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
5 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
10 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
15 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),
20 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 Hdiffusion barrier coatings or simply H-barrier coatings on top of the components to reduce Hingress 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
25 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.
30 [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
3
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.
5 [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-
10 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
15 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.
20 [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
25 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 200o
30 C and is
expected to lose its strength.
4
[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
5 high strength hot rolled steel having a minimum tensile strength of 600 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
10 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: 0.02-0.10, P<0.02, Cr<0.1,
15 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,
Ti<0.005, N<=100 ppm, Nb: 0.10-0.40 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
20 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.
25 [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 %: C:
0.02-0.10, 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, Ti<0.005, N<=100 ppm, Nb: 0.10-0.40 and the balance being
Iron (Fe) and unavoidable impurities. The high strength hot rolled steel comprises a
30 microstructure of 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized
micro alloyed Nb precipitates.
5
[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 ≥ 600 MPa.
In an embodiment, the high strength hot rolled steel has a tensile strength in the range of 600-
5 750 MPa.
[0017] In an embodiment, the high strength hot rolled steel has a yield strength ≥ 450 MPa,
a uniform elongation ≥ 10%, a hardness ≥ 190 Hv.
[0018] In an embodiment, the Nb content of the high strength hot rolled steel is kept in the
concentration range of 0.10-0.40 wt.% to form Nb-precipitates in the form of carbides and
10 carbonitrides. The fine nanometer sized precipitates have high H-trapping ability and provide
excellent resistance to hydrogen embrittlement.
[0019] In an embodiment, the C content of the high strength hot rolled steel is kept in the
concentration range of 0.02-0.10 wt.% to achieve optimum austenite to ferrite transformation
kinetics after the hot rolling along with the calculated quantity of precipitate for good strength
15 in the final microstructure along with excellent H trapping ability.
[0020] In an embodiment, the high strength hot rolled steel comprises the composition
expressed in weight %: C - 0.026, Mn – 0.12, S - 0.008, P - 0.014, Si - 0.042, Cr - 0.023, Nb -
0.35, N – 64 ppm, V – 0.001, Ni – 0.035, Al – 0.002, Cu – 0.006, Mo – 0.001, Ti – 0.001, and
the balance being Iron (Fe) and unavoidable impurities.
20 [0021] In an embodiment, the high strength hot rolled steel has a tensile strength in the range
650 – 736 MPa and hardness ≥ 200 Hv.
[0022] 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: 0.02-0.10, P<0.02, Cr<0.1, V<0.005, Mn: 0.1 – 1.0, Si: 0.001-0.4,
25 Ni: 0.001-0.05, S<0.01, Al<0.05, Cu<0.01, Mo<0.06, Ti<0.005, N<=100 ppm, Nb: 0.10-0.40
and the balance being Iron (Fe) and unavoidable impurities. The method also comprises
reheating the steel slab to a temperature greater than 1250
oC. 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 50o
30 C/s till a coiling temperature (TCT) is reached. TCT varies in the range 600
to 650
oC. The method also comprises coiling the steel sheet at the coiling temperature TCT to
obtain the high strength hot rolled steel sheet.
6
[0023] 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 Nb
precipitates.
[0024] In an embodiment, the ferrite is precipitation strengthened and an average grain size
5 of 10 to 50 µm.
[0025] In an embodiment, the high strength hot rolled steel has a tensile strength ≥ 600 MPa
and yield strength ≥ 450 MPa, hardness ≥ 190 Hv, and a uniform elongation ≥ 10 %.
[0026] 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.
[0027] In an embodiment, the cooling rate is kept higher than 50
o
10 C/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.
[0028] 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.
15 [0029] Other features and aspects of this disclosure will be apparent from the following
description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] 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;
20 [0031] 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;
[0032] Figure 3a illustrates a final microstructure of high strength hot rolled steel sheet of
inventive example composition coiled at TCT of 600
oC, during manufacture of the high strength
25 hot rolled steel sheet, according to an embodiment of present invention;
[0033] Figure 3b illustrates a final microstructure of high strength hot rolled steel sheet of
inventive example composition coiled at TCT of 650
oC, during manufacture of the high strength
hot rolled steel sheet, according to an embodiment of present invention;
7
[0034] Figure 3c illustrates a final microstructure of steel sheet of comparative example
composition coiled at TCT of 600
oC, during manufacture of the high strength hot rolled steel
sheet, according to an embodiment of present invention;
[0035] Figure 3d illustrates a final microstructure of steel sheet of comparative example
composition coiled at TCT of 650
o
5 C, during manufacture of the high strength hot rolled steel
sheet, according to an embodiment of present invention;
[0036] 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;
10 [0037] Figure 4b illustrates a graphical representation of component fractions of Nb, C and
N in the precipitate in the inventive example versus temperature, according to an embodiment
of present invention;
[0038] Figure 4c illustrates a graphical representation of component fractions of V, C and N
in the precipitate in the comparative example versus temperature, according to an embodiment
15 of present invention;
[0039] 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
[0040] Figures 6a and 6b illustrate TEM images showing the nano-sized micro alloyed
20 precipitates in both the inventive and comparative examples respectively, according to an
embodiment of the present invention.
[0041] 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
25 [0042] 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,
30 equivalents, and alternatives falling within the spirit and scope of the present disclosure as
defined by the appended claims.
8
[0043] 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.
5 [0044] 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
10 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.
[0045] 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
15 succession may, in fact, be executed concurrently or may sometimes be executed in the reverse
order, depending upon the functionality/acts involved.
[0046] 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
20 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.
[0047] 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
25 %: Carbon (C): 0.02% - 0.10%, 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): 0.10% - 0.40%, Titanium (Ti):
30 maximum 0.005%, 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. In the illustrated
example, the high strength hot rolled steel comprises nano sized Nb precipitates in the
9
microstructure. In the illustrated example, the ferrite is precipitation strengthened and has a
grain size of 10 to 50 µm. The high strength hot rolled steel has a tensile strength ≥ 600 MPa.
In the illustrated example, the high strength hot rolled steel has a tensile strength in the range
of 600-750 MPa. The high strength hot rolled steel has a yield strength ≥ 450 MPa, hardness ≥
5 190 Hv and a uniform elongation ≥ 10%. The high strength hot rolled steel has an excellent Htrapping ability and provides excellent resistance to hydrogen embrittlement.
[0048] The high strength hot-rolled steel sheet with a minimum 600 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
10 configured to carry petroleum and natural gas products under high pressure conditions.
[0049] 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: 0.02-0.10,
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,
15 Cu<0.01, Mo<0.06, Ti<0.005, N<=100 ppm, Nb: 0.10-0.40 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.
[0050] At step (104), the method (100) comprises reheating the steel slab (steel casting) to a
20 temperature greater than 1250˚C. In the illustrated example, the slab is reheated to temperature
ranging between 1250 to 1350
oC 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 Nb that may have formed in the preceding processing steps.
A reheating temperature greater than 1350oC is also undesirable because it may lead to
25 excessive grain coarsening of austenite and/or scale loss. In one example, the casted steel may
be heated in a furnace.
[0051] 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
30 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
10
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.
[0052] At step (108), the method (100) comprises cooling at a cooling rate greater than
50
oC/s till a coiling temperature (TCT) is reached. The TCT varies in the range 600 to 650
oC.
5 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 50
oC/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.
10 [0053] At step (110), the method (100) comprises coiling the steel sheet at the coiling
temperature. Coiling is carried out at a temperature 600 ≤ TCT ≤ 650 (°C). Coiling below 600
oC
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,
15 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 Nb precipitates. The high strength
hot rolled steel sheet exhibits tensile strength greater than 600 MPa, uniform elongation greater
than 10%, and yield strength greater than 450 MPa. In the illustrated example, the thickness of
20 the steel sheet is in the range of 6 mm - 24 mm.
[0054] According to the disclosed invention method (100), it is possible to manufacture the
high strength hot-rolled steel sheet with a minimum 600 MPa tensile strength consisting of
ferrite + pearlite + nano-sized micro alloyed Nb precipitate microstructure which has excellent
H-trapping ability and provides excellent resistance to hydrogen embrittlement. Such a steel
25 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.
[0055] 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
30 enhancing properties.
[0056] C: 0.02-0.10%: 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
11
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
5 desirable. Also, a higher C causes poor weldability in the final product. Therefore, the C content
in the present invention is restricted to 0.02-0.10 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.
[0057] Si: 0.001-0.4%: Si is a good solid solution strengthening element in steel. Si is also a
10 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.%.
[0058] Mn: 0.1-1.0%: Mn is an efficient austenite stabilizer. Mn also has a high solid solution
15 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
20 restricted to 0.1-1.0 wt.%.
[0059] Ti: <0.005%: Ti forms Ti (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, Ti must remain in austenite solid solution for these beneficial effects to take
place and the Ti-precipitation should not take place before the hot-rolling commences. The Ti
25 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 Nb-carbonitrides. Therefore,
costly addition of Ti is avoided, and its content is restricted to <0.005 wt.%.
[0060] 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.
30 P is an undesired element. Therefore, the P content is being restricted to a maximum of 0.02
wt.%.
12
[0061] 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.
[0062] 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
5 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.
[0063] 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
10 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.%.
[0064] Nb: 0.10-0.40 %: Nb is the most important alloying element for the formation of Nbprecipitates in the form of carbides and carbonitrides. Nb precipitates in the form of NbC and
15 NbCN. Precipitation of these shall be avoided in the slab reheating stage and before the hot
rolling otherwise coarse incoherent precipitates will form which has relatively lower Htrapping 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
20 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 Nb is added in the steel in the concentration range of 0.10-0.40 wt%.
[0065] 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
25 mentioned above.
[0066] 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 Nb 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
30 i.e., ferrite, pearlite and micro alloyed Nb precipitates are described below.
13
[0067] 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.
[0068] Pearlite: A small amount of pearlite (10% maximum) is observed in the final
5 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
Nb-precipitate. Therefore, a maximum of 10% pearlite was formed in the microstructure.
[0069] Nb-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
10 when moving dislocation are impeded by the harder precipitates in the softer ferrite matrix.
Moreover, the precipitates are fine in size (nanometer). 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 650-750
MPa is achieved in the final hot-rolled steel sheet.
15 Examples
[0070] 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.
20 [0071] 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
25 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 0.026 0.014 0.023 0.001 0.12 0.042 0.035 0.008 0.002 0.006
Comparative 0.023 0.013 0.028 0.24 0.15 0.10 0.035 0.010 0.030 0.005
30
14
Mo Nb Ti N
0.001 0.35 0.001 64
0.003 0.012 0.001 36
Table: 1
[0072] The rolled sample were then austenitized at 1300oC for 15 min for the Nb precipitate
dissolution (slab reheating). Then the samples were transferred to salt bath furnaces kept
temperatures 600
oC and 650
o
5 C for 24 h and 3 h, respectively, to emulate the coiling conditions
at the hot-strip mill as well as allow for the Nb-carbide and carbonitride precipitation to be
complete (herein the salt bath furnace is used to emulate the ROT conditions in lab scale).
[0073] 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 600
o
10 C.
Figure 3b illustrates a final microstructure of high strength hot rolled steel sheet of inventive
example composition coiled at TCT of 650
oC. Figure 3c illustrates a final microstructure of high
strength hot rolled steel sheet of comparative example composition coiled at TCT of 600
oC.
Figure 3d illustrates a final microstructure of high strength hot rolled steel sheet of comparative
example composition coiled at TCT of 650o
15 C. 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.
[0074] 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
20 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
25 presence of microalloying element as the major constituents. The component fractions of Nb,
C and N in the precipitate in the inventive example and the component fractions of V, C and N
in the precipitate in the comparative example are shown in Figures 4b and 4c respectively.
[0075] 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
30 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
15
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 1100o
5 C, 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 Htrapping 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
10 the H-trapping ability of the inventive example is more than that of the comparative example.
[0076] 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
15 Table 2.
Sample Coiling
Simulation
Temperature
(
oC)
Hardness (HV) Estimated tensile
strength (MPa)
Inventive example 600 230 736
Inventive example 650 213 682
Comparative example 600 211 675
Comparative example 650 193 618
Table: 2
[0077] It can be clearly noted that the inventive examples have achieved the minimum tensile
20 strength of 600 MPa, and hardness ≥ 190 Hv.
[0078] 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).
25 [0079] The present invention provides the high strength hot rolled steel comprising ferrite +
pearlite + nano-sized micro alloyed Nb precipitate microstructure. The nano-sized Nb
precipitates present in the steel provides excellent H-trapping ability, thereby providing
16
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
5 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.
10 [0080] 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.
15 [0081] 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
20 in the art without departing from the scope of the present disclosure as encompassed by the
following claims.
[0082] 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
25 unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and
others.
[0083] 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
30 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.
17
We Claim:
1. A high strength hot rolled steel having excellent resistance to hydrogen embrittlement
and provides excellent H-trapping ability, the high strength hot rolled steel comprises
the following composition expressed in weight %:
5 Carbon (C): 0.02% - 0.10%,
Manganese (Mn): 0.1% - 1.0%,
Sulphur (S): maximum 0.01%,
Phosphorus (P): maximum 0.02%,
Nitrogen (N): maximum 100 ppm,
10 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%,
15 Copper (Cu): maximum 0.01%,
Molybdenum (Mo): maximum 0.06%
Niobium (Nb): 0.10% - 0.40%, Titanium (Ti): maximum 0.005%, 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,
20 and maximum 0.005% nano-sized micro alloyed Nb precipitates.
2. The high strength hot rolled steel as claimed in the claim 1, wherein the ferrite is
precipitation strengthened and has a grain size of 10 to 50µm.
3. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength
hot rolled steel has a tensile strength ≥ 600 MPa.
25 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 600-750 MPa.
5. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength
hot rolled steel has a yield strength ≥ 450 MPa, a uniform elongation ≥ 10%, and a
hardness ≥ 190 Hv.
30 6. The high strength hot rolled steel as claimed in the claim 1, wherein the Nb content of
the high strength hot rolled steel is kept in the concentration range of 0.10-0.40 wt.%
to form Nb-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.
18
7. The high strength hot rolled steel as claimed in the claim 1, wherein the C content of
the high strength hot rolled steel is kept in the concentration range of 0.02-0.10 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
5 microstructure along with excellent H trapping ability.
8. 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 - 0.026, Mn –
0.12, S - 0.008, P - 0.014, Si - 0.042, Cr - 0.023, Nb - 0.35, N – 64 ppm, V – 0.001, Ni
– 0.035, Al – 0.002, Cu – 0.006, Mo – 0.001, Ti – 0.001, and the balance being Iron
10 (Fe) and unavoidable impurities.
9. The high strength hot rolled steel as claimed in the claim 8, wherein the high strength
hot rolled steel has a hardness ≥ 200 Hv.
10. The high strength hot rolled steel as claimed in the claim 8, wherein the high strength
hot rolled steel has a tensile strength in the range 650 – 736 MPa.
15 11. 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: 0.02-0.10,
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, Ti<0.005, N<=100 ppm, Nb: 0.10-0.40 and the balance
20 being Iron (Fe) and unavoidable impurities;
reheating the steel slab to a temperature greater than 1250
oC;
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 600 to 650
o
25 C; and
coiling the steel sheet at the coiling temperature TCT to obtain the high strength
hot rolled steel sheet.
12. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in
the claim 11, wherein the high strength hot rolled steel comprises a microstructure of
30 90-100% ferrite, maximum 10% pearlite, and maximum 0.005% nano-sized micro
alloyed Nb precipitates.
13. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in
the claim 11, wherein the ferrite is precipitation strengthened and has a grain size of 10
to 50µm.
19
14. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in
the claims 11 to 13, wherein the high strength hot rolled steel has a tensile strength ≥
600 MPa, yield strength ≥ 450 MPa, a hardness ≥ 190 Hv and a uniform elongation ≥
10%.
5 15. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in
the claim 11, wherein the steel slab is reheated to a temperature above 1250
oC-1350oC
for a duration of 20 minutes to 2 hours depending on the slab thickness.
16. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in
the claims 11 to 15, wherein the thickness of the steel sheet is in the range of 6 mm -
10 24 mm.
17. A line pipe produced from the high strength hot rolled steel as claimed in the claims 1
to 16, wherein the line pipe is configured to carry petroleum and natural gas products
under high pressure conditions.