WO2022196196A1

WO2022196196A1 - Duplex stainless steel pipe and method for manufacturing same

Info

Publication number: WO2022196196A1
Application number: PCT/JP2022/005176
Authority: WO
Inventors: 俊輔佐々木; 城吾後藤; 正雄柚賀; 龍郎勝村; 秀夫木島
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2021-03-17
Filing date: 2022-02-09
Publication date: 2022-09-22
Also published as: CA3208799A1; JP7173411B1; US20240309480A1; EP4282990A1; AR125543A1; MX2023010534A; AU2022240057A1; JPWO2022196196A1; CN116917523A; BR112023017868A2

Abstract

Provided are a high-strength duplex stainless steel pipe having an excellent wear resistance or dent resistance on the inner and outer surfaces of a steel pipe, and a method for manufacturing the same.　The present invention has a component composition containing, by mass, 0.005 to 0.150% of C, 1.0% or less of Si, 10.0% or less of Mn, 11.5 to 35.0% of Cr, 0.5 to 15.0% of Ni, 0.5 to 6.0% of Mo, and less than 0.400% of N, with the remainder made up by Fe and unavoidable impurities, and has a steel structure having a ferrite phase and an austenite phase. The pipe axial direction tensile yield strength is 689 MPa or above, and the present invention has, on the steel pipe outer surface and the steel pipe inner surface, an oxide layer having a thickness of 1.0 μm or above.

Description

Duplex stainless steel pipe and manufacturing method thereof

The present invention relates to a stainless steel pipe that has excellent tensile yield strength in the pipe axial direction, as well as excellent wear resistance and dent resistance, and a method for manufacturing the same. The term “excellent tensile yield strength in the tube axis direction” means that the yield strength is 689 MPa or more.

Steel pipes for drilling oil and gas wells (hereafter also referred to simply as oil wells) and geothermal wells have corrosion resistance to withstand highly corrosive environments at high temperatures and pressures, as well as tensile stress due to their own weight when connected to great depths. It is necessary to have high strength properties to withstand the thermal stresses and high pressures associated with high temperatures. Among these, in order to have excellent corrosion resistance performance, it is necessary to adjust the amount of addition of corrosion resistance improving elements such as Cr, Mo, W and N to the steel. For example, SUS329J3L containing 22% by mass of Cr, SUS329J4L containing 25% by mass of Cr, and duplex stainless steel such as ISO S32750 and S32760 containing a large amount of Mo are used.

On the other hand, in order to ensure high strength characteristics, it is necessary to adjust the tube axial tensile yield strength, and this value is the representative value of the product strength specification. The reason for this is that when pipes are connected to a great depth, it is important to be able to withstand tensile stress due to the weight of the pipes themselves. By providing a sufficiently large tensile yield strength in the tube axial direction against the tensile stress due to its own weight, it is possible to suppress plastic deformation and prevent damage to the passive film, which is important for maintaining the corrosion resistance of the tube surface.

In this regard, the above-mentioned duplex stainless steel is composed of two phases, a ferrite phase and an austenite phase with a low yield strength in terms of crystal structure, and can be used for oil well pipes or by heat treatment only by hot forming or heat treatment. The tensile strength required for geothermal wells cannot be secured.
Therefore, in duplex stainless steel pipes used for oil wells or geothermal wells, the tensile yield strength in the pipe axial direction is increased by utilizing dislocation strengthening by various cold rolling. There are two types of cold rolling methods for pipes used for oil wells or geothermal wells: cold drawing rolling and cold pilger rolling. defines cold drawing and cold pilgering. Either cold rolling is a process of extending the tube in the longitudinal direction by thinning and shrinking. When these cold rollings are performed, it is necessary to wash the steel pipe before cold rolling with an acid or to form a lubricating film by chemical treatment in order to suppress product defects and protect tools. Of these, when a lubricating coating is formed, cleaning with an acid is required after cold rolling.

By the way, steel pipes used for oil wells and geothermal wells are often used outdoors and in places where the ground is not leveled. Frequent collisions with hard objects. Furthermore, when steel pipes are passed through steel pipes or when steel pipes are being transported, the steel pipes frequently rub against each other or collide with each other. In addition, when steel pipes are connected, high contact pressure is generated on the surface of the steel pipes by clamping with a fastening jig. Such collisions with hard objects, collisions between steel pipes, rubbing, and contact pressure from fastening jigs cause scratches and dents on the inner and outer surfaces of steel pipes.
These scratches and dents become starting points of corrosion. Furthermore, excessive dents affect the dimensions of the product, for example, the wall thickness is reduced by the depth of the scratches and dents, resulting in a decrease in axial tensile strength, which is important for steel pipes.
As described above, duplex stainless steel pipes used for oil wells and geothermal wells not only have high strength and high corrosion resistance, but also can suppress scratches and dents on the inner and outer surfaces of the steel pipe. Excellent abrasion resistance and dent resistance are required.

In this regard, the duplex stainless steel pipe is obtained through a dislocation strengthening process by cold rolling in order to obtain a high tensile yield strength in the pipe axial direction, as described above. Before cold rolling, the surface of the steel pipe is washed with an acid to remove an oxide layer on the surface for the purpose of suppressing damage to the rolling tool during cold rolling. Alternatively, when a chemically treated coating having high lubricity is formed to prevent seizure during cold rolling, the oxide layer on the surface is removed together with the chemically treated coating after cold rolling. Furthermore, the surface area of the steel pipe increases due to the thinning and stretching in the pipe axial direction due to cold rolling. In this way, the oxide layer is removed from the surface of the steel pipe after cold rolling, and the surface area is increased, resulting in a bare metallic surface with metallic luster.
However, in such a state where the metal surface is bare, scratches and dents as described above are likely to occur. That is, a duplex stainless steel pipe manufactured by conventional cold rolling has an exposed metal surface in order to obtain high strength, so that scratches and dents are likely to occur.

As for conventional steel pipe technologies, for example, Patent Documents 1 and 2 disclose steel pipes with improved hardness and wear resistance of the inner surface of the steel pipe. Further, Patent Document 3 discloses a clad steel pipe in which another material having high hardness and wear resistance is joined to the base material.

JP-A-57-194213 JP-A-1-15323 JP-A-63-290616

However, in the techniques described in Patent Documents 1 to 3, it is not considered to improve all of the strength characteristics, wear resistance, and dent resistance required for oil wells and geothermal wells as described above. However, there is a need for further improvement.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a duplex stainless steel pipe having high strength and excellent wear resistance and dent resistance of the inner and outer surfaces of the steel pipe, and a method for producing the same. .

In the present invention, "high strength" means that, in accordance with JIS Z2241, a round bar tensile test piece having a parallel part diameter of 5.0 mm is cut out from the central part of the wall thickness of the pipe parallel to the pipe axial direction, and , and the tensile yield strength in the tube axial direction obtained by performing a tensile test until breakage at a crosshead speed of 1.0 mm/min is 689 MPa or more.
In addition, excellent "wear resistance and dent resistance" means that the indenter has a carbide tip (conical shape (triangular tip with a cross section perpendicular to the bottom of the cone (contact part with steel pipe) angle: 60°) ), the tube surface was swept at 3 mm/s parallel to the tube axis direction, and a 30 mm scratch test was performed with a load of 59 N, and the unevenness difference between the concave and convex portions formed by the scratches was measured from the center in the length direction. and the height difference of the recesses is 50 μm or less.

In order to achieve the above objects, the present inventors have made extensive studies on duplex stainless steel pipes.
First, in order to improve the corrosion resistance of duplex stainless steel pipes, it is necessary to add corrosion-resistant elements such as Cr and Mo, and reduce C, which causes a decrease in corrosion resistance. Addition of Cr or Mo or reduction of C increases the ferrite phase in the product structure. Therefore, adding elements such as Ni, N, and Mn that increase the austenite phase in a well-balanced manner and making the product structure into an appropriate two-phase state of ferrite phase and austenite phase protects the duplex stainless steel pipe from various forms of corrosion. important to.

In order to make a duplex stainless steel pipe into an appropriate two-phase state, it is necessary to appropriately add chemical components that form a ferrite phase and an austenite phase, and to perform a solid solution heat treatment.
In addition to obtaining an appropriate two-phase fraction by solid solution heat treatment, precipitates and brittle phases harmful to corrosion resistance generated during cooling after solidification and hot forming are dissolved into the steel, and corrosion-resistant elements are added to the steel. It is possible to stabilize the corrosion resistance performance by dispersing it evenly.

A duplex stainless steel pipe with high corrosion resistance performance can be obtained by adjusting the chemical composition and solid solution heat treatment, but the austenite phase reduces the yield strength of the duplex stainless steel pipe. Therefore, the axial tensile yield strength of 689 MPa or more, which is required for steel pipes for oil wells and geothermal wells, cannot be obtained only by adjusting the chemical composition and performing the solid solution heat treatment. Therefore, in the production of duplex stainless steel pipes, a desired strength is obtained by dislocation strengthening by cold rolling following solid solution heat treatment.

Conventionally, cold drawing rolling or cold pilger rolling has been performed as a cold rolling method for the purpose of increasing the strength of a steel pipe. All of the rolling is rolling to reduce the wall thickness of the steel pipe or to elongate it in the pipe axial direction.
The aforementioned solution heat treatment must be performed before these cold rollings. This is because if the steel is exposed to a high temperature such as solution heat treatment after cold rolling, dislocations imparted by cold rolling disappear and the effect of improving yield strength by cold rolling is lost.
After the solid solution heat treatment performed before cold rolling, an oxide layer is formed on the inner and outer surfaces of the steel pipe.
The oxide layer formed on the inner and outer surfaces of the steel pipe before cold rolling may damage the cold rolling tools. will be Furthermore, for the purpose of protecting tools, a lubricating coating having lubricating properties may be formed on the steel pipe surface by chemical treatment, and the coating may be removed together with the oxide layer by washing after cold rolling. By removing the oxide layer before and after cold rolling, the metal surface is exposed on the inner and outer surfaces of the steel pipe.

In addition, the cold rolling process also exposes the metal surface on the steel pipe surface. That is, cold drawing rolling and cold pilger rolling are rolling methods that involve thinning and elongation of a steel pipe, so that the surface area of the metal portion that is the base material increases. Since the oxide layer has no ductility unlike the base material, it cannot follow the deformation, and the surface of the steel pipe after cold rolling becomes more exposed due to the new surface after rolling.

For the above reasons, in order to obtain high corrosion resistance and tensile yield strength in the pipe axial direction, the product surface can only be obtained with the metal surface exposed. When used for oil wells or geothermal wells, this causes friction and collision between hard objects and steel pipes, and the contact pressure of connecting jigs, which causes scratches and dents, which deteriorates the product surface and damages the steel pipes. This is the cause of the decrease in axial compressive yield strength and circumferential tensile yield strength due to reduced dimensional accuracy.

In response to these problems, the present inventors investigated a technique for manufacturing steel pipes without removing the oxide layer on the surface. As a result, solid solution heat treatment under specific conditions is performed, and cold working is performed in the circumferential direction of the pipe without removing the formed oxide layer. It was found that the wear resistance and dent resistance of the material can be obtained.

The present invention was made based on the above findings, and the gist thereof is as follows.
[1] % by mass,
C: 0.005-0.150%
Si: 1.0% or less,
Mn: 10.0% or less,
Cr: 11.5 to 35.0%,
Ni: 0.5 to 15.0%,
Mo: 0.5-6.0%,
N: having a component composition containing less than 0.400% and the balance consisting of Fe and inevitable impurities,
Having a steel structure with a ferrite phase and an austenite phase,
The tube axial direction tensile yield strength is 689 MPa or more,
A duplex stainless steel pipe having an oxide layer with an average thickness of 1.0 μm or more on each of the steel pipe outer surface and the steel pipe inner surface.
[2] The duplex stainless steel pipe according to [1] above, wherein the oxide layer covers 50% or more of each of the steel pipe outer surface and the steel pipe inner surface in area ratio.
[3] The duplex stainless steel pipe according to [1] or [2] above, wherein the axial compressive yield strength/tube axial tensile yield strength is 0.85 to 1.15.
[4] As the component composition, further in mass%,
W: 6.0% or less,
Cu: 4.0% or less,
V: 1.0% or less,
Nb: The duplex stainless steel pipe according to any one of [1] to [3] above, containing one or more selected from 1.0% or less.
[5] As the component composition, further by mass%,
Ti: 0.30% or less,
Al: The duplex stainless steel pipe according to any one of the above [1] to [4], containing one or two selected from 0.30% or less.
[6] As the component composition, further in mass%,
B: 0.010% or less,
Zr: 0.010% or less,
Ca: 0.010% or less,
Ta: 0.30% or less,
Sb: 0.30% or less,
Sn: 0.30% or less,
REM: The duplex stainless steel pipe according to any one of [1] to [5], containing one or more selected from 0.010% or less.
[7] A method for producing a duplex stainless steel pipe according to any one of [1] to [6] above,
Hot rolling is applied to the steel pipe material to make it into a steel pipe shape,
After the hot rolling, the steel pipe material is subjected to a solid solution heat treatment that satisfies the following formula (1),
A method for producing a duplex stainless steel pipe, wherein cold bending and unbending is performed in the circumferential direction of the pipe without removing the oxide layer formed on the steel pipe material after the solid solution heat treatment.
Tmax ² ×t/[Cr] ⁴ >1000 Equation (1)
In formula (1),
Tmax: Maximum heating temperature (° C.) during solid solution heat treatment,
t: holding time (s) at the maximum heating temperature during the solid solution heat treatment,
[Cr]: Cr content (% by mass) in the steel pipe.
[8] The method for producing a duplex stainless steel pipe according to [7] above, wherein the maximum heating temperature in the hot rolling is 1150°C or higher.
[9] By performing the bending and unbending cold working, the diameter of the steel pipe material is reduced,
The duplex stainless steel according to [7] or [8] above, wherein (Di/Do)×100, which is the ratio (%) of the post-processing outer diameter Di to the pre-processing outer diameter Do of the steel pipe material, is 99% or less. A method of manufacturing steel pipes.
[10] By performing the bending and unbending cold working, (Li/Lo) × 100, which is the ratio (%) of the axial length Li after working to the axial length Lo before working of the steel pipe material, is 125. % or less, the method for producing a duplex stainless steel pipe according to any one of [7] to [9].

According to the present invention, it has excellent tensile yield strength in the tube axial direction, as well as high wear resistance and dent resistance. Therefore, even when used for oil wells and geothermal wells, which are high temperature, high pressure, and severe corrosive environments, scratches and dents due to collisions and rubbing can be stably suppressed.

FIG. 1 is a graph for explaining the thickness of an oxide scale layer and the effect of suppressing surface flaws. FIG. 2 is a schematic diagram for explaining bending and unbending processing in the pipe circumferential direction.

The present invention will be described below.

The duplex stainless steel pipe of the present invention has C: 0.005 to 0.150%, Si: 1.0% or less, Mn: 10.0% or less, and Cr: 11.5 to 35.0% in mass %. , Ni: 0.5 to 15.0%, Mo: 0.5 to 6.0%, N: less than 0.400%, with the balance being Fe and inevitable impurities, ferrite It has a steel structure with phases and austenite phases. Further, the duplex stainless steel pipe of the present invention has an axial tensile yield strength of 689 MPa or more, and has an oxide layer having an average thickness of 1.0 μm or more on each of the steel pipe outer surface and the steel pipe inner surface.

First, the reasons for limiting the composition of the duplex stainless steel pipe of the present invention will be explained. In addition, % regarding component content is "mass %."

C: 0.005-0.150%
C deteriorates corrosion resistance. Also, if the C content is high, the austenite phase transforms into the martensite phase, making cold rolling and cold working difficult. Therefore, in order to obtain appropriate corrosion resistance and a two-phase structure, the C content should be 0.150% or less. If the C content is too small, the decarburization cost increases during melting, so the C content is made 0.005% or more. The C content is preferably 0.080% or less.

Si: 1.0% or less Residual Si in steel due to a large amount of Si content may impair workability and low-temperature toughness. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.8% or less. In addition, since Si has a deoxidizing effect on steel, it is effective to include an appropriate amount of Si in molten steel, so the Si content is preferably 0.01% or more. From the viewpoint of achieving both a sufficient deoxidizing effect and suppression of side effects due to excessive Si remaining in the steel, the Si content is more preferably 0.2% or more.

Mn: 10.0% or less Excessive Mn content reduces low temperature toughness. Therefore, the Mn content is set to 10.0% or less. If higher low-temperature toughness is required, the Mn content is preferably less than 1.0%. Also, Mn is a strong austenite phase-forming element and is inexpensive compared to other austenite phase-forming elements. Furthermore, Mn is effective in detoxifying S, which is an impurity element mixed in molten steel. It is preferable to contain 01% or more. On the other hand, when using Mn as an austenite phase-forming element from the viewpoint of achieving both low-temperature toughness and cost reduction, the Mn content is more preferably 2.0% or more. Moreover, the Mn content is more preferably 8.0% or less.

Cr: 11.5-35.0%
Cr is an element that strengthens the passive film of steel and enhances corrosion resistance. Also, it is an element necessary for stabilizing the ferrite phase and obtaining an appropriate two-phase structure. In the present invention, a Cr content of 11.5% or more is required to obtain a two-phase structure and high corrosion resistance. Cr is a basic element that stabilizes the passive film, and the higher the Cr content, the stronger the passive film. For this reason, as the Cr content increases, it contributes to the improvement of corrosion resistance. becomes difficult to mold. Therefore, the upper limit of the Cr content is set to 35.0%. Based on the above, the Cr content is set to 11.5 to 35.0% in the present invention. Moreover, from the viewpoint of ensuring both corrosion resistance and manufacturability, the Cr content is preferably 20% or more. Preferably, the Cr content is 28% or less.

Ni: 0.5-15.0%
Ni is an expensive element among other austenite phase-forming elements, and an increase in its content leads to an increase in production costs. Therefore, the Ni content is set to 15.0% or less. Also, Ni is a strong austenite phase former and improves the low temperature toughness of steel. Therefore, Mn, which is an inexpensive austenitic phase-forming element, should be positively included when low-temperature toughness becomes a problem, and the Ni content should be 0.5% or more. For applications in which low-temperature toughness is not a problem, it is preferable that the Ni content is 0.5 to 5.0% and that Ni is added in combination with other elements. On the other hand, when high low-temperature toughness is required, active addition of Ni is effective, and the Ni content is preferably 5.0% or more. Also, the Ni content is preferably 13.0% or less.

Mo: 0.5-6.0%
Mo increases the pitting corrosion resistance of steel depending on its content. Therefore, it must be uniformly present on the surface of the steel material exposed to the corrosive environment. On the other hand, an excessive Mo content precipitates a brittle phase during solidification from molten steel, generates a large amount of cracks in the solidified structure, and greatly impairs forming stability thereafter. Therefore, Mo content shall be 6.0% or less. Also, the content of Mo improves the pitting corrosion resistance according to the content, but the content must be 0.5% or more in order to maintain stable corrosion resistance in a sulfide environment. From the above, in the present invention, the Mo content is set to 0.5 to 6.0%. Moreover, from the viewpoint of achieving both corrosion resistance and manufacturing stability required for duplex stainless steel pipes, the Mo content is preferably 1.0% or more. Preferably, the Mo content is 5.0% or less.

N: less than 0.400% N itself is inexpensive, but excessive addition of N requires special equipment and addition time, leading to an increase in manufacturing costs. Therefore, the N content should be less than 0.400%. Also, N is a strong austenite phase-forming element and is inexpensive. Also, if dissolved in steel, it is an element useful for improving corrosion resistance and strength. On the other hand, if N can be combined with other austenite phase-forming elements to achieve an appropriate two-phase fraction of the product structure, there is no need to limit the range in particular, but an excessively small N content may cause dissolution or A high degree of vacuum is required during smelting, and there are restrictions on the raw materials that can be used. Therefore, the N content is preferably 0.010% or more.

The balance other than the above composition is Fe and unavoidable impurities.

In addition, in the present invention, the elements described below may be appropriately contained as necessary.

One or more selected from W: 6.0% or less, Cu: 4.0% or less, V: 1.0% or less, Nb: 1.0% or less W: 6.0% or less Like Mo, W increases the pitting corrosion resistance depending on the content, but if it is excessively contained, it impairs the workability during hot working and impairs the manufacturing stability. Therefore, when W is contained, the W content should be 6.0% or less. Since the content of W improves pitting corrosion resistance according to the content, it is not necessary to set a lower limit. . From the viewpoint of the corrosion resistance and production stability required for duplex stainless steel pipes, the W content is more preferably 1.0% or more. More preferably, the W content is 5.0% or less.

Cu: 4.0% or less Cu is a strong austenitic phase-forming element and improves the corrosion resistance of steel. Therefore, Mn and Ni, which are other austenite phase-forming elements, should be positively contained when the corrosion resistance is insufficient. On the other hand, when the content of Cu is too high, the hot workability deteriorates, making molding difficult. Therefore, when Cu is contained, the Cu content is set to 4.0% or less. Although the lower limit of the content does not have to be specified, a corrosion resistance effect can be obtained with a Cu content of 0.1% or more. From the viewpoint of achieving both improved corrosion resistance and hot workability, the Cu content is more preferably 1.0% or more. More preferably, the Cu content is 3.0% or less.

V: 1.0% or less Excessive V content impairs low-temperature toughness, so when V is included, the V content is preferably 1.0% or less. Also, the inclusion of V is effective in improving the strength. Therefore, it can be contained when higher strength is required. The strength improvement effect can be obtained by setting the V content to 0.01% or more. Therefore, when V is contained, the V content is preferably 0.01% or more. Since V is an expensive element, the V content is more preferably 0.40% or less from the viewpoint of the strength improvement effect obtained by addition and the cost. The V content is more preferably 0.10% or less, and even more preferably 0.06% or less. More preferably, the V content is 0.05% or more.

Nb: 1.0% or less Excessive addition impairs low-temperature toughness, so the Nb content is preferably 1.0% or less. Also, the addition of Nb is effective in improving the strength. Therefore, it can be included when higher strength is required. The strength improvement effect can be obtained by setting the Nb content to 0.01% or more. Therefore, when Nb is contained, the Nb content is preferably 0.01% or more. As with V, Nb is also an expensive element, so the Nb content is more preferably 0.40% or less from the viewpoint of the strength improvement effect obtained by addition and cost. The Nb content is more preferably 0.10% or less, and even more preferably 0.06% or less. More preferably, the Nb content is 0.05% or more.

One or two selected from Ti: 0.30% or less and Al: 0.30% or less Ti: 0.30% or less Since the low-temperature toughness of the steel pipe decreases when the Ti content increases, The amount is preferably 0.30% or less. In addition, since Ti can refine the solidification structure and fix excess C and N, it can be appropriately contained when structure control and chemical component adjustment are required. Therefore, when Ti is contained, such an effect can be obtained by setting the Ti content to 0.0001% or more. More preferably, the Ti content is 0.001% or more. More preferably, the Ti content is 0.10% or less.

Al: 0.30% or less If a large amount of Al remains in the steel pipe, the toughness is impaired. Therefore, when Al is contained, the Al content is preferably 0.30% or less. The Al content is more preferably 0.10% or less, more preferably 0.02% or less.
Also, the addition of Al is effective as a deoxidizer during refining. In order to obtain this effect, the Al content is preferably 0.01% or more.

B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, REM: One or two or more selected from 0.010% or less If the content of B, Zr, Ca, and REM is too high, the hot workability is adversely affected, and the rare elements make the alloy unusable. Cost increases. Therefore, the contents of B, Zr, Ca, and REM are each preferably 0.010% or less. Moreover, it is more preferable that the contents of Ca and REM are respectively 0.0015% or less.
B, Zr, Ca, and REM, when added in very small amounts, improve the bonding strength of grain boundaries and change the morphology of oxides on the surface to improve hot workability and formability. Duplex stainless steel pipes are generally a difficult-to-work material, so they are prone to rolling flaws and shape defects due to the amount of processing and the form of processing. elements are effective. It is not necessary to set a lower limit for each content, but if they are contained, by making each content 0.0001% or more, the effect of improving workability and moldability can be obtained.
Also, if the Ta content is too high, the alloy cost increases. Therefore, when Ta is contained, the Ta content is preferably 0.30% or less. Also, when a small amount of Ta is added, it suppresses the transformation to the embrittlement phase and improves the hot workability and corrosion resistance at the same time. In addition, Ta is effective when the embrittlement phase stays in a stable temperature range for a long time during hot working and subsequent cooling. Therefore, when Ta is contained, the Ta content is preferably 0.0001% or more.
On the other hand, if the contents of Sb and Sn are too large, the moldability will be deteriorated. Therefore, when Sb and Sn are contained, the content of each of Sb and Sn is preferably 0.30% or less. Moreover, when Sb and Sn are added in small amounts, the corrosion resistance is improved. Therefore, when Sb and Sn are contained, the content of each of Sb and Sn is preferably 0.0003% or more.

Two Phases of Ferrite Phase and Austenite Phase Next, the ferrite phase and austenite phase that affect corrosion resistance will be described. The ferrite phase and the austenite phase of a duplex stainless steel pipe have different effects on corrosion resistance, and the existence of these phases in the steel in the form of two phases provides high corrosion resistance. Therefore, both the austenite phase and the ferrite phase should be present in the duplex stainless steel. Since the duplex stainless steel pipe of the present invention is used in applications requiring corrosion resistance, it is also preferable to control the duplex fraction state from the viewpoint of corrosion resistance. In the present invention, it is preferable that the ferrite phase fraction (volume fraction) in the duplex stainless steel pipe structure is 20% or more and 80% or less. , it is preferable that the ferrite phase is 35% or more and 65% or less. The residual structure is preferably an austenite phase.

If the structure contains a martensite phase or a brittle phase, the hot workability and cold workability are deteriorated, making it impossible to form into a product shape, so these structures cannot be used. In addition, when the structure is a single phase of ferrite or austenite phase instead of two phases, corrosion resistance performance cannot be obtained, and high tensile strength in the tube axial direction cannot be obtained after cold working. In the present invention, the structure must contain two phases, a ferrite phase and an austenite phase.
That is, the steel structure of the present invention has a ferrite phase and an austenite phase, and is preferably a steel structure composed of the ferrite phase and the austenite phase.

In the observation of the structure described above, first, a test piece for structure observation is taken so that the cross section in the pipe axial direction is the observation surface. The volume fractions of the ferrite phase and the austenite phase are determined by observing the observed surface with a scanning electron microscope. Specifically, the test piece for tissue observation described above was corroded with a Birrella reagent (picric acid, hydrochloric acid, and ethanol mixed in proportions of 2 g, 10 ml, and 100 ml, respectively) and scanned with a scanning electron microscope (SEM) ( 1000x). From the obtained structure photograph, an image analyzer is used to calculate the average value of the area ratios of the ferrite phase and the austenite phase, and this is defined as the respective volume ratios (% by volume).
In the captured image, the ferrite phase is defined as a phase that becomes white by binarization because it is difficult to corrode, and the austenite phase is defined as a phase that is easily corroded and becomes black by binarization. The above binarization is performed on the range of the measurement area (600 μm×800 μm (1920 pixels×2560 pixels)) after converting the captured image into a 256-gradation grayscale image. Regarding the setting of binarization, the minimum brightness between two peaks seen in a histogram with brightness (256 gradations) on the horizontal axis is taken as a threshold.

Pipe axial direction tensile yield strength: 689 MPa or more In the mining of oil wells and hot water, high tensile stress in the pipe axial direction is generated because steel pipes are connected from the ground surface. Therefore, adjustment of the tensile yield strength in the tube axial direction is important among various strengths. In ordinary duplex stainless steel pipes, the axial tensile yield strength after solid solution heat treatment, which is performed to obtain excellent corrosion resistance, does not exceed 689 MPa. Therefore, high yield strength is achieved by strengthening dislocations by cold rolling. If the tensile yield strength in the tube axial direction is 757.9 MPa or more, it is preferable because the thickness of the tube can be reduced by an amount corresponding to the strength improvement, and the material can be saved. More preferably, it is 861.25 MPa or more. There is no upper limit, but if it exceeds 1033.5 MPa, the steel pipe wall thickness reduction effect is lost, so it is preferably 1033.5 MPa or less.

Tube axial compression yield strength/tube axial tensile yield strength: 0.85 to 1.15
Regarding the strength characteristics of steel pipes, it is important to adjust the tensile yield strength in the pipe axial direction. The compressive yield strength in the tube axial direction with respect to the strength is preferably 0.85 or more and 1.15 or less. More preferably, it is 0.90 or more. Moreover, it is more preferably 1.10 or less. A ratio of 0.90 to 1.10 makes it possible to withstand a higher compressive yield stress when connecting steel pipes with screws.

Axial compressive yield strength and axial tensile yield strength are obtained by cutting out a round bar tensile test piece and a cylindrical compression test piece with an outer diameter (diameter) of 5.0 mm from the center of the wall thickness of the end of the pipe used for the pressure test. , the test is performed with compression and tensile speeds of 1.0 mm/min, respectively, and the stress-strain curves are measured in the normal temperature tensile and compression tests. From this stress-strain curve, the tube axis direction tensile yield strength and tube axis direction compressive yield strength are calculated.

Specifically, first, the compression yield strength in the tube axial direction is measured by a cylinder compression test. A cylindrical test piece to be compressed is taken from the center of the wall thickness parallel to the pipe axis direction. A test piece is cut out from the central portion of the tube with a cylinder outer diameter d of 5.0 mm and a cylinder height h of 8.0 mm. In the compression test, a test piece is sandwiched between flat plates at room temperature (25°C) and a load is applied, and the compression yield strength is calculated using the stress-strain curve obtained when the test piece is compressed. A stress-strain curve is obtained by performing 30% compression with a compression tester at a crosshead speed of 1.0 mm/min.

In addition, the tensile yield strength in the tube axial direction is determined according to JIS Z2241. First, as a test piece, a round bar tensile test piece with a parallel part diameter of 5.0 mm is cut from the center of the wall thickness of the tube parallel to the tube axial direction. Then, a tensile test is performed at room temperature (25° C.) at a crosshead speed of 1.0 mm/min until breakage. Using the stress-strain curve thus obtained, the tensile yield strength is calculated.

In order to stably obtain 0.85 to 1.15 for the compressive yield strength in the pipe axial direction with respect to the tensile yield strength in the pipe axial direction, the The average aspect ratio of austenite grains is preferably 9 or less.
Moreover, it is preferable that the austenite grains having an aspect ratio of 9 or less have an area fraction of 50% or more.
Specifically, when assuming a perfect circle (when creating a perfect circle without changing the area), austenite grains having a grain size (diameter) of 10 μm or more are targeted, and the average aspect ratio of the austenite grains is 9. The following are preferable.
In addition, it is preferable that the austenite grains having a grain size of 10 μm or more and an aspect ratio of 9 or less have an area fraction of 50% or more. That is, (1) the total area of austenite grains having a grain size of 10 μm or more, (2) the area of austenite grains having a grain size of 10 μm or more and an aspect ratio of 9 or less, ((2)/(1) )×100(%) is preferably 50% or more.

The duplex stainless steel pipe of the present invention is adjusted to an appropriate two-phase fraction by performing a solution heat treatment.
At this time, the austenite phase becomes a structure having a plurality of crystal grains separated by an azimuth angle of 15° or more due to recrystallization. As a result, the aspect ratio of the austenite grains becomes small. Duplex stainless steel pipes in this state do not have the necessary pipe axial tensile yield strength for oil wells or geothermal wells, but the pipe axial compressive yield strength/pipe axial tensile yield strength is ideally 1 is in a state close to After that, cold working is performed to obtain the tensile yield strength in the tube axial direction necessary for oil country tubular goods or geothermal wells. It has the characteristic that the directional yield strength decreases due to the Bauschinger effect. That is, when the structure is elongated by cold rolling and the aspect ratio increases, it is difficult to obtain a stable relationship between the tube axial direction compressive strength and the tube axial direction yield strength.

For the above reasons, in the present invention, regarding austenite grains having a grain size of 10 μm or more, if the average aspect ratio of the austenite grains is 9 or less, the compressive yield strength in the pipe axial direction relative to the tensile yield strength in the pipe axial direction is 0.85 to 1. Duplex stainless steel pipe with a .15 is likely to be obtained. Further, when the austenite grains having an aspect ratio of 9 or less are 50% or more in area fraction, a stable steel pipe with little strength anisotropy can be obtained. By setting the average aspect ratio to 5 or less, it is possible to obtain a duplex stainless steel pipe having a more stable relationship between the pipe axial direction compressive yield strength and the pipe axial direction tensile yield strength. The smaller the aspect ratio, the more the strength anisotropy can be reduced.

In addition, the aspect ratio of the austenite grains is determined, for example, by observing grains with a crystal orientation angle of 15° or more in the austenite phase by analyzing the crystal orientation of the thick section in the pipe axial direction, and placing the grains in a rectangular frame. Calculated by the ratio of side to short side. Specifically, as a method for measuring the above aspect ratio, crystal orientation analysis is performed by EBSD on the thickness central portion of the obtained steel pipe axial cross section, and the aspect ratio of austenite grains separated by a crystal orientation angle of 15° is calculated. Measure. The measurement area is 1.2 mm × 1.2 mm, and the aspect ratio is measured for austenite grains having a grain size (diameter) of 10 μm or more when assuming a perfect circle (when creating a perfect circle without changing the area). do.

In addition, austenite grains with a small grain size cause a large measurement error, so if austenite grains with a small grain size are included, there is a possibility that the aspect ratio will be erroneous. Therefore, the austenite grains whose aspect ratio is to be measured are preferably austenite grains having a grain size of 10 μm or more assuming that they are perfect circles.

Also, the aspect ratio of the ferrite phase is not particularly limited. The reason is that the austenite phase has a lower yield strength, and the aspect ratio of the austenite grains tends to affect the Bauschinger effect after working, but the aspect ratio of the ferrite grains does not affect the Bauschinger effect. It's for.

Average thickness of oxide layer (surface oxide film) on steel pipe outer surface and steel pipe inner surface: 1.0 μm or more A passivation film that improves corrosion resistance exists on the surface of stainless steel. This passivation film is different from the surface oxide layer of the present invention. Also, the passive film is as thin as 0.01 μm or less. On the other hand, the oxide layer of the present invention is a layer mainly composed of Cr, Fe, and O (oxygen) generated by heating at 600° C. or higher, and is a layer containing iron oxide containing O and Cr. be.
In the case of duplex stainless steel, the oxides forming the oxide layer are spinel type ((Fe, Cr, Si) ₃ O ₄ , (Fe, Cr) ₃ O ₄ , Fe ₃ O ₄ ).
In a region closer to the base material than the oxide layer, the oxide layer may contain a large amount of Si. In some cases, the outer surface layer of the oxide layer contains less Cr and contains hematite composed of Fe and O(OH). Any oxide layer contains O in the composition due to the diffusion of O by heating, and the oxide containing these O has a higher hardness than the base material in any composition, so that the desired effect can be obtained. . In the present invention, it is more preferable to form a spinel-type oxide layer with a thickness (average thickness) of 1.0 μm or more, which has good adhesion to the base material.

In the present invention, the composition of the oxide layer is not particularly limited as described above, and adjustment of the thickness is required. The present inventors have investigated the effect of the chemical composition in the steel and the heat treatment conditions (maximum heating temperature and holding time at the maximum heating temperature) on the thickness of the oxide layer, and the thickness of the oxide layer and the wear resistance of the surface. The dent resistance relationship is clarified and explained below.

First, the present inventors prepared five duplex stainless steel pipes each containing 22.0 to 28.0% by mass of Cr, and made a solid solution by changing the maximum heating temperature and the holding time at the maximum heating temperature. After heat treatment, the thickness of the oxide layer on the surface of the steel pipe was investigated. Furthermore, by conducting other studies, it was confirmed that the thickness (average thickness) of the oxide layer can be stably set to 1.0 μm or more if the following formula (1) is satisfied.
Tmax ² ×t/[Cr] ⁴ >1000 Equation (1)
In formula (1), Tmax: Maximum heating temperature (°C) during solution heat treatment, t: Holding time (s) at maximum heating temperature during solution heat treatment, [Cr]: Content of Cr in the steel pipe ( % by mass).

Next, various solid solution heat treatments were performed under conditions satisfying the values calculated by formula (1), and a steel pipe (steel pipe material) having a surface oxide layer thickness of 1.0 to 45.0 μm was obtained. . Each one of the obtained steel pipes having the same composition was washed with an acid and polished to remove the oxide layer on the surface, so that the thickness of the oxide layer was less than 1.0 μm. The axial tensile yield strength of the steel pipe material obtained at this time is 689 MPa or less.
Outer diameter reduction rate: 10%, elongation in the pipe axis direction for all steel pipe materials with an oxide layer formed and steel pipe materials with an oxide layer thickness of less than 1.0 μm due to cleaning or polishing with acid : Bending back bending cold working was performed at 8%, and the tensile yield strength in the pipe axial direction of the steel pipe was increased from 861 MPa to 931 MPa. As a result of confirming the thickness of the oxide layer after cold working, there was no difference between before and after bending and unbending cold working.
A scratch test was performed on the obtained high-strength steel pipe by applying a load of 59 N to the indenter (needle with a carbide tip) and scratching the steel pipe surface by 30 mm parallel to the pipe axis direction. The difference in height (height difference between the dented portion after scratching and the convex portion caused by the dented portion) was measured to evaluate the wear resistance and denting resistance of the oxide layer on the surface of the steel pipe.

The results are shown in Figure 1. It was confirmed that the steel pipe having an oxide layer with a thickness of 1.0 μm or more could significantly suppress the surface height difference, and improved wear resistance and dent resistance. On the other hand, it was confirmed that steel pipes that did not have an oxide layer after being washed with an acid had a large height difference after the scratch test, generated flaws and irregularities, and reduced dimensional accuracy. In addition, it was confirmed that the dents generated by scratches become stress concentration areas, which may adversely affect corrosion resistance performance such as stress corrosion cracking.

From the above, it was found that excellent wear resistance and dent resistance can be obtained by setting the average thickness of the oxide layer to 1.0 μm or more. Further, from FIG. 1, it is confirmed that the height difference is further reduced as the oxide layer becomes thicker. Therefore, the average thickness of the oxide layer is more preferably 3.0 μm or more, more preferably 5.0 μm or more, as long as the temperature and holding time of the solid solution heat treatment for imparting the oxide layer are within a range where there is no problem. be. There is no upper limit to the thickness of the oxide layer, but if it is too thick, it may lead to peeling of the oxide layer. Therefore, the upper limit of the oxide layer is preferably 200.0 μm or less.

In the present invention, the oxide layer is obtained by cutting a steel pipe into round slices, mirror-polishing the cross section, and measuring the oxygen concentration from the inner and outer surfaces using energy dispersive X-ray analysis. Let the region be an oxide layer. The thickness (average thickness) of the oxide layer is measured at arbitrarily at 5 points (preferably at 5 points at equal intervals in the circumferential direction of the tube), and the average value (total thickness of 5 points/5) is taken.
In addition, by not pickling before the solid solution heat treatment and cold working, that is, by leaving the oxide layer during hot rolling on the surface of the steel pipe, the thickness is effective for wear resistance and dent resistance. is preferable because an oxide layer having

Coverage ratio of oxide layer on steel pipe outer surface and steel pipe inner surface: 50% or more in terms of area ratio The portion covered by the oxide layer has the effect of protecting the steel pipe from rubbing, scratches, and dents. The area covered by the oxide layer on the steel pipe is preferably 50% or more of the surface area of the entire steel pipe. For the purpose of protecting more of the outer surface, it is preferable that 80% or more is covered. Since the inner surface is easily damaged by collision with hard substances flowing inside, it is preferable that 90% or more of the inner surface is covered with an oxide layer.

The coverage ratio of the oxide layer on the steel pipe surface is obtained by dividing the surface area of the pipe where no oxide layer is formed (non-covered area) from the surface area of the entire pipe calculated from the outer diameter, wall thickness and length of the pipe. calculated as a percentage. A region where no oxide layer is formed is produced by polishing with abrasive grains or pickling, and has a metallic luster, so the surface area of that region can be easily measured.
Specifically, the non-coated area draws an enclosure (rectangle) parallel to the pipe circumferential direction and the pipe axial direction so as to enclose the region judged to have been visually polished or pickled. Then, the area is calculated using the length in the pipe circumferential direction (the length of the rectangle) and the length in the pipe axis direction (the width of the rectangle). This area is defined as "the length in the circumferential direction of the tube (the length of the rectangle) x the length in the direction of the tube axis (the width of the rectangle)". Then, for this area, find the sum of one steel pipe.
Next, regarding the surface area of the entire steel pipe (the surface area of the entire pipe other than the pipe end cut portion is the surface area of the entire pipe), the outer diameter and wall thickness of the steel pipe are used to determine the outer and inner circumference lengths of the steel pipe. Calculated by multiplying and adding up. Outer diameter, wall thickness, and length are all calculated using the average length. Then, the coating rate of the steel pipe surface with the oxide layer is obtained by dividing the above uncoated area by the surface area of the entire steel pipe and obtaining it as a percentage (%).

In addition, it is preferable to use a seamless steel pipe that has no joints in the pipe circumferential direction as the duplex stainless steel pipe from the viewpoint of uniforming the characteristics in the pipe circumferential direction.

Next, the method for manufacturing the duplex stainless steel pipe of the present invention will be explained.

First, a steel material having the duplex stainless steel composition described above is produced. Various melting processes can be applied to smelting duplex stainless steel, and there are no restrictions. For example, a vacuum melting furnace or an atmospheric melting furnace can be used to manufacture by electric melting iron scraps or lumps of each element. When hot metal produced by the blast furnace method is used, an Ar—O ₂ mixed gas bottom-blowing decarburizing furnace, a vacuum decarburizing furnace, or the like can be used. The molten material is solidified by static casting or continuous casting to form ingots or slabs, which are then hot rolled to form plates or round billets into raw materials.

In the case of welded steel pipes in which the plate shape is formed into a cylindrical shape and the ends are welded, UOE steel pipes using plates and electric resistance welded pipes by roll forming can be used. When a round billet is used to make a seamless steel pipe, the round billet is heated in a heating furnace, hot piercing-rolled, and then subjected to a thinning and shaping process to form a steel pipe. Any method such as the Mannesmann method or the extrusion tube manufacturing method can be used as the hot forming (perforation process) for forming a round billet into a hollow tube. In addition, elongator, assel mill, mandrel mill, plug mill, sizer, stretch reducer, etc. can be used for thickness reduction and outer diameter sizing.

The maximum heating temperature in the above hot rolling is preferably 1150° C. or higher.
As described later, solid solution heat treatment and cold working treatment are performed, and oxide layer removal treatment such as pickling and surface polishing is not performed, and the maximum heating temperature in hot rolling is set to 1150 ° C. or higher. , a thicker formed oxide layer can be obtained.

Solution heat treatment After hot forming into a steel pipe shape, air cooling produces various carbonitrides and intermetallic compounds in the steel, so solution heat treatment is performed. In other words, the temperature of the duplex stainless steel during hot rolling gradually decreases during hot rolling from a high temperature state during heating. In addition, it is often air-cooled after hot forming, and the temperature history differs depending on the size and type and cannot be controlled. Therefore, the corrosion-resistant element may become thermochemically stable precipitates and be consumed in various temperature ranges while the temperature is decreasing, resulting in deterioration of corrosion resistance. In addition, there is a possibility that phase transformation to an embrittlement phase will occur and the low temperature toughness will be remarkably lowered. Furthermore, since duplex stainless steel can withstand various corrosive environments, it is important that the austenite phase and ferrite phase fractions are in a two-phase state at the time of use. It becomes difficult to control the two-phase fraction, which changes sequentially depending on the holding temperature.
In view of the above problems, after hot forming, the Solid solution heat treatment with rapid cooling is often used.
In solid solution heat treatment, carbonitrides other than the two phases of ferrite phase and austenite phase, and brittle phases are decomposed by heating (for example, heating at a heating temperature of 1000 ° C. or higher), and then rapidly cooled after heating so as not to reprecipitate. It is a process to
This treatment dissolves precipitates and embrittlement phases into the steel and controls the phase fraction to an appropriate two-phase state. The temperature of the solid solution heat treatment is often a high temperature of 900° C. or higher, although the temperature at which precipitates are dissolved, the embrittlement phase is reversely transformed, and the phase fraction is in an appropriate two-phase state varies depending on the added element. Therefore, in the present invention, the solid solution heat treatment temperature is more preferably 900° C. or higher, more preferably 1000° C. or higher. Moreover, the temperature of the solid solution heat treatment is preferably 1150° C. or lower.
After heating, rapid cooling is performed in order to maintain the solid solution state, and cooling with compressed air or cooling with various refrigerants such as mist, oil, and water can be performed. In the present invention, the surface oxide layer, which is important for wear resistance and dent resistance, can be obtained both after hot rolling and after solid solution heat treatment. Not performed.
The fact that the oxide layer is not removed by pickling after the solid solution heat treatment is not particularly limited as long as the average thickness of the oxide layer of the resulting steel pipe is 1.0 μm or more. It may be possible to limit the oxide layer removal treatment to the minimum range without performing the oxide layer removal treatment such as the entire pipe, and before the solid solution heat treatment in which the oxide layer (oxide film) grows, The oxide layer removal treatment such as surface polishing may be performed, and the oxide layer may not be removed by pickling after the solid solution heat treatment.

Tmax ² ×t/[Cr] ⁴ >1000 Equation (1)
In formula (1), Tmax: maximum heating temperature (° C.) during solid solution heat treatment, t: maximum heating temperature holding time (s) during solid solution heat treatment,
[Cr]: Cr content (% by mass) in the steel pipe.
Tmax is preferably 900 to 1150°C. Preferably, t is between 600 and 3600s.
As described above, the solid solution heat treatment is performed so as to satisfy the above formula (1). Thereby, the average thickness of the oxide layers formed on the outer surface and the inner surface of the steel pipe can be 1.0 μm or more. Also, in order to increase the thickness of the oxide layer, the left side in the above formula (1) is preferably more than 2000, more preferably 2500 or more, and even more preferably 3000 or more. . Moreover, if the oxide layer becomes excessively large and grows too much, it may fall off in the furnace.

Bending back cold working in the pipe circumferential direction (hereinafter also referred to as bending back bending)
Since the steel pipe material after solid solution heat treatment contains the austenite phase, which has a low yield strength, the pipe axial tensile yield strength necessary for oil and gas wells and hydrothermal mining cannot be obtained as it is. Therefore, the strength of the pipe is increased by utilizing dislocation strengthening by various cold workings.

In the present invention, as described below, the yield strength of the pipe is increased by bending and returning in the pipe circumferential direction, thereby stably improving the tensile yield strength in the pipe axial direction and improving the wear resistance and resistance. A surface oxide layer necessary for recessability can be obtained at the same time.
The cold working method of the present invention is a new method that utilizes dislocation strengthening by bending back in the pipe circumferential direction. Based on FIG. 2, this processing method will be described. Unlike cold drawing rolling and cold pilger rolling, which improve the tensile yield strength of a steel pipe by utilizing thinning and stretching in the pipe axial direction, this method, as shown in Fig. 2, reduces strain in the pipe. After bending by flattening (first flattening), it is given by bending back when returning to a perfect circle (second flattening). In this method, the amount of strain is adjusted using repeated bending and unbending and changes in the amount of bending without significantly changing the initial shape of the steel pipe. In other words, while the conventional cold rolling method utilizes elongation strain in the pipe axial direction, the strength of the steel pipe is increased by work hardening using the cold working method of the present invention, whereas bending strain in the pipe circumferential direction is used. is used, and the shape of the steel pipe after bending and unbending does not change significantly. In other words, unlike cold drawing rolling and cold pilger rolling, a new surface due to thinning drawing is unlikely to occur in principle, and the yield strength of the steel pipe can be increased while maintaining the oxide layer on the surface. In addition, unlike cold drawing rolling and cold pilger rolling, which involve deformation due to thinning or stretching, this bending process utilizes shear deformation. Even when the same deformation is applied, bending is a form that can be deformed with a small force, so there is little damage to the bending and unbending cold working tools, and cleaning of the oxide layer with acid before bending and unbending cold working is unnecessary. becomes. In addition, since sliding with the tool is small, chemical treatment film treatment for lubrication is not required. Furthermore, since it is not necessary to arrange a tool on the inner surface of the steel pipe, it is easy to maintain the oxide layer provided by the solid solution heat treatment.

2(a) and (b) are cross-sectional views when there are two tool contact portions, and FIG. 2(c) is a cross-sectional view when there are three tool contact portions. The thick arrows in FIG. 2 indicate the directions in which force is applied when the steel pipe is flattened. As shown in Fig. 2, when performing the second flattening process, the tool is moved to rotate the steel pipe or the position of the tool is shifted so that the tool touches the part where the first flattening process has not been performed. (The shaded area in FIG. 2 indicates the first flattened portion.).

As shown in Fig. 2, by intermittently or continuously applying bending and unbending in the circumferential direction of the pipe to flatten the steel pipe, the strain caused by bending near the maximum curvature of the steel pipe was reduced. is applied, and strain due to unbending toward the minimum curvature of the steel pipe is added. As a result, strain is accumulated due to bending and unbending deformation necessary for improving the strength of the steel pipe (dislocation strengthening). In addition, when using this processing mode, unlike the processing mode that compresses the wall thickness and outer diameter of the pipe, it does not require a large amount of power, and because it is deformed by flattening, the shape change before and after processing is minimized. It is characteristic that it can be processed while holding it.

Rolls may be used for the shape of the tool used for flattening the steel pipe as shown in FIG. It is possible to give a strain by Furthermore, if the rotation axis of the roll is inclined within 90° with respect to the rotation axis of the tube, the steel tube advances in the direction of the tube rotation axis while being flattened, which makes it possible to easily carry out continuous processing. In addition, in continuous processing using these rolls, for example, if the interval between rolls is appropriately changed so as to change the amount of flattening as the steel pipe progresses, the first and second steel pipes can be easily processed. You can change the curvature (flatness) of . Therefore, by changing the interval between the rolls, it is possible to change the moving path of the neutral line and homogenize the strain in the thickness direction. Similarly, the same effect can be obtained by changing the flattening amount by changing the roll diameter instead of the roll interval. Moreover, you may combine these. Although the equipment becomes complicated, if the number of rolls is three or more, whirling of the pipe during processing can be suppressed, and stable processing becomes possible.

Regarding the bending and unbending cold working of the present invention, regardless of which processing mode is used, the ratio of the outer diameter after working (steel pipe diameter after working) Di to the outer diameter before working (initial steel pipe diameter) Do of the steel pipe material ( %) (Di/Do)×100 is preferably 99% or less. This suppresses the increase in inner and outer surface areas in the circumferential direction of the pipe, thereby suppressing the exposure of new surfaces due to deformation, and stably covering the entire steel pipe with an oxide layer that is excellent in wear resistance and dent resistance. A more preferable range of (Di/Do)×100 is 80 to 95% from the viewpoint of the strength characteristics of the steel pipe and the stable formation of the oxide layer.
In addition, regarding the bending and unbending cold working of the present invention, the ratio (%) of the axial length Li after working to the axial length Lo before working of the steel pipe material (%) (rate of change in elongation) (Li/Lo) × 100 is preferably 125% or less.
This suppresses an increase in the inner and outer surface areas in the axial direction of the pipe, suppresses the exposure of new surfaces due to deformation, and stably covers the entire steel pipe with an oxide layer that is excellent in wear resistance and dent resistance. From the standpoint of stably obtaining the strength characteristics of the steel pipe and the oxide layer, the above-mentioned elongation change rate is preferably in the range of 105 to 115%.

The duplex stainless steel pipe of the present invention can be obtained by the above manufacturing method.

In this way, the present invention obtains high yield strength characteristics of duplex stainless steel and excellent wear resistance and dent resistance due to the oxide layer by a bending back bending cold working method that can maintain the oxide layer. This makes it possible to provide a duplex stainless steel pipe that is excellent in corrosion resistance and dimensional accuracy by suppressing scratches and dents in the steel pipe when used in oil and gas wells and hot water drilling (geothermal well applications).

The present invention will be described below based on examples.

A steel material having the chemical composition of steels A to O shown in Table 1 was melted in a vacuum melting furnace, and then hot-rolled into a round billet with an outer diameter of φ80 mm. Steels L, M, and N did not have a suitable two-phase structure because the range of added elements was outside the scope of the invention. Also, the steel to which Cr and Mo were added beyond the scope of the invention cracked during the solidification process from melting or during hot rolling.

After obtaining a seamless steel pipe shape by hot rolling, solid solution heat treatment was performed.
The solid solution heat treatment was performed at the maximum heating temperature Tmax (° C.) shown in Table 2 and the maximum heating temperature holding time t (s).
After that, the axial tensile yield strength of the steel pipe was increased by dislocation strengthening by various cold rolling and cold working. In order to increase the strength, the cold working method of the present invention, i.e., bending and unbending cold working in the pipe circumferential direction, and drawing rolling and bilger rolling for comparison were performed. Before cold drawing and cold pilger rolling, the oxide layer on the surface was removed by washing with an acid. For pickling, a mixed acid of nitric acid and hydrofluoric acid was used, and the oxide layer on the inner and outer surfaces of the steel pipe was removed by immersion in a bath. The immersion time was until the oxide layer was visually completely removed.
The bending and unbending process in the pipe circumferential direction was carried out by selectively using an apparatus in which two rolling rolls were arranged opposite to each other or three rolling rolls were arranged at a pitch of 120° in the pipe circumferential direction. In addition, (Di/Do)×100, which is the ratio (%) of the outer diameter after working (the outer diameter of the pipe after cold working) Di to the outer diameter before working (the initial outer diameter of the mother pipe) Do of the steel pipe material, The axial length of the steel pipe material before working (initial axial length) Lo and the axial length after working (axial length after cold working) Li were measured (in Table 2, Di/Do and Li /Lo). In the drawing rolling and bilger rolling, thinning elongation rolling was performed at a thickness reduction rate in the range of 15 to 60%.

The organization was observed as follows. First, a test piece for tissue observation was taken so that the cross section in the tube axial direction was the observation surface. The volume fractions of the ferrite phase and the austenite phase were obtained by observing the observation surface with a scanning electron microscope. Specifically, the test piece for tissue observation described above was corroded with a Birrella reagent (picric acid, hydrochloric acid, and ethanol mixed in proportions of 2 g, 10 ml, and 100 ml, respectively) and scanned with a scanning electron microscope (SEM) ( Tissues were imaged at 1000x). From the obtained structure photograph, the average value of the area ratios of the ferrite phase and the austenite phase was calculated using an image analyzer, and this was defined as the respective volume ratios (% by volume).

In the captured image, the ferrite phase is the phase that becomes white by binarization because it is difficult to corrode, and the austenite phase is the phase that becomes black by binarization because it is easy to corrode. The above binarization was performed on the range of the measurement area (600 μm×800 μm (1920 pixels×2560 pixels)) after converting the captured image into a grayscale image of 256 gradations. Regarding the setting of binarization, the minimum luminance between two peaks seen in a histogram with luminance (256 gradations) on the horizontal axis was used as a threshold value. The martensite phase is easily corroded in the image taken before binarization, so it turns gray, but unlike the austenite phase, which is also colored gray, the shade of gray due to the substructure composed of blocks, laths, etc. can be confirmed. Therefore, the martensite phase was obtained by measuring the area of the range where the substructure was confirmed in the gray part of the captured image. In addition, when an embrittlement phase is generated, it is generated at the grain boundary of the ferrite phase and turns black after corrosion. The embrittlement phase was obtained by measuring the area of the black part.

Table 1 shows the confirmation of the two-phase state by observing the structure of the obtained steel pipe and the results of measuring the ferrite phase fraction.

The oxide layer is obtained by cutting the steel pipe into round slices, mirror-polishing the cross section, and measuring the oxygen concentration from the inner and outer surfaces using energy dispersive X-ray analysis. layered. The thickness (average thickness) of the oxide layer was measured at 5 points (at 5 points equidistantly in the circumferential direction of the tube), and the average value (total thickness of 5 points/5) was obtained. Table 2 shows the thickness of the oxide layer of each steel pipe obtained.

The coverage ratio of the oxide layer on the steel pipe surface is obtained by dividing the surface area of the pipe where no oxide layer is formed (non-covered area) from the surface area of the entire pipe calculated from the outer diameter, wall thickness and length of the pipe. was calculated as a percentage. A region where no oxide layer is formed is produced by polishing with abrasive grains or pickling, and has a metallic luster, so the surface area of that region can be easily measured.
Specifically, the non-coated area was defined by an enclosure (rectangle) parallel to the pipe circumferential direction and the pipe axial direction so as to enclose the region judged to have been visually polished or pickled. Then, the area was calculated using the length in the circumferential direction of the pipe (the length of the rectangle) and the length in the direction of the pipe axis (the width of the rectangle). This area was defined as "the length in the circumferential direction of the tube (the length of the rectangle)×the length in the direction of the tube axis (the width of the rectangle)". Then, for this area, the sum for one steel pipe was obtained.
Next, regarding the surface area of the entire steel pipe (the surface area of the entire pipe other than the pipe end cut portion is the surface area of the entire pipe), the outer diameter and wall thickness of the steel pipe are used to determine the outer and inner circumference lengths of the steel pipe. It was obtained by multiplying and adding together. The outer diameter, wall thickness, and length were all calculated using the average length. Then, the coating rate of the steel pipe surface with the oxide layer was obtained by dividing the above uncoated area by the surface area of the entire steel pipe and obtained as a percentage (%).
Table 2 shows the surface coverage of the oxide layer of each of the obtained steel pipes.

Axial compressive yield strength and axial tensile yield strength are obtained by cutting out a round bar tensile test piece and a cylindrical compression test piece with an outer diameter (diameter) of 5.0 mm from the center of the wall thickness of the end of the pipe used for the pressure test. , respectively, were tested at compression and tensile speeds of 1.0 mm/min, and stress-strain curves were measured in room-temperature tension and compression tests. From this stress-strain curve, the tensile yield strength in the tube axial direction and the compressive yield strength in the tube axial direction were calculated.

Specifically, first, the compression yield strength in the tube axial direction was measured by a cylinder compression test. Cylindrical test pieces to be compressed were taken from the center of the wall thickness parallel to the pipe axis direction. A test piece was cut out from the central portion of the tube with a cylinder outer diameter d of 5.0 mm and a cylinder height h of 8.0 mm. In the compression test, a test piece was sandwiched between flat plates at room temperature (25°C) and a load was applied, and the compression yield strength was calculated using the stress-strain curve obtained during compression. A stress-strain curve was obtained by performing 30% compression with a compression tester at a crosshead speed of 1.0 mm/min.

In addition, the tensile yield strength in the tube axial direction was determined according to JIS Z2241. First, as a test piece, a round bar tensile test piece having a parallel portion diameter of 5.0 mm was cut from the thickness center of the tube parallel to the tube axial direction. Then, a tensile test was carried out at room temperature (25° C.) at a crosshead speed of 1.0 mm/min until breakage. Using the stress-strain curve thus obtained, the tensile yield strength was calculated.
In addition, a needle with a carbide tip (conical shape (angle of triangular tip (contact part with steel pipe) with a cross section perpendicular to the bottom of the cone: 60°)) is used for the indenter, and the tip is parallel to the pipe axis direction. Sweep the tube surface at 3 mm / s and perform a 30 mm scratch test with a load of 59 N, and the unevenness difference (formed by scratching) The maximum value of the height difference in the thickness direction between the projected portion and the recessed portion) was measured. When the difference in dent height was 50 μm or less, the wear resistance and the dent resistance were judged to be excellent, and it was judged as acceptable.

In addition, crystal orientation analysis was performed by EBSD on the thickness center of the obtained steel pipe axial cross section, and the aspect ratio of austenite grains separated by a crystal orientation angle of 15° was measured. The measurement area was 1.2 mm×1.2 mm, and assuming that the grains were perfect circles having the same area, the aspect ratio was measured for austenite grains with a diameter of 10 μm or more. Also, the area fraction of austenite grains having an aspect ratio of 9 or less was calculated. The area fraction was calculated as a percentage of the total area of austenite grains with an aspect ratio of 9 or less with respect to the total area of austenite grains with a grain size of 10 μm or more.

From the results in Table 2, it was confirmed that all the examples of the present invention achieved both a high tensile yield strength in the tube axial direction of 689 MPa or more and the formation of an oxide layer. Obtained. On the other hand, under the conditions of cold drawing and cold pilger rolling, which are conventional cold rolling methods, it is not possible to achieve both high yield strength and an oxide layer, and as a result, the results of the scratch test are inferior. It was shown that the wear resistance and dent resistance when used as steel pipes for geothermal wells (for hot water recovery) are inferior.

Claims

in % by mass,
C: 0.005 to 0.150%,
Si: 1.0% or less,
Mn: 10.0% or less,
Cr: 11.5 to 35.0%,
Ni: 0.5 to 15.0%,
Mo: 0.5-6.0%,
N: having a component composition containing less than 0.400% and the balance consisting of Fe and inevitable impurities,
Having a steel structure with a ferrite phase and an austenite phase,
The tube axial direction tensile yield strength is 689 MPa or more,
A duplex stainless steel pipe having an oxide layer with an average thickness of 1.0 μm or more on each of the steel pipe outer surface and the steel pipe inner surface.
The duplex stainless steel pipe according to claim 1, wherein the oxide layer covers 50% or more of each of the steel pipe outer surface and the steel pipe inner surface in area ratio.
The duplex stainless steel pipe according to claim 1 or 2, wherein the pipe axial compressive yield strength/tube axial tensile yield strength is 0.85 to 1.15.
As the component composition, further by mass%,
W: 6.0% or less,
Cu: 4.0% or less,
V: 1.0% or less,
Nb: The duplex stainless steel pipe according to any one of claims 1 to 3, containing one or more selected from 1.0% or less.
As the component composition, further by mass%,
Ti: 0.30% or less,
Al: The duplex stainless steel pipe according to any one of claims 1 to 4, containing one or two selected from 0.30% or less.
As the component composition, further by mass%,
B: 0.010% or less,
Zr: 0.010% or less,
Ca: 0.010% or less,
Ta: 0.30% or less,
Sb: 0.30% or less,
Sn: 0.30% or less,
REM: The duplex stainless steel pipe according to any one of claims 1 to 5, containing one or more selected from 0.010% or less.
A method for manufacturing a duplex stainless steel pipe according to any one of claims 1 to 6,
Hot rolling is applied to the steel pipe material to make it into a steel pipe shape,
After the hot rolling, the steel pipe material is subjected to a solid solution heat treatment that satisfies the following formula (1),
A method for producing a duplex stainless steel pipe, wherein cold bending and unbending is performed in the circumferential direction of the pipe without removing the oxide layer formed on the steel pipe material after the solid solution heat treatment.
Tmax 2 ×t/[Cr] 4 >1000 Equation (1)
In formula (1),
Tmax: Maximum heating temperature (° C.) during solid solution heat treatment,
t: holding time (s) at the maximum heating temperature during the solid solution heat treatment,
[Cr]: Cr content (% by mass) in the steel pipe.
The method for producing a duplex stainless steel pipe according to claim 7, wherein the maximum heating temperature in the hot rolling is 1150°C or higher.
By performing the bending and unbending cold working, the diameter of the steel pipe material is reduced,
9. The production of a duplex stainless steel pipe according to claim 7 or 8, wherein (Di/Do)×100, which is the ratio (%) of the post-work outer diameter Di to the pre-work outer diameter Do of the steel pipe material, is 99% or less. Method.
By performing the bending and unbending cold working, (Li/Lo) × 100, which is the ratio (%) of the post-working axial length Li to the pre-working axial length Lo of the steel pipe material, is 125% or less. The method for manufacturing a duplex stainless steel pipe according to any one of claims 7 to 9.