Sulfide stress cracking resistant steel, tubular product made from said steel, process for manufacturing a tubular product and use thereof

Abstract

The present invention relates to low alloy steels with high yield strength that present an improved sulfide stress cracking behavior. The present invention also relates to tubular products, such as tubes or pipes, made from said steel, as well as 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, transportation of oil and gas, hydrogen storage and transport, and automotive application.

Description

[0001] Specification for Patent of Invention for “SULFIDE STRESS CRACKING RESISTANT STEEL, TUBULAR PRODUCT MADE FROM SAID STEEL, PROCESS FOR MANUFACTURING A TUBULAR PRODUCT AND USE THEREOF.”

[0002] FIELD OF INVENTION

[0003] The present invention pertains to low alloy steels with high yield strength that exhibit enhanced resistance to sulfide stress cracking. It also encompasses tubular products, such as tubes or pipes, fabricated from said steel, as well as methods for manufacturing these tubular products. Furthermore, the invention includes the application of such tubular products in well drilling, and / or for the production, extraction, and transportation of oil and gas.

[0004] The tubular products of the present invention can also be used for hydrogen storage and transportation, as well as for automotive application.

[0005] BACKGROUND OF INVENTION

[0006] The exploration and development of deeper hydrocarbon wells, subjected to higher pressures, temperatures, and corrosive environments like hydrogen sulfide, increase the need for low alloy tubes with high yield strength and resistant to sulfide stress cracking.

[0007] The presence of hydrogen sulfide (H₂S) is responsible for a dangerous form of cracking in low alloy steels with a high yield strength which is known as sulfide stress cracking (SSC) and may affect both casing and tubing, risers or drill pipes and associated products.

[0008] Sulfide stress cracking resistance is thus of particular importance for oil companies since it is relevant to the safety of equipment.

[0009] The last decades have seen the successive development of low alloy steels which are highly resistant to H₂S with minimum specified yield strengths which are steadily increasing: 552 MPa (80 ksi), 621 MPa (90 ksi), 655 MPa (95 ksi), 758 MPa (110 ksi) and more recently 862 MPa (125 ksi).

[0010] When it comes to steel grades with improved corrosion resistance targeting a yield strength of 862 MPa (125 ksi), the application JP2023160582 provides a steel for steel pipes, said steel having a composition that comprises, on thepercent by mass basis, C:0.20-0.45%, Si:0.60-1.30%, Mn:0.02-1.00%, P:0.050% or less, S:0.0050% or less, AI:0.010-0.100%, N:0.0100% or less, Cr:0.10-3.00%, Mo:0.35-3.00%, Cu:0.01-0.50%, Ni:0.01-0.50%, Zr:0.0010-0.1000%, 0:0.0050% or less, Sb:0-0.50%, Co:0-0.50%, W: 0-0.50%, Ti:0-0.030%, Nb:0-0.150%, V: 0-0.500%, 0 to 0.0030%, Ca:0 to 0.0040%, Mg:0 to 0.0040%, and a rare earth element in an amount of 0 to 0.0040%, with the remainder being Fe and impurities.

[0011] The publication: Air Cooled Bainitic Steels for Strong, Seamless Pipes Part 1 -Alloy Design, Kinetics and Microstructure by G. Gomez, T. Perez, and H. K. D. H. Bhadeshia explains well and in details the kinetics and metallurgy of bainitic steels, but the metallurgical route presented is not fit for industrial production. Hydrocarbon wells today reach depths of several thousand meters, resulting in considerable weight for strings designed to standard yield strength levels. Additionally, hydrocarbon reservoirs can exhibit exceedingly high pressures, often in the range of several hundred bars. The presence of hydrogen sulfide (H₂S), even at relatively low concentrations between 10 to 100 ppm, creates partial pressures from 0.001 to 0.1 bar. These conditions can lead to sulfide stress cracking (SSC) phenomena, particularly when the pH is low and if the tubing material is not appropriately selected.

[0012] Therefore, it is necessary to develop a type of steel that not only offers improved minimum specified yield strength compared to the steel described in JP2023160582 but also maintains or enhances resistance to hydrogen embrittlement, thereby ensuring good SSC performance. This invention aims to achieve these objectives.

[0013] SUMMARY

[0014] An object of the present invention is therefore a steel having a chemical composition comprising, in weight percentage:

[0015] 0.30 < C < 0.45%

[0016] 0.20 < Mo < 0.35%

[0017] 0.50 ≤ Cr ≤ 1.25%

[0018] 0.10 ≤ Mn ≤ 0.25%

[0019] 0.80 < Si < 1.50%2.0 < Ni < 3.50%

[0020] Ti ≤ 0.015%

[0021] Nb ≤ 0.015%

[0022] V ≤ 0.015%

[0023] Al < 0.50%

[0024] Co < 0.50%

[0025] the balance of the chemical composition being constituted by Fe and one or more inevitable impurities, including S, P, H or O and mixtures thereof, the amount of which being as follows:

[0026] S <0.005%

[0027] P <0.020%

[0028] H <0.010%

[0029] O <0.010%.

[0030] This composition of the invention enables the steel to achieve a minimum yield strength that is higher than the minimum yield strength achieved with the steels of the prior art, while still passing the NACE TM0177 Method A Test in a full sour condition (i.e., 100% H₂S).

[0031] The steel of the present invention presents a yield strength greater than or equal to 820 MPa (119 ksi), preferably greater than or equal to 862 MPa (125 ksi), more preferably greater than or equal to 1034 MPa (150 ksi).

[0032] According to an embodiment, the steel of the invention has a chemical composition consisting of, in weight percentage:

[0033] 0.30 < C < 0.45%

[0034] 0.20 < Mo < 0.35%

[0035] 0.50 ≤ Cr ≤ 1.25%

[0036] 0.10 ≤ Mn ≤ 0.25%

[0037] 0.80 < Si < 1.50%

[0038] 2.0 < Ni < 3.50%

[0039] Ti ≤ 0.015%

[0040] Nb ≤ 0.015%

[0041] V ≤ 0.015%

[0042] Al < 0.50%

[0043] Co < 0.50%the balance of the chemical composition being constituted by Fe and one or more inevitable impurities, including S, P, H or O and mixtures thereof, the amount of which being as follows:

[0044] S <0.005%

[0045] P <0.020%

[0046] H <0.010%

[0047] O <0.010%.

[0048] Carbon:

[0049] Carbon (C) is the element that offers the best cost / benefit ratio for increasing mechanical strength. It also determines the initial temperatures of martensite and bainite formation. The chemical composition of the steel according to the present invention contains 0.30 < C < 0.45% by weight, relative to the total weight of said chemical composition.

[0050] In other words, the chemical composition of the steel contains carbon in a content ranging from 0.30 to 0.45% by weight, relative to the total weight of said chemical composition; the lower limit (0.30% by weight) is excluded, while the upper one (0.45% by weight) is included. Such carbon content enables a reduction in the temperature at which the martensitic phase forms. This reduction of the initial temperature promotes the formation of a finer microstructure, which enhances certain mechanical properties such as hardness and yield strength.

[0051] Steel with carbon content of 0.30 < C < 0.45% can undergo the martensite formation process at lower temperatures during quenching. Carbon also determines the maximum volume fraction of bainite and stabilizes the austenitic phase.

[0052] The chemical composition of the steel according to the present invention preferably contains 0.33 < C < 0.45%. Indeed, a carbon content that is equal to or higher than 0.33% enables to work with the most appropriate transformation temperature to obtain the desired microstructure that provides improved yieldstrength and SSC resistance. With a carbon content equal to or higher than 0.45%, this effect of carbon starts to be negatively affected.

[0053] Molybdenum:

[0054] Molybdenum (Mo) has a strong hardenability effect. Mo removes carbon from the interstitial solid solution by precipitation of carbides, which contributes to the increase mechanical properties of the steel, especially the strength of the steel.

[0055] The chemical composition of the steel according to the present invention contains 0.20 < Mo < 0.35% 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 0.20 to 0.35% by weight, relative to the total weight of said chemical composition; the lower limit (0.20% by weight) is excluded, while the upper one (0.35% by weight) is included. To prevent phosphor (P) embrittlement, a content of 0.23 < Mo < 0.27% is optimal.

[0056] Chromium:

[0057] Chromium (Cr) enables to enhance the hardness, tensile strength, and hardenability of the steel. Cr can also contribute to improved corrosion and oxidation resistance, although at the cost of reduced toughness due to the promotion of cementite formation. The chemical composition of the steel according to the present invention therefore contains 0.50 ≤ Cr ≤ 1.25% by weight, relative to the total weight of said chemical composition.

[0058] In other words, the chemical composition of the steel contains chromium (Cr) in a content ranging from 0.50 to 1.25% by weight, relative to the total weight of said chemical composition; both lower (0.50% by weight) and higher (1.25% by weight) limits are included.

[0059] To achieve a balance between SSC resistance, hardenability, and strength while limiting the formation of cementite, a chromium content ranging from 0.85% to0.97% is considered optimal. When chromium content exceeds 0.97%, there is an increased risk of chromium forming cementite.

[0060] Manganese:

[0061] The chemical composition of the steel according to the present invention contains 0.10 ≤ Mn ≤ 0.25% 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 0.25% by weight, relative to the total weight of said chemical composition; both lower (0.10% by weight) and higher (0.25% by weight) limits are included. Mn has an important hardenability effect.

[0062] However, Mn content is kept as low as possible because it is known to segregate within the bainitic structure upon cooling. At the same time, a minimum of 0.10% is needed to prevent sulfur embrittlement as it precipitates with Sulfur. At the same time, a minimum of 0.10% is needed to prevent sulfur embrittlement as manganese form MnS avoiding the formation of FeS and, consequently, hot-shortness during rolling.

[0063] The chemical composition of the steel according to the present invention preferably contains 0.15 < Mn < 0.23%. Above 0.23%, Mn starts to promote blocky retained austenite, which is detrimental for the mechanical properties of the steel. The Mn content is preferably in the range 0.15-0.23% because it is known to segregate within the bainitic structure during cooling. As a result, Mn-rich regions tend to retard the bainitic transformation and may lead to the formation of martensite during continuous cooling.

[0064] Silicon:

[0065] The chemical composition of the steel according to the present invention contains 0.80 < Si < 1.50%. In other words, the chemical composition of the steel contains silicon in a content ranging from 0.80 to 1.50% by weight, relative to the total weight of said chemical composition; the lower limit (0.80% by weight) is included,while the upper one (1.50% by weight) is excluded. Silicon (Si) significantly increases mechanical strength through solid solution hardening.

[0066] The main purpose of silicon is to prevent the formation of the brittle cementite phase. For that purpose, silicon content needs to be at least equal to 0.80%, so that it retards kinetically the formation of carbides, especially when precipitating from austenite, thus enabling the precipitation of carbides within the bainite, and preventing presence of carbide between the laths of bainitic ferrite in the final microstructure of the steel.

[0067] A maximum Si content of 1.50% was chosen because at this concentration and above, bainitic alloys sometimes do not have the ability to form cementite in the steel according to the invention.

[0068] The chemical composition of the steel according to the present invention preferably contains 1.00 < Si < 1.30%. This range has been found to be optimal for preventing the formation of cementite, thereby enhancing mechanical properties and SSC resistance. However, above 1.30%, Si does no longer have benefits for yield strength and SSC resistance of the steel of the invention. Thus, Si content if preferably kept below or equal to 1.30%.

[0069] Nickel:

[0070] The chemical composition of the steel according to the present invention contains 2.00 < Ni < 3.50% by weight, relative to the total weight of said chemical composition. In other words, the chemical composition of the steel contains Nickel (Ni) in a content ranging from 2.00 to 3.50% by weight, relative to the total weight of said chemical composition; the lower limit (2.00% by weight) is included, while the upper one (3.50% by weight) is excluded.

[0071] Nickel improves the toughness and mechanical resistance of steels. If present in the range of 2.00 < Ni < 3.50%, nickel also improves impact resistance. Nickel diffuses into the bainitic ferrite, resulting in solid solution hardening and a reduction in grain size.As the content of this element increases in the material, austenite and bainite become finer, thus the yield strength and tensile strength of the steel also increase. Below 2.00% of Ni, the bainite start temperature is too high and the desired microstructure cannot be achieved. Nickel, as a well-known austenite stabiliser, influences the volume fraction of retained austenite in steel when its content exceeds 3.5%. Increased nickel levels typically lead to higher amounts of retained austenite post-austempering, predominantly in a block morphology. This morphology adversely affects corrosion resistance of the alloy; therefore, it is advisable to maintain the nickel content strictly below 3.5%. Additionally, a higher nickel concentration significantly raises the overall cost of the alloy, reducing its economic feasibility.

[0072] The chemical composition of the steel according to the present invention preferably contains 3.00 < Ni < 3.40%. This preferred range enables to work with the most appropriate transformation temperature to obtain the desired microstructure that provides improved yield strength and SSC resistance.

[0073] Titanium, Niobium and Vanadium:

[0074] At least one of these three optional elements can be added in a concentration that is equal to, or less than 0.015% by weight, relative to the total weight of said chemical composition, to promote the formation of carbides, that are known to be good hydrogen traps, thus improving the steel resistance to SSC.

[0075] Above 0.015%, none of these three elements enables the formation of carbides. Therefore, when the concentration of titanium (Ti), niobium (Nb), and vanadium (V) exceeds 0.015%, these elements remain in solid solution and adversely impact mechanical properties such as yield strength. As a result, if these elements are present in the composition, their combined content should preferably be: Ti + Nb + V ≤ 0.015%.Aluminum and Cobalt:

[0076] The chemical composition of the steel according to the present invention optionally contains Al < 0.50% and / or Co < 0.50% to enhance the kinetics of bainitic phase transformation, especially by reducing the needed isothermal holding time during austempering.

[0077] Therefore, reducing time and costs of the production process. In a preferred embodiment, the chemical composition can contain 0.05 < Al < 0.50% and / or 0.05 < Co < 0.50%. Aluminum and cobalt are also prone to oxidation during the casting process, resulting in the formation of dross that needs to be removed. This phenomenon adversely affects Sulfide Stress Cracking (SSC) resistance. When the content exceeds 0.05%, aluminum and cobalt have an even more pronounced impact.

[0078] Balance and inevitable impurities:

[0079] The balance of the chemical composition of the steel according to the present invention is made of Fe and inevitable impurities resulting from the steel production and casting processes, including sulfur (S), phosphorus (P), hydrogen (H), oxygen (O) and mixtures thereof.

[0080] Sulfur: the presence of sulfur is extremely harmful, reducing weldability and ductility. In common steels, the sulfur content is limited to values below 0.05 wt%. Under sour service conditions, it becomes necessary to control the sulfur content to even lower levels to improve SSC resistance. Consequently, in the present invention sulfur is kept below 0.005%.

[0081] Phosphorus: above certain levels, the presence of phosphorus makes the steel brittle. This impurity tends to segregate during steel solidification, which promotes the formation of hard phases, decreasing resistance to hydrogen embrittlement, and thus the resistance to SSC. Consequently, S is kept below 0.020% in the steel of the present invention.Hydrogen: the presence of hydrogen is harmful, causing brittleness in the steel, reducing its elasticity and tensile strength, and contributing to the formation of the defect called hydrogen blistering. For this reason, hydrogen is kept below 0.010% in the steel of the present invention.

[0082] Oxygen: the presence of oxygen is harmful because it forms oxygen sulfides. These oxygen sulfides, such as FeO-Cr₂O₃ (iron-chromium oxide) or MnS-FeO (manganese sulfide-iron oxide), can be brittle and promote stress cracking under service conditions where the steel is exposed to H₂S. For this, reason, oxygen is kept below 0.010% in the steel of the present invention.

[0083] Microstructural features of the steel:

[0084] The steel of the present invention has a microstructure comprising between 70% and 90% of bainitic ferrite, between 10% and 30% of retained austenite, and carbides in an amount equal to or lower than 0.25%.

[0085] Retained austenite improves the hydrogen embrittlement resistance, because hydrogen permeation in austenite is much lower than in ferrite. Thus, retained austenite works as a trap for hydrogen. It also improves the toughness of the steel.

[0086] The refined austenite / bainitic ferrite interface effectively traps hydrogen and reduces the movement of the mobile untrapped hydrogen by a tortuosity factor caused by the reduction of cross-sectional area free for diffusion. In other words, austenite and bainitic ferrite, when refined, act as an effective trap for hydrogen, reducing its mobility by limiting the available space for diffusion through the material. This is attributed to the increased complexity of the diffusion path caused by the reduction of the free cross-sectional area. These microstructural features improve SSC resistance.

[0087] The alloy needs to have an austenite fraction that represents at least 10% of the microstructure, which corresponds to the percolation threshold necessary for resisting hydrogen ingress. However, to avoid negative effects on toughness, thealloy must also contain a high fraction of bainitic ferrite, thus limiting the size of blocky austenite regions.

[0088] These regions of blocky austenite regions, when in a coarse form, reduce the toughness of the steel. To maintain adequate stability, the volume fraction of austenite should represent between 10% and 30% of the microstructure of the steel, and the volume fraction of bainitic ferrite should be between 70% and 90% of the microstructure. This enables minimizing the adverse effects of blocky austenite while ensuring effective resistance to hydrogen ingress. The figure 1 shows the evolution of the content and shape of residual austenite as a function of the austempering temperature at different scales at half thickness and at¼ of the thickness. This is based on a simulated diagram.

[0089] In an embodiment, the bainitic ferrite has a lath thickness lower than 200nm. This lath thickness, combined with a microstructure comprising between 70% and 90% of bainitic ferrite, and between 10% and 30% of retained austenite, allows the steel to achieve an ultimate tensile strength above 1100MPa, a yield strength above 900MPa, total elongation ranging from 5% to 20%, and fracture toughness exceeding 50J at room temperature. Lath thickness higher than 200 nm could not fulfill the targeted mechanical performance.

[0090] In a preferred embodiment, the bainitic ferrite has a lath thickness lower than 100nm. Compared to a lath thickness of more than 200 nm, a lath thickness lower than 100nm promotes higher mechanical properties to the steel, Yield strength (YS) above 1000MPa, and, more importantly, reduces the hydrogen diffusion in the steels due to both finer grains and higher dislocation density in the bainitic ferrite phase.

[0091] Another object of the present invention concerns a tubular product for applications in the production, transport, and storage of oil and gas, said tubular product being made from a steel as previously defined, and having a microstructure as previously defined.Thus, the present invention also relates to a process for manufacturing a tubular product as previously defined, said process comprising the following steps: (a) providing a steel having a chemical composition as previously defined, (b) casting said steel of step (a) by conventional or continuous casting to obtain a billet or an ingot,

[0092] (c) piercing the billet or the ingot in a heating temperature kept between 1150 °C and 1300 °C to obtain a tubular product,

[0093] (d) rolling of the tubular product obtained in step (c) to obtain a tubular product having the desired dimensions wherein the processing temperature at the end of this step shall be more than 700°C,

[0094] (e) cooling down the tubular product obtained at step (d) by air cooling, or by water cooling, or by oil cooling,

[0095] (f) performing a heat treatment of the cooled tubular product obtained at step (e), said heat treatment comprising at least one sequence of the following steps (g1 ) and (g2):

[0096] (g1) heating up the cooled tubular product obtained at step (e) to an austenitizing temperature ranging from 800°C to 1020°C, and then keeping said tubular product at the austenitizing temperature during a time comprised between 10 minutes and 30 minutes to obtain an austenitized tubular product,

[0097] (g2) cooling down said austenitized tubular product to an austempering temperature ranging from 200°C to 400°C at a cooling rate above 10°C / s, and then keeping said tubular product at the austempering temperature during a time comprised between 20 minutes and 240 minutes, and then cooling said tubular product to room temperature.

[0098] A steel melting step, prior to casting, can be performed by melting in air induction furnace, vacuum induction furnace or electric arc furnace. The selection of the process can be determined based on the availability, capacity, and productivity of each production route.

[0099] The casting step (b) can be performed by conventional or continuous casting. The selection of the process can be determined based on the availability, capacity, and productivity of each production route.Optionally, between casting (b) and piercing (c), the ingot or billet can be homogenized by heating to 1150°C-1280°C for a uniform chemical composition. This optional homogenization step can be performed by homogenizing the ingots or billets between 1 hour and 9 hours, preferably for 1 hour, at a temperature between 1150°C and 1280°C.

[0100] This homogenization step enables us to produce a more homogenized chemical composition within the acceptable industrial ranges. Heating temperatures between 1150 to 1280°C ensure proper solubilization of micro segregation from the casting process.

[0101] Optionally, after the casting step (b) and before the optional homogenization step, the ingot or billet can be hot formed by rolling or forging process to achieve the shape and / or the desired properties.

[0102] The billets or ingots produced by the casting process in step (b) are suitable for applications in the automotive and construction industries. For seamless tube production, they require piercing and heat treatments, which involves performing steps (e) to (g).

[0103] Next step (d) is the rolling of the tube obtained at step (c), followed by the cooling step (e) and the heat treatment of step (f), wherein the heating temperature for the finishing steps shall be higher than 700°C. The hot rolling process enhances the dislocation density in the resulting microstructure and facilitates the recrystallization of the microstructure. This, in turn, leads to a reduction in grain size, which is appropriate prior to heat treatment.

[0104] Optionally, at the end of the at least one sequence of steps (g1) and (g2) of the heat treatment of step (g), it can be performed a tempering step (h) in a temperature ranging from 500°C to 700°C. In this case, the heat treatment step (g) comprises a sequence of steps (hi), (h2) and (h3). This sequence is carried out at least once.The tempering steps (h1,h2,h3) lead to the precipitation of carbides (e.g. Fe3C, MoC, Mo2C, VC). Carbides such as VC, MoC, TiC and Mo2C have a strengthening effect on the steel, and also work as hydrogen trap, thus increasing sulphide stress corrosion resistance. Fe3C has an improving effect on elongation and yield strength.

[0105] In the case of seamless tube production of the present invention, the inventors found that the important parameter for tube rolling is the temperatures at finishing steps of rolling mill and, that the important parameter for the heat treatment is the austempering temperature at step (h2). These temperatures allow to obtain a dual phase microstructure composed mainly by austenite and bainitic ferrite having a lath thickness in the range of 50-200nm, possibly containing carbides.

[0106] The tubular product thus obtained from the steel of the invention presents an improved resistance to hydrogen embrittlement, thus to sulfide stress cracking resistance. It can therefore be used in various applications, such as oil and gas production, as well as hydrogen storage and transport, or automotive application.

[0107] The present invention therefore also relates to the use of a tubular product as previously defined, for well drilling, and / or for production, extraction, transportation of oil and gas, hydrogen storage and transport, and automotive application.

[0108] FIGURES

[0109] The characteristics and advantages of the invention are disclosed in more detail in the following description made with reference to the accompanying illustrations of microstructures and time-temperature-transformation curves.

[0110] Figure 1: shows the microstructures of the steel A according to the invention at different austempering temperatures at 1 / 4thand mid thickness at two different scales.Figure 2: shows a simulated time-temperature-transformation (TTT) diagram of the steel A according to the invention.

[0111] Figure 3: shows a Continuous Cooling Transformation (CCT) diagram of the steel A according to the invention.

[0112] EXAMPLES

[0113] a) Tested steels

[0114] The following compositions of steel, according to the present invention (A) and comparative steel (B), have been prepared from the elements listed in Table 1 below. The amounts are expressed as a percentage by weight relative to the total weight of the chemical composition. Underlined values in Table 1 do not conform to the invention.

[0115] Table 1: tested steels

[0116] St Chemical Composition (Unit: mass%, Balance: Fe and Residual elements) ee M M Ti Nb V Co H 0 C Cr Si Ni Al S p

[0117] 1 o n

[0118] A 0, 0, 0, 0, 1, 3, - - 0,00 0, 0,00 0,0 0,0 <0,0 <0,0 34 25 95 23 10 01 87 01 77 04 18 1 1 B 0, 0, 0, 0, 0, 0,00 0,02 0,13 0, 0,00 0,0 0,0 <0,0 <0,0 43 26 99 31 33 03 4 2 03 38 01 18 1 1

[0119]

[0120] b) Protocol

[0121] Two ingots A1 and A2 having the chemical composition A according to the invention, and one comparative ingot B having the comparative chemical composition B were produced by Vacuum Induced Furnace (VIM), with 140x140x510 mm dimensions. After the ingots were produced, they were submitted to a heat treatment of solubilization at 1200 °C during 8 hours for homogenization.

[0122] Afterwards, each of three the ingots (A1, A2 and B) passed by a thermomechanical processing. Initially the ingots were forged from a 140x140 mm section to round bars each having a diameter of 70 mm. After forging, each of the three ingots were hot rolled six times in order to reach a final diameter of 25 mm. Each hot rolling was preceded by a homogenization step. For each step of forging andhot rolling, the initial and final temperatures were 1190°C and 700°C, respectively.

[0123] The homogenization time before each hot rolling was as follows:

[0124] - 1st rolling: 105 minutes,

[0125] - 2nd rolling: 60 minutes,

[0126] - 3rd rolling: 40 minutes,

[0127] - 4throlling: 40 minutes,

[0128] - 5th rolling: 30 minutes,

[0129] - 6th rolling: 25 minutes.

[0130] As shown in table 2 below, the two ingots A1 and A2 were then submitted to austempering heat treatment as follows: austenitization at 850°C for 20 minutes, followed by air cooling, and then isothermal holding in salt bath during 1 hour at 300°C for ingot A1 and at 320°C for ingot A2.

[0131] As shown in table 3 below, the comparative ingot B was submitted to a different heat treatment than ingots A1 and A2, in order to establish the difference between a steel according to the invention that is especially adapted to have a high hydrogen embrittlement resistance to reach the highest SSC resistance possible, and steel B that is a steel commonly used for OCTG products. Thus, comparative ingot B was submitted to two cycles of the following three steps: austenitization - quenching - tempering.

[0132] Table 2: Heat treatment process conditions for steel A

[0133] Cooling down to austempering Isothermal Ingots Austenitization

[0134] temperature holding

[0135] A1 850°C / 20min Salt bath cooling 300°C / 1h

[0136] A2 850°C / 20min Salt bath cooling 320°C / 1h

[0137]

[0138] Table 3: Heat treatment process conditions for steel B

[0139] Austenitization Quenching Tempering Austenitization Quenching Tempering Ingot 1 iiiiiiiiiiii

[0140] Oil 700°C / 880°C / Oil 725°C / B 850°C / 10min

[0141] Quenching 20m in 15m in Quenching 15m in

[0142]

[0143] c) ResultsThe microstructures of the three ingots A1, A2 and B thus obtained are summarized in the following table 4.

[0144] Table 4: Microstructure results for steels A and B

[0145] Ingots Microstructure Bainite Lath Average Blocky Thickness (nm) Austenite area (pm2) A1 85-90% bainitic ferrite

[0146] 81 ± 15 1.6 ± 0.7

[0147] 10-15% retained austenite

[0148] A2 80-85% bainitic ferrite

[0149] 113 ± 23 7.6 ± 4.8

[0150] 15-20% retained austenite

[0151] B > 95% Martensitic - -

[0152]

[0153] As can be seen in table 4 above, comparative ingot B did not enable to achieve a bainitic ferrite phase. The microstructure obtained with this comparative ingot B has the usual microstructure of steel used for sour service OCTG applications according to the state of the art. The increase in retained austenite content as a function of the austempering temperature is illustrated in Figure 1. In Figure 2, it is evident that a slow cooling rate poses a risk of forming ferrite, which would significantly reduce the steel strength and be detrimental to SSC. Figure 3 demonstrates the impact of the austenitizing temperature on the Continuous Cooling Transformation (CCT) diagram and its effect on the critical cooling rate for Bainite formation. The left-hand figure represents an austenitizing temperature of 850°C, while the right-hand figure represents an austenitizing temperature exceeding 1000°C.

[0154] The mechanical behaviors of the three ingots A1, A2 and B thus obtained are summarized in the tables 5 and 6. The SSC resistances of the three ingots A1, A2 and B thus obtained are summarized in the tables 7a, 7b and 7c.

[0155] Table 5: Tensile results for steels A and B

[0156] YS / U

[0157] Ingots YS (0.2%) UTS El (%)

[0158] TS

[0159] 1096 MPa 1544Mpa

[0160] A1 0.71 17.65

[0161] (159 ksi) (224 ksi)

[0162]

[0163] 1399

[0164] 1061 MPa

[0165] A2 MPa (203 0.76 18.91

[0166] (154 ksi)

[0167] ksi)

[0168] 896 Mpa 944 MPa

[0169] B 0.95 21.7

[0170] (130 ksi) (137 ksi)

[0171]

[0172] YS in MPa is the yield strength obtained in tensile test as defined in standards ASTM A370-17 and ASTM E8 / E8M-13a.

[0173] UTS in MPa is the tensile strength obtained in tensile test as defined in standards ASTM A370-17 and ASTM E8 / E8M-13a.

[0174] As illustrated at table 5 above, the results thus obtained show that steel A according to the present invention enables to achieve a much higher yield strength, such as 1096 MPa (159 ksi), only by adjusting process parameters (i.e., by adjusting the austempering temperature without modifying the holding time). All the ingots had a toughness above 55 Joules and a shear area of 100%, when tested at a temperature of 0°C.

[0175] SSC is the sulfide stress corrosion cracking resistance evaluated according to standard NACE TM0177-2016 Method A. The SSC test consists in immersing the test specimens under load in an aqueous solution adjusted to pH 3.5 or 2.7 with the addition of acetic acid and sodium acetate in a test solution of 5 mass% NaCI. The solution temperature is 24°C, H2S is at 1 bar or 0.1 bar. The testing duration is 720 hours, and the applied stress was in the range of 65%-80% of the actual yield strength. A successful test implies no failure on the specimens after 720 hours.

[0176] Hardness tests were performed using the Vickers test (HV10) in compliance with ISO 6507-1 on the EMCO-test model DuraScan 70 5G testing machine using a 10gf load.

[0177] Charpy V-Notch (CVN) tests were performed on an INSTRON model 750 MP2 V2-J4 machine at 0°C. In accordance with the ASTM A 370, the absorbed energy was evaluated as well as the shear area.Table 6: Hardness results for steels A and B

[0178] Samples HV10

[0179] A1 459

[0180] A2 441

[0181] B1 315

[0182]

[0183] Table 6 shows that the steel of the invention achieves greater hardness compared to conventional OCTG steel B.

[0184] X-ray diffraction (XRD) using Reitveld refinement was used to quantify the austenite phase fraction to a precision of approximately ± 1%. Scanning electron microscopy (SEM) were used for characterization of the structure, and of the bainite lath thickness.

[0185] Table 7a: SSC resistance results for steels A and B in mid sour conditions, pH 3.5, with an applied load of 730 MPa (106 ksi).

[0186] YS (0.2)

[0187] Ingots Solution H2S pH Load Result (h) %

[0188] 3 / 3 passed: 1096

[0189] A1 720h no MPa

[0190] cracks NACE Mid-sour 3 / 3 passed: 1061 730

[0191] A2 TM0177 - (0.1 bar 3,5 720h no MPa MPa

[0192] Solution B H2S) cracks 0 / 3 passed 903

[0193] B Cracks before MPa

[0194] 720h

[0195]

[0196] Table 7b: SSC resistance results for steels A and B in full sour conditions, pH 3.5, with an applied load of 730 MPa (106 ksi).

[0197] YS (0.2)

[0198] Ingots Solution H2S pH Load Result (h)

[0199] %

[0200]

[0201] 3 / 3 passed: 1096

[0202] A1 720h no MPa

[0203] cracks NACE

[0204] Full-sour 3 / 3 passed: 1061 TM0177 - 3,5 730MPa

[0205] A2 (1 bar H2S) 720h no MPa Solution B

[0206] cracks 903 0 / 3 passed B MPa Cracks before

[0207]

[0208] 720h

[0209] Nine ingots were put in the conditions described in tables 7a and 7b during 30 days / 720 hours for a pass / not pass test:

[0210] - three ingots A1 having a yield strength of 1096 MPa, and three ingots A2 having a yield strength of 1061 MPa, all six being according to the invention. Inventors surprisingly observed that all these ingots did pass NACE TM0177 Method A test in full sour and mid-sour conditions despite their greater yield strength.

[0211] - three comparative ingots B having a yield strength of 903 MPa. These three ingots did not pass.

[0212] Table 7c: SSC resistance results for steels A and B in full sour conditions, pH 2.7, with increasing applied loads

[0213] YS (0.2)

[0214] Ingots Solution H2S pH Load Result (h) %

[0215] NACE 3 / 3 passed: 1061 744

[0216] A2 TM0177 - 720h no MPa MPa

[0217] Solution A cracks 1061 NACE 3 / 3 passed:

[0218] Full-sour 799

[0219] A2 MPa TM0177 - 2,7 720h no (1 bar H2S) MPa

[0220] Solution A cracks 1061 NACE 3 / 3 passed:

[0221] 848

[0222] A2 MPa TM0177 - 720h no MPa

[0223] Solution A cracks

[0224]

[0225] NACE 0 / 3 passed: 903 799

[0226] TM0177 - 720h no MPa MPa

[0227] Solution A cracks

[0228]

[0229] Twelve ingots were put in the conditions described in table 7c during 30 days / 720 hours for a pass / not pass test:

[0230] - three ingots A2 having a yield strength of 1061 MPa, all three being according to the invention, that were submitted to a load of 744 MPa.

[0231] - three ingots A2 having a yield strength of 1061 MPa, all three being according to the invention, that were submitted to a load of 744 MPa.

[0232] - three ingots A2 having a yield strength of 1061 MPa, all three being according to the invention, that were submitted to a load of 848 MPa.

[0233] It was observed that these nine ingots A2 passed the NACE TM0177 Method A test in full sour conditions despite their greater yield strength.

[0234] The Three ingots B, each with a yield strength of 903 MPa, were subjected to a load of 799 MPa. These three ingots did not pass the NACE TM0177 Method A test in full sour conditions.

Claims

AMENDED CLAIMSreceived by the International Bureau on 22 June 2026 (22.06.2026)1. Steel having a chemical composition comprising, in weight percentage:0.30 < C < 0.45%0.20 < Mo < 0.35%0.50 ≤ Cr ≤ 1.25%0.10 ≤ Mn ≤ 0.25%0.80 < Si < 1.50%2.00 < Ni < 3.50%Ti ≤ 0.015%Nb ≤ 0.015%V ≤ 0.015%Al < 0.50%Co < 0.50%the balance of the chemical composition being constituted by Fe and one or more inevitable impurities, including S, P, H or O and mixtures thereof, the amount of which being as follows:S <0.005%P <0.020%H <0.010%O <0.010%.

2. Steel according to claim 1, wherein: 0.33 < C < 0.45%.

3. Steel according to any of the preceding claims, wherein: 0.23 < Mo < 0.27%.

4. Steel according to any of the preceding claims, wherein: 0.85 ≤ Cr ≤ 0.97%.

5. Steel according to any of the preceding claims, wherein: 0.15 ≤ Mn ≤ 0.23%.

6. Steel according to any of the preceding claims, wherein: 1.00 ≤ Si ≤ 1.30%.

7. Steel according to any of the preceding claims, wherein: 3.00 ≤ Ni ≤ 3.40.

8. Steel according to any of the preceding claims, wherein: Ti + Nb + V ≤ 0.015%.

9. Steel according to any of the preceding claims, wherein: 0.05 < Al < 0.50%.

10. Steel according to any of the preceding claims, wherein: 0.05 < Co < 0.50%.

11. Steel according to any of the preceding claims, wherein it has a microstructure consisting of bainitic ferrite between 70% and 90% in volume fraction, of retained austenite between 10% and 30% in volume fraction, and carbides in an amount equal to or lower than 0.25% in volume fraction.

12. Steel according to claim 11, wherein the bainitic ferrite has a lath thickness lower than 200nm or even more preferably below 100 nm.

13. Tubular product for applications in the production, transport, and storage of oil and gas, said tubular product being made from a steel having a composition according to any of claims 1 to 10, and having a microstructure according to claim 11 or 12.

14. Process for manufacturing a tubular product according to claim 13, said process comprising the following steps:(a) providing a steel having a chemical composition as defined in any of claims 1 to 10,(b) casting said steel of step (a) by conventional or continuous casting to obtain a billet or an ingot,(c) piercing the billet or the ingot in a heating temperature kept between 1150 °C and 1300 °C to obtain a tubular product,(d) rolling of the tubular product obtained in step (c) to obtain a tubular product having the desired dimensions wherein the processing temperature at the end of this step shall be more than 700°C,(e) cooling down the tubular product obtained at step (d) by air cooling, or by water cooling, or by oil cooling,(f) performing a heat treatment of the cooled tubular product obtained at step (e), said heat treatment comprising at least one sequence of the following steps (g1) and (g2):(g1) heating up the cooled tubular product obtained at step (e) to an austenitizing temperature ranging from 800°C to 1020°C, and then keeping said tubular product at the austenitizing temperature during a time comprised between 10 minutes and 30 minutes to obtain an austenitized tubular product,(g2) cooling down said austenitized tubular product to austempering temperature ranging from 200°C to 400°C at a cooling rate above 10°C / s,and then keeping said tubular product at the austempering temperature during a time comprised between 20 minutes and 240 minutes, and then cooling said tubular product to room temperature.(h) optionally performing a tempering step in a temperature ranging from 500°C to 700°C at the end of at least one sequence of steps (g1 ) and (g2).

15. Use of a tubular product according to claim 13 for well drilling, and / or for production, extraction, transportation of oil and gas, hydrogen storage and transport, and automotive application.

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