Title: Steel composition for expandable tubular articles, expandable tubular article having this steel composition, manufacturing method thereof and use thereof

Technical field

The present invention concerns the technical field of expandable tubular articles from steel. In particular the invention relates to a steel composition for expandable tubular articles, expandable tubular articles having this composition, a manufacturing method of an expandable tubular article from the composition, as well as preferred uses thereof.

Background of the invention

Wellbores for producing oil, gas or other fluids from subsurface formations are drilled into the earth. The wellbore is provided with a casing made up of interconnected tubular steel segments in order to prevent the wellbore from collapsing and to maintain an appropriate fluid flow from the well. The annulus between the wellbore and casing is generally filled with cement to support the casing and isolate downhole formations from one another.

Oil and gas downhole expandable systems are known. These systems comprise tubular segments that can be expanded downhole, mainly in the hoop direction, at the prevailing temperature by applying sufficient internal pressure and force, thereby achieving a diameter larger than the original one, while maintaining their functional properties as strength to withstand the wellbore pressure and open area allowing fluid flow. For example, an expansion tool is inserted into the desired section of the casing/liner. This tool is typically composed by an expandable tubular element made of steel - with a length suitable to cover the desired area - and an expanding element, also known as a mandrel. The mandrel is designed with expansion elements such as slips (a kind of wedges) or cones. When the mandrel is activated, it causes the expansion elements to move within the inner diameter of the expandable tubular element, exerting radial force against the inner wall of the expandable tubular element. The expansion force causes the expandable tubular element to undergo plastic deformation. This expansion process increases the diameter of the expandable tubular element and creates an expanded section in the casing/liner. The expansion rate is defined as the ratio between the inner diameter of the expandable tubular element after and before the expansion process. For instance, an inner diameter expansion rate of 15% means that the inner diameter of the expanded tubular is 15% larger than it originally was before the expansion process. Once the expansion is complete and the desired result is achieved, the expanding element is removed, and the well can proceed with its intended operations. For example, these expandable systems are used for wellbore completion, repairing leaks in existing liners or casings, sealing existing perforations in shale fields as occurs upon reperforating and/or refracturing.

High strength steel tubular products as typically used in Oil Country Tubular Goods (OCTG), given the generally restricted ductility thereof, are less suitable as expandable tubular elements, because such use thereof may bring about drawbacks like localized deformations, hard spots, tearing, cracks and ruptures. Even if an expanded tubular element shows no visible damage, the expanded tubular element may be hardened to a level such that residual ductility is not suitable for the final application. As a result of insufficient strength and/or ductility, toughness may be also compromised. In order to minimize the risk of damage resulting from the downhole expansion, the expansion rate (ratio between the post expansion diameter and the original diameter of the tubular) could be reduced substantially. This limitation reduces the versatility of the expandable tubular element, making it only suitable for jobs where the expansion is not critical.

Lower strength steels would have an increased ductility and therefore could be designed for higher expansion rates. However, due to their lower strength tubular elements manufactured from these lower strength steels are less suitable for withstanding the high wellbore pressure. Thus the versatility of these expandable tubular elements from lower strength steels is also limited. Increasing the wall thickness of the expandable tubular elements without changing the material, in order to allow to withstand the pressure occurring in a wellbore, would reduce the flow area and thus during use result in a reduced flow affecting the productivity.

Expandable tubular systems for downhole operation are commercially available and are typically classified according to their minimum specified yield strength. It is assumed that the high grades having a yield strength in the range of e.g. 552 - 862 MPa (80 - 125 ksi) have a significantly lower expansion rate than can be achieved with lower grades in the yield strength range of 345 - 517 MPa (50 - 75 ksi).

Thus expandable tubular elements are required to have a balance of properties regarding ductility, toughness and strength, in order to allow the elements to be cold expanded downhole to the required diameter and after expansion to provide sufficient resistance to the wellbore pressure, while ensuring an appropriate flow area.

Now US2004/0228679A1 discloses a very low carbon steel alloy for use in manufacturing tubular elements of the 552 MPa (80 ksi) grade, such as oil country tubular goods (OCTG), in particular electric resistance welded products, comprising (in wt.%) C: 0.03 - 0.06; Mn: 1.40 - 1.50; P: < 0.015; S: < 0.005; Si: 0.15 - 0.30; Cu: <0.10; Ni: < 0.10; Cr: < 0.10; Mo: < 0.06; V: 0.05 - 0.08; Sn: < 0.01; Al: 0.015 - 0.040; Ca: 0.0005 - 0.0055; Nb: 0.030 - 0.050; B: < 0.0005; Ti: and N: <0.010, wherein sum of V + Nb + Ti < 0.15. The tubular elements manufactured from this composition, such as a welded pipe after quenching and tempering have a microstructure comprising martensite of at least 90 vol.%. According to US2004/0228679A1 the tubular elements thus formed may be radially expanded approximately 20 - 45 % or greater downhole in the wellbore depending upon overall design of the associated well completion, and still be able to withstand 24131 MPa (3500 psi) internal pressure. US2009/0044882A1 discloses an oil well pipe for expandable tubular product applications comprising, in wt.%, C: 0.03 - 0.14, Si: 0.8 or less, Mn: 0.3 - 2.5, P: 0.03 or less, S: 0.01 or less, Ti: 0.005 - 0.03, Al: 0.1% or less, N: 0.001 - 0.01%, B: 0.0005 - 0.003%, optionally one or more of Nb, Ni, Mo, Cr, Cu and V, and further optionally one or both of Ca and REM, balance of iron and unavoidable impurities, and satisfying the relationship A=2.7C+0.4Si+Mn+0.45Ni+0.45Cu+0.8Cr+2Mo^1.8. The expandable tubular product has a tempered martensite structure. The yield strength amounts to 483 - 689 MPa (70 - 100 ksi). The present invention aims at providing a steel composition that allows manufacturing an article having a useful property combination of ductility (in terms of total elongation) and strength before expansion and strength and toughness after expansion, in particular a downhole expandable tubular element for the oil and gas industry providing a useful combination of resistance to wellbore pressure and adequate inner diameter for production flow area.

The present invention also aims at providing an expandable tubular article having a yield strength of at least 621 MPa (90 ksi), preferably of at least 689 MPa (100 ksi). Furthermore, the invention aims to provide a method of manufacturing an expandable tubular article from the steel composition, as well as its use as an expandable liner for a casing for an oil and/or gas well, such as in refracturing.

Summary of the invention

Accordingly, the present invention concerns a steel composition for expandable tubular articles comprising, in mass %, C: 0.03 - 0.12;

Mn: 0.7 - 1.2; Si: 0.1 - 0.5;

Cr: 0.1 - 1.0;

Mo: 0.1 - 1.0; Ni: >0 and < 1.0;

V: 0 - 0.20;

Nb: 0 - 0.10; iron (Fe) and inevitable impurities with the proviso that V + Nb > 0.01. The steel composition according to the invention allows to manufacture an expandable tubular article, in particular for use in an oil and gas downhole expandable system, having a balanced combination of ductility (in terms of total elongation), strength (620 MPa (90 ksi) minimum specified yield strength) and toughness, that after expansion provides resistance to wellbore pressures and maximized inside diameter for increased production flow area. In the context of this application an expandable tubular article is to be understood as a structure manufactured from a steel composition and having the shape of an elongated tube, such as a pipe having a bore with a circular cross section, that is used in a well operation by expanding the tubular article by permanently increasing the inner diameter of the tubular article at a downhole location in an oil or gas wellbore.

Generally, the steel composition has a low carbon content to ensure toughness and ductility, but high enough to offer elevated tensile properties. Nickel improves the hardenability and toughness, without promoting carbon segregation, which could result in inhomogeneous properties and affect ductility and toughness. Molybdenum and vanadium in the ranges according to the invention ensure elevated tempering resistance, permitting to achieve the strength aimed for in combination with ductility. Hardenability is ensured by nickel, chromium and molybdenum. Toughness is also ensured by microalloying with niobium and vanadium for refining prior austenitic grains size, as well as by the presence of nickel. Furthermore, the total amount of alloying elements nickel, chromium and molybdenum is low, which is beneficial in view of expenses.

Titanium and aluminium may be contained as optional elements in the respective ranges Ti: 0 - 0.10 and Al: 0 - 0.10.

Typically inevitable impurities include nitrogen, phosphorous, sulphur, boron and copper. Typically these impurities may be present within the limited ranges discussed below. In the elemental compositions as discussed below the balance is iron and inevitable impurities.

In a second aspect the invention relates to a method of manufacturing an expandable tubular article comprising the steps of a) providing a tubular component; b) austenitizing the tubular component at a temperature in the range of Ac3 - 1000 °C; c) quenching the austenitized tubular component from step b) to a temperature below Mf at a quenching rate of 20 °C/s or more in the range of 800 - 500 °C; d) tempering the quenched tubular component from step c) at a tempering temperature in the range of 300 °C - Ac1 ; wherein the tubular component has a composition according to the invention.

The invention also relates to an expandable tubular article having the above composition, preferably manufactured as outlined in the above process. In a further preferred embodiment thereof the expandable tubular article having a composition according to the invention and manufactured according to the invention has a microstructure predominantly (sum of martensite and bainite > 55 vol.%) comprising martensite, bainite or a mixture of bainite and martensite wherein the sum of ferrite and pearlite < 10%. Preferably the sum of martensite and bainite amounts to 90 % or more. Most preferably the microstructure comprises > 80 % martensite, up to 20 % bainite and less than 10 % ferrite and/or pearlite. Martensite and bainite are considered important in view of the desired combination of strength and toughness after expansion. The presence of some minor amounts of ferrite and pearlite is allowable, but not desirable in view of the properties aimed for.

Advantageously the expandable article according to the invention is used in expandable tubular application in a wellbore, for example as an expandable liner in refracturing an oil or gas well.

Detailed description of the invention

Composition

In an embodiment the steel composition consists (in mass %) of:

C: 0.03 - 0.12;

Mn: 0.7 - 1.2;

Si: 0.1 - 0.5;

Cr: 0.1 - 1.0;

Mo: 0.1 - 1.0;

Ni: > 0 and < 1.0;

V: 0 - 0.20;

Nb: 0 - 0.10;

Ti: 0 - 0.10;

Al: 0 - 0.10.

Cu: 0 - 1.0;

B: 0 - 0.010;

N: 0 - 0.030;

P: 0 - 0.030;

S: 0 - 0.030. the balance being iron (Fe) and other inevitable impurities with the proviso that V + Nb > 0.01.

In an embodiment the steel composition consists (in mass %) of: C: 0.05 - 0.10;

Mn: 0.7 - 1.2; Si: 0.1 -0.5;

Ni: 0.1 -0.7;

Cr: 0.2 - 0.9;

Mo: 0.2 -0.7;

V: 0.02-0.12;

Nb: 0.01 - 0.06;

Ti: 0-0.05;

Al: 0-0.05;

Cu: 0-1.0;

B: 0 - 0.0050;

N: 0-0.015;

P: 0 - 0.020;

S: 0 - 0.020; the balance being iron (Fe) and other inevitable impurities.

In an embodiment the steel composition consists (in mass %) of:

C: 0.06 - 0.09;

Mn: 0.8- 1.1;

Si: 0.1 -0.3;

Ni: 0.2 - 0.7, preferably 0.3 - 0.6

Cr: 0.4 -0.8;

Mo: 0.25 - 0.60;

V: 0.03 - 0.08;

Nb: 0.02 - 0.04;

Ti: 0-0.04;

Al: 0-0.04;

Cu: 0-1.0;

B: 0-0.030;

N: 0-0.008;

P: 0-0.015;

S: 0-0.015; the balance being iron (Fe) and other inevitable impurities.

Composition details

The following detailed explanation for the elemental chemical composition is provided:

Carbon (C) is required to strengthen the steel by means of precipitation of carbides in the last stage of transformation; however, an excessive amount of carbon results in a loss of toughness and ductility. Therefore, C is in the range of 0.03 - 0.12%; preferably 0.05 - 0.10%; and more preferably 0.06 - 0.09%.

Manganese (Mn) is an important alloying element, with different functions. Upon cooling of austenite, it lowers the transformation temperature of austenite into ferrite, thereby promoting the nucleation of fine grains. Upon accelerated cooling, Mn increases hardenability by slowing the diffusion of C. Excessive amounts of Mn promote C segregation, resulting in inhomogeneous properties in the final product. Therefore Mn is in the range of 0.7 - 1.2%, and preferably 0.8 - 1.1%.

Silicon (Si) is usually added for killing (deoxidizing) the steel. However, large amounts have an adverse effect on toughness. In addition, Si increases the sensitivity to temper embrittlement by enhancing segregation of P at grain boundaries. Therefore the Si content is limited to a maximum of 0.50%; advantageously, a minimum 0.10% Si is included to ensure full killing. More preferably, Si is in the range of 0.10 - 0.30%.

Nickel (Ni) is an austenite stabilizer, which similarly to Mn promotes nucleation of finer ferrite grains upon cooling from austenite by means of lowering the transformation temperature. Upon accelerated cooling, Ni increases the hardenability of the steel. Also, Ni is known to improve toughness., Therefore Ni is present in the steel composition. A minimum amount of 0.10% Ni is desirable, preferably 0.20% and more preferably 0.30% and above. As a generally expensive addition, preferably the Ni content is limited to 1.0% and below. Excellent properties may still be achieved if Ni is 0.7% or less, or even 0.6% or less.

Chromium (Cr) is effective in increasing the hardenability of the steel, and, as a carbide former, allows the formation of bainite upon continuous cooling. Very high amounts of Cr are unnecessary, and increase the cost of steelmaking. Therefore, Cr is in the range of 0.1 - 1.0%, preferably 0.20 - 0.90%, and more preferably 0.40 - 0.80 %, such as 0.30 - 0.60%. Molybdenum (Mo) increases the hardenability of the steel, and is a strong carbide former, therefore allowing the formation of bainite upon continuous cooling. Mo enhances the resistance to tempering due to forming stable carbides, allowing to maintain a desirable strength level while improving toughness and reducing internal stresses. Large amounts of Mo are not desirable mainly due to economic reasons. Mo is included in amounts in the range of 0.1 - 1.0%, preferably 0.20 - 0.70%, and more preferably 0.25 - 0.60%.

Advantageously the sum of the amounts of Ni, Cr and Mo is 2.0% or less.

In an embodiment having Cr and Mo alloying, Cr > 0,40, such as in the range of 0.40 - 0.80% and Mo > 0.25, for example in the range of 0.25 - 0.60.

Vanadium (V) is a strong carbide and nitride former, and is included for increasing hardenability, achieve precipitation hardening, and refining the austenite grain size. Its effectiveness as refining element is limited by its solubility in austenite at higher temperatures. Optionally, V can be included up to 0.20%. A positive effect on the final properties is achieved when V is included in amounts in the range of 0.02 - 0.12%, preferably 0.03 - 0.08%.

Niobium (Nb) and Titanium (Ti) are both strong carbide and nitride formers. Their role is similar to V in controlling austenite grain size, and are more effective than vanadium due to their low solubility in austenite. Titanium is more effective than Nb at higher temperatures (above about 1100 °C), whereas Nb generally results in a finer dispersion of precipitates and therefore allows achieving a very fine prior austenitic grain size. Large amounts of Ti or Nb may result in the precipitation of coarse carbo-nitrides, reducing their effectiveness. The upper limit of Nb is 0.10. If Nb is intentionally added, its content is preferably in the range of 0.01 - 0.06%, and more preferably in the range of 0.02 - 0.04%. If Ti is intentionally added, its amount is limited to 0.10%, preferably 0.05% or less, and more preferably 0.04% or less. In a Ti microalloyed embodiment the steel composition comprises > 0.010% Ti.

The sum of the amounts of V and Nb (in mass %) is at least 0.01%, preferably at least 0.02% and more preferably at least 0.05%. In an embodiment that is V microalloyed, V > 0.05% and Nb < 0.010%. In a Nb microalloyed embodiment Nb > 0.015% and V < 0.030%. In a V Nb microalloyed embodiment V > 0.05% and Nb > 0.015%.

Boron (B) is considered an inevitable impurity. It increases the hardenability of steel and may be used on purpose to achieve a fully martensitic structure over thicker sections. Its effect peaks in the range of 0.0010 - 0.0030% depending on C concentration. Its content is advantageously limited to a maximum of 0.010%, preferably 0.0050% or less, more preferably 0.0030% or less. For thick sections having a thickness of 25.4 mm (1 inch) or more advantageously B is in the range of 0.0010 - 0.0030 and Ti/3.4 + Nb/6.6 > N.

Aluminium (Al) is a deoxidizing element and a nitride former. A minimum amount is advisable to ensure sufficient deoxidation (in combination with Si), and allows to bind residual nitrogen; its addition may not be necessary in the presence of other nitride formers such as, for example, V, Nb or Ti. Excessive amounts may result in large non-metallic inclusions.

Therefore the Al content is limited to 0.10% and less, preferably 0.05% and less, and more preferably 0.04% or less.

Nitrogen (N) is an inevitable impurity in steelmaking. Free N needs to be avoided because it increases the ageing effect, reducing the ductility and toughness of cold formed products. Residual nitrogen therefore needs to be bound in the form of compounds by the addition of nitride formers such as, for example, V, Nb, Ti. N is limited to 0.030% or less, preferably 0.015% or less, and more preferably 0.008% or less. Preferably AI/1.9 + Ti/3.4 + Nb/6.6 + V/3.6 > N, wherein the amounts of the elements are expressed in mass %.

Phosphorous (P) and Sulphur (S) are also typical inevitable impurities in steelmaking. Residual amounts thereof affect ductility and the toughness of the final product, and advantageously should be controlled such that the maximum permitted amount is 0.030% or less, preferably 0.020% or less, and more preferably 0.015% or less.

Copper (Cu) slightly improves hardenability and is inevitably found in scrap steel. Therefore , in the context of this invention Cu is considered an inevitable impurity. However, large amounts of Cu may produce hot shortness, which decreases the surface quality (increased roughness) of hot finished products, and may also result in serious and unrepairable defectiveness. Therefore advantageously the Cu content is limited to a maximum of 1.0%.

Hardenability: To achieve a predominantly martensitic and/or bainitic microstructure in view of the combination of strength and toughness aimed for the chemical composition preferably meets the following formula in order to ensure sufficient hardenability:

■ P _H = f x (C + Ni/41 + Cu/35 + Si/21 + Mn/16 + Cr/12 + Mo/10 + V/8) x 100

If B > 0.0010, f= 3.5; else, f = 1, wherein the amounts of the elements are expressed in mass %.

PH is at least 20, preferably 24 and more preferably 27 or higher.

In alternative to meeting the formula above, the microstructure of the article contains 10 vol. % ferrite or less.

Tempering resistance: To achieve good ductility and toughness, the article is tempered at elevated temperature. However, the steel should be resistant to tempering, to ensure the required strength is maintained. Tempering resistance can be calculated by the following formula:

■ PT = 485x V + 113x Mo + 4x In(Mo) + 18x In(Cr) + 12x In(Si) + 67, wherein the function “In” indicates the natural logarithm, wherein the amounts of the elements are expressed in mass %;

Advantageously PT is at least 90, preferably 100 or higher, and more preferably 110 or higher.

Inclusions

Reducing the amount of non-metallic inclusions, and controlling the size and shape thereof improves toughness, fatigue resistance, and reduces sensitivity to hydrogen embrittlement. In order to achieve a low non-metallic inclusions content vacuum degassing is performed during preparation of the composition.

Advantageously the maximum content of non-metallic inclusions, if any, conforms to (ASTM E45):

A (sulphide)

Thin: 0;

Heavy: 0;

B (alumina) Thin: 2.0, preferably 1.0;

Heavy: 1.0, preferably 0;

C (silicates)

Thin: 1.5, preferably 1.0;

Heavy: 1.0, preferably 0;

D (globular oxide)

Thin: 2.0, preferably 1.5;

Heavy: 1.0, preferably 0.5.

Oversize inclusions can be present up to a maximum size of 50 pm, preferably less than 30 pm.

Process

In the method of manufacturing an expandable tubular article according to the invention in a first step a) a tubular component having a steel composition according to the invention as presented above is provided. Typically, this step a) comprises preparing the steel composition, casting the composition into a billet and forming the tubular component from the billet, e.g. by hot forming (hot rolling) or cold forming (cold rolling, cold drawing), and combinations thereof. The tubular component may comprise a weld, e.g. Electric Resistance Welding. A seamless tubular component is preferred.

In an embodiment the billet, casted from the composition, is subjected to piercing at elevated temperature, and then to hot rolling the pierced billet in at least one hot rolling pass, optionally comprising an intermediate reheating step between two hot rolling passes to a temperature above Ac3.

For example, a starting product from a low carbon steel composition according to the invention, typically a solid steel bar or billet made by casting in the steel shop that can be pierced, is shaped into a hollow (seamless) length of tubing. The solid billet has e.g. a circular shape. Then the solid billet is heated and pierced, e.g. using the Mannesmann process, and subsequently hot rolled in one or more subsequent hot rolling passes in a hot rolling mill, during which the outside diameter and wall thickness are substantially reduced, while the length is substantially increased.

The tubular product thus obtained is austenized in step b) at a temperature in the range of Ac3 - 1000 °C, such as 860 - 940 °C, and subsequently quenched and tempered. Quenching after sufficient soaking time at the austenitizing temperature is typically performed at a high cooling rate down to a temperature below the martensite finish temperature Mf, for example at a rate of 20 °C/s or more in the relevant temperature range, typically 500 - 800 °C, to avoid excessive ferrite formation in the relevant temperature range, e.g. down to ambient temperature thereby ensuring the transformation into a predominantly martensitic and/or bainitic microstructure, while preventing the formation of undesired ferrite. Typically tempering is performed at a tempering temperature in the range of 300 ° - Ac1, e.g. 300 °C - Ac1 minus 10 °C, such as 300 - 700 ° C, for example 400 - 700 °C, in particular 600 - 700 °C, and preferably in the range between about the temperature of secondary hardening given by precipitation of carbides and lower than the martensite recrystallization temperature, for a period of time of at least 10 minutes, and preferably at least 2 minutes per mm of thickness. The expandable tubular article thus obtained may be subjected to standard finishing operations. In order to preserve the expandability capacity advantageously any cold deformation in the finishing operations after the tempering treatment is 10% or less, preferably 5% or less,. These finishing operations do not include the cold deformation in the final application in in-well expansion, which expansion can be as high as 20 - 25%.

A preferred use of the expandable article according to the invention in expandable tubular applications downhole in a well, is as an expandable liner for a casing for an oil and/or gas well, such as in refracturing.

Microstructure

The expandable tubular article according to the invention obtained from the above steel composition using the above process preferably has a predominantly martensitic and/or bainitic microstructure, wherein the sum of martensite and bainite >55 vol.%, preferably > 90 vol.%, and the sum of ferrite and pearlite < 10%. Preferably the sum of martensite and bainite amounts to 90 % or more. Most preferably the microstructure comprises > 80 % martensite and up to 20 % bainite in view of the combination of strength and toughness aimed for. It is assumed that some ferrite and/or pearlite may be present in the microstructure without seriously affecting these properties, but as they do not show an additional positive effect on these properties, preferably their presence is limited.

Mechanical Properties

The expandable tubular article according to the invention could also be characterized by its properties regarding strength, ductility and/or toughness. In an embodiment an expandable tubular article having a composition according to the invention has after expansion of the inner diameter of the expandable tubular article of 15%, the properties

Yield strength (YS): > 620 MPa (90 ksi), preferably > 689 MPa (100 ksi); and

Charpy impact test: absorbed energy at -20 °C: > 80 J, preferably > 100 J (ASTM A370 full size specimen of 10 mm x 10 mm x 55 mm).

The expansion test can be performed as a full scale test, wherein a mandrel of a suitable size (expansion ratio) is inserted in a pre-expanded end of a tubular element. The mandrel is attached to a rod that extends through the bore of the tubular element beyond the non- expanded end. The pre-expanded end is clamped in a stationary die head of a cold drawn bench and the rod is pulled by the bench car, thereby passing the mandrel through the tubular element over the full length thereof to expand the inner diameter of the tubular element. On laboratory scale, similarly drawing a mandrel through a section of the tubular element using a suitable hydraulic press can be performed.

Samples of the tubular element thus expanded or tubular element section thus expanded are tested for their properties using standard test methods.

In a further embodiment thereof before expansion an expandable tubular article has the properties of

Yield strength (YS): > 620 MPa (90 ksi), preferably > 689 MPa (100 ksi); and

Charpy impact test: absorbed energy at -60 °C : > 120 J, preferably > 150 J (ASTM A370 full size specimen).

Advantageously before expansion the expandable tubular element also has at least one of the properties selected from the group consisting of:

Total elongation > 18%, preferably > 20%;

Lateral expansion: > 1.8 mm, preferably > 2.0 mm;

Shear area: > 85%, preferably 100% ductile down to -40 °C, preferably down to - 60 °C, more preferably down to -80 °C.

The above tensile properties are determined according to the respective standards ASTM A370, E8 and E23, the impact properties according to ASTM A370 and ASTM E22.

Examples

The invention is also illustrated by means of the following Examples.

Table 1 lists the chemical composition of a steel according to the invention and that of an expandable tubular article of a commercially available 110 ksi grade as comparative example.

Table 1. composition

The composition of the example according to the invention (‘Invention Ex. T) and the commercially available composition as a Comparative Example (‘Comp. Ex.’) were cast into a billet and made into tubular articles having dimensions of 107.95 mm (4.25 inch) Outer Diameter (OD) and Wall Thickness (WT) of 6.35 mm (0.25 inch). The conditions applied included an austenitizing temperature of 890 °C, soaking time of 15 minutes, cooling with a water spray (cooling rate in the range of 800-500 °C assumed to be at least 20 °C/s, tempering temperature of 640 °C and soaking time of 26 minutes. For the Invention Ex. 1 eight billets were manufactured. From each hot rolled billet 3 lengths of pipe were obtained. The microstructure of the Invention Ex. 1 after tempering was predominantly constituted by tempered martensite in more than 80% with clusters of bainite being the remainder. Neither ferrite, nor pearlite were observed. The prior austenite grain size number was 9-10 as determined according to ASTM E112. Each length thus obtained was subjected to various tests in order to determine their properties. The yield strength, tensile strength and elongation are determined according to ASTM A370. Table 2 lists the range of the measured properties for the twenty four lengths of pipe and the Comp. Ex. Table 2. Properties of expandable tubular articles I I | I |

Some of the lengths of pipe according to the invention were also subjected to a full scale expansion test on a cold drawing bench to an internal diameter increase of 15% (‘Exp. 15%’) under surface conditions A (Surf. Cond. A) and B (Surf. Cond. B). In these tests surface condition A means that lubricant was applied both on the mandrel and on the length of pipe. In surface condition B lubricant was applied only to the mandrel. Properties of the expanded tubular articles are listed in the below tables. The listed values are the average of three specimens.

Table 3. Yield Strength (YSO.2%)

Table 4. Tensile Strength (UTS) Table 5. Total Elongation

Table 6. Absorbed Energy Table 7. Shear Area

Table 8. Lateral Expansion These experiments show that the test tubes as originally manufactured according to the invention (as received) have an improved combination of properties, in particular regarding elongation and toughness compared to the commercial example. Upon expansion of the test tubes according to the Invention Example and the Comparative Example a similar loss in yield strength occurs, but the Invention Example shows a larger increase in tensile strength indicating a stronger strain hardening effect. The reduction in elongation properties of the expanded Invention Example is larger, but as in downhole practice expansion occurs only once this is less significant compared to the increase in tensile strength.

From the impact and lateral expansion data it appears that the Invention Example is superior to the Comparative Example at all measurement temperatures, evidencing the enhanced residual toughness.

In summary, the Invention Example showed superior properties, in particular regarding elongation and toughness before the expansion process. After the 15% internal diameter expansion the Invention Example gained a strength advantage due to the better strain hardening properties, while preserving the superior impact toughness absorbed energy values, resulting in an expanded steel article having an overall better performance.