Claims:We claim:
1. A low carbon non-peritectic line pipe steel comprising in weight percentage (wt%):
Carbon (C) at about 0.02% to about 0.05%;
Manganese (Mn) at about 0.7% to about 1.25%;
Niobium (Nb) at about 0.07% to about 0.11%;
Molybdenum (Mo) at about 0.10% to about 0.20;
Titanium (Ti) at about 0.015% to about 0.025%;
Aluminium (Al) at about 0.03% to about 0.05%;
Silicon (Si) at about 0.1% to about 0.5%;
Nitrogen (N) at about 0.0001% to about 0.0060%;
Sulphur (S) at 0.0010% to about 0.0020%; and
Phosphorus (P) at about 0.001% to about 0.015%.

2. The non-peritectic line pipe steel as claimed in claim 1, wherein Carbon is present preferably in the range of 0.02-0.04.
3. The non-peritectic line pipe steel as claimed in claim 1, wherein Manganese is present preferably in the range of 0.70-1.20.
4. The non-peritectic line pipe steel as claimed in claim 1, wherein Niobium is present preferably in the range of 0.07-0.10.
5. The non-peritectic line pipe steel as claimed in claim 1, wherein Molybdenum is present preferably in the range of 0.10-0.15.
6. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel conforms to properties as per API 5L PSL-2 X70 specifications.
7. The non-peritectic line pipe steel as claimed in claim 1, wherein said steel composition 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%.
8. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has ferrite potential of at least 1.05.
9. The non-peritectic 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 room temperature yield strength of steel.
10. The non-peritectic line pipe steel as claimed in claim 1, wherein the carbon equivalence of the steel composition is less than 0.35.
11. The non-peritectic line pipe steel as claimed in claim 1, wherein the cumulative concentration of Nb, Mo, Ti and N is less than 0.25% wt%.
12. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has yield strength (YS) ranging from about 480 MPa to about 625 MPa; Ultimate Tensile Strength (UTS) ranging from about 570 MPa to about 700 MPa; and elongation values greater than 22%.
13. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has polygonal ferrite and bainitic ferrite microstructure.
14. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has average grain size ranging from about 2 µm to about 4µm.
15. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has an impact toughness ranging from about 280J to about 360J at -60°C temperature in longitudinal direction and about 280J to about 340J at -60°C in transverse direction.
16. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has a hardness value ranging from about 180Hv to about 210 Hv
17. The non-peritectic line pipe steel as claimed in claim 1, wherein the steel has a fracture toughness (CTOD - crack tip opening displacement) value of at least 0.80.

18. A method for manufacturing non-peritectic line pipe steel, the method comprising:
Casting a steel slab with the steel composition comprising, in weight percentage:
Carbon (C) at about 0.02% to about 0.05%, Manganese (Mn) at about 0.7% to about 1.25%, Niobium (Nb) at about 0.07% to about 0.11%, Molybdenum (Mo) at about 0.10% to about 0.20, Titanium (Ti) at about 0.015% to about 0.025%, Aluminium (Al) at about 0.03% to about 0.05%, Silicon (Si) at about 0.1% to about 0.5%, Nitrogen (N) at about 0.0001% to about 0.0060%, Sulphur (S) at 0.0001% to about 0.0020%, and
Phosphorus (P) at about 0.0001% to about 0.015%;
followed by heating the slab to a temperature ranging from about 1100°C to about 1250°C;
hot rolling of the slab with about 70% to about 90% reduction below recrystallization stop temperature (TNR) with finish hot rolling temperature ranging from about Ae3 - 50 (°C) to about Ae3 + 50 (°C); and
controlled cooling of the hot rolled steel sheet to a coiling temperature ranging from about 520°C to about 600°C.
19. The method for manufacturing non-peritectic line pipe steel as claimed in claim 16, wherein the heating is carried out for a duration ranging from about 20 minutes to about 2 hours.
20. The method for manufacturing non-peritectic line pipe steel as claimed in claim 16, wherein the cooling is carried out at a rate ranging from about 10°C to about 50°C per second.
21. The method for manufacturing non-peritectic line pipe steel as claimed in claim 16, wherein the cooling at said temperature results in steel having polygonal ferrite and bainitic/acicular ferrite microstructure.
, Description:“HIGH SRENGTH LINE PIPE STEEL WITH ENHANCED HYDROGEN INDUCED CRACKING (HIC) RESISTANCE AND SULFIDE STRESS CORROSION CRACKING RESISTANCE FOR SOUR APPLICATION”
TECHNICAL FIELD
The present disclosure relates to designing of linepipe steel grade conforming to API 5L PSL-2 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.
BACKGROUND OF THE DISCLOSURE
The transportation of petroleum and the natural gas over the longer distances through pipelines is the most effective and economical mode of fuel transportation. The gradual depletion of non-sour grade of crude reserves demands the utilization of sour grade (sulphur content > 0.5%) through advanced refining techniques. The transportation of crude from such reserves requires the line pipe steel grades with superior resistance to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC).
Prior arts suggest methods by which HIC properties in high strength linepipe steel may be improved, one of them being through substantial addition of copper (Cu) in the steel. The Cu content of up to 1% by weight is suggested in the steel composition. However, such a high level of Cu in the steel increases the susceptibility of steel to hot shortness, which may cause the cracking of steel surface while hot forming/rolling of the steel slab. While Nickle (Ni) can be added to overcome the issue of hot shortness, it’s addition beyond 0.5% not only impacts the weldabilty of steel but significantly increases the cost of the steel as well. Other prior arts suggest melting and refining of steel to limit the number of inclusions for improving the HIC properties of line pipe steel.

Further, prior art also discloses steel composition in weight percent with C: 0.02~0.08%, Si: 0.01~0.50%, Mn: 0.5~1.8%, P: 0.01% or less, S: 0.002% or less, Mo: 0.05~0.50%, Ti: 0.005~0.04% , Al: contains 0.01 to 0.07%, Nb: 0.005 to 0.05% and / or V: containing from 0.005 to 0.10 percent. 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.

In the light of the above discussed prior art, there is a need of a steel composition and the microstructure which overcomes the limitations of prior art and exhibits superior resistance to hydrogen induced cracking, coupled with superior formability and weldability.
SUMMARY OF THE DISCLOSURE
The present disclosure relates to non-peritectic steel made of a composition which provides high resistance to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC) and has enhanced strength and toughness. The said composition comprises Carbon (C) at a concentration ranging from about 0.02wt% to about 0.05wt%; Manganese (Mn) at a concentration ranging from about 0.6 wt% to about 1.25 wt%; Niobium (Nb) at a concentration ranging from about 0.06wt% to about 0.10wt%; Molybdenum (Mo) at a concentration ranging from about 0.10 wt% to about 0.20 wt%; Titanium (Ti) at a concentration ranging from about 0.015wt% to about 0.025wt%; Aluminum (Al) at a concentration ranging from about 0.03wt% to about 0.05 wt%; Silicon (Si) at a concentration ranging from about 0.1wt% to about 0.5wt%; Nitrogen (N) at a concentration ranging from about 0.0010wt% to about 0.060wt%; Sulphur (S) at a concentration ranging from about 0.0001wt% to about 0.0020wt%; and Phosphorus (P) at a concentration ranging from about 0.0001wt% to about 0.015wt%.
The steel as per the current invention possesses 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%. Also, the said steel possesses superior resistance to sulfide stress corrosion cracking (SSCC) by exhibiting the value of threshold stress greater than 80 % of yield strength (YS) of the steel. As per the current invention, the ferrite potential of said composition is more than 1.05, thereby making the resultant steel non-peritectic; whereas the carbon equivalence of the composition is less than 0.30, ensuring that the steel exhibits excellent weldability. The steel manufactured as per the current invention has polygonal ferrite and bainitic ferrite microstructure; and possesses tensile properties in accordance with API 5L PSL-2 X70 specification.
The steel of the present disclosure is designed such that it is readily hot/cold formed, HIC and SSCC resistant and is welded to form linepipe tubes to be used for the transportation of natural gas or crude oil, especially of sour grade. The present invention also provides a method for manufacturing steel having composition as described above, wherein said method involves casting of the composition in steel slab, hot rolling of the steel slab at specific conditions, and controlled cooling of the hot rolled steel sheet to obtain the steel.
In embodiments of the present disclosure, recrystallization stop temperature (TNR) with finish hot rolling temperature (FRT), along with the coiling temperature are critical to arrive at the steel of the present disclosure.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 provides a schematic illustrating the designed thermo-mechanical processing for the manufacturing of API 5L PSL-2 X70 hot rolled steel grade for welded steel pipe to be used for sour service application.
Figure 2 provides (a) optical and (b) SEM micrograph showing the ferrite- bainite (bainitic ferrite) microstructure of steel-1 coiled at 520°C.
Figure 3 provides (a) optical and (b) SEM micrograph showing predominantly polygonal ferrite with acicular or bainitic ferrite microstructure of steel-1 coiled at 600°C.
Figure 4 provides bright field TEM micrographs showing the fine bainitic ferrite/acicular ferrite microstructure for the steel-1 coiled at 520°C.
Figure 5 provides (a) bright field TEM micrograph illustrating the fine ferrite and bainitic ferrite/acicular ferrite microstructure, (b) bright field TEM micrograph illustrating the dispersion of fine precipitates in the steel-1 coiled at 600°C and (c) Energy dispersive spectrum showing the composition of fine scale precipitates,
DETAILED DESCRIPTION
In view of the problems of the prior art highlighted above, the present disclosure aims to provide a steel made of a composition which provides high resistance to hydrogen induced cracking (HIC), is non-peritectic and has enhanced strength and toughness. The steel having the said composition must also conform to the properties laid in API 5L PSL-2 X70 specification (API - American Petroleum Institute / PSL – Product Standard Level). The steel as per the present disclosure discloses a micro alloyed steel composition and thermo-mechanical processing to manufacture the hot rolled sheets/plates which exhibits superior HIC and SSCC resistance and conforms to the properties laid down for API 5L PSL-2 X-70 specification.
The present disclosure provides a steel composition comprising Carbon (C) at a concentration ranging from about 0.02wt% to about 0.05wt%; Manganese (Mn) at a concentration ranging from about 0.7 wt% to about 1.25wt%; Niobium (Nb) at a concentration ranging from about 0.07wt% to about 0.11wt%; Molybdenum (Mo) at a concentration ranging from about 0.10 wt% to about 0.20 wt%; Titanium (Ti) at a concentration ranging from about 0.015wt% to about 0.025wt%; Aluminium (Al) at a concentration ranging from about 0.03wt% to about 0.05wt%; Silicon (Si) at a concentration ranging from about 0.1wt% to about 0.5wt%; Nitrogen (N) at a concentration ranging from about 0.0001wt% to about 0.060wt%; Sulphur (S) at a concentration ranging from about 0.0001wt% to about 0.0020wt%; and Phosphorus (P) at a concentration ranging from about 0.0001wt% to about 0.015wt%.
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.
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:



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 possess superior resistance to sulfide stress corrosion cracking. 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 TM0177-2016. The ferrite potential of the steel as per the current invention is more than 1.05.
The said composition of the present disclosure is specifically designed to ensure that the concentrations of the constituent elements provide optimum desired results. Thus, it is important to understand the role of each of the critical elements with respect to their concentrations as provided below:
Carbon: The carbon in the steel as per the current invention is present in the range of about 0.02wt% to about 0.05wt%. 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 to HIC resistance of steel. Also, the increased carbon content decreases the toughness and weldabilty 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 0.02wt% to about 0.05wt%.
Manganese: Mn in the steel of the current invention varies in the range of about 0.7wt% to about 1.25 wt%. Preferably, Mn is present less than 1.20 weight%. 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 resistance. Higher level of manganese in steel also increases the carbon equivalence and impairs the weldability of steel.
Silicon: Silicon is present in the range of about 0.20wt% to about 0.40wt%. 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.
Niobium: Nb in the steel of the current invention varies in the range of about 0.07 wt% to about 0.11 wt%. Preferably, Nb is present less than 1.10 weight%. 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.
However, Niobium content more than 0.12 % 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.
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 0.15 weight%. Mo. The presence of Mo promotes the formation of acicular ferrite/bainitic ferrite because of it increases the 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 and SSCC. 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.
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/carbonitirdes. 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.
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
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.
Sulphur: Sulphur needs to be limited to about 0.0010wt% to avoid high level of MnS inclusions, as they cause severe deterioration of HIC properties.
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.
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.
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 0.24wt%. In an embodiment of the current invention, the alloying elements like Ni and Cu can be added to the total content of 0.5 wt% to improve the corrosion properties of the steel.
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 90-95% ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn and Si. With the application of high Nb and Mo coupled with controlled thermo-mechanical processing conditions, the average grain size is restricted to about 2.0 and 2.5 ?m for coiling temperature of 520 and 600°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 4 which shows bright and dark field TEM micrographs 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 µm to about 5µm.

Accordingly, in embodiments of the present disclosure, the steel having the said composition has polygonal ferrite and bainitic ferrite microstructure. The amount of bainitic ferrite /acicular ferrite in the microstructure ranges between about 10% to about 20%. The strengthening from bainite/acicular ferrite is derived from its fine structure and higher dislocation density.
This microstructure, formed by the composition, lends enhanced strength and quality to the resultant steel of the present disclosure. More particularly, the steel of the present disclosure possesses high yield strength (YS) and ultimate tensile strength (UTS), as required by API 5L PSL-2 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 570 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.
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 280J to about 360J at -60°C in longitudinal direction and about 280J to about 300J in transverse direction. 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 0.80.
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:
Pcm = + + + + + + +B

On the other hand, the formula based on International Institute of Welding (IIW) is:
CE =
In embodiments of the present disclosure, 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.
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 X-70 grade steel, and capable of being used for sour 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 linepipe tubes to be used for the transportation of natural gas or crude oil.
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 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.

Figure 1 shows the schematic that defines the thermo-mechanical processing employed for the production of hot rolled strips of the designed chemistry used for manufacturing of X-70 sour grade.
The steel as per the current invention is manufactured by casting a steel slab as per the above specified steel composition followed by heating the slab to a temperature ranging from about 1100°C to about 1250°C. The hot rolling of the slab with about 70% to about 90% reduction is controlled below recrystallization stop temperature (TNR) with predefined finish hot rolling temperature (FRT) from about Ae3 - 50 (°C) to about Ae3 + 50 (°C). Thereafter, hot rolled steel sheet is cooled in control fashion to a coiling temperature ranging from about 520°C to about 600°C t to obtain the said steel.
In further embodiments of the present disclosure, the heating of the slab as aforementioned is carried out for a duration ranging from about 20 minutes to about 2 hours; and wherein the cooling in the step is carried out at a rate ranging from about 10°C to about 50°C per second.
In embodiments of the present disclosure, the said method is carried out under specific conditions and parameters, which help achieve the desired steel of the present disclosure. Initially, the specified composition is first casted either through conventional continuous caster or a thin slab casting route. The non-peritectic steel composition of the present disclosure ensures smooth casting of steel through either route. After casting the slab with the specified composition, the slabs are reheated to a temperature greater than 1100°C (preferably in the range of about 1100°C to 1250°C) for a duration of about 20 minutes to about 2 hours. The reheating temperature is above 1100°C to ensure complete dissolution of any precipitates Niobium and Molybdenum carbide/carbonitrides may have formed in the preceding processing steps. A reheating temperature greater than about 1250°C is also undesirable as it may lead to grain coarsening of austenite and lead to yield loss due to excessive scale formation.

After casting and reheating the steel slab with the specified composition, hot-rolling of the slab is carried out. 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 an embodiment, 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 Ae3 - 50 (°C) to about Ae3 + 50 (°C).
Thereafter, the hot rolled steel sheet is subjected to accelerated cooling strategy on the Run-Out-Table (ROT), at a cooling rate ranging from about 10°C/s to about 50°C/s to a coiling temperature (CT) ranging from about 520°C to about 600°C, 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.
In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
EXAMPLES
Example 1: Manufacturing of steel of the present disclosure and analysis of the Microstructure thereof
Steels with the composition as defined in Table 1 below was cast into two billets. The cast billets were then reheated to a temperature of 1200°C for a period of 1 hour to ensure the complete dissolution of Niobium precipitates. Billets for both the steel composition were hot rolled with the identical deformation schedule, with the 80% reduction below TNR (1030-1040°C) and finish rolled to a temperature of 870°C. Post hot rolling, the hot rolled sheets processed from the billets of both the steel composition were cooled to a coiling temperature of 520°C and 600°C, respectively at cooling rate of 28-30°C/s.
Table 1 – Steel composition
Elements/Properties Concentrations (wt%)/Values
Steel-1 Steel-2
C 0.025 0.04
Mn 1.20 0.85
Si 0.30 0.35
Nb 0.10 0.08
Mo 0.10 0.12
Ti 0.017 0.018
Al 0.03 0.04
S <0.0010 <0.0010
P 0.005 0.005
N 0.0050 0.0060
Ferrite Potential (Fp) 1.20 1.22
Pcm 0.10 0.10
C (IIW) 0.245 0.205
Total Micro alloying content
(Nb +Mo+ Ti + N) 0.22 0.224

The resulting hot rolled sheets coiled at 520°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 details
Coiling Temperature
(CT) Microstructure Average Grain size
(µm) Hardness
(Hv)
Steel-1 520°C Ferrite + Bainitic ferrite 2±1 185±4
600°C Ferrite+ Bainitic ferrite 2.5±1.5 200±5
Steel-2 520°C Ferrite + Bainitic ferrite 2.5±1 180±5
600°C Ferrite+ Bainitic ferrite 3.0±1.5 202±7

Results: For both cases of steel coiled at 520°C and 600°C, the microstructure, grain size and hardness correspond to the values required by the steel of the present disclosure.
While, figure 2 provides optical (2a) and SEM micrograph (2b) showing the ferrite-bainite microstructure of steel 1 coiled at 520°C; figure 3 provides optical (3a) and SEM micrograph (3b) showing predominantly polygonal ferrite with acicular or bainitic ferrite microstructure of steel 1 coiled at 600°C.
Further, while figure 4 shows bright and dark field TEM micrographs revealing the dispersion of fine precipitates of niobium rich carbide/carbonitride for the steel 1 coiled at 600°C, figure 5 shows bright field TEM micrographs showing the fine bainitic ferrite/acicular ferrite microstructure for the steel 1 coiled at 520°C. The presence of fine precipitates of the order of few nanometre in size not only increase the strength of the steel but also increase the resistance of the developed steel to hydrogen induced cracking and sulphide stress corrosion cracking.
ANALYSIS OF THE TENSILE PROPERTIES OF STEEL OF THE PRESENT DISCLOSURE
Steels with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°C and 600°C for both the 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 sheets along the longitudinal and transverse direction
Coiling Temperature Longitudinal direction Transverse direction
YS (MPa) UTS (MPa) % El. (YS/UTS) Ratio YS (MPa) UTS (MPa) % El. (YS/UTS) Ratio
Steel-1 CT 520°C 515 575 30 0.89 545 610 28.5 0.893
CT 600°C 560 620 29 0.90 600 675 27.8 0.88
Steel-2 CT 520°C 505 590 30 0.85 540 620 29 0.87
CT 600°C 540 645 28 0.84 580 680 27 0.85

Results: For both cases of steels, hot rolled sheets coiled at 520°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 X70 specification.
Analysis of the Impact Toughness of steel of the present disclosure
Steel 1 with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°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 from Steel 1 coiled at 520°C and 600°C
Impact Toughness
(In Joules) CT-520°C CT-600°C
Temperature Longitudinal Transverse Longitudinal Transverse
25°C 402 378 390 348
0°C 384 360 350 320
-40°C 370 336 312 300
-60°C 360 300 340 280

Results: For both cases of steel coiled at 520°C and 600°C, the impact toughness was found to be ranging between 300J to 360J at -60°C in longitudinal direction and 280J to 340 J in transverse direction, as required by the steel of the present disclosure.

Analysis of the fracture toughness (CTOD) of steel of the present disclosure

Steel 1 with the composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°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 of steel 1 coiled at 520 and
600 °C
Target as per API specification Properties achieved
Fracture Toughness
CTOD (mm) Fracture Toughness
CTOD (mm) CT-520°C CT-600°C
Room Temperature 0.35 0.95 0.89
0°C 0.35 1.02 1.01
-40°C 0.35 0.97 0.99

Results: For both cases of hot rolled sheets for steel 1coiled at 500°C and 600°C, the fracture toughness was found to be greater than 0.85, 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
Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°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. The steel was also tested in the much aggressive environment by lowering the pH of the solution to 2±0.5.
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 6 below.

HIC properties for steel 1
Solution-A, pH of 3 ± 0.5 Solution-A, pH of 2 ± 0.5
Index CT-520°C CT-600°C CT-520°C CT-600°C
Crack Length Ratio (CLR) 0 0 0 0
Crack Thickness Ratio (CTR) 0 0 0 0
Crack Sensitivity Ratio (CSR) 0 0 0 0
Table 6
Results: For both cases of steel coiled at 520°C and 600°C for steel-1, no cracks were generated and all the three parameters CLR, CTR and CSR were found to be zero for the developed steel, even for the solution with pH of 2 ± 0.5.

Example 6: Analysis of the sulfide stress corrosion cracking (SSCC) properties of steel of the present disclosure
Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°C and 600°C are tested as per NACE standard TM0177-2016, for the evaluation of SSCC properties. A sub-size test specimen with gauge section of 3.81 ±0.05 mm in diameter and 25.4 mm (1.00 in.) long is tested under un-axial 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 distilled water. The steel samples were subjected to the test for a period of 720 hours, until the sample fractured before this period. The results are provided in Table 7 below:

SSCC properties of developed steel
Steel 1 Target as per API specification Properties Achieved
CT 520°C 72% YS 85%YS
CT 600°C 72% YS 85%YS
Table 7
Results: For both cases of steel coiled at 520°C and 600°C for steel 1, no cracks were generated/failure occurred after exposure of 720 hours under uni-axial loading, and in both the case the steel exhibited the threshold stress greater than 85% of its room temperature yield strength.
Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.