High-strength, high-toughness bainite geological drilling pipe and method for manufacturing the same

Abstract

The present disclosure provides a bainite geological drilling pipe containing the following chemical elements in the following mass percentages: 0.14 to 0.22% C, 0.2 to 0.55% Si, 2.1 to 2.9% Mn, 0.01 to 0.04% Nb, 0.015 to 0.04% Al, 0.001 to 0.005% B, 0 < N ≤ 0.007%, and the balance being Fe and unavoidable impurities; the content ratio of Al to N is Al / N ≥ 3. Further, the present disclosure provides a method for manufacturing a bainite geological drilling pipe, which includes the following steps: (1) a step of smelting and casting molten steel to obtain a raw pipe; (2) a step of heating, piercing, continuously rolling, and sizing the raw pipe to obtain a pipe body; and (3) a step of performing two-stage air cooling on the pipe body: in the first-stage air cooling, air circulation blowing cooling is performed on the outer surface of the pipe body, the temperature before cooling is Ar3 + 50°C or higher, the cooling rate is 5 to 15°C / s, and it is cooled to a temperature range of Bs - 100°C to Bs - 50°C; in the second-stage air cooling, the pipe body is naturally air cooled, and the cooling rate is 0.5 to 4°C / s.

Description

Technical Field

[0001] The present disclosure relates to seamless steel pipes and a manufacturing method thereof, and particularly to geological excavation pipes and a manufacturing method thereof.

Background Art

[0002] In recent years, due to the gradual mining of shallow mineral resources, China has proposed strategies such as "Three Deep Seas and One Continent (Three Depths and One Earth)" and "Deep Sea Excavation and Maritime Rights". Geological excavation has begun to be developed towards larger diameters and greater depths step by step. Due to this development trend, excavation pipes will have to withstand increasingly severe tensile forces, pressures, bending forces, torsional forces, impact forces, friction, and other complex stresses during use. Excavation pipes are very vulnerable to problems such as deformation, fracture, and wear failures caused by overload, resulting in high accident treatment costs and low excavation efficiency. Therefore, it is urgent to develop higher-performance excavation pipe products to meet the development needs of the industry.

[0003] Currently, conventional geological excavation pipe products can no longer meet the development needs towards deeper depths in the geological industry. To achieve higher-level geological excavation pipe products, it is necessary to adopt Cr-Mo steel grades subjected to quenching and tempering heat treatment. Such methods in the prior art not only have high alloy and process costs, but also the heat treatment procedures are restricted by environmental protection requirements, so many users do not have the production conditions. In addition, there are problems such as the tendency of deformation and cracking after quenching and tempering, and the variation in the performance of upset ends.

[0004] In view of the drawbacks and defects of the prior art, it is expected to obtain high-strength and high-toughness bainite geological excavation pipes with low manufacturing costs, good harmony between strength and toughness, without the need for quenching and tempering heat treatment, and capable of achieving the level of Cr-Mo steel grades subjected to quenching and tempering heat treatment, with good solid tensile performance and torque resistance.

Summary of the Invention

[0005] One of the objectives of the present disclosure is to provide a bainite geological drilling pipe that has low manufacturing costs, good strength and toughness, can achieve the level of Cr-Mo steel grades without the need for quenching and tempering heat treatment, and has good solid tensile performance and torque resistance. The bainite geological drilling pipe can be effectively applied to the geological drilling industry, helps to promote the green and efficient development of the geological drilling industry, and has a very broad application prospect.

[0006] To achieve the above objectives, the present disclosure provides a bainite geological drilling pipe containing the following chemical elements in the following mass percentages: 0.14 - 0.22% C, 0.2 - 0.55% Si, 2.1 - 2.9% Mn, 0.01 - 0.04% Nb, 0.015 - 0.04% Al, 0.001 - 0.005% B, 0 < N ≤ 0.007%, the balance being Fe and unavoidable impurities, and the content ratio of Al to N is Al / N ≥ 3.

[0007] In one embodiment, the bainite geological drilling pipe contains the following chemical elements in the following mass percentages: 0.14 - 0.22% C, 0.2 - 0.55% Si, 2.1 - 2.9% Mn, 0.01 - 0.04% Nb, 0.015 - 0.04% Al, 0.001 - 0.005% B, 0 < N ≤ 0.007%, the balance being Fe and unavoidable impurities, and the content ratio of Al to N is Al / N ≥ 3; and the bainite geological drilling pipe does not contain the elements Cr, Mo and W.

[0008] In the above technical solution of the present disclosure, by adopting a reasonable chemical composition design, that is, by including medium to high contents of Mn and / or a chemical composition that does not contain Cr, Mo, W and other precious metal elements, not only is the alloy cost reduced, but also due to the dragging effect of element Mn on the element diffusion at the phase transformation interface, the bainite phase transformation point is significantly reduced, so that the refinement of the microstructure is realized and the strength and toughness of the product are improved.

[0009] Furthermore, the chemical composition of the bainite geological drilling pipes of this disclosure is further enriched with element B. The combined addition of element B and Mn further improves air-cooling hardening properties and ensures the formation of a stable granular bainite structure. At the same time, element B may further strengthen grain boundaries, prevent excessive precipitation of island martensite (MA islands), and improve the toughness of the material.

[0010] In the bainite geological drilling pipes of this disclosure, the design principles for each chemical element are specifically explained below.

[0011] C: In the bainite geological drilling pipes of this disclosure, element C is an important element for ensuring the strength of the pipe. After the addition of element C, the bainite structure will be stabilized, and the air-cooling hardening properties of the material will be effectively improved. If the element C content in the steel is too low, the bainite structure may become unstable, and the strength and toughness of the material may deteriorate. At the same time, care must be taken not to ensure that the element C content in the steel is too high. If the element C content in the steel is too high, the toughness and plasticity of the steel may decrease. Therefore, in the bainite geological drilling pipes of this disclosure, the mass percentage content of element C is controlled to 0.14% to 0.22%.

[0012] Si: In the bainite geological drilling pipe of this disclosure, element Si is both a ferrite-forming element and a deoxidizing element. Element Si promotes ferrite formation within the microstructure and can improve the purity of molten steel while suppressing carbide precipitation. However, it should be noted that if the element Si content in the steel is too low, the corresponding above effects cannot be obtained, and if the element Si content in the steel exceeds 0.55%, improvement in the microstructure cannot be obtained. Therefore, in the bainite geological drilling pipe of this disclosure, the mass percentage content of element Si is controlled to 0.2% to 0.55%.

[0013] Mn: In the bainite geological drilling pipes of this disclosure, Mn is an important element that improves the air-cooling hardening properties of the material. Compared to elements such as Mo, Cr, and W, element Mn is very inexpensive and readily available. Compared to other elements, element Mn has the potential to significantly lower the transformation point of the bainite structure, thus refining the structure and improving strength and toughness. If the element Mn content in the steel is less than 2.1%, the hardening properties decrease, leading to the formation of a superbainite structure and a decrease in the toughness of the material. If the element Mn content in the steel exceeds 2.9%, Mn segregation becomes severe, which can worsen the toughness of the material and simultaneously deteriorate weldability. Based on this, in the bainite geological drilling pipes of this disclosure, the mass percentage content of Mn is controlled to 2.1% to 2.9%.

[0014] Nb: In the bainite geological drilling pipes of this disclosure, element Nb is bonded with element C to form Nb carbides, which suppresses grain growth, thereby refining the granular bainite structure and improving the strength of the material. At the same time, Nb can further suppress the precipitation of protereminate ferrite and upper bainite, allowing for a stable granular structure at a lower cooling rate, reducing the size of island martensite, and improving the toughness of the material. Therefore, in the bainite geological drilling pipes of this disclosure, the mass percentage content of element Nb is controlled to 0.01% to 0.04%.

[0015] Al: In the bainite geological drilling pipes of this disclosure, Al is a good deoxidizing element. However, care must be taken not to increase the elemental Al content in the steel. Too much elemental Al addition can easily lead to the formation of alumina inclusions. Therefore, it is necessary to maximize the specific gravity of acid-soluble aluminum in the total aluminum and supply an appropriate amount of Al wire after vacuum degassing. Accordingly, in the bainite geological drilling pipes of this disclosure, the mass percentage content of elemental Al is controlled to 0.015% to 0.04%.

[0016] B: In the bainite geological drilling pipe of the present disclosure, element B can effectively enhance the hardenability of the material. The combined addition of element B and element Mn can further improve the air-cooling hardenability and ensure the formation of a stable granular bainite structure in the steel. Furthermore, element B can further strengthen the grain boundaries, suppress the formation of island-like martensite, and improve the harmony between the strength and toughness of the material. When the content of element B in the steel is less than 0.001%, its effect is not obvious. When the content of element B in the steel is too high, for example, higher than 0.005%, it is difficult to accurately control the steelmaking process. Therefore, in the bainite geological drilling pipe of the present disclosure, the mass percentage content of element B is controlled to be 0.001% - 0.005%.

[0017] N: In the bainite geological drilling pipe of the present disclosure, element N in the steel may combine with element Al to form carbonitrides. Therefore, while controlling the mass percentage content of a single chemical element, it is necessary to control the mass percentage contents of element Al and element N so that Al / N ≥ 3 is achieved to ensure the content of acid-soluble aluminum in the steel. Element Al in the steel binds sufficiently with element N, thereby preventing element N from combining with element B to form a brittle low melting point phase, ensuring the effect of improving the hardenability of the steel by element B, and preventing embrittlement of the grain boundaries. Based on this, in the bainite geological drilling pipe of the present disclosure, the mass percentage content of element N is controlled to be 0 < N ≤ 0.007%.

[0018] In one embodiment, for the inevitable impurities in the bainite geological drilling pipe of the present disclosure, S ≤ 0.01% and P ≤ 0.006%.

[0019] In the above technical solution, both P and S are inevitable impurity elements in the steel. If technical conditions permit, in order to obtain a bainite geological drilling pipe with better performance and quality, the content of impurity elements in the steel should be reduced as much as possible.

[0020] In this disclosure, an excessive amount of element P in the steel can cause segregation and embrittlement at grain boundaries, thereby severely reducing the toughness of the material. Furthermore, an excessively high amount of element S in the steel may lead to an increase in the amount of inclusions, which does not contribute to the low-temperature toughness of the material. Therefore, where technical conditions permit, the content of elements P and S in the steel should be reduced as much as possible.

[0021] In one embodiment, the main component of the microstructure of the bainite geological drilling pipe is granular bainite. The phase ratio of granular bainite is 95% or more, and the size of the granular bainite is 4 to 10 μm.

[0022] In one embodiment, the microstructure of the bainite geological borehole further contains austenite in a phase ratio of 3-5%.

[0023] In one embodiment, the wall thickness of the bainite geological drilling pipe is 12 to 30 mm.

[0024] In one embodiment, a bainite geological drilling pipe can achieve the following properties without requiring quenching and tempering heat treatment: yield strength of 750 MPa or more; tensile strength of 1100 MPa or more; hardness of 35 HRC or more; toughness of 60 J or more; and residual stress of 40 MPa or less.

[0025] Another object of this disclosure is to provide a method for manufacturing bainite geological drilling pipes. This manufacturing method has a simple process. The bainite geological drilling pipes obtained by this manufacturing method have good strength and toughness and can achieve the same level of quenching and tempering as Cr-Mo steel without requiring quenching and tempering heat treatment. The bainite geological drilling pipes obtained by this manufacturing method have a yield strength of 750 MPa or more, a tensile strength of 1100 MPa or more, a hardness of 35 HRC or more, a toughness of 60 J or more, and a residual stress of 40 MPa or less, and have good solid tensile performance and torque resistance. The bainite geological drilling pipes obtained by this manufacturing method can be effectively applied to the geological drilling industry, promote the green and efficient development of the geological drilling industry, and have very broad application prospects.

[0026] To achieve the above object, the present disclosure provides a method for manufacturing a bainite geological drilling pipe including the following steps:

[0027] (1) A step of smelting and casting molten steel to obtain a pipe blank; (2) A step of heating, piercing, continuously rolling, and sizing the pipe blank to obtain a pipe body; and (3) A step of performing two-stage air cooling on the pipe body: In the first-stage air cooling, air circulation blowing cooling is performed on the outer surface of the pipe body. The temperature before cooling is Ar3 + 50 °C or higher, the cooling rate is 5 - 15 °C / s, and it is cooled to the temperature range of Bs - 100 °C to Bs - 50 °C; in the second-stage air cooling, the pipe body is naturally air cooled. The cooling rate is 0.5 - 4 °C / s. Here, Ar3 represents the ferrite precipitation temperature during cooling, and Bs represents the start temperature of bainite phase transformation.

[0028] In the above technical solution of the present disclosure, the manufacturing method of the present disclosure has a short manufacturing process flow and low manufacturing cost, greatly improves economic benefits, eliminates the need for subsequent heat treatment by users, and simultaneously improves the processing efficiency of the final product and the stability of product quality.

[0029] In the first-stage air cooling of the above (3), when air circulation blowing cooling is performed on the outer surface of the pipe body, the temperature before cooling is controlled to be Ar3 + 50 °C or higher, and the cooling rate is controlled to be 5 - 15 °C / s to cool to the temperature range of Bs - 100 °C to Bs - 50 °C. It should be noted that in this way, the phase transformations of proeutectoid ferrite and upper bainite can be effectively avoided, the supercooling degree for the formation of granular bainite increases, and the granular bainite structure is refined.

[0030] In step (3) of the manufacturing method of the present disclosure, the sized pipe is treated by two-stage air cooling. By using an air-cooled, highly hardenable composition design that is harmonized with cooling over a wide cooling rate range, the pipe can obtain a stable granular bainite structure (which contributes to the structural and performance stability of the thick-walled steel pipe), and the pipe can obtain low residual stress. With two-stage air cooling, the method of the present disclosure can effectively control the mutual cancellation of thermal stress and phase transformation stress in the cooling phase transformation process, thereby reducing the final residual stress and improving the deformation resistance of the steel pipe.

[0031] In one embodiment, in step (1) of the manufacturing method of the present disclosure, the degree of superheating of the molten steel is less than 30°C, and / or the pull-up speed of the continuous casting is 1.8 to 2.2 m / min.

[0032] In one embodiment, in step (2) of the manufacturing method of the present disclosure, the raw tube obtained in step (1) is cooled and then heated in a heating furnace (such as an annular heating furnace). The heating temperature is 1240 to 1300°C, and the heating time is 3 to 6 hours; then, perforation is performed. The perforation temperature is 1180 to 1240°C; continuous rolling is performed after perforation. The continuous rolling temperature is 1000 to 1100°C; thereafter, sizing is performed. The sizing temperature is in the range of Ac3 + 100°C to Ac3 + 200°C, where Ac3 represents the austenitization temperature.

[0033] Compared to prior art, the method for manufacturing bainite geological boreholes according to this disclosure has the following advantages and beneficial effects: By rationally optimizing and designing the chemical composition of the steel pipe and coordinating it with the manufacturing process of this disclosure, a high-strength, high-toughness bainite geological drilling pipe with good strength and toughness can be obtained. This bainite geological drilling pipe not only has good mechanical performance at room temperature but also low residual stress.

[0034] The bainite geological drilling pipe of this disclosure has good strength and toughness, and can achieve the quenching and tempering level of Cr-Mo steel without requiring quenching and tempering heat treatment. The bainite geological drilling pipe of this disclosure has a yield strength of 750 MPa or more, a tensile strength of 1100 MPa or more, a hardness of 35 HRC or more, a toughness of 60 J or more, and a residual stress of 40 MPa or less, and has good solid tensile performance and torque resistance.

[0035] The method for manufacturing bainite geological drilling tubes described herein has a short manufacturing process flow, significantly improves economic benefits, eliminates the need for subsequent heat treatment by the user, simultaneously improves the processing efficiency and stability of product quality of the final product, promotes the green and efficient development of the geological drilling industry, and has very broad application prospects. [Brief explanation of the drawing]

[0036] [Figure 1] Figure 1 shows a typical metallographic diagram of the bainite geological borehole from Example 1 under a 500x microscope. [Figure 2] Figure 2 is a microstructure photograph of the bainite geological borehole of Example 1 under a scanning electron microscope at 2000x magnification. [Modes for carrying out the invention]

[0037] The bainite geological drilling pipes and their manufacturing methods described herein will be explained and described below with reference to specific examples and accompanying drawings. However, this explanation and description will not unduly limit the technical solutions of this disclosure. [Examples]

[0038] Examples 1-6 and Comparative Examples 1-6 The bainite geological drilling pipes of Examples 1-6 and the comparative steel pipes of Comparative Examples 1-5 are manufactured by the following process:

[0039] (1) According to the chemical composition shown in Table 1, smelting and casting are carried out using an electric furnace or converter to obtain a raw tube: a mixing method of scrap steel and blast furnace molten iron is adopted. The proportion of molten iron is 50-60%. Smelting is carried out through an electric furnace, refined outside the furnace, degassed under vacuum, stirred with argon gas, and then inclusion modification is performed by Ca treatment to reduce the content of O and H. The alloy is cast into a circular raw tube, and in the casting process the superheating of the molten steel is controlled to less than 30°C, and the pull-up speed of continuous casting is controlled to 1.8-2.2 m / min to reduce segregation of components.

[0040] (2) Heating, perforation, hot rolling, and sizing: After the raw tube is cooled, it is heated in an annular heating furnace. The heating temperature is controlled to 1240-1300°C, and the heating time is controlled to 3-6 hours; perforation is performed. The perforation temperature is controlled to 1180-1240°C; continuous rolling is performed after perforation. The continuous rolling temperature is controlled to 1000-1100°C; then sizing is performed. The sizing temperature is controlled to be in the range of Ac3+100°C to Ac3+200°C (Ac3 indicates the austenitization temperature).

[0041] (3) Two-stage air cooling: The sized tube is subjected to two-stage air cooling. In the first stage of air cooling, the outer surface of the tube is cooled by air circulation. The temperature before cooling is Ar3 + 50°C or higher, and the cooling rate is 5 to 15°C / s, cooling to a temperature range of Bs - 100°C to Bs - 50°C; In the second stage of air cooling, the tube is air-cooled naturally. The cooling rate is 0.5 to 4°C / s. Ar3 represents the ferrite deposition temperature during cooling, and Bs represents the onset temperature of the bainite phase transformation.

[0042] The comparative steel pipe in Comparative Example 6 is manufactured in the same manner as in Example 1, except that the cooling of the pipe body after sizing is performed using only natural air cooling instead of two-stage air cooling.

[0043] The chemical composition design and related processes of the bainite geological drilling pipes in Examples 1-6 satisfy the design specification requirements of this disclosure. The chemical composition design or related processes of the comparative steel pipes in Comparative Examples 1-6 contain parameters that do not satisfy the design specification requirements of this disclosure.

[0044] The element C content in the comparative steel pipe of Comparative Example 1 is below the design range; the element Mn content in the comparative steel pipe of Comparative Example 2 is below the design range; the element Nb content in the comparative steel pipe of Comparative Example 3 is below the design range; the element C content in the comparative steel pipe of Comparative Example 4 exceeds the design range; the Al / N value in the comparative steel pipe of Comparative Example 5 does not meet the design range; the chemical composition design of the comparative steel pipe of Comparative Example 6 meets the design range of this disclosure, but it should be noted that in the manufacturing process, two-stage air cooling is not performed after sizing, and only natural air cooling is performed.

[0045] Table 1 lists the mass percentage ratios of each chemical element in the bainite geological drilling pipes of Examples 1 to 6 and the comparative steel pipes of Comparative Examples 1 to 6.

[0046] [Table 1]

[0047] Tables 2-1 and 2-2 list the specific process parameters for the bainite geological drilling pipes of Examples 1-6 and the comparative steel pipes of Comparative Examples 1-6 in the above process steps.

[0048] [Table 2]

[0049] [Table 3]

[0050] The bainite geological drilling pipes from Examples 1-6 and the comparative steel pipes from Comparative Examples 1-6 were sampled, and the mechanical performance of the completed pipes from each example and comparative example was tested at room temperature. The results of the mechanical performance tests for each example and comparative example are shown in Table 3.

[0051] The relevant performance test methods are as follows: Mechanical performance test: Testing conditions: temperature 23°C, humidity 56%, tensile speed 3 mm / min before yield point, 28 mm / min after yield point. The test is performed according to the conditions of "GB / T 228.1-2010 Tensile testing of metallic materials Part 1: Tensile testing at room temperature".

[0052] Table 3 shows the results of mechanical performance tests for the bainite geological drilling pipes of Examples 1-6 and the comparative steel pipes of Comparative Examples 1-6.

[0053] [Table 4]

[0054] As can be seen from Table 3, compared to the comparative steel pipes of Comparative Examples 1 to 6, the bainite geological drilling pipes of Examples 1 to 6 of this disclosure have excellent mechanical properties, good strength and toughness, a yield strength of 780 MPa to 900 MPa, a tensile strength of 1120 MPa to 1200 MPa, a hardness of 35 HRC to 40 HRC, a residual stress of 0 MPa to 35 MPa, and a longitudinal impact toughness of 65 J to 100 J at room temperature.

[0055] In contrast, the overall performance of the comparative steel pipes in Comparative Examples 1-5 is clearly inferior to that of the bainite geological drilling pipes in Examples 1-6. The comparative steel pipes in Comparative Examples 1-2 have very weak yield strength and tensile strength, and also inferior longitudinal impact toughness and hardness at room temperature; the comparative steel pipe in Comparative Example 3 has very weak yield strength and tensile strength, inferior hardness, and toughness that does not meet the requirements; the comparative steel pipe in Comparative Example 4 has high yield strength, tensile strength, and hardness, but inferior longitudinal impact toughness at room temperature and high residual stress; the comparative steel pipe in Comparative Example 5 has inferior yield strength, longitudinal impact toughness and hardness at room temperature, and high residual stress.

[0056] The comparative steel pipe in Comparative Example 6 meets the design specifications requirements of this disclosure in terms of its chemical composition, but because it does not employ a two-stage air cooling after sizing during the process, it has very high residual stress and poor longitudinal impact toughness at room temperature.

[0057] In summary, the bainite geological drilling pipes of Examples 1-6 possess good strength and toughness, can achieve the same quenching and tempering levels as Cr-Mo steel without requiring quenching and tempering heat treatment, have a yield strength of 750 MPa or higher, a tensile strength of 1100 MPa or higher, a hardness of 35 HRC or higher, a toughness of 60 J or higher, a residual stress of 40 MPa or lower, good solid tensile performance and torque resistance, a short manufacturing process flow, significantly improving economic efficiency, eliminating the need for further heat treatment by the user, simultaneously improving the processing efficiency and stability of the final product quality, promoting the green and efficient development of the geological drilling industry, and having very broad application prospects.

[0058] Figure 1 shows a typical metallographic diagram of the bainite geological borehole from Example 1 under a 500x microscope.

[0059] Figure 2 is a microstructure photograph of the bainite geological borehole of Example 1 under a scanning electron microscope at 2000x magnification.

[0060] As shown in Figures 1 and 2, the main component of the bainite geological borehole in Example 1 is granular bainite with a homogeneous granular bainite structure, and its size is 4 to 10 μm. The bainite geological borehole also contains a small amount of austenite with a phase ratio of 3 to 5%.

[0061] The morphology of the bainite geological drilling pipe in Example 1 after residual stress can be tested using the slitting method. The residual stress in Example 1 is small, and the pipe body is basically closed after slitting, which effectively prevents deformation during subsequent processing and use.

[0062] It should be noted that the various combinations of technical features in this case are not limited to the combinations described in the claims or the combinations described in the specific embodiments of this case. All technical features described in this disclosure can be freely combined or combined in any way, provided that they do not conflict with each other.

[0063] It should also be noted that the above embodiments are merely specific embodiments of the present disclosure. Clearly, the present disclosure is not limited to the above embodiments, and similar changes or modifications made accordingly would be readily conceivable to those skilled in the art or directly derived from the content of the present disclosure, and all should be included within the scope of protection of the present disclosure.

Claims

1. The following chemical elements are present in the following mass percentages: 0.14–0.22% C, 0.2–0.55% Si, 2.1–2.9% Mn, 0.01–0.04% Nb, 0.015–0.04% Al, 0.001–0.005% B, 0 < N ≤ 0.007%, the remainder being Fe and unavoidable impurities; The Al to N content ratio is Al / N ≥ 3, in a bainite geological drilling pipe. The bainite geological drilling pipe achieves the following mechanical properties without requiring quenching and tempering heat treatment: yield strength of 750 MPa or more; tensile strength of 1100 MPa or more; and hardness of 35 HRC or more. The aforementioned mechanical properties were measured according to the conditions described in "GB-T 228.1-2010 Metallic materials Tensile testing Part 1: Tensile testing at room temperature," A bainite geological drilling pipe whose microstructure is mainly composed of granular bainite, with a phase ratio of granular bainite of 95% or more, and granular bainite particles measuring 4 to 10 μm.

2. The following chemical elements are present in the following mass percentages: 0.14–0.22% C, 0.2–0.55% Si, 2.1–2.9% Mn, 0.01–0.04% Nb, 0.015–0.04% Al, 0.001–0.005% B, 0 < N ≤ 0.007%, the remainder being Fe and unavoidable impurities; Al / N ≥ 3; Bainite geological drilling pipes do not contain Cr, Mo, and W. The bainite geological drilling pipe according to claim 1.

3. The bainite geological drilling pipe according to claim 1 or 2, wherein the unavoidable impurities are S ≤ 0.01% and P ≤ 0.006%.

4. The bainite geological drilling pipe according to claim 1 or 2, wherein the microstructure of the bainite geological drilling pipe further contains austenite in a phase ratio of 3-5%.

5. The bainite geological drilling pipe according to claim 1 or 2, wherein the wall thickness of the bainite geological drilling pipe is 12 to 30 mm.

6. A method for manufacturing a bainite geological drilling pipe according to claim 1 or 2, comprising the following steps: (1) The process of smelting and casting molten steel to obtain a billet; (2) A process of heating, drilling, continuous rolling, and sizing the billet to obtain a pipe; and, (3) Step of two-stage air cooling of the tube: In the first stage air cooling, the outer surface of the tube is cooled by air circulation, the temperature before cooling is Ar3 + 50°C or higher, the cooling rate is 5 to 15°C / s, and the temperature is cooled to the range of Bs - 100°C to Bs - 50°C; In the second stage air cooling, the tube is air cooled naturally, the cooling rate is 0.5 to 4°C / s (wherein Ar3 represents the ferrite deposition temperature during cooling, and Bs represents the start temperature of the bainite phase transformation).

7. The method according to claim 6, wherein in step (1), the degree of superheating of the molten steel is less than 30°C, and / or the pull-up speed of the continuous casting is 1.8 to 2.2 m / min.

8. The method according to claim 6, wherein in step (2), the billet obtained in step (1) is cooled and then heated in a heating furnace to a heating temperature of 1240 to 1300°C for a heating time of 3 to 6 hours; then, perforation is performed to a perforation temperature of 1180 to 1240°C; after perforation, continuous rolling is performed to a continuous rolling temperature of 1000 to 1100°C; thereafter, it is cooled to generate ferrite, and then heated to a temperature of Ac3 + 100°C to Ac3 + 200°C to perform sizing (wherein Ac3 represents the austenitization temperature).

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