Field of the Invention
The present invention relates to design of chemistry including heat treatment schedule for
high strength dual phase steel with minimum 600 MPa tensile strength with addition of
phosphorus to act as a solid solution strengthening element. More particularly, the invention
relates to high strength phosphorus added dual phase steel of 600 MPa tensile strength.
Background of the Invention
In recent times more and more focused research is ongoing on to reduce weight of
automotive vehicles to improve the fuel consumption by utilizing high strength low alloyed
steels. These steels are produced in different microstructures with addition of costly alloying
elements. Addition of these alloying elements in steel leads to increase in the strength with
associated increase in the cost of production and subsequent manufacturing processes such
as welding. So the approach to reduce the weight of the steel in-service by reducing the
thickness of the section must be coupled with steel design with low alloying elements.
Various high strength thinner section steels, such as hot rolled dual phase grade steels, with
various chemistries, microstructure and processing routes have been applied in auto wheels
to achieve significant weight reduction [T. Irie, K. Tsunoyama, M. Shinozaki and T. Kato: SAE
Paper No. 880695,1988].
An automotive wheel consists of a disk and a rim. The wheel disk is press formed. The rim is
flared and then roll formed after flash butt welding. The rim material needs good formability
post-welding as well as good fatigue strength for durability. Good fatigue strength can be
achieved by increasing the strength. However, it is a general phenomenon that increasing
strength levels is accompanied with a concomitant decrease in ductility (and hence
formability). The ductility decreases almost linearly with increasing strength. There is a trend
towards the use of hot rolled sheets of a minimum tensile strength of 600 MPa for wheel rim
application.

WO2005005670-A1 discloses a method of producing a hot rolled dual phase steel
(ferrite/bainite + martensite) to be produced through hot strip mill route, having minimum
tensile strength range of 490-500 MPa for automotive application. Though the proposed steel
is claimed to have excellent shape fixability, no spring back phenomenon and no wall
chamber, the fatigue performance is expected to be quite low considering the low strength of
the steel.
EP1398392A1 discloses a method of producing a hot rolled dual phase (ferrite + martensite)
steel of minimum tensile strength of 590 MPa for wheel disk applications. The steel is
subjected to baking after press forming. According to the proposed method the steel must
contain 0.5 to 2.0 wt% Si and 1.0 to 3.0% Mn. The addition of this large amount of alloying
elements is costly and the alloying elements segregate at the center of the hot rolled strip.
EP2053139B1 describes a method in which a hot rolled steel sheet is subjected to heat
treatment after forming so as to achieve a tensile strength varying in the range of 440 to 640
MPa. However, the heat treatment after forming, which is an essential part of the invention,
is likely to add to the processing cost and hence is not suitable for mass production.
EP2578714A1 teaches a method of producing hot-rolled steel sheets with a minimum tensile
strength of 590 MPa with excellent bake hardenability and stretch-flangeability. According to
the proposed method the steel must contain 1.7 to 2.5 wt% of Mn. When added in such large
amounts, Mn tends to segregate in the central portion in the thickness direction, which not
only induces cracking during press forming but also leads to inconsistency in achieving the
desired stretch-flangeability. Moreover, additions of Cr/Mo/Ni/B for increasing hardenability
and Nb/Ti/V for refining austenite grain size and precipitation strengthening are
incorporated, all of which leads to an increase in cost of the steel.
EP2586886A1 discloses a method of producing a high-strength hot-rolled steel sheet of a
minimum tensile strength of 590 MPa and possessing excellent stretch flangeability. The
proposed steel relies on precipitation strengthening of ferrite by Titanium. The dissolution
temperature of Titanium 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.

It is known that elements like Si, Cr, Mn etc can be used as a solid solution strengtheners in
ferrite-martensite dual phase grade steels. But addition of these alloying elements increases
the cost of production as well as post-production treatments such as welding. Therefore
there is a need to replace the existing grade of dual phase steels (produced through hot strip
mill) with addition of P as a solid solution strengthening element substituting costly alloying
elements. Si is a cheap alloying element (than Cr, Mn etc) which can act as a strengthener in
ferrite-martensite dual phase steel, but it generates hard SiO2 scale during hot rolling which
is not easily removable. P is also a cheap alloying element which is generally available in iron
ore itself. However addition of P leads to formation of Fe3P compound which causes cold
work embrittlement (CWE) in steel. So a judicious P- content with addition of Mo, which
prevents P segregation in microstructure and associated formation of Fe3P, can be designed
for producing dual phase grade steel with 600 MPa minimum tensile strength. This grade of
steel contains a very small amount of Cr for hardenability as it already has Mo which also
provides hardenability. The heat treatment schedule for such steel also needs to be
designed. P-contained dual phase steels with 60-75% ferrite and 25-40%
bainite+martensite as second phase is a suitable material in this regard.
Objects of the Invention
It is therefore an object of the present invention to propose high strength phosphorus added
dual phase steel of 600 MPa tensile strength which eliminates the disadvantages of the prior
art.
Another object of the invention is to propose high strength phosphorus added dual phase
steel of 600 MPa tensile strength which can be achieved through a heat treatment is
schedule applicable in conventional hot rolling mills.
Another object of the invention is to propose high strength phosphorus added dual phase
steel of 600 MPa tensile strength which includes an optimum quantity of Mo to prevent P
segregation in microstructure and associated formation of Fe3P to prevent cold work
embrittlement.

A further object of the present invention is to propose high strength phosphorus added dual
phase steel of 600 MPa tensile strength which includes a very less Si content (<0.1%) to
prevent hard sticky scale formation during hot rolling.
A still another object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 600 MPa, which possesses a microstructure consisting of ferrite
and bainite + martensite as second phase.
Another object of the present invention is to propose a heat treatment schedule for this steel
which can be easily applied in conventional hot rolling mills to produce a dual phase steel
with 600 MPa minimum tensile strength.
A still further object of the present invention is to propose a method of producing a dual
phase steel sheet which yields a hardness value of minimum 200 VHN (>600 MPa tensile
strength).
Detailed description of the invention
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention contains in weight percent 0.06-0.1% of C, 1.15-1.25% of Mn,
<0.1% of Si, 0.013-0.018% of Nb, 0.25-0.35% of Cr, 0.04-0.06 % P, 0.1-0.15% Mo,
maximum 0.005% of S, 0.007 - 0.01% of N, maximum 0.05% of Al, maximum 0.01% of Cu,
the remaining being substantially iron and incidental impurities.
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has a microstructure comprising 60-75% ferrite and 25-40% bainite
+ martensite.
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has a P of 0.04-0.06 %, which is decided on the basis of the solid
solution strengthening contributions of Mn, and P.
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has Si content less than 0.1% so as to avoid the possibility of hard
sticky scale formation in hot rolling mill.
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has a Mo content of 0.1-0.15% to prevent grain boundary
segregation of P with associated improvement in hardenability.

The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has a very less amount of Cr: 0.25-0.35% to achieve hardenability.
The high strength dual phase steel having a minimum tensile strength of 600 MPa, according
to the present invention has a Mn addition of 1.15-1.25 % to compensate for the removal of
Si and its associated effect on the austenite to ferrite transformation behaviour.
The method of manufacturing the high strength dual phase steel with a ferrite + 2nd phase
(martensite+bainite) microstructure with a minimum tensile strength of 600 MPa consists of
heating the steel in the austenitizing temperature of 950 °C for 5min, then cooling to 930 °C
at cooling rate of 3°C/s, then cooling to the intercritical temperature of 700 °C at cooling rate
of 50°C/s, then slow cooling at a rate of 3°C/s to 670 °C, then final quenching to room
temperature.
According to the present invention, it is possible to produce a high strength dual phase steel
with a minimum tensile strength of 600 MPa, consisting of a ferrite + martensite
microstructure. Such a steel sheet is adaptable in a wide spectrum of industrial fields
including the automobile industry. It is particularly suited to manufacture automotive parts
and components which demand high strength, good formability and weldability, especially
parts like auto wheels, suspension and chassis components.
Brief description of the accompanying drawings Drawings:
Fig. 1: Schematic diagram of cooling profile.
Fig. 2: Transformation temperatures of the experimental heat.
Fig. 3: Continuous cooling transformation (CCT) diagram of the experimental heat.
Fig. 4: RoT cooling schedules 1 and 2 superimposed on CCT diagram.
Fig. 5: Microstructures corresponding to the RoT cooling schedules 1 (a) and 2 (b).
Fig. 6: RoT cooling schedules 3 and 4 superimposed on CCT diagram.
Fig. 7: Microstructures corresponding to the RoT cooling schedules 3 (a) and 4 (b).
Fig. 8: RoT cooling schedules 5 and 6 superimposed on CCT diagram.
Fig. 9: Microstructures corresponding to the RoT cooling schedules 5 (a) and 6 (b).

Brief description of tables
Table l:Composition of the experimental cast.
Table 2: The various cooling strategies simulated on the Gleeble 3800D using the lab made
heat.
Detail description of the invention
The present invention relates to a high strength dual phase steel which has a specific alloying
composition and is manufactured with a precise control of the heat treatment schedules to
produce the target microstructure, such that a minimum tensile strength of 600 MPa is
achieved.
The basic components constituting the hot rolled steel sheet produced according to the
present invention are described below.
1. Alloying additions: The addition of each alloying element and the limitations imposed on
each element are essential for achieving the target microstructure and properties.
C: 0.06-0.1%: Carbon is always present in steel to provide effective and economical
strengthening. Carbon combines with Nb to form carbides or carbonitrides which bring about
precipitation strengthening and grain refinement. This requires a minimum of 0.06%C in the
steel. However, in order to avoid the peritectic reaction during casting (especially for
continuous strip production or CSP facilities) and considering voidability issues, the carbon
content has to be restricted to less than 0.1%.
Mn: 1.15-1.25%: Manganese not only imparts solid solution strengthening to the ferrite but
it also lowers the austenite to ferrite transformation temperature thereby refining the ferrite
grain size. However, the Mn level cannot be increased to beyond 1.25% as at such high levels
it enhances centerline segregation during continuous casting.

Si: <0.1%: Silicon like Mn is a very efficient solid solution strengthening element but forms
hard scale at RoT. So Si content should be restricted to less than 0.1% in order to prevent the
formation of surface scales.
P: 0.04-0.06%: Phosphorus is a very efficient and cheap strengthening element. P should be
added more than 0.04% to achieve strengthening. But P content should be restricted to
0.06% maximum as higher P levels can lead to reduction in toughness and voidability due to
segregation of P into grain boundaries.
Mo: 0.1-0.15 %: Molybdenum is added to prevent (1) the segregation of P in the
microstructure and (2) precipitation of Fe3P which causes cold work embrittlement. A
minimum 0.1% Mo is required to be added for the P-content mentioned above. However Mo
also provides hardenability. A very high hardenability shall produce a more than required
amount of martensite in the microstructure. Also Mo is a costly alloying element. So a
maximum of 0.15% of Mo is restricted in the steel.
Cr: 0.25-0.35%: Chromium is a very effective alloying element to improve the hardenability
of the steel. A minimum of 0.25% of Cr is required (along with 0.1-0.15% Mo) to impart the
hardenability such that austenite to martensite transformation can take place during cooling
at RoT. However, the Cr level cannot be increased to beyond 0.35% as it adds to difficulty in
the weldability of steel.
Nb: 0.013-0.018%: Niobium is an effective micro alloying element for grain refinement even
when it is added in very small amounts. Nb in austenite lowers the ferrite transformation
temperature and refines the ferrite grain size. However, to ensure that, Nb should not be
allowed to precipitate as NbC/NbCN before the transformation temperature is reached and
the entire Nb content remains in solution before rolling commences, the maximum Nb
content is restricted to 0.018%. This limit has been specifically set keeping in mind the low
equalization temperatures possible in CSP (compact strip production) processes.
S: 0.005% maximum: The Sulphur content has to be limited to avoid the deleterious effect
on formability.

N: 0.007 - 0.01%: Nitrogen forms precipitate like nitride or carbo-nitride with Nb and
precipitates at the high temperatures during hot rolling. So to achieve the grain refinement
by Nb, it is essential to maintain a minimum of 0.007% N in the steel. On the contrary, too
high a N content raises the dissolution temperature of Nb(CN) and hence reduces the
effectiveness of Nb. So an upper limit is restricted to 0.01 %. Reducing N 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 0.001% or more specifically below 0.009%.
2. Microstructure:
The microstructure requirement as outlined above requires 60-75% ferrite and 25-40%
bainite + martensite for high strength dual phase steel having a minimum tensile strength of
600 MPa. The presence of hard second phase shall provide the requisite strength in the steel.
The presence of soft ferrite shall provide the ductility and the associated formability in the
steel. The steel is aimed for use in automotive components. So the steel shall have sufficient
durability which is measured through the fatigue strength. The fatigue performance
improves as the strength of ferrite is improved in dual phase grade steels. This is achieved by
adding solid solution strengthening elements like P & Mn in steel.
Ferrite: The high strength dual phase steel according to the present invention has 60-75%
ferrite. The ferrite is strengthened by solid solution strengthening contributions from P and
Mn.
Second phase : The high strength dual phase steel according to the present invention has
25-40% bainite + martensite as second phase. This hard second phase adds to the overall
strength of the dual phase steel. A minimum of 25% second phase is required to achieve 600
MPa UTS.
3. Heat treatment schedule
In order to design the cooling strategy which can be easily adopted in the RoT of a hot strip
mill or CSP facilities, the following steps were followed:
• An experimental heat was made in the laboratory with the proposed chemistry.
• Continuous Cooling transformation (CCT) diagram for the laboratory heat was
generated using dilatometry data generated using Gleeble 3800D thermo-mechanical
simulator.

• On the basis of the CCT diagram generated thus, several cooling schedules were
simulated using Gleeble 3800D thermo-mechanical simulator to arrive at the
appropriate cooling schedule which would generate the desired dual phase
microstructure.
• Vickers hardness of the most suitable microstructure was measured, as three times
the Vickers hardness is equal to the tensile strength of steel.
The schematic cooling profile that is followed to produce hot rolled high strength dual phase
steel at any hot strip mill or CSP facilities is shown in the Fig. 1.
. The definitions of the various temperatures marked on the figure are given below:
a) Ti: Finish rolling temperature or FRT.
b) TV The temperature of the strip at the beginning of the laminar cooling section.
c) T2: The temperature of the strip at the end of the first rapid cooling step which roughly
corresponds to a temperature at which the austenite to ferrite transformation kinetics
is fastest.
d) T3: The temperature at the end of the second stage of cooling. Between T2 and T3, the
water headers are kept closed and so the strip is subjected to air cooling only. This
slow cooling facilitates 60-75% transformation of austenite to ferrite, and is therefore
the most critical step for obtaining DP microstructure. After point d) the strip is again
subjected to very rapid cooling so that the untransformed austenite can transform to
martensite.
COMPARATIVE EXAMPLES
The composition of the cast and forged experimental heat is given below: Table 1:


The figure below shows the transformation temperatures at various cooling rates as
obtained from the dilatometry data. The 0.75a indicates the locus of temperatures at which
75% transformation to ferrite has completed. It was generated using microstructure analysis
and the dilatation curve obtained at the different cooling rates. The Vickers hardness
(measured using 1 kg load) of the resultant microstructures is also indicated on the diagram.
From the transformation temperatures obtained the OCT diagram for the experimental heat
was generated and the same is shown in Fig. 3.
On the basis of the CCT diagram, RoT cooling strategies were simulated using Gleeble
3800D. The various cooling strategies simulated on the Gleeble 3 800D using the lab made
heat are given in the table below:


The cooling schedules and the corresponding microstructures generated are shown.
From the test results it is clear that a ferrite fraction of ~65% can be obtained using cooling
schedule 2. The Vickers hardness of the said microstructure at 1 Kg load is 232. The tensile
strength of the steel is calculated using the following relation (Ref: STN EN ISO 18265):
UTS~3.21*VHN (in MPa) ...equation (1).
The calculated tensile strength of the material is 744MPa, which is well above the minimum
tensile strength of 600 MPa.

WE CLAIM:

1. A process for making a hot rolled high strength steel (HRHSS) product, the process
comprising steps of:
casting a steel slab with composition C: 0.06-0.1,Mn: 1.15 -1.25,Si: < 0.1,Cr: 0.25 -0.35,

S: 0.005 max, P: 0.04-0.06,AI: 0.05 max,N: 0.007-0.01,Nb: 0.013-0.018,Mo: 0.1- 0.15,
Cu: max 0.01 rest iron (Fe) and unavoidable impurities (all in wt. percentage);

hot rolling the steel slab into strip at finish rolling temperature (FRT) of 930 - 950 °C;
cooling the hot rolled strip at 50°C /s or more to inter-critical temperature of 700°C;
slow cooling at rate of 3 °C/sec or below to 670 °C;
quenching at a rate of 40°C/sec or more to the coiling temp of 200°C or below; and
coiling the hot rolled strip and then air cooling to room temperature.

2. A hot rolled high strength steel (HRHSS) product comprising:
composition of C: 0.06- 0.1,Mn: 1.15- 1.25, Si: < 0.1, Cr: 0.25- 0.35, S: 0.005
max,P: 0.04-0.06,AI: 0.05 max,N: 0.007- 0.01,Nb: 0.013-0.018,Mo: 0.1- 0.15,CU: max
0.01 rest iron (Fe) and unavoidable impurities (all in wt. percentage), tensile strength
1000-1200 MPa and total elongation of 16-17%.

3. The hot rolled high strength steel (HRHSS) product claimed in claim 2, wherein the
composition of the HRHSS product is C: 0.094,Mn: 1.15,Si:0.004,Cr: 0.31,P: 0.057,N:
0.0058, Nb: 0.016, Mo: 0.13 rest iron (Fe) and unavoidable impurities (all in wt.

percentage).

4. The hot rolled high strength steel (HRHSS) product as claimed in claim 2, wherein the
microstructure of the produced steel comprises ferrite: 60-75 and bainite + martensite:
25-40 (all in vol. %), and wherein the ferrite is the solution is strengthened by

phosphorus and manganese.

5. The hot rolled high strength steel (HRHSS) product as claimed in claim 2, whereinthe
tensile strength of the product is > 600 MPa.