JP7559728B2 - Seamless steel pipe and method for manufacturing steel pipe - Google Patents

Description

The present invention relates to a steel pipe that has high axial tensile yield strength, a small difference between axial tensile yield strength and axial compressive yield strength, and excellent fatigue properties, and a method for manufacturing the same.

Cold rolling of steel pipes includes cold drawing (cold drawing, punch rolling) and cold pilger rolling. In this cold rolling, the steel pipe is strained by elongating it in the axial direction and reducing its wall thickness, which improves the axial yield strength of the steel pipe, which is one of its mechanical properties.
Cold rolling is carried out in the manufacture of steel pipes used in applications where a high tensile load is applied in the axial direction of the pipe. For example, Patent Document 1 discloses a method for manufacturing a pipe having improved fatigue strength by cold drawing.

Patent No. 4285053

However, the technology described in Patent Document 1 has a problem with respect to fatigue characteristics when a load is applied by compressing the tube in the axial direction.
That is, cold rolling such as cold drawing and cold pilger rolling, which stretches the tube in the axial direction, improves the yield strength in the axial direction of the tube and also improves the fatigue strength in the axial direction of the tube. On the other hand, the load generated by compression in the axial direction of the tube is significantly inferior to the yield strength characteristics in the tensile direction in the axial direction due to the Bauschinger effect. Fatigue strength is highly correlated with the tensile strength of the material. In other words, these methods of improving the strength of steel tubes using cold rolling have the problem that the yield strength when compressed in the axial direction of the tube (compressive yield strength in the axial direction of the tube) is lower than the yield strength due to tension (tensile yield strength in the axial direction of the tube), and the fatigue characteristics (compressive fatigue characteristics) when a load is generated by compression in the axial direction of the tube are also inferior.

Steel pipes in which the tensile yield strength in the axial direction has been increased using conventional cold rolling are effective in improving fatigue strength (tensile fatigue strength) under repeated loads in the tensile direction only, but are weaker against loads in the compressive direction, where the yield strength is relatively poor.

The load that affects the fatigue properties of steel pipes is rarely axial tension alone, but rather a combination of axial tension and axial compression.
In other words, the conventional method of increasing the strength of steel pipes by using cold drawing or cold pilger rolling is effective in improving the tensile yield strength in the pipe axis direction (tensile yield strength in the pipe axis direction) and the fatigue strength under a tensile load only in the pipe axis direction (tensile fatigue strength), but it is difficult to increase the yield strength when compressed in the pipe axis direction (compressive yield strength in the pipe axis direction) and the fatigue strength (compressive fatigue strength) compared to the tensile yield strength in the pipe axis direction and the fatigue strength obtained under a tensile load in the pipe axis direction.

For the above reasons, the conventional method of increasing the strength of steel pipes by cold rolling cannot solve the problem that the mechanical properties (axial compressive yield strength and compressive fatigue strength) when compressed in the axial direction are inferior to the axial tensile yield strength and tensile fatigue strength. Applications requiring excellent mechanical properties when compressed in the axial direction include those in which a compressive load is applied in the axial direction, such as wheel axles, where the pipe is repeatedly subjected to bending loads in the axial direction (where a compressive load is applied in the axial direction on the inner diameter side of the bending curvature).
Furthermore, when steel pipes are repeatedly connected with screws, a compressive load is applied to the threaded portion when the pipes are fastened, and a tensile load is applied when the pipes are used after fastening.
Furthermore, when used as steel pipes for hot wells or oil well drilling, repeated compressive and tensile loads are generated in the axial direction of the pipe due to thermal expansion caused by high temperatures of geothermal energy and thermal contraction when the well cools down.
In addition, when used as piping in chemical plants or for burying carbon dioxide underground for CCS (Carbon dioxide Capture and Storage) or CCUS (Carbon dioxide Capture, Utilization and Storage), temperature changes and internal and external pressure changes occur due to pressure fluctuations in the fluid inside the pipe, and thermal stress and pressure cause repeated tensile and compressive loads in the axial direction of the pipe.
In these operating environments where repeated tension-compression fatigue loads are applied, sufficient fatigue strength cannot be obtained with steel pipes having low axial compressive yield strength obtained by conventional cold rolling.

The present invention was made in consideration of the above circumstances, and aims to provide a steel pipe that has high axial tensile yield strength, a small difference between axial tensile yield strength and compressive yield strength, and excellent fatigue properties, and a manufacturing method thereof.

Here, a high axial tensile yield strength means that the axial tensile yield strength is 400 MPa or more, and a small difference between the axial tensile yield strength and the compressive yield strength means that the axial compressive yield strength/axial tensile yield strength is in the range of 0.85 to 1.15.
"Excellent fatigue properties" means that the same steel pipe material has a higher fatigue strength than a steel pipe manufactured by cold drawing or cold pilgering.

As shown in Patent Document 1, cold drawing or cold pilger rolling has been conventionally used as a cold rolling method for improving the axial strength, which is the most important mechanical property of a steel pipe. The present inventors have noticed that the direction in which plastic strain is imparted to the steel pipe by these cold working methods is the cause of the problem.
In other words, in cold drawing and cold pilger rolling, plastic strain is applied to the steel pipe in the cold state, and the steel pipe is strengthened by dislocation strengthening. The plastic strain generated in cold drawing and cold pilger rolling causes diameter reduction or pipe expansion deformation on the outer periphery of the pipe.
In addition, in the thickness direction of the pipe, a decrease or increase in thickness occurs due to plastic strain.
On the other hand, in the axial direction of the tube, plastic strain is imparted only in the elongation direction.
In metal materials, the Bauschinger effect generally occurs at the end of forming, which means that the material becomes weak against a load in the opposite direction to the direction in which plastic strain is applied. Therefore, in conventional cold rolling such as cold drawing and cold pilger rolling, the axial compressive strength of the tube inevitably decreases due to the Bauschinger effect.

The inventors therefore conducted extensive research into the direction of plastic strain in the axial direction of steel pipes, in contrast to the conventional cold rolling method for steel pipes, which only caused stretching in the axial direction. As a result, they came up with a method for increasing the axial compressive yield strength and associated fatigue strength of steel pipes by cold working that does not rely on plastic strain caused by stretching in the axial direction.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.01-1.15%,
Si: 0.01-2.50%,
Mn: 0.01 to 2.50%,
N: 0.001 to 0.050%, with the balance being Fe and unavoidable impurities;
The steel has a structure having a ferrite phase and a retained austenite phase of 10% or less in area ratio,
The axial tensile yield strength is 400 MPa or more,
A steel pipe having a ratio of compressive yield strength in the pipe axis direction to tensile yield strength in the pipe axis direction of 0.85 to 1.15.
[2] The composition further includes, in mass%, Cr: 2.00% or less;
Mo: 2.00% or less.
[3] The component composition further includes, in mass%, Ti: 0.50% or less;
Al: 0.30% or less,
V: 0.55% or less,
The steel pipe according to the above [1] or [2], containing one or more selected from the following: Nb: 0.75% or less.
[4] The component composition further includes, in mass%, Ni: less than 2.5%;
W: less than 1.0%
Cu: less than 2.5%
B: 0.010% or less,
Zr: 0.10% or less,
Ca: 0.010% or less,
Ta: 0.01% or less,
REM: 0.10% or less,
Mg: 0.10% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
The steel pipe according to any one of the above [1] to [3], containing one or more selected from the following: Ag: 0.30% or less.
[5] A method for producing a steel pipe according to any one of [1] to [4] above, comprising bending and unbending a steel pipe material in a pipe circumferential direction to obtain a steel pipe.
[6] A method for manufacturing a steel pipe as described in [5] above, in which, after bending and unbending in the circumferential direction of the pipe, a heat treatment is performed in which the holding temperature is 300°C or less and the holding time is 300 seconds or more and less than 3600 seconds, followed by air cooling.

The present invention provides a steel pipe that has high axial tensile yield strength, a small difference between axial tensile yield strength and compressive yield strength, and excellent fatigue properties, and a method for manufacturing the same.

FIG. 1 is a schematic diagram showing a method of cold bending and unbending in the circumferential direction of a pipe.

The present invention is described below.

The steel pipe of the present invention has a composition, by mass%, of C: 0.01-1.15%, Si: 0.01-2.50%, Mn: 0.01-2.50%, N: 0.001-0.050%, with the balance being Fe and unavoidable impurities, has a ferrite phase, and has a structure in which the area ratio of the retained austenite phase is 10% or less, has an axial tensile yield strength of 400 MPa or more, and has an axial compressive yield strength/axial tensile yield strength of 0.85-1.15.

First, we will explain the reasons for limiting the composition of the steel pipe of the present invention. Hereinafter, unless otherwise specified, mass% will simply be written as %.

C: 0.01-1.15%
C is important for improving the strength and fatigue properties of steel, and must be present at 0.01% or more. However, excessive C content forms an austenite phase and excessive carbides, which reduces the strength properties. , fatigue properties, and furthermore, toughness are deteriorated. Therefore, the C content is set to 1.15% or less. From the viewpoint of obtaining high strength and fatigue properties while maintaining good toughness, the C content is set to 0.05%. The C content is preferably 0.55% or less.

Si: 0.01~2.50%
Since silicon has a deoxidizing effect on steel, it is effective to include an appropriate amount of silicon in molten steel. In addition, silicon is effective in improving the strength properties of steel, so it is necessary to include silicon. However, a large amount of silicon The inclusion of Si impairs cold workability and low temperature toughness. Therefore, the Si content is set to 2.50% or less. On the other hand, excessive reduction of Si after deoxidation leads to an increase in manufacturing costs. The Si content is set to 0.01% or more. In order to obtain a sufficient deoxidizing effect, the Si content is preferably set to 0.10% or more. Excessive Si remaining in the steel From the viewpoint of suppressing deterioration in cold workability and low temperature toughness due to hardening, the Si content is preferably set to 0.80% or less.

Mn: 0.01-2.50%
Mn must be present in an amount of 0.01% or more to improve the strength properties of steel. However, since Mn is a strong austenite phase forming element, excessive Mn content can cause the formation of a retained austenite phase. The Mn content is increased and the strength is decreased. Therefore, the Mn content is set to 2.50% or less. In order to obtain a desired structure while improving the strength characteristics, the Mn content is set to 0.10% or more. The Mn content is preferably 1.50% or less.

N: 0.001-0.050%
In order to obtain a fine structure and strength properties, the N content must be 0.001% or more. On the other hand, if the N content is excessive, bubbles will form in the steel. To reduce the N content too much would require a lot of time and cost for processing, and from the viewpoint of stably obtaining a fine structure without bubbles, the content should be 0.005 to 0.050%. preferable.

The balance is Fe and unavoidable impurities. Examples of unavoidable impurities include P: 0.05% or less, S: 0.05% or less, and O (oxygen): 0.01% or less. P, S, and O are impurities that are inevitably mixed in during refining. If too many of these elements remain as impurities, various problems may occur, such as reduced hot workability, corrosion resistance, and low-temperature toughness. For this reason, it is preferable to keep the amounts of P to 0.05% or less, S to 0.05% or less, and O to 0.01% or less, respectively.

In addition to the above composition, the present invention may contain the following elements as necessary.

One or two selected from the following: Cr: 2.00% or less, Mo: 2.00% or less Cr: 2.00% or less (including 0%)
Cr improves the strength properties of steel and is effective in improving corrosion resistance and wear resistance, so if Cr is contained, it is preferable to contain 0.05% or more. On the other hand, if Cr is contained in excess, the hardness increases and the cold workability decreases. Therefore, the Cr content is preferably 2.00% or less. From the viewpoint of obtaining a strength improvement effect and excellent cold workability, the Cr content is more preferably 0.5% or more. In addition, the Cr content is more preferably 1.3% or less.

Mo: 2.00% or less (including 0%)
Mo is effective in improving high-temperature strength, so if it is contained, it is preferable to contain 0.05% or more. On the other hand, if the Mo content is high, the hardness increases and the cold workability decreases. Therefore, the Mo content is preferably 2.00% or less. From the viewpoint of obtaining high-temperature strength and excellent cold workability, the Mo content is more preferably 0.1% or more. In addition, the Mo content is more preferably 1.5% or less.

The present invention may further contain the elements described below as necessary.

One or more selected from Ti: 0.50% or less (including 0%), Al: 0.30% or less (including 0%), V: 0.55% or less (including 0%), and Nb: 0.75% or less (including 0%). When contained in an appropriate amount, Ti, Al, V, and Nb form nitrides or carbides and are effective in refining the structure, and can be contained as necessary. When contained, the Ti content is preferably 0.001% or more. The Al content is preferably 0.001% or more. The V content is preferably 0.01% or more. The Nb content is preferably 0.01% or more. When contained, in order to prevent a decrease in toughness, it is preferable to contain Ti at 0.50% or less, Al at 0.30% or less, V at 0.55% or less, and Nb at 0.75% or less. Also, from the viewpoint of suppressing alloy costs, the Ti content is preferably 0.05% or less, the Al content is preferably 0.10% or less, the V content is preferably 0.30% or less, and the Nb content is preferably 0.30% or less.

The present invention may further contain the elements described below as necessary.

One or more selected from the following: Ni: less than 2.5% (including 0%), W: less than 1.0% (including 0%), Cu: less than 2.5% (including 0%) Ni: less than 2.5% (including 0%)
Ni is effective in improving the toughness of steel. When Ni is contained, it is preferable to contain 0.01% or more. On the other hand, if Ni is contained excessively, the tensile strength and fatigue strength are reduced. Therefore, the Ni content is preferably less than 2.5%. From the viewpoint of obtaining toughness and excellent strength characteristics, the Ni content is more preferably 0.05% or more. In addition, the Ni content is preferably 1.50% or less.

W: less than 1.0% (including 0%), Cu: less than 2.5% (including 0%)
W and Cu improve the corrosion resistance of steel. When contained, the W content is preferably 0.01% or more and the Cu content is preferably 0.01% or more. If the content of each element is high, it may cause deterioration of the surface skin or defects during hot forming. Therefore, it is preferable that W is contained at less than 1.0% and Cu is contained at less than 2.5%. It is more preferable that the W content is 0.10% or less. It is more preferable that the Cu content is 1.50% or less.

The present invention may further contain the elements described below as necessary.

One or more selected from B: 0.010% or less (including 0%), Zr: 0.10% or less (including 0%), Ca: 0.010% or less (including 0%), Ta: 0.01% or less (including 0%), REM: 0.10% or less (including 0%), and Mg: 0.10% or less (including 0%). When B, Zr, Ca, Ta, REM, and Mg are contained in very small amounts, they affect the workability and surface scale during hot forming, and are useful for suppressing surface defects. Therefore, when contained, the B content is preferably 0.001% or more. The Zr content is preferably 0.01% or more. The Ca content is preferably 0.001% or more. The Ta content is preferably 0.001% or more. The REM content is preferably 0.001% or more. The Mg content is preferably 0.001% or more. However, excessive content may worsen surface defects. It is preferable that B is 0.010% or less, Zr is 0.10% or less, Ca is 0.010% or less, Ta is 0.01% or less, REM is 0.10% or less, and Mg is 0.10% or less. It is more preferable that the B content is 0.005% or less. It is more preferable that the Zr content is 0.03% or less. It is more preferable that the Ca content is 0.005% or less. It is more preferable that the Ta content is 0.005% or less. It is more preferable that the REM content is 0.005% or less. It is more preferable that the Mg content is 0.005% or less.

The present invention may further contain the elements described below as necessary.

One or more selected from Sn: 0.30% or less (including 0%), Sb: 0.30% or less (including 0%), Ag: 0.30% or less (including 0%) Sn, Sb, and Ag improve corrosion resistance when added in small amounts. There is no need to set a lower limit for the content, but when contained, the effect of improving corrosion resistance can be obtained by making the content of each of Sn, Sb, and Ag 0.0001% or more. On the other hand, if the content is too high, hot workability is reduced. Therefore, when contained, the content of each of Sn, Sb, and Ag is 0.30% or less.

Next, we will explain the structure of the steel pipe in this invention.

The structure of the present invention has a ferrite phase (including a bainite phase), and the area ratio of the retained austenite phase is 10% or less. The structure of the present invention may also contain a cementite phase (including pearlite). Steel pipes having these structures do not have high axial tensile yield strength when left to cool after hot rolling or when treated by annealing, but when cold worked, they are strengthened by dislocation strengthening caused by plastic strain, and also exhibit excellent fatigue strength properties.

Because the Bauschinger effect is at work in this structure, when cold drawing or cold pilger rolling is performed to increase the strength of steel pipes by utilizing the elongation plastic strain in the axial direction, the compressive yield strength in the axial direction decreases, and as a result, the fatigue strength under a compressive load in the axial direction is inferior to the fatigue strength under a tensile load in the axial direction. Therefore, in order to obtain excellent fatigue strength in both tension and compression in the axial direction, bending and unbending is required as cold working.

The area ratio of the ferrite phase is not particularly limited, but is preferably 50% or more since this provides good workability during cold working. There is no particular upper limit to the area ratio of the ferrite phase, but from the viewpoint of obtaining good workability while also obtaining high strength characteristics, it is preferable that the area ratio of the ferrite phase is 95% or less.

The area ratio of the cementite phase is not particularly limited, but from the viewpoint of obtaining high strength and high hardness, it is preferable that it is 5% or more. Furthermore, from the viewpoint of obtaining excellent toughness and cold workability, the area ratio of the cementite phase is preferably 50% or less.

In addition, if the area ratio of the retained austenite phase present in the ferrite phase or cementite phase exceeds 10%, it will have a detrimental effect on cold workability and strength characteristics, but if it is 10% or less, there will be no problem. Therefore, in the structure of the present invention, the area ratio of the retained austenite phase is 10% or less.

The method for measuring the structure is as follows. After cold working, the steel pipe is mirror-polished and etched with an acid such as Nital or Virela's etching solution. Using an optical microscope, it can be confirmed that the structure contains ferrite phase, or ferrite phase and cementite phase (including pearlite); pearlite is colored black, so its fraction can be measured. The accuracy of the area ratio can be improved by using a magnification of 100 to 400 times and a field of view that allows observation of 50 or more ferrite grains, or a total of 50 or more ferrite grains and pearlite grains.

In addition, the structure can be observed using an electron microscope except for the ferrite phase, or the ferrite phase and cementite phase. The percentage of retained austenite phase can be easily measured by separating the peaks of the body-centered cubic lattice of the ferrite phase and the face-centered cubic lattice of the austenite phase using X-ray diffraction on a mirror-polished test piece. The volume fraction of the retained austenite phase is measured using X-ray diffraction, and this is treated as the area fraction of the retained austenite phase.

In addition, the steel pipe of the present invention has the same structure containing ferrite phase or ferrite phase and cementite phase before and after cold working.

The structure of the steel pipe of the present invention can be controlled by adjusting the composition.

In the steel pipe of the present invention, the axial tensile yield strength is set to 400 MPa or more. Normally, a structure containing a ferrite phase is soft, so an axial tensile yield strength of 400 MPa or more cannot be obtained without cold working. Therefore, in the present invention, dislocation strengthening is performed by cold working to obtain a high axial tensile yield strength of 400 MPa or more. There is no upper limit to the axial tensile yield strength, but since delayed fracture may occur if the strength is too high, it is preferable that it be 870 MPa or less.

In addition, in the present invention, the ratio of tube axial compressive yield strength to tube axial tensile yield strength, i.e., tube axial compressive yield strength/tube axial tensile yield strength, is set to 0.85 to 1.15. By setting the tube axial compressive yield strength/tube axial tensile yield strength to 0.85 to 1.15, excellent fatigue strength can be obtained in the tube axial direction, even when a compressive load is included, compared to the fatigue strength of a tensile load. The ratio of tube axial compressive yield strength to tube axial tensile yield strength, i.e., tube axial compressive yield strength/tube axial tensile yield strength, is preferably closer to 1, since excellent fatigue properties can be obtained, and the tube axial compressive yield strength/tube axial tensile yield strength is preferably 0.95 or more. Also, the tube axial compressive yield strength/tube axial tensile yield strength is preferably 1.10 or less.

The tube axial compressive yield strength can be measured by a cylindrical compression test. The cylindrical test piece to be compressed is taken from the center of the wall thickness parallel to the tube axial direction. The outer diameter d [mm] of the cylinder and the height h [mm] of the cylinder should be h/d≦2.0. Specifically, a test piece with an outer diameter d = 3.0 mm and a height h = 5.0 mm is cut out from the center of the tube thickness. The compression test is performed at room temperature (25°C) by sandwiching the test piece between flat plates and applying a load, and the compressive yield strength is calculated using the stress-strain curve obtained during compression. The stress-strain curve is obtained by compressing 30% with a compression tester at a compression speed of 1.0 mm/min.

The axial tensile yield strength is measured according to JIS Z2241 (2011). First, a round bar tensile test piece with a parallel diameter of 3.0 mm is cut from the center of the wall thickness of the tube parallel to the axial direction. A tensile test is then performed at room temperature (25°C) and a crosshead speed of 1.0 mm/min until fracture. The tensile yield strength is calculated using the stress-strain curve obtained from this.

In the steel pipe of the present invention, the axial compressive yield strength/axial tensile yield strength can be controlled to 0.85 to 1.15 by performing the bending and unbending process described below on the steel pipe material having the above-mentioned composition.

The fatigue environment in which good fatigue strength can be obtained with the present invention will be explained. In conventional cold rolling, such as cold drawing and cold pilger rolling, the fatigue characteristics are good against stress in the axial direction of the tube as the tensile direction. In other words, it is good in a unilateral tensile stress environment in which the stress σ ratio R (=σmin/σmax) in the fatigue test is 0. On the other hand, most normal fatigue environments are alternating fatigue environments in which tension and compression are applied, R=-1, but conventional cold rolling, which has poor compressive yield strength, does not provide good fatigue strength in an alternating fatigue environment. Since the present invention is excellent in axial compressive yield strength/axial tensile yield strength, it exhibits excellent fatigue strength even in an alternating fatigue environment in which stress in the compressive direction is loaded. Therefore, the steel pipe of the present invention is suitable for use in axles, steel pipes with threaded joints, steel pipes for hot wells and oil wells, chemical plant piping, etc.

Next, we will explain the manufacturing method of the steel pipe of the present invention.

First, a steel material having the above-mentioned composition is prepared. Various melting processes can be applied to the production of steel, and there is no limitation. For example, when producing steel by electrically melting scrap iron or a lump of each element, a vacuum melting furnace or an atmospheric melting furnace can be used. When using molten pig iron by the blast furnace method, an Ar- _O2 mixed gas bottom blowing decarburization furnace or a vacuum decarburization furnace can be used. The melted material is solidified by static casting or continuous casting to form an ingot or slab, and then formed into a plate-shaped steel material by hot rolling, or into a round billet shape by forging or rolling to form the steel material.

Next, in the case of a plate-shaped steel material, it is formed into an approximately tubular shape, and the ends are welded to form a steel pipe. There are no particular limitations on the steel pipe forming process, and forming techniques such as UOE forming and roll forming, welding using a molten material, and electric resistance welding using induction heating can be used. In the case of a round billet-shaped steel material, it is heated in a heating furnace and undergoes various hot rolling processes to form a steel pipe. Then, hot forming (piercing process) is performed to make the round billet into a hollow pipe. Any method such as the Mannesmann method or the extrusion pipe method can be used for hot forming. In addition, if necessary, an elongator, Assel mill, mandrel mill, plug mill, sizer, stretch reducer, etc., which are hot rolling processes that reduce the wall thickness and determine the outer diameter of the hollow pipe, may be used.

Next, it is preferable to perform annealing heat treatment on the formed steel pipe before cold working, as this homogenizes the structure and removes residual stress. Cooling after heat treatment is preferably performed by furnace cooling or natural cooling at a rate of 5°C/s or less in order to maintain the uniform state with little residual stress obtained by annealing.

In the present invention, the yield strength of steel pipes is increased by bending and unbending the pipe in the circumferential direction.

This cold working method uses dislocation strengthening caused by plastic strain caused by bending and unbending in the circumferential direction of the tube to increase the strength of the steel tube. This working method is explained based on the drawings. Unlike cold drawing and cold pilger rolling, in which plastic strain caused by cold working occurs in the longitudinal direction of the tube axis, as shown in Figure 1, this method is given by bending the tube to flatten it (first flattening process) and then bending it back to return it to a perfect circle (second flattening process). In this method, the amount of strain is adjusted by using repeated bending and unbending and changes in the amount of bending, but the applied strain does not significantly change the shape before and after processing. Furthermore, since there is almost no strain in the axial direction of the tube, and strength is increased by dislocation strengthening caused by strain applied in the circumferential direction and the wall thickness direction of the tube, the occurrence of the Bauschinger effect in the axial direction of the tube can be suppressed. In other words, there is no significant decrease in the axial compressive strength relative to the axial tensile strength, as occurs with cold drawing rolling or cold pilger rolling, resulting in excellent axial compressive fatigue strength properties.

Figures 1(a) and (b) are cross-sectional views when there are two tool contact areas, and Figure 1(c) is a cross-sectional view when there are three tool contact areas. The thick arrow in Figure 1 indicates the direction in which force is applied when flattening the steel pipe. As shown in Figure 1, when performing the second flattening, the tool can be moved to rotate the steel pipe or the tool can be shifted so that the tool comes into contact with areas that have not been flattened in the first flattening (the shaded areas in Figure 1 indicate the areas that were flattened in the first flattening).

As shown in Figure 1, by performing bending and unbending in the circumferential direction of the pipe to flatten it intermittently or continuously around the entire circumference of the pipe, bending strain is applied near the maximum value of the steel pipe's curvature, and unbending strain is applied toward the minimum value of the steel pipe's curvature. As a result, strain due to bending and unbending deformation necessary for improving the strength of the steel pipe (dislocation strengthening) accumulates. In addition, when using this processing method, unlike processing methods that compress the pipe's wall thickness and outer diameter, it is characterized by the fact that it does not require a large amount of power, and because the deformation is due to flattening, processing can be performed while minimizing shape changes before and after processing.

In bending and unbending, in order to control the strength of the steel pipe in the axial direction, it is important to manage the flattening amount of the pipe. The more flattened the pipe, the more distortion there is, and the greater the improvement in strength in the axial direction in one bending and unbending process. To obtain sufficient strength in the axial direction by bending and unbending, it is preferable to give the steel pipe a flattening amount of 5% or more (flattening to an outer diameter that is 95% or less of the initial outer diameter) compared to the outer diameter before processing, although this depends on the material composition. In addition, a flattening amount of 10% or more will provide stable strength in the axial direction. There is no particular upper limit to the flattening amount, but if the flattening amount is too large, the roundness after bending and unbending will deteriorate. In addition, there is a possibility of cracks occurring during processing. Therefore, it is preferable to perform bending and unbending with a flattening amount of 25% or less.

As shown in Figure 1, rolls may be used as the tool shape used to flatten the steel pipe. If the steel pipe is flattened and rotated between two or more rolls arranged in the circumferential direction of the steel pipe, it is possible to easily give strain by repeated bending and unbending deformation. Even in this case, the strength in the pipe axial direction can be controlled by the amount of flattening. It is preferable to set the roll spacing to 95% or less of the initial outer diameter of the steel pipe so that the amount of flattening of the steel pipe relative to the initial outer diameter is 5% or more. If the roll spacing is 90% or less of the initial outer diameter of the steel pipe and the amount of flattening is 10% or more, stable strength characteristics in the pipe axial direction can be obtained. If the roll spacing is set to a flattening amount of more than 25% of the initial outer diameter, cracks may occur, the roundness may deteriorate, and the bite into the rolls may decrease, so the amount of flattening is preferably 25% or less.

If the rotation axis of the roll is inclined within 90° to the rotation axis of the tube, the steel tube advances in the direction of the tube rotation axis while undergoing flattening processing, making it easy to continue processing. In addition, continuous processing using this roll can easily change the curvature (flattening amount) of the steel tube for the first and second times by appropriately changing the roll spacing so that the flattening amount changes as the steel tube advances. Therefore, by changing the roll spacing, the movement path of the neutral line can be changed, making it possible to homogenize the strain in the wall thickness direction. Similarly, the same effect can be obtained by changing the roll diameter instead of the roll spacing to change the flattening amount. These can also be combined. Although the equipment becomes complicated, if the number of rolls is three or more, the tube can be prevented from moving around during processing, making stable processing possible.

As mentioned above, it is preferable to use tools such as rolls to clamp the outer periphery of the tube with a gap that is 5 to 25% smaller than the outer diameter of the tube, and then rotate the tube to flatten it.

As for the processing temperature during bending and unbending in the circumferential direction of the tube, if the required axial strength can be obtained without significant loss of dislocations, heat generation when heating or increasing the processing speed will not be a problem. From these points of view, it is preferable to keep the processing temperature at 300°C or less, and it is preferable to keep it at room temperature in order to prevent accumulated dislocations from disappearing.

In addition, even if processing heat occurs with an increase in processing speed, the processing time is not a problem as long as dislocations do not disappear significantly and the required strength in the tube axis direction is obtained. It is preferable that the temperature of the steel pipe during processing does not exceed 300°C. When bending and unbending processing is performed using inclined rolls, it is preferable that the bending and unbending processing is performed for 10 seconds/m or more (1 m of bending and unbending processing in the axial length of the steel pipe is performed over a period of 10 seconds or more).

In the present invention, heat treatment may be performed within a range in which dislocations do not disappear significantly after cold working. By performing heat treatment, residual stress after cold working can be reduced, fatigue strength can be improved, and the ratio of axial compressive yield strength to axial tensile yield strength can be brought closer to 1, so heat treatment is effective for obtaining a steel pipe with a smaller strength ratio. The upper limit of the heat treatment temperature (holding temperature) is 300°C or less, so there is no problem with strength reduction, and the heat treatment temperature is preferably 300°C or less. From the viewpoint of avoiding blue brittleness, this heat treatment temperature is preferably 100 to 250°C.
From the viewpoint of maintaining strength after bending and unbending, the heat treatment time (holding time) is preferably 300 seconds or more and less than 3600 seconds. After the heat treatment (holding), air cooling (average cooling rate (= (cooling start temperature (°C) - cooling end temperature (°C)) / cooling time (s)): 0.1 to 5.0°C/s) is performed.

The present invention will be explained below based on examples. Note that this example is directed to seamless steel pipes, but the same effect can be obtained with steel pipes obtained by forming and welding plate-shaped steel material.

Materials with the composition of steel types A to N shown in Table 1 were melted in a vacuum melting furnace and then hot rolled into round billets with an outer diameter of φ60 mm.

After hot rolling, the round billet was again inserted into the heating furnace and held at a high temperature of 1200°C or more, and then hot-formed into a seamless blank pipe with an outer diameter of φ70 mm and an inner diameter of 58 mm (wall thickness 6 mm) using a Mannesmann piercing mill. The blank pipes of each component were allowed to cool after hot forming. For steel types B, D, E, F, G, H, I, J, K, L, M, and N, they were allowed to cool and then reheated to 850°C, followed by annealing heat treatment in which they were slowly cooled. The blank pipes (steel pipe materials) after hot rolling or annealing heat treatment were cold worked to increase the strength in the tensile direction of the pipe axis. As shown in Table 2, the cold working method was either one of two types: drawing rolling, which is one of the conventional cold working methods, or bending and bending back processing according to the present invention. In addition, the number of times of processing was increased to one, and a condition was added in which processing was performed twice for the purpose of further increasing strength. The drawing rolling was performed under conditions of a wall thickness reduction of 10-25% and a 20% reduction in outer periphery. The bending and unbending process was performed by preparing a rolling mill with three cylindrical rolls arranged at a 120° pitch around the outer periphery of the tube (see Figure 1 (c)), clamping the outer periphery of the tube with the roll spacing set to 10-20% smaller than the outer diameter of the tube, and rotating the tube. The speed of the steel tube in the traveling direction during bending and unbending was 0.02 m/s, and it was confirmed that the temperature of the outer surface of the tube during processing was 150°C or less.

After the cold working during rolling, the material was left as is, and in some cases, heat treatment was performed at a heat treatment temperature (holding temperature) of 250°C and a heating holding time of 20 minutes (1200 seconds).

[Textural observation]
Test specimens were cut out from the center of the wall thickness (center of the wall thickness in a cross section perpendicular to the axial direction of the tube) of the steel tube after cold working or subsequent heat treatment, mirror polished, and then subjected to nital etching. The structure was identified by observing the specimens under an optical microscope at magnifications of 100 to 400 times to confirm whether or not they had a suitable structure containing a ferrite phase.
The fraction of the retained austenite phase can be easily measured by separating the peaks of the body-centered cubic lattice of the ferrite phase and the face-centered cubic lattice of the austenite phase by X-ray diffraction of the mirror-polished test piece. The volume fraction of the retained austenite phase was measured by X-ray diffraction, and this was taken as the area fraction of the retained austenite phase. The measurement area was in the range of 5 ^mm2 .

The chemical composition and microstructural observation results for each steel type are shown in Table 1. As shown in Table 1, steel types J, K, and L had retained austenite phase exceeding 10% in the structure, or bubbles thought to be due to the influence of _N2 gas were confirmed in the steel. In addition, steel types M and N (No. 27 and 28) had axial tensile yield strength below 400 MPa after cold working.
These steel types M and N (Nos. 27 and 28, respectively) did not have high mechanical properties even after cold working, so the evaluation of the compressive yield strength in the tube axial direction/tensile yield strength in the tube axial direction and the fatigue strength in the tube axial direction, which will be described later, was not carried out.
In addition, steel types J, K, and L (Nos. 24, 25, and 26, respectively) were not capable of cold working due to poor cold workability or the presence of defects such as voids in the steel. For these steels as well, the evaluation of the compressive yield strength in the tube axial direction/tensile yield strength in the tube axial direction and the fatigue strength in the tube axial direction, which will be described later, was not carried out.

[evaluation]
The mechanical properties and fatigue properties in the axial direction of the obtained steel pipes were evaluated.
The mechanical properties were evaluated by evaluating the tensile and compressive yield strength in the axial direction of the tube, and the fatigue strength (stress ratio R=-1) was measured by alternately applying tensile and compressive stresses.
The tensile yield strength and compressive yield strength were evaluated by cutting round bar test pieces with a diameter of 3 mm from the center of the wall thickness of the steel pipe after cold working, or after heat treatment if the steel pipe was heat treated after cold working, parallel to the pipe axis. Both tension and compression tests were performed at a processing speed of 1.0 mm/min, and the tensile yield strength and the ratio of compressive yield strength to tensile yield strength (compressive yield strength in the pipe axis direction/tensile yield strength in the pipe axis direction) were calculated.

Specifically, the tube axial compressive yield strength was measured by a cylindrical compression test. The cylindrical test piece to be compressed was taken from the center of the wall thickness parallel to the tube axial direction. In addition, the outer diameter d [mm] of the cylinder and the height h [mm] of the cylinder were set to h/d≦2.0. Specifically, a test piece was cut out from the center of the wall thickness of the tube with an outer diameter d = 3.0 mm and a height h = 5.0 mm. The compression test was performed at room temperature (25°C) by sandwiching the test piece between flat plates and applying a load, and the compressive yield strength was calculated using the stress-strain curve obtained during compression. The stress-strain curve was obtained by performing 30% compression at a compression speed of 1.0 mm/min using a compression tester.

The axial tensile yield strength was measured in accordance with JIS Z2241 (2011). First, a round bar tensile test piece with a parallel diameter of 3.0 mm was cut out from the center of the wall thickness of the tube parallel to the axial direction. A tensile test was then performed at room temperature (25°C) at a crosshead speed of 1.0 mm/min until fracture. The tensile yield strength was calculated using the stress-strain curve obtained from this.

In addition, fatigue strength was investigated using test pieces of the same size. Various tensile and compressive stresses were applied to the evaluation part of a φ3 mm round bar in air at room temperature (25°C) at a stress ratio R = -1 and a stress loading rate of 2 Hz, and the stress value obtained when the number of times of application was ¹⁰⁷ or more was evaluated as fatigue strength. The obtained fatigue strength was relatively compared between the values of drawing rolling and bending and unbending processing according to the steel type and cold processing conditions.

The results in Table 2 show that under the conditions in which the bending and unbending process of the present invention was carried out, the difference between the tensile yield strength and compressive yield strength in the tube axis direction was small compared to the conventional cold rolling method of draw rolling, and the fatigue properties were excellent.

Claims (6)

In mass percent,
C: 0.01-1.15%,
Si: 0.01-2.50%,
Mn: 0.01 to 2.50%,
N: 0.001 to 0.050%, with the balance being Fe and unavoidable impurities;
The steel has a structure having a ferrite phase and a retained austenite phase of 10% or less in area ratio,
The axial tensile yield strength is 400 MPa or more,
A seamless steel pipe having a ratio of compressive yield strength in the pipe axis direction to tensile yield strength in the pipe axis direction of 0.85 to 1.15. The composition further includes, in mass%, Cr: 2.00% or less,
2. The seamless steel pipe according to claim 1, further comprising one or two selected from the group consisting of Mo: 2.00% or less. The composition further includes, in mass%, Ti: 0.50% or less,
Al: 0.30% or less,
V: 0.55% or less,
The seamless steel pipe according to claim 1 or 2, further comprising one or more selected from the group consisting of Nb: 0.75% or less. The composition further includes, in mass%, Ni: less than 2.5%;
W: less than 1.0%
Cu: less than 2.5%
B: 0.010% or less,
Zr: 0.10% or less,
Ca: 0.010% or less,
Ta: 0.01% or less,
REM: 0.10% or less,
Mg: 0.10% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
The seamless steel pipe according to any one of claims 1 to 3, further comprising one or more selected from the following: Ag: 0.30% or less. In mass percent,
C: 0.01-1.15%,
Si: 0.01-2.50%,
Mn: 0.01 to 2.50%,
N: 0.001 to 0.050%;
Or even more so:
Cr: 2.00% or less,
Mo: 2.00% or less,
Ti: 0.50% or less,
Al: 0.30% or less,
V: 0.55% or less,
Nb: 0.75% or less,
Ni: less than 2.5%
W: less than 1.0%
Cu: less than 2.5%
B: 0.010% or less,
Zr: 0.10% or less,
Ca: 0.010% or less,
Ta: 0.01% or less,
REM: 0.10% or less,
Mg: 0.10% or less,
Sn: 0.30% or less,
Sb: 0.30% or less,
Ag: 0.30% or less,
The balance is Fe and unavoidable impurities.
The steel has a structure having a ferrite phase and a retained austenite phase of 10% or less in area ratio,
The axial tensile yield strength is 400 MPa or more,
A method for producing a steel pipe having a ratio of compressive yield strength in the pipe axis direction to tensile yield strength in the pipe axis direction of 0.85 to 1.15 , comprising bending and unbending a steel pipe material in the pipe circumferential direction to obtain the steel pipe. The method for manufacturing steel pipes according to claim 5, wherein after the bending and unbending process in the circumferential direction of the pipe, a heat treatment is performed in which the holding temperature is set to 300°C or less for 300 seconds or more and less than 3600 seconds, and then air-cooled.

Images