Description:2
TECHNICAL FIELD
[0001] The present disclosure in general relates to the field of material science and metallurgy.
Particularly, but not exclusively, the present disclosure relates to a high-strength hot-rolled
steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the
high-strength hot-rolled steel sheet which exhibits tensile strength greater than 600 MPa and
yield strength ranging from 450 MPa – 550 MPa with total elongation greater than 15%.
BACKGROUND OF THE DISCLOSURE
[0002] Steel is an alloy of iron, carbon and other elements such as Phosphorous (P), Sulphur
(S), Nitrogen (N), Manganese (Mn), Silicon (Si), Chromium (Cr), etc. Because of its high
strength and low cost, steel may be considered as a major component in wide variety of
applications. Some of the applications of the steel may include buildings, maritime vehicles,
tools, automobiles, machines, bridges and numerous other industrial applications. The steel
obtained from steel making process may not possess all the desired properties. Therefore, the
steel may be subjected to secondary processes such as heat treatment for controlling material
properties to meet various needs in the intended applications.
[0003] Generally, heat treatment may be carried out using techniques including but not limiting
to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process,
the material undergoes a sequence of heating and cooling operations, thus, the microstructures
of the steel may be modified during such operation. As a result of heat treatment, the steel
undergoes phase transformation, influencing mechanical properties like strength, ductility,
wear resistance, toughness, hardness, drawability, and amongst others. The purpose of heat
treatment is to increase service life of a product by improving its strength, hardness etc., or
prepare the material for improved manufacturability.
[0004] In the automotive industry, strict norms of legislation regarding fuel consumption and
emission have forced the industry to develop lighter, more fuel-efficient vehicles. Some of the
objectives in the automobile industry are reduction in vehicle weight and improvement in
safety. In order to achieve such objections, the automobile manufacturers may prefer high
strength steel materials having formability properties, to make the parts such as side body in
white, body inner panels, door inner panels and the like.
3
[0005] There has been an ever-increasing demand for development of high strength steel sheets
to ensure safety of passengers in the vehicle, in case of collision. One such method for
increasing energy absorption capacity of the vehicle is to absorb the energy generated during a
collision by suppressing large deformation of a structural component during the collision and
suppress the transmission of energy around the vehicle cabin. The structural component is
required to form a complicated shape and hence to develop a steel sheet that improves these
collision characteristics without impairing formability.
[0006] Conventionally, the high strength steel such as dual phase steel (ferrite +
martensite/bainite), single phase precipitation strengthened steel and the like have been
developed to improve collision resistance of the vehicles. However, such steels are generally
expensive due to close tolerance of parameters required to produce at an industrial scale and
high alloying quantity and associated costs. On the other hand, the precipitation strengthened
steel may suffer from yield point phenomenon (YPP), which may result in conditions such as
coil breaks, edge cracking, stretcher strain, and reel kinks, which are undesirable. Sometimes
conditions are severe enough to affect flatness and results in poor surface finish in the final
product after forming.
[0007] In some of the conventional method of manufacturing steel disclose about high strength
steel with yield point elongation as low as 0.5 %. However, such a small amount of yield point
elongation may deteriorate the surface quality and compromise quality requirements of the
automotive industry.
[0009] Another document discloses a method for manufacturing a steel with lower yield point
elongation, but the procedure adopts to have second phase which may result in lower yield
strength and lower hole expansion ratio.
[0010] The present disclosure is directed to overcome one or more limitations stated above or
any other limitation associated with the prior arts.
SUMMARY OF THE DISCLOSURE
[0011] One or more shortcomings of the prior art are overcome by method as disclosed and
additional advantages are provided through the method as described in the present disclosure.
4
[0012] Additional features and advantages are realized through the techniques of the present
disclosure. Other embodiments and aspects of the disclosure are described in detail herein and
are considered a part of the claimed disclosure.
[0013] In one non-limiting embodiment, a method for manufacturing high-strength hot-rolled
steel sheet is disclosed. The method comprising casting, a steel slab comprising of a
composition of: Carbon (C) in a range of 0.03 wt.% to 0.10 wt.%, Silicon (Si) less than 0.10
wt.%, Manganese (Mn) in a range of 0.2 wt.% to 2 wt.%, Chromium (Cr) in a range of 0.01
wt.% to 0.7 wt.%, Phosphorus (P) up-to 0.025 wt%, Sulphur (S) up-to 0.008 wt%, Aluminium
(Al) in a range of 0.01 to 0.1 wt.%, Niobium (Nb) in a range of 0.005 wt.% to 0.04 wt.%,
Titanium (Ti) in a range of 0.005 wt.% to 0.10 wt.%, Molybdenum (Mb) in a range of 0.005
wt.% to 0.2 wt.%, vanadium (V) up to 0.06 wt.%, nitrogen (N) up to 0.007 wt.%, and Iron (Fe)
being remainder of the composition along with incidental elements. Then, reheating the steel
slab to a 1100-1300 oC for 20-120 min. Further, hot working the reheated steel slab at a second
predetermined temperature to form a steel sheet. Then, first cooling the hot worked steel sheet
to a 450-585 oC (at a cooling rate 40-70 oC/s) and second cooling the steel sheet at a 450-585
oC for a 5-15 sec. After cooling, coiling the steel sheet at a fourth predetermined temperature
to obtain a high-strength hot-rolled stee sheet including ferrite, bainite and pearlite
microstructure.
[0014] In an embodiment, the high-strength hot-rolled steel sheet exhibits tensile strength
greater than 600 MPa and yield strength ranging from 450 MPa – 550 MPa.
[0015] In an embodiment, the high-strength hot-rolled steel sheet exhibits zero yield point
elongation.
[0016] In an embodiment, the high-strength hot-rolled steel sheet exhibits uniform elongation
greater than 10% and total elongation greater than 15%.
[0017] In an embodiment, the high-strength hot-rolled steel sheet exhibits yield strength to
tensile strength ratio ranging from 0.8 – 0.9.
[0018] In an embodiment, the high-strength hot-rolled steel sheet exhibits grain size ranging
from 2 – 3 μm.
[0019] In an embodiment, the steel slab is hot charged into a furnace for heating.
5
[0021] In an embodiment, the hot working is performed in a finish rolling mill, and the second
predetermined temperature ranges from 830 ℃ - 890℃.
[0022] In an embodiment, reduction ratio of the steel slab after finish rolling ranges from 30%
- 40%.
[0023] In an embodiment, the cooling rate ranges from 40℃/s - 70℃/s.
[0024] In an embodiment, the predetermined time ranges from 5 seconds to 15 seconds.
[0025] In an embodiment, the fourth predetermined temperature ranges from 400℃ to 500℃.
[0026] In an embodiment, the microstructure of the high-strength hot-rolled steel sheet consists
of ferrite of 90 - 95 vol.%, bainite of less than 5 vol.% and balance being pearlite
microstructure.
[0027] In an embodiment, the high-strength hot-rolled steel sheet is strengthened by fine
precipitates of Ti, Nb, Mo and V.
[0028] In an embodiment, the high-strength hot-rolled steel sheet exhibits continuous yielding.
[0029] In another non-limiting embodiment of the disclosure, a high strength hot-rolled steel
sheet is disclosed. The steel sheet comprising composition of Carbon (C) in a range of 0.03
wt.% to 0.10 wt.%, Silicon (Si) less than 0.10 wt.%, Manganese (Mn) in a range of 0.2 wt.%
to 2 wt.%, Chromium (Cr) in a range of 0.01 wt.% to 0.7 wt.%, Phosphorus (P) up-to 0.025
wt%, Sulphur (S) up-to 0.008 wt%, Aluminium (Al) in a range of 0.01 to 0.1 wt.%, Niobium
(Nb) in a range of 0.005 wt.% to 0.04 wt.%, Titanium (Ti) in a range of 0.005 wt.% to 0.10
wt.%, Molybdenum (Mb) in a range of 0.005 wt.% to 0.2 wt.%, vanadium (V) up to 0.06 wt.%,
nitrogen (N) up to 0.007 wt.%, and Iron (Fe) being remainder of the composition along with
incidental elements. The high strength hot-rolled steel sheet includes ferrite, bainite, and
pearlite microstructure.
[0030] In an embodiment, the high-strength hot-rolled steel sheet exhibits tensile strength
greater than 600 MPa and yield strength ranging from 450 MPa – 550 MPa.
6
[0031] In an embodiment, the high-strength hot-rolled steel sheet exhibits zero yield point
elongation.
[0032] In an embodiment, the high-strength hot-rolled steel sheet exhibits uniform elongation
greater than 10% and total elongation greater than 15%.
[0033] In an embodiment, the high-strength hot-rolled steel sheet exhibits yield strength to
tensile strength ratio ranging from 0.8 – 0.9.
[0034] In an embodiment, the high-strength hot-rolled steel sheet exhibits grain size ranging
from 2 – 3 μm.
[0035] In an embodiment, the microstructure of the high-strength hot-rolled steel sheet consists
of ferrite of 90 - 95 vol.%, bainite of less than 5 vol.% and balance being pearlite
microstructure.
[0036] In an embodiment, the high-strength hot-rolled steel sheet is strengthened by fine
precipitates of Ti, Nb, Mo and V.
[0037] In an embodiment, the high-strength hot-rolled steel sheet exhibits continuous yielding.
[0038] It is to be understood that the aspects and embodiments of the disclosure described
above may be used in any combination with each other. Several of the aspects and embodiments
may be combined to form a further embodiment of the disclosure.
[0039] The foregoing summary is illustrative only and is not intended to be in any way limiting.
In addition to the illustrative aspects, embodiments, and features described above, further
aspects, embodiments, and features will become apparent by reference to the drawings and the
following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
[0040] The novel features and characteristics of the disclosure are set forth in the appended
description. The disclosure itself, however, as well as a preferred mode of use, further
objectives, and advantages thereof, will best be understood by reference to the following
detailed description of an illustrative embodiment when read in conjunction with the
accompanying figures. One or more embodiments are now described, by way of example only,
7
with reference to the accompanying figures wherein like reference numerals represent like
elements and in which:
[0041] Figure 1 is a flowchart illustrating a method for producing high strength hot rolled steel
sheet, according to an exemplary embodiment of the present disclosure.
[0042] Figure 2 illustrates a graphical representation of cooling profile followed during the
manufacturing of high strength hot rolled steel sheet.
[0043] Figure 3 illustrates graphical representation of stress versus strain, obtained during
tensile test of the steel, according to an exemplary embodiment of the present disclosure.
[0044] Figure 4 illustrates an optical micrograph of the high strength hot rolled steel sheet,
according to an exemplary embodiment of the present disclosure.
[0045] Figure 5 illustrates graphical representation of size distribution of precipitates,
according to an exemplary embodiment of the present disclosure.
[0046] Figure 6 illustrates an optical image of the high strength hot rolled steel sheet, according
to an exemplary embodiment of the present disclosure.
[0047] Figure 7 illustrates SEM (Scanning Electron Microscope) micrograph of the high
strength hot rolled steel sheet, according to an exemplary embodiment of the present disclosure.
[0048] Figure 8a illustrates bright field TEM (Transmission Electron Micrograph) of the high
strength hot rolled steel sheet, according to an exemplary embodiment of the present disclosure.
[0049] Figure 8b illustrates selected area diffraction (SAD) pattern of the high strength hot
rolled steel sheet confirming the presence of precipitates in the ferrite matrix, according to an
exemplary embodiment of the present disclosure.
[0050] The figures depict embodiments of the disclosure for purposes of illustration only. One
skilled in the art will readily recognize from the following description that alternative
embodiments of the methods illustrated herein may be employed without departing from the
principles of the disclosure described herein.
DETAILED DESCRIPTION
8
[0051] The foregoing has broadly outlined the features and technical advantages of the present
disclosure in order that the detailed description of the disclosure that follows may be better
understood. Additional features and advantages of the disclosure will be described hereinafter
which form the subject of the description of the disclosure. It should also be realized by those
skilled in the art that such equivalent methods do not depart from the scope of the disclosure.
The novel features which are believed to be characteristic of the disclosure, as to method of
operation, together with further objects and advantages will be better understood from the
following description when considered in connection with the accompanying figures. It is to
be expressly understood, however, that each of the figures is provided for the purpose of
illustration and description only and is not intended as a definition of the limits of the present
disclosure.
[0052] In the present disclosure, the word "exemplary" is used herein to mean "serving as an
example, instance, or illustration." Any embodiment or implementation of the present subject
matter described herein as "exemplary" is not necessarily to be construed as preferred or
advantageous over other embodiments.
[0053] While the disclosure is susceptible to various modifications and alternative forms,
specific embodiment thereof has been shown by way of example in the drawings and will be
described in detail below. It should be understood, however that it is not intended to limit the
disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all
modifications, equivalents, and alternative falling within the spirit and the scope of the
disclosure.
[0054] The terms “comprises”, “comprising”, or any other variations thereof, are intended to
cover a non-exclusive inclusion, such that a method that comprises a list of acts does not
include only those acts but may include other acts not expressly listed or inherent to such
method. In other words, one or more acts in a method proceeded by “comprises… a” does not,
without more constraints, preclude the existence of other acts or additional acts in the method.
[0055] Embodiments of the present disclosure discloses a high strength hot rolled steel sheet
and a method for manufacturing or producing a high strength hot rolled steel sheet. The
manufacturing of advanced high strength steels generally exhibits yield point elongation, and
involves additional method steps such as cold rolling, skin pass rolling followed by coating
which may increase the overall cost of the steel. The present disclosure discloses a high strength
9
hot rolled steel sheet which exhibits zero yield point elongation and minimizes the method
steps involved during manufacturing. Further, the high strength hot rolled steel sheet exhibits
continuous yielding. Accordingly, the method of present disclosure discloses a high strength
hot rolled steel sheet, exhibits tensile strength greater than 600 MPa, yield strength ranging
from 450 MPa – 550 MPa and total elongation greater than 15%.
[0056] Henceforth, the present disclosure is explained with the help of figures for a method of
manufacturing high strength hot rolled steel sheet. However, such exemplary embodiments
should not be construed as limitations of the present disclosure, since the method may be used
on other types of steels where such need arises. A person skilled in the art can envisage various
such embodiments without deviating from scope of the present disclosure.
[0057] Figures 1 and 2 are exemplary embodiments of the present disclosure illustrating a
flowchart depicting a method for manufacturing high strength hot rolled steel sheet and a
graphical representation of cooling profile followed during the method of manufacturing high
strength hot rolled steel sheet. In the present disclosure, mechanical properties such as strength,
ductility, formability, surface quality, corrosion resistance and of the steel may be improved.
The steel produced by the method of the present disclosure, includes ferrite, bainite and pearlite
microstructure. The method is now described with reference to the flowchart blocks and is as
below. The order in which the method is described is not intended to be construed as a
limitation, and any number of the described method blocks can be combined in any order to
implement the method. Additionally, individual blocks may be deleted from the methods
without departing from the scope of the subject matter described herein. The method is
particularly applicable to high strength hot rolled steel sheet and it may also be extended to
other type of steels as well.
[0058] At block 101, a steel of desired alloy composition is formed by any of the manufacturing
process including but not limiting to casting process. In an embodiment, the steel is made in
the form of slabs, and the alloy may be prepared in at least one air-melting furnace, a vacuum
furnace and the like. The steel slab may have composition of in weight percentage (wt.%) of:
Carbon (C) in a range of 0.03 wt.% to 0.10 wt.%, Silicon (Si) less than 0.10 wt.%, Manganese
(Mn) in a range of 0.2 wt.% to 2 wt.%, Chromium (Cr) in a range of 0.01 wt.% to 0.7 wt.%,
Phosphorus (P) up-to 0.025 wt.%, Sulphur (S) up-to 0.008 wt.%, Aluminium (Al) in a range of
0.01 to 0.1 wt.%, Niobium (Nb) in a range of 0.005 wt.% to 0.04 wt.%, Titanium (Ti) in a range
of 0.005 wt.% to 0.10 wt.%, Molybdenum (Mb) in a range of 0.005 wt.% to 0.2 wt.%, vanadium
10
(V) up to 0.06 wt.%, nitrogen (N) up to 0.007 wt.%, and Iron (Fe) being remainder of the
composition along with incidental elements. In an embodiment, lower limit of elements such
as silicon (Si), phosphorus (P), sulphur (S), vanadium (V) and nitrogen (N) may be negligible
for consideration.
[0059] The method then includes the step of heating as shown in block 102. After casting the
steel slab with the desired composition, the steel slabs may be reheated in a furnace to a 1100-
1300 oC for 20-120 min. In an embodiment, the steel slab may be hot charged into the
furnace for reheating and the 1100-1300 oC may be ranges from 1100 ºC to 1300 ºC.
[0060] The method further includes a step or a stage of hot working the steel slab at a second
predetermined temperature [shown in block 103] immediately after heating. The hot working
process may be a hot rolling process. Rolling is a mechanical process, which involves passing
the metal stock through one or more pairs of rolls to refine the grain size in the structure. As
an example, the steel slab may be passed through the one or more pair of rolls for at least 5 to
6 times. In an embodiment, during hot rolling process the thickness of the steel may be reduced
up-to 40% (i.e., overall reduction ratio may be in a range of 25% to 40% in the last three stands
during finishing mill). In an embodiment, the rolling process may be carried out in a finish
rolling mill at a second predetermined temperature ranging from 830 ºC – 890 ºC.
[0061] Referring to block 104 in combination with Figure. 2, the method comprises of first
cooling the hot worked steel sheet to a 450-585 oC at a 40℃/s - 70℃/s. In an embodiment,
the hot rolled steel sheet may be subjected to first cooling process which may be a rapid cooling
process carried out on a run out table at 40 ºC/s to 70 ºC/s. The hot rolled steel sheet may be
cooled to 450 ºC to 585 ºC. At block 105, the method comprises of second cooling the steel
sheet for a predetermined time. In an embodiment, the second cooling may be natural cooling
in air and the predetermined time ranges from 5 seconds to 15 seconds.
[0062] After second cooling process, the cooled steel sheet is subjected to coiling [shown in
block 106] at a fourth predetermined temperature. In an embodiment, the fourth predetermined
temperature ranges from 400 ºC to 500 ºC. After coiling, the coiled steel sheets may be cooled
in air to obtain the high strength hot rolled steel sheet.
[0063] The steel processed by the method of the present disclosure results in microstructural
changes to form high strength hot rolled steel sheet. A schematic diagram of the cooling
11
employed in the method of manufacturing is shown in Figure. 2. This ensures that the
microstructure of the high strength hot rolled steel sheet comprises ferrite, bainite and pearlite
microstructure. In an embodiment, the high-strength hot-rolled steel sheet exhibits grain size
ranging from 2 – 3 μm.
[0064] In an embodiment, the cooling rate may be maintained greater than 40 ºC/s to obtain
fine grains of ferrite. Higher cooling rate may result in lowering the ferrite start temperature
which may lead to the refinement of the ferrite grain size. By increasing the cooling rate and
controlling rolling schedule, the grain size of 2-3 μm may be obtained. Further, the cooling rate
may not be more than 70°C/s as the desired amount of ferrite may not be formed. As shown in
Figure 2, the 5-15 sec of the second cooling process i.e., natural cooling may be ranging from
5 seconds to 15 seconds, in which cooling below 5 seconds may not form enough ferrite and
cooling for more than 15 seconds may result in lower coiling temperature. During the period
of natural cooling, austenite may transform to ferrite and also results in the formation of
carbonitrides during phase transformation. Further, depending on the time period of natural
cooling, small amount of either pearlite or bainite may form.
[0065] As shown in Figure 2, coiling of the steel sheet after natural cooling may ensure to
precipitate out if any excess Nb, Ti, Mo, V may not form precipitates during phase
transformation. The coiling step may enhance the consumption of excess solutes (Nb, Ti, Mo,
V) to form carbides and to consume the total carbon from ferrite. Further, the fine precipitates
in the of carbides formed during coiling stage may result in excess dislocations as the
precipitates formed would be semi-coherent which may require geometrically necessary
dislocations. This is to be appreciated that when the precipitates are small, the interfaces are
semi-coherent - results in large dislocations. In case of large precipitates, the interfaces are
incoherent and hence a smaller number of dislocations are generated which further results in
yield point phenomenon (YPP).
This is also to be appreciated that excess dislocations due to formation of very fine carbides
(less than 5 nm) which may results no yield point elongation. The locking and unlocking result
in yield point elongation. The generated dislocations as per the embodiment of the present
disclosure are more in number compared to the conventional precipitation strengthened steel
in which precipitates are of 10 to 20 nm.
12
[0066] Strength may be primarily obtained from the strength of ferrite, bainite and pearlite
microstructures. The steel processed by the method of present disclosure comprises ferrite of
90 – 95 vol.%, bainite of less than 5 vol.%, and balance being pearlite microstructure. In an
embodiment of the present disclosure, the high strength hot rolled steel sheet including ferrite,
bainite and pearlite microstructure exhibits tensile strength greater than 600 MPa and upto
800MPa and yield strength ranging from 450 MPa – 550 MPa. Further, the high strength hot
rolled steel sheet exhibits uniform elongation greater than 10% and upto 15%, and total
elongation greater than 15% and upto 30%. Also, the high- hot rolled steel exhibits yield
strength to tensile strength ratio ranging from 0.8 – 0.9.
[0067] In an embodiment of the present disclosure, the high strength hot rolled steel sheet may
be strengthened by the precipitates of Ti, Nb, Mo and V. The size of such precipitates may be
less than 5 nm and may be between 1 nm to 5 nm. Further, the high strength hot rolled steel
sheet exhibits zero yield point elongation or yield point phenomena and continuous yielding.
Due to such nano size precipitates and the secondary phase like bainite microstructure, may
result in excess dislocations and hence, the high strength hot rolled steel sheet exhibits zero
yield point elongation or yield point phenomena.
[0068] The following portion of the present disclosure provides details about the proportion of
each alloying element in a composition of the steel and their role in enhancing properties.
[0069] Carbon (C) may be used in the range of 0.03 wt.% to 0.1 wt.%. Carbon is one of the
most effective and economical strengthening elements. Carbon combines with Nb, Ti, Mo, or
V to form carbides or carbonitrides which bring about precipitation strengthening. This requires
a minimum of 0.03%C in the steel. However, to have good weldability and formability, the
carbon content has to be restricted to less than 0.1%.
[0070] Silicon (Si) may be used in the range of less than 0.1 wt.%. Silicon like Mn is a very
efficient solid solution strengthening element. However, Si may lead to surface scale problems
in hot rolling due to formation of complex Fe and Si oxides and hence Si should be restricted
to less than 0.1% to prevent the formation of surface scales.
[0071] Manganese (Mn) may be used in the range of 0.2 wt.% to 2 wt.%. Manganese not only
imparts solid solution strengthening to the ferrite, but also lowers the austenite to ferrite
transformation temperature thereby refining the ferrite grain size. However, the Mn level
cannot be increased to beyond 2% as at such high levels it enhances centerline segregation
13
during continuous casting. Further, high amount of Mn may increase hardenability and hence
formation of hard phase like martensite.
[0072] Niobium (Nb) may be used in the range of 0.005 wt.% to 0.04 wt.%. Niobium is the
most potent microalloying element for grain refinement even when it is added in very small
amounts. When in solid solution Nb lowers the austenite to ferrite transformation temperature
which not only refines the ferrite grain size but also promotes the formation of lower
transformation products like bainite. However, to ensure the effectiveness of Nb, it should not
be allowed to precipitate before the transformation temperature is reached. To ensure that the
entire Nb content remains in solution before rolling commences and it is alone added, the
maximum Nb content is restricted to 0.04% depending upon the drop out temperature of the
slab after reheating.
[0073] Vanadium (V) may be used up to 0.06 wt.%. Microalloying by Vanadium also leads to
precipitation strengthening as well as grain refinement. The solubility of Vanadium in austenite
is more than that of other microalloying elements and so V is more likely to remain in solution
prior to transformation. During phase transformation, vanadium precipitates as carbides and/or
nitrides, depending on the relative carbon and nitrogen contents, at grain boundaries resulting
in precipitation strengthening as well as grain refinement. To achieve the desired strengthening,
it is required to add either Nb or V. Both can also be added. If V alone is added, it is required
up to 0.06 wt.%.
[0074] Phosphorus (P) may be used up to 0.025 wt.%. Phosphorus content should be restricted
to 0.025 wt.% maximum as higher phosphorus levels can lead to reduction in toughness and
weldability due to segregation of P into grain boundaries.
[0075] Sulphur (S) may be used up to 0.008wt.%. Sulphur content must be limited otherwise
it results in a very high inclusion level that deteriorates formability.
[0076] Nitrogen (N) may be used less than 0.007 wt.%. Too high N content raises the
dissolution temperature of Nb(C, N) and hence reduces the effectiveness of Nb. Reducing
nitrogen levels also positively affects ageing stability and toughness in the heat-affected zone
of the weld seam, as well as resistance to inter-crystalline stress-corrosion cracking. Thus, N
levels should be preferably kept below 0.007 wt.%.
[0077] Aluminium (Al) may be used in the range of 0.01 wt.% to 0.1 wt.%. Aluminium (Al) is
used to remove undesirable oxygen from molten steel and hence steel contains some amount
14
of Al, may be up to 0.05 wt.%. Excess (high) Al in steel making is a major problem as it
decreases hot deformation of cast slab besides nozzle clogging during casting. Therefore, Al
needs to be restricted to 0.1 wt.%.
[0078] Titanium (Ti) may be used in the range of 0.005 wt.% to 0.10 wt.%: Ti is used to mainly
control grain growth during reheating furnace. The carbides or nitrides of Ti reduces growth of
austenite grains in the slab during reheating. However, excess amount can cause formation of
large carbides which can deteriorate formability of the steel. Hence, the amount should be
restricted to 0.10 wt.%
[0079] Molybdenum (Mo) may be used in the range of 0.005 wt.% to 0.2 wt.%, and Chromium
(Cr) may be added in the range of 0.01 wt.% to 0.7 wt.%. Mo and Cr elements are mainly added
to increase hardenability of the steel. This is particularly important in low cooling rate in thick
steel plates. Further, Mo also helps in obtaining fine precipitates of carbonitrides. However,
Mo and Cr decrease weldability and hence need to add in limited quantity. Maximum amount
of Mo and Cr is 0.3 and 0.7, respectively.
Example:
[0080] Further embodiments of the present disclosure will be now described with an example
of a particular composition of the steel. Experiments have been carried out for a specific
composition of the steel formed by using method of the present disclosure. Results have been
compared on various fronts to show the improvement of strength of the steel. The composition
of the steel for which the tests are carried out is as shown in below table 1.
Strip
1 C Mn Si Ti Nb Al P S N Cr, Mo, V
0.065 1.0 0.01 0.03 0.03 0.036 0.02 0.003 <0.005 traces
Table - 1
[0081] In an embodiment of the present disclosure, various experiments were carried out on
the steel sample for composition as mentioned in Table - 1. The composition mentioned in the
above table - 1 may be considered for experimental purposes, while variation in composition
of said elements is being highlighted which produces steel of substantially similar properties
and hence, is considered to be within experimental range of the present disclosure. For
15
conducting the experiment, the steel specimens of pre-determined dimensions may be prepared
by the method of the present disclosure.
A slab (230 mm thick) was casted with the composition mentioned in the table. When the slab
was reached below 600 oC, it was charged into reheating furnace. The reheated slab was
discharged at 1200 oC after 180 min. The slab was further processed including descaling. The
slab was reduced to 44 mm in thickness during roughing mill. Then it was transferred to 7 stand
finishing mill. In the finishing, the thickness was reduced to 9 mm by providing appropriate
reduction at each stand but 30 % reduction in the last three stands. About 10% reduction was
given in the last stand. The finishing rolling temperature was 850 o. Then the strip was cooled
at 40 oC/sec to 570 oC and then allowed to cool at ambient conditions/natural cooling for 8
seconds and coiled at 500 oC.
In an embodiment, the tensile samples (size of gauge length 50 mm following ASTM E8
standard) are prepared keeping the tensile axis parallel to rolling direction. After the
experiments, test results have been compared. In subsequent paragraphs of the disclosure, the
method of carrying out the experiment and test results in conjunction with the figures is
disclosed.
[0082] Referring to Table 2 below, mechanical properties of hot-rolled steel samples
manufactured using the method of the present disclosure is depicted.
Sample ID Description Yield
strength
(MPa)
Tensile
strength
(MPa)
Total
Elongation
(%)
Yield
Strength/Tensile
Strength
Yield
point
elongation
Strip 1 Rolling
Direction
(RD)
500 600 18 0.83 0%
Table 2
[0083] In an embodiment, the above Table 2 indicates the mechanical properties of the steel
sample with the composition mentioned in the Table 1. The steel sample exhibits yield strength
of 500 MPa, tensile strength of 600 MPa, total elongation of 18% and the ratio of yield strength
to tensile strength of 0.83. The mechanical properties of the steel sample mentioned in the
above Table 2 are within the range of properties mentioned in the present disclosure.
16
[0084] Now referring to Figure 3, which is an exemplary embodiment of the present disclosure
illustrating a graph with stress versus strain plot obtained during tensile test of the steel samples
as per Table - 2 above. The tensile test may be carried using standard tensile test samples. As
an example, the test sample may be with a gauge length of 50mm and E8/E8M-09
configuration, in accordance to ASTM standards. As evident from the graph illustrated in
Figure. 3, the steel with composition of Table 1 exhibits zero yield point phenomena without
any serrations after the yield point, and hence exhibits continuous yielding. The presence of
nano precipitates in the steel sample with size less than 5 nm results in large number of
dislocations, due to which the steel sample exhibits continuous yielding.
[0085] Referring to Table 3 below, microstructure of hot-rolled steel samples manufactured
using the method of the present disclosure is depicted.
Sample ID Ferrite (%) Pearlite
(%)
Bainite or
martensite
(%)
Grain Size
(μm)
Hardness
(VHN with
10 kgf)
Precipitate
size
Strip 1 92±3 5±5 <5 3 200±10 <5 nm
Table 3
[0086] From the above Table 3, the microstructure of the steel sample comprises 90 - 95 vol.%,
less than 5 vol.% of bainite or martensite and balance being pearlite microstructure. Further,
the ferrite grain size of the steel sample may be 3 μm and the precipitate size may be less than
5 nm. The steel sample exhibits hardness ranging from 190 – 210 VHN.
[0087] Figure 4 illustrates the optical micrograph of the steel sample in accordance with an
embodiment of the present disclosure. The steel sample may be etched with nital solution, and
the resulting optical micrograph illustrate that the steel sample exhibits ferrite, bainite and
pearlite phases with the precipitates uniformly distributed in the ferrite matrix.
[0088] Now referring to Figure 5, which is an exemplary embodiment of the present disclosure
illustrating a graphical representation of size distribution of precipitate size. The size of the
precipitates may be less than 5 nm and such precipitates with semi-coherent interfaces produce
large of number of dislocations enough to have zero yield point elongation.
17
[0089] The steel sample may be etched with Le Pera solution, and the resulting optical
micrograph illustrate that the steel sample exhibits fine grain size of 2 μm and negligible
amount of second phase. Figure 6 illustrates an optical image of the high strength hot rolled
steel sheet, according to an exemplary embodiment of the present disclosure. The bright areas
represent bainite, while dark areas indicate ferrite and pearlite being non-traceable.
[0090] Figure 7 illustrates the scanning electron microscope (SEM) micrograph of the steel
sample, in accordance with an embodiment of the present disclosure. In an embodiment, the
SEM microstructure illustrate that the steel sample exhibits precipitates distributed
homogeneously in the ferrite matrix, and sub-divide the entire matrix i.e., refinement of the
individual grains. The strengthening of the steel sample mainly contributed from solid solution
elements and microalloying elements which may be exploited during natural cooling and
coiling step. Further, the extent of possible grain refinement, by controlled rolling and cooling
is limited to 2 μm due to which the high strength steel may be obtained.
[0091] As seen from the Figure 8a, the TEM micrograph illustrates that the precipitates are
uniformly distributed in the ferrite matrix. Further, Figure 8b illustrates the SAD (selected area
diffraction) pattern of the precipitate, according to an exemplary embodiment of the present
disclosure. From the Figure 8b, SAD pattern of the precipitates present in the ferrite matrix is
illustrated and the size of the precipitates is obtained.
[0092] It should be understood that the experiments are carried out for a particular composition
of the steel and the results brought out in the previous paragraphs are for the composition shown
in Table – 1. However, this composition should not be construed as a limitation to the present
disclosure as it could be extended to other compositions of the steel as well.
[0066] In an embodiment of the present disclosure, the high strength hot rolled steel sheet of
the present disclosure may be used any application including but not limiting to automotive
applications to manufacture structural components like chassis, pillars, outer and inner panels,
and the like. The high strength hot rolled steel sheet may be used in any other industrial
structural applications.
Equivalents:
[0093] 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
18
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.
[0094] It will be understood by those within the art that, in general, terms used herein, and
especially in the appended claims (e.g., bodies of the appended claims) are generally intended
as "open" terms (e.g., the term "including" should be interpreted as "including but not limited
to," the term "having" should be interpreted as "having at least," the term "includes" should be
interpreted as "includes but is not limited to," etc.). It will be further understood by those
within the art that if a specific number of an introduced claim recitation is intended, such an
intent will be explicitly recited in the claim, and in the absence of such recitation no such intent
is present. For example, as an aid to understanding, the following appended claims may
contain usage of the introductory phrases "at least one" and "one or more" to introduce claim
recitations. However, the use of such phrases should not be construed to imply that the
introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular
claim containing such introduced claim recitation to inventions containing only one such
recitation, even when the same claim includes the introductory phrases "one or more" or "at
least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be
interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite
articles used to introduce claim recitations. In addition, even if a specific number of an
introduced claim recitation is explicitly recited, those skilled in the art will recognize that such
recitation should typically be interpreted to mean at least the recited number (e.g., the bare
recitation of "two recitations," without other modifiers, typically means at least two recitations,
or two or more recitations). Furthermore, in those instances where a convention analogous to
"at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense
one having skill in the art would understand the convention (e.g., "a system having at least one
of A, B, and C" would include but not be limited to systems that have A alone, B alone, C
alone, A and B together, A and C together, B and C together, and/or A, B, and C together,
etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is
used, in general such a construction is intended in the sense one having skill in the art would
understand the convention (e.g., "a system having at least one of A, B, or C" would include but
not be limited to systems that have A alone, B alone, C alone, A and B together, A and C
together, B and C together, and/or A, B, and C together, etc.). It will be further understood by
those within the art that virtually any disjunctive word and/or phrase presenting two or more
alternative terms, whether in the description, claims, or drawings, should be understood to
19
contemplate the possibilities of including one of the terms, either of the terms, or both
terms. For example, the phrase "A or B" will be understood to include the possibilities of "A"
or "B" or "A and B."
[0095] While various aspects and embodiments have been disclosed herein, other aspects and
embodiments will be apparent to those skilled in the art. The various aspects and embodiments
disclosed herein are for purposes of illustration and are not intended to be limiting, with the
true scope and spirit being indicated by the following claims.
Referral Numerals
Referral Numerals Description
101-106 Flowchart blocks
101 Casting stage
102 Reheating stage
103 Hot working stage
104 First cooling stage
105 Second cooling stage
106 Coiling stage , Claims:We claim:
1. A method for manufacturing high-strength hot-rolled steel sheet, the method
comprising:
casting, a steel slab comprising an alloying composition of:
carbon (C) in a range of 0.03 wt.% to 0.10 wt.%,
silicon (Si) less than 0.1 wt.%,
manganese (Mn) in a range of 0.2 wt.% to 2 wt.%,
chromium (Cr) in a range of 0.01 wt.% to 0.7 wt.%,
phosphorous (P) up to 0.025 wt.%,
sulphur (S) up to 0.008 wt.%,
aluminium (Al) in a range of 0.01 to 0.1 wt.%,
niobium (Nb) in a range of 0.005 wt.% to 0.04wt.%,
titanium (Ti) in a range of 0.005 wt.% to 0.10 wt.%,
molybdenum (Mo) in a range of 0.005 wt.% to 0.2 wt.%,
vanadium (V) up to 0.06 wt.%,
nitrogen (N) up to 0.007 wt.%, and
iron (Fe) being remainder of the composition along with incidental
elements; and
reheating, the steel slab to a 1100-1300 oC for a 20 – 120 min;
hot working, the reheated steel slab at 830-890 oC to form a steel sheet;
first cooling, the hot worked steel sheet to a temperature of 450-585 oC at a
predetermined cooling rate of 40℃/s – 70℃/s;
second cooling, the steel sheet undergoing natural cooling under ambient
condition at a temperature of 450-585 oC for 5-15 sec under natural cooling conditions;
and
coiling, the cooled steel sheet at a coiling temperature at 400-500oC to obtain a
high-strength hot-rolled steel sheet including ferrite, bainite, and pearlite
microstructure.
2. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits tensile strength greater than 600 MPa and yield strength ranging from 450 MPa
– 550 MPa.
21
3. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits zero yield point elongation.
4. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits uniform elongation greater than 10% and total elongation greater than 15%.
5. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits yield strength to tensile strength ratio ranging from 0.8 – 0.9.
6. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits grain size ranging from 2 – 3 μm.
7. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits precipitates (carbides or carbonitrides or nitrides of either or together of Nb,
Mo, V, Ti) ranging from 2 – 5 nm, with average size of 2 nm.
8. The method as claimed in claim 1, wherein the steel slab is hot charged into a furnace
for heating.
9. The method as claimed in claim 1, wherein the hot working is performed in a finish
rolling mill, being performed at temperature ranging from 830 ℃ - 890℃.
10. The method as claimed in claim 9, wherein the reduction ratio of the steel slab after
finish rolling ranges from 30% - 40%.
11. The method as claimed in claim 1, wherein the microstructure of the high-strength hotrolled
steel sheet consists of ferrite of 90 - 95 vol.%, bainite of less than 5 vol.% and
balance being pearlite microstructure.
12. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet is
strengthened by fine precipitates of Ti, Nb, Mo and V by way of at least one of carbides,
nitrides, combination of carbides and nitrides, and carbonitrides.
13. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet
exhibits continuous yielding.
14. A high-strength hot-rolled steel sheet, comprising an alloying composition of:
22
carbon (C) in a range of 0.03 wt.% to 0.10 wt.%,
silicon (Si) less than 0.1 wt.%,
manganese (Mn) in a range of 0.2 wt.% to 2 wt.%,
chromium (Cr) in a range of 0.01 wt.% to 0.7 wt.%,
phosphorous (P) up to 0.025 wt.%,
sulphur (S) up to 0.008 wt.%,
aluminium (Al) in a range of 0.01 to 0.1 wt.%,
niobium (Nb) in a range of 0.005 wt.% to 0.04wt.%,
titanium (Ti) in a range of 0.005 wt.% to 0.10 wt.%,
molybdenum (Mo) in a range of 0.005 wt.% to 0.2 wt.%,
vanadium (V) up to 0.06 wt.%,
nitrogen (N) up to 0.007 wt.%, and
iron (Fe) being remainder of the composition along with incidental
elements,
wherein, the high-strength hot-rolled steel sheet includes ferrite, bainite and
pearlite microstructure.
15. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits tensile strength greater than 600 MPa and yield
strength ranging from 450 MPa – 550 MPa.
16. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits zero yield point elongation.
17. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits uniform elongation greater than 10% and total
elongation greater than 15%.
18. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits yield strength to tensile strength ratio ranging
from 0.8 – 0.9.
19. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits grain size ranging from 2 – 3 μm.
23
20. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the
microstructure of the high-strength hot-rolled steel sheet consists of ferrite of 90 - 95
vol.%, bainite of less than 5 vol.% and balance being pearlite microstructure.
21. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet is strengthened by fine precipitates of Ti, Nb, Mo and V.
22. The high-strength hot-rolled steel sheet as claimed in claim 14, wherein the highstrength
hot-rolled steel sheet exhibits continuous yielding.