A 110 ksi strength level seamless steel tube and method of making the same

Abstract

The present application discloses a kind of 110ksi strength level seamless steel tube, it contains Fe and inevitable impurity element, it also contains the following each chemical element with mass percentage as follows: C:0.16~0.18%, Si:0.56~1%, Mn:1.5~1.75%, Cr:1.35~2.25%, Al:0.015~0.04%, B:0.001~0.005%, 0 In addition, the present application also discloses the manufacturing method of the above-mentioned 110ksi strength level seamless steel tube, which comprises the following steps: (1) smelting and billet continuous casting; (2) piercing, continuous rolling and sizing; (3) two-stage cooling is carried out on the sized pipe: the open cooling temperature is 780-850℃, the cooling rate is maintained at 6-10℃ / s before cooling to 400℃; after cooling to 400℃, natural air cooling is carried out. The 110ksi strength level seamless steel tube designed in the present application can obtain good strength and toughness matching in rolled 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 are avoided.

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] It is well known that seamless steel pipe is a kind of pipe material with very broad application prospect.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] For the seamless steel pipe product, it is found that in the current prior art, in order to be able to prepare the yield strength of 110 ksi and have a certain toughness, the steel pipe usually needs to be treated by quenching and tempering C-Mn or Cr-Mo steel heat treatment process.

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

[0005] Therefore, in order to solve the problem that the 110 ksi seamless steel pipe in the prior art needs to be treated by quenching and tempering heat treatment to obtain strength and toughness matching, the present application expects to develop and obtain a new 110 ksi strength grade bainite seamless steel pipe product, which can obtain good strength and toughness matching in as-rolled or single tempering state without quenching and tempering heat treatment through reasonable composition matching and process design. The process flow of this new 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 110 ksi strength grade seamless steel pipe, which can obtain good strength and toughness matching in as-rolled 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 110 ksi strength grade seamless steel pipe, which contains Fe and inevitable impurity elements, and further contains the following chemical elements in mass percentage:

[0008] C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.5-1.75%, Cr: 1.35-2.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0

[0009] Further, in the 110 ksi seamless steel pipe according to the present application, the mass percentage of each chemical element is as follows:

[0010] C: 0.16-0.18%, Si: 0.56-1%, Mn: 1.5-1.75%, Cr: 1.35-2.25%, Al: 0.015-0.04%, B: 0.001-0.005%, 0

[0011] In the above technical solution, the 110 ksi seamless steel pipe according to the present application adopts a composition design of air-cooling high hardenability in the chemical composition design, and 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.

[0012] In addition, in the chemical composition design, the 110 ksi seamless steel pipe according to the present application does not contain noble metals and micro-alloying elements such as Mo, Ni, Nb, V and Ti, but adopts conventional Mn, Cr and B elements, which has good economy and low alloy cost.

[0013] In the 110 ksi seamless steel pipe according to the present application, the design principles of each chemical element are as follows:

[0014] C: In the 110 ksi seamless steel pipe according to the present application, C element is an important element for ensuring the strength of the steel, which can stabilize the bainite structure and improve the air-cooling hardenability after being added. In addition, C can also move the CCT curve to the right, and the appropriate amount of C element added in the steel can ensure that the steel obtains bainite structure under a lower cooling rate. 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 will be poor. Therefore, in order to exert the beneficial effects of C element, the mass percentage of C element in the 110 ksi seamless steel pipe according to the present application is controlled to be between 0.16-0.18%.

[0015] Si: In the 110 ksi strength level seamless steel pipe described in the present application, 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 carbide. Adding appropriate amount of Si element in the steel can not only ensure the solid solution strengthening of C element, but also reduce the number of martensite-austenite islands, refine the martensite-austenite island structure, and improve the strength and toughness matching of the steel. In addition, the content of Cr element in the steel of the present application is high, and Si element has an inhibiting effect on the growth of Cr carbide, thereby improving the strength and toughness matching. Of course, in order to ensure the beneficial effect of Si element, the addition amount of Si element should not be too low. When the content of Si in the steel is too low, it cannot play the corresponding role, and when the content of Si in the steel exceeds 1%, it has no improvement on the structure. Therefore, in order to play the beneficial effect of the above-mentioned Si element, the mass percentage content of Si element in the 110 ksi strength level seamless steel pipe described in the present application is controlled between 0.56% and 1%.

[0016] Mn: In the 110 ksi strength level seamless steel pipe described in the present 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, thereby effectively refining the structure and improving the strength and toughness of the steel. When the content of Mn element in the steel is less than 1.5%, the hardenability will decrease, forming upper bainite structure, resulting in poor toughness of the material; and when the content of Mn element in the steel is higher than 1.75%, the yield strength will exceed the upper limit, 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 content of Mn element in the 110 ksi strength level seamless steel pipe described in the present application is controlled between 1.5% and 1.75%.

[0017] Cr: In the 110 ksi strength level seamless steel pipe described in the present application, Cr is an important element for improving controlled cooling hardenability, which can ensure the formation of stable controlled cooling bainite structure with Mn and B elements at a cooling rate of more than 2℃ / s, and has certain solid solution strengthening and precipitation 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. If the content of Cr in the steel is too low, it cannot ensure the formation of stable bainite structure, and if the content of Cr in the steel is too high, it will cause waste of alloy. In addition, higher Cr is easy to cause the precipitation of more M23C6 carbide during tempering, and form chain carbide in the range of 480-550℃, resulting in temper brittleness. Therefore, in the 110 ksi strength level seamless steel pipe described in the present application, the mass percentage content of Cr element is controlled between 1.35% and 2.25%.

[0018] Al: In the 110 ksi seamless steel pipe described in the present application, Al is a good deoxidizing element, which can play a deoxidizing role. However, Al element is not easy to add too much, and too much Al can easily cause alumina inclusions, so it is necessary to try to increase the proportion of acid-soluble aluminum in the total aluminum, and then feed Al wire after vacuum degassing. Therefore, in order to play the beneficial effect of Al element, in the present application, the mass percentage content of Al element is controlled between 0.015-0.04%.

[0019] B: In the 110 ksi seamless steel pipe described in the present application, B element can increase the hardenability of the steel, and the combined addition of B and Mn, Cr elements can further improve the air-cooling hardenability of the steel to 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 to improve the strength and toughness matching of the steel. Of course, the addition amount of B element should not be too low, when the content of B element in the steel is less than 0.0015%, the role of B is not obvious, and when the content of B in the steel is too high, the steelmaking is difficult to control accurately. Therefore, in order to play the beneficial effect of B element, in the 110 ksi seamless steel pipe described in the present application, the mass percentage content of B element is controlled between 0.001-0.005%.

[0020] N: In the 110 ksi seamless steel pipe described in the present application, N element can form carbonitride with Al and play a role in refining grains. Therefore, in order to play the beneficial effect of N element and avoid its harmful effect, in the 110 ksi seamless steel pipe described in the present application, the mass percentage content of N element is controlled to meet: 0

[0021] Correspondingly, in the present application, while controlling the mass percentage content of a single chemical element, the mass percentage content of Al element and N element also needs to be further controlled to meet: Al / N≥3. The purpose of controlling the mass percentage content of Al element and N element to meet this relationship is to ensure that the acid-soluble aluminum content can be combined with N element sufficiently, thereby preventing N from combining with B to form a brittle low-melting point phase, to ensure the improvement effect of B element on the hardenability of the steel, and to prevent the grain boundary from being embrittled.

[0022] In addition, it should be noted that in the present application, Ti element is not added to the steel, and Ti element is not used to remove N, which avoids the problem that Ti element forms coarse carbides and combines with inclusions to cause poor impact toughness.

[0023] Further, in the 110 ksi seamless steel pipe described in the present application, among the unavoidable impurities, S≤0.01%, P≤0.01%.

[0024] In the 110 ksi strength grade seamless steel pipe, the P element and the S element are both harmful impurity elements in the seamless steel pipe, and the content of the harmful impurity elements in the seamless steel pipe should be reduced as much as possible to obtain a better performance and a better quality seamless steel pipe under the condition that the technical conditions are allowed.

[0025] In the present application, when the content of the impurity element P is too high, the P will be segregated at the grain boundary, and the grain boundary will be embrittled, thereby seriously deteriorating the toughness of the steel. When the content of the impurity element S is too high, the content of the inclusions in the steel will be increased, which is not conducive to the low-temperature toughness of the steel. Therefore, in the present application, the content of the P and S elements in the steel must be strictly controlled, and the mass percentage content of the S element satisfies S≤0.01%, and the mass percentage content of the P element satisfies P≤0.01%.

[0026] Further, in the 110 ksi strength grade seamless steel pipe, the microstructure is granular bainite.

[0027] Further, in the 110 ksi strength grade seamless steel pipe, the as-rolled performance satisfies: yield strength≥760 MPa, tensile strength≥1000 MPa, longitudinal impact toughness≥50 J, yield strength ratio is 0.65-0.85, elongation≥12%, the local residual stress of the outer wall of the seamless steel pipe in the thickness direction is≤100 MPa within a range of≤500 μm, and the overall residual stress of the pipe body is≤100 MPa.

[0028] Further, in the 110 ksi strength grade seamless steel pipe, after tempering at a temperature of 120-190℃, the yield strength is≥760 MPa, the tensile strength is≥1000 MPa, and the longitudinal impact toughness is≥60 J.

[0029] Further, in the 110 ksi strength grade seamless steel pipe, after tempering at a temperature of 300-550℃, the yield strength is≥760 MPa, and the tensile strength is≥1000 MPa.

[0030] Correspondingly, another object of the present application is to provide a manufacturing method of the above-mentioned 110 ksi strength grade seamless steel pipe, which does not use quenching and tempering heat treatment process, has a short process flow and low energy consumption, avoids a series of problems such as size deterioration and surface oxidation caused by quenching and tempering, and has a good application prospect.

[0031] In order to achieve the above-mentioned objects, the present application provides a manufacturing method of the above-mentioned 110 ksi strength grade seamless steel pipe, which comprises the following steps:

[0032] (1) smelting and continuous casting of a pipe blank;

[0033] (2) piercing, continuous rolling and sizing;

[0034] (3) two-stage cooling of the sized pipe: open cooling temperature is 780-850℃, and the cooling rate is controlled to be 6-10℃ / s before cooling to 400℃; and natural air cooling is performed after cooling to 400℃.

[0035] In the method for manufacturing the 110ksi strength grade seamless steel pipe, the inventors control the two-stage cooling of the sized pipe and optimize the two-stage cooling process.

[0036] In step (3), the cooling rate is controlled to be 6-10℃ / s before cooling to 400℃ in the first-stage cooling. This is because: in this cooling rate range, the ferrite phase transformation is not prematurely occurred, and the stable bainite phase transformation of the supercooled austenite is ensured at 400℃; and the use of this cooling rate control can prevent the upper bainite structure of the 110ksi strength grade seamless steel pipe from being formed in the 500μm range of the wall thickness direction of the oxide decarburized layer, and ensure the transformation difference between the oxide decarburized layer and the normal position phase transformation structure to be reduced, thereby avoiding the larger phase transformation stress at the interface caused by the transformation difference, and reducing the cracking tendency of the outer surface and the tolerance to the outer surface defects.

[0037] If the cooling rate in the first-stage cooling is too low, the oxide decarburized layer will form the upper bainite structure, and the oxide decarburized layer and the normal position phase transformation will be out of synchronization and have a large difference in structure, thereby causing the outer surface residual stress to be high, and the outer surface of the pipe body to crack during use. This cooling rate can improve the stability of the oxide decarburized layer and the supercooled austenite at the normal pipe body position, ensure the bainite to have a structure transformation at a lower temperature, form stable fine bainite structure, reduce the size of the M-A island, thereby ensuring the strength and toughness matching, and effectively control the mutual offset of the thermal stress and the phase transformation stress during the cooling and phase transformation process, thereby reducing the final residual stress and effectively improving the deformation resistance of the product.

[0038] Correspondingly, in the second-stage air cooling process, the natural air cooling is performed after cooling to 400℃, the bainite phase transformation occurs in this process, the thermal stress caused by the rapid cooling is reduced, and the overall residual stress level of the 110ksi strength grade seamless steel pipe can be effectively reduced.

[0039] In summary, in the 110 ksi strength level seamless steel pipe product prepared in the application, through the control of the two-stage cooling process, the local residual stress of the outer wall of the 110 ksi strength level seamless steel pipe in the thickness direction is ≤100 MPa within the range of ≤500 μm, and the overall residual stress of the pipe body is ≤100 MPa, which not only improves the fault tolerance of the outer surface of the steel pipe, but also improves the resistance to crack propagation after surface scratching in the subsequent use process, and improves the overall performance and service life of the pipe body in the use process.

[0040] Further, in the manufacturing method described in the application, in step (1), the charging scheme of scrap steel + blast furnace molten iron is adopted during smelting, and the mass percentage of blast furnace molten iron is 50-60%.

[0041] In the manufacturing method described in the application, during the smelting process of step (1), the charging scheme of scrap steel + blast furnace molten iron can be adopted, and the proportion of blast furnace molten iron is controlled to be 50-60%. The molten steel can be smelted by an electric furnace, and after external refining, vacuum degassing and argon stirring, the inclusions are modified by Ca treatment to reduce the contents of O and H elements. Then in the pipe blank continuous casting process of step (1), the alloy is cast into round billets, and the superheat of the molten steel is controlled to be lower than 30℃ during continuous casting, and the continuous casting speed is 1.8-2.2 m / min to reduce the composition segregation.

[0042] Further, in the manufacturing method described in the application, in step (1), the superheat of the molten steel is controlled to be lower than 30℃ during continuous casting, and the continuous casting speed is 1.8-2.2 m / min.

[0043] Further, in the manufacturing method described in the application, in step (2), the continuous casting round billet is cooled and then heated in a ring-shaped heating furnace at 1240-1300℃, and the heating time is 3-6 hours.

[0044] Further, in the manufacturing method described in the application, in step (2), after heating, piercing is performed at a piercing temperature of 1180-1240℃, and then continuous rolling is performed at a continuous rolling temperature of 1000℃-1100℃.

[0045] Further, in the manufacturing method described in the application, in step (2), after continuous rolling, the pipe body is naturally cooled, and then reheated to 950-980℃ in a reheating furnace, and then discharged for sizing at a sizing temperature of 850-950℃.

[0046] Further, in the manufacturing method described in the application, it further includes step (4): tempering, wherein the tempering temperature is 120-190℃ or 300-550℃.

[0047] In some preferred embodiments, based on the 110 ksi strength grade seamless steel pipe in the rolled state obtained according to the above steps (1)-(3), the operator can further perform a tempering treatment, and the obtained 110 ksi strength grade seamless steel pipe after the tempering treatment also has a good strength-toughness match.

[0048] Compared with the prior art, the 110 ksi strength grade seamless steel pipe and the manufacturing method thereof have the following advantages and beneficial effects:

[0049] The present application develops a new 110 ksi strength grade seamless steel pipe product, which can obtain a good strength-toughness match in the rolled state or in the single-tempered state without quenching and tempering heat treatment, has a short process flow, low energy consumption, and avoids a series of problems such as size deterioration and surface oxidation caused by quenching and tempering.

[0050] In the chemical composition design, the inventors specifically adopt a high hardenability air-cooled composition design, which can obtain stable granular bainite structure in a wide range of cooling rates, 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, etc., which has low alloy cost and good economy.

[0051] Correspondingly, in the manufacturing process, the present application also designs a process control at different stages of air cooling after rolling, and performs a two-stage cooling process control on the pipe after sizing. By adopting this two-stage cooling process, the formation of upper bainite structure in the 500 μm range of the outer wall of the 110 ksi strength grade seamless steel pipe can be effectively prevented, and the transformation difference between the oxide decarburized layer and the normal position phase transformation structure is reduced, avoiding the phase transformation stress of the two sections caused by the transformation difference, thereby reducing the cracking tendency of the outer surface and the tolerance to defects on the outer surface. In addition, this two-stage cooling process can also control the microstructure transformation in the stable bainite transformation interval, increase the supercooling degree of bainite transformation, to ensure the overall microstructure and strength-toughness match of the 110 ksi strength grade seamless steel pipe, and prevent the problem of large macroscopic residual stress of the pipe body, to ensure the performance during subsequent use.

[0052] The 110 ksi strength level seamless steel pipe product developed by the present application has good strength and toughness matching in the rolled state and the single tempering state. In the rolled state, the yield strength is ≥ 760 MPa, the tensile strength is ≥ 1000 MPa, the longitudinal impact toughness is ≥ 50 J, the yield strength ratio is 0.65-0.85, the elongation is ≥ 12%, the local residual stress of the outer wall of the seamless steel pipe in the thickness direction is ≤ 100 MPa within a range of ≤ 500 μm, and the overall residual stress of the pipe body is ≤ 100 MPa. The 110 ksi strength level seamless steel pipe in the rolled state is subjected to single tempering treatment at a temperature of 120-190 ℃, and the yield strength and tensile strength and the yield strength ratio do not change significantly, but the longitudinal impact toughness is increased to more than 60 J, at which time the best strength and toughness matching is achieved.

[0053] In addition, the 110 ksi strength level seamless steel pipe product in the rolled state is subjected to single tempering treatment at a temperature of 300-550 ℃, and the strength can also meet the requirements, but when the tempering temperature is in the range of 480-550 ℃, there is obvious temper brittleness, and the longitudinal impact toughness is ≤ 30 J. DETAILED DESCRIPTION

[0054] The 110 ksi strength level seamless steel pipe and the manufacturing method thereof described in the present application will be further explained and described below in conjunction with specific examples, but the explanation and description do not constitute undue limitations on the technical solutions of the present application.

[0055] Examples 1-6 and Comparative Examples 1-8

[0056] The 110 ksi strength level seamless steel pipe of Examples 1-6 and the seamless steel pipe of Comparative Examples 1-8 described in the present application are both prepared by the following steps:

[0057] (1) The chemical composition shown in Table 1 is smelted and the pipe blank is continuously cast: a charge scheme of scrap steel + blast furnace molten iron is used during smelting, 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 being subjected to outside furnace refining, vacuum degassing and argon stirring, the inclusions are modified by Ca treatment to reduce the O and H contents. Then the alloy is cast into a round billet, and during the 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.

[0058] (2) Piercing and rolling and sizing: after the obtained continuous casting round billet is cooled, it is input into a ring heating furnace for heating, the continuous casting round billet is soaked at 1240-1300°C in the ring furnace, and the heating time is controlled to be 3-6 hours. After heating, piercing is performed, the piercing temperature is controlled to be 1180-1240°C, after piercing, rolling is performed, and the rolling temperature is controlled to be 1000-1100°C; after rolling, the pipe body is naturally cooled for 30s, and then is input into a reheating furnace for heating to 950-980°C, and then is output from the furnace for sizing, and the sizing temperature is controlled to be 850-950°C.

[0059] (3) Two-stage cooling is performed on the pipe after sizing: the open cooling temperature is 780-850°C, before cooling to 400°C, the cooling rate is maintained to be 6-10°C / s; after cooling to 400°C, natural air cooling is performed.

[0060] In the present application, the chemical element components and the related process design of the 110 ksi strength grade seamless steel pipes of Examples 1-6 all meet the design specification requirements of the present application. Although the seamless steel pipes of Comparative Examples 1-8 are also prepared by using the above process steps, the chemical element components and / or the related process parameters thereof do not meet the design parameters of the present application.

[0061] Table 1 lists the mass percentages of each chemical element in the 110 ksi strength grade seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-8.

[0062] Table 1. (the balance is Fe and other unavoidable impurities except P and S)

[0063]

[0064]

[0065] Table 2-1 and Table 2-2 list the specific process parameters of the manufacturing method of the 110 ksi strength grade seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-8 in the above process steps.

[0066] Table 2-1.

[0067]

[0068] Table 2-2.

[0069]

[0070] From the above Table 1, Table 2-1 and Table 2-2, it can be seen that, in the present application, the content of C element of Comparative Example 1 is lower than the design requirement; the content of Mn element of Comparative Example 2 is lower than the design requirement; the content of Cr element of Comparative Example 3 is lower than the design requirement; the content of C element of Comparative Example 4 is higher than the design requirement; the ratio of Al / N of Comparative Example 5 is less than 3; the content of B element of Comparative Example 6 is lower than the design requirement; the first-stage cooling rate of the seamless steel pipe of Comparative Example 7 after sizing is lower than the design requirement; and the second-stage cooling rate of the seamless steel pipe of Comparative Example 8 after sizing is higher than the design natural cooling rate.

[0071] The obtained 110 ksi strength grade seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-8 were sampled respectively, and various performance tests were carried out, and the obtained test results are listed in Table 3.

[0072] The relevant performance detection means are as follows:

[0073] (1) Tensile test: According to GB / T228.1-2010 “Metallic materials—Tensile test methods”, the yield strength, tensile strength, yield ratio and elongation values of the seamless steel pipes of each example and comparative example at room temperature were tested. Among them, the yield ratio = yield strength / tensile strength.

[0074] (2) Impact test: According to GB / T229 “Charpy impact test methods”, the longitudinal impact toughness of the seamless steel pipes of each example and comparative example at room temperature was tested.

[0075] (3) Residual stress test: The residual stress was measured according to ISO_TR10400-2018 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 within the range of ≤500 μm in the thickness direction.

[0076] Table 3 lists the performance test results of the 110 ksi strength grade seamless steel pipes of Examples 1-6 and the seamless steel pipes of Comparative Examples 1-8.

[0077] Table 3.

[0078]

[0079] As can be seen from Table 3, compared with the seamless steel pipes of Comparative Examples 1-8, the comprehensive performance of the 110 ksi strength grade seamless steel pipes of Examples 1-6 of the present application is obviously better.

[0080] As shown in Table 3, after the steps (1)-(3) of the manufacturing method of the present application are completed, the as-rolled 110 ksi strength grade seamless steel pipes of Examples 1-6 have excellent mechanical properties, and the strength and toughness match well, the yield strength is between 785-945 MPa, the tensile strength is between 1050-1150 MPa, the yield strength ratio is between 0.75-0.84, the elongation is between 13-15%, and the longitudinal impact toughness at room temperature is between 75-95 J.

[0081] Correspondingly, the as-rolled 110 ksi strength grade seamless steel pipes of Examples 1-6 have excellent resistance to crack propagation after surface scratching during subsequent use, the overall residual stress of the pipe body is between 25-85 MPa, and the local residual stress of the outer wall in the thickness direction is between 0-92 MPa within a range of ≤500 μm.

[0082] After the analysis of the as-rolled 110 ksi strength grade seamless steel pipes of Examples 1-6 is completed, the 110 ksi strength grade seamless steel pipes obtained by the present application also have good strength and toughness match after subsequent tempering treatment.

[0083] In the present application, after the as-rolled 110 ksi strength grade seamless steel pipes are subjected to single tempering treatment at a temperature of 120-190°C, the yield strength and tensile strength, and the yield strength ratio do not change significantly, but the longitudinal impact toughness is increased to more than 60 J, at which time the strength and toughness match is best. Correspondingly, after the as-rolled 110 ksi strength grade seamless steel pipes are subjected to single tempering treatment at a temperature of 300-550°C, the strength can also meet the requirements, but when the tempering temperature is in the range of 480-550°C, there is obvious temper brittleness, and the longitudinal impact toughness is ≤30 J.

[0084] In the present application, the as-rolled 110 ksi strength grade seamless steel pipes of Example 1 are taken as an example, and the 110 ksi strength grade seamless steel pipes prepared by the above steps (1)-(3) are further subjected to step (4): tempering treatment.

[0085] The as-rolled 110 ksi strength grade seamless steel pipes of Example 1 are sampled and classified into 6 groups, and the 6 groups are subjected to tempering treatment at different tempering temperatures (120°C, 190°C, 300°C, 350°C, 440°C, 550°C), respectively.

[0086] After the above tempering treatment, the six groups of the finished tempering treatment seamless steel pipes are sampled respectively, and the six 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 seamless steel pipes in the tempering state of each group, and the test results are listed in Table 4 below. The experimental means of the related tensile test and impact test are the same as those in Table 3 above, which will not be described here.

[0087] Table 4 lists the mechanical properties of the rolled 110 ksi strength grade seamless steel pipe of Example 1 and the 110 ksi strength grade seamless steel pipe after being tempered at five different tempering temperatures.

[0088] Table 4.

[0089]

[0090] As shown in Table 4, taking the rolled 110 ksi strength grade seamless steel pipe of Example 1 as an example, after being tempered at a temperature of 120-190℃, the yield strength and tensile strength are slightly improved, and the room temperature longitudinal impact toughness is greatly improved, the room temperature longitudinal impact toughness is improved from 80J to 95J (tempered at 120℃) and 100J (tempered at 190℃), at this time, the best strength and toughness matching is achieved.

[0091] Taking the rolled 110 ksi strength grade seamless steel pipe of Example 1 as an example, after being tempered at a temperature of 300-550℃, the strength can also meet the requirements, but when tempered at 480-550℃, there is obvious temper brittleness, and the room temperature longitudinal impact toughness is greatly reduced. And when the tempering temperature is 550℃, the longitudinal impact toughness is only 22J.

[0092] In summary, it can be seen that in the present application, the 110 ksi strength grade seamless steel pipe designed by the inventor can obtain good strength and toughness matching in the rolled state or single tempering state without quenching and tempering heat treatment, 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.

[0093] It should be noted that the combination of the technical features in the present case is not limited to the combination mode described in the claims of the present case or the combination mode described in the specific embodiments, all the technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0094] It should also be noted that the above-mentioned examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned examples, 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 all should belong to the protection scope of the present application.

Claims

1. A seamless steel pipe with a strength grade of 110ksi, characterized in that, Its mass percentage of each chemical element is as follows: C: 0.16~0.18%, Si: 0.56~1%, Mn: 1.5~1.75%, Cr: 1.35~2.25%, Al: 0.015–0.04%, B: 0.001–0.005%, 0 < N ≤ 0.006%, where Al / N ≥ 3; The balance consists of Fe and unavoidable impurities; The rolled properties of the 110ksi strength grade seamless steel pipe meet the following requirements: yield strength ≥760MPa, tensile strength ≥1000MPa, longitudinal impact toughness ≥50J, yield-to-tensile ratio of 0.65-0.85, elongation ≥12%, local residual stress on the outer wall of the seamless steel pipe within the wall thickness direction ≤500μm ≤100MPa, and overall residual stress of the pipe body ≤100MPa.

2. The seamless steel pipe with a strength grade of 110ksi as described in claim 1, characterized in that, In unavoidable impurities, S ≤ 0.01%, P ≤ 0.01%.

3. The seamless steel pipe with a strength grade of 110ksi as described in claim 1, characterized in that, Its microstructure is granular bainite.

4. The seamless steel pipe with a strength grade of 110ksi as described in claim 1, characterized in that, After tempering at 120-190℃, its yield strength is ≥760MPa, tensile strength is ≥1000MPa, and longitudinal impact toughness is ≥60J.

5. The seamless steel pipe with a strength grade of 110ksi as described in claim 1, characterized in that, After tempering at 300-550℃, its yield strength is ≥760MPa and its tensile strength is ≥1000MPa.

6. A method for manufacturing a seamless steel pipe with a strength grade of 110ksi as described in any one of claims 1-5, characterized in that, Including the following steps: (1) Smelting and continuous casting of tube blanks; (2) Piercing, continuous rolling and sizing; (3) The pipe after sizing is cooled in two stages: the initial cooling temperature is 780-850℃, and the cooling rate is maintained at 6-10℃ / s before cooling to 400℃; after cooling to 400℃, natural air cooling is carried out.

7. The manufacturing method as described in claim 6, characterized in that, In step (1), a feedstock of scrap steel and molten iron is used during smelting, wherein the mass percentage of molten iron is 50-60%.

8. The manufacturing method as described in claim 6, characterized in that, In step (1), the superheat of the molten steel is controlled to be below 30°C during the continuous casting process, and the continuous casting speed is 1.8-2.2 m / min.

9. The manufacturing method as described in claim 6, characterized in that, In step (2), the continuously cast round billet is cooled and then heated in an annular heating furnace at 1240-1300℃ for 3-6 hours.

10. The manufacturing method as described in claim 6, characterized in that, In step (2), after heating, piercing is performed at a temperature of 1180-1240℃, and after piercing, continuous rolling is performed at a temperature of 1000℃-1100℃.

11. The manufacturing method as described in claim 6, characterized in that, In step (2), the tube body is naturally cooled after continuous rolling, and then heated to 950-980℃ in a reheat furnace, and then removed from the furnace for sizing at a sizing temperature of 850-950℃.

12. The manufacturing method as described in claim 6, characterized in that, It also includes step (4): tempering, wherein the tempering temperature is 120-190℃ or 300-550℃.

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