JP6597038B2 - Cutting method for seamless steel pipe - Google Patents

Description

  The present invention relates to a seamless steel pipe cutting method, and more particularly to a thread cutting method for a seamless high alloy steel pipe.

  Seamless steel pipes used as oil well pipes are connected to each other and used for oil wells. Therefore, a male screw or a female screw is formed at both ends of the seamless steel pipe. Each screw is formed by thread cutting. Thread cutting is performed using a high-speed rotary lathe device. Thread cutting is performed by fixing one of a steel pipe or a cutting tool as a work material and rotating the other at a high speed. Specifically, the steel pipe is rotated at high speed around the pipe axis, and the cutting edge of the cutting tool is pressed against the surface of the steel pipe for processing. Alternatively, the steel pipe body is fixed, and the cutting edge of a single cutting tool or a plurality of cutting tools is rotated at high speed around the pipe axis while being in contact with the steel pipe surface.

  When the hardness of the seamless steel pipe is high, the cutting resistance during thread cutting increases, and the load on the cutting tool increases. Increasing the cutting speed further increases the load on the cutting tool. When the load applied to the cutting tool is too large, the shear stress of the seamless steel pipe during the thread cutting process exceeds the cutting edge strength of the cutting tool, and the cutting tool is lost. Therefore, the cutting speed of the thread cutting process needs to be set in an appropriate range.

  On the other hand, in recent years, along with the deepening of oil and natural gas drilling environment, oil well pipes are required to be further strengthened in strength and corrosion resistance. If steel contains alloy elements, such as Cr and Ni, the intensity of steel will increase. Therefore, high Cr content steel, nickel alloy, etc. are used as a material of the seamless steel pipe used as an oil well pipe. High Cr content steel is, for example, 13Cr steel and 17Cr steel. 13Cr steel is a martensitic stainless steel containing 12-14% Cr. 17Cr steel is a duplex stainless steel containing 16 to 18% Cr. For example, 17Cr steel is disclosed in JP2012-149317A (Patent Document 1), JP2005-336595A (Patent Document 2), JP2010-209402A (Patent Document 3), and International Publication No. 2010/050519. No. 4 (Patent Document 4).

  In the case of steel containing an alloy element such as Cr, the strength and hardness are increased, and cutting is difficult. When screw cutting is performed on difficult-to-cut materials that are difficult to cut, cutting resistance tends to increase. Therefore, the above problem is particularly likely to occur.

  By controlling the cutting speed, it is possible to suppress the load applied to the cutting tool even when screw cutting is performed on a difficult-to-cut material. For example, when thread cutting is performed on a seamless steel pipe containing an alloy element such as Cr, the cutting speed is reduced as the content of the alloy element such as Cr increases. Thereby, the load concerning a cutting tool can be suppressed.

For example, Japanese Patent Application Laid-Open No. 2005-313306 (Patent Document 5) proposes a method of threading a difficult-to-cut material under appropriate cutting conditions. In the steel pipe thread cutting method disclosed in Patent Document 5, the steel pipe is thread-cut using a chaser provided with a plurality of blade portions for thread cutting. In this steel pipe thread cutting method, the average cutting area of the blade is S (mm ² ), the width of the blade for finishing cutting is B (mm), the height of the blade for finishing cutting is H (mm), cutting When the speed is V (m / min) and the Cr content of the steel pipe to be cut is C (%), cutting is performed under conditions that satisfy the following formula.
S ≦ 5.68 × 10 ⁻¹ × B-6.76 × 10 ⁻² × H−1.12 × 10 ⁻⁵ × V ² −1.68 × 10 ⁻² × C−5.82 × 10 ⁻¹

  By setting the cutting conditions according to the above formula, even when cutting steel (13Cr steel) with a Cr content of 9-18% and a Ni content of 0.5% or less, the service life of the tether blade can be improved This is described in Patent Document 5.

  JP 2007-268656 A (Patent Document 6), JP 2005-118925 A (Patent Document 7), JP 2001-293601 A (Patent Document 8), JP 2014-144533 A (Patent Document 9). ) And Japanese Patent Laid-Open No. 2005-281779 (Patent Document 10) propose a cutting tool that can improve the tool life even for difficult-to-cut materials. These documents describe specific examples of cutting conditions when cutting a difficult-to-cut material.

  Patent Document 6 and Patent Document 7 describe specific examples of cutting conditions in thread cutting. Patent Document 6 describes that a 5% Ni-containing alloy (S13Cr steel) is thread-cut at a cutting speed of 90 m / min. Patent Document 7 describes that an API standard L80 grade steel pipe is thread-cut at a cutting speed of 100 m / min.

  Patent Documents 8 to 10 describe specific examples of cutting conditions in cutting other than thread cutting. Cutting other than screw cutting is, for example, cutting of seamless steel pipe (Japanese Patent No. 4385349 (Patent Document 11)). Patent Document 8 describes cutting with respect to SCM435 (chromium molybdenum steel) at a cutting speed of 250 m / min. Patent Document 9 describes that Inconel 718 is cut at a cutting speed of 200 m / min. In Patent Document 10, in mass%, C: 0.46%, Si: 0.27%, Mn: 0.99%, P: 0.018%, S: 0.068%, Al: 0.021 % And N: 0.008%, with the balance being described as being cut at a cutting speed of 200 m / min with respect to a sample made of Fe and impurities.

JP 2012-149317 A JP 2005-336595 A JP 2010-209402 A International Publication No. 2010/050519 JP-A-2005-313306 JP 2007-268656 A JP 2005-118925 A JP 2001-293601 A JP 2014-144533 A Japanese Patent Laid-Open No. 2005-281779 Japanese Patent No. 4385349

  However, even if the methods and cutting conditions disclosed in the above-mentioned patent documents are used, depending on the steel type, an increase in cutting resistance during thread cutting, occurrence of adhesion, or a decrease in production efficiency of seamless steel pipes may occur. There is a case.

  An object of the present invention is to provide a seamless steel pipe cutting method capable of improving production efficiency while suppressing cutting resistance and adhesion during thread cutting.

The seamless steel pipe cutting method according to the present embodiment includes a step of preparing a base material and a step of cutting. In the step of preparing the base material, a base material having a structure containing Cr: 16.0 to 18.0% by mass% and including a ferrite phase, a martensite phase, and a retained austenite phase is prepared. In the step of cutting, the base material is cut using a cutting tool. In the cutting process, when the contact area (mm ² ) between the cutting tool and the substrate is S, the cutting speed is as follows. When 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is 150 to 350 m / min, and when 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min, and 4.0 < In the case of S ≦ 6.0 mm ² , the cutting speed is 150 to 200 m / min.

  The seamless steel pipe cutting method according to the present embodiment can improve production efficiency while suppressing cutting resistance and adhesion during thread cutting.

FIG. 1 is a diagram illustrating an example of a cutting process. FIG. 2 is a diagram illustrating an example of a cutting tool. FIG. 3 is an enlarged view of the blade portion of the cutting tool. FIG. 4 is a diagram showing the relationship between the cutting speed and the cutting resistance. FIG. 5 is an SEM observation of the cutting tool surface after thread cutting. FIG. 6 is a diagram showing the relationship between the cutting speed and the amount of adhesion.

  Hereinafter, this embodiment will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  The inventors have made various studies on cutting conditions for seamless steel pipes. As a result, the following knowledge was obtained.

  Generally, a seamless steel pipe containing an alloy element such as Cr increases in hardness as the alloy element content increases. Therefore, when content of alloy elements, such as Cr, is high, it is necessary to slow down the cutting speed in screw cutting, and to suppress the load concerning a cutting tool.

  However, a seamless steel pipe using 17Cr steel (mass%, Cr: containing 16.0 to 18.0%, and having a structure including a ferrite phase, a martensite phase and a retained austenite phase) is screwed. In the case of cutting, the cutting resistance is low even when screw cutting is performed at a higher cutting speed than before. Therefore, the load applied to the cutting tool can be suppressed even when the cutting speed is increased. Increasing the cutting speed improves production efficiency.

  During thread cutting, heat is generated due to friction between the seamless steel pipe and the cutting tool. By this frictional heat, a local liquid layer is formed on the surface layer of the seamless steel pipe and the surface layer of the cutting tool, and a film having lubricity (referred to as a component cutting edge) is formed.

  As the cutting speed in thread cutting increases, the frictional heat increases. Therefore, generally, the higher the cutting speed, the larger the amount of liquid layer on the surface layer of the seamless steel pipe. When the cutting speed is too high, the transfer component generated by melting the seamless steel pipe is excessively supplied to the constituent cutting edges. As a result, adhesion occurs. If the amount of adhesion is too large, the design dimensions of the cutting tool will be incorrect, and the surface roughness of the work surface of the seamless steel pipe will increase. Therefore, it is necessary to suppress the adhesion by slowing the cutting speed. However, in the case of the 17Cr steel described above, the amount of adhesion decreases if the cutting speed during thread cutting is increased. Therefore, the amount of adhesion is reduced by increasing the cutting speed.

  Generally, thread cutting includes a plurality of cutting stages (cutting paths). In the initial stage of screw cutting, screw cutting called roughing is performed. Roughing is thread cutting for the purpose of increasing the amount of cutting rather than the dimensional accuracy after the material is machined. In the final stage of thread cutting, thread cutting called finishing is performed. The finishing process is a thread cutting process that requires dimensional accuracy according to a desired quality and a machined surface roughness of the seamless steel pipe. Thread cutting includes intermediate processing between roughing and finishing as required.

  In the initial stage of thread cutting (rough machining), the contact area between the cutting tool and the seamless steel pipe is relatively large. If the contact area is large, the cutting resistance tends to increase. On the other hand, in the final stage of thread cutting (finishing), the contact area between the cutting tool and the seamless steel pipe is relatively small, and the cutting resistance is also small. The cutting speed needs to be appropriately set in consideration of the size of the contact area between the cutting tool and the seamless steel pipe.

The seamless steel pipe cutting method according to the present embodiment, completed based on the above knowledge, includes a step of preparing a base material and a step of cutting. In the step of preparing the base material, a base material having a structure containing Cr: 16.0 to 18.0% by mass% and including a ferrite phase, a martensite phase, and a retained austenite phase is prepared. In the step of cutting, the base material is cut using a cutting tool. In the cutting process, when the contact area (mm ² ) between the cutting tool and the substrate is S, the cutting speed is as follows. When 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is 150 to 350 m / min, and when 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min, and 4.0 < In the case of S ≦ 6.0 mm ² , the cutting speed is 150 to 200 m / min.

  The seamless steel pipe cutting method according to the present embodiment can improve production efficiency while suppressing cutting resistance and adhesion during thread cutting.

  Hereinafter, the cutting method of the seamless steel pipe of this embodiment will be described in detail.

[Manufacturing process]
The seamless steel pipe cutting method according to the present embodiment includes a step of preparing a base material and a step of cutting.

[Preparation process]
In the preparation step, a substrate is prepared. The substrate is 17Cr steel. Below, an example of the chemical composition and structure of 17Cr steel will be described.

[Base material]
The 17Cr steel as a base material contains 16.0 to 18.0% of chromium (Cr), and its microstructure includes a ferrite phase, a martensite phase, and a retained austenite phase. “%” For chemical composition means “mass%”.

[Chemical composition of substrate]
Cr: 16.0 to 18.0%
Chromium (Cr) increases the hardenability of the substrate and increases the strength of the substrate. Cr further enhances the corrosion resistance of the substrate, particularly the SSC resistance. If the Cr content is too low, the above effect cannot be obtained. On the other hand, if the Cr content is too high, the hot workability of the substrate is lowered. Therefore, the Cr content is 16.0 to 18.0%. A preferred lower limit of the Cr content is 16.5%. The upper limit with preferable Cr content is 17.5%.

  The chemical composition of 17Cr steel contains, for example, the following elements other than Cr.

C: 0.03% or less Carbon (C) stabilizes the austenite phase. C further increases the strength of the substrate by solid solution strengthening. However, if the C content of the substrate is too high, carbides are precipitated excessively and the corrosion resistance of the substrate is lowered. Therefore, the C content is preferably 0.03% or less. Considering the decarburization cost, the lower limit of the preferable C content is 0.001%. A more preferable upper limit of the C content is 0.01%.

Si: 0.50% or less Silicon (Si) deoxidizes the base material. Si further increases the strength of the substrate. If the Si content is too low, this effect may not be obtained. On the other hand, if the Si content is too high, the SSC resistance and hot workability of the base material are lowered. Therefore, the Si content is preferably 0.50% or less. The lower limit of the preferred Si content is 0.05%.

Mn: 2.0% or less Manganese (Mn) deoxidizes the base material. Mn further increases the strength of the substrate. However, if the Mn content is too high, Mn segregates at grain boundaries together with impurity elements such as phosphorus (P) and sulfur (S). In this case, the SSC resistance and toughness of the substrate are reduced. Therefore, the Mn content is preferably 2.0% or less. The minimum of preferable Mn content is 0.05%. A more preferable upper limit of the Mn content is 0.50%.

Cu: 1.0 to 3.5%
Copper (Cu) increases the strength of the substrate. Cu further enhances the SSC resistance of the substrate. If the Cu content is low, these effects may not be obtained. On the other hand, if Cu content is too high, the hot workability of a base material will fall. Therefore, the Cu content is preferably 1.0 to 3.5%. A more preferable lower limit of the Cu content is 2.0%. A more preferable upper limit of the Cu content is 3.0%.

Ni: 2.5-6.0%
Nickel (Ni) increases the corrosion resistance of the substrate. On the other hand, if the Ni content is too high, the production cost increases. Therefore, the Ni content is preferably 2.5 to 6.0%. A more preferable lower limit of the Ni content is 4.0%. A more preferable upper limit of the Ni content is 5.5%.

Mo: 1.0-3.5%
Molybdenum (Mo) increases the strength and corrosion resistance of the substrate. If the Mo content is too low, these effects may not be obtained. On the other hand, if the Mo content is too high, the hot workability of the substrate decreases. If the Mo content is too high, an unnecessary cost increase is caused. Therefore, the Mo content is preferably 1.0 to 3.5%. A more preferable lower limit of the Mo content is 2.0%. A more preferable upper limit of the Mo content is 3.0%.

  In an example of the chemical composition of 17Cr steel, the balance is Fe and impurities. The impurity here refers to an ore and scrap used as a raw material of steel, or an element mixed from the environment of the manufacturing process. The 17Cr steel may contain the following selective elements instead of a part of Fe.

V: 0 to 0.1%
Vanadium (V) is a selective element. V combines with carbon and nitrogen to form a carbide, nitride or carbonitride. These carbides, nitrides and carbonitrides precipitate and strengthen the substrate. V further increases the SSC resistance of the substrate. However, if the V content is too high, the toughness of the substrate decreases. Therefore, the V content is preferably 0 to 0.1%. The minimum of preferable V content in the case of containing is 0.005%.

W: 0 to 3.0%
Tungsten (W) is a selective element. W increases the SCC resistance in a high temperature environment. If W is contained even a little, the above effect can be obtained to some extent. On the other hand, if the W content exceeds 3.0%, the ferrite fraction tends to be excessive, and high strength may not be obtained. Therefore, the W content is preferably 0 to 3.0%. The minimum of preferable W content in the case of containing is 0.05%. A more preferable upper limit of the W content is 2.0%.

[Base material structure]
As described above, the 17Cr steel as the base material has a structure including a ferrite phase, a martensite phase, and a retained austenite phase. The base material is preferably in volume%, the ferrite phase is 10 to 15%, the residual austenite phase is 15% or less, and the balance is the martensite phase. When it has the above-mentioned structure, the yield strength of the base material increases.

[Manufacturing method of substrate]
A substrate having the above chemical composition is produced by a well-known method. The base material is a seamless steel pipe. The manufacturing method of a base material is not specifically limited. For example, a molten metal having the above-described chemical composition is manufactured. Subsequently, a slab (bloom) is manufactured by continuous casting, and the resulting slab is rolled into a billet. The billet is piercing-rolled and stretch-rolled using a Mannesmann-mandrel mill method. After producing the molten metal, an ingot may be produced by an ingot-making method. Instead of the Mannesmann-Mandrel Mill method, the substrate may be produced by a hot extrusion method.

  Well-known heat processing is implemented with respect to the obtained base material. The heat treatment includes, for example, quenching and tempering. For example, the quenching is performed by heating to 850 ° C. to 1050 ° C. and then cooling with water. Tempering after quenching is performed, for example, at a tempering temperature of 510 ° C to 650 ° C. A base material can be manufactured according to the above process.

[Cutting process]
In the cutting process, thread cutting is performed on the base material obtained by the above-described manufacturing method using a cutting tool. Thread cutting can be performed using a CNC lathe device that can control the cutting speed, cutting depth, feed amount, and the like with a computer. FIG. 1 is a diagram illustrating an example of a cutting process. With reference to FIG. 1, the substrate 1 rotates in the direction X around the axis of the substrate 1. The cutting tool 2 is fixed and pressed against the surface of the substrate 1. Thread cutting is performed by advancing the cutting tool 2 in the axial direction Y of the substrate 1 while rotating the substrate 1. The thread cutting process can also be performed by fixing the substrate 1 and rotating the cutting tool 2. The cutting process may be completed at one time or may include a plurality of stages (cutting passes). When a plurality of stages (cutting paths) are included, the cutting process includes an initial stage (rough machining) and a final stage (finishing process). As necessary, the cutting process includes an intermediate stage (intermediate process).

[Cutting tool 2]
An example of the cutting tool 2 is shown in FIG. Referring to FIG. 2, the cutting tool 2 is a thread cutting chether having a plurality of blade portions 3. The cutting tool 2 includes a cemented carbide. The cemented carbide contains tungsten carbide (WC) as a main component and contains cobalt (Co) as a sintering aid. The cemented carbide may contain titanium carbide (TiC) and tantalum carbide (TaC) in addition to WC and Co. A hard protective film is formed on the outermost layer of the cutting tool 2. The hard protective film includes a nitride of titanium (Ti). The thickness of the hard protective film is a maximum of 5 μm.

[Contact area]
The contact area between the cutting tool 2 and the substrate 1 is S (mm ² ). FIG. 3 is an enlarged view of the blade portion 3 of the cutting tool 2. Referring to FIG. 3, the contact area S is the total contact amount of the cutting tool 2 and the base material 1 obtained from the shape of the thread of the cutting tool 2 and the depth of cut (the depth at which the rake face is difficult to squeeze the base material 1). It is an area. That is, the total area of contact between the cutting tool 2 and the substrate 1 on the rake face 4, the flank face 5, the side face 6, and the side face 7 is defined as a contact area S.

It is preferable to appropriately change the contact area S as the cutting process proceeds from the initial stage to the final stage. In the initial stage (rough machining), it is preferable to increase the contact area S in order to increase the amount of cutting. In the final stage (finishing), it is preferable to reduce the contact area S in order to increase the dimensional accuracy and reduce the surface roughness of the work surface. The contact area S in the initial stage is, for example, 4.0 <S ≦ 6.0 mm ² . The contact area S at the final stage is, for example, 0.2 ≦ S ≦ 2.0 mm ² . When the cutting process includes an intermediate stage, the contact area S in the intermediate stage is, for example, 2.0 <S ≦ 4.0 mm ² . It is preferable that the contact area S described above be sequentially reduced as the cutting process proceeds. However, the contact area S in each cutting process is not limited to a specific value and can be appropriately selected depending on the required dimensional accuracy of the screw.

[Cutting speed]
The cutting speed is set to an appropriate value according to the contact area S. When the contact area S is 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is 150 to 350 m / min, and when 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min, and In the case of 4.0 <S ≦ 6.0 mm ² , the cutting speed is 150 to 200 m / min. The base material 1 targeted by the present invention has low cutting resistance and little adhesion even when the cutting speed is increased. Therefore, by increasing the cutting speed, it is possible to improve production efficiency while suppressing cutting resistance and adhesion.

If the cutting speed is less than 150 m / min, the production efficiency cannot be improved. On the other hand, when the cutting speed is faster than the above range, the cutting resistance becomes too high. Specifically, when the contact area S is 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is higher than 350 m / min, and when 2.0 <S ≦ 4.0 mm ² , the cutting speed is 300 m. This is a case where the cutting speed is faster than 200 m / min, and a case where the cutting speed is higher than 200 m / min when 4.0 <S ≦ 6.0 mm ² . In this case, the cutting resistance becomes too high. Therefore, the shear stress of the base material 1 at the time of thread cutting may exceed the cutting edge strength of the cutting tool 2 in some cases. In this case, a cutting edge defect of the cutting tool 2 occurs.

Production efficiency can be increased by increasing the cutting speed. Therefore, in the case of emphasizing production efficiency in the case of 0.2 ≦ S ≦ 2.0mm ^2, cutting speed 250 to 350 / min, for 2.0 <S ≦ 4.0mm ^2, the cutting speed 200 In the case of ˜300 m / min, 4.0 <S ≦ 6.0 mm ² , the cutting speed is preferably 180 to 200 m / min.

[Cutting resistance]
The shearing stress applied to the cutting edge of the cutting tool 2 is the cutting resistance. Referring to FIG. 1, the component force in the rotation direction of the base material 1 is the main component force Fc, the component force in the feed direction of the cutting tool 2 is the feed force Ft, and the normal line of the base material 1 surface to be pressed against the cutting tool 2 is referred to. The component force in the direction is defined as the back component force Fn. The cutting resistance R is a vector sum of the component forces in these three directions expressed by the equation (1).
R ² = Fc ² + Fn ² + Ft ² (1)
The lower the cutting resistance R, the easier the substrate 1 is cut. On the contrary, the higher the cutting resistance R, the harder the substrate 1 is cut. In the seamless steel pipe cutting method according to this embodiment, the cutting resistance R is low even when the cutting speed in thread cutting is increased.

[Other cutting condition settings]
Thread cutting is usually performed by a plurality of cutting passes (rough machining-(intermediate machining)-finishing). Setting of the number of cutting passes, the amount of cutting and the number of rotations in each cutting pass is performed by input to a computer program. As the computer controller, for example, a controller made by FANUC (model number; FANUC Series Oi-TC) is used.

  In general, the depth of cut is gradually reduced from roughing to finishing. For example, the depth of cut is set to 0.5 mm to 1.0 mm during rough machining, 0.2 mm or less during finishing, and 0.2 mm to 0 during intermediate machining. .5mm is set. The contact area is determined by the depth of cut.

  The number of rotations refers to the rotation speed (rpm) of the substrate 1 or the cutting tool 2. The rotational speed is obtained from the cutting speed and the outer diameter of the substrate 1. In other words, the cutting speed can be calculated from the number of rotations and the outer diameter of the substrate 1. The number of rotations (or cutting speed) can be changed in stages from rough cutting to finishing according to the depth of cut (or contact area). That is, the number of revolutions can be increased (that is, the cutting speed can be increased) in accordance with the reduction in the amount of cut (ie, the reduction in contact area) from roughing to finishing.

The number of rotations (or cutting speed) can be set to be the same for all cutting passes or a plurality of cutting passes of all cutting passes from roughing to finishing, because of easy setting. When the cutting speed is the same, the cutting resistance increases as the cutting area increases. Accordingly, the rotation speed when the rotation speed is the same in a plurality of cutting passes is set according to the contact area in the cutting pass having the largest cutting depth (the largest contact area) among the plurality of cutting passes. . For example, when the rotational speed is made the same in all cutting passes, the amount of cutting is the largest during rough machining and the contact area is large. Therefore, the cutting speed is set according to the contact area during rough machining, and the rotation speed is set from the cutting speed and the outer diameter of the substrate 1. Thus, the rotation speed (or cutting speed) when the rotation speed (or cutting speed) is not changed in a plurality of cutting passes is an appropriate rotation speed (the cutting speed having the largest contact area among the plurality of cutting passes ( Or cutting speed). That is, among the plurality of cutting passes having the same rotation speed (or cutting speed), when the contact area in the cutting pass having the largest contact area is 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is 150 to 350 m. / Min (preferably 250 to 350 m / min), 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min (preferably 200 to 300 m / min), 4.0 <S ≦ In the case of 6.0 mm ² , the cutting speed is set to 150 to 200 m / min (preferably 180 to 200 m / min), and the rotation speed is calculated from the cutting speed and the outer diameter of the workpiece (base material 1). .

  The distance traveled by the cutting tool 2 per one rotation of the substrate is referred to as the feed amount. The feed amount is appropriately set according to the target screw shape. The general shape of the screw provided at the end of the steel pipe used as the oil well pipe is determined, and the feed amount in this case is in the range of 3.1 to 6.5 mm / rotation. When the feed amount is within this range, the cutting speed defined in the present invention can be applied.

[Preparation process]
Two types of base materials were prepared. First, two types of molten metal having chemical compositions shown in Table 1 were produced. Blooms were produced from each molten metal by continuous casting, and the resulting blooms were subjected to partial rolling to produce billets. The billet was subjected to piercing and stretching using the Mannesmann-mandrel mill method. The obtained base material was quenched and tempered to obtain a base material A (17Cr steel) and a base material B (13Cr steel).

[Vickers hardness measurement test]
The Vickers hardness of the base material A and the base material B was implemented based on JIS Z2244 (2009). The test load was 2.94 N (0.3 kgf). The Vickers hardness of the base material A was 345 Hv _0.3 or more. The Vickers hardness of the base material B was 239 Hv _0.3 or more.

[Cutting process]
The obtained base material A and base material B were subjected to thread cutting using a lathe machine for rotating the work steel material. The lathe device for rotating the work steel material was a CNC lathe device. Specifically, the base material was fixed to the headstock, and the cutting tool was mounted on a jig called a cutting holder. The cutting tool was a cutting tool provided with a hard protective layer containing Ti nitride on the outermost layer of a cemented carbide composed of WC and Co.

[Cutting resistance measurement test]
Cutting resistance in each contact area was measured. The above-mentioned cutting holder was attached to a dynamometer (mechanical strain gauge) of a CNC lathe device, and thread cutting was performed under the conditions shown in Table 2 using the CNC lathe device. At this time, the feed amount was 6.0 mm / rotation. Mechanical strain was measured using a dynamometer. The mechanical strain is replaced with a piezoelectric signal with a correction value for each dynamometer, and the main component force Fc (component force in the tool rake face direction), feed component force Ft (component force in the tool feed direction), and back component force Fn ( It was measured as the component force in the tool pushing direction. The vector sum (the resultant force) of the obtained main component force Fc, feed component force Ft, and back component force Fn was calculated by the above-described equation (1) and used as the cutting force. The results are shown in Table 2 and FIG. FIG. 4 is a diagram showing the relationship between the cutting speed and the cutting resistance when the contact area is 2.0 mm ² . For steel type B, the test was stopped because the measurement limit of the dynamometer was exceeded at a cutting speed of 300 m / min or more.

[Adhesion amount measurement test]
The relationship between the cutting speed and the amount of adhesion in thread cutting was investigated. Specifically, using the base material A, thread cutting including roughing, intermediate processing and finishing was performed. Cutting conditions were as follows. Roughing: contact area 6.0 mm ² , intermediate processing: contact area 4.0 mm ² , finishing: contact area 2.0 mm ² . The cutting speed was set to the same speed through roughing to finishing, and was set as shown in Table 3 to perform thread cutting. Next, the cutting edge part of the cutting tool after thread cutting was analyzed using SEM-EDS. FIG. 5 is an SEM observation of the cutting tool surface after thread cutting. With reference to FIG. 5, it analyzed using EDS with respect to the blade edge | tip part 10 of a cutting tool. Specifically, the measurement was performed using a scanning electron microscope with an energy dispersive X-ray analyzer (JSM-7100 type FESEM, manufactured by JEOL Ltd.). Arbitrary five places were observed with the scanning electron microscope, and the composition was analyzed. The electron beam of accelerating voltage: 15 kV, irradiation current: maximum 200 nA was irradiated at a measurement magnification of 1,000 times, and the X-ray intensity of kα rays was measured. Based on the X-ray intensity of each element, the content (% by mass) of each metal element was calculated and used as a measured value. The mass% of Fe, Cr, and Ti per 1 mm ^{2 of} contact area was calculated as an average value measured at five locations. The results are shown in Table 3 and FIG.

[Evaluation results]
Using the base material A, when the contact area S is 2.0 mm ² , the cutting speed is 150 to 350 m / min, 4.0 mm ² , the cutting speed is 150 to 300 m / min, and 6.0 mm ² , When screw cutting was performed at a cutting speed of 150 to 200 m / min, the value of the cutting resistance (N) was 2000 N or less. Since the cutting resistance increases as the contact area increases, when thread cutting is performed at an appropriate cutting speed using the substrate A, specifically, the contact area S is 0.2 ≦ S ≦ 2.0 mm ² . In this case, when the cutting speed is 150 to 350 m / min, 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min, and when 4.0 <S ≦ 6.0 mm ² , the cutting speed is 150 to When screw cutting is performed at 200 m / min, the value of the cutting resistance (N) is 2000 N or less. The substrate A had a Vickers hardness higher than that of the substrate B. Nevertheless, the cutting resistance of the base material A was lower than that of the base material B at any cutting speed.

  Referring to Table 3 and FIG. 6, when thread cutting is performed using the base material A, the ratio of the amount of Fe and Cr transferred from the base material to the cutting tool decreases as the cutting speed increases. did. On the other hand, the proportion of Ti amount, which is a component of the protective coating on the cutting tool surface, increased. Therefore, the amount of adhesion was suppressed as the cutting speed increased.

On the other hand, when the cutting speed of the thread cutting process using the substrate A is not appropriate, specifically, when the contact area S is 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is higher than 350 m / min. When 2.0 <S ≦ 4.0 mm ² , the cutting speed is 200 m / min when the cutting speed is higher than 300 m / min and when 4.0 <S ≦ 6.0 mm ^2. At higher speeds, the value of cutting resistance (N) exceeds 2000N.

Although the Cr content of the base material B is 13% and is lower than the base material A, it is 200 m even when the contact area S is 2.0 mm ² when threaded using the base material B. The cutting resistance exceeded 2000 N at a cutting speed of 1 min / min or more.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

1 Base material 2 Cutting tool

Claims (1)

A seamless steel pipe cutting method,
A step of preparing a base material having a structure containing Cr: 16.0 to 18.0% , Ni: 2.5 to 6.0%, and containing a ferrite phase, a martensite phase, and a retained austenite phase. When,
A step of cutting the base material using a cutting tool,
The method of cutting a seamless steel pipe, wherein, in the step of cutting, when the contact area (mm ² ) between the cutting tool and the substrate is S, the cutting speed is as follows.
In the case of 0.2 ≦ S ≦ 2.0 mm ² , the cutting speed is 150 to 350 m / min,
When 2.0 <S ≦ 4.0 mm ² , the cutting speed is 150 to 300 m / min, and
In the case of 4.0 <S ≦ 6.0 mm ² , the cutting speed is 150 to 200 m / min.