FIELD OF THE INVENTION
The present invention relates to designing a novel composition and determining
the processing parameters to produce hot rolled dual phase steel with minimum
600 MPa UTS and excellent formability adaptable to automotive structural and
wheel disc applications.
BACKGROUND OF THE INVENTION
The growing demand for lighter vehicles with reduced fuel consumption in the
recent years has led to an increasing use of new and lighter materials like fibre
reinforced plastics and aluminium alloys [E.E Tuttle: SAE Paper No. 800065,
1980: J.R. Kinstler: SAE Paper No.800065, 1980; D.C. Wei: SAE Paper No.
820341, 1982; M Kawachi, T. Okita and K. Kanbayashi; SAE Paper No.820339,
1982]. But these materials are quite costly and exhibit inferior mechanical
properties as compared to steels. Besides, the weight of the automobile can be
reduced appreciably without compromising on cost, passenger safety and
comfort by substituting conventional low strength steels with higher strength
steels ["Ultra Light Steel Auto body (ULSAB) Final Report", AISI-North American
Steel Industry, 1998]. Evidently, the superior strength makes it possible to use
steel products of thinner gauges, resulting in reduction of the weight of vehicles
while retaining the desired rigidity of the structure.
Among the various auto-components, the weight reduction achievable for auto
wheels by substituting mild steel with high strength steels is found to be the
largest [T. Irie, K. Tsunoyama, M. Shinozaki and T. Kato: SAE Paper No. 880695.
1988], Furthermore, the corresponding energy savings is estimated to be 1.2-1.3
times, being the highest amount possible for any non-rotating part [M. Mizui, T.

Sekine, S. Soneda and T. Herai: SAE Paper No. 850540,1985]. As a result, in the
past few years, a large number of researches has been dedicated to
development of high strength steels for automotive wheel applications, which
can serve as potential replacements for the traditionally used mild steel.
The automotive wheel is normally composed of a disk and a rim. While the disc
is press formed, the rim is flared and then roll formed after flash butt welding.
Therefore, the material needed to form the disk needs to have deep drawability,
stretch formability and stretch-ability, whereas the material needed to form the
rim needs to have good formability after welding. After the wheel discs and rims
are formed by their respective processes, they are assembled by spot welding or
arc welding. Hence the materials used for both rim and disc needs to have a
good spot weldability. From the point of view of application, the most important
functional requirement for auto-wheels is durability, which can be increased by
enhancing the fatigue strength of the wheel material.
The various studies conducted in the recent past show that while precipitation
hardened steels (PHS) and duel phase (DP) steels (DP steels usually contains
two phases, one of them being harder the other being softer). Normally, the
harder phase is martensite or bainite, the softer one being ferrite. The DP steels
as eell as PHS are both suitable for wheel disc applications. However, DP steels
are not that suitable for wheel rim application, because during the welding

operation, tempering of martensite occurs which causes softening of the steel
[M. Shinozaki, Y. Matsumoto, T. Kato, M. Nishida and N.Sudo; SAE Paper
No.830279, 1983]. From the fatigue strength considerations, the upper limit of
the tensile strength of steels for wheel use is - 600 MPa (or 85 ksi) [ T. Irie,
K.Tsuroyama, M. Shinozaki and T Kato: SAE Paper No.880695, 1988]. This is
because when the tensile strength is increased beyond 600 MPa, consequent
increased notch sensitivity results in lowering of the fatigue strength. Hot rolled
DP steels with a tensile strength of 600 MPa (or HR-DP 600) have become a very
popular choice for wheel disc applications owing to their superior strength and
formability and at the same time good stretchability (high n value ) and spot
weldability. However, it is difficult to produce the HR-DP 600 steel in any mill
because many parameters, e.g. the finish rolling temperature, cooling rate etc
are needed to be optimized and fine tuned keeping in mind the mill configuration
e.g. the length of the run out table, water volume available etc in order to obtain
the desired microstructural features which in turn will decide the final mechanical
properties.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved process
to produce hot rolled dual phase (DP) steel with a mixed microstructure of about
85-90% polygonal ferrite and 10-15% martensite to deliver a minimum 600 MPa.
Another object of the present invention is to propose an improved process to

produce hot rolled dual phase (DP) steel with a mixed microstructure of about
85-90% polygonal ferrite and 10-15% martensite to deliver a minimum 600 MPa,
which maintains process parameters for example, finish rolling temperature
(FRT), coiling temperature (CT), cooling rate, time interval between each process
steps such that a mixture of 85-90% polygonal ferrite and 10-15% martensite
can be obtained.
A further object of the present invention is to propose an improved process of
producing hot rolled dual phase steel sheet which yields a hardness value of
•minimum 160 VHN (>600 MPa tensile strength).
A still further object of the invention is to propose a composition of dual phase
steel which provides a minimum 600 MPa and hardness value of at least 160
Vl+N.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Schematic diagram of the time-temperature combination on the run
out table after hot rolling, according to the invention.
Figure 2 Microstructure of the steel produced in the improved process of the
Invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING TABLES
Table 1 Composition of steel

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the invention, a new process is disclosed to develop hot rolled dual
phase microstructure in steel whose composition is shown in Table 1.
A hot rolled sheet of the composition as shown in Table 1 was used in an
improved process keeping in mind the final objective to develop a microstructure
with 10-15% martensite ( a ) along with 85-90% ferrite (a ) in order to develop
a steel which can deliver high UTS (minimum 600 MPa) with high elongation
(minimum 20%). All the processing parameters were tuned keeping the target to
fulfill such objective.
From the experimental work it was found that the steel needs to be finish rolled
at a temperature Ti where (A3 + 50 °C) < Ti < (A3 + 70 °C). After the finish
rolling, maximum of 10-15 °C (depending on the thickness of the strip) drop
takes place before the strip reaches the run out table. A first cooling step begins
on the run out table at TV This first fast cooling step continues till T2 such that
(A3 - 150 °C ) > T2 > (A3 - 170 °C ) after which there is another slow cooling
upto T3 where (A3 - 200 °C) > T3 > (A3 - 230 °C ). Once this temperature is

reached, there is a second fast cooling step upto a coiling temperature which is
<350 °C. Such is thermal profile is schematically shown in Figure 1.
The first fast cooling (T'1→ T2) brings down the temperature of austenite ( γ)
to a temperature range where austenite starts transforming to ferrite (α ) at
the earliest. In order to facilitate such γ→α transformation, normal air cooling
upto T3 is employed. It is expected that due to such cooling rate, about 85-90%
of austenite is transformed to ferrite by the time the temperature comes down to
T3. Thus, it is important to transform the remaining enriched austenite with
carbon to martensite for which the second fast cooling rate is needed to be
employed after T3. It is to be kept in mind that the cooling rate at each cooling
step are to be selected in such a way that the total cooling time does not exceed
the time spent by the hot rolled sheet. Similar cooling profile was replicated in
dilatometry study and Figure 2 represents the optical microstructure showing the
presence of about 85-90% ferrite along with 10-15% martensite. Such a
microstructure yielded a hardness value of 163 VHN which is expected to provide
a tensile strength of minimum 600 MPa following Eqn 1 (Ref. 4, 12 and 13 of
report).
UTS (MPa) = 0.0189(VHN)2 - 4.5466VHN + 862.7 (1)
FEM simulations carried out using microstructural details as inputs also confirmed
that a minimum tensile strength of 600 MPa can be achieved with the above
microstructure.

WE CLAIM
1. A method for producing hot rolled dual phase (DP) steel where at least 85
- 90% of austenite is transformed to ferrite during a two stage cooling step
carried-out subsequent to the finish rolling operation and where the
remaining 1-15% austenite sets directly transformed to martensite leading
to a mixed final microstructure of about 85-90% ferrite along with 10-15%
martensite,
wherein the first stage of controlled cooling of the two stage cooling steps
is conducted at a cooling rate of 20 - 100°C s-1 commensing after 1-15 °C
below the finish rolling operations on T1 where (A3 + 50 °C) < T1 < (A3 +
70 °C) when the steel is still in austenite range and continues till about
(A3 - 150 °C) > T2 > (A3-170 °G) within 0.5 - 5.0 second time interval so
that austenite starts transforming to ferrite after T2;
wherein after the first fast cooling of the two stage controlled cooling
steps, the cooling rate applied within the temperature of T2 where (A3 -
150 °C) > T2 > (A3-170 °C) and T3 where (A3 - 200 °C) would be 3 - 30
°C s-1 so that the total time spent within the temperature range from T2 to
T3 remains 3.5 - 9.5 seconds in order to allow transformation of 85-90%
austenite to ferrite;
wherein after the first stage of fast cooling followed by the slow cooling,
the steel is fast cooled from the temperature T3 where (A3 - 200 °C)
>T3 > (A3 - 230 °C) at a cooling rate of 20-100 °C s-1 so that the final strip
temperature becomes <350 °C within 0.5 - 4.40 second in order to
transform the 10-15% austenite.
2. The method for producing DP steel as claimed in claim1, wherein
remaining untransformed microstructure after formation of 85-90% ferrite

is transformed to martensite so that the final microstructure of the strip at
room temperature consists of 85-90% ferrite along with 10-15%
martensite.
3. The method as claimed in claim 1, wherein that chemical composition of
said DP steel is

ABSTRACT

The invention relates to a method for producing hot rolled dual phase (DP) steel
at room temperature wherein at least 85 - 90% of austenite is transformed to
ferrite during a two stage cooling step carried-out subsequent to the finish rolling
operation to a temperature where the remaining 1-15% austenite sets directly
transformed to martensite leading to a mixed final microstructure of about 85 -
90% ferrite along with 10 - 15% martensite starting from a steel with a chemical
composition as