Baillard / horse multiphase petroleum casing pipe and preparation method thereof

Abstract

The invention belongs to the field of steel for petroleum casings, and discloses a bainite / martensite multiphase petroleum casing and a preparation method of the bainite / martensite multiphase petroleum casing. The preparation method comprises the following steps: smelting and casting a bainite steel billet containing C, Mn, Si, Cr, Ni, Mo, Nb, V, Ti, Cu and Fe to obtain bainite round steel; the bainite round steel is heated, hot-rolled and air-cooled to obtain a steel pipe; the hot-rolled steel pipe is naturally cooled to the bainite phase transformation starting temperature at a first cooling speed, cooled to the martensite phase transformation ending temperature from the bainite phase transformation starting temperature at a second cooling speed and then naturally cooled to the room temperature, and the bainite / martensite multiphase steel pipe is obtained; and tempering and straightening the bainite / horse complex-phase steel pipe to obtain the bainite / horse complex-phase petroleum casing pipe. According to the invention, through reasonable component design and phase change latent heat of the casing, redistribution of carbon is promoted, so that the corrosion-resistant bainite / martensite multiphase oil casing with ultrahigh toughness is prepared.

Description

Technical Field

[0001] The invention belongs to the field of steel for petroleum casing, and in particular relates to a Bayer / Massachusetts complex phase petroleum casing and a preparation method thereof. Background Art

[0002] Oil and gas production requires a large amount of oil pipes, and with the development of ultra-deep wells and shale gas wells, oil and gas pipes need to have both ultra-high strength and toughness. Traditional high-strength pipes are made using the quenching-tempering technology to form martensite, tempered martensite or troostite / troostite structures. However, the alloy design of steel in this technology is complex and the process is long.

[0003] Compared with martensite, bainite structure has both high strength and high toughness, and has attracted the attention of scholars at home and abroad. However, the currently disclosed oil pipe steel containing bainite structure all adopts controlled cooling (water cooling or stage cooling, etc.), which has complex process and is not suitable for the production of thick-walled pipelines.

[0004] Under this circumstance, the development of short-process, quenching-free, ultra-high strength and toughness steel for oil casing has become an important issue in the industry. Summary of the invention

[0005] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a bainite / martensite duplex oil casing and a preparation method thereof. Through reasonable composition design, the steel pipe is prevented from pearlite transformation during the cooling process, and is naturally cooled to the bainite transformation start temperature (Bs) at a first cooling rate; when cooled to the martensite transformation end temperature (Mf), through reasonable composition design, the phase transformation latent heat of bainite and martensite is regulated, and cooling is performed at an extremely low cooling rate between Bs and Mf, that is, a "slow cooling interval", which approximately forms a "temperature platform" to promote the redistribution of carbon atoms and alloying elements between bainite, martensite and residual austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite. Bainite and martensite ensure the high strength of the steel, and the residual austenite ensures the high toughness of the steel. At the same time, compared with the traditional martensitic structure, the "carbon-poor" martensite is hard but not brittle, and has both high strength and toughness.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing a Bei / Ma composite petroleum casing, comprising:

[0008] The bainite steel billet containing C, Mn, Si, Cr, Ni, Mo, Nb, V, Ti, Cu and Fe is smelted and cast to obtain bainite round steel;

[0009] The bainite round steel is heated, hot rolled, and air cooled to obtain a steel pipe;

[0010] The hot-rolled steel pipe is naturally cooled at a first cooling rate to a bainite phase transformation start temperature, cooled at a second cooling rate from the bainite phase transformation start temperature to a martensite phase transformation end temperature, and then naturally cooled to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0011] The B / M duplex steel pipe is tempered and straightened to obtain the B / M duplex oil casing.

[0012] Furthermore, the mass percentage of each element in the bainite steel billet is: C: 0.15-0.25wt%; Mn: 2.0-2.4wt%; Si: 0.8-1.7wt%; Cr: 0.2-1.2wt%; Ni: 0-2.0wt%; Mo: 0.1-0.3wt%; Nb: 0-0.06wt%; V: 0-0.10wt%; Ti: 0-0.05wt%; Cu: 0-0.5wt%; the rest is Fe and unavoidable impurities.

[0013] Further, the bainite round steel is heated and hot rolled, including:

[0014] Heat the bainitic round steel to 1150-1250℃ and keep it at this temperature for 2-5 hours.

[0015] Furthermore, heating and hot rolling the bainite round steel further comprises:

[0016] The heated bainite round steel is punched at 1000-1150°C, hot rolled at 950-1050°C, and sized at 900-950°C.

[0017] Furthermore, the average value of the first cooling rate is 0.3-1.5° C. / s.

[0018] Furthermore, the starting temperature of the bainite phase transformation is 350-450°C, and the ending temperature of the martensite phase transformation is 100-250°C.

[0019] Furthermore, the average value of the second cooling rate is 0.05-0.15° C. / s.

[0020] Further, the B / M duplex steel pipe is tempered and straightened, including:

[0021] The Bayer / Massachusetts duplex steel pipe is kept at 260-360°C for 6-30 hours and then naturally cooled to room temperature.

[0022] Further, the tempering and straightening of the Bayer / Massachusetts duplex steel pipe also includes:

[0023] The tempered Besten / Mastenite duplex steel pipe is straightened, and the maximum deformation of straightening is less than 5%. After straightening, the transformation amount of the retained austenite in the organization is less than 30%.

[0024] On the other hand, the present invention discloses a Bayer / Masonite composite petroleum casing, which is made by the above method. The Bayer / Masonite composite petroleum casing has a yield strength of 1000-1250MPa, an elongation of 15-20%, and an impact toughness greater than 120J.

[0025] Technical effects and advantages of the present invention:

[0026] 1. The present invention regulates the start and end temperatures of bainite phase transformation and martensite phase transformation, regulates the energy and speed of latent heat release of phase transformation, and synchronously completes the formation of bainite / martensite multiphase structure and carbon distribution through natural cooling without any external cooling medium and reheating. The process flow is simple, quenching and reheating are eliminated, and energy conservation and emission reduction are achieved.

[0027] 2. The present invention innovates the design of composition-process-organization performance. Through reasonable composition design, the latent heat of phase change (not waste heat) is used to promote the redistribution of carbon, and "carbon-poor" bainite and martensite and "carbon-rich" residual austenite are obtained. Bainite and martensite ensure the high strength of steel, and residual austenite ensures the high toughness of steel. At the same time, compared with traditional martensitic structure, "carbon-poor" martensite is hard but not brittle, and has high strength and toughness. The yield strength reaches 1000-1250MPa (150-170ksi), while maintaining high plasticity and toughness, the elongation is 15-20%, and the impact toughness is greater than 120J; on the other hand, due to the "carbon-poor" design and the addition of Cu element, the corrosion resistance of bainite / martensite composite oil casing is improved.

[0028] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of a method for preparing a Bei / Ma composite petroleum casing according to the present invention;

[0030] Figure 2 This is a schematic diagram of preparing a Bayer / Matz complex phase steel pipe by natural cooling according to the present invention;

[0031] Figure 3 is the cooling curve of the steel pipe after hot rolling in Example 2 of the present invention;

[0032] Figure 4 This is a schematic diagram of the microstructure of the Bei / Ma composite oil casing produced in Example 3 of the present invention. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] like Figure 1 As shown, the present invention provides a method for preparing a Bei / Ma complex phase petroleum casing, comprising:

[0035] The bainite steel billet containing C, Mn, Si, Cr, Ni, Mo, Nb, V, Ti, Cu and Fe is smelted and cast to obtain bainite round steel;

[0036] The bainite round steel is heated, hot rolled, and air cooled to obtain a steel pipe;

[0037] The hot-rolled steel pipe is naturally cooled to the starting temperature of bainite phase transformation at a first cooling rate, cooled from the starting temperature of bainite phase transformation to the ending temperature of martensite phase transformation at a second cooling rate, and then naturally cooled to room temperature to obtain a bainite / martensite multiphase steel pipe. The temperature change curve of the steel pipe over time is shown in FIG. Figure 2 As shown;

[0038] The B / M duplex steel pipe is tempered and straightened to obtain the B / M duplex oil casing.

[0039] In some embodiments of the present invention, the mass percentage of each element in the bainite steel billet is: C: 0.15-0.25wt%; Mn: 2.0-2.4wt%; Si: 0.8-1.7wt%; Cr: 0.2-1.2wt%;

[0040] Ni: 0-2.0wt%; Mo: 0.1-0.3wt%; Nb: 0-0.06wt%; V: 0-0.10wt%; Ti: 0-0.05wt%; Cu: 0-0.5wt%; the rest is Fe and unavoidable impurities.

[0041] It should be noted that adding C element to bainitic steel billet can ensure the strength of steel through interstitial solid solution strengthening on the one hand, and enrich austenite through carbon partitioning of latent heat of phase transformation on the other hand, stabilize the residual austenite and ensure the high toughness of steel; if C is lower than 0.15%, it is not enough to play the above two roles, but if C is higher than 0.25%, it will lead to the formation of twin martensite, resulting in reduced toughness of steel.

[0042] Mn is the main element that inhibits the ferrite / pearlite phase transformation. When the content is 2.0-2.4wt%, it can maximize the "solute drag and quasi-drag" effect and promote the formation of bainite. When it is lower than 2.0wt%, ferrite structure may be formed during the cooling process. When it is higher than 2.4wt%, too much martensite structure will be formed, which is not conducive to strength and toughness.

[0043] Si is one of the most effective elements for improving solid solution strengthening and is also an element that inhibits the precipitation of cementite. After its addition, on the one hand, the strength of the steel is improved, and on the other hand, the formation of residual austenite is promoted to improve the toughness of the steel. If the Si content is lower than 0.8wt%, the above two effects cannot be exerted. If the Si content is higher than 1.7wt%, it will cause smelting difficulties and hot rolling cracks, affecting toughness.

[0044] Cr is an important element for regulating the starting temperature of bainite transformation and the ending temperature of martensite transformation, and is the key to controlling the kinetics of bainite / martensite phase transformation. Experiments have found that controlling the Cr content at 0.2-1.2wt% can keep the "slow cooling interval" in a reasonable temperature range and promote carbon redistribution. The "slow cooling interval" refers to the cooling interval from the starting temperature of bainite phase transformation to the ending temperature of martensite phase transformation.

[0045] The role of alloying elements Ni, Mo, Nb, V, and Ti is to further regulate the phase transition temperature range of bainite / martensite and the release amount and release rate of phase transition latent heat, so that Bs, Mf, and "slow cooling range" are within a reasonable range, and the corrosion resistance of bainite / martensite composite oil casing is improved by adding Cu element.

[0046] In some embodiments of the present invention, heating and hot rolling bainite round steel include:

[0047] Heat the bainitic round steel to 1150-1250℃ and keep it at this temperature for 2-5 hours.

[0048] In some embodiments of the present invention, heating and hot rolling bainite round steel further comprises:

[0049] The heated bainite round steel is punched at 1000-1150°C, hot rolled at 950-1050°C, and sized at 900-950°C.

[0050] In some embodiments of the present invention, the average value of the first cooling rate is 0.3-1.5° C. / s, so as to avoid pearlite and ferrite phase transformation.

[0051] In some embodiments of the present invention, the starting temperature of the bainite phase transformation is 350-450°C, and the ending temperature of the martensite phase transformation is 100-250°C.

[0052] In some embodiments of the present invention, the average value of the second cooling rate is 0.05-0.15° C. / s. During this cooling stage, carbon atoms are partitioned to the untransformed austenite to form "carbon-poor" bainite and martensite and "carbon-rich" residual austenite.

[0053] In some embodiments of the present invention, tempering and straightening the Bayer / Massachusetts duplex steel pipe comprises:

[0054] The Bayer / Massachusetts duplex steel pipe is kept at 260-360°C for 6-30 hours and then naturally cooled to room temperature.

[0055] In some embodiments of the present invention, tempering and straightening the Bayer / Matz duplex steel pipe further comprises:

[0056] The tempered Besten / Mastenite duplex steel pipe is straightened, and the maximum deformation of straightening is less than 5%. After straightening, the transformation amount of the retained austenite in the organization is less than 30%.

[0057] On the other hand, the present invention discloses a Bayer / Masonite composite petroleum casing, which is made by the above method. The Bayer / Masonite composite petroleum casing has a yield strength of 1000-1250MPa, an elongation of 15-20%, and an impact toughness greater than 120J.

[0058] In order to better illustrate the present solution, the following embodiments and comparative examples are provided, wherein the mass percentage of each element in the bainite steel billet of each embodiment and comparative example is shown in Table 1.

[0059] Table 1 Mass percentage of each element in different bainite billets (wt%)

[0060]

[0061] Example 1

[0062] S1. Smelting and casting a bainite steel billet having the composition shown in Example 1 in Table 1 to obtain a bainite round steel;

[0063] S2, heating the bainite round steel to 1200°C and keeping it warm for 2 hours; then punching at 1100-1150°C, hot rolling at 950-1050°C, and finally sizing at 900°C-950°C to obtain a steel pipe;

[0064] S3, placing the hot-rolled steel pipe on a cooling bed, cooling it naturally at an average speed of 1°C / s to the starting temperature of 450°C for bainite phase transformation, and then cooling it at an average speed of 0.15°C / s to the ending temperature of 250°C for martensite phase transformation, so that bainite and martensite phase transformations occur, controlling the latent heat of phase transformation released during the phase transformation process, forming a "slow cooling zone", promoting the distribution of carbon atoms to untransformed austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and then cooling it naturally to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0065] S4. Straighten the above-mentioned Besso / Marietosecond composite steel pipe, and temper it at 260° C. for 30 hours to obtain an ultra-high strength and toughness Besso / Marietosecond composite oil casing.

[0066] Example 2

[0067] S1. Smelting and casting a bainite steel billet having the composition shown in Example 2 of Table 1 to obtain a bainite round steel;

[0068] S2, heating the bainite round steel to 1200°C and keeping the temperature for 3 hours; then punching at 1050-1100°C, hot rolling at 950-1050°C, and finally sizing at 900°C-950°C to obtain a steel pipe;

[0069] S3. Place the hot-rolled steel pipe on a cooling bed and cool it naturally at an average rate of 0.85°C / s to the start temperature of bainite phase transformation, 400°C, and then cool it at an average rate of 0.12°C / s to the end temperature of martensite phase transformation, 200°C. The temperature-time curves of the cooling process obtained by the two measurements are as follows: Figure 3 As shown, by cooling, bainite and martensite phase transformation occurs, the latent heat of phase transformation released during the phase transformation process is controlled, a "slow cooling zone" is formed, and carbon atoms are promoted to be distributed to the untransformed austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and then naturally cooled to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0070] S4. Straighten the above-mentioned Besso / Marietosecond composite steel pipe, and temper it at 280°C for 15 hours to obtain an ultra-high strength and toughness Besso / Marietosecond composite oil casing.

[0071] Example 3

[0072] S1. Smelting and casting a bainite steel billet having the composition shown in Example 3 of Table 1 to obtain a bainite round steel;

[0073] S2, heating the bainite round steel to 1250°C and keeping it warm for 4 hours; then punching at 1100-1150°C, hot rolling at 950-1050°C, and finally sizing at 900°C-950°C to obtain a steel pipe;

[0074] S3, placing the hot-rolled steel pipe on a cooling bed, cooling it naturally at an average speed of 1.5°C / s to the starting temperature of 380°C for bainite phase transformation, and then cooling it at an average speed of 0.1°C / s to the ending temperature of 200°C for martensite phase transformation, so that bainite and martensite phase transformations occur, controlling the latent heat of phase transformation released during the phase transformation process, forming a "slow cooling zone", promoting the distribution of carbon atoms to untransformed austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and then cooling it naturally to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0075] S4. Straighten the above-mentioned Besso / Marietosecond composite steel pipe, and temper it at 320°C for 20 hours to obtain an ultra-high strength and toughness Besso / Marietosecond composite oil casing.

[0076] The microstructure of the Bei / Ma composite oil casing prepared in this embodiment is as follows: Figure 4 As shown, the microstructure includes bainite and martensite, wherein retained austenite is distributed between bainite and martensite laths.

[0077] Example 4

[0078] S1. Smelting and casting a bainite steel billet having the composition shown in Example 4 of Table 1 to obtain a bainite round steel;

[0079] S2, heating the bainite round steel to 1200°C and keeping the temperature for 5 hours; then punching at 1100-1150°C, hot rolling at 950-1050°C, and finally sizing at 900°C-950°C to obtain a steel pipe;

[0080] S3, placing the hot-rolled steel pipe on a cooling bed, cooling it naturally at an average rate of 0.45°C / s to the starting temperature of 390°C for bainite phase transformation, and then cooling it at an average rate of 0.05°C / s to the ending temperature of 150°C for martensite phase transformation, so that bainite and martensite phase transformations occur, controlling the latent heat of phase transformation released during the phase transformation process, forming a "slow cooling zone", promoting the distribution of carbon atoms to untransformed austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and then cooling it naturally to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0081] S4. Straighten the above-mentioned Besso / Marietosecond composite steel pipe, and temper it at 340°C for 10 hours to obtain an ultra-high strength and toughness Besso / Marietosecond composite oil casing.

[0082] Example 5

[0083] S1. Smelting and casting a bainite steel billet having the composition shown in Example 5 of Table 1 to obtain a bainite round steel;

[0084] S2, heating the bainite round steel to 1150°C and keeping it warm for 2 hours; then punching at 1100-1150°C, hot rolling at 950-1050°C, and finally sizing at 900°C-950°C to obtain a steel pipe;

[0085] S3, placing the hot-rolled steel pipe on a cooling bed, cooling it naturally at an average speed of 0.3°C / s to the starting temperature of 350°C for bainite phase transformation, and then cooling it at an average speed of 0.05°C / s to the ending temperature of 100°C for martensite phase transformation, so that bainite and martensite phase transformations occur, controlling the latent heat of phase transformation released during the phase transformation process, forming a "slow cooling zone", promoting the distribution of carbon atoms to untransformed austenite, forming "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and then cooling it naturally to room temperature to obtain a bainite / martensite complex phase steel pipe;

[0086] S4. Straighten the above-mentioned Besso / Marietosecond composite steel pipe, and temper it at 320°C for 6 hours to obtain an ultra-high strength and toughness Besso / Marietosecond composite oil casing.

[0087] Comparative Example 1

[0088] The alloy composition of Comparative Example 1 is shown in Table 1 Comparative Example 1, and the process control is consistent with Example 1.

[0089] Comparative Example 2

[0090] The alloy composition of Comparative Example 2 is shown in Table 1 Comparative Example 2, and the process control is consistent with Example 2.

[0091] Comparative Example 3

[0092] The alloy composition of Comparative Example 3 is shown in Table 1 Comparative Example 3, which is consistent with Example 3. The preparation process steps are as follows:

[0093] S1, smelting and casting the bainite steel billet with the composition shown in Comparative Example 3 of Table 1 to obtain round steel;

[0094] S2, heating and hot rolling the round steel to obtain a steel pipe;

[0095] S3, quenching the hot-rolled steel pipe to room temperature;

[0096] S4. Straighten the steel pipe treated in S3 and temper it at 320°C for 20 hours.

[0097] Comparative Example 4

[0098] The alloy composition of Comparative Example 4 is shown in Table 1, which is consistent with Example 4. Comparative Example 4 adopts forced cooling in the phase change latent heat release stage, without a "slow cooling zone", and the preparation process steps are as follows:

[0099] S1, smelting and casting the bainite steel billet with the composition shown in Comparative Example 4 in Table 1 to obtain round steel;

[0100] S2, heating and hot rolling the round steel to obtain a first steel pipe;

[0101] S3, placing the hot-rolled steel pipe on a cooling bed, cooling it to 390°C at an average speed of 0.45°C / s, and then forced cooling it to room temperature to obtain a second steel pipe;

[0102] S4. Straighten the second steel pipe and temper it at 340°C for 10 hours to obtain a petroleum casing.

[0103] The oil casing obtained in the above examples and comparative examples was sampled and processed into standard tensile and impact samples, and the yield strength, tensile strength, elongation and impact toughness at room temperature were tested. The corrosion resistance was tested by 5% sodium chloride aqueous solution for 7 days of immersion. The rust was mechanically removed and weighed. The corrosion resistance was evaluated by the weight loss. In order to normalize the comparison, the tempered N80 steel pipe was used as the substrate in the test, that is, the weight loss ratio of the test sample to the weight loss of the N80 steel pipe was used as the corrosion resistance. The smaller the relative weight loss, the higher the corrosion resistance. The test results are shown in Table 2.

[0104] Table 2 Mechanical properties of oil casing obtained in the embodiments and comparative examples

[0105]

[0106] It can be seen from Table 2 that the C content in Comparative Example 1 is too low and the Mn content is too high. Although the two have close Bs and Mf points, the interstitial solid solution of C has a greater impact on the strength. Therefore, compared with Example 1, the strength of Comparative Example 1 is lower. At the same time, the Si content in Comparative Example 1 is low, and the precipitation of cementite cannot be effectively inhibited, resulting in a serious decrease in toughness compared with Example 1.

[0107] In Comparative Example 2, the C content is too high. Although the strength is greatly improved, the toughness is deteriorated.

[0108] Although the composition of Comparative Example 3 is the same as that of Example 3, water quenching is performed after hot rolling of the steel pipe to form a single martensitic structure, and the strength is slightly increased, but the impact toughness and corrosion resistance are significantly deteriorated.

[0109] The composition of Comparative Example 4 is consistent with that of Example 4. Natural cooling is also first adopted after hot rolling of the steel pipe. However, during the phase change latent heat release stage, forced cooling is adopted, which does not fully promote the redistribution of carbon. Its impact toughness and corrosion resistance are lower than those of Example 4.

[0110] In summary, the present invention avoids pearlite transformation during natural cooling of the steel pipe through reasonable composition design, regulates the latent heat of phase transformation of bainite and martensite, forms a "slow cooling zone" between Bs and Mf, promotes the redistribution of carbon atoms and alloy elements between bainite, martensite and residual austenite, forms "carbon-poor" bainite and martensite, and "carbon-rich" residual austenite, and finally obtains a bainite / martensite complex phase oil casing with a yield strength of 1000-1250MPa, an elongation of 15-20%, an impact toughness of more than 120J, and a corrosion resistance of more than 30% higher than that of traditional quenched and tempered oil casing.

[0111] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a Bayer / Madison composite petroleum casing, characterized in that: include: Smelting and casting a bainite steel billet containing C, Mn, Si, Cr, Ni, Mo, Nb, V, Ti, Cu, and Fe to obtain a bainite round steel; The bainite round steel is heated, hot-rolled, and air-cooled to obtain a steel pipe; The hot-rolled steel pipe is naturally cooled to a bainite transformation start temperature at a first cooling rate, cooled from the bainite transformation start temperature to a martensite transformation end temperature at a second cooling rate, and then naturally cooled to room temperature to obtain a bainite / martensite complex phase steel pipe; The B / M duplex steel pipe is tempered and straightened to obtain a B / M duplex petroleum casing.

2. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 1, characterized in that: The mass percentage of each element in the bainite steel billet is: C: 0.15-0.25wt%; Mn: 2.0-2.4wt%; Si: 0.8-1.7wt%; Cr: 0.2-1.2wt%; Ni: 0-2.0wt%; Mo: 0.1-0.3wt%; Nb: 0-0.06wt%; V: 0-0.10wt%; Ti: 0-0.05wt%; Cu: 0-0.5wt%; the rest is Fe and unavoidable impurities.

3. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 1, characterized in that: The heating and hot rolling of the bainite round steel comprises: The bainite round steel is heated to 1150-1250° C. and kept at this temperature for 2-5 hours.

4. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 3, characterized in that: The heating and hot rolling of the bainite round steel further comprises: The heated bainite round steel is punched at 1000-1150°C, hot rolled at 950-1050°C, and sized at 900-950°C.

5. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 1, characterized in that: The average value of the first cooling rate is 0.3-1.5° C. / s.

6. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 1, characterized in that: The starting temperature of the bainite phase transformation is 350-450°C, and the ending temperature of the martensite phase transformation is 100-250°C.

7. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 1, characterized in that: The average value of the second cooling rate is 0.05-0.15° C. / s.

8. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 7, characterized in that: The tempering and straightening of the B / M duplex steel pipe comprises: The Bayer / Matz complex phase steel pipe is kept at 260-360° C. for 6-30 hours and then naturally cooled to room temperature.

9. The method for preparing a Bei / Ma complex phase petroleum casing according to claim 8, characterized in that: The tempering and straightening of the Bayer / Matz complex phase steel pipe further includes: The tempered Beta / Mastenite complex phase steel pipe is straightened, and the maximum deformation of straightening is less than 5%. After straightening, the transformation amount of the retained austenite in the structure is less than 30%.

10. A Bayer / Madison composite oil casing, characterized in that: The Bayer / Massachusetts multiphase oil casing is made by the method described in any one of claims 1 to 9. The Bayer / Massachusetts multiphase oil casing has a yield strength of 1000-1250 MPa, an elongation of 15-20%, and an impact toughness greater than 120J.

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