BAINITIC STEEL OF HIGH STRENGTH AND HIGH ELONGATION AND
METHOD TO MANUFACTURE SAID BAINITIC STEEL
FIELD OF THE INVENTION
The present invention relates to high strength bainitic steel with a minimum
ultimate tensile strength (UTS) of 1300 MPa and an elongation of at least 20% as
well as to a method for manufacturing such a steel. The bainitic steel according to the
invention is suitable for use in the automotive industry as well as for other structural
applications.
BACKGROUND OF THE INVENTION
Environmental concerns in the recent times are forcing automobile industries
to reduce the weight of the vehicles by lowering down the thickness of the steels
used in different parts of an automobile. However, this weight reduction may not
compromise passenger safety. Passenger safety is directly related to the energy
absorbed during any possible collision and in turn related to the steel thickness for
the same strength level. One way of achieving both conditions (reducing the weight
of the automobile and stringent safety parameters) can be met by using higher
strength steel grade. Thus, the challenge is to develop stronger steel with better
ductility.
Several high strength and high elongation steel grades, providing a wide
range of strength/elongation combination from 600-1400 MPa UTS with 30-5%
elongation, are available worldwide. However, in most cases when the strength of the
steel goes up the elongation value goes down and it is difficult to achieve a good
combination of high strength and at the same time high elongation.

In the prior art bainitic steel is disclosed with nano-structured bainitic
microstructure and C-enriched austenite which can provide very high strengths of
about 2200 MPa but with a maximum elongation of approximately 7%. See for
instance:
- C. G. Mateo, F. G. Caballero and H. K. D. Bhadeshia, Journal de Physique IV, Vol.
112, pp. 285-288,2003;
- F. G. Caballero, H. K. D. Bhadeshia, K. J. A. Mawella, D. G. Jones and P. Brown,
Materials Science and Technology, Vol. 18, pp. 279 - 284,2002, and
- H. K. D. H. Bhadeshia, Materials Science and Engineering A, Vol. 481 - 482, pp.
36-39,2008.
In the composition of these known bainitic steels about 0.9 wt% C is used in
combination with costly alloying elements like Co and Ni. The steel is rapidly cooled
from austenite region to avoid any diffusional transformation and isothermally
transformed to bainitic steel by holding at a certain temperature or temperature range
for a long time, for instance 7 days at 200°C.
Although high strength bainitic steel with lower C are known also, these
steels however have a composition with high amounts of costly alloying elements
like Ni and Mo. See for instance:
- F. G. Caballero, M. J. Santofima, C. Capdevila, C. G. -Mateo and C. G. De Andres,
ISIJ International, Vol. 46, pp. 1479 - 1488,2006, and
- F. G. Caballero, M. J. Santofima, C. G. -Mateo J. Chao and C. G. De Andres,
Materials and Design, Vol. 30, pp. 2077 - 2083,2009.
According to prior art methods to manufacture bainitic steel, the steel is held
under isothermal conditions for a prolonged period of time to maximize the bainitic
transformation. However, due to slower kinetics at lower temperature such methods
are not ideal for continuous production of bainitic steel sheets and, moreover, due to
the prolonged period of time the process becomes very energy intensive.

Air-cooled bainitic steel is known from the works by G. Gomez, T. Perez and
H. K. D. H. Bhadeshia, Strong steels by continuous cooling transformation in
"International Conference on New Developments on Metallurgy and Applications of
High Strength Steels", Buenos Aires, Argentina, 2008. This bainitic steel is obtained
through continuous air cooling after hot rolling and the final product has a UTS of
about 1400 MPa with 15% elongation. However, also this composition has a
considerable amount of alloying elements like Mo and Ni. The purpose of adding
costly elements like Ni is to stabilize the retained austenite to provide the elongation
and Mo is added to increase the toughness of the steel.
Thus, the prior art lacks the development of a continuously cooled bainitic
steel which can deliver more than 1300 MPa UTS and at least 20% elongation
without the addition of costly alloying addition like Ni and/Mo.
OBJECTS OF THE INVENTION
It is therefore the prime concern of the present invention to propose a suitable
steel composition for producing high strength carbide-free bainitic steel which
overcomes the disadvantages of having to add costly alloying elements as known
from the prior art.
The isothermal holding at a fixed temperature for the bainite transformation
requires a huge quantity of energy and is thus not very environmental friendly. This
known method is also not feasible for higher productivity and continuous production.
An object of the current innovation is to produce the steel in an environment friendly
way by having, the bainite transformation taking place during cooling of the steel. In
this manner isothermal holding at a fixed temperature is no longer necessary which
results in saving energy costs, reducing pollution and allows to produce through an
existing industrial route.
Another object of the current invention is to propose a suitable chemistry of
the steel which can deliver UTS of minimum 1300 MPa and at least 20% elongation.

Another object of the invention is to ensure the presence of 70-80% nano-
structured bainite in the matrix along with 20-30% C enriched stable austenite to .
provide an excellent combination of strength and ductility.
Another object of the invention is to propose a method that can be carried out
in an existing hot strip mill like plant.
DESCRIPTION OF THE INVENTION
According to a first aspect of the invention, one or more of the above
objectives are met by providing a bainite steel with the following elements in weight
%:
C: 0.25-0.55
Si: 0.5-1.8
Mn: 0.8-3.8
Cr: 0.2-2.0
Ti: 0.0-0.1
Cu: 0.0-1.2
V: 0.0-0.5
Nb: 0.0-0.06
Al: 0.0-2.75
N: < 0.004
P: < 0.025
S: <0.025
the balance being iron and unavoidable impurities.
With this composition it has proven that a high strength bainite steel can be obtained
without the necessity of adding alloying elements like Ni and Mo as is known from
the prior art.
In this composition the C content has a crucial role in developing the final
microstructure and thus controls to a considerable extent the mechanical properties

of the bainite steel. The C content is a very effective solid solution strengthener and
has great effect on the stability of the retained austenite. To meet the objectives of
the present invention the C content should be in the range as above indicated, but
according to a preferred embodiment the C content of the bainite steel is in the range
of 0.30 - 0.40 wt% and even more preferable in the range of 0.30 - 0.40 wt%. With
these ranges an optimum of the effect of C in the composition according to the
invention is obtained.
The Si content in the composition prevents the formation of cementite (iron
carbide) due to its very low solubility in cementite. In the composition according to
the invention the Si content is needed to realize a carbide-free bainite. At the same
time Si enhances the solid solution strengthening effect.
The element Al in the composition also effectively hinders the formation of
cementite for the same reason as Si, and can be used to at least partly replace Si for
that purpose. For that reason the Si content may vary in the composition over a wide
range dependent on the Al content.
If the Si content is taken at a level of 1.0 - 1.8 weight % or a more limited
range of 1.2 -1.7 weight %, which gives very good results with the final bainite steel,
the Al content may be taken lower. The range of the Al content could be limited to
0.0 -1.50 weight % or even as low as 0.0 - 0.2 weight % depending on the amount of
Si.
Another reason to have a certain amount of Al in the composition is that it
acts to deoxidize the steel during the steel making process. This helps in getting a
more fluid slag which is easier to remove from the liquid steel bath.
The Mn in the composition of the bainite steel helps in avoiding the possible
formation of polygonal ferrite by shifting the diffusional bay of the time-temperature-
transformation (111) diagram to the right side on the time scale so that even with a
moderate cooling rate ferrite is not allowed to form. A further effect of Mn content is

that the bainite formation temperature can be lowered significantly by increasing the
Mn content. This will facilitate the formation of fine bainite. However, the Mn
content should not be'too high since that could result in a steel that is difficult to
weld.
Further Mn is an effective solid solution strengthener and can improve the
yield strength of the steel significantly.
With a Mn content in the range of 0.8 - 3.8 weight % the diffusional bay of
the time-temperature-transformation (TTT) diagram is shifted sufficiently to the right
side so that the cooling rate normally applicable in a hot strip mill will not lead to the
formation of ferrite, sufficiently fine bainite can be formed and also the solid solution
strength will be high.
According to a preferred embodiment the Mn content is within a range of 1.0
- 2.5 weight %. In tests very good results were obtained with Mn in the range of 1.6 -
2.1 weight %.
The addition of Cr to the composition helps to improve the hardenability of
the steel. During welding Cr can form carbides with the C present which will reduce
the softening of the steel in the heat affected zone (HAZ). Good results with the
composition according to the invention have been obtained with a Cr content of 0.7 -
1.5 weight % and also with a content of 0.9 -1.2.
The Ti in the composition will react with the available N to form TiN which
in turn forms fine TiCN precipitates which can improve the strength significantly by
precipitation strengthening. The addition of Ti should however be limited because
too much Ti would reduce the amount of C available to stabilize the retained
austenite. For that reason the amount is kept low and tests have shown that the
amount may even be lowered further to 0.08 or 0.07 weight % and even an amount
of 0.04 weight % has shown to give the desired results.
Also the addition of Cu contributes to strengthening of the steel through

precipitation strengthening. However, there is maximum to the Cu content since too
much Cu will result in difficulties with coiling and moreover the use of Cu will
increase the costs. Therefore a maximum is set at 1.2 weight %. Test samples
without addition of Cu have shown to fulfil the objectives of the invention.
The elements Nb and V have great effect on the yield strength through the
formation of fine sized carbides and carbo-nitrides which precipitate during or after
coiling. These carbides can improve the strength of the steel significantly without
deteriorating ductility. However, to avoid excessive strengthening and removal of
carbon of the matrix the content is restricted to the given upper limit.
The invention further provides a method for manufacturing a bainite steel
according to the above composition by heat treating the steel to form bainite steel
comprising the steps of:
- hot rolling a cast slab into strip,
- cooling the strip to a temperature above the bainite start temperature,
- coiling the strip at a temperature above the bainite start temperature,
- cooling the coiled strip by natural cooling.
It has turned out that with the above method the bainite formation takes place
when the strip is coiled, that is a situation wherein no further, heat is applied. In the
process of letting the coiled strip cool by natural cooling to ambient temperature the
transformation to bainite takes place without the necessity of having to apply extra
heat. This is a great advantage over the know methods wherein to have the bainite
. transformation take place large quantities of heat have to be applied to keep the
temperature constant at 200°C or higher for prolonged periods of time. Not only the
advantage of considerable energy savings that are realized with the method, another
clear advantage of the method is that the whole process can be a continuous process
instead of a batch process.
. The method further comprises the steps of

- preparing liquid steel of the required composition,
- casting the steel into a slab,
- cooling the slab.
The cast and cooled slab may be reheated to 1250°C for starting of the hot
rolling operation. The final hot rolling temperature is at least 850°C.
After rolling the hot rolled strip is rapidly cooled to a temperature in the range
of 400 - 550°C, which is well above the start temperature of the bainite formation.
This allows to coil the strip at a temperature in the range of 350 - 500°C which is
still for the greater part above the start temperature of the bainite formation and
prevents that the strip is cooled too rapidly which may result in an incomplete bainite
transformation.
With the method of the invention the final bainite steel obtained after cooling
the coiled steel to ambient temperature is carbide-free and has a microstructure with
15 - 30% of retained austenite and with bainite plates with a thickness of less than
100 nm. With 70 .- 85% carbide free bainite and 15 - 30% retained austenite in the
final bainite steel according to the invention a strength of at least 1300 MPa and an
elongation of at least 20% is realized. The hardness of the steel is at least 415 HVN.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 Calculated TTT diagram for the designed steel
Figure 2 Calculated To curve for the designed steel composition
Figure 3a Calculated amount of retained austenite as a function of isothermal
transformation temperature
Figure 3b Calculated ratio of film type to blocky type austenite as a function of
isothermal transformation temperature
Figure 4 Calculated strength of the designed steel
Figure 5 Schematic diagram of the hot rolling operation

Figure 6 Microstructure of the bainitic steel (a) Optical and (b) SEM
Figure 7 TEM photograph of the microstructure showing nanoscale bainite
with high dislocation density
Figure 8 XRD profile (experimental along with simulated) of the continuously .
cooled sample
Figure 9 Tensile test results of three samples exposed to continuous cooling
transformation after hot rolling.
DESCRIPTION OF THE FIGURES
In fig. 1 a TTT diagram is shown for a sample with a composition within the
ranges given in Table 1 below.

Table 1 Range of compositions
In the diagram Bs and Ms stand for respectively bainite start temperature and
martensite start temperature. It can be seen from this figure that a minimal cooling
rate 20 °C sec"1, which is typical of any hot rolling mill, is capable enough to avoid
the diffusional bay and in turn avoid the chance of formation of high temperature
products like ferrite. The difference between Bs and Ms temperatures provides a
reasonably wide processing window to carry out the method for producing bainite.
The Ms will further be suppressed by the formation of bainite where due to
the rejection of C from bainitic ferrite, adjacent austenite gets enriched with C, as
denoted by the 7o curve presented in Figure 2.

From Fig.2, it can be seen that the lower the transformation temperature, the
higher is the enrichment of C in austenite. Consequently all the austenite is expected
to be retained till the cessation of bainitic transformation. A sufficiently lower Bs
also provides the chance to produce lower bainite which is finer in nature and can
contribute for higher strengthening.
During the progress of bainitic transformation, the whole of the austenite
grain does not transform instantaneously to bainite. It is a gradual process; when the
first plate of bainite forms, it rejects its excess carbon which it can not accommodate
into the adjacent austenite. Further advancement of transformation therefore is
associated with a lowering of free energy due to the higher carbon content of
austenite from which bainite forms. Finally a time is reached when the free energies
of both residual austenite and bainitic ferrite of the same composition becomes
identical and therefore any further transformation becomes thermodynamically
impossible. To represents the locus of all the points, on a temperature versus carbon
concentration plot, where the stress-free austenite and ferrite of identical composition
have the same free energy. The bainitic transformation can progress by successive
nucleation of subunits of bainitic ferrite till the carbon concentration in the remaining
austenite reaches to its limit which is defined by the To curve. The maximum amount
of bainite which can be produced at any given transformation temperature is
restricted by the retained austenite carbon concentration which can not exceed the
limit given by the To curve.
In this approach, bainitic transformation is made to occur at such a
temperature where the diffusion of any elements except carbon is extremely
negligible. Hence it can be considered that during bainitic transformation no other
diffusional reaction interacts with it and the temperature is high enough for
restricting other diffusionless transformation product. The carbon enrichment in
austenite from adjacent bainitic-ferrite plates makes it thermally stable at room
temperature and it will only transform to martensite during deformation exhibiting a
TRansformation Induced Plasticity (TRIP) effect. .

Fig. 3a represents a theoretical calculation of the amount of retained austenite
after bainitic transformation at different isothermal temperatures whereas Fig. 3b
shows the calculated ratio between the blocky and film type austenite. In the Fig. 3b,
volume fraction of blocky and film type austenite are represented by Vr_b and Vy_f,
respectively. From Fig. 3a and Fig. 2b it is evident that the lower the transformation
temperature is, the lower will be the amount of austenite which is detrimental for
the expected TRIP effect and final elongation value. On the other hand, lower the
transformation temperature, higher the ratio between films to blocky austenite
which is required for the good ductility behavior. During TRIP effect, austenite
transforms to martensite and the material gets work hardened. As a consequence, it
is essential to have a certain amount of austenite remain uhtransformed at ambient
temperature so that TRIP effect can occur.
It can also be found from Fig. 3 that at temperature 350°C, the calculated
amount of retained austenite is approximately 24% and the ratio between the thin to
blocky austenite is 0.9. At further lower temperature, the kinetics of the
transformation becomes very sluggish and further reduction in the amount of retained
austenite is not very much expected.


Figure 4 represents the strength of the alloy which shows that the calculated
total strength of the designed steel could exceed 1500 MPa. The major source of
strengthening is coming from the ultra fine bainite plates. Another major source of
strengthening is from the dislocation density which was calculated to be in the range
of 4-6 x 106. Since there are some approximations and assumptions, the actual
strength will be below the calculated strength. As there is very little knowledge
available for bainitic transformation during continuous cooling, all the calculations
were carried out at many different temperatures considering isothermal nature of
transformation and then extrapolated to the continuous cooling situation.
Four 40 kg heats were made in vacuum induction furnace. The chemical
compositions of these four casts are given in Table 2 below.
Subsequently, the cast steels were forged to 40 mm thickness and
homogenized at 1100 °C for 48 hours after which the steels were cooled along with
the furnace. All the experiments were carried out with this homogenized steel.
Small pieces of samples (150 mm x 100 mm x 20 mm) were cut for hot
rolling in an experimental rolling mill. The soaking was done at 1200 ° for 3 hours.
The rolling operation was completed within 6-7 passes, keeping the final rolling
temperature at about 850 - 900°C. Throughout the experiments, temperature was
monitored with laser radiation pyrometer. After the hot rolling, the samples were
kept on run-out table where water jet cooling was applied till a temperature of 400 -
550°C is reached and finally the samples were kept inside a programmable furnace
where very slow cooling rate was applied to simulate the actual coil cooling
situation. The cooling rate of a coil after coiling in downcoiler in hot strip mill was
first measured with radiation pyrometer over a long period of time and similar
cooling rate was simulated in furnace for the simulation purpose. The temperature of
the furnace for coiling simulation was kept within 350-500°C. Schematic diagram of

the entire hot rolling process is shown in Fig. 5. The hot rolled thickness was about
3.0 mm.
Samples for metallographic observation were cut from the rolling plane of
one end of the heat treated samples. The samples were polished using standard
procedure, etched with nital and the microstructures are reproduced here in Fig. 6
where Fig. da is the optical microstructure and Fig. 6b is the SEM photograph. Image
analysis of the optical microstructures was carried out with the help of Axio-Vision
Software version 4 equipped with Zeiss 80 DX microscope and shows the presence
of significant amount of bainite (~75%) along with some retained (~ 25%) austenite.
The products of diffusional transformation, e.g. ferrite, cementite were not seen and
the bainite thus produced is a carbide-free bainite. The bainite plate thicknesses, as
can be observed from the TEM photograph presented in Fig. 7, are less than 100 nm
and the structure is highly dislocated.
The volume fraction and the lattice parameter of retained austenite were
calculated from the X-ray data by using commercial software, X'Pert High Score
Plus. The X-Ray Diffraction analysis results are shown in Table 3 below.

Table 3 Volume fraction of different phases along with C in austenite
Fig. 8 represents the calculated and experimentally obtained XRD profiles
along with the differences between these two. During the XRD analysis, it was
assumed that whatever ferrite is present is only bainitic ferrite as the diffusional bay
and its products were bypassed. From the Table 3, it is apparent that the C content of

retained austenite is higher than that predicted from calculated To curve shown in
fig.2. It should be kept in mind that the To curve was calculated at isothermal
condition and the actual experiments were carried out in continuous cooling form
producing different austenites with different C concentration. These different
austenites are not separable by XRD and XRD indicates average C concentration
only.
After continuous cooling to room temperature, hardness measurement was
carried out in Vicker's Hardness tester using 30 kg load. The hardness value turned
out to be 425 ± 9 VHN which is an averaged out value of 100 readings from four
different hot rolled and continuously cooled samples. See Table 4 below for all the
mechanical properties (hardness, YS, UTS, uniform elongation, total elongation).
The ultimate tensile strength is even more than 1350 MPa.

Table 4 Mechanical properties of the 4 casts
Standard tensile samples were prepared from the steel following the ASTM
procedure [ASTM E8] for standard samples of 50 mm gauge length and tested in
Instron tensile testing machine (Model number: 5582). Figure 9 shows the results of
the first three samples. From this figure, it is evident that the bainite steel according
to the invention has an outstanding combination of tensile strength (>1300 MPa)
with more than 20% elongation.

We Claim:
1. Bainite steel consisting of the following elements in weight %:
C: 0.30-0.50
Si: 1.0-1.8
Mn: 1.0-2.5
Cr: 0.7-1.5
Ti: 0.0 - 0.08
Cu: 0.0-1.2
V: 0.0-0.5
Nb: 0.0 - 0.06
Al: 0.0-1.50
, N: < 0.004
P: < 0.025
S: < 0.025
the balance being iron and unavoidable impurities.
2. Bainite steel according to claim 1, Wherein one or more of the following
elements are present in weight %:
C; 0.30-0.40
Si: 1.2-1.7
Mn: 1.6-2.1
Cr: 0.9-1-2
Ti: 0.0-0.07
Al: 0.0-0.2
3. Bainite steel according to one or more of claims 1-2, wherein the steel has a
hardness of at least 415 VHN.
4. Bainite steel according to one or more of claims 1-3, wherein the steel has a
ultimate tensile strength of at least 1300 MPa,
5. Bainite steel according to one or more of claims 1-3, wherein the steel has ft
ultimate tensile strength of at least 1350 MPa.
6. Bainite steel according to one or more of claims 1-5, wherein the steel has at
least a total elongation of 20%.

7. Bainite steel according to one or more of claims 1-6, wherein the bainite is
carbide-free and with a microstructure with bainlte plates with a thickness of
Jess than 100 nm.,
8. Bainite steel according to one or more of claims 1-7, wherein the steel has a
microsimcture with 15 - 30% of retained austenifce.
9. Method for manufacturing a bainite steel consisting of the following elements
in Weight %:
C: 0.25-0.55
Si: 0.5-1.8
Mn: 0.8-3.8
Cr: 0.2-2.0
Ti: 0.0-0.1
Cu: 0.0 -1.2
V: 0.0-0.5
Nb:. 0.0-0.06
Al:. 0.0-2.75
N: < 0.004
P: < 0.025
S: < 0.025
thebaiance being iron and unavoidable Impurities,
by heat treating the steel to form bainite steel comprising the steps of:
- hot rolling a cast slab into strip,
- cooling the strip to a temperature above the bainite start temperature,
- cooling the strip at a temperature above the bainite start temperature,
- cooling the colled strip by natural cooling.
10. Method according to claim 9, wherein the method further comprises the steps
of:
- preparing liquid steel of the required composition,
-casting the steel into a slab,
- cooling the slab.
11. Method according/to claim 10,.wherein the cast and cooled slab is reheated to
an austenltic state.

12. Method according to one or more of claims 9-11, wherein the final hot rolling
temperature is at least 850° C.
13. Method according to one or more of claims 9-12, wherein the hot rolled strip
is rapidly cooled to a temperature in the range of 400 - 550°C.
14. Method according to one or more of claims 9-13, wherein the strip isjcoiled at
a strip temperature in the range of 350 - 500°C.
15. Method according to one or more of claims 9-14, wherein the coiled strip is
naturally cooled to ambient temperature,
16. Method according to one or more of claims 9-15, wherein one or more of the
following elements are present in weight % in the bainite steel:
C: 0.30 - 0.50
Si: 1.0-1.8
Mn: 1.0-2.5
Cr: 0.7-1.5
Ti: 0.0-0.08
Al: 0.0-1.50
17. Method according to one or more of claims 9-15, wherein one or more of the
following elements are present in weight % in the bainite steel:
C: 0.30-0.40
Si: 1.2-1.7
Mn:" 1.6-2.1
Cr: 0.9-1.2
Ti: 0.0-0,07
Al: 0.0-0.2