Method for drilling seamless steel pipes, rolled materials, and manufacturing method for seamless steel pipes
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- JFE STEEL CORP
- Filing Date
- 2024-06-03
- Publication Date
- 2026-06-30
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to seamless steel pipes, a piercing method for a material to be rolled, and a method for manufacturing seamless steel pipes, and particularly to a technique for realizing high strength and high toughness of seamless steel pipes by refining crystal grains contained in seamless steel pipes.
Background Art
[0002] From the perspective of the depletion of energy resources expected in the near future, development of oil fields at high depths and oil wells in severe corrosion environments, which were not considered in the past, has been actively carried out. Seamless steel pipes used in such environments are required to have excellent mechanical properties and corrosion resistance.
[0003] In response to such requirements, in Patent Document 1, in terms of mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and less than 2.8%, Cu: 0.3 to 3.5%, Ni: 3.0% or more and 5.0% or less, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N: 0.010 to 0.100%, O: 0.01% or less, B: 0.0005 to 0.0100%, and Nb, Ta, C, N, and Cu satisfy a specific formula. In terms of volume ratio, it has a structure composed of 45% or more tempered martensite phase, 20 to 40% ferrite phase, and more than 10% and 25% or less retained austenite phase. When crystal grains with a crystal orientation difference within 15° are defined as the same crystal grains, the maximum crystal grain size of ferrite crystal grains is 500 μm or less, and it is stated that a high-strength stainless steel seamless steel pipe for oil wells with excellent mechanical properties and corrosion resistance can be obtained.
[0004] Furthermore, Patent Document 2 states that a high-strength stainless steel seamless pipe for oil wells can be obtained that contains, by mass%, C: 0.002~0.05%, Si: 0.05~0.50%, Mn: 0.04~1.80%, P: 0.030% or less, S: 0.002% or less, Cr: over 14.0% and 17.0% or less, Ni: 4.0~8.0%, Mo: 1.5~3.0%, Al: 0.005~0.10%, V: 0.005~0.20%, Co: 0.01~1.0%, N: 0.002~0.15%, and O: 0.006% or less, satisfies a specific formula, has a structure in which the average grain size of prior austenite is 40 μm or less, and has a yield strength of 1034 MPa or more. [Prior art documents] [Patent Documents]
[0005] [Patent Document 1] International Publication No. 2019 / 035329 [Patent Document 2] Patent No. 7201094 [Overview of the project] [Problems that the invention aims to solve]
[0006] The technologies described in Patent Documents 1 and 2 allow for the acquisition of excellent mechanical properties and corrosion resistance by optimizing the composition. However, in product sizes where the reduction ratio shown in the following formula (1) cannot be increased, it is sometimes not possible to secure sufficient processing strain to refine the crystal grains of the ferrite phase, resulting in the failure to obtain the desired strength and toughness. Area reduction ratio (%) = 100 × (Circumferential cross-sectional area of round steel piece (mm²) 2 )-Circumferential cross-sectional area of the raw tube after perforation and rolling (mm 2 )) / Circumferential cross-sectional area of a round steel piece (mm 2 )···(1)
[0007] This invention was developed in view of the above-mentioned problems, and aims to provide a seamless steel pipe having excellent strength and toughness, a method for drilling a rolled material to obtain the seamless steel pipe, and a method for manufacturing a seamless steel pipe. In this invention, superior strength refers to a yield strength of 758 MPa or higher, measured in accordance with the API 5CT standard, and superior toughness refers to a toughness value of 60 J or higher at room temperature (25°C), measured in accordance with the JIS Z 2242 (2023) standard. [Means for solving the problem]
[0008] As a result of our investigation, we found that even when the reduction ratio is low, by optimizing the perforation rolling conditions, it is possible to refine the ferrite particle size of seamless steel pipes and obtain excellent mechanical properties. Specifically, the inventors investigated methods for reducing particle size during the manufacturing process of seamless steel pipes and found that even for product sizes where the reduction ratio cannot be increased, particle size reduction can be achieved by introducing appropriate circumferential shear strain into the rolled material. Therefore, the inventors investigated optimizing the perforation rolling conditions in order to finen the raw tubes. The present invention was made based on the above findings, and its gist is as follows.
[0009] [1] In terms of volume ratio, More than 40% tempered martensite phase, A ferrite phase of 25% to 40%, It has a structure consisting of a residual austenite phase that is more than 10% but less than 25%, When crystal grains with a crystal orientation difference of 15° or less are defined as the same crystal grain, the average ferrite grain size is 35 μm or less. The yield strength is 758 MPa or higher. A seamless steel pipe with a toughness value of 60 J or more at 25°C. [2] In terms of component composition, in mass%, C: 0.002~0.05%, Si: 0.05~0.6%, Mn: 0.1~0.5%, Cu: 0.9~3.5%, Ni: 3.0~6.0%, Cr: 16.5~19%, Mo: 1 to 4%, N: 0.01 to 0.2%, contains Furthermore, P: 0.02% or less, S: 0.002% or less, Al: 0 to 1%, V: 0 to 0.6%, Nb: 0 to 0.5%, W: 0 to 3.50%, Co: 0 to 1.5% contains The balance consists of Fe and unavoidable impurities, seamless steel pipe according to [1] above. [3] A piercing method of the material to be rolled when manufacturing the seamless steel pipe according to [1] or [2] above, A piercing method of the material to be rolled, in which the circumferential shear strain applied to the central part of the thickness of the material to be rolled by piercing rolling satisfies 0.48 or more. [4] A method for manufacturing a seamless steel pipe using the piercing method of the material to be rolled according to [3] above.
Effect of the Invention
[0010] According to the present invention, a seamless steel pipe excellent in strength and toughness, a piercing method of the material to be rolled for obtaining the seamless steel pipe, and a method for manufacturing the seamless steel pipe are provided.
Brief Explanation of Drawings
[0011] [Figure 1] [Figure 1] Figure 1 is a diagram for explaining the sampling position of the sample for tissue observation. [Figure 2] Figure 2 is a schematic diagram showing the process of the Mannesmann-Mandrel mill method. [Figure 3] Figure 3 is a diagram for explaining the API5CT arc-shaped tensile test piece and V-notch test piece used in the evaluation.
Embodiment for Carrying out the Invention
[0012] [Seamless Steel Pipe] Hereinafter, the present invention will be described in detail. <The seamless steel pipe of the present invention has a structure consisting of a tempered martensite phase of 40% or more by volume fraction, a ferrite phase of 25% to 40%, and a retained austenite phase of more than 10% to 25%. When crystal grains with a crystal orientation difference of 15° or less are defined as the same crystal grain, the average ferrite grain size is 35 μm or less, the yield strength is 758 MPa or more, and the toughness value at 25°C is 60 J or more.
[0013] <Organization> Volume fraction of tempered martensite phase: 40% or more Volume fraction of ferrite phase: 25% to 40% Volume fraction of the retained austenite phase: greater than 10% and less than or equal to 25% In the seamless steel pipe of the present invention, the tempered martensite phase is set to 40% or more by volume in order to ensure the desired strength. Furthermore, it is preferable that the tempered martensite phase is set to less than 65%. Then, the ferrite phase is precipitated to a volume fraction of 25% or more. This suppresses the concentration of strain during hot rolling in the ferrite phase. On the other hand, if a large amount of ferrite phase precipitates to a volume fraction exceeding 40%, it may become impossible to achieve the desired strength. Therefore, the ferrite phase is kept between 25% and 40% by volume fraction.
[0014] Furthermore, in the seamless steel pipe of the present invention, a retained austenite phase is precipitated in addition to the ferrite phase. The presence of the retained austenite phase improves ductility and toughness. In order to obtain such improvements in ductility and toughness while ensuring the desired strength, the retained austenite phase is precipitated to a volume fraction of more than 10%. On the other hand, precipitation of a large amount of austenite phase exceeding 25% by volume makes it impossible to ensure the desired strength. For this reason, the retained austenite phase is kept at 25% or less by volume. Preferably, the retained austenite phase is between 10% and 20% by volume.
[0015] Figure 1 is a diagram illustrating the sampling locations for tissue observation. Here, to measure the microstructure of the seamless steel pipe of the present invention, first, a specimen for microstructure observation was taken from the center of the wall thickness (t / 2 position (t: wall thickness)) of the cross section perpendicular to the pipe axis direction (circumferential cross section of the raw pipe) of the seamless steel pipe (see symbol X in Figure 1). The collected sample (specimen for microstructure observation) was etched with Virela's reagent (a reagent prepared by mixing picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 10 ml, and 100 ml, respectively), and the microstructure at the center of the wall thickness was imaged using a scanning electron microscope (magnification: 1000x, field of view: 450 μm × 450 μm), and the microstructure fraction (volume %) of the ferrite phase was calculated using an image analysis device.
[0016] Then, the X-ray diffraction specimen is ground and polished so that the measurement surface is a cross section perpendicular to the tube axis (circumferential cross section of the original tube), and the amount of retained austenite (γ) is measured using the X-ray diffraction method. The amount of retained austenite is determined by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α, and is given by the following equation. γ(volume ratio)=100 / (1+(IαRγ / IγRα)) (Here, Iα:integrated intensity of α, Rα:crystallographic theoretical calculation of α, Iγ:integrated intensity of γ, Rγ:crystallographic theoretical calculation of γ) The conversion is performed using the following method. The fraction (volume fraction) of the tempered martensite phase is the remainder of the ferrite phase and retained austenite phase determined by the above measurement method.
[0017] Ferrite average particle size: 35 μm or less The seamless steel pipe of the present invention is characterized in that, when crystal grains with a crystal orientation difference of 15° or less are defined as the same crystal grain, the average ferrite grain size (hereinafter also referred to as the average ferrite crystal grain size) is 35 μm or less. If the average ferrite crystal grain size exceeds 35 μm, the ferrite becomes coarser, and at least one of the strength and toughness deteriorates. Therefore, in this invention, the average ferrite grain size is 35 μm or less. The average ferrite grain size (average ferrite grain size) is preferably 30 μm or less, and more preferably 27 μm or less. The lower limit is not particularly limited, but the average ferrite grain size (average ferrite crystal grain size) may be 20 μm or more, and may also be 24 μm or more.
[0018] The measurement area for the average ferrite particle size of the seamless steel pipe 10 is the central part of the wall thickness of the seamless steel pipe 10 (see symbol X in Figure 1), as shown in Figure 1. The average grain size of ferrite was determined to be 100 mm using backscattered electron diffraction (EBSD). 2 The average value of ferrite crystals (number average value (= sum of grain sizes / number of crystal grains)) is calculated by performing crystal orientation measurements on a continuous region and defining grains with a crystal orientation difference of 15° or less as the same crystal grain.
[0019] <Component composition> The component composition of the seamless steel pipe of the present invention is not particularly limited, but preferably it contains, by mass%, C: 0.002~0.05%, Si: 0.05~0.6%, Mn: 0.1~0.5%, Cu: 0.9~3.5%, Ni: 3.0~6.0%, Cr: 16.5~19%, Mo: 1~4%, N: 0.01~0.2%, and further contains P: 0.02% or less, S: 0.002% or less, Al: 0~1%, V: 0~0.6%, Nb: 0~0.5%, W: 0~3.50%, Co: 0~1.5%, with the remainder being Fe and unavoidable impurities. The following describes each component. Unless otherwise specified, the percentages indicating the content of each component refer to mass percentages.
[0020] C: 0.002~0.05% Carbon (C) is an element that increases the strength of stainless steel. In this invention, the desired strength can be achieved by including 0.002% or more of carbon. On the other hand, toughness decreases when the carbon content exceeds 0.05%. Therefore, the C content is set to 0.002 to 0.05%. Preferably, the C content is 0.005 to 0.04%, and more preferably 0.015 to 0.035%.
[0021] Si: 0.05~0.6% Si is an element that acts as a deoxidizing agent. The deoxidizing effect can be achieved with a Si content of 0.05% or more. On the other hand, if the Si content exceeds 0.6%, the hot workability decreases. Therefore, the Si content is set to 0.05-0.6%. Preferably, the Si content is 0.09-0.5%, and more preferably 0.2-0.4%.
[0022] Mn: 0.1~0.5% Mn is an element that increases the strength of martensitic stainless steel, and this effect can be obtained with a content of 0.1% or more. On the other hand, toughness decreases when the Mn content exceeds 0.5%. Therefore, the Mn content is set to 0.1-0.5%. Preferably, the Mn content is 0.1-0.4%, and more preferably 0.2-0.35%.
[0023] Cu: 0.9~3.5% Cu is an element that strengthens the protective film, suppresses hydrogen penetration into the steel, and enhances resistance to sulfide stress cracking and sulfide stress corrosion cracking. This effect can be obtained with a Cu content of 0.9% or more. On the other hand, if the Cu content exceeds 3.5%, it leads to the precipitation of CuS at grain boundaries, reducing hot workability. Therefore, the Cu content is set to 0.9-3.5%. Preferably, the Cu content is 1.2-3.0%, and more preferably 1.5-2.8%.
[0024] Ni: 3.0~6.0% Ni is an element that strengthens protective coatings and contributes to improved corrosion resistance. It also increases the strength of steel through solid solution strengthening. This effect becomes significant with a Ni content of 3.0% or more. On the other hand, when the Ni content exceeds 6.0%, the stability of the martensite phase decreases, and the strength decreases. Therefore, the Ni content is set to 3.0-6.0%. Preferably, the Ni content is 3.0-5.5%, and more preferably 3.5-4.5%.
[0025] Cr: 16.5~19% Cr is an element that forms a protective film on the surface of steel pipes, contributing to improved corrosion resistance. This effect becomes particularly noticeable when the Cr content is 16.5% or higher. On the other hand, if the Cr content exceeds 19%, the ferrite fraction may become too high, making it impossible to secure the desired strength. Therefore, the Cr content should be 16.5 to 19%. Preferably, the Cr content is 16.8 to 18.0%. More preferably, the Cr content is 17.0 to 17.5%.
[0026] Mo: 1-4% Mo increases resistance to pitting corrosion by stabilizing the protective film, thereby enhancing resistance to sulfide stress cracking and sulfide stress corrosion cracking. A Mo content of 1% or more is necessary to achieve these effects. On the other hand, since Mo is an expensive element, a Mo content exceeding 4% leads to increased material costs and a decrease in toughness and resistance to sulfide stress corrosion cracking. Therefore, the Mo content should be 1-4%. Preferably, the Mo content is 1.5-3.5%, and more preferably 2.0-3.0%.
[0027] N: 0.01~0.2% N is an element that inexpensively suppresses the formation of δ-ferrite and improves hot workability. This effect can be obtained with a N content of 0.01% or more. On the other hand, an N content exceeding 0.2% forms nitrides and reduces toughness. Therefore, the N content should be 0.01 to 0.2%. Preferably, the N content is 0.02 to 0.15%, and more preferably 0.05 to 0.1%.
[0028] P:0.02% or less P is an element that reduces carbon dioxide corrosion resistance, pitting corrosion resistance, and SSC resistance. It is preferable to reduce the content as much as possible, but extreme reduction leads to a surge in manufacturing costs. For this reason, the P content is set to 0.02% or less, within a range that can be implemented industrially at a relatively low cost without causing an extreme decrease in properties. Preferably, the P content is between 0% and 0.01%.
[0029] S: 0.002% or less Since sulfur (S) significantly reduces hot workability, it is preferable to reduce its content as much as possible; however, extreme reduction leads to a significant increase in manufacturing costs. Therefore, the S content is set to 0.002% or less, which is a range that can be implemented industrially at a relatively low cost without causing an extreme decrease in properties. Preferably, the S content is 0% to 0.001%.
[0030] Al: 0-1% Although the Al content may be 0%, it is an element that acts as an oxygen absorber, and in order to obtain this effect, the Al content may be 0.001% or more. On the other hand, if the Al content exceeds 1%, the amount of oxides increases, and the toughness deteriorates. Therefore, the Al content should be 1% or less. The Al content is preferably 0.015 to 0.8%, and more preferably 0.025 to 0.7%.
[0031] V: 0~0.6% Although the V content may be 0%, it is an element that improves the strength of seamless steel pipes through precipitation strengthening, and in order to obtain this effect, the V content may be 0.01% or more. On the other hand, if the V content exceeds 0.6%, the toughness decreases. Therefore, the V content should be 0.6% or less. The V content is preferably 0.03 to 0.5%, and more preferably 0.07 to 0.45%.
[0032] Nb: 0~0.5% Nb may be present at 0%, but since it is an element that improves yield strength by precipitating as Nb carbide, the Nb content may be 0.01% or more in order to obtain this effect. On the other hand, an Nb content exceeding 0.5% leads to a decrease in toughness and resistance to sulfide stress cracking. Therefore, the Nb content should be 0.5% or less. The Nb content is preferably 0.07 to 0.4%, and more preferably 0.1 to 0.3%.
[0033] W: 0~3.50% While the W content may be 0%, it is an element that improves strength, and in order to obtain this effect, the W content may be 0.01% or more. On the other hand, if the W content is higher than 3.50%, the toughness deteriorates. Therefore, the W content should be 3.50% or less. The W content is preferably 0.05 to 0.3%, and more preferably 0.07 to 0.2%.
[0034] Co: 0~1.5% Co may be present at 0%, but since it is an element that improves SSC resistance, the Co content may be 0.01% or more in order to obtain this effect. On the other hand, if the Co content exceeds 1.5%, the hot workability decreases. Therefore, the Co content should be 0.01 to 1.5% or less. Preferably, the Co content is 0.03 to 0.15%, and more preferably 0.05 to 0.1%.
[0035] In the component composition, the remainder other than those mentioned above consists of Fe and unavoidable impurities. Examples of unavoidable impurities in the remainder include Sn, As, and Sb. However, as long as the effects of the present invention are not impaired, the content of Sn, As, and Sb may be 0.1% or less each.
[0036] The seamless steel pipe of the present invention is not particularly limited, but the pipe thickness (wall thickness) is preferably 15 mm or more, and more preferably 25 mm or more. Furthermore, the seamless steel pipe of the present invention is not particularly limited, but the pipe thickness (wall thickness) is preferably 45 mm or less, and more preferably 40 mm or less.
[0037] The seamless steel pipe of the present invention can be a stainless steel seamless steel pipe. Furthermore, the seamless steel pipe of the present invention can be a stainless steel seamless steel pipe for oil wells.
[0038] [Manufacturing method for seamless steel pipes and method for drilling rolled material] Figure 2 is a schematic diagram showing the Mannesmann-Mandrelmill process, one of the methods for manufacturing seamless steel pipes. Referring to Figure 2, an example of the manufacturing method for seamless steel pipes according to the present invention will be mainly described.
[0039] First, as shown in (1) in Figure 2, in the first step of the manufacturing method for seamless steel pipes, a rolled material (round steel billet) 1 having the above-mentioned component composition as an example is heated in a rotary heating furnace 11. Next, as shown in (2), in the second step, the rolled material (round steel billet) 1 heated in the rotary heating furnace 11 is subjected to perforation rolling by a piercer (PCM) 13 to obtain a raw pipe 3. Next, as shown in (3), in the third step, the raw pipe 3 is stretch-rolled by a mandrel mill (MDM) 15. This reduces the wall thickness of the raw pipe 3. Next, as shown in (4), in the fourth step, the raw pipe 3 that has been thinned by the mandrel mill 15 is heated again in a reheating furnace (WBF) 17. Next, as shown in (5), in the fifth step, the raw pipe 3 heated in the reheating furnace 17 is subjected to constant diameter rolling to a predetermined wall thickness and outer diameter by a hot stretch reducer (HSR) 19.
[0040] Circumferential shear strain applied to the center of the rolled material's thickness by perforated rolling: 0.48 or greater Here, we will explain the reason for specifying the circumferential shear strain generated during perforation rolling as shown in (2) in Figure 2. In this invention, it is necessary to set the circumferential shear strain applied to the center of the thickness of the rolled material by perforation rolling to 0.48 or higher. If the circumferential shear strain is less than 0.48, the ferrite grain size will become coarser, making it impossible to achieve the desired value for at least one of the strength or toughness. While no upper limit is specifically specified, from the viewpoint of the outer surface quality of seamless steel pipes, the circumferential shear strain is preferably 1.40 or less. That is, the circumferential shear strain is preferably 0.48 or more and 1.40 or less. More preferably, the circumferential shear strain is 0.48 or more and 1.30 or less, and even more preferably 0.48 or more and 1.10 or less. Even more preferably, it is 0.48 or more and 0.59 or less.
[0041] In this invention, it is possible to obtain excellent mechanical properties even when the reduction ratio is low, and the invention may be applied to product sizes with a reduction ratio of 50% or less. Here, the reduction ratio is expressed by equation (1) relating the circumferential cross-sectional area of the round steel billet (area of the cross-section perpendicular to the pipe axis of the round steel billet) and the circumferential cross-sectional area of the raw pipe after perforation and rolling (area of the cross-section perpendicular to the pipe axis of the raw pipe). Area reduction ratio (%) = 100 × (Circumferential cross-sectional area of round steel piece (mm²) 2 )-Circumferential cross-sectional area of the raw tube after perforation and rolling (mm 2 )) / Circumferential cross-sectional area of a round steel piece (mm 2 )···(1)
[0042] The circumferential shear strain applied to the center of the rolled material's thickness can be calculated using the finite element method after drilling and rolling.
[0043] The seamless steel pipe of the present invention is manufactured using the perforation rolling of the seamless steel pipe described above. In the manufacturing method of the seamless steel pipe of the present invention, conditions other than the circumferential shear strain applied to the center of the wall thickness of the rolled material are not particularly limited during perforation rolling. For example, the quenching conditions for the raw pipe obtained by perforation rolling may be a quenching temperature of 900 to 1150°C and a quenching time of 20 to 80 minutes, and the subsequent tempering conditions may be a tempering temperature of 450 to 700°C and a tempering time of 30 to 90 minutes. [Examples]
[0044] The embodiments of the present invention will be described in detail below. The present invention is not limited by the embodiments described below, and can be modified as appropriate within the scope that is consistent with the spirit of the present invention, and all of these modifications fall within the technical scope of the present invention. From the starting material obtained by preparing molten steel having the component composition shown in Table 1, a round steel billet with an outer diameter of 58 mm and a length of 250 mm was prepared. In the second step shown in Figure 2 (2), the raw pipe manufactured under the perforation rolling conditions shown in Table 1 was heat-treated under the conditions of quenching: 960°C × 30 minutes (water cooling) and tempering: 615°C × 70 minutes (air cooling) to obtain a seamless steel pipe.
[0045] To measure the microstructure of the obtained seamless steel pipes, first, a specimen for microstructure observation was taken from the center of the wall thickness (t / 2 position (t: wall thickness)) of the cross section perpendicular to the pipe axis (circumferential cross section of the raw pipe) of the seamless steel pipe. The collected sample (specimen for microstructure observation) was etched with Virela's reagent (a reagent prepared by mixing picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 10 ml, and 100 ml, respectively), and the microstructure at the center of the wall thickness was imaged using a scanning electron microscope (magnification: 1000x). The microstructure fraction (volume %) of the ferrite phase was calculated using an image analysis device. The X-ray diffraction specimens were then ground and polished so that the measurement surface was the cross-section perpendicular to the tube axis (circumferential cross-section of the original tube), and the amount of retained austenite (γ) was measured using the X-ray diffraction method. The amount of retained austenite is determined by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α, and is given by the following equation. γ(volume ratio)=100 / (1+(IαRγ / IγRα)) (Here, Iα:integrated intensity of α, Rα:crystallographic theoretical calculation of α, Iγ:integrated intensity of γ, Rγ:crystallographic theoretical calculation of γ) The conversion was performed using the following method. The fraction (volume fraction) of the tempered martensite phase was the remainder of the ferrite phase and retained austenite phase determined by the above measurement method.
[0046] The measurement area for the average ferrite particle size of the seamless steel pipe was defined as the area where the thickness is 10% of the wall thickness, with the center of the wall thickness of the seamless steel pipe as the center in the wall thickness direction. The average grain size of ferrite was determined to be 100 mm using backscattered electron diffraction (EBSD). 2 The crystal orientation was measured over a continuous region, and the average value (number mean) of ferrite crystals was calculated by defining grains with a crystal orientation difference of 15° or less as the same crystal grain.
[0047] For yield strength, API5CT arc-shaped tensile test specimens (see (1) in Figure 3) and V-notch test specimens (5 mm thick) (see (2) in Figure 3) were taken from seamless steel pipes and measured. Specifically, for the API5CT arc tensile test, the sampling start point was set at 50 mm in the axial direction from the trailing end in the rolling direction of the seamless steel pipe, and the sampling area was defined as extending to 200 mm in the axial direction of the pipe. Furthermore, for the V-notch test specimens (5 mm thick), three specimens were taken, each with a length of 55 mm in the axial direction of the pipe, starting at positions 100 mm, 155 mm, and 210 mm from the rear end in the rolling direction of the seamless steel pipe. Then, tensile tests were conducted in accordance with the API5CT standard to determine the strength. In addition, V-notch test specimens (5 mm thick) were taken from the obtained test specimen material in accordance with the JIS Z 2242 (2023) standard, with the longitudinal direction of the test specimen oriented in line with the axial direction of the pipe, and Charpy impact tests were performed. The tests were conducted at room temperature (25°C) to evaluate toughness. For the Charpy impact test, the numerical average of the toughness values of three test specimens was evaluated as the toughness value.
[0048] [Table 1]
[0049] As can be seen from Table 1, by satisfying the conditions of the present invention, it is possible to manufacture seamless steel pipes with excellent strength and toughness through fine graining, even with a low reduction ratio (50% or less). [Explanation of symbols]
[0050] 1 round steel strip 3. Raw tube 10 Seamless steel pipe 11 Rotary heating furnace 13 Piercer 15 Mandrel Mill 17 Reheating furnace 19 Hot Stretch Reducer X Test specimen for tissue observation
Claims
1. In mass%, C: 0.002-0.05%, Si: 0.05-0.6%, Mn: 0.1 to 0.5%, Cu: 0.9 to 3.5%, Ni: 3.0 to 6.0%, Cr: 16.5-19%, Mo: 1-4%, N: 0.01-0.2%, It contains, moreover, P: 0.02% or less, S: 0.002% or less, Al: 0-1%, V: 0-0.6%, Nb: 0 to 0.5%, W: 0-3.50%, Co: Contains 0-1.5%, The remainder has a component composition consisting of Fe and unavoidable impurities. In terms of volume ratio, More than 40% tempered martensite phase, A ferrite phase of 25% to 40%, It has a structure consisting of a residual austenite phase that is more than 10% but less than 25%, When crystal grains with a crystal orientation difference of 15° or less are defined as the same crystal grain, the average ferrite grain size is 35 μm or less. The yield strength is 758 MPa or higher. A seamless steel pipe with a toughness value of 60 J or more at 25°C.
2. A method for drilling a rolled material when manufacturing a seamless steel pipe as described in claim 1, A method for drilling a rolled material, wherein the circumferential shear strain applied to the center of the thickness of the rolled material by drilling and rolling is 0.48 or greater.
3. A method for manufacturing a seamless steel pipe, comprising manufacturing a seamless steel pipe using the method for drilling a rolled material described in claim 2.