Steel composition, manufacturing method, steel article and uses hereof

Abstract

A steel composition allowing to manufacture a weldable product for compressed hydrogen storage and transportation comprises in wt. %: C: 0.07-0.20; Mn: 1.00-1.60, Si: 0.30-0.50; Cr: 0.30-1.50; Mo: 0.30-0.80; P: ≤ 0.020; S: ≤ 0.020; Al: 0.01-0.20; Ni: ≤ 1.20; W: ≤ 1.60; V: ≤ 0.30; Nb: ≤ 0.20; Ta: ≤ 0.20; Ti: ≤ 0.20; Zr: ≤ 0.20; B: ≤ 0.005; Cu: ≤ 0.50; and Fe and inevitable impurities.

Description

[0001] STEEL COMPOSITION, MANUFACTURING METHOD, STEEL ARTICLE AND USES

[0002] THEREOF

[0003] Technical Field

[0004] The present invention relates to a steel composition, manufacturing method, steel article and uses thereof, in particular for storage and transportation of compressed hydrogen.

[0005] Technical Background.

[0006] In the transition of a natural fuel based economy due to the limited resources and the concerns and problems regarding the emission of greenhouse gases to a greenhouse gas free energy economy hydrogen as an alternative fuel has gained interest over the recent years. Hydrogen presents a good energy density per unit of mass, but a very low energy density per unit of volume at low pressure. From a technological point of view hydrogen gas is only interesting when it is stored and transported at elevated pressure or in liquid state.

[0007] In the development of large scale facilities and installations for storage and transportation of pressurized hydrogen gas a balance between capacity (vessel size and hydrogen pressure) and weight (vessel size, wall thickness in view of required strength) is sought, Also the productivity of the manufacturing process including welding steps plays an important role. Therefore, high strength steel is required for these applications.

[0008] However, exposure to hydrogen may have a detrimental effect on the steel properties. In particular, it is well known in the art that hydrogen causes embrittlement (i.e. a reduction of ductility and toughness) and reduction of fatigue resistance of the steel. The sensitivity of the steel to embrittlement is higher, the higher the steel strength (for the same chemical composition of the steel).

[0009] In the art various steel types for use with hydrogen are known.

[0010] Ferritic steels are limited by hydrogen embrittlement due to elevated hydrogen diffusivity and low solubility resulting in accumulation of hydrogen at the grain boundaries and interfaces with non-metallic phases (inclusions), thereby weakening the bonds and enhancing embrittlement Therefore the ferritic steels operating with pressurized hydrogen are limited in terms of tensile strength.

[0011] Austenitic steels have a low hydrogen diffusivity and high solubility and these steels are less sensitive to hydrogen accumulation at the grain boundaries resulting in a limited sensitivity to hydrogen embrittlement. Compared to ferritic steels a higher tensile strength can be achieved, as well as a better fatigue resistance and / or lower fatigue crack growth. Austenitic steels can be either stable or metastable. Metastable austenitic steels may undergo a strain induced martensitic transformation. Fresh martensite resulting from this strain induced transformation constitutes a weak spot in the steel structure, because it is highly sensitive to embrittlement due to being untempered martensite. Stable austenitic steels do not suffer from these drawbacks, but require a high amount of alloying elements. These steels have poor tensile strength properties, requiring a larger thickness at a given design pressure. Therefore these steels are not competitive regarding Cr-Mo steels. Austenitic steels having strength properties similar to Cr-Mo steels and reduced sensitivity to hydrogen embrittlement are highly alloyed steel composition. These ‘rich’ compositions, however, are expensive. Micro-alloyed ferritic steels can achieve a high strength at a lean chemical composition. However, these steels require low temperature tempering, resulting in poor toughness and eventually poor resistance to hydrogen embrittlement, as well as a short fatigue life. Thus these micro-alloyed ferritic steels, although being less expensive, are not suitable for operating in high pressure hydrogen environments.

[0012] Hydrogen storage vessel parts and transportation pipes are typically joined by welding. Therefore in addition to strength and toughness, weldability is another desired property of the steel composition. Hydrogen storage vessels are typically exposed to an alternating load as a result of the cycles of loading and discharging and thus hydrogen fatigue resistance plays a significant role.

[0013] Generally, strength can be increased by cold working or heat treatment. For high pressure hydrogen storage and transportation steel components cold working is not suitable as this cold working reduces ductility and toughness. A heat treatment comprising quenching followed by tempering of the steel (also known as Q & T steel) is effective for achieving a desired combination of strength and toughness. However, the strength of a Q & T steel having a given composition decreases at increasing the tempering temperature and / or holding time, while toughness increases at increasing the tempering temperature and / or holding time. Due to the opposite effects of tempering temperature and / or holding time on strength and toughness respectively a balance between strength and toughness needs to be found. As described above, increasing the amounts of alloying elements allows to achieve an increase of strength and toughness, however the weldability is significantly reduced. The reduced weldability manifests itself in increased sensitivity to cold cracking and reheat cracking.

[0014] From US10106875B2 a steel material and hydrogen container are known. The steel includes a composition containing, by mass, C: 0.05% - 0.60%, Si: 0.01% - 2.0%, Mn: 0.3% - 3.0%, P: 0.001% - 0.040%, S: 0.0001% - 0.010%, N: 0.0001% - 0.0060%, Al: 0.01% - 1.5%, one or more elements selected from Ti: 0.01% - 0.20%, Nb: 0.01% - 0.20% and V: 0.01% or more and less than 0.05%, and one or more elements selected from B: 0.0001% - 0.01%, Mo: 0.005% - 2.0%, and Cr: 0.005% - 3.0%, with the balance being Fe and inevitable impurities, as well as having a steel microstructure that includes 95% or more of tempered martensite on a volume fraction basis, that includes a precipitate having a diameter of 100 nm or less and including one or more elements selected from Ti, Nb, and V and one or more elements selected from carbon and nitrogen at a density of 50 particles / pm2 or more, and that includes prior austenite having a grain diameter of 3 pm or more. This known steel material is not weldable. Its toughness and fatigue properties in hydrogen leave something to be desired. The invention aims at providing a steel composition allowing to manufacture a steel article, in particular such as a pressurized hydrogen storage vessel or a pressurized hydrogen transportation pipe, having a balanced combination of properties regarding strength, toughness and weldability.

[0015] It is an object of the invention to provide a high strength, weldable steel that is suitable for manufacturing hydrogen pressure vessels and piping for compressed hydrogen gas storage and transportation.

[0016] It is another object of the invention to provide such a steel that has a yield strength of at least 689 MPa (100 ksi),

[0017] It is yet another object of the invention to provide such a steel having a good fracture toughness in hydrogen.

[0018] It is a further object of the invention to provide such a steel that is able to withstand hydrogen gas pressures of 80 bar or more, in particular 120 bar or more.

[0019] It is yet another object of the invention to provide such a steel that is suitable for manufacturing seamless products as used in integrally forged or welded constructions. It is still another object of the invention to provide such a steel that is weldable and therefore suitable for manufacturing welded constructions such as joining pipe length, mounting plates, flanges and the like.

[0020] Summary of the Invention

[0021] In a first aspect the present invention provides a steel composition comprising, in wt. %, preferably consisting of:

[0022] C: 0.07-0.20;

[0023] Mn: 1.00-1.60,

[0024] Si: 0.10-0.50;

[0025] Cr: 0.30-1.50;

[0026] Mo: 0.30-0.80;

[0027] P: ≤ 0.020;

[0028] S: ≤ 0.020;

[0029] Al: 0.01-0.20;

[0030] Ni: ≤ 1.20; W: ≤1.60;

[0031] V: ≤ 0.30;

[0032] Nb: ≤ 0.20;

[0033] Ta: ≤ 0.20;

[0034] Ti: ≤ 0.20;

[0035] Zr: ≤ 0.20;

[0036] B: ≤ 0.005

[0037] Cu ≤ 0.50; and

[0038] Fe and inevitable impurities, wherein P* ≥ 22, wherein P* =78x %C + 7x %Cr + 29x (%Mo + %W / 2) + 137 %V.

[0039] In a further aspect the invention provides a method of manufacturing a steel article, comprising the steps of: a) providing a semi-finished product having a steel composition according to the invention; b) austenitizing the semi-finished product at a temperature and during a time sufficient to achieve a fully austenitized semi-finished product; c) quenching the austenitized semi-finished product to ambient temperature; d) tempering the quenched semi-finished product at a temperature above 600 °C and below the austenite transformation start temperature; e) cooling the tempered semi-finished product to room temperature.

[0040] In yet another aspect the invention provides a steel article that is manufactured according to the method according to the invention.

[0041] The invention allows to manufacture a steel article that can be welded in further construction steps, that shows a high strength, in particular yield strength and comparatively outstanding fracture toughness, allowing to use the steel article in hydrogen pressure vessels and transportation pipes.

[0042] Brief description of the drawings

[0043] Fig. 1 is a diagram showing the relationship between tensile strength and fracture toughness for Inventive Examples and Comparative Examples;

[0044] Fig. 2 is a diagram showing the fracture toughness as function of the tempering temperature for Inventive Examples and Comparative Examples;

[0045] Fig. 3 shows the relationship between the yield strength and tensile strength respectively as a function of the tempering parameter P* temperature for Inventive Examples and Comparative Examples; and

[0046] Fig. 4 shows the relationship between the cold cracking sensitivity and the ratio C / Ceq temperature for Inventive Examples and Comparative Examples. Detailed description of the Invention

[0047] Composition

[0048] The steel composition according to the invention is explained hereinbelow in detail.

[0049] Carbon, manganese, silicon, aluminium, chromium and molybdenum are mandatory elements in the steel composition. Nickel, tungsten, vanadium, niobium, tantalum, titanium and zirconium are optional elements. Phosphor and sulphur belong to the inevitable impurities. Typically boron is a residual element, that is not intentionally added. Copper is also a residual element, frequently occurring in scrap material.

[0050] Carbon (C)

[0051] Carbon is present is required for strength and hardenability by precipitation of carbides in the last stage of heat treatment. Therefore the lower limit is 0.07, such as 0.09, advantageously 0.10 wt.%. The carbon content is limited by weldability. If the carbon content is above 0.20 wt.%, weldability is affected. Therefore, carbon is present at 0.10 - 0.20 wt. %. A preferred range is 0.12 -0.18.

[0052] Manganese (Mn)

[0053] Manganese is required for hardenability, toughness and grain size control. In the context of this specification grain size control involves two characteristics 1) a fine average grain size; and 2) a small variance in grain size. Therefore the lower limit is 1.00 wt. %. Manganese promotes carbon segregation upon solidification, which is an undesired phenomenon as it is a cause of gradients of hardness and toughness wherein some of the steel sections are below or above the expected ranges, Also local carbon concentration may result in more severe hard spots upon welding. Therefore the upper limit of manganese is 1.6 wt.%. thus in the steel according to the invention manganese is in the range of 1.00-1.60, preferably in the range of 1.00 -1.50.

[0054] Silicon (Si)

[0055] The primary purpose of silicon is deoxidation (killing) of the molten steel composition. The silicon content is therefore in the range of 0.10 -0. 50 wt.%. Above 0.50 wt.% toughness is affected. Therefore the silicon content is in the range of 0.10-0.50, preferably in the range of 0.10 - 0.40.

[0056] Phosphor (P) and Sulphur (S)

[0057] Phosphor and sulphur are considered inevitable impurities resulting from the ores, as well as scrap material. Above 0.020 wt. % they are contemplated to affect the toughness properties of the steel. Therefore the phosphor content is in the range of up to 0.0020 wt.%, preferably the phosphor content is up to 0.015 wt.%, including 0 wt.%. The sulphur content is also in the range of up to 0.0020, zero included, preferably in the range up to 0.015 wt.%, including 0 wt.%. Aluminium (Al):

[0058] Aluminium is present for the purpose of deoxidation during preparation, like silicon. It may also bind nitrogen. A minimum amount of 0.01 wt.% is required. Above 0.20 wt.%, hot workability is deteriorated. Therefore aluminium is present in the range of 0.01 - 0.20 wt.%. Nickel (Ni)

[0059] Nickel has a beneficial effect on the hardenability of the composition. In addition nickel contributes to toughness and grain size control. Nickel also affects retained austenite and weldability. Nickel is also considered an expensive element. Therefore nickel might be present in the range up to 1.20 wt.%, such as up to 1.00 wt. %, preferably up to 0.50 wt.%, zero included. The higher Ni range above 0.50 wt.% can be advantageous in case of low carbon content, such as C is in the range of 0.07 up to and exclusive 0.10.

[0060] Chromium (Cr)

[0061] Chromium is effective for increasing the hardenability of the composition and strength. A minimum amount of 0.30 wt.% is required. Above 1.50 wt.% weldability of the composition is reduced. Therefore chromium is present in the range of 0.30 - 1.50 wt.%, preferably 0.30 - 0.80 wt.%.

[0062] Molybdenum (Mo)

[0063] Molybdenum is also effective for the same reasons as chromium. A minimum amount of 0.30 wt.% is required. The upper limit of 0.80 wt.% is dictated by weldability. Molybdenum is also considered an expensive element and thus increases costs. Therefore molybdenum is present in the range of 0.30 - 0.80 wt.%, preferably in the range of 0.30 - 0.70 wt.%, such as in the range of 0.30 - 0.60 wt.%.

[0064] In an embodiment, in particular at low carbon content such as C is in the range of 0.07 up to and exclusive 0.10, the sum of Cr + Mo is 0.90 wt.% or more, such as 1.00 wt.% or more. Tungsten (W)

[0065] Tungsten is another element that may be present to increase hardenability and strength.. However, it is less effective than molybdenum. In order to ensure sufficient weldability the upper limit of tungsten is 1.60 wt.%. Therefore tungsten may be present in the range up to 1.60 wt.%, preferably up to 1.20 wt.%, including 0 wt.% Vanadium (V)

[0066] Vanadium may be present to increase hardenability, strength and grain size control. The upper limit is set in view of weldability and costs. Therefore vanadium may be present in the range up to 0.30 wt.%, preferably up to 0.15 wt.%.

[0067] Chromium, molybdenum, tungsten and vanadium are strong carbides forming elements. The balance of these elements ensure tempering resistance and therefore allow tempering at sufficiently elevated temperature to improve toughness. Niobium (Nb)

[0068] Niobium is an optional element for grain size control and toughness. The upper limit is set in view of weldability and costs. Therefore niobium may be present in the range up to 0.20 wt.%, preferably up to 0.10 wt.%.

[0069] Tantalum (Ta)

[0070] Tantalum is another optional element for grain size control and toughness. The upper limit is set in view of weldability and costs. Therefore tantalum may be present in the range up to 0.20 wt.%, preferably up to 0.10 wt.%.

[0071] Titanium (Ti)

[0072] Titanium is also an optional element having the function of grain size control and ensuring toughness in the heat affected zone (HAZ) upon welding. The upper limit is set in view of overall toughness. Therefore titanium may be present in the range up to 0.20 wt.%, preferably up to 0.10 wt.%.

[0073] Zirconium (Zr)

[0074] Zirconium is another optional element having the function of grain size control and ensuring toughness in the heat affected zone (HAZ) upon welding. The upper limit is set in view of overall toughness. Therefore titanium may be present in the range up to 0.20 wt.%, preferably up to 0.10 wt.%.

[0075] Boron (B)

[0076] Typically boron is considered a residual element as it is not intentionally added. Boron can be used optionally to increase hardenability. The maximum content in the steel according to the invention is 0.005 wt. %. Preferably its content is 0.0035 wt.% or less.

[0077] Copper (Cu)

[0078] Typically copper is considered a residual element as is not intentionally added. Copper may be present in scrap steel used for preparing the composition. Copper has a very minor effect on increasing strength and increasing resistance to atmospheric corrosion, Copper may result in hot shortness (brittleness at hot working temperatures). The maximum amount is 0.50 wt. %, preferably 0.30 wt % or less.

[0079] Inclusions

[0080] 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.

[0081] Advantageously the maximum content of metallic inclusions, if any, conforms to (ASTM E45) A (sulphide)

[0082] Thin: 0;

[0083] Heavy: 0;

[0084] B (alumina)

[0085] Thin: 1.5, preferably 1.0;

[0086] Heavy: 1.0, preferably 0;

[0087] C (silicates)

[0088] Thin: 1.5, preferably 1.0;

[0089] Heavy: 1.0, preferably 0;

[0090] D (globular oxide)

[0091] Thin: 2.0, preferably 1.5;

[0092] Heavy: 1.0, preferably 0.5.

[0093] Oversize inclusions can be present up to a maximum size of 50 pm, preferably less than 30 pm.

[0094] Preferably the steel composition according to the invention comprises, preferably consists of, in wt.%:

[0095] C: 0.09-0.20, such as 0.10-0.20, in particular 0.12-0.18;

[0096] Mn: 1.00-1.50,

[0097] Si: 0.10-0.40;

[0098] Cr: 0.30-0.80;

[0099] Mo: 0.30-0.70, in particular 0.30-0.60;

[0100] P: ≤0.015;

[0101] S: ≤0.015;

[0102] Al: 0.01-0.05;

[0103] Ni: ≤ 1.00, in particular ≤ 0.50;

[0104] W: ≤1.20;

[0105] V: ≤0.15;

[0106] Nb: ≤0.10;

[0107] Ta: ≤0.10;

[0108] Ti: ≤0.10;

[0109] Zr: ≤0.10;

[0110] B: ≤ 0.0035;

[0111] Cu: s 0.30; and

[0112] Fe and inevitable impurities. Tempering resistance

[0113] Generally, sufficient toughness can be achieved by performing a final tempering heat treatment at a sufficiently high temperature, which temperature may slightly vary as a function of the chemical composition of the steel and actual holding time applied. To ensure meeting a desired minimum yield strength of at Ieast689 MPa (100 ksi) in combination with sufficient tempering to achieve high toughness a tempering resistance parameter P* =78* %C + 7x %Cr + 29x (%Mo + %W / 2) + 137 %V (% is wt.%) is at least 22. Preferably P* is at least 23, more preferably at least 25, most preferably at least 28, such as at least 28.7 or at least 29.1.

[0114] Weldability

[0115] In the context of this invention weldability is to be understood as a qualitative indication that an ordinary skilled welder can make the weld without having to apply excessively controlled welding conditions and that no defect is detected on the finished welded joint by means of non-destructive testing, such as using radiographs, penetrant liquid testing, magnetic particle inspection and the like. Welding does not affect the mechanical properties, that is to say the mechanical properties are in compliance with a design specification, e.g. a cross-weld tensile strength of at least 689 MPa (100 ksi); impact test at -40 °C of all portions of the welded joint (WM, HAZ, BM) showing predominantly ductile fracture (>50% shear area) and / or impact energy test results > 40 J average, ≥ 27 J for 3 specimens.

[0116] Cold cracking

[0117] For weldability the equivalent carbon content CE may be used to define the combined effect of the alloying elements on hardenability, toughness and strength and in particular susceptibility to (hydrogen) cold cracking.

[0118] High hardenability results in hard microstructures and poor toughness in the as-welded condition at the heat-affected zone. Together with hydrogen absorption and residual stresses of thermal origin from welding, this may result in cracking at the heat affected zone of the weldments. Cracking can be delayed up to 48 h after welding due to hydrogen diffusing and concentrating at weak spots in the microstructure, such as inclusions, grain and sub-grain boundaries, as well as dislocation stacks.

[0119] Advantageously .in order to ensure limited susceptibility to cold cracking the steel composition according to the invention meets the formulas of %C a 0.125 - 0.05* Ceq; and

[0120] %C ≥ -1.15 + 2.35x Ceq, wherein

[0121] Ceq = %C + A(%C)x(%Mn / 6 + %Si / 24 + %Cu / 15 + %Ni / 20 + (%Cr+%Mo+%V+%Nb+%W) / 5 + 5x %B) with A(%C) = 0.75 + 0.25x tanh(20x(%C-0.12)) (% is wt.%). These formulas show the correlation of the sensitivity to cold cracking to the ratio between carbon content and carbon equivalent. (Graville, B. A survey review of weld metal hydrogen cracking. Weld World 24, 1986, pp.190-198).

[0122] Meeting the above formulas ensures that the susceptibility to cold cracking of the steel composition is limited and that qualified welding procedure specifications can be used for welding.

[0123] Reheat cracking

[0124] Residual stresses from welding, plastic deformation upon reheating during post-welding heat treatment (to reduce internal stress resulting from welding), and temper embrittlement may result in the occurrence of cracking in the heat affected zone (HAZ). Tempering embrittlement itself may occur in a certain range of temperatures (typically below the tempering temperature range, and the post-welding heat treatment range) due to a combination of a hard microstructure in the HAZ, accelerated ageing and segregation of impurities at grain and subgrain boundaries.

[0125] For defining the cracking sensitivity in relation to the chemical composition of the steel a cracking sensitivity parameter CS = Cr + 3.3x %Mo + 8.1 x %V - 3 may be used, wherein CS ≤ 0.5. According to H. Nakamura, T. Naiki, H. Okabayashi. Fracture in the Process of Stress Relaxation. Proc. 1st Int. Conf. Fracture, Sendai, Japan. (1965) Vol.2, pp 863-878 a steel is considered sensitive to reheat cracking if CS > 0. Therefore, advantageously CS < 0.

[0126] Manufacturing method

[0127] The method of manufacturing a steel article according to the invention starts with preparing a steel composition as defined above, which is processed into a semi-finished product, such as a hot-rolled semi-finished product. Typically such a hot-rolled seamless semi-finished tubular product is obtained through a process comprising: i) Heating a round billet, preferably by means of a rotary hearth furnace; ii) Piercing the billet, in particular according to the Mannesmann process; iii) Hot rolling e.g. on a retained mandrel; iv) Sizing, advantageously mandrel-free sizing; v) Optionally, reheating and expanding; vi) Cooling to room temperature, preferably in still air; vii) Cutting in particular to a desired length; viii) If applicable, ends forging, advantageously by hot spinning.

[0128] The semi-finished product thus obtained has a tubular body, optionally with forged ends.

[0129] A semi-finished sheet or strip of steel may be obtained using well-known cold roll and hot roll techniques. The semi-finished product thus obtained is subjected to a heat treatment at a temperature and using a holding time that allows to achieve a fully austenitized semi-product, typically above Ac3 up to the abnormal grain coarsening temperature. During the austenitizing step internal stresses are released and stable carbides and / or carbonitrides precipitates are formed.

[0130] Next the fully austenitized semi-product is quenched to room temperature. Preferably quenching is performed between 800 to 500 °C at a quenching rate of at least 5°C / s, more preferably at least 10 °C / s, most preferably at least 25 °C / s. Thereafter the quenched semifinished product is tempered at a tempering temperature above 600 °C and below the austenite transformation start temperature Ac1. Water spray is an example of an appropriate quenching medium. Typically, the higher the tempering temperature, the lower the strength, as well as the higher the toughness and ductility. For Q & T steels the tempering temperature has a direct correlation to the fracture toughness in hydrogen. To ensure a fracture toughness KJQ according to ASTM E1820 of 80 MPa.m14 in hydrogen at 206 bar (3000 psi) a minimum tempering temperature of 600 °C is required. In order to avoid martensite recrystallisation during tempering, advantageously the tempering temperature is 700 °C or less, preferably less than 670 °C.

[0131] Subsequent to the tempering step the thus quenched semi-finished product is allowed to cool to ambient temperature thereby resulting in a steel article according to the invention.

[0132] Weldable steel article according to the invention

[0133] A weldable steel article having a steel composition according to the invention and manufactured according to the manufacturing method of the invention preferably has a microstructure essentially consisting of, in area %, 30 - 70% martensite, preferably 50% or more, and 30 - 70 % bainite, preferably up to 50%, and the sum of ferrite and pearlite is 10% or less, preferably 3% or less, and most preferably about 0 %.

[0134] In view of a homogenous microstructure throughout the article and thus constant mechanical properties advantageously the grain size (ASTM E112) is 6 or finer, such as 7 or more. The finer the grain size, the better the fracture toughness and fatigue resistance.

[0135] The steel article preferably has at least one property from

[0136] Yield strength:(MPa (ksi)) ≥ 689 (100);

[0137] Tensile strength :(MPa (ksi)) ≥ 758 (110);

[0138] Elongation (%): a 15;

[0139] Absorbed energy of Charpy impact test at -40 °C (J / cm2): a 240, preferably a 250; Fracture toughness (100% H2, 200 bar, RT; MPa^mVa): KJQ ≥ 80, preferably ≥ 90; Fracture toughness (air, -40 °C; MPa-m%) KJQ ≥ 200; preferably all these properties. Advantageously the fracture toughness (air; room temperature) KJQ ≥ 250 MPa-m!4.

[0140] The yield strength, tensile strength and elongation are determined according to ASTM E8. Fracture toughness is measured according to ASTM E1820. In general, the wall thickness of a tubular product like a vessel or pipe is not particularly limited. Typically the wall thickness may be up to 51.8 mm (2 inches).

[0141] Preferred applications of the steel article comprise a (welded) pressure vessel for compressed hydrogen gas storage or components thereof like flanges and / or plates, a (weldable) tubular product, such as a line pipe for compressed hydrogen gas transportation. Another contemplated application concerns a riser.

[0142] Examples

[0143] Inventive Examples 1-6 and Comparative Examples 1-7 were manufactured and tested. A steel composition as indicated in Table 1 was prepared. From the steel composition a hot- rolled hollow cylindrical tube was manufactured as outlined above. The resulting tube having the dimensions shown in Table 1 as semi-finished product was subjected to a quenching and tempering heat treatment (Q &T) under the conditions shown in Table 2. Inventive Examples 5-6 concern plates .Some of the Comparative Examples were subjected to a thermomechanical treatment instead of a Q&T treatment. The mechanical properties of the final product, as well as weldability are also included in Table 2. The grain size number (ASTM E112) of the Inventive Examples was 7-8.

[0144] From the results it can be seen that the Inventive Examples have a TS > 758 MPa and YS > 689 MPa. The Inventive Examples 1-6 also show an Absorbed Energy @ -40 °C of > 240 J / cm2. Comparative Example 3 (A4) shows a similar behaviour, but is not weldable.

[0145] Further test were performed.

[0146] Fig. 1 shows the fracture toughness behaviour in hydrogen at 206 bar in relation to tensile strength, Inventive Examples 1 and 2 (resp. A1 (■) and A1B (•)) and 5 and 6 (A13 and A14, both A ) show a stable crack propagation. □ represent Comparative Examples A2, A5, A6 and A8 thatshow stable crack propagation. 0 indicate Comparative Examples A3 and A4 showing an unstable crack propagation. The weldable Inventive Examples show a very high fracture toughness in hydrogen at high strength. Comparative Example A2 shows an even higher strength and fracture toughness, but is not weldable. Comparative Example A4 is difficult to weld and shows an unstable crack propagation.

[0147] Fig. 2 shows the relation between the fracture toughness in hydrogen at 206 bar and the tempering temperature of the quenched and tempered steel. It appears that a minimum tempering temperature of 600 °C allows to achieve a minimum Kjoof 80 MPa m1 / -. Fig. 3 show the relation between the yield strength and tensile strength as a function of the tempering parameter. A P* value of at least 22 allows to achieve a yield strength of at least 689 MPa.

[0148] Fig. 4 shows the relationship between the cold cracking sensitivity and the ratio C / Ceq temperature for Inventive Examples and Comparative Examples in a Granville diagram. The inventive Examples are in the weldable section of the diagram. Weldable low carbon (<0.10 wt. C), low alloyed (low Ceq) steel like Comparative Examples 4 and 5 (A5 and A6) is easily weldable, but show lower strength. Comparative Example 7 being also a weldable low carbon, low alloyed steel shows inferior fracture toughness as shown in Fig. 1. Comparative Examples 4 , 5 and 7 are manufactured by thermomechanically rolling, wherein strength is derived from the combination of microstructure and work hardening. Table 1. Composition and Formulas

[0149] Comp. Ex 4 (A5) and Comp. Ex 5 (A6): data obtained from C. San Marchi et al., Fracture and Fatigue of Commercial Grade API Pipeline Steels in Gaseous Hydrogen DOI:10.1115 / PVP2010-25825; and Comp. Ex 7 (A8): data obtained from J. Ronevich et al., Oxygen Impurity Effects on Hydrogen Assisted Fatigue and Fracture of x100 Pipeline Steel, PVP2018-84163.

[0150] Table 2. Formulas, method conditions and properties

Claims

CLAIMS1. Steel composition, comprising, in wt. %,C: 0.07-0.20;Mn: 1.00-1.60,Si: 0.10-0.50;Cr: 0.30-1.50;Mo: 0.30-0.80;P: ≤ 0.020;S: ≤ 0.020;Al: 0.01-0.20;Ni: ≤1.20;W: ≤1.60;V: ≤ 0.30;Nb: ≤ 0.20;Ta: ≤ 0.20;Ti: ≤ 0.20;Zr: ≤ 0.20;B: ≤ 0.005;Cu: : ≤ 0.50; andFe and inevitable impurities, wherein P* ≥ 22, wherein P* =78* %C + 7* %Cr + 29* (%Mo + %W / 2) + 137 %V.

2. Steel composition according to claim 1 , wherein:C: 0.09-0.20; preferably 0.10-0.20; more preferably 0.12-0.18;Mn: 1.00-1.50,Si: 0.10-0.40;Cr: 0.30-0.80;Mo: 0.30-0.70; preferably 0.30-0.60;P: ≤0.015;S: ≤0.015;Al: 0.01-0.05;Ni: ≤ 1.00; preferably ≤ 0.50;W: ≤1.20;V: ≤0.15;Nb: ≤0.10;Ta: ≤0.10;Ti: ≤0.10;Zr: ≤0.10;B: ≤ 0.0035;Cu: ≤ 0.30; andFe and inevitable impurities.

3. Steel composition according to claim 1 or claim 2, wherein P* ≥ 23.

4. Steel composition according to any one of the preceding claims, wherein P* ≥ 25, preferably P* ≥ 28.

5. Steel composition according to any one of the preceding claims, wherein%C ≥ 0.125 - 0.05x Ceq; and%C ≥ -1.15 + 2.35x Ceq, whereinCeq = %C + A(%C)x(%Mn / 6 + %Si / 24 + %Cu / 15 + %Ni / 20 + (%Cr+%Mo+%V+%Nb+%W) / 5 + 5x %B) with A(%C) = 0.75 + 0.25x tanh(20x(%C-0.12)).

6. Steel composition according to any one of the preceding claims, wherein CS < 0.5, wherein CS= Or + 3.3X%Mo + 8.1 x %V - 3, preferably CS < 0.

7. Method of manufacturing a steel article, comprising the steps of: a) providing a hot-rolled semi-finished product having a steel composition as defined in any one of the preceding claims 1-6; b) austenitizing the semi-finished product at a temperature and during a time sufficient to achieve a fully austenitized semi-product; c) quenching the austenitized semi-product to ambient temperature d) tempering the quenched semi-product at a temperature above 600 °C and below the austenite transformation start temperature; e) cooling the tempered semi-product to room temperature.

8. Method according to claim 7, wherein step b) is performed in the temperature range of Ac3 up to the grain coarsening temperature.

9. Method according to claim 7 or 8, wherein step c) is performed between 800-500 °C at a quenching rate of at least 5 °C / s, preferably at least 10 °C / s, more preferably at least 25 °C / s.

10. Method according to any one of claims 7-9, wherein step d) is performed in the temperature range of 600-700 °C, preferably in the range of 600 - 670 °C.

11. Steel article manufactured according to the method of any one of claims 7-10.

12. Steel article according to claim 11 , having a microstructure comprising (in area %)(tempered) martensite: 30-70%, preferably 50-70%; bainite: 30-70%, preferably 30-50%; and sum ferrite and pearlite: less than 10%, preferably less than 5%, more preferably 3% or less.

13. Steel article according to any one of the preceding claims 11 -12, having at least one, preferably all, property fromYield strength:(MPa (ksi)) ≥ 689 (100);Tensile strength :(MPa (ksi)) ≥ 758 (110);Elongation (%): a 15;Absorbed energy of Charpy impact test at -40 °C (J / cm2): ≥ 240, preferably ≥ 250;Fracture toughness (100% H2, 200 bar, RT): KJQ ≥ 80 MPa m!4, preferably ≥ 90MPa-mYz;Fracture toughness (air, -40 °C) KJQ a 200 MPa-nYA14. Steel article according to claim 13, whereinFracture toughness (air, RT) KJQ ≥ 250 MPa-m1 / 2.

15. Steel article according to any one of the preceding claims 11-14, wherein the steel article is a pressure vessel for compressed hydrogen gas storage, a linepipe for compressed hydrogen gas transportation.

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