Bainitic seamless steel pipe and its manufacturing method

Abstract

Bainitic seamless steel pipe and method for manufacturing the same. The bainitic seamless steel pipe further contains the following chemical elements in mass percentage in addition to Fe and inevitable impurities: 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 < N ≤ 0.006%, where Al / N ≥ 3. The bainitic seamless steel pipe can obtain a good strength-toughness balance in the rolled state or in a single tempered state without the need for quenching and tempering heat treatment.

Description

[Technical field]

[0001] Technical Field The present invention relates to a steel pipe and a manufacturing method thereof, and more particularly to a bainitic seamless steel pipe and a manufacturing method thereof. [Background technology]

[0002] Background technology In recent years, in order to meet the market demands, more and more seamless steel pipe products are applied in various trades and industries, and play a very important role.

[0003] In the prior art, quenching and tempering heat treatment of steel pipe is required to prepare seamless steel pipe products with 80 ksi yield strength and certain toughness.

[0004] However, this quenching and tempering heat treatment process not only consumes high energy, but also has a series of problems such as severe surface oxidation, and the steel pipe is prone to dimensional deterioration, cracks, and straightness deterioration during the quenching and tempering process, which does not conform to the current trend toward high-quality and environmentally friendly development.

[0005] Therefore, in order to solve the problem in the prior art that seamless steel pipe having 80 Ksi steel grade is required to be subjected to quenching and tempering heat treatment to obtain strength-toughness matching, the present invention is expected to develop and obtain a new bainite seamless steel pipe product. Summary of the Invention [Means for solving the problem]

[0006] Summary of the Invention One object of the present invention is to provide a bainitic seamless steel pipe. The bainitic seamless steel pipe can obtain a good strength-toughness balance in the rolled state or in a single annealing state by using a reasonable chemical composition and process design without the need for quenching and tempering heat treatment. The bainitic seamless steel pipe has a low alloy cost and good economic efficiency.

[0007] To achieve the above object, in one aspect, the present invention provides a bainitic seamless steel pipe further comprising the following chemical elements in mass percent in addition to Fe and inevitable impurities: 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 < N ≤ 0.006%, where Al / N ≥ 3.

[0008] Preferably, the present invention provides a bainitic seamless steel pipe comprising the following chemical elements in mass percent: 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 < N ≤ 0.006%, the balance being Fe and inevitable impurities, where Al / N ≥ 3.

[0009] In the chemical composition design, the bainitic seamless steel pipe according to the present invention uses an air-cooled high hardenability composition design capable of obtaining a stable granular bainite structure at a wide range of cooling rates, which brings about the stability of the structure and properties of the thick-walled steel pipe.

[0010] Furthermore, the bainitic seamless steel pipe according to the present invention does not contain precious metals and microalloying elements such as Mo, Ni, Nb, V, Ti, etc., and uses ordinary elements such as Mn, Cr, and B, which has good economic efficiency and a low alloy cost.

[0011] In the bainite seamless steel pipe according to the present invention, the design principle of each chemical element is described as follows:

[0012] C: Element C is an important element to ensure the strength of steel. The addition of element C can stabilize the bainite structure and improve the air-cooled hardenability. In addition, C also causes the continuous cooling transformation (CCT) curve to shift to the right. By adding an appropriate amount of element C in steel, it can be ensured that the steel can obtain the bainite structure at a low cooling rate. If the content of element C in steel is too low, the bainite structure may be unstable, and the strength and toughness of the steel may deteriorate; if the content of element C in steel is too high, the amount of martensite-austenite islands may increase, and the toughness of the steel may deteriorate. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percentage of element C is controlled to be 0.16-0.18%.

[0013] Si: element Si is both a ferrite-forming element and a deoxidizing element. Element Si can improve the purity of molten steel while inhibiting the precipitation of carbides. Adding an appropriate amount of element Si in steel can not only ensure the solid solution strengthening of element C, but also reduce the amount of martensite-austenite islands and refine the structure of martensite-austenite islands, so that the strength-toughness balance of steel is improved. If the content of Si in steel is too low, it has no corresponding effect; if the content of Si in steel is more than 1%, there is no improvement in structure. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percentage of element Si is controlled to be 0.56-1%, preferably 0.66-0.85%, and even more preferably 0.75-0.85%.

[0014] Mn: Mn is an important element for improving air-cooled hardenability. Compared with elements such as Mo, Cr and W, Mn is cheaper and more readily available. Compared with other elements, Mn can lower the transformation point of bainite structure more significantly, so that the structure is effectively refined and the strength and toughness of the steel are improved. When the content of element Mn in the steel is lower than 1.8%, the upper bainite structure is formed due to the decrease of hardenability, which leads to poor toughness; when the content of element Mn in the steel is higher than 2.05%, severe segregation of Mn occurs and the amount and size of martensite-austenite islands increase, which results in the deterioration of the toughness of the steel. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percent content of element Mn is controlled to be 1.8-2.05%, preferably 1.85-2%.

[0015] Cr: Cr is an important element for improving the air-cooled hardenability. The combination of Cr with elements Mn and B can ensure the formation of a stable air-cooled bainite structure under the condition of 2-5°C / s. In addition, Cr has a certain solid solution strengthening effect. It can also improve the corrosion resistance of the material while increasing the strength of the steel, and therefore improve the applicable working conditions of the steel pipe. If the content of Cr in the steel is too low, the formation of a stable bainite structure cannot be guaranteed. If the content of Cr in the steel is too high, it will lead to alloy waste. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percentage of element Cr is controlled to be 0.85-1.25%, preferably 0.9-1.1%.

[0016] Al: Al is a good deoxidizing element, which can have the effect of deoxidization. However, excessive addition of Al tends to produce alumina inclusions. Therefore, it is necessary to increase the proportion of acid-soluble aluminum in the total aluminum as much as possible and provide an appropriate amount of Al wire after vacuum degassing. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percentage of element Al is controlled to be 0.015-0.04%.

[0017] B: Element B can increase the hardenability of steel. The combined addition of Element B and Mn can further improve the air-cooling hardenability of steel and ensure the formation of a stable granular bainite structure. At the same time, Element B can further strengthen the grain boundaries, inhibit the formation of martensite-austenite islands, and improve the strength-toughness harmony of the steel. When the content of Element B in the steel is less than 0.0015%, its effect is not obvious. When the content of Element B in the steel is too high, it is difficult to accurately control steelmaking. Therefore, in order to exert the beneficial effect of Element B, in the bainitic seamless steel pipe according to the present invention, the mass percentage of Element B is controlled to be 0.001 - 0.005%, preferably 0.002 - 0.0045%.

[0018] N: Element N can form carbonitrides with Al and play a role in strengthening to a certain extent. However, when the content of Element N is too high, it will lead to a decrease in impact toughness. Therefore, in the bainitic seamless steel pipe according to the present invention, the mass percentage of Element N is controlled such that 0 < N ≤ 0.006%, preferably 0.004% ≤ N ≤ 0.006%.

[0019] In the present invention, it is necessary to further control the mass percentages of Elements Al and N to satisfy the following formula: Al / N ≥ 3, preferably Al / N ≥ 3.33. The purpose of controlling the mass percentages of Elements Al and N to satisfy this relational expression is to ensure the content of acid-soluble aluminum and ensure that the acid-soluble aluminum completely combines with Element N, thereby preventing N from combining with B to form a brittle low-melting point phase. As a result, the improvement effect of Element B on the hardenability of the steel is ensured, and embrittlement of the grain boundaries is prevented.

[0020] In the present invention, Element Ti is not added to the steel, and Element Ti is not used for N removal, thereby avoiding the problem that Element Ti forms coarse carbides, which combine with inclusions to cause deterioration of impact toughness.

[0021] Preferably, in the bainitic seamless steel pipe according to the present invention, the unavoidable impurities include P and S; where S≦0.01%, P≦0.005%.

[0022] In the bainite seamless steel pipe according to the present invention, the elements P and S are both impurity elements in the seamless steel pipe. If technical conditions permit, the content of impurity elements in the seamless steel pipe should be reduced as much as possible in order to obtain seamless steel pipes with better properties and quality. If the content of the impurity element P is too high, P will cause grain boundary segregation and embrittlement, thereby significantly deteriorating the toughness of the steel. In addition, if the content of the impurity element S is too high, it will result in an increase in the content of inclusions in the steel, which is detrimental to the low-temperature toughness of the steel. Therefore, in the bainite seamless steel pipe according to the present invention, the mass percentage of the element S is controlled to be S≦0.01%, and the mass percentage of the element P is controlled to be P≦0.005%.

[0023] Preferably, the bainitic seamless steel pipe of the present invention has the following properties in the rolled state: yield strength of 552-758 MPa (preferably 650-758 MPa), tensile strength of ≧980 MPa (preferably ≧1010 MPa), longitudinal impact toughness of ≧40 J (preferably ≧60 J, more preferably ≧65 J), yield ratio of 0.6-0.75, elongation of 12-15%, local residual stress of ≦200 MPa (preferably ≦190 MPa, more preferably ≦120 MPa) within 1-2 mm through the wall thickness of its outer wall, and total residual stress of ≦100 MPa (preferably ≦90 MPa, more preferably ≦50 MPa) throughout the tube.

[0024] Preferably, the bainite seamless steel pipe of the present invention, after tempering at a temperature of 200-350°C, has the following properties in the tempered state: yield strength of 650-860 MPa, tensile strength of ≥ 980 MPa (preferably ≥ 1050 MPa), longitudinal impact toughness of ≥ 60 J (preferably ≥ 65 J), and yield ratio of 0.7-0.83.

[0025] Preferably, the bainite seamless steel pipe of the present invention, after tempering at a temperature of 400-460°C, has the following properties in the tempered state: yield strength of 650-860 MPa, tensile strength of ≥ 980 MPa.

[0026] In this specification, "performance in the rolled state" refers to the performance exhibited by the bainite seamless steel pipe of the present invention after three stages of air cooling without tempering; "performance in the tempered state" refers to the performance exhibited by the bainite seamless steel pipe of the present invention after further tempering after three stages of air cooling.

[0027] In another aspect, the present invention also provides a method for producing the above-mentioned bainitic seamless steel pipe, comprising the steps of: (1) smelting molten steel and continuously casting it to obtain a steel billet; (2) piercing, continuous rolling and sizing of the steel billet to obtain a steel pipe; (3) The steel pipe after sizing is air-cooled in three stages: in the first stage, the cooling is carried out in the range of 500-850°C at a controlled cooling rate of 2-5°C / s; after cooling to 500°C, the cooling rate in the second stage is controlled to be 5-15°C / s (preferably 8.5-15°C / s); after cooling to 300°C, natural air cooling is carried out to obtain a bainite seamless steel pipe.

[0028] The method of the present invention does not adopt the quenching and tempering heat treatment process. This method has a short process and low energy consumption, and avoids a series of problems such as size degradation and surface oxidation caused by quenching and tempering. The bainite seamless steel pipe products produced by this method can obtain good strength-toughness harmony in the rolled state or in the single tempered state, and have good application prospects. In the method of the present invention, the inventors carried out three-stage air cooling control for the sized steel pipe. Through the optimization design for the three-stage air cooling process, the problem that the transformation of the outer wall surface is not synchronized with the transformation of the center and inner wall of the bainite seamless steel pipe is effectively prevented, and thus the problem of longitudinal cracks on the outer surface caused by the local residual stress being too large within the range of 1-2 mm in the wall thickness direction of the outer wall of the tube body is avoided.

[0029] In addition, the three-stage controlled cooling process of the present invention can also control the structural transformation within the stable bainite phase transformation range, thereby ensuring the harmony of the overall structure and strength-toughness of the bainite seamless steel pipe while preventing the problem of large macroscopic residual stress of the tube body, which ensures the performance of the bainite seamless steel pipe during subsequent use.

[0030] In step (3) of the present invention, the cooling rate of the first stage is controlled to be 2-5°C / s. Within this cooling rate range, it can be ensured that the ferrite phase transformation does not occur first, and the undercooled austenite undergoes a stable bainite phase transformation below 500°C. At the same time, the use of this cooling rate can prevent the difference between the cooling rate within the range of 1-2 mm in the wall thickness direction of the outer wall of the bainite seamless steel tube and the cooling rate at other positions of the tube body from being too large, thereby avoiding the problem of asynchronous phase transformation and preventing the residual stress within the range of 1-2 mm in the wall thickness direction of the outer wall of the bainite seamless steel tube from being too large.

[0031] If the local residual stress is too large within 1-2 mm in the wall thickness direction of the outer wall of the bainite seamless steel pipe, the failure resistance of the outer surface of the steel pipe will be poor, and the outer wall of the tube will crack at a position within 1-2 mm in the wall thickness direction during use. Therefore, the process design of the present invention can reasonably control the local residual stress to be ≦200 MPa within 1-2 mm in the wall thickness direction of the outer wall of the steel pipe.

[0032] The cooling rate in the second stage is controlled to be 5-15°C / s. The use of cooling rate can improve the stability of undercooled austenite, ensure that bainite undergoes structural transformation at low temperature, form a stable fine bainite structure, reduce the size of martensite-austenite islands, and therefore ensure the strength-toughness balance. At the same time, the use of cooling rate can effectively control the mutual elimination 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 product.

[0033] In the third stage of cooling, after cooling to 300°C, natural air cooling is carried out, because after cooling to 300°C, the bainite phase transformation is completed, and self-tempering can be carried out with the residual heat, which further improves the strength and toughness while reducing the total residual stress level of the tube body of the bainite seamless steel pipe.

[0034] In the present invention, by controlling the three-stage air cooling process, it can be ensured that the local residual stress is ≦200MPa within 1-2 mm in the wall thickness direction of the outer wall of the bainite seamless steel tube, and the total residual stress is ≦100MPa throughout the tube body, which not only improves the failure resistance of the outer surface of the steel tube, but also improves the resistance of the tube body to crack propagation after surface scratches occur during subsequent use, and at the same time improves the overall performance and service life of the tube body during use.

[0035] Preferably, in step (1) of the method according to the invention, scrap steel and blast furnace molten iron are used to produce molten steel, where the blast furnace molten iron is used in a mass percentage of 50-60%.

[0036] Preferably, in step (1) of the method according to the present invention, the molten steel can be smelted by an electric furnace, and after secondary refining, vacuum degassing and argon stirring, Ca treatment is carried out to modify the inclusions, thereby reducing the contents of elements O and H.

[0037] Preferably, in step (1) of the method according to the present invention, in continuous casting, the alloy is cast into a round billet, the superheat of the molten steel is controlled to be lower than 30°C during continuous casting, and the drawing speed during continuous casting is controlled to be 1.8-2.2 m / min, which can reduce the segregation of the components.

[0038] Preferably, in the method according to the present invention, in step (2), the obtained round billet is cooled and then heated in a tube furnace, the heating temperature is 1240-1300° C., and the heating time is 3-6 h.

[0039] Preferably, in step (2) of the method according to the invention, the perforation is carried out after heating at a temperature of 1180-1240°C.

[0040] Preferably, in step (2) of the method according to the present invention, continuous rolling is carried out after piercing at a temperature of 1000-1100°C; after continuous rolling, the tube is naturally cooled and then heated to 950-980°C in a reheating furnace, and then removed for sizing at a temperature of 850-950°C.

[0041] Preferably, the method according to the present invention further comprises step (4): the bainitic seamless steel pipe obtained in step (3) is subjected to a tempering process. The bainitic seamless steel pipe obtained after the tempering treatment has a good strength-toughness balance.

[0042] Preferably, in step (4) of the method according to the invention, the tempering temperature is 200°C-350°C or 400°C-460°C.

[0043] Compared with the prior art, the bainite seamless steel pipe and its manufacturing method according to the present invention have the following advantages and beneficial effects:

[0044] The present invention provides a new bainite seamless steel pipe product, which can obtain good strength-toughness balance in rolled or single tempered state without the need for quenching and tempering heat treatment through reasonable chemical composition and process design. The product has a short process flow and low energy consumption, and avoids a series of problems such as size degradation and surface oxidation caused by quenching and tempering.

[0045] In designing the chemical composition, the inventors use an air-cooled high hardenability composition design that can obtain a stable granular bainite structure in a wide range of cooling rates, which leads to the stability of the structure and properties of the thick-walled steel pipe. In addition, in the design of the chemical element composition, the seamless steel pipe according to the present invention does not contain precious metals and micro-alloying elements such as Mo, Ni, Nb, V, Ti, and adopts the rational design of ordinary elements such as Mn, Cr, and B, which has good economic efficiency and low alloying cost.

[0046] The present invention also designs parameter control of different air-cooling stages after rolling in the manufacturing process.Three-stage air-cooling control of the steel pipe after sizing effectively prevents the problem that the transformation of the outer wall surface is not synchronized with the transformation of the center and inner wall of the steel pipe, and thus avoids the problem of longitudinal cracks on the outer surface caused by the local residual stress being too large within the range of 1-2 mm in the wall thickness direction of the outer wall of the tube body.In addition, this three-stage cooling control process can also control the structural transformation within the stable bainite phase transformation range, thereby ensuring the harmony of the overall structure and strength-toughness of the bainite seamless steel pipe while preventing the problem of the macroscopic residual stress of the tube body being large, which ensures the performance during subsequent use.

[0047] The bainite seamless steel pipe product provided by the present invention has good strength-toughness balance in the rolled state and in the single tempered state. In the rolled state, the yield strength is 552-758MPa, the tensile strength is ≧980MPa, the longitudinal impact toughness is ≧40J, the yield ratio is 0.6-0.75, the local residual stress is ≦200MPa within 1-2 mm in the wall thickness direction of the outer wall of the bainite seamless steel pipe, and the total residual stress is ≦100MPa throughout the tube body. After the bainite seamless steel pipe in the rolled state is subjected to a single tempering treatment at a temperature of 200-350°C, the yield strength increases to 650-860MPa, the tensile strength is ≧980MPa, the longitudinal impact toughness is ≧60J, and the yield ratio is 0.7-0.83. In this case, the bainite seamless steel pipe has the best strength-toughness balance.

[0048] In addition, after the rolled bainite seamless steel pipe is subjected to a single tempering treatment at a temperature of 400-460℃, the strength of the bainite seamless steel pipe can meet the requirements, but the bainite seamless steel pipe has obvious temper brittleness. In this case, the longitudinal impact toughness is ≦20J. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Detailed Description Hereinafter, the bainitic seamless steel pipe and its manufacturing method according to the present invention will be further described and illustrated below with reference to specific examples, but the description and illustration will not unduly limit the technical solutions of the present invention.

[0050] Examples 1-6 and Comparative Examples 1-9 The bainite seamless steel pipes of Examples 1-6 of the present invention and the seamless steel pipes of Comparative Examples 1-9 are manufactured by the following steps: (1) According to the chemical composition shown in Table 1, smelting and continuous casting: scrap steel and blast furnace molten iron are used to produce molten steel, where blast furnace molten iron is used in mass percentage of 50-60%%; the molten steel is smelted in an electric furnace, and after secondary refining, vacuum degassing and argon stirring, Ca treatment is carried out to modify inclusions, thereby reducing the content of elements O and H. Then in continuous casting, the superheat of the molten steel is controlled to be lower than 30°C, and the drawing speed during continuous casting is controlled to be 1.8-2.2 m / min. (2) Piercing, continuous rolling and sizing: After cooling the obtained steel billet, the steel billet is heated in a ring heating furnace, the temperature in the ring heating furnace is controlled to be 1240-1300℃, and the heating time is controlled to be 3-6h. After heating, piercing is carried out at a temperature of 1180-1240℃. After piercing, continuous rolling is carried out at a temperature of 1000-1100℃. After continuous rolling, the tube is naturally cooled for 30 seconds, then placed in a reheating furnace to be heated to 950-980℃, and then taken out of the furnace for sizing at a temperature of 850-950℃. (3) The steel pipe after sizing is air-cooled in three stages: in the first stage, cooling is performed in the range of 500-850°C with a controlled cooling rate of 2-5°C / s; after cooling to 500°C, in the second stage, the cooling rate is controlled to be 5-15°C / s; after cooling to 300°C, natural air cooling is performed in the third stage.

[0051] The bainite seamless steel pipes of Examples 1-6 of the present invention are all manufactured by the method including the above-mentioned steps, and their chemical element compositions and related process designs meet the design specification requirements of the present invention. The seamless steel pipes of Comparative Examples 1-9 are manufactured by basically the same method as that of Examples 1-6 of the present invention, except that at least one of the chemical compositions and / or process parameters of the seamless steel pipes of Comparative Examples 1-9 do not meet the requirements of the present invention.

[0052] Table 1 shows the mass percentages of each chemical element in the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9.

[0053] [Table 1]

[0054] The specific process parameters in the above process steps of the method for producing the bainitic seamless steel pipes of Examples 1-6 and Comparative Examples 1-9 are shown in Tables 2-1 and 2-2.

[0055] [Table 2-1]

[0056] [Table 2-2]

[0057] From the above Tables 1, 2-1 and 2-2, it is not difficult to understand that in the present invention, the content of element C in Comparative Example 1 is lower than the design requirement; the content of element Mn in Comparative Example 2 is lower than the design requirement; the content of element Cr in Comparative Example 3 is lower than the design requirement; the content of element C in Comparative Example 4 is higher than the design requirement; the Al / N ratio in Comparative Example 5 is less than 3; the cooling rate in the first stage after sizing of the seamless steel pipe in Comparative Example 6 is lower than the design requirement; the cooling rate in the first stage after sizing of the seamless steel pipe in Comparative Example 7 is higher than the design requirement; the cooling rate in the second stage after sizing of the seamless steel pipe in Comparative Example 8 is lower than the design requirement; and the cooling rate in the second stage after sizing of the seamless steel pipe in Comparative Example 9 is higher than the design requirement.

[0058] The rolled bainite seamless steel pipes of Examples 1-6 and Comparative Examples 1-9 were each taken as samples and subjected to various performance tests. The test results are shown in Table 3.

[0059] The relevant performance test methods are listed as follows: (1) Tensile test: According to GB / T 228.1 - Metallic materials - Tensile test, the yield strength, tensile strength, yield ratio, and elongation of the bainite seamless steel pipe of each example and the seamless steel pipe of each comparative example at room temperature are measured, where yield ratio = yield strength / tensile strength. (2) Impact test: According to GB / T 229 - Metallic materials - Charpy pendulum impact test, the longitudinal impact toughness at room temperature of the bainite seamless steel pipe of each embodiment and the seamless steel pipe of each comparative example is measured. (3) Residual stress test: According to the standard ISO / TR 10400, the residual stress of the bainite seamless steel pipe of each embodiment and the seamless steel pipe of each comparative example is measured to obtain the total residual stress throughout the pipe body and the local residual stress within a range of 1-2 mm in the wall thickness direction of the outer wall of the seamless steel pipe.

[0060] Table 3 shows the performance test results of the bainite seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-9.

[0061] [Table 3]

[0062] It can be seen from Table 3 that the comprehensive performance of the bainitic seamless steel pipe in Example 1-6 of the present invention is significantly better than that of the seamless steel pipe in Comparative Example 1-9.

[0063] From Table 3, it can be seen that after completing steps (1)-(3) of the manufacturing method of the present invention, the as-rolled bainitic seamless steel pipes of Examples 1-6 obtained have the following excellent mechanical properties: the strength-toughness balance is good, the yield strength is 560-745 MPa, the tensile strength is 980-1030 MPa, the yield ratio is 0.61-0.74, the elongation is 12-15%, and the longitudinal impact toughness at room temperature is 65-95 J.

[0064] From Table 3, it can be seen that the as-rolled bainitic seamless steel pipes of Examples 1-6 have the following performance: the total residual stress is 20-90 MPa throughout the tube, and the local residual stress within 1-2 mm in the wall thickness direction of the outer wall is 80-120 MPa. Accordingly, during subsequent use, the as-rolled bainitic seamless steel pipes of Examples 1-6 have excellent resistance to crack propagation after the surface is scratched.

[0065] After the above performance tests and analyses are completed, the as-rolled bainite seamless steel pipes of Examples 1-6 are tempered. The obtained tempered bainite seamless steel pipes also have good strength-toughness harmony. In the present invention, after the as-rolled bainite seamless steel pipes are tempered at a temperature of 200-350 ° C, the yield strength increases by 50-100 MPa, the tensile strength increases slightly, the yield ratio reaches 0.73-0.83, the longitudinal impact toughness is ≧ 60J, and it has the best strength-toughness harmony. However, after the as-rolled bainite seamless steel pipes are tempered at a temperature of 400-460 ° C, although the strength can meet the requirements, the bainite seamless steel pipes have obvious temper brittleness, and the longitudinal impact toughness is ≦ 20J.

[0066] Taking the rolled bainitic seamless steel pipe of Example 1 as an example, the bainitic seamless steel pipe prepared through the above steps (1)-(3) is further subjected to step (4): tempering treatment.

[0067] The as-rolled bainitic seamless steel pipes of Example 1 are sampled and divided into five groups, which are tempered at different tempering temperatures (200°C, 300°C, 350°C, 400°C, 460°C).

[0068] Five groups of bainite seamless steel pipes that have completed the tempering treatment are taken as samples respectively, and tensile tests and impact tests are carried out on the samples of the five groups respectively to obtain the yield strength, tensile strength, yield ratio, longitudinal impact toughness and elongation of the tempered bainite seamless steel pipes of each group. The test results are shown in Table 4 below. The relevant test methods of the tensile test and impact test are the same as those in Table 3 above.

[0069] The mechanical properties of the as-rolled bainitic seamless steel pipe of Example 1 and the as-tempered bainitic seamless steel pipe of Example 1 after subjecting the as-rolled bainitic seamless steel pipe of Example 1 to tempering treatment at five different tempering temperatures are shown in Table 4.

[0070] [Table 4]

[0071] As shown in Table 4, taking the rolled bainitic seamless steel pipe of Example 1 as an example, after the bainitic seamless steel pipe is tempered at a temperature of 200-350°C, the yield strength is greatly improved, the tensile strength is slightly improved, and the bainitic seamless steel pipe has good strength-toughness consistency. Specifically, the yield strength is 825-845MPa, the tensile strength is 1050-1153MPa, the yield ratio reaches 0.73-0.79, the elongation is 13-14%, the longitudinal impact toughness is 82-90J, and the strength-toughness consistency is the best.

[0072] Taking the rolled bainite seamless steel pipe of Example 1 as an example, after the bainite seamless steel pipe is tempered at a temperature of 400-460°C, the strength of the bainite seamless steel pipe can meet the requirements, but the bainite seamless steel pipe has obvious temper brittleness, and the longitudinal impact toughness is only 15-19J.

[0073] In summary, the bainite seamless steel pipe of the present invention can obtain a good strength-toughness balance in the rolled state or in the single tempered state without the need for quenching and tempering heat treatment through reasonable composition matching and process design. The bainite seamless steel pipe has a short process flow and low energy consumption, and can effectively avoid a series of problems such as size degradation and surface oxidation caused by quenching and tempering.

[0074] It should be noted that the combinations of various technical features in this application are not limited to the combinations described in the claims or the combinations described in the specific embodiments of this application. All technical features described in this application can be freely combined in any way as long as there is no contradiction between them.

[0075] It should be noted that the above examples are merely specific implementations of the present invention. Obviously, the present invention is not limited to the above examples, and those skilled in the art can easily conceive of or directly obtain similar modifications and variations made accordingly from the content of the present invention, all of which should be included in the protection scope of the present invention.

Claims

1. A bainitic seamless steel pipe, which in addition to Fe and unavoidable impurities further contains the following chemical elements in mass percent: 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<N≦0.006%, where Al / N≧3.

2. 2. A bainitic seamless steel pipe according to claim 1, comprising the following chemical elements in mass percent: 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<N≦0.006%, the balance being Fe and unavoidable impurities, where Al / N≧3.

3. 3. The bainitic seamless steel pipe according to claim 1 or 2, wherein the unavoidable impurities include S and P; where S≦0.01% and P≦0.005%.

4. 3. A bainitic seamless steel tube as claimed in claim 1 or 2, having the following properties in the rolled state: yield strength of 552-758 MPa, tensile strength of ≥ 980 MPa, longitudinal impact toughness of ≥ 40 J, yield ratio of 0.6-0.75, elongation of 12-15%, local residual stress of ≤ 200 MPa within 1-2 mm through the wall thickness of its outer wall, and total residual stress of ≤ 100 MPa throughout the tube.

5. The bainite seamless steel pipe according to claim 1 or 2, after tempering at a temperature of 200-350°C, has the following properties in the tempered state: yield strength of 650-860 MPa, tensile strength of ≥ 980 MPa, longitudinal impact toughness of ≥ 60 J, and yield ratio of 0.7-0.

83.

6. 3. The bainite seamless steel pipe according to claim 1 or 2, which, after tempering at a temperature of 400-460°C, has the following properties in the tempered state: yield strength of 650-860 MPa, tensile strength of ≥ 980 MPa.

7. A bainitic seamless steel pipe according to claim 1 or 2, which does not contain Mo, Ni, Nb, V and / or Ti; and / or satisfies one or more of the following: the Si content is in the range of 0.66-0.85%, preferably 0.75-0.85%; the Mn content is in the range of 1.85-2%; the Cr content is in the range of 0.9-1.1%; the B content is in the range of 0.002-0.0045%; the N content is in the range of 0.004-0.006%.

8. 3. A bainite seamless steel pipe according to claim 1 or 2, having a bainite structure.

9. A method for producing a bainitic seamless steel pipe according to any one of claims 1 to 8, comprising the steps of: (1) smelting molten steel and continuously casting it to obtain a steel billet; (2) piercing, continuous rolling and sizing of the steel billet to obtain a steel pipe; (3) The steel pipe after sizing is air-cooled in three stages: in the first stage, the cooling is carried out in the range of 500-850°C with a controlled cooling rate of 2-5°C / s; after cooling to 500°C, the cooling rate in the second stage is controlled to be 5-15°C / s, preferably 8.5-15°C / s; after cooling to 300°C, natural air cooling is carried out to obtain a bainite seamless steel pipe.

10. 10. The method of claim 9, wherein one or more of the following are satisfied: a: in step (1), scrap steel and blast furnace molten iron are used to produce molten steel, where the blast furnace molten iron is used in a mass percentage of 50-60%; and / or in the continuous casting, the molten steel has a superheat of less than 30°C, and the drawing speed during the continuous casting is 1.8-2.2 m / min; b: Before step (2), the steel billet is cooled and then heated, the heating temperature is 1240-1300°C, and the heating time is 3-6h; c) in step (2), piercing is performed at a temperature of 1180-1240°C, and continuous rolling is performed at a temperature of 1000°C-1100°C; d: In step (2), after continuous rolling and before sizing, the tube is naturally cooled, then heated to 950-980°C, and then removed from the furnace for sizing at a temperature of 850-950°C.

11. The method according to claim 9, further comprising: step (4): tempering the bainite seamless steel pipe obtained in step (3) at a temperature of 200-350°C or 400-460°C.

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