FIELD OF INVENTION:
The present invention relates to the designing of easily formable and weldable high
strength hot rolled Cu bearing steel, having excellent fatigue properties with endurance
strength greater than 230 MPa and fatigue ratio greater than 0.5 and the manufacturing
method for the same.
BACKGROUND OF THE INVENTION:
Fatigue is the major failure mechanism in the components subjected to the repeated
and fluctuating loads, which accounts for a large percentage of failure in automotive
components. As the automotive components are designed to withstand the service loads
(generally well below the yield strength of the material) for a large number of cycles (>
10^6 cycles), high cycle properties of steels becomes important, Thus the realization of
fatigue property becomes important in design and performance evaluation of automotive
components. The fatigue properties of the existing standard steel grade used for auto
structural and wheel rim application exhibits the fatigue ratio less than 0.5. The present
invention relates to the development of the steel with the improved fatigue performance
by attaining fatigue ratio in excess of 0.5.
The fatigue life of the metal components consists of two basic time-domain regimes:
crack-initiation and crack-growth. The third regime of rapid crack growth leading to
failure of the component is relatively small. Out of the two, the former takes around
2/3rd of the total fatigue life. Thus improving the resistance of steel/component to
crack-initiation gives clear advantage of improved fatigue-life. Crack-initiation, most
prominently, occurs by Persistent Slip Band (PSB) formation. Due to continuous cyclic
loading, the surface of the component develops ridges/steps as a result of slip on
preferred planes. These steps, when sufficiently sharp, act as stress concentrators and
subsequently, areas of crack-initiation. The refinement in these steps formed during the
intrusion and extrusion process under fatigue loading significantly improve the fatigue
performance of the steel component.

Japanese patent JP2000063986 discloses a method of producing high strength hot
rolled steel sheets with improved fatigue properties having dual phase microstructure
comprising of ferrite and martensite. However the application of this steel is limited by
the fact that components like wheel rim are subjected to forming operation post
welding. Welding results in the tempering of martensite causing the steel to soften up
and hence deteriorate the tensile as well as fatigue properties of the end product.
US Patent US20020162613A1 discloses a method of producing a high-strength hot-rolled
steel sheet with excellent fatigue properties and stretch frangibility achieved by the
precipitation strengthening by alloying elements like Ti and Mo in granular bainitic
structure. The dissolution temperature of Titanium and Niobium precipitates (nitrides
and carbo-nitrides) is quite high and hence, this method can be applied only to
conventional hot rolling facilities which are equipped with a reheating furnace and not
continuous strip production facilities. Moreover the addition of Mo in the steel makes it
expensive.
Some of the common methods used for the improvement in fatigue performance of a
component are shot peening ,laser peening which induces the compressive stresses on
the metal surface. These compressive stresses are effective in increasing the resistance
to the fatigue failures.
In light to the above discussed prior art, there is a need for an invention which
overcomes the limitation of prior art and disclose the steel composition capable of
deriving intrinsic improved fatigue properties along with the method of its production.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a steel composition capable of improving the
fatigue characteristics of hot rolled steel sheets by attainment of fatigue ration in excess
of 0.5.
Another object of the present invention is to propose the chemistry for a high strength
hot rolled Cu bearing steel, of which ferrite potential is >1.05.
Another object of the present invention is to propose a composition with carbon
equivalence (as defined by IIW) less than 0.35 that will enable the steel to exhibit
excellent weldability.
Still another object of this invention is to propose a thermo mechanical processing that
is effective in attaining the desired microstructure with retention of Cu in solid solution in
order to achieve improved fatigue performance of developed steel over conventional
steel.
Further object of present invention is to propose the micro alloyed steel with room
temperature properties YS: 310-400 MPa, UTS: 420-500 MPa and percentage elongation
in excess of 25.
Another object of the present invention is to design the Cu bearing steel which is easily
cast able and can be rolled through conventional hot rolling mill as well as thin slab
casting route without encountering the issues of hot shortness.

SUMMARY OF THE INVENTION:
The invention discloses a high-strength hot-rolled steel sheet with a tensile strength of
at least 420 MPa and method of manufacturing the same. A steel slab of the
composition comprising, in weight percent 0.04-0.08% of C, 0.3-1.05% of Mn, 0.1-0.5%
of Si, 0.25-1.5% of Cu, 0.25-1.0% of Ni (Ni/Cu ratio in arrange of 0.5-1), maximum
0.005% of S, maximum 0.030% of P, maximum 0.005% of N, the balance being iron
and inevitable impurities, is reheated to a temperature of 1100 to 1250° C to
homogenize the cast structure and ensure that the alloying elements are dissolved as
solid solution. The reheated slab is subsequently hot rolled with finish hot rolling in the
temperature range of (Ar3 -30) °C to (Ar3 +30) °C and followed by lamellar water cooling
over the run out table to a coiling temperature of 550-580°C with the average cooling
rate of 10-20°C/s.
The present invention is aimed to develop the steel capable with the fatigue ratio
greater than 0.5 and also meet the tensile properties of high strength steels that are
being used for structural and wheel components of automobiles like ASTM A1011-
Grade50,ASTMA1011-Grade60 ,Fe 360, E-36, E38 with YS:310-400; UTS:420-540 and
percentage elongation values greater than 20.
The steel was designed with ferrite potential in excess of 1.05 to ensure that the
composition of the designed steel remains non-peritectic. The steel with peritectic
composition have the tendency to breakouts and cracking of slab during solidification in
the caster. And finally the steel composition was designed to aim for carbon equivalence
to be less than 0.3 to make the hot rolled steel readily weldable, and to enable the
usage of steel for various component of automobile to be used as long and cross
members, connecting road wheel rim and disc, suspension and chassis components

In the present invention, the fatigue properties were improved by the addition of Cu and
Ni in the steel coupled with controlled thermo mechanical processing. Room temperature
strength was achieved with the presence of pearlite as a second phase, solid solution
strengthening derived from C, N, Mn, Cu, Ni and Si. The thermo mechanical processing
was designed to restrict the Cu in solid solution by employing the faster cooling over
the run out table and coiling at lower temperature.
The steel of the current invention exhibits the endurance strength greater than 230 MPa
with fatigue ratio in excess of 0.5
The proposed chemical composition, in weight percent of the steel consists of
C: < 0.10%;
Mn: <1.2 %;
Cu: <1.5%;
Ni: <1.0%;
Si: < 0.50%;
N: <0.005%;
S<0.005 %; and P<0.030%,
remaining Fe along with the unavoidable impurities.
The proposed steel essentially consists of ferritic microstructure with pearlite volume
fraction less than 10% and ferrite grain size of 4-12 urn. The steel consists of Cu in solid
solution with little or no precipitation of Cu in ferrite matrix.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 is the schematic illustrating the thermo mechanical processing employed for the
development of Cu bearing steel.
Figure 2 shows the sample geometry employed for conducting stress controlled fatigue
testing.
Figure 3 is the bright field TEM micrograph showing the gain boundary triple junction .
Steel 3 with the lowest Cu content (0.25 wt%) exhibits the precipitate free ferrite matrix
indicating the complete solid solution state of Cu in ferrite.
Figure 4 shows the bright field TEM micrograph of steel 6 (a) precipitates distribution in
as rolled condition (b) precipitate distribution for the sample aged at 600 °C for 4 hours.
Figure 5 highlights the effect of Cu content on fatigue performance of the developed
steel.
Figure 6 shows the comparison in S-N curve behaviour of Cu bearing steel and Fe-360
Steel grade
DETAILED DESCRIPTION OF THE INVENTION:
The present invention relates to the designing of the chemical composition of steel
coupled with the controlled thermo mechanical processing method to develop formable
and welding friendly Cu bearing High strength steel. The developed steel exhibits the
fatigue ratio greater than 0.5. The developed hot rolled steel is hot/cold formed and
welded to produce structural and wheel components of automobiles used as long and
cross members, connecting road wheel rim and disc, suspension and chassis
components.

The detailed research was carried out to study the effect and role of Cu alloying on the
fatigue performance on the steel and accordingly the composition was designed .The
steel composition of Cu bearing steel in the present invention consists of Cu , Ni,
besides C, Si, Mn, P and S. The proposed chemical composition, in weight percent of the
steel, consists of
C: < 0.10%;
Mn: <1.2 %;
Cu: <1.5%;
Ni: <1.0%;
Si: < 0.50%;
N: <0.005%;
S<0.005 %; and P<0.030%,
Nb: 0.015-0.025 and V:0.03-0.05 can be optionally added to further improve upon the
tensile properties of the developed steel. Ni/Cu greater than 0.5 was maintained to
ensure the issues related to the hot shortness.
Table 1 provides a few examples of the composition of steel as per the current
invention.


* Ferrite Potential ** Carbon Equivalence
The ferrite potential for a given composition of steel is a parameter that defines whether
steel composition is peritectic or non-peritectic. Ferrite potential (FP) is calculated by the
following empirical formula:

are expressed in wt%.
Figure 1 shows the schematic that defines the thermo mechanical processing employed
for the production of hot rolled strips of the designed chemistry. The cast slab with the
following composition in weight percent
C: < 0.10%;
Mn: <1.2 %;
Cu: <1.5%;
Ni: <1.0%;
Si: < 0.50%;
N: <0.005%;
S<0.005 %; and P<0.030%,
is heated to a temperature of 1100 to 1250° C to homogenize the cast structure and
ensure that the alloying elements are dissolved as solid solution. The reheated slab is
subsequently hot rolled with finish hot rolling in the temperature range of (Ar3 -30) °C)
to (Ar3 +30) °C and followed by lamellar water cooling over the run out table to a coiling
temperature of 550-580°C with the average cooling rate of 10-20°C/s. The

coiling temperature for the hot rolled strips should not exceed 600°C as it will result in
the nucleation of Cu precipitates when the coil is allowed to cool naturally. Figure 3 is
the bright field TEM micrograph showing the gain boundary triple junction . Steel 3 with
the lowest Cu content (0.25 wt%) exhibits the precipitate free ferrite matrix indicating
the complete solid solution state of Cu in ferrite
Salient features on the role primary alloying elements in the present invention for the Cu
bearing steel with the improved fatigue performance are described below:
C:<0.10 wt%: The preferable range for the carbon in the steel is 0.04-0.08%wt. C is
added to derive the strength in steel through solid solution strengthening, along with
the formation of second phase i.e. pearlite in the ferrite matrix. The carbon content of
the developed steel is restricted to 0.08 in order to avoid the peritectic reaction during
the casting (particularly when steel is manufactured by thin slab casting route where the
casting speed is much higher in comparison to the convention continuous casting route).
Also considering weldability issues, the carbon content must be restricted to 0.08%.
Mn: <1.2 wt%: The preferable range for the Mn in the steel is 0.3-1.05 %wt.
Manganese apart from imparting solid solution strengthening, also lowers the austenite
to ferrite transformation temperature leading to refinement of the ferrite grain size .
However higher Mn content beyond 1.2 % enhances the possibility of centerline
segregation and formation of MnS inclusion during the continuous casting of the steel.
Increased Mn content also impairs the weldability of the steel.

Si: 0.05-0.50 wt%: The preferable range for the Si in the steel is 0.05-0.30 %wt. Silicon
imparts the solid solution strengthening effect and is also effective in limiting the
chemistry to non peritectic composition. The presence of Si becomes more significant
when the steel is being rolled to higher sheet thicknesses. Si is also being employed as a
deoxidizing element. .However the Si content in the steel should be restricted to 0.5% in
order to prevent the formation of surface scales. Also the higher Si content will impair
the weldability of the steel.
Cu: 0.25-1.5 wt%: The preferable range for the Cu in steel is 0.25-1.0 %wt. Cu also
imparts solid solution strengthening effect if retained in solution. During continuous
cyclic loading, the surface of the component develops ridges/steps as a result of slip on
preferred planes. These steps, when sufficiently sharp, act as stress concentrators and
subsequently, areas of crack-initiation. The presence of Cu delays the crack initiation by
the refinement of these steps formed on the steel surface as a result of intrusion and
extrusion process of fatigue loading and results in the improved fatigue performance of
steel in comparison to convention steel grades. Increasing the Cu content beyond 1.5%
increases the severity of hot shortness which is encountered during the casting and
rolling of steel slabs. Moreover increased Cu content significantly deteriorates the
weldability of steel.
Ni:<1.00: The presence of copper in steel causes problem of the hot shortness and
makes slab more sensitive to the surface cracking during the bending operation
employed in the continuous casting process. Nickel in steel is effective in preventing
the hot shortness by increasing the solubility of copper in y -Fe and thus reduces the
precipitation of liquid copper at the scale/metal interface. The Ni/Cu ratio in range of
0.5-1.0 was maintained for the steel containing Cu greater than 0.25. to prevent the
issue of hot shortness. Ni also imparts the solid solution strengthening of ferrite and
improves the room temperature tensile properties in steel.

Nitrogen :< 0.050 wt%: Higher nitrogen content leads to the embrittlement of the heat
affected zone (HAZ) of weld joints.
Sulphur: <0.005 by wt%: Sulphur has to be limited to 0.005% to avoid high level of
inclusions which induces the in homogeneity in the steel and deteriorates the formability
of the steel.
Phosphorous: <0.03 wt%: The Phosphorous content should be restricted to 0.03%
maximum as higher phosphorus content leads to the reduction in toughness and
voidability of steel as a result of phosphorus segregation at the grain boundaries.
The steels developed using the process and composition as per the current invention
were tested for tensile properties such as YS, UTS, EI and fatigue properties for
evaluation of endurance strength and generation of S-N curve. Fatigue performance of
the designed steels was evaluated via stress controlled fatigue tests at a stress ratio (R)
of -1. Figure 2 shows the sample geometry employed for the fatigue testing of the steel
samples. Table 2 summarizes the tensile properties of standard (Steel 1 and Steel 2)
and the developed steel grades.


Table 3 summarizes the fatigue behaviors of standard steel grade and the Cu alloyed
steel. The increase in fatigue properties with increasing Cu content is evident from the
data. The fatigue ratio in excess of 0.5 is achieved by 0.25%Cu steel while the
maximum of 0.69 was achieved for the steel with 1.5%. Figure 5 brings out the
comparison of the fatigue properties of investigated steel.

It is evident from the examples of the steels described above that current invention
provides steel with fatigue ratio greater than 0.5. Further, the steels as per the current
invention exhibit desired room temperature properties with YS: 310-400 MPa and UTS:
420-540MPa. The steel of the current invention is capable of cold formed and weldable
to form the various components of automobile used as long and cross members,
connecting road wheel rim and disc, suspension, chassis components etc.

Figure 6 shows S-N curve (Stress and Number of cycles to fatigue failure) for the
standard steel and 0.5% Cu steel at room temperature and high temperature for a
holding period of 60 minutes and it is evident from the curve that steel as per the
current invention exhibits fatigue ratio of 0.6 in comparison to 0.43 of conventional
steel.

WE CLAIM:
1. A Cu bearing high strength hot-rolled steel for structural applications, the steel
comprising, in terms of weight %:
C: < 0.10%;
Mn:<1.2%;
Cu: <1.5%;
Ni: <1.0%;
Si: < 0.50%;
N: <0.005%;
S<0.005 %; and P<0.030%,
wherein ferrite potential of the developed steel is greater than 1.05 and fatigue ratio is
greater than 0.5.
2. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein one
or more of the following elements are preferably present in the developed steel
in weight %:
C: 0.04-0.08%;
Mn: 0.3 -1.05 %;
Cu: 0.25-1.5%, more preferably 0.25 to 1 %;
Ni: 0.10-1.0%; and
N: 0.0030-0.005%.
3. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein the
steel has YS of at least 310 MPa, UTS 410 to 540 MPa and %EI> 20.

4. The Cu bearing high strength hot rolled steel according to one or more of claims
1-3 further comprising optionally up to 0.025% Nb or 0.05 % of Vanadium.
5. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein the
developed steel is hot- rolled to a thickness of up to 10 mm.
6. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein
pearlite volume fraction is less than 10 %.
7. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein
ferrite grain size varies in the range 4 to 12 pm.
8. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein
carbon equivalence is less than 0.35.
9. The Cu bearing high strength hot rolled steel as claimed in claim 1, wherein the
steel further comprises, in weight %, Niobium (Nb) in the range of 0.015 to
0.025 and Vanadium (V) in the range of 0.03 to 0.05
10. A structural automobile component manufactured as per the steel composition
claimed in claims 1 to 9.

11. Method of production of Cu bearing high strength hot rolled steel, the method
comprising: reheating a steel slab with a composition in weight %
C: < 0.10%;
Mn: <1.2%;
Cu: <1.5%;
Ni: <1.0%;
Si: < 0.50%;
N: <0.020%;
S<0.005 %; and P<0.030%,
to a temperature of 1050 to 1250° C; hot rolling to a finish rolling temperature range
(Ar3 -30) °C to (Ar3 +30) °C, Ar3 denotes austenite to ferrite ; and coiling at a
temperature from 550-580°C with cooling rate from 10 to 20°C/s.