FIELD OF INVENTION

[0001] The present invention relates to a high strength hot rolled steel, and more particularly to the high strength hot rolled steel having excellent H-trapping ability and excellent hydrogen embrittlement resistance, and method of manufacturing the high strength hot rolled steel.

BACKGROUND
[0002] Various grades of steels are used in the energy industries. Notable among them are the line pipe grade of steels which carry petroleum and natural gas products under high pressure conditions. These steels are used for both on-shore and off-shore applications. Moreover, there are power plant grade of steels which operate under severe conditions like pressure, temperature in wet or humid conditions. These grades of steels are exposed to nascent and atomic hydrogen. The atomic hydrogen may come from the continuous exposure in petroleum products, which contains hydrocarbons, hydrogen sulfide etc. or may be from the corrosion in moisture environment. It is also known that the atomic and diffusible H causes the H related failures mentioned above.
[0003] It is known that few ppm of hydrogen can cause abrupt and drastic failures in steels even well within designed mechanical stress and fracture limits. The phenomenon is broadly known as hydrogen embrittlement (HE). There are various other terminologies, which are frequently used in scientific literature for example, hydrogen induced cracking (HIC), hydrogen environment assisted cracking (HEAC) etc. Recent literature study suggests that HE can be prevented in steels using two broad strategies. The first one includes application of H-diffusion barrier coatings or simply H-barrier coatings on top of the components to reduce H-ingress during service. Few examples of such coatings are metaling Ni or Cd plating, ceramic coatings like TiO2, Al2O3 etc., and amorphous coatings like electroless NiP coatings. These coatings slow down the H-ingress into the components during service and hence HE is delayed. The second approach includes design of steel microstructures with H-traps. These traps are micro alloyed precipitates like carbides and carbonitrides. The precipitates trap the H in steel microstructure rendering them immovable. Therefore, the propensity towards H related failures is delayed. Present invention is based on the second approach.
[0004] Chinese patent No. CN105583548 discloses an invention high-strength low-alloy steel metal-cored welding wire (with minimum UTS 700 MPa) which when welded using Ar gas or CO2 gas, provides excellent anti-hydrogen-induced cracking property and anti-hydrogen sulfide corrosion property. However, utilization of high concentration Ni, Fe-Mo, Mn, Fe-Mn, Fe-Si, Fe-Ti powders (costly powders) to impart the resistance to hydrogen embrittlement is not cost effective. Further it may not be feasible to add the costly alloying elements in a continuous production plant like hot-strip mill.
[0005] Japanese patent application publication No. JP2004231992 discloses an invention of high strength steel sheet with excellent resistance to hydrogen embrittlement and its manufacturing method. The steel sheet with a minimum tensile strength of 980 MPa is intended for primarily automotive applications. The steel contains a huge quantity of alloying elements such as 0.05-0.3 C, 0.01-3.0 Si, 0.01-4.0 Mn, 0.01-3.0 Al, 0.001-5.5 Ni, 0.001-3.0 Cu, 0.001-5.0 Cr and 0.005-5.0 Mo (all wt.%) which is very difficult to add and manufacture in hot-strip mills. Moreover, the cost of such steel sheet is expected to be high.
[0006] Japanese patent application publication No. JP2005097725 discloses a method of producing high strength steel sheet for hot-press forming having hydrogen embrittlement resistance for automobile member application and its production method. Again the resistance to hydrogen embrittlement in this invention is achieved by adding one or more alloying elements like 0.01-0.40 C, Si<=2.0, 0.01-3.5 Mn, P<0.1, S<0.05, 0.005-4.0 Al, N<=0.01 and comprising one or more kinds of selected carbide forming elements like Nb, V, Cr, Ti and Mo in an amount of 0.001-3.0 % in total (all wt.%). The addition of such expensive carbide forming alloying elements in such a high quantity raises the overall cost of the steel product.
[0007] Korean patent application publication No. KR20150047042 discloses a method of producing hot rolled steel plate having excellent hydrogen embrittlement resistance. The disclosed steel comprises a moderate quantity of alloying element of 0.15-0.20 C, 1.0-1.5 Si, 2.0-2.5 Mn, 0.01-0.03 Mo, 0.5-1.5 Cr, 0.5-1.5 Ni and the balance being Fe and unavoidable impurities. The said steel is produced in the hot strip mill and have a minimum tensile strength of 1200 MPa. The steel is intended for automotive parts and home appliance. However, the disclosed invention is said to have 80% or more of martensite phase in the microstructure. Such a microstructure may be suitable for automotive or home appliances application. However, for line pipe applications, presence of martensite may cause severe corrosion due to its high energy structure with high density of defects and dislocations. Moreover, in power plant applications the disclosed steel may have to be tempered above a moderate temperature like 200oC and is expected to lose its strength.
[0008] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
OBJECTIVE OF INVENTION
[0009] It is an object of the invention to solve the problems of the prior art and to provide a high strength hot rolled steel having a minimum tensile strength of 400 MPa, which possesses a microstructure consisting of ferrite, pearlite and micro alloyed precipitate.
[0010] Another objective of the present invention is to provide the steel having excellent hydrogen embrittlement resistance due to its enhanced hydrogen trapping ability.
[0011] Another objective of present invention is to provide a new easier manufacturing method combining thermomechanical, and heat treatment processes for the proposed chemical composition to manufacture the high strength hot rolled steel having excellent H-trapping ability and excellent hydrogen embrittlement resistance.
[0012] It is yet another objective of the present invention, to provide a high strength hot rolled steel sheet, having the following composition in weight%:C: 0.08-0.12, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4, Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Nb<0.005, N<=100 ppm, Ti: 0.10-0.15 and the balance being Iron (Fe) and unavoidable impurities.
SUMMARY OF INVENTION
[0013] This summary is provided to introduce concepts related to a high strength hot rolled steel having excellent H-trapping ability and excellent hydrogen embrittlement resistance and a method of manufacturing the high strength hot rolled steel sheet. 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.
[0014] In one aspect of the present invention, a high strength hot rolled steel is provided. The high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.08% - 0.12%, Manganese (Mn): 0.1% - 1.0%, Sulphur (S): maximum 0.01%, Phosphorus (P): maximum 0.02%, Nitrogen (N): maximum 100 ppm, Chromium (Cr): maximum of 0.1%, Vanadium (V): maximum 0.005%, Silicon (Si): 0.001%-0.4%, Nickel (Ni): 0.001%-0.05%, Aluminium (Al): maximum 0.05%, Copper (Cu): maximum 0.01%, Molybdenum (Mo): maximum 0.06%, Niobium (Nb): maximum 0.005%, Titanium (Ti): 0.10%-0.15%, and the remaining being substantially iron and incidental impurities. The high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates.
[0015] In an embodiment, the ferrite is precipitation strengthened and has an average grain size of 10 to 50 µm.
[0016] In an embodiment, the high strength hot rolled steel has a tensile strength = 400 MPa. In an embodiment, the high strength hot rolled steel has a tensile strength in the range of 400-500 MPa.
[0017] In an embodiment, the high strength hot rolled steel has a yield strength = 250 MPa, a uniform elongation = 10%, a hardness = 130 Hv.
[0018] In an embodiment, the high strength hot rolled steel comprises nano-sized Ti precipitates.
[0019] In an embodiment, the high strength hot rolled steel has an excellent H-trapping ability and provides excellent resistance to hydrogen embrittlement.
[0020] In an embodiment, the Ti content of the high strength hot rolled steel is kept in the concentration range of 0.10-0.15 wt.% to form Ti-precipitates in the form of carbides and carbonitrides. The fine nanometer sized precipitates have high H-trapping ability and provide excellent resistance to hydrogen embrittlement.
[0021] In an embodiment, the C content of the high strength hot rolled steel is kept in the concentration range of 0.08-0.12 wt.% to achieve optimum austenite to ferrite transformation kinetics after the hot-rolling along with the calculated quantity of precipitate for good strength in the final microstructure along with excellent H trapping ability.
[0022] In an embodiment, the Si content of the high strength hot rolled steel is kept in the range between 0.001-0.4 wt.% to avoid the scale formation.
[0023] In an embodiment, the high strength hot rolled steel comprises the composition expressed in weight %: C - 0.085, Mn – 0.12, S - 0.008, P - 0.017, Si - 0.105, Cr - 0.012, Nb - 0.001, N – 90 ppm, V – 0.003, Ni – 0.025, Al – 0.018, Cu – 0.005, Mo – 0.057, Ti – 0.11, and the balance being Iron (Fe) and unavoidable impurities.
[0024] In an embodiment, the high strength hot rolled steel has a tensile strength in the range 400 – 500 MPa.
[0025] In another aspect of the present invention, a method for manufacturing high strength hot rolled steel sheet is provided. The method comprises casting steel slab having a composition expressed in weight %: C: 0.08-0.12, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4, Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Nb<0.005, N<=100 ppm, Ti: 0.10-0.15 and the balance being Iron (Fe) and unavoidable impurities. The method also comprises reheating the steel slab to a temperature greater than 1250oC. The method further comprises hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT). TFRT varies in the range 830oC to 890oC. The method comprises cooling at a cooling rate greater than 50oC/s till a coiling temperature (TCT) is reached. TCT varies in the range 650 to 700oC. The method also comprises coiling the steel sheet at the coiling temperature TCT to obtain the high strength hot rolled steel sheet.
[0026] In an embodiment, the high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates.
[0027] In an embodiment, the ferrite is precipitation strengthened and an average grain size of 10 to 50 µm.
[0028] In an embodiment, the high strength hot rolled steel has a tensile strength = 400 MPa and yield strength = 250 MPa, hardness = 130 Hv, and a uniform elongation = 10 %.
[0029] In an embodiment, the steel slab is reheated to a temperature above 1250oC-1350oC for a duration of 20 minutes to 2 hours depending on the slab thickness.
[0030] In an embodiment, the cooling rate is kept higher than 50oC/s to prevent formation of excessive pearlite. In an embodiment, the thickness of the steel sheet is in the range of 6 mm - 24 mm.
[0031] In an embodiment, the C content of the high strength hot rolled steel is kept in the concentration range of 0.08-0.12 wt.% to achieve optimum austenite to ferrite transformation kinetics after the hot-rolling along with the calculated quantity of precipitate for good strength in the final microstructure along with excellent H trapping ability.
[0032] In yet another aspect of the present invention, a high strength hot rolled steel having a tensile strength = 400 MPa is provided. The high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.08% - 0.12%, Manganese (Mn): 0.1% - 1.0%, Sulphur (S): maximum 0.01%, Phosphorus (P): maximum 0.02%, Nitrogen (N): maximum 100 ppm, Chromium (Cr): maximum of 0.1%, Vanadium (V): maximum 0.005%, Silicon (Si): 0.001%-0.4%, Nickel (Ni): 0.001%-0.05%, Aluminium (Al): maximum 0.05%, Copper (Cu): maximum 0.01%, Molybdenum (Mo): maximum 0.06%, Niobium (Nb): maximum 0.005%, Titanium (Ti): 0.10%-0.15%, and the remaining being substantially iron and incidental impurities. The high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates. The produced high strength hot rolled steel has an excellent H-trapping ability and gives excellent resistance to hydrogen embrittlement during service conditions.
[0033] A line pipe produced from the high strength hot rolled steel. The line pipe is configured to carry petroleum and natural gas products under high pressure conditions.
[0034] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Figure 1 illustrates a flowchart of a method of manufacturing a high strength hot rolled steel sheet, according to an embodiment of the present invention;
[0036] Figure 2 illustrates a schematic diagram of cooling technique followed after rolling (in the run-out table of the hot-strip mill) during manufacture of the high strength hot rolled steel sheet, according to an embodiment of the present invention;
[0037] Figure 3a illustrates a final microstructure of high strength hot rolled steel sheet of inventive example composition coiled at TCT of 650oC, during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0038] Figure 3b illustrates a final microstructure of high strength hot rolled steel sheet of inventive example composition coiled at TCT of 700oC, during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0039] Figure 3c illustrates a final microstructure of steel sheet of comparative example composition coiled at TCT of 650oC, during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0040] Figure 3d illustrates a final microstructure of steel sheet of comparative example composition coiled at TCT of 700oC, during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0041] Figure 4a illustrates a graphical representation of micro alloyed precipitate volume fractions versus temperature of the inventive and comparative examples simulated using Thermocalc© software and TCFE8 database, according to an embodiment of present invention;
[0042] Figure 4b illustrates a graphical representation of component fractions of Ti, C and N in the precipitate in the inventive example versus temperature, according to an embodiment of present invention;
[0043] Figure 4c illustrates a graphical representation of component fractions of Mo, C and N in the precipitate in the comparative example versus temperature, according to an embodiment of present invention;
[0044] Figure 5 illustrates a graphical representation of H-trapping ability of the inventive and the comparative example versus temperature, according to an embodiment of present invention; and
[0045] Figures 6a and 6b illustrate TEM images showing the nano-sized micro alloyed precipitates in both the inventive and comparative examples respectively, according to an embodiment of the present invention.
[0046] 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
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] The high strength hot rolled steel having a minimum tensile strength of 400 MPa according to the present invention comprises the following composition expressed in weight %: C: 0.08-0.12, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4, Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Nb<0.005, N<=100 ppm, Ti: 0.10-0.15 and the balance being Iron (Fe) and unavoidable impurities. The high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates. In the illustrated example, the high strength hot rolled steel comprises nano-sized Ti precipitates in the microstructure. In the illustrated example, the ferrite is precipitation strengthened and has a grain size of 10 to 50 µm. In the illustrated example, the high strength hot rolled steel has a tensile strength in the range of 400-500 MPa. The high strength hot rolled steel has a yield strength = 250 MPa, hardness = 130 Hv and a uniform elongation = 10%. The high strength hot rolled steel has an excellent H-trapping ability and provides excellent resistance to hydrogen embrittlement.
[0053] The high strength hot-rolled steel sheet with a minimum 400 MPa tensile strength consisting of ferrite + pearlite + nano-sized micro alloyed precipitate microstructure provides excellent resistance to hydrogen embrittlement and is suitable for producing line pipes configured to carry petroleum and natural gas products under high pressure conditions.
[0054] Referring to Figures 1 and 2, the method (100) of manufacturing a high strength hot rolled steel sheet of the desired composition is illustrated. At step (102), the method (100) comprises casting molten steel having composition expressed in weight %: C: 0.08-0.12, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4, Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Nb<0.005, N<=100 ppm, Ti: 0.10-0.15 and the balance being Iron (Fe) and unavoidable impurities. The molten steel is casted in a casting apparatus to obtain steel slabs (cast ingots). In the illustrated example, the steel is cast in a conventional continuous caster. The temperature of the slab is not allowed to drop below 1200oC to avoid the Ti-carbonitride precipitation. In case, the precipitation occurs, it may become difficult to completely dissolve the precipitates in the subsequent reheating process rendering them ineffective for precipitation strengthening.
[0055] At step (104), the method (100) comprises reheating the steel slab (steel casting) to a temperature greater than 1250°C. In the illustrated example, the slab is reheated to temperature ranging between 1250 to 1350oC for a duration of 20 minutes to 2 hours depending on the slab thickness. The reheating temperature must be on or above 1250oC, to ensure complete dissolution of any precipitates of Ti that may have formed in the preceding processing steps. A reheating temperature greater than 1350oC is also undesirable because it may lead to excessive grain coarsening of austenite and/or scale loss. In one example, the casted steel may be heated in a furnace.
[0056] At step (106), the method (100) comprises hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) (also shown in Figure 2). The TFRT varies in the range 830oC to 890oC. After the steel slab is cast in the specified composition and reheated, it is hot-rolled. The slabs of higher thicknesses are rough rolled in roughing stands in a conventional hot-rolling mill. The rough rolling is done above the recrystallization temperature. Then the hot rolling is done in the tandem rolling mill below the recrystallization temperature. The rolling is finished at the finish rolling temperature, TFRT given by such that 830 = TFRT = 890oC. The above range of the finish rolling temperature (TFRT) range is chosen to finish the hot rolling in the austenitic range.
[0057] At step (108), the method (100) comprises cooling at a cooling rate greater than 50oC/s till a coiling temperature (TCT) is reached. The TCT varies in the range 650 to 700oC. After the hot rolling, the rolled sheet is subjected to laminar cooling on the Run-Out-Table (RoT) at a cooling rate of greater than 50°C/s till the desired coiling temperature (TCT) is reached. The cooling rate should be higher than 50oC/s to prevent formation of excessive pearlite. High cooling rate also results in lowering the ferrite start temperature which leads to refinement of the ferrite grain size. It also prevents the growth of the ferrite.
[0058] At step (110), the method (100) comprises coiling the steel sheet at the coiling temperature. Coiling is carried out at a temperature 650 = TCT = 700 (°C). Coiling below 650oC is avoided as at lower temperature the precipitation kinetics is slow and desired degree of precipitation is not achieved. The steel is cooled rapidly and coiled at the coiling temperature (TCT) to allow development of nano-sized micro alloyed precipitates in the final microstructure, which will give rise to the excellent hydrogen trapping ability. The obtained high strength hot rolled steel sheet has microstructure represented by, in area%, 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates. In the illustrated example, the high strength hot rolled steel comprises nano-sized Ti precipitates in the microstructure. The high strength hot rolled steel sheet exhibits tensile strength greater than 400 MPa, uniform elongation greater than 10%, and yield strength greater than 250 MPa. In the illustrated example, the thickness of the steel sheet is in the range of 6 mm - 24 mm.
[0059] According to the disclosed invention method (100), it is possible to manufacture the high strength hot-rolled steel sheet with a minimum 400 MPa tensile strength consisting of ferrite + pearlite + nano-sized micro alloyed precipitate microstructure which has excellent H-trapping ability and provides excellent resistance to hydrogen embrittlement. Such a steel sheet is suited for producing line pipes configured to carry petroleum and natural gas products under high pressure conditions where good hydrogen trapping ability is a requirement along with the high tensile strength.
[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 sheet and their role in enhancing properties.
[0061] C: 0.08-0.12%: Carbon is an essential component of steel and is present in any commercial iron and steel making process. C also the essential component of forming the carbide and carbo-nitride precipitates in the steel. Also, the C content in austenite determines the ferrite formation kinetics. Presence of specified quantity of C is required in the microstructure for the formation of the precipitates. However, a higher C content than the above specified range causes peritectic reaction during the continuous casting stage which is not desirable. Also, a higher C causes poor weldability in the final product. Therefore, the C content in the present invention is restricted to 0.08-0.12 wt.% to achieve optimum austenite to ferrite transformation kinetics after the hot-rolling along with the calculated quantity of precipitate for good strength in the final microstructure along with excellent H trapping ability.
[0062] Si: 0.001-0.4%: Si is a good solid solution strengthening element in steel. Si is also a cheap alloying element. Moreover, higher Si content causes a faster austenite to ferrite transformation kinetics. However, presence of Si also causes surface scale formation during the hot-rolling stage which is not desired. Therefore, the Si content is restricted to 0.001-0.4 wt.%.
[0063] Mn: 0.1-1.0%: Mn is an efficient austenite stabilizer. Mn also has a high solid solution strengthening ability in ferrite. Moreover, Mn reduces austenite to ferrite transformation temperature which is helpful is finishing the hot-rolling in the austenite stage at lower temperature and helps to reduce the ferrite grain size. However, at higher Mn content there is a chance of center-line segregation in the hot-rolled steel sheet, which is undesirable for the perspective of hydrogen embrittlement. Therefore, in the present invention Mn content was restricted to 0.1-1.0 wt.%.
[0064] Nb: <0.005%: Nb forms Nb(CN) precipitates in the steel microstructure during the hot rolling stage. These fine precipitates pin the grain boundaries and causes fine austenite grain size. However, Nb must remain in austenite solid solution for these beneficial effects to take place and the Nb-precipitation should not take place before the hot-rolling commences. The Nb shall remain dissolved in austenite in the slab-reheating stage. However, in the present invention the H-trapping ability is achieved by the precipitation of Ti-carbonitrides. Therefore, costly addition of Nb is avoided and Nb content is restricted to <0.005 wt.%.
[0065] P: 0.02% maximum: P has a deleterious effect on the toughness and weldability of the steel by segregating at the grain-boundaries during the steel making and hot-rolling stages. P is an undesired element. Therefore, the P content is being restricted to a maximum of 0.02 wt.%.
[0066] S: 0.01% maximum: The S content is restricted to a maximum of 0.01 wt.% to limit the deleterious effect of sulfide inclusions on formability.
[0067] N: 100 ppm maximum: Presence of high N content causes formation of TiN and then Ti(CN) at higher temperature and raises the dissolution temperature of the same during the slab-reheating stage. Moreover, at higher N content the ageing stability and toughness of the heat-affected zone in weld seam reduces. Therefore, N content is restricted to a maximum of 100 ppm.
[0068] Cr: 0.1% maximum: Cr improves the hardenability during austenite to martensite transformation. It also acts as the solid solution strengthening element for ferrite. Presence of Cr helps in avoiding the austenite to pearlite formation during continuous cooling from the finish rolling temperature. In the present work no martensite is desired in the final microstructure and therefore, the Cr content was restricted to a maximum of 0.1 wt.%.
[0069] Ti: 0.10-0.15 %: Ti is the most important alloying element for the formation of Ti-precipitates in the form of carbides and carbonitrides. TiN has a very low solubility product and therefore, it precipitates in the continuous casting stage itself. Moreover, Ti also precipitates in the form of TiC and TiCN. Precipitation of these shall be avoided in the slab reheating stage and before hot rolling otherwise coarse incoherent precipitates will form which has relatively lower H-trapping ability. The slab reheating temperature is to be maintained at a temperature >1250oC for dissolution of such precipitates. In the case of precipitation in ferrite at relatively lower temperatures, the precipitate size is small and number density high. These fine nanometer sized precipitates have high H-trapping ability and expected to give excellent resistance to HE in service conditions. Therefore, in the present invention a slightly higher than stoichiometric ratio of Ti is added in the steel in the concentration range of 0.10-0.15%.
[0070] All the other alloying elements like Mo, Al, Cu, Ni, V are incidental in the steel making process and the respective quantities are to be restricted within the specified limit as mentioned above.
[0071] Microstructure: The final set of desired properties in the high strength hot-rolled steel is achieved by the presence of ferrite, pearlite and micro-alloyed precipitate, described above. All the hot-rolling, controlled cooling and coiling conditions have significance in achieving the final microstructure and properties. The contribution of the each of the phases i.e. ferrite, pearlite and micro alloyed precipitates are described below.
[0072] Ferrite: The final hot-rolled microstructure contains nearly 90-100% ferrite, which is strengthened by the contributions from the alloying elements mentioned above. Ferrite is a softer phase.
[0073] Pearlite: A small amount of pearlite (10% maximum) is observed in the final microstructure. A small pearlite impart strength in the final microstructure. However, a large pearlite fraction would consume the C from the steel microstructure which would make less Ti-precipitate. Therefore, a maximum of 10% pearlite was formed in the microstructure.
[0074] Ti-precipitate: The precipitates are present in the microstructure in 0.005% maximum. Precipitate is a harder phase. The precipitation strengthening is achieved in the steel when moving dislocation are impeded by the harder precipitates in the softer ferrite matrix. Moreover, the precipitates are fine in size (nano-meter). The precipitates maintain coherent interface with the ferrite matrix with coherency strains which help in excellent H-trapping. Presence of the above specified quantity of precipitate ensures the tensile strength of 400-500 MPa is achieved in the final hot-rolled steel sheet.
Examples
[0075] Further embodiments of the present disclosure will be now described with examples of compositions of the high strength hot rolled steel, which are illustrated in Table 1. Various experiments and tests were conducted on a laboratory scale in order to evaluate various conditions.
[0076] For illustration purpose, experimental cast with the specified compositions mentioned in Table 1 were made in the laboratory. Since, the cast ingot was of larger size with 150 x 150 mm cross-section, the following steps were taken to prepare the steel samples from the ingots before the coiling simulation. The ingot was cut to pieces with a size of 60x150x150 mm. The cut pieces of the ingot were then hot forged to remove the cast structure and thickness was brought down to 35 mm. After wards the forged plate was rolled to a thickness of 2.5 mm in successive steps. The composition of the inventive example and a comparative example, prepared following the same steps, is listed in Table 1.
Composition 1:

Example C P Cr V Mn Si Ni S Al Cu
Inventive 0.085 0.017 0.012 0.003 0.12 0.105 0.025 0.008 0.018 0.005
Comparative 0.012 0.017 0.02 0.001 0.09 0.007 0.022 0.018 0.002 0.009

Mo Nb Ti N
0.057 0.001 0.11 90
0.88 0.001 0.001 35

Table: 1
[0077] The rolled sample were then austenitized at 1300oC for 15 min for the Ti-precipitate dissolution (slab reheating). Then the samples were transferred to salt bath furnaces kept temperatures 650oC and 700oC for 3 h and 0.5 h, respectively, to emulate the coiling conditions at the hot-strip mill as well as allow for the Ti-carbide and carbonitride precipitation to be complete (herein the salt bath furnace is used to emulate the ROT conditions in lab scale).
[0078] The SEM (scanning electron microscopy) micrographs are shown in Figures 3a to 3d for the inventive and the comparative example. Figure 3a illustrates a final microstructure of high strength hot rolled steel sheet of inventive example composition coiled at TCT of 650oC. Figure 3b illustrates a final microstructure of high strength hot rolled steel sheet of inventive example composition coiled at TCT of 700oC. Figure 3c illustrates a final microstructure of high strength hot rolled steel sheet of comparative example composition coiled at TCT of 650oC. Figure 3d illustrates a final microstructure of high strength hot rolled steel sheet of comparative example composition coiled at TCT of 700oC. As seen from the Figures 3a, 3b, 3c, and 3d, both the inventive and the comparative examples have the respective micro alloyed precipitates in the final microstructure.
[0079] Referring to Figures 4a, 4b, and 4c, the expected precipitate volume fractions in both the inventive and the comparative examples and the component fractions in the precipitates of the both the examples are depicted. A thermodynamic simulation using Thermocalc© software using TCFE8 database was used to carry out to compute the expected precipitate volume fractions in both the inventive and the comparative examples. It can be seen that both the examples are expected to have a comparable quantity of precipitates in the microstructure. Moreover, the component fractions in the precipitates of the both the examples show the presence of microalloying element as the major constituents. The component fractions of Ti, C and N in the precipitate in the inventive example and the component fractions of Mo, C and N in the precipitate in the comparative example are shown in Figures 4b and 4c respectively.
[0080] Referring to Figure 5, the H-trapping ability of the inventive and the comparative example are illustrated. The H-trapping ability of the inventive and the comparative examples are measured after the saturation H-charging. The heat-treated samples were subjected to the electrochemical H-charging experiment at constant galvanostatic current density of -0.5 mA/cm2 in an aqueous solution containing 0.05 M H2SO4 with 250 mg/l As2O3 for 72 h. After the H-charging experiment, the samples were kept at room temperature for 5 days to allow for the diffusible H to escape from the samples. Then the samples were taken to a duly calibrated LECO DH603 H-determinator to measure the trapped H. In this instrument, the sample is inserted in a tubular furnace at 1100oC and the trapped H desorbs from the steel which is then measured in a thermal conductivity detector (TCD). At this temperature all the trapped H in the steel comes out. Therefore, the measured H in this method represents the maximum H-trapping ability of the steel. The H-trapping ability of the inventive and the comparative example is shown in Figure 5 within the statistical measurement error. It can be observed that the H-trapping ability of the inventive example is more than that of the comparative example.
[0081] Furthermore, the mechanical properties of the inventive and the comparative examples were estimated by the hardness measurement in Vickers scale and the tensile strength were estimated from the average hardness values following hardness conversion method according to the ISO 18265 standard. The hardness and the tensile values are mentioned in Table 2.

Sample Coiling Simulation
Temperature (oC) Hardness (HV) Estimated tensile strength (MPa)

Inventive example 650 142±2.08 458
Inventive example 700 133±2.38 427
Comparative example 650 150±4.72 480
Comparative example 700 130±1.03 418
Table: 2

[0082] It can be clearly noted that the inventive examples have achieved the minimum tensile strength of 400 MPa, and hardness = 130 Hv.
[0083] The presence of micro alloyed precipitates in both the inventive and the comparative examples are shown in Figures 6a and 6b. The samples of the inventive and the comparative examples were investigated under the bright field (BF) transmission electron microscope (TEM).
[0084] The present invention provides the high strength hot rolled steel comprising ferrite + pearlite + nano-sized micro alloyed precipitate microstructure. The nano-sized Ti precipitates present in the steel provides excellent H-trapping ability, thereby providing excellent resistance to hydrogen embrittlement. The disclosed steel is suitable for producing line pipes configured to carry petroleum and natural gas products under high pressure conditions. The invention also provides a new easier manufacturing method combining thermomechanical, and heat treatment processes for the proposed chemical composition to manufacture the high strength hot rolled steel having excellent resistance to hydrogen embrittlement. The high strength hot rolled steel makes an important contribution towards durable, cost effective, futuristic and strategic application of steel with greater factor of safety. Further the steel may be used to manufacture pipes used for oil tankers, reactors and vessels, or oil-country tubular-goods for crude oil or gas.
[0085] It should be understood that the experiments are carried out for particular compositions of the high strength hot rolled steel sheet 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 strip, as well.
[0086] 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.
[0087] 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.
[0088] 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 to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims:
CLAIMS
We Claim:

1. A high strength hot rolled steel comprising the following composition expressed in weight %:
Carbon (C): 0.08% - 0.12%,
Manganese (Mn): 0.1% - 1.0%,
Sulphur (S): maximum 0.01%,
Phosphorus (P): maximum 0.02%,
Nitrogen (N): maximum 100 ppm,
Chromium (Cr): maximum of 0.1%,
Vanadium (V): maximum 0.005%,
Silicon (Si): 0.001%-0.4%,
Nickel (Ni): 0.001%-0.05%,
Aluminium (Al): maximum 0.05%,
Copper (Cu): maximum 0.01%,
Molybdenum (Mo): maximum 0.06%
Niobium (Nb): maximum 0.005%, Titanium (Ti): 0.10%-0.15%, and the remaining being substantially iron and incidental impurities, wherein the high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates.
2. The high strength hot rolled steel as claimed in the claim 1, wherein the ferrite is precipitation strengthened and has an average grain size of 10 to 50 µm.
3. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength hot rolled steel has a tensile strength = 400 MPa.
4. The high strength hot rolled steel as claimed in the claim 3, wherein the high strength hot rolled steel has a tensile strength in the range of 400-500 MPa.
5. The high strength hot rolled steel as claimed in the claims 1 to 4, wherein the high strength hot rolled steel has a yield strength = 250 MPa, a uniform elongation = 10%, and a hardness = 130 Hv.
6. The high strength hot rolled steel as claimed in the claims 1 to 5, wherein the high strength hot rolled steel comprises maximum 0.005% nano-sized Ti precipitates.
7. The high strength hot rolled steel as claimed in the claims 1 to 6, wherein the high strength hot rolled steel has an excellent H-trapping ability and provides excellent resistance to hydrogen embrittlement.
8. The high strength hot rolled steel as claimed in the claim 7, wherein the Ti content of the high strength hot rolled steel is kept in the concentration range of 0.10-0.15 wt.% to form Ti-precipitates in the form of carbides and carbonitrides, wherein the fine nanometer sized precipitates have high H-trapping ability and provide excellent resistance to hydrogen embrittlement.
9. The high strength hot rolled steel as claimed in the claim 7, wherein the C content of the high strength hot rolled steel is kept in the concentration range of 0.08-0.12 wt.% to achieve optimum austenite to ferrite transformation kinetics after the hot-rolling along with the calculated quantity of precipitate for good strength in the final microstructure along with excellent H trapping ability.
10. The high strength hot rolled steel as claimed in the claim 7, wherein the Si content of the high strength hot rolled steel is kept in the range between 0.001-0.4 wt.% to avoid the scale formation.
11. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength hot rolled steel comprises the composition expressed in weight %: C - 0.085, Mn – 0.12, S - 0.008, P - 0.017, Si - 0.105, Cr - 0.012, Nb - 0.001, N – 90 ppm, V – 0.003, Ni – 0.025, Al – 0.018, Cu – 0.005, Mo – 0.057, Ti – 0.11, and the balance being Iron (Fe) and unavoidable impurities.
12. The high strength hot rolled steel as claimed in the claims 11, wherein the high strength hot rolled steel has a tensile strength in the range 400 – 500 MPa.
13. The high strength hot rolled steel as claimed in the claims 11, wherein the high strength hot rolled steel has a hardness = 130 Hv.
14. A method (100) for manufacturing high strength hot rolled steel sheet, the method (100) comprising:
casting steel slab having a composition expressed in weight %: C: 0.08-0.12, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4, Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Nb<0.005, N<=100 ppm, Ti: 0.10-0.15 and the balance being Iron (Fe) and unavoidable impurities;
reheating the steel slab to a temperature greater than 1250oC;
hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT), wherein TFRT varies in the range 830oC to 890oC;
cooling at a cooling rate greater than 50oC/s till a coiling temperature (TCT) is reached, wherein TCT varies in the range 650 to 700oC; and
coiling the steel sheet at the coiling temperature TCT to obtain the high strength hot rolled steel sheet.
15. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 14, wherein the high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates.
16. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 15, wherein the ferrite is precipitation strengthened and an average grain size of 10 to 50 µm.
17. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claims 14 to 16, wherein the high strength hot rolled steel has a tensile strength = 400 MPa and yield strength = 250 MPa, a uniform elongation = 10 %, and a hardness = 130 Hv.
18. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 14, wherein the steel slab is reheated to a temperature above 1250oC-1350oC for a duration of 20 minutes to 2 hours depending on the slab thickness.
19. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 14, wherein the cooling rate is kept higher than 50oC/s to prevent formation of excessive pearlite.
20. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claims 14 to 19, wherein the thickness of the steel sheet is in the range of 6 mm - 24 mm.
21. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 14, wherein the C content of the high strength hot rolled steel is kept in the concentration range of 0.08-0.12 wt.% to achieve optimum austenite to ferrite transformation kinetics after the hot-rolling along with the calculated quantity of precipitate for good strength in the final microstructure along with excellent H trapping ability.
22. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 15, wherein the high strength hot rolled steel comprises nano-sized Ti precipitates.
23. A high strength hot rolled steel having a tensile strength = 400 MPa comprising the following composition expressed in weight %:
Carbon (C): 0.08% - 0.12%,
Manganese (Mn): 0.1% - 1.0%,
Sulphur (S): maximum 0.01%,
Phosphorus (P): maximum 0.02%,
Nitrogen (N): maximum 100 ppm,
Chromium (Cr): maximum of 0.1%,
Vanadium (V): maximum 0.005%,
Silicon (Si): 0.001%-0.4%,
Nickel (Ni): 0.001%-0.05%,
Aluminium (Al): maximum 0.05%,
Copper (Cu): maximum 0.01%,
Molybdenum (Mo): maximum 0.06%
Niobium (Nb): maximum 0.005%, Titanium (Ti): 0.10%-0.15%, and the remaining being substantially iron and incidental impurities, wherein the high strength hot rolled steel comprises a microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro alloyed precipitates, wherein the produced high strength hot rolled steel has an excellent H-trapping ability and gives excellent resistance to hydrogen embrittlement during service conditions.
24. A line pipe produced from the high strength hot rolled steel as claimed in the claims 1 to 23.
25. The line pipe as claimed in the claim 24, wherein the line pipe is configured to carry petroleum and natural gas products under high pressure conditions.