FIELD OF INVENTION
The present invention relates to a method of producing advance high strength
steel. More specifically the invention relates to development of advance high
tensile strength steel and articles including sheets, rods, bars and other articles
produced thereof in hot rolled condition having a compatible ductility maintained
through elongation and low yield strength/ultimate tensile strength ratio
characteristics.
BACKGROUND OF THE INVENTION
The structural parts of automotive (long member, cross member, structural
panels etc.) body require high strength. There have been a number of steels
developed for this purpose. However, elongation of these high strength steels is
not adequate, and these cannot be formed easily. In order to have good
formability, the yield strength has to be low at the same time tensile strength of
these steels must be high enough to improve crash worthiness.
In the known art High strength ferrite-martensite dual phase steels have been
developed keeping the above requirements in mind. For the said requirements
hot rolled ferrite-martensite dual phase steels have to be coiled at rather low
temperatures in order to form the martensite. In many conventional hot rolling
mills low coiling temperature is quite difficult to achieve. Therefore there is a
long need to produce comparable steels using the high coiling temperature
option in this field of applications.

To solve the prior art difficulties the present invention proposes to develop high
strength steel with superior ductility and low yield to tensile ratio that can be
produced by hot rolling.
Additions of alloying elements such as nickel, manganese, cobalt and others
sharply increase austenite stability. On the other hand chromium, Vandium,
Tungsten etc are known as ferritic stabiliser elements.
Role of constituents added as alloying elements in production of steel and heat
treatment of the present steel is enumerated as fa follows:-
Si: Si is an element effective for solid solution strengthening. It increases the
strength of ferrite, and when used in conjunction with other alloys can help
increase the toughness.
Mn: Manganese plays an important role as it lowers the temperature at which
austenite transforms into ferrite, thus avoiding cementite precipitation at ferrite
grain boundaries, and by refining the resulting pear lite structures. When the
cooling process is accelerated by quenching, austenite transforms into structures
with high strength such as bainite and martensite. Manganese improves the
response of steel to quenching by its effect on the transformation temperature.
Manganese is also weak carbide former. Another important property of
manganese is its ability to stabilise the austenite in steel. The effect of
manganese in forming austenite can be reinforced by combining it with nitrogen,
which is also an austenite-forming element. Manganese also increases
hardenability rate, used to significant advantage, depending on the steel type
and the end product, to improve mechanical properties. The content of Mn
should be 0.5% or higher from the viewpoint of solid solution strengthening.

P: P is effective for solid solution strengthening.
S: A lower content of S is more desirable.
Al: Al is added as a deoxidizer. If the content of Al is higher than 0.1% both of
the elongation and the stretch flangeability deteriorate. Therefore, the content of
Al should be 0.1% or lower.
N: A lower content of N is more desirable. If the content of N is higher than
0.006%, coarse nitrides increase, so that the stretch flangeability degrades.
Therefore, the content of N should be 0.006% or lower.
Mo: Mo forms fine composite carbides, and thus strengthens the steel while the
high elongation and the excellent stretch flangeability are maintained. In the
case of a steel sheet having tensile strength of around 780 MPa, the content of
Mo should be in the range of 0.05 to 0.6%, and in the case of a steel sheet
having tensile strength of around 950 MPa, the content of Mo should be in the
range of 0.3 to 0.7%.
V: V is effective in making the structure fine, and also form composite
precipitates together with Mo, which contributes to the increase in elongation
and stretch flangeabiiity. However, if the content of V is higher than 0.15%, the
elongation decreases.
Nb: Nb is a typical microalloying element and is usually added to structural
steels in amounts of less than 0.05%. Nb has a wide range of effects on
microstructural evolution in every step of sheet processing and consequently, on
the resulting mechanical properties.

Nb precipitates in austenite as Nb (CN). The amount of precipitates depends on
temperature and the contents of C and N. When steel is reheated, fine Nb(CN) is
precipitated immediately above Ac3. As the heating temperature is raised.
Nb(CN) precipitates grow and are gradually dissolved into γ. Fine Nb(CN)
precipitates at lower temperatures in γ and can inhibit grain growth by a pinning
mechanism acting on the grain boundaries, resulting in the achievement of a
finer grain structure. As the Nb content increases, this refinement effect becomes
more pronounced.
Nb in solution in y precipitates in a as very fine Nb(CN) during and after the γ ->
a transformation in air cooling. Since the size of the precipitates is very small,
they contribute greatly to precipitation hardening in α. However, this
phenomenon takes place only when the steel is reheated to high temperature,
where a sufficient amount of Nb can be dissolved, as in the case of hot rolling.
Ti: High strength low alloy steels rely on a combination of precipitation of
carbides and nitrides, and grain refining for their strengthening mechanism.
Titanium is used especially for the precipitation of TiCN, following controlled
rolling and rapid cooling. TiCN is the only micro-alloy carbo-nitride that is stable
at the high temperatures attained in the HAZ during welding, where it reduces
grain growth and increases toughness. Ti also forms its nitride at very high
temperatures and is therefore used to reduce grain growth of austenite during
hot rolling of plates. Ti levels are kept sufficiently high that, after formation of
TiN to remove N from solid solution, excess Ti is available to magnify the
precipitation strengthening of fine carbides formed by microalloying elements.

In low alloy steels that have been grain refined with aluminum, AIN can cause
intergranular fracture, known as "panel cracking". An addition of Ti causes TiN to
be precipitated uniformly in the matrix and increases ductility. A Ti content of
0.05% would be typical for this application.
By various choice of addition of alloying elements and their behaviour
characteristics during steel making and during heat treatment and cooling rates
employed to increase hardenability of steel by solution heat treatment it is
known in development of structural steel, tools steels and steels of special
physical properties producing steels of dual and multiphase by influencing
decomposition characteristics of austenite to precipitate carbides trotstite,
acicular trotsite (bainite) by employing different course of heat treatment
methods viz quenching, martempering, austempering as known in the art.
Bainite is formed as a result of austempering of carbon steels in which austenite
decomposition is fully completed in the intermediate zone. Such phase
distribution ensures a very high strength with sufficient toughness.
According to one objective of the invention the low alloy steel is proposed to be
developed on thorough studies and experiments on behaviour of addition of
alloying elements and opting for course of hardenability to produce micro-
structure consisting in combination of ferrite, bainite and martensite phase in the
resulted steel structure.
According to another objective of the invention it is proposed to develop a high
tensile strength steel with compatible ductility by reducing high amount of
martensite and strengthening ferrite phase resulted through precipitation during
solution hardening.

According to still another objective of the invention it is proposed to avoid yield
point phenomenon by maintaining limited amount of martensite in the resulted
steel but without sacrificing the tensile strength characteristics by strengthening
ferrite phase during heat treatment.
According to still further objective of the invention producing high strength steel
with superior ductility is proposed by maintaining low yield to tensile strength
ratio of the resultant steel.
According to yet another objective of the invention it is proposed to develop a
high strength with compatible ductility steel structure to be produced by hot
rolling during on line production of the same in the working floor.
According to yet further objective of the invention it is proposed to produce a low
yield to tensile strength ratio by producing a steel slab from low alloyed carbon
steel having added alloying elements from the group of silicon, manganese,
aluminium, cromium, vanadium, molybdenum, titanium and niobium.

The present invention is directed to the production off extra low yield to tensile

strength ratio, high strength and ductility hot rolled steels having compositions of
0.1 to 0.2 wt.% of C, 0.3 to 0.7 wt% Si, about 1.7 wt% or less of Mn, 0.02 wt%
or less P, 0.02 wt% or less S, less than 0.005 wt% N, about 0.04wt% of Al,
0.15wt% or less Cr, 0.15 wt% or less V, 0.2 wt% or less Mo bout 0.05 wt% or
less Ti, about 0.05 wt% or less Nb, the balance being substantially Fe and
incidental impurities.

The present invention is distinct from the said invention while producing steel of
different category of alloy group to produce steel sheet of microstructure with
enhanced precipitation of phase.
According to one aspect of the present invention it involves the method of
manufacturing a low yield to tensile ratio, advance high strength and ductile hot
rolled steel having a structure consisting of ferrite, bainite and martensite. The
method comprises manufacturing steel slab of the above described composition
as a raw material, hot rolling the steel slab, finish rolling at a temperature of
about 870 deg C or higher, retaining the steel sheet in the range of temperature
of about 730 deg to 780 deg C, cooling the steel at a cooling rate of about 30 deg
C /sec or higher and coiling the resultant steel sheet at a temperature in the
range of 450°C-520°C.
According to another aspect of the invention the high strength and low YS/UTS
ratio are derived in the steel from a combination of mechanisms including solid
solution strengthening, optimization of phase mixture and precipitation hardening.
According to the invention there is provided a method of producing a hot rolled
advance high strength ductile steel with low yield to tensile strength ratio to
generate articles comprising making a steel slab of compositions in wt%
consisting of carbon 0.1 to 0.2, silicon 0.30 to 0.70, manganese 1.2 to 1.70,
phosphorus 0.005 to 0.02, sulphur 0.005 to 0.03, nitrogen less than 0.005,
aluminium 0.02 to 0.06, chromium 0.05 to 0.15, vanadium 0.05 to 0.15,
molybdenum 0.05 to 0.20, titanium 0.03 to 0.06, niobium 0.03 to 0.06 and the

balance being iron and incidental impurities, the said slab being hot rolled to a
steel article at finishing roll at a temperature 870°C - 900°C, cooling the article at
a temperature in the range of 730°C - 780°C, retaining the article at this
temperature for a while, cooling the article to 450°C - 520°C at a cooling rate of
30°C to 35°C/sec and coiling the article at the same temperature followed by air
cooling to room temperature to result multiphase microstructure consisting of
ferrite, bainite and martensite.
The invention will be better understood by the following description with
reference to the accompanying drawings in which
Figure 1 represents a stress - strain diagram prepared from steel specimens of
steel prepared according to the present invention and
Figure 2 represents a typical microstructure of the advance high strength steel
consisting of ferrite, bainite and martensite phase, the specimen being etched
with La Pera reagent.
Ten steel slab compositions were prepared to conduct the different experiments
for the present invention. The compositions consisting essentially in wt % of 0.1
to 0.2 wt% of C, 0.30 to 0.70 Si, 1.2 to 1.70 Mn, 0.005 to 0.02 P, 0.005 to 0.02
S, less than 0.005 N, 0.02 to 0.06 Al, 0.05 to 0.15 Cr, 0.05 to 0.15 V, 0.05 to
0.20 Mo, 0.03 to 0.06 Ti, 0.03 to 0.06 Nb, the balance being substantially Fe and
incidental impurities. All the steel slabs were hot-rolled under various conditions
to manufacture steel sheets each having a thickness of 3.00 mm. Mechanical
properties and microstructural characteristics were determined for the hot-rolled
steels. One typical chemistry and the respective tensile properties are listed in
the following tables:

As is apparent from Table 2, the steel of the present Invention has a tensile
strength of 751 MPa, has a low yield to tensile ratio and good ductility. Presence
of Cr and Mo suppress the formation of pearlite and hence promotes bainitic
transformation, chromium being known as highest in promoting hardenability.
The steel produced is apt for oil or air hardening, the alloying element - Cr
reduces critical cooling rate required for martensite formation, increases
hardenability and thus improves the aptitude for heat treatment. Mo while
present in steel in more than 0.05 wt% forms very fine precipitates, and retards
the pearlite formation. Presence of Vanadium increases strength by grain
refinement and as well as by precipitation hardening. Ti in combination of C
and/or N form precipitates, which increases strength by precipitation hardening.
Fig 1. shows the engineering stress-strain diagram of one of the invented steels
produced according to the invention. This plot essentially shows continuous
yielding, high rate of strain hardening and very good ductility. These are the
features of conventional dual phase steels. These properties have been achieved
in the steels produced by the invention due to the appropriate phase formation

of ferrite, bainite and martensite. By test datas from stress-strain curve for
various compositions of the steel it has been found that tensile strength in the
range of 700 MPa to 900 MPa is possible. Total elongation and YS/UTS ratio
being observed as 18-22% and 0.45 to 0.60 respectively.
Fig. 2 shows a typical microstructure of one of the composition according to the
invented steel. Presence of a mixture of ferrite, bainite and martensite is the
special feature of this microstructure. A small amount of martensite is sufficient
to suppress the yield point phenomenon. In the microstructure shown ferrite
phase is the matrix, bainite is shown as dark contrast and martensite is shown as
white streaks. By standard metallurgical test methods grain sizes and grain
counts are carried out and it is observed that the ferrite, bainite and martensite
phase in volume % are in the ranges of 3045%, 45-60% and 5-15%
respectively.
While comparing and studying the test results of the steel specimens according
to the present invention it has been strikingly observed that the low yield/tensile
strength ratio is maintained with the phase distribution formed on heat treatment
of the low alloy steel compositions developed by the present invention. The
reason of advance high strength characteristics with ductility is attributed due to
the fact of ferrite phase being strengthened by the precipitation of Vanadium
carbide and solid solution hardening by the alloying constituents Si, Mn and Mo,
Ti and Nb beside the fact that the steel is basically strengthened due to presence
of martensite and bainite.
The invention as disclosed herein and illustrated should not be read in a
restrictive manner as various adaptations, modifications and changes are
possible as encampused within the scope of the appended claims.

WE CLAIM:
1. A method of producing a hot rolled advance high strength ductile steel
with low yield to tensile strength ratio to generate articles comprising
making a steel slab of compositions in wt% consisting of carbon 0.1 to 0.2,
silicon 0.30 to 0.70, manganese 1.2 to 1.70, phosphorus 0.005 to 0.02,
sulphur 0.005 to 0.02, nitrogen less than 0.005, aluminium 0.02 to 0.06,
chromium 0.05 to 0.15, vanadium 0.05 to 0.15, molybdenum 0.05 to 0.20,
titanium 0.03 to 0.06, niobium 0.03 to 0.06 and the balance being iron and
incidental impurities, the said slab being hot rolled to steel article at
finishing roll at a temperature 870°C - 900°C, cooling the article at a
temperature in the range of 730°C- 780°C, retaining the article at this
temperature for a while, cooling the article to 450°C - 520°C at a cooling
rate of 30°C to 35°C/sec and coiling the article at the same temperature
followed by air cooling to room temperature to result multiphase
microstructure consisting of ferrite, bainite and martensite.
2. A hot rolled advance high strength ductile steel having the elongation and
yield strength/ultimate tensile strength (UTS) ratio characteristic as 18-
22% and 0.45 to 0.60 respectively.

3. The hot rolled advance high strength ductile steel as claimed in claim 2,
wherein ferrite phase of the resultant steel is strengthened by Vanadium,
Molybdenum, Titanium and Niobium, precipitation and solution hardened
by addition of Silicon and Manganese.
4. The hot rolled advance high strength ductile steel as claimed in the
preceeding claims wherein ferrite, bainite and martensite phases formed
are in the ranges by volume % of 30-45%, 45-60% and 5-15%
respectively.
5. The hot rolled advance high strength ductile steel as claimed in the
preceeding claims wherein the steel produced and has UTS in the range
of 700 MPa to 900 Mpa.
6. A method as claimed in claim 1, wherein coiling temperature of the
resultant article from the steel is maintained at 450° to 520°C and the
article is coiled without any difficulty at the same temperature range.

7. The hot rolled advance high strength ductile steel as claimed in claim 2,
wherein the yield point phenomenon of the resultant article produced from
the steel is avoided due to presence of limited amount of martensite in the
resulted article.
8. A hot rolled advance high strength ductility with low yield/tensile strength
ratio steel produced according to method claim 1 having compositions in
wt% 0.1 to 0.2 wt.% of C, 0.30 to 0.70 wt% Si, 1.2 to 1.70 Mn, 0.005 to
0.02 P, 0.005 to 0.03 S, less than .0.005 N, 0.02 to 0.06 Al, 0.05 to 0.15
Cr, 0.05 to 0.15 V, 0.05 to 0.20 Mo, 0.03 to 0.06 Titanium, 0.03 to 0.06
Niobium and the balance being substantially Fe and incidental impurities.
9. The hot rolled advance high strength ductile steel as claimed in claim 2,
wherein the article resulted is coiled steel sheets or bar, rod or other steel
articles adapted to be coiled.
10.The hot rolled advance high strength ductile steel with low yield to tensile
strength ratio to generate articles as described herein and illustrated.

A method of producing a hot rolled advance high strength ductile steel with low
yield to tensile strength ratio to generate articles comprising making a steel slab
of compositions in wt% consisting of carbon 0.1 to 0.2, silicon 0.30 to 0.70,
manganese 1.2 to 1.70, phosphorus 0.005 to 0.02, sulphur 0.005 to 0.02,
nitrogen less than 0.005, aluminium 0.02 to 0.06, chromium 0.05 to 0.15,
vanadium 0.05 to 0.15, molybdenum 0.05 to 0.20, titanium 0.03 to 0.06, niobium
0.03 to 0.06 and the balance being iron and incidental impurities, the said slab
being hot rolled to steel article at finishing roll at a temperature 870°C - 900°C,
cooling the article at a temperature in the range of 730°C- 780°C, retaining the
article at this temperature for a while, cooling the article to 450°C - 520°C at a
cooling rate of 30°C to 35°C/sec and coiling the article at the same temperature
followed by air cooling to room temperature to result multiphase microstructure
consisting of ferrite, bainite and martensite.