High strength line pipe steel having enhanced hydrogen resistance and method of manufacturing thereof

Abstract

The present disclosure relates to a steel composition comprising Carbon (C) at 0.02% to 0.06%, Manganese (Mn) at 0.6% to 1.30 %, Niobium (Nb) at 0.05 % to 0.10%, Molybdenum (Mo) at 0.10% to 0.20, Titanium (Ti) at 0.015% to 0.025%, Aluminium (Al) at 0.03% to 0.05%, Silicon (Si) at 0.1% to 0.5%, Nickel (Ni) at 0.1% to 0.3%, Nitrogen (N) at 0.0001% to 0.0060%, Hydrogen (H) at a concentration ranging up to 0.0002 wt.%, Sulphur (S) at 0.0001% to 0.0020%, Phosphorus (P) at 0.0001% to 0.015%; and the balance being Fe. The developed steel of the present disclosure exhibits enhanced tensile properties in accordance with API 5L PSL-2 specification for X65 / X-70 grade steel, along with superior hydrogen induced cracking resistance. The developed steel welded to form line pipe tubes used for the transportation of natural gas or crude oil, especially of sour grade.

Description

HIGH STRENGTH LINE PIPE STEEL HAVING ENHANCED HYDROGEN RESISTANCE AND METHOD OF MANUFACTURING THEREOF FIELD OF INVENTION

[0001] The present invention relates to high strength line pipe steel, and more particularly to the high strength line pipe steel having excellent resistance against hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), along with the superior low temperature toughness, formability, and weldability. The said HIC resistant steel is accordingly tough enough to transport sour grade of crude reserves. The present disclosure also provides a method for manufacturing the said steel.BACKGROUND

[0002] The transportation of petroleum and natural gas via pipelines is the most efficient and economical method. However, dwindling reserves of non-sour crude necessitate utilizing sour crude (sulfur content greater than 0.5%), requiring advanced refining and pipeline steels resistant to hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC). The increasing demand for hydrogen transportation further necessitates hydrogen-resistant steel grades.

[0003] Prior art proposes improving HIC resistance in high-strength line pipe steel through copper (Cu) additions (up to 1%). However, this increases hot shortness, causing cracking during hot forming. Other prior art focuses on minimizing inclusions to enhance HIC resistance. Further, existing compositions (e.g., containing Mo and Ti to form Ti-Mo precipitates) often feature high manganese (Mn), leading to Mn segregation, microstructural banding, and harder phases susceptible to HIC. These harder phases, localized in segregated regions, increase the risk of hydrogen-induced cracking.

[0004] Further, some prior arts suggest compositions comprising Molybdenum (Mo) along with Titanium (Ti), which allows formation of Ti-Mo precipitates in the steel microstructure, which are used to improve the HIC properties of steel. However, these compositions contain high concentration of Manganese (Mn)which leads to the Mn segregation which promotes microstructural banding in the steel microstructure. This causes the microstructural inhomogeneity and makes the steel prone to the formation of harder phase at the mid thickness of steel grade. The presence of harder phase localized in the segregated regions is known to make steel susceptible to hydrogen induced cracking.

[0005] Therefore, a need exists for a steel composition and microstructure that overcomes these limitations. The ideal material would exhibit superior HIC and SSCC resistance, along with excellent formability and weldability.

[0006] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.SUMMARY OF INVENTION

[0007] This summary is provided to introduce concepts related to high strength line pipe steel grade conforming to API 5L PSL-2 X65 / X70 specification having excellent resistance against hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), along with the superior low temperature toughness, formability and weldability, suitable for the applications related to transportation of petroleum / natural gas (sour service) and pure hydrogen / hydrogen blend gases under high pressure environment. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0008] In one aspect of the present invention, a high strength line pipe steel is provided. The high strength line pipe steel comprises the following composition expressed in weight %: Carbon (C) at 0.02% to 0.06%, Manganese (Mn) at 0.6% to 1.30 %, Niobium (Nb) at 0.05 % to 0.10%, Molybdenum (Mo) at 0.10% to 0.20, Titanium (Ti) at 0.015% to 0.025%, Aluminium (Al) at 0.03% to 0.05%, Silicon (Si) at 0.1% to 0.5%, Nickel (Ni) at 0.1% to 0.3%, Nitrogen (N) at 0.0001% to 0.0060%, Hydrogen (H) at a concentration ranging up to 0.0002 wt.%, Sulphur (S) at 0.0001% to 0.0020%, Phosphorus (P) at 0.0001% to 0.015% and the remaining being iron and incidental impurities. The high strength line pipe steel comprises amicrostructure of polygonal ferrite as primary phase and a secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination.

[0009] In an embodiment, the steel having the said composition is non-peritectic, conforms to API 5L PSL-2 X65 / X70 specifications and provides high resistance to hydrogen induced cracking (HIC).

[0010] In an embodiment, the steel exhibits crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, crack sensitivity ratio (CSR) of less than 2%.

[0011] In an embodiment, the steel has ferrite potential of at least 1.05.

[0012] In an embodiment, the steel provides high resistance to sulfide stress corrosion cracking (SSCC) with threshold stress greater than 80% of yield strength of steel.

[0013] In an embodiment, the steel provides high resistance to fracture toughness (K1H) in high pressure hydrogen environment with values greater than 55 MPa(m)1 / 2.

[0014] In an embodiment, the carbon equivalence of the steel composition is less than 0.35.

[0015] In an embodiment, the cumulative concentration of Nb, Mo, Ti and N is less than 0.25% wt%.

[0016] In an embodiment, the steel has yield strength (YS) ranging from 490 MPa to 625 MPa; Ultimate Tensile Strength (UTS) ranging from 570 MPa to 700 MPa; and elongation values 22 - 42%.

[0017] In an embodiment, the steel has microstructure of greater than 75% polygonal ferrite phase and less than 25% either of acicular ferrite or bainitic ferrite or any combination phase.

[0018] In an embodiment, the steel has microstructure of 75 - 90 % polygonal ferrite phase and 10 - 25% either of bainitic ferrite or acicular ferrite phase.

[0019] In an embodiment, the steel has average grain size ranging from 2 pm to 6 pm.

[0020] In an embodiment, the steel has an impact toughness ranging from 250J to 360J at -60°C.

[0021] In an embodiment, the steel has a hardness value ranging from 180Hv to 210 Hv.

[0022] In an embodiment, the steel has a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.75 at -30°C.

[0023] In another aspect of the present invention, a method for manufacturing a high strength line pipe steel is provided. The method comprises casting a steel slab with the steel composition comprising, in weight percentage: Carbon (C) at 0.02% to 0.06%, Manganese (Mn) at 0.6% to 1.30 %, Niobium (Nb) at 0.05 % to 0.10%, Molybdenum (Mo) at 0.10% to 0.20, Titanium (Ti) at 0.015% to 0.025%, Aluminium (Al) at 0.03% to 0.05%, Silicon (Si) at 0.1% to 0.5%, Nickel (Ni) at 0.1% to 0.3%, Nitrogen (N) at 0.0001% to 0.0060%, Hydrogen (H) at a concentration ranging up to 0.0002 wt.%, Sulphur (S) at 0.0001% to 0.0020%, Phosphorus (P) at 0.0001% to 0.015% and the remaining being iron and incidental impurities. The method also comprises reheating the slab to a temperature ranging from 1100°C to 1250°C. The method further comprises hot rolling of the slab with 70% to 90% reduction below recrystallization stop temperature (TNR) with finish hot rolling temperature ranging from 860°C to 910°C. The method comprises controlled cooling of the hot rolled steel sheet to a coiling temperature ranging from 540°C to 600°C and coiling thereafter. The method further comprises cooling the hot rolled coil to room temperature to obtain high strength line pipe sheet. The high strength line pipe steel comprises a microstructure of polygonal ferrite as primary phase and a secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination.

[0024] In an embodiment, the heating is carried out for a duration ranging from 20 minutes to 3 hours.

[0025] In an embodiment, the cooling is carried out at a rate ranging from 10°C / s to 50°C / s.

[0026] In an embodiment, the steel has microstructure of greater than 75% polygonal ferrite phase and less than 25% either of bainitic ferrite or acicular ferrite phase.

[0027] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flowchart illustrating a method for producing high strength line pipe steel, according to an exemplary embodiment of the present disclosure;

[0029] Figure 2 illustrates a schematic thermo-mechanical processing followed during the manufacturing of high strength line pipe steel, according to an embodiment of the present invention;

[0030] Figures 3a-3b illustrates microstructure of steel sheet manufactured by method of present disclosure in which (3a) is an Optical micrograph, and (3b) is a SEM micrograph, according to an exemplary embodiment of the present disclosure;

[0031] Figure 4 illustrates a pipe processed from the developed steel, in accordance with some embodiments of the present disclosure;

[0032] Figure 5 presents bright field transmission electron microscope (TEM) micrographs, with (a) displaying the fine bainitic ferrite / acicular ferrite microstructure, and (b) showcasing nanoscale precipitates distributed throughout the ferrite. These micrographs pertain to the steel coiled at 580°C manufactured by the method described in the present disclosure;

[0033] Figure 6 presents bright field transmission electron microscope (TEM) micrographs, with (a) displaying the fine bainitic ferrite / acicular ferrite microstructure, and (b) showcasing nanoscale precipitates distributed throughout the ferrite. These images pertain to the HAZ region of the electric resistance welded (ERW) pipes, which were produced using the steel manufactured by the method described in the present disclosure;

[0034] Figure 7 presents bright field transmission electron microscope (TEM) micrographs, with (a) displaying the fine bainitic ferrite / acicular ferrite microstructure, and (b) showcasing nanoscale precipitates distributed throughout the ferrite. These micrographs pertain to the weld region of the electric resistance welded (ERW) pipes, which were produced using the steel manufactured by the method described in the present disclosure; and

[0035] Figure 8 illustrates microhardness corresponding to the different regions across the ERW weld (including base, HAZ and weld zones).

[0036] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.DETAILED DESCRIPTION

[0037] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0038] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.

[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and / or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0040] It should also be noted that in some alternative implementations, the functions / acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality / acts involved.

[0041] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0042] The present disclosure provides a high strength line pipe steel comprising the following composition expressed in weight %: Carbon (C): 0.02% to 0.06%, Manganese (Mn): 0.6% to 1.30 %, Niobium (Nb):0.05% to 0.10%, Molybdenum (Mo):0.10% to 0.20%, Nickel (Ni):0.10% to 0.35%, Titanium (Ti):0.015% to 0.025%, Aluminium (Al):0.03% to 0.05%, Silicon (Si):0.2 % to 0.5%, Nitrogen (N):0.0001% to 0.0060%, Hydrogen (H): 0.0000% to 0.0002%, Sulphur (S):0.0001% to 0.0020%, Phosphorus (P):0.0001% to 0.015%, and the remaining being iron and incidental impurities. The carbon equivalence of the steel composition is less than 0.35. The cumulative concentration of Nb, Mo, Ti and N is less than 0.25% wt%.

[0043] The high strength line pipe steel comprises a microstructure of polygonal ferrite as primary phase and a secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination (Fig. 3a and Fig.3b). The steel having the said composition is non-peritectic, conforms to API 5L PSL-2 X65 / X70 specifications and provides high resistance to hydrogen induced cracking (HIC).

[0044] The high strength line pipe steel (alternatively referred to as low carbon non-peritectic line pipe steel) has yield strength (YS) ranging from 490 MPa to 625 MPa, Ultimate Tensile Strength (UTS) ranging from 570 MPa to 700 MPa, andelongation values in the range of 22 - 42 %. The high strength line pipe steel exhibits crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, crack sensitivity ratio (CSR) of less than 2%. The steel has ferrite potential of at least 1.05. The steel provides high resistance to sulfide stress corrosion cracking (SSCC) with threshold stress greater than 80% of yield strength of steel.

[0045] The steel composition as per the current invention restricts the segregation of elements like Mn and C which promotes microstructural banding in mid-section thickness of hot rolled steels. The lower carbon and manganese content in conjunction with accelerated cooling at run-out table (ROT) to produce segregation free hot rolled steel with superior HIC properties.

[0046] The disclosed steel composition allows the resulting steel to possess superior resistance to hydrogen induced cracking. This performance of steel against HIC is expressed in terms of three parameters - crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR) when subjected to the HIC testing as per NACE standard TM0284-2005 (NACE - National Association of Corrosion Engineers). The CLR, CTR and CSR are defined by the following equations:SaCrack Length Ratio (CLR) = — * 100%wSbCrack Thickness Ratio (CTR) = — * 100%S(a * b)Crack Sensitvity Ratio (CSR) = — - - * 100%S( w* T)

[0047] The steel as per the current invention possess superior resistance to hydrogen induced cracking by exhibiting crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, and crack sensitivity ratio (CSR) of less than 2% when subjected to the HIC testing as per NACE standard TM0284-2005. The steel further possesses superior resistance to sulfide stress corrosioncracking. This performance of steel against SSCC is expressed in terms of threshold stress, which is defined as the stress the steel can withstand without failure when subjected to the external loading in the acidic environment containing H2S. The test is conducted as per NACE standard TMO 177-2016. The ferrite potential of the steel as per the current invention is more than 1.05.

[0048] The high strength line pipe steel provides high resistance to fracture toughness (represented in terms of stress intensity factor i.e., KIH) in high pressure hydrogen environment with values greater than 55 MPa(m)1 / 2.

[0049] The high strength line pipe steel has average grain size ranging from 2 pm to 6 pm. The steel has an impact toughness ranging from 250J to 360J at -60°C. The steel has a hardness value ranging from 180 Hv to 210 Hv. The steel has a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.75 at -30°C.

[0050] Figures 1 and 2 are exemplary embodiments of the present disclosure illustrating a flowchart depicting a method (100) for manufacturing a high strength line pipe steel sheet, and a thermos-mechanical processing followed during the manufacturing of high strength line pipe steel, respectively. In the present disclosure, mechanical properties such as resistance against hydrogen induced cracking (HIC), sulfide stress corrosion cracking (SSCC), along with the superior low temperature toughness, formability, weldability, and final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a microstructure which is primarily a mixture of polygonal ferrite and acicular or bainitic ferrite. The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein. The method is particularly applicable to high strength line pipe steel, and it may also be extended to other type of steels as well.

[0051] The method of manufacturing the high strength line pipe steel sheet according to the present disclosure consists of a casting step followed by a hot working and controlled cooling, where the steel sheet is then coiled to satisfy component composition described below. It should not be construed that the following steps to be limited, rather, coiling may be performed after hot working and prior cooling, based on variation in temperature and cooling rate, respectively. The various processing steps of Figures. 1 and 2 respective, are described below:

[0052] As shown in block 101, the method starts with the process of casting. In the method of present disclosure, the steel of the specified composition including in weight percentage of Carbon (C) at 0.02% to 0.06%, Manganese (Mn) at 0.6% to 1.30 %, Niobium (Nb) at 0.05 % to 0.10%, Molybdenum (Mo) at 0.10% to 0.20, Titanium (Ti) at 0.015% to 0.025%, Aluminium (Al) at 0.03% to 0.05%, Silicon (Si) at 0.1% to 0.5%, Nickel (Ni) at 0.1% to 0.3%, Nitrogen (N) at 0.0001% to 0.0060%, Hydrogen (H) at a concentration ranging up to 0.0002 wt.%, Sulphur (S) at 0.0001% to 0.0020%, Phosphorus (P) at 0.0001% to 0.015% and the remaining being iron and incidental impurities by any manufacturing process including but not limiting to casting. In an embodiment, the casting process is a continuous casting process.

[0053] The method then includes the step of heating as shown in block 102. After casting the steel slab with the specified composition, the slabs are heated to a first predetermined temperature for a first predetermined time. In an embodiment, the first predetermined temperature is greater than 1100 °C, preferably in the range of 1100 °C to 1250 °C, and the first predetermined time ranges from 20 minutes to 3 hours. In an embodiment, the first predetermined temperature is above 1150 °C, to ensure complete dissolution of precipitates that may have formed in the preceding processing steps. A first predetermined temperature greater than 1250 °C is also not desirable because it may lead to grain coarsening of austenite and / or excessive scale loss. In some embodiment, heating of the steel slab may be carried out in a furnace.

[0054] Once the steel slab is heated as per the block 102, it is subjected for hot working as shown in block 103 to form a steel sheet. In an embodiment, the hotworking process is a hot rolling process. As shown in block 103, after casting and heating the steel slab with the specified composition, it is hot rolled.

[0055] The hot rolling constitutes a roughing step above the recrystallization temperature and a finishing step below the recrystallization temperature when rolling is done in a conventional hot strip mill. The recrystallization stop temperature (TNR in degree centigrade) is a critical parameter in defining the final microstructure of the developed steel in terms of grain size and second phase formation. The rolling is done with percentage reduction greater than about 70% to about 90 % below TNR with specific finish rolling temperature (FRT). In some embodiments, where a CSP (compact strip processing) / TSCR (thin slab casting) is used for producing the steel (where there is no separate roughing mill) the deformation schedule should be designed in order to break the cast structure during the initial stands of hot rolling, and finishing must be done below the recrystallization temperature such that the percentage reduction below TNR is between about 70% to about 90% with FRT ranging from about 860°C to 910°C.

[0056] After finish rolling, the rolled steel is cooled to a third predetermined temperature (T3) in the range of temperature ranging from 540°C to 600°C as shown in block 104. In the illustrated embodiment, the steel sheet is subjected to intensive cooling or laminar cooling (as shown in Figure 2) at a predetermined cooling rate in the range of 10 - 30 °C / s till the third predetermined temperature (T3) is reached. In an embodiment, the cooling is performed on a run-out-table. This is done in order to suppress the pearlite formation and encourage the formation of bainitic ferrite or acicular ferrite in the microstructure. Higher coiling temperature of around 600°C allows increase in the strength of steel by the precipitation of fine carbides in supersaturated ferrite. Once the rolled steel is in the range of temperature ranging from 540°C to 600°C it is coiled. Once the steel slab is coiled as per the block 105, the hot rolled coil is cooled to room temperature to obtain high strength line pipe sheet.

[0057] The obtained high strength line pipe steel comprises a microstructure of polygonal ferrite as primary phase and a secondary phase comprising at least oneof acicular ferrite or bainitic ferrite or any combination. More particularly, the steel has microstructure of greater than 75% polygonal ferrite phase and less than 25% either of bainitic ferrite or acicular ferrite phase. The steel having the said composition is non-peritectic, conforms to API 5L PSL-2 X65 / X70 specifications and provides high resistance to hydrogen induced cracking (HIC).

[0058] The obtained high strength line pipe steel (alternatively referred to as low carbon non-peritectic line pipe steel) has yield strength (YS) ranging from 490 MPa to 625 MPa, Ultimate Tensile Strength (UTS) ranging from 570 MPa to 700 MPa, and elongation values in the range of 22 - 42 %. The obtained high strength line pipe steel exhibits crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, crack sensitivity ratio (CSR) of less than 2%. The steel has ferrite potential of at least 1.05. The steel provides high resistance to sulfide stress corrosion cracking (SSCC) with threshold stress greater than 80% of yield strength of steel.

[0059] The high strength line pipe steel provides high resistance to fracture toughness (KIH) in high pressure hydrogen environment with values greater than 55 MPa(m)1 / 2. The high strength line pipe steel has average grain size ranging from 2 pm to 6 pm. The steel has an impact toughness ranging from 250J to 360J at -60°C. The steel has a hardness value ranging from 180 Hv to 210 Hv. The steel has a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.75 at -30°C.

[0060] Following portions of the present disclosure, provides details about the proportion of each element in a composition of the high strength hot rolled steel and their role in enhancing properties.

[0061] Carbon (C) may be added in the range of about 0.02 wt% to about 0.6 wt%. C is added to derive the strength in steel through solid solution strengthening, second phase formation along with the formation of precipitates in the form of carbides / carbonitrides. The carbon content in the current steel composition is limited to a range which limits the segregation of carbon in steel which causes the formation of martensite or martensite / austenite (MA) constituents in the steel microstructure. The presence of martensite and MA constituents is detrimental toHIC and SSCC resistance of steel. Also, the increased carbon content decreases the toughness and weldability of steel. In addition, the lower carbon content also allows the designing of non-peritectic steel composition. Preferably, carbon is present in the range if 0.02 wt.% to about 0.05 wt.%.Manganese (Mn) may be added in the range of about 0.6 wt% to about 1.3 wt%. Mn in the steel of the current invention varies in the range of about 0.6 wt% to about 1.30 wt.%. Preferably, Mn is present less than 1.25 wt.%. Mn, apart from imparting solid solution strengthening, also lowers the austenite to ferrite transformation temperature and helps in refining the ferrite grain size. Manganese at higher level enhances the centerline segregation during the process of continuous casting. Moreover, it leads to the higher number of MnS inclusions which are detrimental to hydrogen induced cracking and sulfide stress corrosion cracking resistance. Higher level of manganese in steel also increases the carbon equivalence and impairs the weldability of steel.

[0062] Silicon is present in the range of about 0.20 wt.% to about 0.50 wt.%. Silicon imparts the solid solution strengthening effect like Mn. Si is also being employed as a deoxidizing element. However, in order to prevent the formation of surface scales, the Si content in the steel is restricted to a maximum content of 0.5%. Also, higher Si content impairs the weldability of the steel by increasing carbon equivalence.

[0063] Niobium: Nb in the steel of the current invention varies in the range of about 0.05 wt% to about 0.10 wt%. Preferably, Nb is present less than 0.10 wt.%. Nb in steel helps in the grain refinement because of its solute drag effect and allows lowering the carbon content of the steel. Niobium significantly increases the recrystallization stop temperatures and allows the higher amount of deformation below recrystallization stop temperature (TNR) during the hot rolling of the steel. This allows significant reduction in grain size and remarkably increases the toughness of steel. The role of Niobium in the present disclosure is also extended to increase the hardenability of austenite to form the bainitic ferrite or acicular ferrite at relatively lower cooling rates. Furthermore, an important role of Nb in thecurrent invention is the formation of Nb rich fine precipitates, which acts as a hydrogen trap and thus improve the resistance of steel against HIC, SSCC and enhances KIH performance at high pressure hydrogen environment. However, Niobium content more than 0.10 % can significantly increase the mill load which may drastically reduce the life of the rolls in the rolling mill or in some cases it may be beyond the capacities of the rolling mills.

[0064] Molybdenum: Mo in the steel of the current invention varies in the range of about 0.10 wt% to about 0.20 wt%. Preferably, Mo is present less than 0.18 weight%. The presence of Mo promotes the formation of acicular ferrite / bainitic ferrite as a result of increase in hardenability of steel. Its presence also significantly suppresses the pearlite formation, which is an undesirable phase in the present invention. Presence of Mo also contributes towards strengthening of steel through precipitation by limiting the coarsening of fine precipitates. These fine precipitates act as hydrogen traps and thus improve the resistance of steel against HIC, SSCC and enhances KIH performance at high pressure hydrogen environment. The upper limit of Mo in the present steel is restricted to 0.20 by weight percent. Further increase in the Mo content causes the weld embrittlement by the formation of phases like martensite or martensite-austenite (MA) which not only deteriorates the toughness of weld heat affected zone but significantly impairs the HIC and SSCC performance in heat affected region.

[0065] Nickel: Ni in the steel of the current invention varies in the range of about 0.10 wt.% to about 0.35 wt.%. Preferably, Ni is present less than 0.30 weight%. Presence of Ni improves low temperature toughness and imparts solid solution strengthening especially becomes important in defining the strength and toughness of heat affected zone (HAZ) and weld. Further increase in the Ni content beyond 0.35 wt., % can lead to the weld embrittlement by the formation of phases like martensite or martensite-austenite (MA). The presence of such phases is undesirable as it deteriorates the toughness of weld and heat affected zone and significantly impairs the HIC and SSCC performance of the respective regions.

[0066] Nitrogen: The preferable range for the nitrogen in the steel is about 0.0040wt% to about 0.0050wt%. Nitrogen combines with Titanium and Niobium to form nitrides / carbonitrides. Accordingly, Ti / N ratio should be maintained at or less than (>) 3.14 to limit the grain coarsening when material is subjected to end application process of SAW (submerged arc- welding) or ERW (electric resistance welding). However, increasing the nitrogen content above 0.010wt% may lead to the embrittlement of the heat affected zone (HAZ) of weld joints.

[0067] Titanium: The preferable range of titanium in the steel is 0.015-0.025 wt%. Titanium in steel combines with nitrogen to form TiN precipitates which inhibits the austenite grain coarsening when the steel is reheated prior to rolling. Also, the presence of TiN restricts the prior austenite grain coarsening in the heat affected zone, when the steel is subjected to the welding operation, this prevents the deterioration of toughness in the heat affected zone of the welded steel.

[0068] Aluminum: The preferable range of aluminum 0.03-0.05 wt%. Aluminum in steel is used for de-oxidation of steel. The content of Al was limited to restrict the content of aluminum oxide, the presence of which mat deteriorate the hydrogen induced cracking resistance.

[0069] Sulphur: Sulphur needs to be limited to about 0.0010wt% to avoid high level of MnS inclusions, as they cause severe deterioration of HIC and SSCC properties.

[0070] Phosphorous: Phosphorus content as per the current invention needs to be restricted to a maximum of 0.015wt% as higher phosphorus levels can lead to reduction in resistance to hydrogen induced cracking, toughness, and weldability due to segregation of P at grain boundaries.

[0071] Hydrogen: Hydrogen needs to be limited to about 0.0002 wt.% The dissolved hydrogen beyond this limit deteriorate hydrogen resistant of steel.

[0072] Calcium: The preferable range of calcium is about 0.0020-0.0050 wt%. Calcium treatment of steel is important to change the size and morphology of MnS inclusions. Ca / S ratio should be in a range of 2-3.

[0073] In an embodiment of the present disclosure, the total micro alloying content of the composition is restricted to less than 0.25wt%. Particularly, in the composition of the present disclosure, the cumulative concentration of Nb, Mo, Ti and N does not exceed 0.25 wt.%.

[0074] This specific concentration of the components in the composition of the present disclosure lead to specific microstructure formation, that helps in providing the desired HIC properties to the steel. In embodiments of the present disclosure, the steel sheet according to the present disclosure has 80-85 % ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn, Si and Ni. With the application of high Nb and Mo coupled with controlled thermomechanical processing conditions, the average grain size is restricted to about 3.0 and 3.5 pm for coiling temperature of 560 and 580 °C, respectively. This grain refinement significantly increases the strength of ferrite governed by the Hall-Petch relationship. Also, the finer grain size results in remarkable toughness of the steel at room temperature and at sub-zero temperatures. The dispersion of fine precipitates Niobium rich carbides, which are few nanometers in size, also contribute towards the strength of the ferrite. This can be seen from figure 5 (b) which shows bright field TEM micrograph revealing the dispersion of fine precipitates of niobium carbide / carbonitride for the steel coiled at 600°C (as provided by example 1 below). In various embodiments of the present disclosure, the steel having the said composition has average grain size ranging from about 2 pm to about 8 pm.

[0075] Accordingly, in embodiments of the present disclosure, the steel having the said composition has polygonal ferrite and either of bainitic ferrite or acicular ferrite microstructure. The amount of bainitic ferrite / acicular ferrite in the microstructure ranges between about 10% to about 25%. The strengthening from bainite / acicular ferrite is derived from its fine structure and higher dislocation density.

[0076] This microstructure, formed by the composition, lends enhanced strength and quality to the resultant steel of the present disclosure. More particularly, thesteel of the present disclosure possesses high yield strength (YS) and ultimate tensile strength (UTS), as required by API 5L PSL-2 X65 / X70 specifications. In various embodiments of the present disclosure, the steel having the composition of the present disclosure has yield strength ranging from about 480 MPa to about 625 MPa; ultimate tensile strength (UTS) ranging from about 535 MPa to about 700 MPa; and elongation value of at least 22%. Accordingly, the YS / UTS ratio of the steel is also kept below 0.93.

[0077] In addition to YS and UTS, in embodiments of the present disclosure, the steel having the said composition has an impact toughness ranging from about 250 J to about 350 J at -60°C. Further, the steel also has a hardness value ranging from about 180Hv to about 210 HV and a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.75 mm at -30 °C.

[0078] The steel of the present disclosure has a ferrite potential of either less than 0.85 or greater than 1.05, thereby making the steel non-peritectic. This ferrite potential (FP) is calculated by the following empirical formula:FP = 2.5 * (0.5 - Ceq),where Ceq is carbon equivalence of the composition, and defined by the following equations:Ceq = C + 0.04*Mn + 0.1 *Ni + 0.7*N - 0.14*Si - 0.04*Cr - 0.1 *Mo - 0.24*Ti -0.7*S;whereas the critical metal parameter (Pcm) for weld cracking is calculated by:Si Mn Cu Ni Cr Mo VPcm =30 + 20 -|- 20 + 60 + 20 4- 20 4- 104-gOn the other hand, the formula based on International Institute of Welding (IIW) is:Mn Cr + Mo + V Cu + NiC + - + - + -

[0079] In some embodiments, the carbon equivalence of the composition is less than 0.35. Said carbon equivalence ensures that the steel exhibits excellent weldability during the process of tube manufacturing and other end applications.

[0080] Various embodiments covered in the current invention are precise synergistic interplay of elements at specific concentrations, that allow the steel of the present disclosure to exhibit high resistance to HIC and SSCC along with the tensile properties in accordance with the specifications laid down by API 5L PSL-2 for X65 / X-70 grade steel, and capable of being used for sour service environment and transportation of hydrogen gas under high pressure environment. Hence, while the developed steel is designed such that it is readily hot / cold formed, HIC and SSCC resistant and is welded to form line pipe tubes to be used for the transportation of natural gas / crude oil or gaseous hydrogen over long distances.

[0081] The present disclosure thus also relates to the designing of the chemical composition of steel coupled with the controlled thermo-mechanical processing and accelerated cooling method to develop the line pipe steel grade conforming to the properties specified in API PSL-2 X65 / X-70 specification with excellent resistance to hydrogen induced cracking and sulphide stress corrosion cracking, superior low temperature toughness along with excellent weldability and formability.

[0082] The high strength hot rolled line pipe steel may be processed through any one of electric resistance welding (ERW), longitudinal submerged arc welding (LSAW), and helical submerged arc welding (HSAW) to form the line-pipe tubes.

[0083] Further embodiments of the present disclosure will be now described with an example of a particular composition of the steel. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 1.Elements / Properties Concentrations (wt.%) / Values C 0.05Mn 1.21Si 0.28Nb 0.065Mo 0.10Ti 0.011Al 0.021Ni 0.25S 0.0010P 0.011N 0.005H 0.00015 Ferrite Potential (Fp) 1.064Pcm 0.129C (IIW) 0.29Total Micro alloying content0.18(Nb +Mo+ Ti + N)Table - 1Example 1: Manufacturing of steel of the present disclosure and analysis of the Microstructure thereof

[0084] Steels with the composition as defined in Table 1 below was cast into multiple slabs through conventional continuous casting route. The cast slabs were then reheated to a temperature of 1200°C for a period of at least 1 hour to ensure the complete dissolution of Niobium precipitates. The slabs were subsequently hot rolled with the 80% reduction below TNR (1030-1040°C) and finish rolled to atemperature of 870°C. Post hot rolling, the hot rolled sheets processed from the different slabs were cooled to a coiling temperature of 560°C and 600°C, respectively at cooling rate of 28-30°C / s.

[0085] The resulting hot rolled sheets coiled at 560°C and 600°C are analysed for their microstructure details, grain sizes and hardness values, and the results are provided in Table 2 below:Table 2 - Microstructural detailsCoiling Microstructure Average Grain Hardness Temperature size(HV10) (CT) (pm)Ferrite + Bainitic 2.5 ±1 185 ± 4 CT 560°CferriteFerrite+ Bainitic 3.5± 1.5 194 ± 6 CT 600°CferriteTable - 2

[0086] For both cases of steel coiled at 560 °C and 600°C, the microstructure, grain size and hardness correspond to the values required by the steel of the present disclosure.

[0087] While figure 3 provides optical (3a) and SEM micrograph (3b) showing the ferrite-bainite microstructure of steel coiled at 600 °C. Further, while figure 5 (b) shows bright field TEM micrographs revealing the dispersion of fine precipitates of niobium rich carbide / carbonitride for the steel at 600 °C.Example 2: Analysis of the tensile properties of steel of the present disclosure

[0088] Steels with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 560 °C and 600°C for the said steel composition are analysed for their yield strength (YS), ultimate tensile strength (UTS), % Elongation and YS / UTS ratio, and the results are provided in Table 3 below:Table -3 Tensile properties of hot rolled sheetsCoiling Temperature YS (MPa) UTS % El. (YS / UTS) Ratio (°C) (MPa)CT 560°C 525 598 40 0.88CT 600°C 543 607 38 0.89Table - 3

[0089] For both cases, hot rolled sheets coiled at 560°C and 600°C, the tensile properties correspond to the values required by the steel of the present disclosure, and as per the API 5L PSL-2 X65 / X70 specification.Example 3: Analysis of the Impact Toughness of steel of the present disclosure

[0090] Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 560°C and 600°C are analysed for their impact toughness, and the results are provided in Table 4 below:Table 4 - Impact toughness of hot rolled strips coiled at 560°C and 600°C CT-560°C CT-600°C TemperatureImpact Toughness (In Joules)0°C 347 339-40°C 330 297-60°C 277 260Table - 4

[0091] For both cases of steel coiled at 560°C and 600°C, the impact toughness was found to be ranging between 250 J to 330 J at -60°C as required by the steel of the present disclosure.Example 4: Analysis of the fracture toughness (CTOD) of steel of the present disclosure

[0092] Steel 1 with the composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 560°C and 600°C are analysed for their fracture toughness measured in terms of Crack tip opening displacement (CTOD), at room temperature and sub-zero temperature, and the results are provided in Table 5 below:Table 5 -Fracture toughness of hot rolled strips coiled at 560 and 600 °C Target as perProperties achieved API specificationTemperature FractureToughness CT-560°C CT-600°C CTOD (mm)Room0.35 0.94 1.03 Temperature0°C 0.35 0.91 0.97 -30°C 0.35 0.78 0.83

[0093] Results: For both cases of hot rolled sheets coiled at 560°C and 600°C, the fracture toughness was found to be greater than 0.75, as required by the steel of the present disclosure.Example 5: Analysis of the Hydrogen Induced Cracking (HIC) properties of steel of the present disclosure

[0094] Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 560°C and 600°C are tested as per NACE standard TM0284-2005, for their HIC properties. The standard samples of 100*20*T (where T is the thickness of hot rolled strip) were exposed to a test solution comprising of 0.5% acetic acid and 5% sodium chloride dissolved in distilled water with pH of 3 ± 0.5, saturated with H2S under a positive pressure for a period of 96 hours.

[0095] After the test, exposed samples were polished and subjected to metallography examination for the cracks generated in the sample. Theperformance of steel against HIC is expressed in terms of three parameters - CLR, CTR and CSR (as defined previously). Said results are provided in Table 6 below.Table 6 HIC Testing of hot rolled sheetsSolution-A, pH of 3 ± 0.5Index CT-560°C CT-600°C Crack Length Ratio (CLR) 0 0Crack Thickness Ratio0 0(CTR)Crack Sensitivity Ratio0 0(CSR)

[0096] Results: For both cases of steel coiled at 560°C and 600°C for steel in present disclosure, no cracks were generated and all the three parameters CLR, CTR and CSR were found to be zero for the developed steel, for the solution with pH of 3 ± 0.5.Example 6: Analysis of the sulfide stress corrosion cracking (SSCO propertiesof steel of the present disclosure

[0097] Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 560°C and 600°C are tested as per Four-point bend test- (Solution A), complying to NACE standard TM0177-2016, for the evaluation of SSCC properties. A standard sample having length 150 mm, width 25 mm and 5 mm thickness is tested under loading for different stress levels (70,80,85,90 % of yield strength) in a solution A comprising of 5.0 wt.% sodium chloride and 0.5 wt.% acetic acid dissolved in deionized water. The steel samples were subjected to the test for a period of 720 hours, post which the samples are polished and subjected to metallography examination for the cracks generated in the sample. The results are provided in Table 7 below:Table - 7 SSCC properties of developed steelTarget as per API Properties Achieved Steel in present disclosurespecificationCT 560 °C 72% YS85%YS CT 600 °C 72% YS >= 85%YS

[0098] Results: For both cases of steel coiled at 560°C and 600°C for steel 1, no cracks were generated / failure occurred after exposure of 720 hours under loading, and in both the case the steel exhibited the threshold stress greater than 85% of its room temperature yield strength.Example 7: Analysis of the tensile properties of samples from ERW processed pipes from the steel of the present disclosure

[0099] ERW pipes with outer diameter of 406.4 mm (16 inches) with a wall thickness of 12.6 mm were processed from hot rolled steel sheets with composition and process details as defined in Example 1, and the resulting ERW pipes are analysed for their yield strength (YS), ultimate tensile strength (UTS), % Elongation, YS / UTS ratio and Hardness for the base,weld and HAZ regions of the processed pipes, and the results are provided in Table 8 below:Table -8 Tensile and hardness properties of samplesYS UTS % El. (YS / UTS) Average Location of the (MPa) (MPa) Ratio Hardness sample (HV10)195 BASE 0.87546 626 28.0196 WELD - - 593 - 197 HAZ - - - -

[0100] Results: For the samples tested for base and weld, the tensile properties correspond to the values required by the steel of the present disclosure, and as per the API 5L PSL-2 X65 / X70 specification. The ERW pipes exhibited consistent hardness across the base, weld and HAZ regions of the pipe (Fig.8).Example 8: Analysis of the Hydrogen Induced Cracking (HIC) properties of ERW pipe processed from the steel of the present disclosure

[0101] ERW pipes with outer diameter of 406.4 mm (16 inches) with a wall thickness of 12.6 mm were processed from hot rolled steel sheets with composition and process details as defined in Example 1. Three samples each from the different regions of the pipe (as marked in the figure 4) i.e., weld, 90° from the weld (base) and 180° from the weld (base) in longitudinal direction were tested as per NACE standard TM0284-2005, for evaluation of HIC properties. The standard samples of 100*20*T (where T is the thickness of hot rolled strip) were exposed to a test solution comprising of 0.5% acetic acid and 5% sodium chloride dissolved in distilled water with pH of 3 ± 0.5, saturated with H2S under a positive pressure for a period of 96 hours.

[0102] After the test, exposed samples were polished and subjected to metallography examination for the cracks generated in the sample. The performance of steel against HIC is expressed in terms of three parameters - CLR, CTR and CSR (as defined previously). Said results are provided in Table 9 below.Table -9 HIC Testing of ERW processed tubes% Crack Crack Thickness Crack Sample Location Length Ratio Ratio (CTR) Sensitivity Ratio (CLR) (CSR) Weld0 0 0 Base -900fromthe weld 0 0 0 Base -1800fromthe weld 0 0 0

[0103] Results: For all the cases pertaining to different regions of ERW pipe, no cracks were generated and all the three parameters CLR, CTR and CSR were found to be zero for the developed steel, for the solution with pH of 3 ± 0.5. TheT1PCT-2793consistent performance of the different regions of ERW pipes is attributed to the corresponding microstructure (Fig.5 to Fig.7).Example 9: Analysis of the sulfide stress corrosion cracking (SSCC) properties of ERW pipe processed from the steel of the present disclosure

[0104] ERW pipes with outer diameter of 406.4 mm (16 inches) with a wall thickness of 12.6 mm were processed from hot rolled steel sheets with composition and process details as defined in Example 1 are tested as Four-point bend testsolution A), complying to NACE standard TM0177-2016, for the evaluation of SSCC properties of base and weld region of pipe. A standard sample having length 150 mm, width 25 mm and 5 mm thickness is tested under loading for different stress levels (70,80,85,90% of yield strength) in a solution A comprising of 5.0 wt% sodium chloride and 0.5 wt% acetic acid dissolved in deionized water. The samples were subjected to the test for a period of 720 hours, post which the samples are polished and subjected to metallography examination for the cracks generated in the sample. The results are provided in Table 10.Table -10 SSCC properties of ERW processed tubesTarget as per API Properties Achieved Steel 1specificationBASE 72% SMYS > = 85%YS WELD 72% SMYS >= 85%YS

[0105] Results: For both cases corresponding to base and weld region of ERW processed pipe, no cracks were generated / failure occurred after exposure of 720 hours, and in both the case the steel exhibited the threshold stress greater than 85% of its room temperature yield strength.Example 10: Hydrogen qualification test of ERW pipe processed from the steel of the present disclosure

[0106] ERW pipes of 16-inch outer diameter and 12.6 mm wall thickness, the samples were sent to RINA-CSM S.p. A, Italy for KIH testing. Specimens pertaining to the base, HAZ (Heat affected zone) and weld regions of ERW processed pipe were subjected to KIH (Stress intensity factor in hydrogen environment)) test i.e., evaluation of fracture toughness in 100 % hydrogen environment at a pressure of 100 bars for a period of 1000 hours in compliance to ASME B31.12, ASME BPVC Section VIII, Division 3 and ASTME1681-03 specification. No crack growth was observed in the specimens after their exposure in high pressure environment for a period of 1000 hours. The results for KIH testing are provided in Table 11.Table -11 KIH properties of ERW processed tubesTarget Properties Steel 1(MPa^(m)) Achieved BASE > 55 > 78HAZ > 55 >84 WELD > 55 >84

[0107] Results: All samples pertaining to the base, HAZ (Heat affected zone) and weld regions of ERW processed pipe significantly exceeded the minimum qualification criteria of 55 MPa (m) for KIH testing.

[0108] The present disclosure relates to designing of steel composition for line pipe steel to be used for long distance transportation of oil and natural gas (especially for sour service) and gaseous hydrogen under high pressure. The developed steel of the present disclosure exhibits enhanced tensile properties in accordance with API 5L PSL-2 specification for X65 / X-70 grade steel, along with superior hydrogen induced cracking resistance with crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, crack sensitivity ratio (CSR) of less than 2% and enhanced resistance against sulfide stress corrosion cracking (SSCC) with threshold stress greater than 80% of yield strength. The develop steel is designed to meet the fracture toughness greater than 55 MPa(m)1 / 2in a high-pressure hydrogen environment. The developed steel is designed such that it isreadily hot / cold formed and welded to form line pipe tubes to be used for the transportation of natural gas or crude oil, especially of sour grade and gaseous hydrogen under high pressure. The present disclosure also provides a method of manufacturing the said steel having the composition of the present disclosure. The hot rolled line pipe steel may be processed through any one of electric resistance welding (ERW), longitudinal submerged arc welding (LSAW), and helical submerged arc welding (HSAW) to form the line-pipe tubes.

[0109] It should be understood that the experiments are carried out for particular compositions of the high strength hot rolled steel and the results brought out in the previous paragraphs are for the composition shown in Table 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the high strength hot rolled steel as well.

[0110] Furthermore, the terminology used herein is for describing embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.

[0111] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants / patentees and others.

[0112] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions, or examples, which are included to enable a person having ordinary skill in the art tomake and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims

I / We claim:

1. A high strength line pipe steel comprising the following composition expressed in weight %:Carbon (C): 0.02% to 0.06%;Manganese (Mn): 0.6% to 1.30 %;Niobium (Nb):0.05%to 0.10%;Molybdenum (Mo):0.10% to 0.20%;Nickel (Ni):0.10% to 0.35%;Titanium (Ti):0.015% to 0.025%;Aluminium (Al):0.03% to 0.05%;Silicon (Si):0.2 % to 0.5%;Nitrogen (N):0.0001% to 0.0060%;Hydrogen (H): 0.0000% to 0.0002%;Sulphur (S):0.0001% to 0.0020%;Phosphorus (P):0.0001% to 0.015%; and the remaining being iron and incidental impurities, wherein the high strength line pipe steel comprises a microstructure of polygonal ferrite as primary phase and a secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination.

2. The high strength line pipe steel as claimed in claim 1, wherein the steel having the said composition is non-peritectic, conforms to API 5L PSL-2 X65 / X70 specifications and provides high resistance to hydrogen induced cracking (HIC).

3. The high strength line pipe steel as claimed in claim 1, wherein the steel exhibits crack length ratio (CLR) of less than 10%, crack thickness ratio (CTR) of less than 5%, crack sensitivity ratio (CSR) of less than 2%.

4. The high strength line pipe steel as claimed in claim 1, wherein the steel has ferrite potential of at least 1.05.

5. The high strength line pipe steel as claimed in claim 1, wherein the steel provides high resistance to sulfide stress corrosion cracking (SSCC) with threshold stress greater than 80% of yield strength of steel.

6. The high strength line pipe steel as claimed in claim 1, wherein the steel provides high resistance to fracture toughness (KIH) in high pressure hydrogen environment with values greater than 55 MPa(m)1 / 2.

7. The high strength line pipe steel as claimed in claim 1, wherein the carbon equivalence of the steel composition is less than 0.35.

8. The high strength line pipe steel as claimed in claim 1, wherein the cumulative concentration of Nb, Mo, Ti and N is less than 0.25% wt%.

9. The high strength line pipe steel as claimed in claim 1, wherein the steel has yield strength (YS) ranging from 490 MPa to 625 MPa; Ultimate Tensile Strength (UTS) ranging from 570 MPa to 700 MPa; and elongation values 22 - 42%.

10. The high strength line pipe steel as claimed in claim 1, wherein the steel has microstructure of greater than 75% polygonal ferrite phase and less than 25% at least one of bainitic ferrite or acicular ferrite phase or any combination.

11. The high strength line pipe steel as claimed in claim 10, wherein the steel has microstructure of 75 - 90 % polygonal ferrite phase and 10 - 25% either of bainitic ferrite or acicular ferrite phase or any combination.

12. The high strength line pipe steel as claimed in claim 1, wherein the steel has average grain size ranging from 2 pm to 6 pm.

13. The high strength line pipe steel as claimed in claim 1, wherein the steel has an impact toughness ranging from 250J to 360J at -60°C.

14. The high strength line pipe steel as claimed in claim 1, wherein the steel has a hardness value ranging from 180 Hv to 210 Hv.

15. The high strength line pipe steel as claimed in claim 1, wherein the steel has a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.75 at -30°C.

16. A method for manufacturing a high strength line pipe steel, the method comprising:casting a steel slab with the steel composition comprising, in weight percentage: Carbon (C) at 0.02% to 0.06%, Manganese (Mn) at 0.6% to 1.30%, Niobium (Nb) at 0.05 % to 0.10%, Molybdenum (Mo) at 0.10% to 0.20, Titanium (Ti) at 0.015% to 0.025%, Aluminium (Al) at 0.03% to 0.05%, Silicon (Si) at 0.1% to 0.5%, Nickel (Ni) at 0.1% to 0.3%, Nitrogen (N) at 0.0001% to 0.0060%, Hydrogen (H) at a concentration ranging up to 0.0002 wt.%, Sulphur (S) at 0.0001% to 0.0020%, Phosphorus (P) at 0.0001% to 0.015% and the remaining being iron and incidental impurities;reheating the slab to a temperature ranging from 1100°C to 1250°C; hot rolling of the slab with 70% to 90% reduction below recrystallization stop temperature (TNR) with finish hot rolling temperature ranging from 860°C to 910°C; andcontrolled cooling of the hot rolled steel sheet to a coiling temperature ranging from 540°C to 600°C and coiling thereafter;cooling the hot rolled coil to room temperature to obtain high strength line pipe, wherein the high strength line pipe steel comprises a microstructure of polygonal ferrite as primary phase and a secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination.

17. The method for manufacturing high strength line pipe steel as claimed in claim 16, wherein the heating is carried out for a duration ranging from 20 minutes to 3 hours.

18. The method for manufacturing high strength line pipe steel as claimed in claim 16, wherein the cooling is carried out at a rate ranging from 10°C / s to 50°C / s.

19. The method for manufacturing high strength line pipe steel as claimed in claim 16, wherein the steel has microstructure of greater than 75% polygonal ferrite primary phase and less than 25% of secondary phase comprising at least one of acicular ferrite or bainitic ferrite or any combination.

20. The method for manufacturing high strength line pipe steel as claimed in claim 16, wherein the steel composition further comprises calcium in therange of 0.0020-0.0050 wt% to maintain the Ca / S ration in the range of 2- 3.Dated This 6thFebruary 2025.GOPINATH ARENUR SHANKARARAJ IN / PA - 1852 OF K& S PARTNERS ATTORNEY FOR THE APPLICANT

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