A bainite seamless steel pipe and a method for manufacturing the same

Abstract

The application discloses a bainite seamless steel tube containing Fe and inevitable impurity elements, and also containing the following chemical elements with mass percentage as follows: C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.8-2.05%, Cr: 0.85-1.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0 The application also discloses a manufacturing method of the bainite seamless steel tube, which comprises the following steps: (1) smelting and pipe blank continuous casting; (2) piercing and continuous rolling and sizing; (3) performing three-stage air cooling on the sized pipe: in the range of 500-850 DEG C, the cooling speed is controlled in the range of 2-5 DEG C / s; after cooling to 500 DEG C, the cooling speed is controlled as 5-15 DEG C / s; after cooling to 300 DEG C, natural air cooling is performed. The bainite seamless steel tube can be obtained without quenching and tempering heat treatment, and has good strength and toughness matching in the rolled state or single tempering state.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of steel pipe and its manufacturing method, and more particularly to a kind of seamless steel pipe and its manufacturing method. BACKGROUND

[0002] In recent years, in order to meet market demand, more and more seamless steel pipe products begin to be applied to various industries, and play a very important role.

[0003] Research found that, in order to be able to prepare yield strength reaches 80 ksi, and has a certain toughness of seamless steel pipe product, in the current prior art, must be treated by quenching and tempering heat treatment process to steel pipe.

[0004] However, this quenching and tempering heat treatment process not only has high energy consumption, but also has serious surface oxidation, and steel pipe is prone to size deterioration, cracking, straightness deterioration and a series of problems in the quenching and tempering process, which does not meet the current high-quality green development direction.

[0005] Therefore, in order to solve the problem that 80 ksi seamless steel pipe needs to be treated by quenching and tempering heat treatment to obtain strength and toughness matching in the prior art, the present application expects to develop and obtain a new bainite seamless steel pipe product, which can obtain good strength and toughness matching in rolling state or single tempering state without quenching and tempering heat treatment through reasonable composition matching and process design. The process flow of this new bainite seamless steel pipe product is short, the energy consumption is low, and a series of problems such as size deterioration and surface oxidation caused by quenching and tempering are avoided. SUMMARY

[0006] One of the purposes of the present application is to provide a bainite seamless steel pipe, which can obtain good strength and toughness matching in rolling state or single tempering state without quenching and tempering heat treatment through reasonable composition matching and process design, and has lower alloy cost and good economy.

[0007] In order to achieve the above purpose, the present application provides a bainite seamless steel pipe containing Fe and inevitable impurity elements, which also contains the following chemical elements with mass percentage as follows:

[0008] C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.8-2.05%, Cr: 0.85-1.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0

[0009] Further, in the bainite seamless steel pipe of the present application, the mass percentage of each chemical element is:

[0010] C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.8-2.05%, Cr: 0.85-1.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0

[0011] In the above technical solution, the bainite seamless steel tube has high hardenability and can obtain stable granular bainite structure in a wide range of cooling speed, which is beneficial to the stability of the microstructure and performance of the thick-walled steel tube.

[0012] In addition, the bainite seamless steel tube does not contain noble metals and micro-alloying elements such as Mo, Ni, Nb, V, Ti, etc., but uses conventional Mn and Cr, B, etc., which has good economy and low alloy cost.

[0013] In the bainite seamless steel tube, the design principles of each chemical element are as follows:

[0014] C: In the bainite seamless steel tube, C element is an important element for ensuring the strength of the steel, which can stabilize the bainite structure and improve the hardenability after adding. In addition, C can also move the CCT curve to the right, and appropriate amount of C element added in the steel can ensure that the steel obtains bainite structure at a lower cooling speed. If the content of C element in the steel is too low, the bainite structure will be unstable, and the strength and toughness of the steel will be poor; and if the content of C element in the steel is too high, the number of M-A islands will increase, and the toughness of the steel will be poor. Therefore, in the bainite seamless steel tube, the mass percentage of C element is controlled between 0.16-0.18%.

[0015] Si: In the bainite seamless steel tube, Si element is a ferrite forming element and a deoxidizing element, which can not only improve the purity of molten steel, but also inhibit the precipitation of carbides. Appropriate amount of Si element added in the steel can not only ensure the solid solution strengthening of C element, but also reduce the number of M-A islands and refine the M-A structure, thereby improving the strength and toughness matching of the steel. Of course, the addition amount of Si element should not be too low, and the Si content in the steel is too low to have the corresponding effect, and the Si content in the steel exceeds 1% after which the structure is not improved. Therefore, in the bainite seamless steel tube, the mass percentage of Si element is controlled between 0.56-1%.

[0016] Mn: In the bainite seamless steel pipe described in the application, Mn is an important element for improving air-cooling hardenability, which is more inexpensive and easy to obtain than Mo, Cr, W and other elements, and can more obviously reduce the bainite structure transformation point than other elements, so as to effectively refine the structure and improve the strength and toughness of the steel. When the content of Mn element in the steel is less than 1.8%, the upper bainite structure will be formed due to the decrease of hardenability, and the toughness is poor; when the content of Mn element in the steel is higher than 2.05%, Mn segregation will be caused, in addition, Mn will increase the number and size of martensite-austenite islands, resulting in poor toughness of the steel. Therefore, considering the influence of the content of Mn element on the performance of the steel, the mass percentage of Mn element in the bainite seamless steel pipe described in the application is controlled to be between 1.8% and 2.05%.

[0017] Cr: In the bainite seamless steel pipe described in the application, Cr is an important element for improving controlled-cooling hardenability, which can ensure the formation of stable controlled-cooling bainite structure under the condition of 2-5 ℃ / s in combination with Mn and B elements, and has a certain solid solution strengthening effect, which can improve the strength of the steel and the corrosion resistance of the material, thereby improving the working conditions of the steel pipe. However, it should be noted that the addition amount of Cr element in the steel should be appropriate, and the content of Cr in the steel should not be too low to ensure the formation of stable bainite structure, and the content of Cr in the steel should not be too high to cause waste of alloy. Therefore, in the bainite seamless steel pipe described in the application, the mass percentage of Cr element is controlled to be between 0.85% and 1.25%.

[0018] Al: In the bainite seamless steel pipe described in the application, Al is a good deoxidizing element, which can play a deoxidizing role. However, Al element is also not easy to add too much, and too much Al can easily cause alumina inclusions, so it is necessary to try to improve the proportion of acid-soluble aluminum in the total aluminum, and then feed Al wire after vacuum degassing. Based on this, in order to play the beneficial effect of Al element, the mass percentage of Al element in the application is controlled to be between 0.015% and 0.04%.

[0019] B: In the bainite seamless steel pipe described in the application, B element can increase the hardenability of the steel, and the combined addition of B and Mn elements can further improve the air-cooling hardenability of the steel and ensure the formation of stable granular bainite structure. At the same time, B element can also strengthen the grain boundary and inhibit the formation of martensite-austenite islands, thereby improving the strength and toughness matching of the steel. Of course, the addition amount of B element should not be too low, and when the content of B element in the steel is less than 0.0015%, the effect is not obvious, and when the content of B in the steel is too high, the steelmaking is difficult to accurately control. Therefore, in order to play the beneficial effect of B element, the mass percentage of B element in the bainite seamless steel pipe described in the application is controlled to be between 0.001% and 0.005%.

[0020] N: In the bainite seamless steel tube described in the application, the element N can form nitrides with Al and has a certain strengthening effect. Therefore, in order to play the beneficial effect of the element N and avoid its harmful effect, the mass percentage of the element N in the bainite seamless steel tube described in the application is controlled to satisfy: 0 < N ≤ 0.006%.

[0021] Correspondingly, in the application, while controlling the mass percentage of a single chemical element, the mass percentages of the elements Al and N also need to be further controlled to satisfy: Al / N ≥ 3. The purpose of controlling the mass percentages of the elements Al and N to satisfy this relationship is to ensure the acid-soluble aluminum content, which can sufficiently combine with the element N, thereby preventing N from combining with B to form a brittle low-melting phase, to ensure the effect of the element B on the hardenability of the steel, and to prevent grain boundary embrittlement.

[0022] In addition, it needs to be noted that, in the application, the element Ti is not added to the steel, and Ti is not used to remove N, thereby avoiding the problem of the formation of coarse carbides by the element Ti and the combination of the coarse carbides with inclusions, which leads to poor impact toughness.

[0023] Further, in the bainite seamless steel tube described in the application, among the unavoidable impurities, S ≤ 0.01% and P ≤ 0.005%.

[0024] In the bainite seamless steel tube described in the application, the elements P and S are impurity elements in the seamless steel tube, and in the case where the technical conditions permit, in order to obtain a seamless steel tube with better performance and higher quality, the content of the impurity elements in the seamless steel tube should be as low as possible.

[0025] In the application, when the content of the impurity element P is too high, P will segregate at the grain boundaries and embrittle the grain boundaries, thereby seriously deteriorating the toughness of the steel. When the content of the impurity element S is too high, the content of inclusions in the steel will increase, which is not conducive to the low-temperature toughness of the steel. Therefore, in the application, the contents of the elements P and S in the steel must be strictly controlled, the mass percentage of the element S is controlled to satisfy: S ≤ 0.01%, and the mass percentage of the element P is controlled to satisfy: P ≤ 0.005%.

[0026] Further, in the bainite seamless steel tube described in the application, the as-rolled performance satisfies: the yield strength is 552-758 MPa, the tensile strength is ≥980 MPa, the longitudinal impact toughness is ≥40 J, the yield strength ratio is 0.6-0.75, the elongation is 12-15%, the local residual stress of the outer wall of the bainite seamless steel tube in the thickness direction is ≤200 MPa in the range of 1-2 mm, and the overall residual stress of the tube body is ≤100 MPa.

[0027] Further, in the bainite seamless steel pipe according to the present application, when tempered at a temperature of 200-350℃, the yield strength is 650-860MPa, the tensile strength is ≥980MPa, the longitudinal impact toughness is ≥60J, and the yield strength ratio is 0.7-0.83.

[0028] Further, in the bainite seamless steel pipe according to the present application, when tempered at a temperature of 400-460℃, the yield strength is 650-860MPa, and the tensile strength is ≥980MPa.

[0029] Correspondingly, another object of the present application is to provide the manufacturing method of the above-mentioned bainite seamless steel pipe, which does not use the quenching and tempering heat treatment process, has a short process flow and low energy consumption, and avoids a series of problems such as size deterioration and surface oxidation caused by quenching and tempering. The bainite seamless steel pipe product obtained by using the manufacturing method can obtain good strength and toughness matching in the rolled state or single tempering state, and has good application prospect.

[0030] In order to achieve the above-mentioned objects, the present application provides the manufacturing method of the above-mentioned bainite seamless steel pipe, which comprises the following steps:

[0031] (1) smelting and continuous casting of a pipe blank;

[0032] (2) piercing and continuous rolling and sizing;

[0033] (3) three-stage air cooling of the sized pipe: in the range of 500-850℃, the cooling rate is controlled in the range of 2-5℃ / s; after cooling to 500℃, the cooling rate is controlled in the range of 5-15℃ / s; after cooling to 300℃, natural air cooling is performed.

[0034] In the manufacturing method of the bainite seamless steel pipe according to the present application, the inventor performs three-stage cooling control on the sized pipe, and optimizes the three-stage air cooling process. By using the three-stage air cooling process, the problem that the transformation of the outer wall surface and the center of the bainite seamless steel pipe and the inner wall are not synchronized can be effectively prevented, and the problem of longitudinal cracking of the outer surface caused by excessive local residual stress in the thickness direction of the pipe body wall in the range of 1-2mm can be avoided.

[0035] In addition, the three-stage cooling control process can also control the transformation of the structure in the stable bainite phase transformation range, so as to ensure the overall structure and strength and toughness matching of the bainite seamless steel pipe, and prevent the problem of large macro residual stress of the pipe body, and ensure the performance of the bainite seamless steel pipe in subsequent use.

[0036] In the step (3) of the application, the cooling rate is controlled in the range of 2-5℃ / s in the first air cooling process in the range of 500-850℃. This is because: in this cooling rate range, the ferrite phase transition is ensured not to occur in advance, and the stable bainite phase transition of the supercooled austenite below 500℃ is ensured; at the same time, this cooling rate control can prevent the difference of the cooling rate in the wall thickness direction of 1-2mm of the outer wall of the bainite seamless steel pipe from the other positions of the pipe body, thereby avoiding the problem of asynchronous phase transition and preventing the excessive residual stress of the outer wall of the bainite seamless steel pipe in the wall thickness direction of 1-2mm.

[0037] When the residual stress of the outer wall of the bainite seamless steel pipe in the wall thickness direction of 1-2mm is too large, the defect tolerance of the outer surface of the steel pipe is poor, and the outer wall of the pipe body in the wall thickness direction of 1-2mm will crack during use. Therefore, based on the design of the application, the local residual stress of the outer wall of the steel pipe in the wall thickness direction of 1-2mm can be reasonably controlled and controlled to be ≤200MPa.

[0038] Correspondingly, in the second air cooling process, the cooling rate is controlled in the range of 5-15℃ / s after cooling to 500℃. This is because: this cooling rate can improve the stability of the supercooled austenite, ensure the microstructure transition of the bainite at a lower temperature, form stable fine bainite structure, reduce the size of the M-A island, and thus ensure the strength and toughness matching. At the same time, by controlling in this cooling rate, the mutual offset of thermal stress and phase transition stress during the cooling and phase transition process can be effectively controlled, thereby reducing the final residual stress and improving the deformation resistance of the product.

[0039] And in the third cooling process, after cooling to 300℃, natural air cooling is carried out, because: after cooling to 300℃, the bainite phase transition has ended, and self-tempering can be carried out by using the residual heat to further improve the strength and toughness, and at the same time reduce the overall residual stress level of the bainite seamless steel pipe.

[0040] In summary, in the bainite seamless steel pipe product prepared by the application, through the control of the three-stage air cooling process, the local residual stress of the outer wall of the bainite seamless steel pipe in the wall thickness direction of 1-2mm is ≤200MPa, and the overall residual stress of the pipe body is ≤100MPa, which not only improves the defect tolerance of the outer surface of the steel pipe, but also improves the resistance to crack propagation after surface scratching during subsequent use of the pipe body, and at the same time improves the overall performance and service life of the pipe body during use.

[0041] Further, in the manufacturing method of the application, in step (1), the charging scheme of scrap steel + blast furnace molten iron is adopted during smelting, and the mass percentage of the blast furnace molten iron is 50-60%.

[0042] In the manufacturing method, in the smelting process of step (1), the proportion of the blast furnace molten iron is controlled to be 50-60%, the molten steel is smelted by the electric furnace, and after the external refining, vacuum degassing and argon stirring, the molten steel is treated by Ca to change the inclusions and reduce the contents of O and H. In the pipe blank continuous casting process of step (1), the alloy is cast into round blanks, the overheat degree of the molten steel is controlled to be lower than 30℃, and the continuous casting speed is 1.8-2.2 m / min, so that the composition segregation can be reduced.

[0043] Further, in the manufacturing method, in step (1), the overheat degree of the molten steel is controlled to be lower than 30℃, and the continuous casting speed is 1.8-2.2 m / min.

[0044] Further, in the manufacturing method, in step (2), the round blank is cooled and heated in a ring heating furnace at 1240-1300℃ for 3-6 hours.

[0045] Further, in the manufacturing method, in step (2), after heating, the piercing is performed at a piercing temperature of 1180-1240℃, and then the continuous rolling is performed at a continuous rolling temperature of 1000-1100℃.

[0046] Further, in the manufacturing method, in step (2), after the continuous rolling, the pipe body is naturally cooled, then heated to 950-980℃ in a reheating furnace, and then discharged to be sized at a sizing temperature of 850-950℃.

[0047] Further, in the manufacturing method, step (4) of tempering is further included, wherein the tempering temperature is 200-350℃ or 400-460℃.

[0048] In some preferred embodiments, based on the rolled bainite seamless steel pipe prepared according to steps (1)-(3), the tempering treatment can be further performed, and the bainite seamless steel pipe obtained after the tempering treatment also has good strength and toughness matching.

[0049] Compared with the prior art, the bainite seamless steel pipe and the manufacturing method thereof have the following advantages and beneficial effects:

[0050] The bainite seamless steel pipe product is developed, and through reasonable component matching and process design, the heat treatment can be avoided, and good strength and toughness matching can be obtained in the rolled state or the single tempering state. The process flow is short, the energy consumption is low, and the series of problems such as size deterioration and surface oxidation caused by the quenching and tempering are avoided.

[0051] In the chemical composition design, the inventors specifically adopt the air-cooling high hardenability composition design, which can obtain stable granular bainite structure in a wide range of cooling rate, which is beneficial to the stability of the microstructure and performance of the thick-walled steel pipe. In addition, in the chemical element composition design, the seamless steel pipe of the present application does not contain noble metals and micro-alloying elements such as Mo, Ni, Nb, V, Ti, etc., but adopts a reasonable design of conventional Mn and Cr, B and other elements, which has low alloy cost and good economy.

[0052] Correspondingly, in the manufacturing process, the present application also designs the process control of different stages of air cooling after rolling. After sizing, the pipe is controlled by three-stage air cooling. This three-stage air cooling process effectively prevents the problem of out-of-sync transformation of the outer wall surface and the center of the steel pipe and the inner wall, and avoids the problem of longitudinal cracking of the outer surface caused by excessive local residual stress in the wall thickness direction of the pipe body within 1-2mm. In addition, this three-stage cooling control process can control the microstructure transformation in the stable bainite transformation interval to ensure the matching of the overall microstructure and toughness of the bainite seamless steel pipe, and prevent the problem of large macro residual stress of the pipe body, thereby ensuring the subsequent use performance.

[0053] The bainite seamless steel pipe product developed by the present application has good strength and toughness matching in the as-rolled and single tempering states. In the as-rolled state, the yield strength is 552-758MPa, the tensile strength is ≥980MPa, the longitudinal impact toughness is ≥40J, the yield strength ratio is 0.6-0.75, the local residual stress of the outer wall of the bainite seamless steel pipe in the wall thickness direction within 1-2mm is ≤200MPa, and the overall residual stress of the pipe body is ≤100MPa. After single tempering treatment at a temperature of 200-350℃, the yield strength of the as-rolled bainite seamless steel pipe is increased to 650-860MPa, the tensile strength is ≥980MPa, the longitudinal impact toughness is ≥60J, and the yield strength ratio reaches 0.7-0.83, which has the best strength and toughness matching.

[0054] In addition, the strength of the as-rolled bainite seamless steel pipe product can also meet the requirements after single tempering treatment at a temperature of 400-460℃, but there is obvious temper brittleness, and the longitudinal impact toughness is ≤20J. DETAILED DESCRIPTION

[0055] The bainite seamless steel pipe and the manufacturing method thereof according to the present application will be further explained and described below in combination with specific examples, but the explanation and description do not constitute undue limitation on the technical solutions of the present application.

[0056] Examples 1-6 and Comparative Examples 1-9

[0057] The bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9 according to the present application are prepared by the following steps:

[0058] (1) Smelting and pipe blank continuous casting are performed according to the chemical composition shown in Table 1: a charging scheme of scrap steel + blast furnace molten iron is adopted, and the mass percentage of blast furnace molten iron is controlled to be 50-60%, the molten steel is smelted by an electric furnace, after external refining, vacuum degassing and argon stirring, the inclusions are modified by Ca treatment to reduce the contents of O and H. Then in the pipe blank continuous casting process, the superheat of the molten steel is controlled to be lower than 30℃, and the continuous casting speed is controlled to be 1.8-2.2 m / min.

[0059] (2) Piercing and continuous rolling and sizing: the obtained continuous casting round billet is cooled and input into a ring heating furnace for heating, the continuous casting round billet is controlled to be soaked at 1240-1300℃ in the ring furnace, and the heating time is 3-6 hours. After heating, piercing is performed, the piercing temperature is 1180-1240℃, after piercing, continuous rolling is performed, the continuous rolling temperature is 1000-1100℃; after continuous rolling, the pipe body is naturally cooled for 30S, and then input into a reheating furnace for heating to 950-980℃, and then output to perform sizing, and the sizing temperature is controlled to be 850-950℃.

[0060] (3) The pipe after sizing is subjected to three-stage air cooling: in the range of 500-850℃, the cooling rate is controlled to be in the range of 2-5℃ / s; after cooling to 500℃, the cooling rate is controlled to be 5-15℃ / s; after cooling to 300℃, natural air cooling is performed.

[0061] It should be noted that the chemical element composition and the related process design of the bainite seamless steel pipes of Examples 1-6 meet the design specification requirements of the present application. Although the seamless steel pipes of Comparative Examples 1-9 are also prepared by using the above process steps, the chemical element composition and / or the related process parameters do not meet the design parameters of the present application.

[0062] Table 1 lists the mass percentages of various chemical elements in the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9.

[0063] Table 1. (the balance is Fe and other unavoidable impurities except P and S)

[0064]

[0065]

[0066] Table 2-1 and Table 2-2 list the specific process parameters of the manufacturing method of the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9 in the above process steps.

[0067] Table 2-1.

[0068]

[0069] Table 2-2.

[0070]

[0071]

[0072] From the above Table 1, Table 2-1 and Table 2-2, it can be seen that, in the present application, the content of element C of Comparative Example 1 is lower than the design requirement; the content of element Mn of Comparative Example 2 is lower than the design requirement; the content of element Cr of Comparative Example 3 is lower than the design requirement; the content of element C of Comparative Example 4 is higher than the design requirement; the ratio of Al / N of Comparative Example 5 is less than 3; the first-stage cooling rate of the seamless steel pipe of Comparative Example 6 after sizing is lower than the design requirement; the first-stage cooling rate of the seamless steel pipe of Comparative Example 7 after sizing is higher than the design requirement; the second-stage cooling rate of the seamless steel pipe of Comparative Example 8 after sizing is lower than the design requirement; and the second-stage cooling rate of the seamless steel pipe of Comparative Example 9 after sizing is higher than the design requirement.

[0073] The obtained finished products of the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9 were sampled respectively, and various performance tests were carried out, and the obtained test results are listed in Table 3.

[0074] The relevant performance detection means are described as follows:

[0075] (1) Tensile test: according to GB / T 228.1 metal material tensile test, the yield strength, tensile strength, yield strength ratio and elongation value of the seamless steel pipes of each example and comparative example at room temperature were obtained. Among them, the yield strength ratio = yield strength / tensile strength.

[0076] (2) Impact test: according to GB / T 229 metal material Charpy pendulum impact test, the longitudinal impact toughness of the seamless steel pipes of each example and comparative example at room temperature was obtained.

[0077] (3) Residual stress test: the residual stress was measured according to ISO / TR 10400 standard, so as to obtain the overall residual stress of the pipe body of the seamless steel pipes of each example and comparative example and the local residual stress of the outer wall of the seamless steel pipe in the range of 1-2mm in the thickness direction.

[0078] Table 3 lists the performance test results of the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9.

[0079] Table 3.

[0080]

[0081] As can be seen from Table 3, compared with the seamless steel pipes of Comparative Examples 1-9, the comprehensive performance of the bainite seamless steel pipes of Examples 1-6 of the present application is obviously better.

[0082] As can be seen from Table 3, after the steps (1)-(3) of the manufacturing method of the present application are completed, the rolled state bainite seamless steel pipes of Examples 1-6 have excellent mechanical properties, and the strength and toughness match well, the yield strength is between 560-745 MPa, the tensile strength is between 980-1030 MPa, the yield strength ratio is between 0.61-0.74, the elongation is between 12-15%, and the room temperature longitudinal impact toughness is between 65-95 J.

[0083] Correspondingly, the rolled state bainite seamless steel pipes of Examples 1-6 have excellent resistance to crack propagation after surface scratching during subsequent use, and the overall residual stress of the pipe body is between 20-90 MPa, and the local residual stress of the outer wall in the thickness direction is between 80-120 MPa within 1-2 mm.

[0084] After the analysis of the rolled state bainite seamless steel pipes of Examples 1-6 is completed, the bainite seamless steel pipes obtained by the present application also have good strength and toughness match after subsequent tempering treatment. In the present application, after the rolled state bainite seamless steel pipes are tempered at a temperature of 200-350°C, the yield strength is increased by 50-100 MPa, the tensile strength is slightly increased, the yield strength ratio is in the range of 0.73-0.83, and the longitudinal impact toughness is ≥60 J, at this time the strength and toughness match is best. Correspondingly, after the rolled state bainite seamless steel pipes are tempered at a temperature of 400-460°C, the strength can also meet the requirements, but there is obvious temper brittleness, and at this time the longitudinal impact toughness is ≤20 J.

[0085] In the present application, the rolled state bainite seamless steel pipe of Example 1 is taken as an example, and the bainite seamless steel pipe prepared by the above steps (1)-(3) is further subjected to step (4): tempering treatment.

[0086] The rolled state bainite seamless steel pipe of Example 1 is sampled and classified into 5 groups, and the 5 groups are respectively tempered at different tempering temperatures (200°C, 300°C, 350°C, 400°C, 460°C).

[0087] After the above tempering treatment is completed, the five groups of bainite seamless steel pipes subjected to tempering treatment are respectively sampled, and the five groups of samples are respectively subjected to tensile test and impact test to obtain the yield strength, tensile strength, yield strength ratio, longitudinal impact toughness and elongation of the bainite seamless steel pipes of the five groups in the tempered state, and the test results are listed in Table 4 below. The experimental methods of the related tensile test and impact test are the same as those described above in Table 3, which will not be described here.

[0088] Table 4 lists the mechanical properties of the rolled state bainite seamless steel pipe of Example 1 and the tempered state bainite seamless steel pipe after tempering treatment at five different tempering temperatures.

[0089] Table 4.

[0090]

[0091] As shown in Table 4, taking the rolled state bainite seamless steel pipe of Example 1 as an example, after tempering treatment at a temperature of 200-350℃, the yield strength is greatly improved, and the tensile strength is slightly improved. After tempering treatment at a temperature of 200-350℃, the tempered state bainite seamless steel pipe of Example 1 has a good strength and toughness match, with a yield strength of 825-845 MPa, a tensile strength of 1050-1153 MPa, a yield strength ratio of 0.73-0.79, an elongation of 13-14%, and a longitudinal impact toughness of 82-90 J, which has the best strength and toughness match.

[0092] Taking the rolled state bainite seamless steel pipe of Example 1 as an example, after tempering treatment at a temperature of 400-460℃, the strength can also meet the requirements, but there is obvious temper brittleness, and the longitudinal impact toughness is 15-19 J.

[0093] In summary, in the present application, the bainite seamless steel pipe designed by the inventor can obtain a good strength and toughness match without quenching and tempering heat treatment and in the rolled state or single tempering state, the process flow is short, the energy consumption is low, and the series of problems such as size deterioration and surface oxidation caused by quenching and tempering can be effectively avoided.

[0094] It should be noted that the combination of the technical features in the present application is not limited to the combination mode described in the claims or the combination mode described in the specific embodiments, and all the technical features described in the present application can be freely combined or combined in any mode, unless contradictory to each other.

[0095] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made on the basis of the disclosure of the present application are directly derived or easily conceived by those skilled in the art, and should all fall within the protection scope of the present application.

Claims

1. A bainite seamless steel pipe characterized by, The mass percentage of each chemical element is as follows: C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.8-2.05%, Cr: 0.85-1.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0 The bainite seamless steel pipe is prepared by the following steps: Smelting and continuous casting of pipe blank; Piercing and continuous rolling and sizing; (3) The pipe after sizing is subjected to three-stage air cooling: in the range of 500-850℃, the cooling rate is controlled in the range of 2-5℃ / s; in the range of 300℃ to below 500℃, the cooling rate is controlled in the range of 5-15℃ / s; in the range of cooling to below 300℃, natural air cooling is performed.

2. The bainite seamless steel tube according to claim 1, characterized in that, In the inevitable impurities, S≤0.01%, P≤0.005%.

3. The bainite seamless steel tube as recited in claim 1, characterized by The as-rolled properties meet: yield strength is 552-758MPa, tensile strength≥980MPa, longitudinal impact toughness≥40J, yield strength ratio is 0.6-0.75, elongation is 12-15%, local residual stress of the bainite seamless steel pipe outer wall in the range of 1-2mm in the wall thickness direction is≤200MPa, and the overall residual stress of the pipe body is≤100MPa.

4. The bainite seamless steel tube according to claim 3, characterized in that, When it is tempered at a temperature of 200-350℃, the yield strength is 650-860MPa, the tensile strength≥980MPa, the longitudinal impact toughness≥60J, and the yield strength ratio is 0.7-0.

83.

5. The bainite seamless steel tube as recited in claim 3, characterized by When it is tempered at a temperature of 400-460℃, the yield strength is 650-860MPa, and the tensile strength≥980MPa.

6. A method of producing a bainite seamless steel pipe as claimed in any one of claims 1 to 3, characterized by, The steps include: Smelting and continuous casting of pipe blank; Piercing and continuous rolling and sizing; The pipe after sizing is subjected to three-stage air cooling: in the range of 500-850℃, the cooling rate is controlled in the range of 2-5℃ / s; in the range of 300℃ to below 500℃, the cooling rate is controlled in the range of 5-15℃ / s; in the range of cooling to below 300℃, natural air cooling is performed.

7. The production method according to claim 6, wherein In step (1), during smelting, the charging scheme of scrap steel + blast furnace molten iron is adopted, wherein the mass percentage of blast furnace molten iron is 50-60%.

8. The production method according to claim 6, wherein In step (1), during continuous casting, the superheat of molten steel is controlled to be lower than 30℃, and the continuous casting speed is 1.8-2.2m / min.

9. The production method according to claim 6, wherein In step (2), after cooling the continuous casting round blank, heating is performed in a ring-shaped heating furnace at 1240-1300℃, and the heating time is 3-6 hours.

10. The production method according to claim 6, wherein In step (2), after heating, piercing is performed at a temperature of 1180-1240℃, and then continuous rolling is performed at a temperature of 1000℃-1100℃.

11. The production method according to claim 6, wherein In step (2), after continuous rolling, the pipe body is naturally cooled, and then reheated to 950-980℃ in a reheating furnace, and then taken out for sizing at a sizing temperature of 850-950℃.

12. The production method according to claim 6, wherein It also includes step (4): tempering, wherein the tempering temperature is 200-350℃ or 400-460℃.

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