A bimetallic petroleum casing and a method for manufacturing the same

Abstract

The application belongs to the field of steel for oil casing, and discloses a bainite / martensite dual-phase oil casing and a preparation method thereof. The preparation method comprises the following steps: smelting and casting a bainite steel blank containing C, Mn, Si, Cr, Ni, Mo, Nb, V, Ti, Cu and Fe to obtain a bainite round steel; heating and hot-rolling the bainite round steel to obtain a steel pipe through air cooling; naturally cooling the hot-rolled steel pipe to a bainite phase transition starting temperature at a first cooling speed, cooling the steel pipe from the bainite phase transition starting temperature to a martensite phase transition ending temperature at a second cooling speed, and then naturally cooling the steel pipe to room temperature to obtain a bainite / martensite dual-phase steel pipe; and tempering and straightening the bainite / martensite dual-phase steel pipe to obtain the bainite / martensite dual-phase oil casing. The bainite / martensite dual-phase oil casing with super-high strength and toughness and corrosion resistance is prepared by reasonable component design and self latent heat of phase transition, so as to promote the redistribution of carbon.

Description

Technical Field

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

[0002] Oil and gas production requires a large amount of oil pipe. With the development of ultra-deep wells and shale gas wells, oil and gas pipes must combine ultra-high strength and toughness. Traditional high-strength pipes are produced using a quenching and tempering process to form martensite, tempered martensite, or troostite / troostite structures. However, this process involves complex steel alloy design and a lengthy manufacturing process.

[0003] Compared to martensite, bainite offers both high strength and high toughness, attracting considerable attention from researchers both domestically and internationally. However, currently available bainite-containing oil pipe steels all utilize controlled cooling (water cooling or staged cooling), resulting in complex processes that are unsuitable for thick-walled pipeline production.

[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 avoids pearlite phase transformation during the cooling process, and is naturally cooled to the bainite transformation start temperature (Bs) at a first cooling rate; when cooling 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 carried out at an extremely low cooling rate between Bs and Mf, that is, a "slow cooling range", which approximately forms a "temperature platform", promotes the redistribution of carbon atoms and alloying elements among bainite, martensite and retained austenite, and forms "carbon-poor" bainite and martensite, and "carbon-rich" retained austenite. Bainite and martensite ensure the high strength of the steel, and retained 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 Beima / Ma composite oil casing, comprising:

[0008] 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;

[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 transformation start temperature, then cooled at a second cooling rate from the bainite transformation start temperature to a martensite transformation end temperature, and then naturally cooled to room temperature to obtain a bainite / martensite duplex 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%; and the rest is Fe and unavoidable impurities.

[0013] Furthermore, 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 pierced 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 transformation is 350-450°C, and the ending temperature of the martensite transformation is 100-250°C.

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

[0020] Furthermore, 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] Furthermore, the tempering and straightening of the B / M duplex steel pipe also includes:

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

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

[0025] The technical effects and advantages of the present invention are as follows:

[0026] 1. The present invention regulates the start and end temperatures of the bainite phase transformation and the martensite phase transformation, and regulates the energy and speed of the latent heat released by the phase transformation. Without any external cooling medium or reheating, the present invention simultaneously completes the formation of the bainite / martensite duplex structure and carbon distribution through natural cooling. The process is simple, eliminates quenching and reheating, and saves energy and reduces emissions.

[0027] 2. The present invention innovates the design of composition-process-organization performance. Through reasonable composition design, the latent heat of phase transformation (not waste heat) is used to promote the redistribution of carbon, and "carbon-poor" bainite and martensite and "carbon-rich" residual austenite are obtained. The bainite and martensite ensure the high strength of the steel, and the retained 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 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 the bainite / martensite duplex oil casing is improved.

[0028] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be 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 oil casing according to the present invention;

[0030] Figure 2 Schematic diagram of the preparation of a Bayer / Massachusetts duplex steel pipe by natural cooling according to the present invention;

[0031] Figure 3 : is the cooling curve of the hot-rolled steel pipe in Example 2 of the present invention;

[0032] Figure 4 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 clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts 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 composite oil casing, comprising:

[0035] 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;

[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 bainite transformation start temperature at a first cooling rate, cooled from the bainite transformation start temperature to the 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 temperature change curve of the steel pipe with 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 percentages of the elements in the bainite steel billet are: 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 the addition of C element to the bainite steel billet, on the one hand, ensures the strength of the steel through interstitial solid solution strengthening, and on the other hand, enriches the carbon to the austenite through carbon partitioning of the latent heat of phase transformation, stabilizes the residual austenite, and ensures the high toughness of the steel; if the C content is lower than 0.15%, it is not enough to play the above two roles, but if the C content is higher than 0.25%, it will lead to the formation of twinned martensite, resulting in a decrease in the toughness of the steel.

[0042] Mn is the main element that inhibits the ferrite / pearlite phase transformation. When its 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%, excessive 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, it improves the strength of steel on the one hand, and promotes the formation of retained austenite and improves the toughness of steel on the other hand. 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 range" within a reasonable temperature range and promote carbon redistribution. The "slow cooling range" refers to the cooling range 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 transformation temperature range of bainite / martensite and the release amount and release rate of phase transformation latent heat, so that Bs, Mf, and "slow cooling range" are within a reasonable range. The addition of Cu element can also improve the corrosion resistance of bainite / martensite duplex oil casing.

[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 pierced 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 to avoid pearlite and ferrite phase transformations.

[0051] In some embodiments of the present invention, the bainite transformation start temperature is 350-450°C, and the martensite transformation end temperature 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 partition into untransformed austenite to form "carbon-poor" bainite and martensite and "carbon-rich" retained austenite.

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

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

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

[0058] In order to better illustrate the present solution, the following examples and comparative examples are provided, wherein the mass percentages of the various elements in the bainite steel billets of the examples and comparative examples are shown in Table 1.

[0059] Table 1 Mass percentage of each element in different bainite steel 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. The bainite round steel is heated to 1200° C. and kept at this temperature for 2 hours; then, perforated at 1100-1150° C., hot rolled at 950-1050° C., and finally sized at 900-950° C. to obtain a steel pipe;

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

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

[0066] Example 2

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

[0068] S2. The bainite round steel is heated to 1200° C. and kept at this temperature for 3 hours; then, perforated at 1050-1100° C., hot rolled at 950-1050° C., and finally sized at 900-950° 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 transformation, 400°C. Then cool it at an average rate of 0.12°C / s to the end temperature of martensite transformation, 200°C. The temperature-time curves of the cooling process obtained from the two measurements are shown in Figure 2. Figure 3 As shown in the figure, by cooling, bainite and martensite phase transformation occurs, the latent heat released during the phase transformation is controlled, a "slow cooling zone" is formed, and carbon atoms are promoted to be partitioned into 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 duplex steel pipe;

[0070] S4. Straighten the above-mentioned Besso / Marshmallow complex phase steel pipe and temper it at 280° C. for 15 hours to obtain an ultra-high strength and toughness Besso / Marshmallow complex phase 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. The bainite round steel is heated to 1250° C. and kept at this temperature for 4 hours; then, perforated at 1100-1150° C., hot rolled at 950-1050° C., and finally sized at 900-950° C. to obtain a steel pipe;

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

[0075] S4. Straighten the above-mentioned Besso / Marshmallow complex phase steel pipe and temper it at 320° C. for 20 hours to obtain an ultra-high strength and toughness Besso / Marshmallow complex phase 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, in which retained austenite is distributed between the bainite and martensite laths.

[0077] Example 4

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

[0079] S2. The bainite round steel is heated to 1200° C. and kept at this temperature for 5 hours; then, perforated at 1100-1150° C., hot rolled at 950-1050° C., and finally sized at 900-950° C. to obtain a steel pipe;

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

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

[0082] Example 5

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

[0084] S2. The bainite round steel is heated to 1150° C. and kept at this temperature for 2 hours; then, perforated at 1100-1150° C., hot rolled at 950-1050° C., and finally sized at 900-950° C. to obtain a steel pipe;

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

[0086] S4. Straighten the above-mentioned Besso / Marshmallow complex phase steel pipe and temper it at 320° C. for 6 hours to obtain an ultra-high strength and toughness Besso / Marshmallow complex phase 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, 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 in 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 after S3 treatment and temper it at 320℃ for 20 hours.

[0097] Comparative Example 4

[0098] The alloy composition of Comparative Example 4 is shown in Table 1, which is consistent with that of Example 4. Comparative Example 4 adopts forced cooling during 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 rate 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] Oil casing samples obtained in the above examples and comparative examples were processed into standard tensile and impact specimens and tested for yield strength, tensile strength, elongation, and impact toughness at room temperature. Corrosion resistance was tested using a cyclic immersion test in a 5% sodium chloride aqueous solution for seven days. Rust was mechanically removed and weighed, with weight loss used to evaluate corrosion resistance. For normalized comparison, this test used a quenched and tempered N80 steel pipe as the substrate. The ratio of the weight loss of the test specimen to the weight loss of the N80 steel pipe was used as the corrosion resistance. The lower 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 Examples 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, which cannot effectively inhibit the precipitation of cementite, 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. The strength is slightly increased, but the impact toughness and corrosion resistance are significantly deteriorated.

[0109] Comparative Example 4 has the same composition as 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 both lower than those of Example 4.

[0110] In summary, the present invention, through reasonable composition design, prevents pearlite transformation during natural cooling of the steel pipe, regulates the latent heat of phase transformation of bainite and martensite, forms a "slow cooling zone" between Bs and Mf, and promotes the redistribution of carbon atoms and alloying elements among bainite, martensite, and retained austenite, forming "carbon-poor" bainite and martensite and "carbon-rich" retained austenite. The result is a bainite / martensite duplex oil casing with a yield strength of 1000-1250 MPa, an elongation of 15-20%, an impact toughness greater than 120 J, and corrosion resistance that is 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, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or 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 scope of protection of the present invention.

Claims

1. A method for preparing a Bayer / Madison composite oil casing, characterized in that: include: A 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, wherein the mass percentage of each element in the bainite steel billet is as follows: 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%; and the remainder is Fe and unavoidable impurities; The bainite round steel is heated, hot-rolled, and air-cooled to obtain a steel pipe; The hot-rolled steel pipe is naturally cooled at a first cooling rate to a bainite transformation start temperature, cooled at a second cooling rate from the bainite transformation start temperature to a martensite transformation end temperature, and then naturally cooled to room temperature to obtain a bainite / martensite complex phase steel pipe, wherein the average value of the first cooling rate is 0.3-1.5°C / s, and the average value of the second cooling rate is 0.05-0.15°C / s; The B / M duplex steel pipe is tempered and straightened to obtain a B / M duplex oil casing.

2. The method for preparing a Bei / Ma complex phase oil 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.

3. The method for preparing a Bei / Ma complex phase oil casing according to claim 2, characterized in that: The heating and hot rolling of the bainite round steel further comprises: The heated bainite round steel is pierced at 1000-1150° C., hot rolled at 950-1050° C., and sized at 900-950° C.

4. The method for preparing a Bei / Ma complex phase oil 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.

5. The method for preparing a Bayer / Massac multiphase oil casing according to claim 1, characterized in that: The tempering and straightening of the B / M duplex steel pipe comprises: The Bayer / Massachusetts duplex steel pipe is kept at 260-360° C. for 6-30 hours and then naturally cooled to room temperature.

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

7. A Bayer / Mass multiphase oil casing, characterized in that: The Bayer / Masonry composite oil casing is produced by the method according to any one of claims 1 to 6. The Bayer / Masonry composite oil casing has a yield strength of 1000-1250 MPa, an elongation of 15-20%, and an impact toughness greater than 120 J.

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