Sulphide stress cracking resistant steel, tubular product made from said steel, process for manufacturing a tubular product and use thereof The present invention relates to low alloy steels with a high yield strength that present an improved sulphide stress cracking behaviour.

The present invention also relates to tubular products, such as tubes or pipes, made from said steel, as well as a process for manufacturing such tubular products.

In addition, the present invention concerns use of such tubular products for well drilling, and/or for production, extraction and/or transportation of oil and gas .

Exploring and developing ever and ever deeper hydrocarbon wells which are subjected to ever higher pressures at ever higher temperatures and in ever more corrosive media, in particular when loaded with hydrogen sulphide, means that the need to use low alloy tubes with both a high yield strength and high sulphide stress cracking resistance is constantly increasing .

The presence of hydrogen sulphide (H ₂ S) is responsible for a dangerous form of cracking in low alloy steels with a high yield strength which is known as sulphide stress cracking (SSC) and may affect both casing and tubing, risers or drill pipes and associated products.

Sulphide stress cracking resistance is thus of particular importance for oil companies since it is relevant to the safety of equipment.

The last decades have seen the successive development of low alloy steels which are highly resistant to H ₂ S with minimum specified yield strengths which are steadily increasing : 552 MPa (80 ksi) , 621 MPa (90 ksi) , 655 MPa (95 ksi), 758 MPa ( 1 10 ksi) and more recently 862 MPa ( 125 ksi) .

Today's hydrocarbon wells reach depths of several thousand meters and the weight of the strings treated for standard levels of yield strength is thus very high. Further, the pressures in the hydrocarbon reservoirs may be very high, of the order of several hundred bars, and the presence of H ₂ S, even at relatively low levels of the order of 10 to 100 ppm, results in partial pressures of the order of 0.001 to 0.1 bar, which are sufficient when the pH is low to cause SSC phenomena if the material of the tubes is not suitable. In addition, the use of low alloy steels combining a minimum specified yield strength of 862 MPa (125 ksi) with good sulphide stress cracking resistance would be particularly welcome in such strings.

For this reason, there is a need to develop a low alloy steel, which presents a minimum specified yield strength of 862 MPa (125 ksi), as well as a good SSC behaviour.

Steels having a yield strength of 862 MPa are already mentioned in WO 2010/100020. However, their SSC resistance can further be improved.

When it comes to steel grades with improved corrosion resistance targeting a yield strength of 862 MPa (125 ksi), the application US2006016520 provides a steel for steel pipes which comprises, on the percent by mass basis, C: 0.2 to 0.7%, Si: 0.01 to 0.8%, Mn: 0.1 to 1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005 to 0.1%, Ti: 0.005 to 0.05%, Ca: 0.0004 to 0.005%, N: 0.007% or less, Cr: 0.1 to 1.5%, Mo: 0.2 to 1.0%, Nb: 0 to 0.1%, Zr: 0 to 0.1%, V: 0 to 0.5% and B: 0 to 0.005%, with the balance being Fe and impurities, in which non-metallic inclusions containing Ca, Al, Ti, N, O, and S are present, and in the said inclusions (Ca %)/(Al %) is 0.55 to 1.72, and (Ca %)/(Ti %) is 0.7 to 19 can be used as a raw material for oil country tubular goods, being used at a greater depth and in severer corrosive circumstances, such as casings and tubings for oil and/or gas wells, drilling pipes and drilling collars for excavation, and the like.

The presence of non metallic inclusions in this steel reduces its hot formability, toughness as well as its SSC resistance. Moreover, hardenability of such steel should be improved.

Then comes application US2011186188 with a steel pipe with excellent expandability, comprising, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and balance being Fe and impurities, having a mixed microstructure comprising ferrite and one or more selected from fine pearlite, bainite and martensite, and having a tensile strength of more than or equal to 600 MPa and a uniform elongation satisfying following formula (A):

U-el > 28-0.0075 x TS formula (A)

in which, U-el means uniform elongation (%), and TS means tensile strength (MPa).

This steel pipe, having the above described chemical composition, can be obtained, for example, by being heated at temperatures from 700 to 790°C, then being forced-cooled down to a temperature of lower than or equal to l00°C with the cooling rate of greater than or equal to l00°C/min at the temperature from 700 to 500°C.

Significant amounts of ferrite, bainite and pearlite in this steel could be detrimental to toughness properties in highly corrosive media. In addition, high Mn contents could lead to massive segregation and thus reduce the SSC resistance.

There is therefore a real need to provide steels compositions suitable to enable the production of tubular products having high yield strength of at least 862 MPa (125 ksi) and improved SSC resistance.

The steels should also present improved sulphide stress cracking performance, as well as corrosion resistance.

An object of the present invention is therefore a steel having a chemical composition consisting of, in weight%, relative to the total weight of said chemical composition,

0.32 < C < 0.46

0.10 < Si < 0.45

0.10 < Mn < 0.50

0.30 < Cr < 1.25

1.10 < Mo < 2.10 0.10 < V < 0.30

0.01 < Nb < 0.10;

the balance of the chemical composition of said steel being constituted by Fe and one or more residual elements, including Cu; and the chemical composition of said steel satisfying the following formula (1) between C, Si, Mn, Cr, Mo, V, Nb and Cu, the contents of which are expressed in weight%,

b + 15*a- 165 > 0 formula (1) in which,

a = -90+274*C-25*Si-64*Mn+22*Cr+l7*Mo+268*V-225*Nb + l 84*Cu, and

b = 54+l62*C-86*Si-49*Mn-31 *Cr+22*Mo+20* V - l72*Nb-364*Cu.

The steel of the invention presents an improved sulphide stress cracking resistance than steels of the prior art.

The steel of the present invention presents a yield strength preferably greater than or equal to 862 MPa (125 ksi).

The yield strength is determined by tensile tests as defined in standards ASTM A370-17 and ASTM E8/E8M-l3a.

The steel of the present invention is particularly useful for the production of tubular products for hydrocarbon wells containing hydrogen sulphide (H ₂ S).

Thus another object of the present invention concerns a tubular product, and in particular a tube or a pipe, made from a steel as previously defined.

The present invention also relates to a process for manufacturing a tubular product, and in particular a tube or a pipe, comprising:

(a) providing a steel having a chemical composition as previously defined,

(b) heating up the steel provided at (a) to a temperature ranging from

1100 to l300°C,

(c) hot forming the steel heated at (b) through hot forming processes, such as forging, rolling or extrusion, at a temperature ranging from 900 to l300°C to obtain a tubular product, (d) cooling down the tubular product obtained at (c) to room temperature, before carrying out the following sequences (e) and (f) at least once :

(e) heating up the cooled tubular product to an austenitization temperature (AT) ranging from Ac3 to l 000°C before keeping said tubular product at the temperature AT during a time comprised between 2 and 60 minutes to obtain an austenitized tubular product, and then cooling said austenitized tubular product down to ambient temperature to obtain a quenched tubular product,

and either repeating sequence (e) one more time or carrying out the following sequence (f) :

(f) heating up the quenched tubular product to a tempering temperature (TT) ranging from 500°C to Ac l before keeping said tubular product at the temperature TT during a tempering time (Tt) comprised between 5 and 120 minutes, and then cooling said tubular product down to ambient temperature to obtain a quenched and tempered tubular product;

it being understood that:

Ac l =723 - l 0.7

Ac3=910-203

Ac l and Ac3 being expressed in °C .

The tubular product thus obtained from the steel of the invention presents an improved sulphide stress cracking resistance. It can therefore be used in oil and gas production.

Thus, the present invention also relates to the use of such a tubular product for well drilling, and/or for production, extraction, transportation of oil and gas .

Other subj ects, characteristics, aspects and advantages of the invention will emerge even more clearly on reading the description and the examples that follow.

In what follows and unless otherwise indicated, the limits of a range of values are included within this range, in particular in the expressions“of between” and“ranging from... to ...”. Moreover, the expressions“at least one” and“at least” used in the present description are respectively equivalent to the expressions “one or more” and“greater than or equal to”.

Figure 1 disclo ses a graph displaying coefficient a on the x- axis and coefficient b on the y-axis, both coefficients a and b being determined according to the formulae previously defined. This figure 1 shows that the steel of the present invention (o), wherein both coefficients a and b satisfy formula ( 1 ) defined previously, present a better sulphide stress cracking resistance than comparative steels (x) .

Carbon

The chemical composition of the steel according to the present invention contains 0.32 < C < 0.46 % by weight, relative to the total weight of said chemical composition . In other words, the chemical composition of the steel contains carbon (C) in a content ranging from 0.32 to 0.46% by weight, relative to the total weight of said chemical composition; it being understood that the lower limit (0.32% by weight) being included, while the upper one (0.46 % by weight) being excluded.

Indeed, if the carbon content is lower than 0.32% by weight, the steel thus obtained is less resistant to stress cracking . High carbon content, meaning a content higher than 0.32% by weight, relative to the total weight of the chemical composition, enables a higher tempering temperature, which leads to a lower dislocation density and thus po sitive effect on SSC resistance. However, if the carbon content is greater than or equal to 0.46% by weight, quench cracks can occur as well as the formation of coarse precipitates that are detrimental to SSC resistance.

The carbon content is preferably higher than or equal to 0.34% (0.34% < C) by weight, more preferentially higher than or equal to 0.41 % (0.41 % £ C) by weight, relative to the total weight of the chemical composition.

The carbon content is preferably lower than or equal to 0.44% (C < 0.44% ) by weight, relative to the total weight of the chemical composition. Preferably, the carbon content is higher than or equal to 0.34% by weight and lower than or equal to 0.44% by weight, relative to the total weight of the chemical composition . In other words, the chemical composition of the steel according to the present invention preferably contains 0.34 < C < 0.44% by weight, relative to the total weight of said chemical compo sition .

More preferably, the carbon content is higher than or equal to 0.41 % by weight and lower than or equal to 0.44% by weight, relative to the total weight of the chemical composition. In other words, the chemical composition of the steel according to the present invention more preferably contains 0.41 < C < 0.44% by weight, relative to the total weight of said chemical composition.

Silicon

The chemical composition of the steel according to the present invention further contains 0. 10 < Si < 0.45 % by weight, relative to the total weight of said chemical composition . In other words, the chemical composition of the steel contains silicon (Si) in a content ranging from 0. 10 to 0.45 % by weight, relative to the total weight of said chemical composition; it being understood that both lower (0. 10% by weight) and higher (0.45 % by weight) limits being included.

A minimum content of 0. 10% by weight of silicon comes from steel de-oxidation. This element is also needed to retard softening phenomenon during high temperature tempering . Eventually, it helps to increase the strength after quenching and tempering .

Above 0.45 % by weight, silicon makes the steel brittle.

The silicon content is preferably higher than or equal to 0. 12% (0. 12% < Si) by weight, relative to the total weight of the chemical composition.

The silicon content is preferably lower than 0.38 % (Si < 0.38 % ) by weight, relative to the total weight of the chemical composition.

Preferably, the silicon content is higher than or equal to 0. 12% by weight and lower than 0.38 % by weight, relative to the total weight of the chemical composition. In other words, the chemical composition of the steel according to the present invention preferably contains 0. 12 < Si < 0.38 % by weight, relative to the total weight of said chemical composition.

Manganese

The chemical composition of the steel according to the present invention further contains 0. 10 < Mn < 0.50 % by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel contains manganese (Mn) in a content ranging from 0. 10 to 0.50 % by weight, relative to the total weight of said chemical composition; it being understood that both lower (0. 10% by weight) and higher (0.50% by weight) limits being included.

Manganese is required to avoid free sulfur (S ) in the steel by the formation of MnS . This element is beneficial to hot workability as well as hardenability due to solute solution strengthening, thus indirectly also to SSC by enabling a more homogeneous through thickness tempered martensite microstructure.

When the content of manganese is higher than 0.50% by weight, Mn segregates at steel mid thickness and affects negatively SSC resistance.

The manganese content is preferably higher than or equal to 0.20% (0.20 % < Mn) by weight, relative to the total weight of the chemical composition.

The manganese content is preferably lower than or equal to 0.40% (Mn < 0.40 %) by weight, relative to the total weight of the chemical composition.

The manganese content preferably ranges from 0.20 to 0.40% by weight, relative to the total weight of the chemical compo sition . In other words, the chemical composition of the steel according to the present invention preferably contains 0.20 < Mn < 0.40% by weight, relative to the total weight of said chemical composition.

Chromium

The chemical composition of the steel according to the present invention further contains 0.30 < Cr < 1 .25 % by weight, relative to the total weight of said chemical composition . In other words, the chemical composition of the steel contains chromium (Cr) in a content ranging from 0.30 to 1 .25 % by weight, relative to the total weight of said chemical composition; it being understood that both lower (0.30% by weight) and higher ( 1 .25 % by weight) limits being included.

When the chromium content is lower than 0.30% by weight, the steel thus obtained is less resistant to corrosion .

Cr improves the SSC resistance by limiting the corrosion rate and thus the hydrogen production rate. This beneficial role of Cr is attributed to the formation of a thin layer of enriched Cr oxide, identified as mainly CrOOH, between base material and iron sulphide scales.

When the chromium content is higher than 1 .25 % by weight, coarse carbides such as M23 C6 _, in which Cr is the metal (M) , will form and precipitate. Such precipitates are however detrimental to SSC .

The chromium content is preferably higher than or equal to 0.70% (0.70% < Cr) by weight, relative to the total weight of the chemical composition.

The chromium content is preferably lower than or equal to 1.20 % (Cr < 1 .20% ) by weight, and more preferentially lower than or equal to 1 . 10% (Cr < 1. 10 % ) by weight, relative to the total weight of the chemical compo sition.

The chromium content preferably ranges from 0.30 to 1 .20% by weight, and more preferentially from 0.30 to 1 . 10% by weight, relative to the total weight of the chemical composition. In other words, the chemical composition of the steel according to the present invention preferably contains 0.30 < Cr < 1 .20% by weight, and more preferentially 0.30 < Cr < 1 . 10% by weight, relative to the total weight of said chemical composition.

Molybdenum

The chemical composition of the steel according to the present invention further contains 1 . 10 < Mo < 2. 10 % by weight, relative to the total weight of said chemical composition . In other words, the chemical composition of the steel contains molybdenum (Mo) in a content ranging from 1.10 to 2.10% by weight, relative to the total weight of said chemical composition; it being understood that the lower limit (1.10% by weight) being excluded, while the upper one (2.10% by weight) being included.

Indeed, more than 1.10% by weight of Mo are required to improve corrosion performance by increasing the corrosion resistance of the protective scale. In addition, Mo is beneficial to hardenability. The presence of molybdenum also makes it possible to increase the tempering temperature, without changing other process parameters, improving thus SSC resistance.

When the molybdenum content is higher than 2.10% by weight, the formation of M ₆ C precipitates, in which Mo is the metal (M), is favored, which is then detrimental to toughness and SSC.

The molybdenum content is preferably higher than or equal to 1.15% (1.15% < Mo) by weight, relative to the total weight of the chemical composition.

The molybdenum content is preferably lower than or equal to 1.60% (Mo < 1.60%) by weight, and more preferentially lower than or equal to 1.51% (Mo < 1.51%) by weight, and even more preferentially lower than or equal to 1.40% (Mo < 1.40%) relative to the total weight of the chemical composition.

The molybdenum content is preferably higher than 1.10% by weight and lower than or equal to 1.60% by weight, and more preferentially this content ranges from 1.15 to 1.60% by weight, and even more preferentially from 1.15 to 1.51% by weight relative to the total weight of the steel. In other words, the chemical composition of the steel according to the present invention preferably contains 1.10 < Mo < 1.60, and more preferentially 1.15 < Mo < 1.60% by weight, and even more preferentially 1.15 < Mo < 1.51% by weight relative to the total weight of said chemical composition.

Vanadium

The chemical composition of the steel according to the present invention further contains 0.10 < V < 0.30% by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel contains vanadium (V) in a content ranging from 0.10 to 0.30% by weight, relative to the total weight of said chemical composition; it being understood that both lower (0.10% by weight) and higher (0.30% by weight) limits being included.

According to the invention, a minimum of 0.l0%by weight of vanadium is required to reach 862 MPa (l25ksi) along with high tempering temperatures. Moreover, vanadium forms fine carbides that have a positive impact on SSC resistance.

A saturation effect occurs when vanadium represents more than 0.30% by weight of the total weight of the steel.

The vanadium content is preferably higher than or equal to 0.11% (0.11% < V) by weight, and more preferentially higher than or equal to 0.125% (0.125% < V) by weight, relative to the total weight of the chemical composition.

The vanadium content is preferably lower than or equal to

0.25% (V < 0.25%) by weight, and more preferentially lower than or equal to 0.21% (V < 0.21%) by weight, relative to the total weight of the chemical composition.

The vanadium content preferably ranges from 0.11 to 0.25% by weight, and more preferentially from 0.125 to 0.25% by weight, and even more preferentially from 0.125 to 0.21% by weight, relative to the total weight of the chemical composition. In other words, the chemical composition of the steel according to the present invention preferably contains 0.11 < V < 0.25%, more preferentially 0.125 < V < 0.25% by weight, and even more preferentially 0.125 < V < 0.21% by weight, relative to the total weight of said chemical composition.

Niobium

The chemical composition of the steel according to the present invention further contains 0.01 < Nb < 0.10% by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel contains niobium (Nb) in a content ranging from 0.01 to 0.10% by weight, relative to the total weight of said chemical composition; it being understood that both lower (0.01% by weight) and higher (0.10% by weight) limits being included. The maximum niobium content is limited to 0. 10% by weight of the total weight of the steel to avoid that coarse primary NbC carbides form. Indeed, these precipitates are deleterious to SSC resistance .

In addition, at least 0.01 % by weight of niobium are needed to limit prior austenitic grain size.

The niobium content is preferably higher than or equal to 0.022% (0.022% < Nb) by weight, relative to the total weight of the chemical composition.

The niobium content is preferably lower than or equal to 0.05 % (Nb < 0.05 % ) by weight, and more preferentially lower than or equal to 0.045 % (Nb < 0.045 % ) by weight relative to the total weight of the chemical composition.

The niobium content preferably ranges from 0.01 to 0.05 % by weight, and more preferentially from 0.022 to 0.045 % by weight, relative to the total weight of the chemical composition . In other words, the chemical composition of the steel according to the present invention preferably contains 0.01 < Nb < 0.05 % by weight, and more preferentially 0.022 < Nb < 0.045 % by weight, relative to the total weight of said chemical composition.

Balance and residual elements

The term “residual elements” refers to inevitable elements resulting from the steel production and casting processes .

The sum of residual element contents is preferably lower than 0.4% by weight of the total weight of the chemical composition.

The balance of the chemical composition of the steel according to the present invention is made of Fe and residual elements resulting from the steel production and casting processes, including Cu, and also anyone of As, P, S , N, Ni, Al, Co, Sn, B , Ti, W and mixtures thereof. Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Cu is lower than or equal to 0. 10% in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of As is lower than or equal to 0.05 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of P is lower than or equal to 0.03 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of S is lower than or equal to 0.01 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of N is lower than or equal to 0.01 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Ni is lower than or equal to 0. 10% in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Al is lower than or equal to 0. 10% in weight. Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Co is lower than or equal to 0. 10% in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Sn is lower than or equal to 0.03 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of B is lower than or equal to 0.003 % in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of Ti is lower than or equal to 0. 10% in weight.

Preferably, when present as a residual element, alone or in combination with one or more other elements in the chemical composition of the steel according to the invention, the amount of W is lower than or equal to 0.05 % in weight.

Preferably, when the chemical composition of the steel according to the present invention contains one or more residual elements including Cu, P, S , N, Ni, Al, Co, Sn, B, Ti, W and mixtures therefore, the amounts of said elements, expressed in weight% , relative to the total weight of said chemical composition, are as follows : Cu < 0. 1 0

P < 0.03

S < 0.01

N < 0.01

Ni < 0. 10

Al < 0. 10

Co < 0. 1 0

Sn < 0.03

B < 0.003

Ti < 0. 1 0

W < 0.05.

Tungsten

The chemical composition of the steel according to the present invention may further contain W preferably < 0.05 % by weight, and more preferably < 0.04% by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel may further contain tungsten (W) preferably in a content lower than or equal to 0.05 % by weight, and more preferably lower than or equal to 0.04% by weight, relative to the total weight of said chemical composition.

M ₆ C carbides, where the metal (M) is tungsten (W) , trigger deleterious effects on SSC resistance. The content of tungsten has therefore to remain preferably lower than or equal to 0.05 % by weight, and more preferably lower than or equal to 0.04% by weight, relative to the total weight of said chemical composition.

Copper

The chemical composition of the steel according to the present invention may further contain preferably Cu < 0. 10% by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel may further contain copper (Cu) preferably in a content lower than or equal to 0. 10 % by weight, relative to the total weight of said chemical composition. Above 0. 10% by weight, copper may lead to undesirable increase of hardness for a given level of yield strength.

In some embodiments, B , Ti and Al may be added on purpose, meaning that these elements may be added or not in a deliberate way, but in any case limited below specific amounts .

Boron

When boron (B) is present in the chemical composition, its content is preferably lower than or equal to 0.003 % by weight, and more preferentially lower than or equal to 0.0025 % by weight, relative to the total weight of the chemical composition of the steel. In other words, the chemical composition of the steel according to the present invention preferably contains B < 0.003 % , and more preferentially B < 0.0025 % by weight, relative to the total weight of said chemical composition.

Titanium

The chemical composition of the steel according to the present invention may further contain titanium (Ti) . In other words, the chemical composition may further contain Ti, even when said chemical composition is free from boron (B) .

However, when the chemical compo sition further contains B ,

Ti is also contained in a content preferably lower than or equal to 0. 10% by weight, and more preferably lower than or equal to 0.04% by weight, relative to the total weight of said chemical compo sition . In other words, when the chemical composition further contains B , Ti is added on purpose and its content is limited, i.e. preferably lower than or equal to 0. 10 % by weight, relative to the total weight of said chemical composition.

Ti may be useful when it comes to grain growth during elaboration, since such grain growth is deleterious to SSC resistance . When the content of titanium is higher than 0. 10% by weight of the total weight of the chemical compo sition of the steel, coarse nitrides precipitates form that are then deleterious to SSC resistance. Aluminium

The chemical composition of the steel according to the present invention may further contain aluminium (Al) as this element is used for de-oxidation during the melting process. When the chemical composition further contains aluminium, its contents is preferably lower than or equal to 0. 10 % by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel according to the present invention preferably contains Al < 0. 10% by weight, relative to the total weight of said chemical composition. Above 0. 10% by weight, aluminium forms inclusions that may be detrimental to SSC resistance.

Nitrogen

The content of nitrogen (N) is preferably lower than or equal to 0.01 % by weight, relative to the total weight of the chemical composition. N forms coarse carbo-nitrides that are deleterious to SSC resistance. Other elements such as Ca and REM (rare earth metals) can also be present as unavoidable residual elements . P and S decrease grain cohesion and are therefore detrimental to toughness.

Microstructural features of the steel

The microstructure of the steel according to the present invention is preferably made of at least 90 % of tempered martensite, more preferentially more than 95 % of tempered martensite, and better still more than 99% of tempered martensite .

Such tempered martensite is obtained after final cooling of the steel. A tempered martensite is a martensite that has undergone a tempering treatment as defined in the process according to the invention.

In a preferred embodiment, the quenched and tempered tubular product obtained by the process of the invention, after final cooling, is made of steel presenting a tempered martensitic micro structure, meaning that the microstructure of this steel is preferably made of at least 90 % , more preferentially more than 95 % , and better still more than 99% of tempered martensite. The steel according to the present invention has a microstructure made of tempered martensite with a prior austenite grain size of less than or equal to 22.4p m that is greater than or equal to 8 in Standard ASTM E l 12- 13 format.

The prior austenite grain size corresponds to the grain size of the austenite from which martensite is formed.

The present invention also relates to a tubular product, and in particular a tube or a pipe, made from a steel as previously defined.

More particularly the tubular product is a seamless tube or a seamless pipe .

Another object of the present invention concerns a process for manufacturing a tubular product as previously defined.

More particularly, a tubular product made from steel according to the present invention is obtained according to conventional hot forming methods known by the man skilled in the art.

For example, the steel according to the present invention may be melted by commonly-used melting practices and commonly-used casting process such as the continuous casting or the ingot casting blooming methods .

The steel is then heated to a temperature ranging from 1 100 °C and l 300°C, so that at all points the temperature reached is favorable to the high rates of deformation the steel will undergo during hot forming .

Preferably the maximum temperature is lower than l 300°C to avoid burning . Below H 00°C, the hot ductility of the steel is negatively impacted. The semi finished product is then hot formed between 900°C and l 300°C in at least one step .

A tubular product having the desired dimensions is thus obtained.

The tubular product is then austenitized, i.e . heated up to an austenitization temperature (AT) where the microstructure is austenitic. This temperature should be within the austenitic range.

The austenitization temperature (AT) ranges from Ac3 (°C) to l 000 °C; if AT is less than Ac3 , the microstructure will not be fully austenitic and we may not reach the minimum of 90% martensitic steel after quench. Above l 000 °C, the austenite grains grow undesirably large and lead to a coarser final structure, which impacts negatively toughness and SSC resistance .

The tubular product made of steel according to the present invention is then kept at the austenitization temperature AT for an austenitization time At of at least 2 minutes, the obj ective being that at all points of the tube, the temperature reached is at least equal to the austenitization temperature. The temperature should be homogeneous throughout the tube . The austenitization time At shall not be above 60 minutes because above such duration, the austenite grains grow undesirably large and lead to a coarser final structure. This would be detrimental to toughness and SSC resistance.

The austenitized tubular product made of steel according to the present invention is then cooled to the ambient temperature. This cooling may either be performed in water (water quench) or in oil (oil quench) . In this manner, a quenched tubular product made of steel is obtained which preferably comprises in percentage of at least 90% of martensite, more preferentially at least 95 % of martensite, and better still at least 99% of martensite.

The quenched tubular product made of steel according to the present invention is then tempered, i.e. heated up at a tempering temperature (TT) ranging from 500°C to Ac l (°C) , and preferably ranging from 600°C to Ac l (°C) . Tempering must be done below Ac l to avoid any phase transformation.

Such tempering step is done during a tempering time Tt between 5 and 120 minutes . Preferably, the tempering time is between 10 and 60 min. This leads to a quenched and tempered steel tubular product.

The quenched and tempered steel tubular product according to the invention is then cooled down to the ambient temperature using either water or air cooling . At the end of the process of the present invention, the tubular product thus obtained may further undergo additional finishing steps, such as sizing or straightening .

According to a preferred embodiment of the present invention, at the end of sequence (e), said sequence (e) is repeated at least one more time. In other words, the sequence (e) is performed successively at least two times during the process of the present invention.

When the sequence (e) is successively repeated two times during the process of the present invention, it means that the tubular product obtained at the end of the cooling down to room temperature

(d) is subjected to a double quenching treatment.

According to this particular embodiment, the process of the present invention comprises :

(a) providing a steel having a chemical composition as previously defined,

(b) heating up the steel provided at (a) to a temperature ranging from 1 100 to l 300°C,

(c) hot forming the steel heated at (b) through hot forming processes, such as forging, rolling or extrusion, at a temperature ranging from 900 to l 300 °C to obtain a tubular product,

(d) cooling down the tubular product obtained at (c) to room temperature, before carrying out the following sequences :

(e) heating up the cooled tubular product to an austenitization temperature (AT) ranging from Ac3 to l 000°C before keeping said tubular product at the temperature AT during a time comprised between 2 and 60 minutes to obtain an austenitized tubular product, and then cooling said austenitized tubular product down to ambient temperature to obtain a quenched tubular product,

and then repeating said sequence (e) at least one more time before carrying out the following sequence (f) :

(f) heating up the quenched tubular product to a tempering temperature (TT) ranging from 500°C to Ac l before keeping said tubular product at the temperature TT during a tempering time (Tt) comprised between 5 and 120 minutes, and then cooling said tubular product down to ambient temperature to obtain a quenched and tempered tubular product;

it being understood that:

Ac l and Ac3 being expressed in °C .

According to another preferred embodiment of the present invention, at the end of sequence (f), the sequences (e) and (f) of the process are repeated at least one more time . In other words, the sequences (e) and (f) are performed at least two times during the process of the present invention.

When the sequences (e) and (f) are repeated two times during the process of the present invention, it means that the tubular product obtained at the end of (d) is subjected to a double quenching- tempering treatment.

According to this particular embodiment, the process of the present invention comprises :

(a) providing a steel having a chemical composition as previously defined,

(b) heating up the steel provided at (a) to a temperature ranging from

1 100 to l 300°C,

(c) hot forming the steel heated at (b) through hot forming processes, such as forging, rolling or extrusion, at a temperature ranging from 900 to l 300 °C to obtain a tubular product,

(d) cooling down the tubular product obtained at (c) to room temperature, before carrying out the following sequences (e) and (f) at least once :

(e) heating up the cooled tubular product to an austenitization temperature (AT) ranging from Ac3 to l 000 °C before keeping said tubular product at the temperature AT during a time comprised between 2 and 60 minutes to obtain an austenitized tubular product, and then cooling said austenitized tubular product down to ambient temperature to obtain a quenched tubular product, and (f) heating up the quenched tubular product to a tempering temperature (TT) ranging from 500°C to Ac l before keeping said tubular product at the temperature TT during a tempering time (Tt) comprised between 5 and 120 minutes, and then cooling said tubular product down to ambient temperature to obtain a quenched and tempered tubular product;

said sequences (e) and (f) being repeated at least one more time;

it being understood that:

Ac l and Ac3 being expressed in °C .

The quenched and tempered steel tubular product is then useful for well drilling, and/or for production, extraction, transportation of oil and gas.

Thus the present invention also concerns use of a tubular product as previously defined for well drilling, and/or for production, extraction, transportation of oil and gas .

Other characteristics and advantages of the invention are given in the following experimental section, which should be regarded as illustrative and not limiting the scope of the present application.

EXAMPLES a) Tested steels

The following compositions of steels according to the present invention (A-E) and comparative steels (F-P) have been prepared from the elements indicated in the table 1 below, the amounts of which are expressed as percent by weight, relative to the total weight of the chemical composition . Underlined values in the following table 1 are not in conformance with the invention. Table 1: tested steels

5

The coefficients a and b corresponding to each steel (A-P), as well as the result after computing the formula b + 1 5 * a- 165 , are mentioned in the table 2 below. Underlined values in the following table 2 are not in conformance with the invention, i.e. the chemical composition of the steel does not satisfy formula ( 1 ) .

Table 2

b) Protocol

The steels (A-P) having the chemical compositions described in the table 1 above have been heated and then hot formed into seamless steel pipes of the desired dimensions by hot working using the Mannesmann-plug mill process.

After hot forming, the seamless steel pipes thus obtained have undergone the following process conditions summarized in the table 3 , with :

AT (°C) : Austenitization temperature in °C

At: Austenitization time in minutes TT: Tempering temperature in °C

Tt: Tempering time in minutes

The following steps, defined in table 3 and corresponding to steps (e) and (f) of the process of the present invention have been performed twice. In others words, the step of austenitization, cooling and tempering (AT1, Atl, cooling Al, TT1, Ttl and cooling Tl) have been repeated (AT2, At2, cooling A2, TT2, Tt2 and cooling T2). Table 3: process conditions

c) Results

The microstructure, mechanical behavior and SSC resistance of the seamless steel pipes (A-P) thus obtained are summarized in the following table 4. The SSC resistance of the seamless steel pipes (A-K and N) is also shown in figure 1.

- PAG is the prior austenite grain size index as defined in standard ASTM El 12-13. - YS in MPa and ksi is the yield strength obtained in tensile test as defined in standards ASTM A370- 17 and ASTM E8/E8M- l 3a.

- UTS in MPa and ksi is the tensile strength obtained in tensile test as defined in standards ASTM A370- 17 and ASTM E8/E8M- l 3a.

- SSC is the sulphide stress corrosion cracking resistance evaluated according standard NACE TM0 177 -2016 Method A. The SSC test consists in immersing the test specimens under load in an aqueous solution adjusted to pH 3.5 with the addition of acetic acid and sodium acetate in a test solution of 5 mass% NaCl. The solution temperature is 24°C, H ₂ S is at 0. 1 atm. , C02 is at 0.9 atm. The testing duration is

720 hours, and the applied stress is 90% of the yield strength. A successful test implies no failure on the specimens after 720 hours.

Table 4: results obtained for the seamless steel pipes (A-P)

The results thus obtained show that the steels according to the present invention (A-E), having particular contents of C, Si, Mn, Cr, Mo, V and Nb and satisfying formula ( 1 ) , present a better yield strength as well as a better sulphide stress cracking resistance than comparative steels (F-P) .

As displayed in figure 1 , surprisingly, comparative steels (G-K and N) having elements contents outside the composition ranges of the invention and having chemical compositions that do not satisfy formula ( 1 ), exhibit lower yield strength and worse sulphide stress cracking resistance than steels according to the present invention (A- E) . Furthermore, the inventors were flabbergasted to observe that comparative steel F having elemental contents within the composition ranges of the invention but having a chemical composition that does not satisfy formula ( 1 ) , also exhibit lower yield strength and worse sulphide stress cracking resistance than steels according to the present invention (A-E) .

Astonishingly, comparative examples (M, O and P) having chemical compositions that do satisfy formula ( 1 ) , but having contents outside the composition ranges of the invention, also exhibit lower yield strength and worse sulphide stress cracking resistance than steels according to the present invention (A-E) .