FIELD OF THE INVENTION
The present invention relates to a process of producing a hot-rolled microalloyed steel
with high strength, high ductility and low YS/UTS ratio. More particularly, the invention
relates to a process of producing hot-rolled high strength steel for automotive
applications with YS ≤ 400 MPa, UTS > 500 MPa, %EI > 40, YS/UTS < 0.8.
BACKGROUND OF THE INVENTION
Conventionally microalloying in automotive steels is known to increase the strength of
the steel. However, it is noted that microalloying also increases (typically YS/UTS
>0.85) the yield strength of the automotive steel. Therefore, the YS/UTS ratio is always
found to be higher in microalloyed steels. A high YS/UTS ratio leads to various
problems during forming of the steel sheets, such as die wear out and spring back
issues, which restricts to impart higher amount of deformation due to a lower uniform
elongation. Therefore, lower YS/UTS ratio is always desired by automakers to maintain
a higher degree of work hardening in the steel. This parameter can be obtained by
producing a dual phase (ferrite + martensite) structure where normally >0.06 wt % C is
used, and a highly efficient cooling is maintained to coil the material normally below
300°C which requires a strong coiler. Accordingly, the production process become
expensive and further pose a technical challenge to produce consistent uniform
microstructures which influence the final mechanical properties.

Prior Art
US Patent US 4188241 discloses the method of producing a plain C-Mn steel with low
yield ratio. However, the processing requires coiling of the strip at ≤300°C. It also
shows that if the coiling temperature increased, the yield ratio increases to ≥ 0.8.
Moreover, the higher amount of carbon (≥0.1wt%) has disadvantages to welding and
low temperature coiling also add to the production cost.
US Patent US 4614551 discloses a process for producing a low yield ratio, high
strength two-phase steel sheet having an excellent artificial aging property after
working. This work mainly deals with C-Mn grades with FRT ranging from 750-890°C
range and very low temperature coiling (<250°C). This aims to improve the artificial
ageing properties. However, the complicacy with step cooling and low temperature
coiling makes it a costly and difficult process to handle in conventional rolling process.
US patent US 4437903 discloses a method for producing two-phase steel sheets with
the addition of small amount of boron in the steel. However, the properties are
achieved with low finish rolling temperature and lower coiling temperature (<300°C).
Although the amount of Mn and Si can be reduced with the addition of boron, however,
the low temperature rolling and coiling add to cost and complexity of production of such
product using conventional rolling practices.
In view of the said prior art, there is a need for an invention to overcome the limitations
of the prior art and disclose a process that does not need higher carbon or specialized
processing conditions to achieve step cooling, low temperature coiling.

SUMMARY OF THE INVENTION
Accordingly, there is provided a process of producing a hot-rolled high strength steel for
automotive application with YS ≤ 400 MPa, UTS > 500 MPa, %EI > 40, YS/UTS ≤ 0.8.
The present invention proposes vanadium microalloyed grade steel based on aluminum-
killed low carbon chemistry containing manganese, vanadium and nitrogen with
ultimate tensile strength of at least 500 MPa. The new grade steel material has been
produced in an air induction steel making furnace including a hot-rolling unit. The
material was characterized and found to record an attractive combination of strength,
ductility and lower YS/UTS ratio. Thus, the inventive process achieves the object
through:
(1) use of optimum chemistry of the steel to achieve high strength and ductility,
coupled with low YS/UTS ratio after hot-rolling, suitable for automotive steel
products
(2) developing a lower YS/UTS ratio <0.8 by virtue of larger ferrite grain size (>6
urn) and optimum pearlite content (10-15 %).
The strength of the steel mainly relies on sufficient alloying addition of manganese and
strengthening though precipitates. In addition, a higher elongation is achieved through
a low carbon (<0.05 wt%) chemistry. To form the vanadium precipitates, nitrogen was
used in the form of nitrovan ferroalloys. A lower YS/UTS ratio was achieved through
relatively larger ferrite grains (average ~9 urn) and pearlite phases (~10 -15 %).

Objects of the Invention:
An object of the invention is to develop a hot-rolled high strength steel for automotive
application with YS ≤ 400 MPa, UTS > 500 MPa, %EI > 40, YS/UTS ≤ 0.8.
Another object of the invention is to develop a hot-rolled high strength steel for
automotive applications with low carbon (<0.07 wt%) chemistry.
Still another object of the invention is to propose a process to produce a hot-rolled high
strength steel with YS/UTS ratio ~ 0.8 or less.
Further object of the invention is to propose a process which does not need specialized
processing conditions such as step cooling, low temperature coiling for developing a
steel with YS/UTS ratio ~ 0.8or less.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- shows a tensile stress-strain curve of the hot-rolled high strength steel according
to the invention
Fig. 2- shows a microstructure of the hot-rolled high strength steel after hot-rolling.
DETAIL DESCRIPTION OF THE INVENTION
The specifications of the hot-rolled high strength steel as per the current invention are
as follows:

Material specification:
(i) Steel plate with thickness in the range of 5 to 10 mm, as rolled in a hot
rolling mill.
(ii) Chemistry of steel as specified in Table 1
(iii) Mechanical properties of the product specified are as follows:
YS ≤ 400 MPa, UTS > 500 MPa, %EI > 40, YS/UTS ≤ 0.8.
Table 1: Chemistry of steel specified for a hot-rolled high strength steel as per the
current invention (wt%)

The various technical details in terms of addition of steel composition and process route
followed to obtain the desired properties in the present invention is as under.
Carbon (C): carbon in the steel composition of the present invention is ≤ 0.07.
preferably the carbon varies in the range 0.04-0.07%: Carbon is one of the most
effective and economical strengthening elements. Carbon is necessary for precipitation
strengthening and pearlite formation. Carbon combines with V to form carbides or
carbonitrides which bring about precipitation strengthening. This requires a minimum of
0.04%C in the steel. However, in order to avoid the preitectic reaction during casting
(especially for continuous strip production or CSP facilities) and considering weldability
issues, the carbon content has to be restricted to less than 0.07%.

Manganese (Mn): Manganese content in the steel composition of the present
invention is ≤ 2.5%. Preferably Mn varies in the range of 1.4-1.8 %: Manganese is an
excellent solid
solution strengthener. However, very high Mn enhances centerline segregation during
continuous casting and also increases the carbon equivalent and thereby creating
problems during welding.
Silicon (Si): 0.2-0.5%: Silicon like Mn is a very efficient solid solution strengthening
element. Si is also a ferrite stabilizer. However, additions of Si should be restricted to
less than 0.5% in order to prevent the formation of surface scales.
Vanadium (V): Vanadium content as per the current invention should be ≤ 0.1.
Preferably Vanadium is present in the range of 0.04-0.06 weight %: Vanadium is the
most potent microalloying element for precipitate formation in ferrite. Due to higher
solubility in austenite vanadium normally doesn't form precipitate in austenite unlike
other microalloying elements such as Nb or Ti. Therefore, total vanadium content in the
steel can be utilized to form fine precipitate in ferrite. However, higher amount of
vanadium is responsible for coarsening of precipitates and therefore V content was
restricted to 0.06 wt%.
Phosphorous (P): Phosphorous content in the steel composition of the current
invention is ≤ 0.12. Preferably Phosphorous content is 0.03% maximum: Phosphorus
content should be restricted to 0.03% maximum as higher phosphorus levels can lead
to reduction in toughness and weldability due to segregation of P into grain boundaries.

Sulphur (S):Sulphur content in the steel composition as per the current invention is <
0.012. Preferably Sulphur is added to maximum quantity of 0.01 weight %. The Sulphur
content has to be limited otherwise it results in a very high inclusion level that
deteriorates formability.
Nitrogen (N): Nitrogen content in the steel composition as per the current invention is
≤ 0.02. Preferably nitrogen content varies in the range 0.01-0.02%: Nitrogen combines
with vanadium to form vanadium nitrides (VN) or vanadium carbonitrides (V(CN)).
Vanadium nitrides is more stable than vanadium carbides and therefore added in
conjunction with vanadium for higher strengthening through precipitates. However,
high
nitrogen levels negatively affects ageing stability and toughness in the heat-affected
zone of the weld seam, as well as resistance to inter-crystalline stress-corrosion
cracking. Therefore, the amount of nitrogen addition is more preferably restricted to
0.015wt%.
Microstructure: Effective utilization of various strengthening mechanism including
control of microstructure is essential in order to obtain the targeted strength properties.
As outlined above the majority of the strengthening contributions come from the solid
solution strengthening and precipitation strengthening. Also, to obtain lower YS/UTS
ratio it is desired to have larger ferrite grains and suitable amount of second phase. In
view of the above, the only way by which the target strength with lower YS/UTS ration
can be achieved is by tailoring the microstructure and hence a microstructure consisting

of precipitation strengthened coarse grained ferrite as matrix and pearlite as the second
phase, was targeted in the present invention. The contribution of each of the
microstructural components is described below:
Ferrite: The hot-rolled steel plate according to the present invention has 85-90 %
ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn
and Si.
Using suitable processing conditions, the grain size achieved was >6 μm. The
precipitation strengthening was achieved by predominantly fine Vanadium Nitride (VN)
precipitates.
Pearlite: The amount of pearlite in the microstructure is 10-15%. The pearlite adds up
to the strength as well as induces dislocations in ferrites to reduce the YS/UTS ratio.
Production process: The method of manufacturing the hot-rolled steel according to
the present invention consists of a steel making by air induction furnace followed by
ingot casting. Finally the ingot was forged, hot rolled and coiling simulation was done
using a salt bath furnace. The various processing steps are described in their respective
order below:

Steel Making and Casting: The new steel grade with an optimum chemical
composition (Table 1) was produced and processed. The steel was made in an air
induction furnace and cast into a 25 kg ingot.
Forging: The material was then homogenized at about 1200 °C for more than 3 hours
in a muffle furnace, forged and cut into a plurality of 50 mm square bars. These bars
were further hot forged to reduce from 50 mm to 20 mm by forging. Sufficient amount
of soaking at 1200 °C was done in order to homogenize the composition throughout.
Reheating and Hot Rolling: Finally, the sample was reheated to around 1200 °C and
rolled to a final thickness in the range of 5 to 10 mm at a finish rolling temperature of
890 °C to 900°C. A reheating temperature greater than 1200°C is also not desirable
because it may lead to grain coarsening of austenite and/or excessive scale loss.
The finish rolling temperature (FRT) was selected based on the equilibrium temperature
of austenite to ferrite transformation (Ae3). More specifically the finish rolling
temperature was set above Ae3 temperature i.e FRT ≤ Ae3 + 50 (°C).
Coiling: The steel plate was then put in a salt bath furnace at 650-700°C for about 60
minutes to simulate the coiling condition. After this temperature is attained, the steel
sheet is subjected to natural air cooling which facilitates the remaining austenite to
transform to pearlite. Coiling was carried out at a coiling temperature (CT) given by
600°C < CT < 700°C. Coiling below 600°C is avoided to prevent the formation of non
polygonal ferrite.

Metallography and Mechanical tests:
Tensile properties (Figure 1) of the end product were measured using test specimens
with 25 mm gage length (prepared according to ASTM E8 specification), fitted with an
extensometer. All tests were performed at room temperature.
Experimental Results:
The chemical composition of the alloy in an embodiment of the invention is given in
Table 2. Mechanical properties of the material in hot-rolled condition are presented in
Table 4. The results indicate that the new steel grade with the optimum chemistry
recorded excellent combination of strength and ductility. The high tensile strength of
the steel as achieved reduces the gauge of the body panels and hence the weight of
the automobile. The material is also revealed to possess a predominantly ferritic
microstructure, with a second phase of pearlite 10-15 wt% (Figure 2). The process of
the current invention enables production of hot rolled high strength steel with YS/UT
ratio less than 0.8. The examples given below are even able to achieve YS/UT ratio of
0.74.
Table 2: Chemical composition of the hot-rolled high strength steel as per the current
invention.


The preferable range of the micro alloying elements that achieve the desired properties
of the invention are given in table 3.
Table 3: Preferable range of the micro alloying elements
1 i i i i
The newly developed HSS grade with low YS/UTS ratio can be used in manufacture of
structural parts of automobiles. Achieving favorable combination of mechanical
properties through judicious selection of chemistry.
High tensile strength of the material would allow usage of thinner gauge component
and help reduce the weight of the car body.
Low YS/UTS ratio will help in forming with lower load and increase the life of die.

We claim:
1. A process of producing a hot-rolled high strength steel, the process comprising,
- selecting a steel composition comprising, in terms of weight %, Carbon (C) ≤
0.07, Manganese(Mn) ≤ 2.5, Sulphur(S) ≤ 0.012, Phosphorous(P) ≤ 0.12,
Silicon(Si) ≤0.5, Aluminum(AI) ≤ 0.1, Nitrogen(N) ≤ 0.02, Vanadium(V) ≤0.1,
remainder being iron and unavoidable impurities;
- processing the steel composition for steel making in an air induction furnace;
- casting the steel composition in to ingots; and
- homogenizing the steel composition at about 1200 °C in a muffle furnace and
forging in to steel bars;
- reheating the steel bars to a temperature of around 1200°C and rolling at a
finish rolling temperature of 890 °C to 900°C;
- coiling of steel sheets in a salt bath furnace at a temperature of 600-700°C; and
- subjecting the steel sheets to natural air cooling.

2. The process as claimed in any of the preceding claims, wherein the hot-rolled
high strength steel exhibits mechanical properties of YS≤400MPa, UTS≥500 MPa,
%EL≥40, and YS/UTS ≤ 0.8.
3. The process claimed in claim 1, wherein the steel composition comprises of mico-
alloying elements preferably in the range of, in terms of weight %, Carbon (C)
0.04 to 0.07%, Manganese(Mn) 1.4 to 1.8 %, Aluminum(AI) <0.1, Nitrogen(N)
0.01 to

0.02%, Vanadium(V) 0.04-0.06 %, Silicon(Si) 0.2-0.5%, Sulphur to a maximum
of 0.01 %. and Phosphorous to a maximum of 0.03%.
4. The process as claimed in claim 1, wherein steel comprises a microstructure
comprising of 85-90% ferrite and is 10-15% pearlite.
5. The process as claimed in claim 1, wherein thickness of the steel sheet after
rolling step is in the range of 5 to 10 mm.
6. A hot-rolled high strength steel with YS≤400MPa, UTS≥500 MPa, %EL≥40, and
YS/UTS ≤ 0.8, the steel comprising, in terms of weight %: Carbon (C) ≤ 0.07,
Manganese(Mn) ≤ 2.5, Sulphur(S) ≤0.012, Phosphorous(P) ≤0.12, Silicon(Si)
≤0.5, Aluminum(AI) ≤0.1, Nitrogen(N) ≤0.02, Vanadium(V) ≤0.1, remainder being
iron and unavoidable impurities; and a microstructure comprising of 85-90%
ferrite and is 10-15% pearlite.
7. The hot-rolled high strength steel as per the claim 6, wherein ferrite has a grain
size of >6 μm.
8. The hot-rolled high strength steel as per the claim 6, wherein steel comprises of
mico- alloying elements preferably in the range of, in terms of weight %, Carbon
(C) 0.04 to 0.07%, Manganese(Mn) 1.4 to 1.8 %, Aluminum(AI) ≤0.1,

Nitrogen(N) 0.01 to 0.02%, Vanadium(V) 0.04-0.06 %, Silicon(Si) 0.2-0.5%, Sulphur
to a maximum of 0.01 %. and Phosphorous to a maximum of 0.03%.
9. The hot-rolled high strength steel as per the claim 6, wherein the YS/UTS ratio is
preferably less than 0.74.

ABSTRACT

The invention relates to a process of producing hot-rolled high strength steel for
automotive application with YS ≤ 400 MPa, UTS > 500 MPa, %EI > 40, YS/UTS ≤ 0.8.
The steel composition as per the current invention comprises, in terms of weight %,
Carbon (C) ≤ 0.07, Manganese(Mn) ≤ 2.5, Sulphur(S) ≤0.012, Phosphorous(P) ≤0.12,
Silicon(Si) ≤0.5, Aluminum(AI) ≤0.1, Nitrogen(N) ≤0.02, Vanadium(V) ≤0.1, remainder
being iron and unavoidable impurities. The method of manufacturing the hot-rolled
steel according to the present invention consists of a steel making by air induction
furnace followed by ingot casting. Finally, the ingot was forged, hot rolled and coiling
simulation was done using a salt bath furnace. A lower YS/UTS ratio was achieved
through relatively larger ferrite grains (>6 μm) and pearlite phases (~10 -15 %)