of Nb element is 0.01 - 0.03%, the content of Al element is 0.01 - 0.03%, the content of S element is 0.003 - 0.005%, the content of P element is 0.008 - 0.01%, the total content of inevitable impurities ≤ 0.15wt%, the content of a single impurity is less than 0.05wt%, and the balance is the Fe element.

7. A steel pipe, characterized in that, The steel pipe is prepared by the preparation method of the steel pipe according to any one of claims 1 to 6.

8. The steel pipe according to claim 7, characterized in that, The steel pipe has a tensile strength ≥ 793MPa and an impact energy ≥ 170J at 25°C.

9. Application of the steel pipe according to claim 7 or 8 as an oil casing on oil exploration equipment.

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