TITLE:
A method for producing cold rolled transformation induced plasticity aided
steel.
FIELD OF INVENTION:
This invention relates to a method for producing cold rolled transformation
induced plasticity aided steel.
BACKGROUND OF THE INVENTION:
The need for addressing stringent environment and safety norms has
compelled the automakers to design light-weight auto bodies with enhanced
crash resistance. A new family of multi-phase steels having higher strength
and better formability, named Transformation induced plasticity (TRIP)
aided steel is gaining popularity to satisfy the demand. The TRIP effect
arises from deformation induced transformation of retained austenite to
martensite. It is accompanied by an invariant plane strain shape deformation
as well as a volume expansion. This results in a higher strain hardening rate
that delays the onset of necking, eventually resulting into higher uniform and
total elongation. An important feature of steels of this genre is enhanced
ductility at a very high strength level. This steel is ideal for passive safety
structural applications like bumper reinforcement, door impact beam etc.,
due to its high strain hardening rate and dynamic energy absorption. The
microstructure of conventional TRIP aided steels comprises ferritic matrix

(~55-65%) along with bainite (~25-35%) and metastable retained austenite
(~5-20%). The TRIP effect depends on the amount of retained austenite and
its stability to deformation induced transformation.
Conventionally, TRIP aided steels contain about 1.5 wt% silicon which
enhances the volume fraction and stability of the retained austenite by
suppressing cementite formation during the isothermal bainitic
transformation. Published results show that Si level should be more than
0.8% to obtain reasonable amount of retained austenite. However, the
conventional TRIP aided steel with high silicon (1,5 wt%) suffers from poor
wettability during galvanizing due to deleterious effect of silicon. It is
reported that silicon in concentration higher than 0.5 wt% is detrimental to
coatability. High Si leads to poor weldability as well. To avoid this problem
silicon can be partially replaced by aluminium without giving any harmful
effect on coatability. It helps in retarding cementite formation like silicon
and is also insoluble in cementite.
A judicious modification of chemistry by reduction of Si and addition of Al
and some micro alloying elements, such as, Nb combined with proper heat
treatment cycles can result in a material with the required strength. Addition
of Nb helps to refine austenite formed during the intercritical annealing. It
leads to formation of some pro-eutectoid ferrite during cooling to the
isothermal transformation temperature, which enhances the carbon
enrichment of the untransformed austenite. The finer grain size helps to

increase the stability of austenite by reduction of Ms (martensite start)
temperature.
In the current invention, such a steel sheet/strip have been proposed which is
having an excellent strength elongation combination (Ultimate Tensile
Strength (UTS) >850 Mpa, Uniform Elongation > 20%) with an improved
wettability (coatability during hot dip galvanizing).
Such steel sheet/strip have been proposed in US 2007/0020478 Al which
specifies UTS > 600 Mpa (which is lesser than the present invention) and
without any specific mention about the uniform elongation which is a very
crucial value for this sort of steel genre. It also shows that silicon was almost
replaced by aluminium whereas for the present proposed work, silicon is
partially replaced (for better wettability) so that the strengthening effect
offered by silicon is also sustained. There is no claim of addition and impact
of niobium in the afore-said patent. Such steel sheet/strip have also been
proposed in US 2006/0140814 Al which guarantees a UTS of the tune of
980 Mpa and strain hardening coefficient of 0,14, with a carbon level having
a larger range (between 0.13-0.26%), where as published data and
theoretical understanding, proposes that carbon should be restricted within
0.2% for better weldability.
Such steel sheet/strip have still been proposed in US 7,294,412 B2/ Nov. 13,
2007 which confirms addition of nickel (costlier additive) and upper limit of
carbon to the extent of 0.25%.

The proposed invention has been developed to solve the difficulties of
achieving mechanical properties to the desired level with alloy engineering,
with the help of multi layered perceptron neural network method, as well as
enhancing the coatability through alloy engineering and experiments with
the operational modalities during heat treatment process within a hot dip
galvanizing simulator, popularly known to be Rhesca simulator.
OBJECTS OF THE INVENTION:
An object of mis invention is to propose a method for producing cold-rolled
transformation induced plasticity aided steel;
Another object of this invention is to propose a method of producing cold-
rolled high strength high elongation steel with UTS>800 MPa;
Still another object of this invention is to propose a method of producing
cold rolled steel having uniform elongation >20% and strain hardening
coefficient greater than equal to 0.20;
Further, object of this invention is to propose a method of depicting the
impact of different chemical compositions on mechanical properties vis-a-
vis wettability of cold rolled TRIP-aided steel while carbon content is same
for all cases;

A stil! further object of this invention is to propose a method of depicting the
impact of different chemical compositions and dew points on wettability of coid
rolled TRIP-aided steel while carbon content is same for all cases.
BRIEF DESCRIPTION OF THE INVENTION :
According to this invention there is provided a method of producing high
strength high uniform elongation cold-rolled Transformation Induced Plasticity
(TRIP)-aided hot dip galvanized steel sheets for automotive application as crash
resistant components comprising the steps of:


Grade C-Mn-Si-Nb
C-0.18 to 0.20 Mn-1.4 to 1.55 Si-1.55 to 1.65
S-not less than 0.01 P-not less than 0.02 Al-not less than 0.01
N-not less than 0.005 Nb - 0.03 - 0.04
casting the liquid steel in to ingots;
soaking the ingot at 1150°C to 1180°C and forging to 25mm to 30 mm thick
plates,
hot rolling the forged plates to an average thickness of 3.5 mm to 4mm at FRT
880-920°C;
air cooling the hot rolled plate,
pickling to remove the surface scales and cold rolling by 70% to 75%,
surface cleaning of the cold rolled sheets followed by two step heat treatment
(intercritical annealing and isothermal bainitic transformation) in controlled
atmospheres (and different dew points) and hot dip galvanizing (in Rhesca
simulator).

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figs, la and lb: Shows a graph indicating the relationship between dilatation
and transformation temperature.
Figs. 2a and 2b: Shows contours of predicted retained austenite for different
intercritical annealing (IA) time and temperatures for a fixed isothermal
bainitic transformation (IBT) parameter [IBT temperature: 450°C and time:
120 seconds].
Fig. 3: Shows the schematic diagram for heat treatment followed by
galvanizing in Rhesca simulator.
Figs. 4a and 4b: shows the engineering stress strain diagrams.
Figs 5a and 5b: shows the microstructures.
Fig 6: shows the x-ray diffraction results.
Fig 7: shows appearance of coated areas of samples from both the grades at
different operating conditions.

DETAILED DESCRIPTION OF THE INVENTION:
The present invention relates to a process of preparing a method of
producing high strength- high uniform elongation cold-rolled
Transformation Induced Plasticity (TRIP)-aided hot dip galvanized steel
sheets (in laboratory) for automotive application as crash resistant
components comprising the steps of:


- casting the liquid steel in to ingots;
- soaking the ingot at 1150°C and forging to 30mm thick plates,
- hot rolling the forged plates to an average thickness of 4mm at
soaking temperature of 1150-1180°C andFRT 880-920°C;
- air cooling the hot rolled plate,
- pickling to remove the surface scales and cold rolling by 70%,
- surface cleaning of the cold rolled sheets followed by two step heat
treatment (intercritical annealing and isothermal bainitic
transformation) in controlled atmospheres (and different dew points)
and hot dip galvanizing (in Rhesca simulator).
Amongst the compositions of the steels selected for the study, one has higher
aluminium with partially replaced silicion, and the other has conventional
silicon content, to compare the wettability under identical process flow. Bom
the steels are also containing a small amount of Nb for the reasons as
discussed in the previous section. As already discussed, TRIP aided steels
are given a two-stage heat treatment consisting of intercritical annealing and
austempering treatment to obtain the desired microstructure. It is well
established that excellent combination of mechanical properties are obtained
when the amount of retained austenite in the final product is adequate.
Accordingly, the composition of the steel was selected so as to obtain about
10-15% retained austenite in the steel after the heat treatment; the details of
this estimation is described below. The carbon content of the steel was

restricted to 0.2 wt% to ensure good weldability. However, based on
operating conditions of the simulator (referred as Rhesca in the following
section), some constraints were envisaged due to restrictions on process
parameters during the present invention for one amongst the two grades.
The dialatation behaviour against temperature (with a heating and cooling
rate 0.5°C s"1 selected for homogeneity) and transformation temperatures
were observed from Gleeble 3500D work (Ref. Fig. la and lb) which
revealed the intercritical boundaries [for CMnSiAINb grade between 735°C,
and 1035°C, for CMnSiNb, between 704 °C and 880°C].
Based on the existing facilities of the Rhesca simulator (IEHK, RWTH
UNIVERSITY, GERMANY THROUGH DST-BMBF PROJECT], the
intercritical annealing time was restricted within 1 minute. Based on the
findings from previous works [COMMUNICATION in MSE A, by the
inventor], the inetrcritical annealing temperatures for the CMnSiAINb grade
was fixed at 800°C and 860°C, and for CMnSiNb, inetrcritical annealing
temperatures were fixed at 770°C and 790°C respectively.
As shown in Figs. 2a and 2b, the contour of retained austenite in matrix,
after isothermal bainitic transformation (IBT, temperature fixed here for
450°C and time, 120 seconds, temperature and time fixed at those levels,
considering galvanizing after IBT], was predicted through Multilayered
Perceptron Neural Network with the following architecture:

Total data used for modeling: 216 Nos.
Total data used for training: 50% of the total data
Total data used for testing: 25% of the total data
Total data used for validation: 25% of the total data MSE=0.033217
Technical / Architectural input for paper
Nodes in Input Layer: 11
Nodes in Output Layer: 1
Nodes in Hidden Layer: 15
Input to hidden layer transfer function: Hyperbolic tangent
Hidden layer to output transfer function: Linear
Training Algorithm: Feed forward network trained with scale conjugate
gradient back propagation algorithm.
The contours indicated that operating between 790-800°C for the
CMnSiADMb grade and 765-770°C for CMnSiNb grade respectively with 60
seconds time would yield a higher amount of retained austenite amongst the
choices. Accordingly, process parameters were selected as shown in Table 1
and Table 2. Cooling parameters from intercritical annealing (IA) to
isothermal bainitic transformation were judiciously selected to avoid any
ferrite formation during that period. Fig. 3 shows the schematic heat
treatment cycle.

The heat treatment followed by the hot dip galvanizing operation were
carried out in a Hot dip process simulator (HDPS), named as Rhesca.
The hot dip process simulator (HDPS) or Rhesca simulator allows to
simulate the annealing and hot dipping process of steel strip in accordance
with the commercial production of hot dip coated steel strip. The simulator
serves for development of new steel grades on one hand and for simulation
of a continuous hot dip galvanisning line on the other hand. Complex heat
treatment cycles with subsequent coatings are possible. Steel sheets samples
(120mm x 200mm) are heat treated and subsequently coated by hot dipping
in laboratory scale.
The apparatus is characterized by a vertical arrangement of a sample drive
unit, and infra-red heating furnace a cooling box and separated by a gate
valve from the upper heat treatment zone a melting bath. The whole system
is sealed. The sample is moved by a spindle which is attached to a servo
drive. The maximum sample speed is 1000 mm/sec within the modules.
Sample temperature is controlled with the help of a carried along thermo
couple (Pt/PtRh) which is spot-welded on the sample surface. The heat cycle
can be freely defined and is monitored and adjusted by a high speed
controller. Hie maximum heating rate is about 40 K to a maximum
temperature of 900°C referring to cold-rolled steel strip (thickness-0.7mm).

Process gas is supplied by a gas mixing station. The atmosphere of the upper
section during annealing can be set under programme control using N2, H2
and optionally CO, C02, CH4 or NH3 as input gases. The use of this reactive
gases allows a surface enrichment of the steel strip with C or N. The dew
point of the atmosphere can be set in the range between -60°C to + 5°C. A
variation of annealing atmospheres during a single heating cycle is limited
because of maximum gas flow and the over all gas volume of the upper
section. For example, an oxidation/reduction cycle can not be performed in
one state. For this, two separate stages with intermediate cooling of the
sample are necessary. For oxidation processes, oxygen can be used as an
additional gas.
The gas jet cooling uses N2, H2 and /He depending on the required cooling
rates. Maximum cooling rates are about 100 K/s (900°C-400°C, sheet
thickness of 0.7 mm). The cooling plates are water cooled to provide steady
cooling conditions.
The melting bath separated by a pneumatically operated gate valve. The
lower chamber is permanently purged using N2 as protective gas during
operating. The graphite crucible by a conductive heating device. The melting
bath temperature is adjusted with the help of a thermo couple which is
placed in a bath and a controller. Maximum temperatures of 800°C can be
achieved. A sample guiding system is installed. Wiping nozzles are to
control the thickness of the coating using as wiping gas.

Subsequently the coated sheet can be annealed in the infra -red heating
furnace (called galvannealing, for this invention not followed) or directly
moved in to the cooling box (conventional hot dipping process, for this
invention followed). The melting bath can be run in two different
modifications: without or with bath agitation. Without bath agitation the top
dross on the surface of the bath can be removed using the dross removing
paddle.
The crucible then contains approximately 4.6 1 of liq zinc. With bath
agitation, the volume of zinc is reduced to 2.2 1. A zinc stirring equipment is
mounted in the crucible, allowing a zinc movement in a duct.
The stimulator is equipped with a vacuum pump to evacuate the upper
system before the experiments start.
The process control system mainly consists of a programmable logic
controller (PLC) unit for high speed control tasks associated with the heating
system and a PC for programming, visualization and documentation of the
process.

Further components as a positioning controlling unit for the traversing
system and dew point control system employ an additional Siemens S7PLC.
The control system has to ensure various functions as preparing the
simulation by adjusting all necessary parameters, performing the simulation
process, monitoring the process, visualizing online and displaying the
results.
The dew point is the temperature to which a given parcel of air must be
cooled, at constant barometric pressure, for water vapor to condense into
water. Higher dew point leads to higher humidity and higher oxygen
potential and lower dew point results in dry atmosphere and lower oxygen
potential. It is understood that for typical Mn-Si grades, formation of
Mn2Si04 oxide film on the steel surface during heat treatment leads to poor
wettability. Increasing humidity in furnace atmosphere (i.e. dew point)
facilitates internal oxidation of Mn and Si, controlling Mn2Si04, resulting in
improved coating. Based on the facilities of the simulator, two dew point
ranges were selected (- 40 °C and +5 °C). Table 1 and Table 2 are showing
all relevant data.

Table 3: is showing the mechanical properties achieved and other relevant
technical information. It shows a clear.
Figs. 4a and 4b, shows the stress strain diagram for the available property
ranges.
Table 3: Mechanical properties of steel

Figs. 5a and 5b are showing distinct presence of retained austenite in the
matrix through optical microscopy which is supported by the x-ray
diffraction (Ref. 6).

The retained austenite% found in CMnSiAINb grade was between 11-14%
by experimental procedures while through the MLP Artificial neural
network, the same was predicted around 12-14%. For the CMnSiNb grade,
the experimental processes had shown presence of retained austenite against
of 6-10% against a prediction of 6-8%. A satisfactory corroboration between
the prediction and experimental procedures have been observed in this
process.
Fig. 7 shows the coating condition and Table 4 depicts the result of
evaluation of coating quality.


Coating thickness was measured by coating thickness gauge "Delta Scope"
For higher dew points, both the grades have shown better coatability
(wettability) which is clear from Fig. 7. It is also evident that number of bare
spots are higher in CMnSiNb both from scanning and analysis through the
Program Open Image J The value refer to reference values, when samples
were scanned (Table 4). Fig. 7 also revealed more bare spots (though
nominal in amount) for the CMnSiNb grade. "Tesa Test" for adherence had
shown better performances for the coating in CMnSiAINb over CMnSiNb
grade, though both of them qualified for application.

A scientific approach towards the observations reveals the following:
• While the oxygen potential of the atmosphere is high enough to
diffuse in to the steel and selective internal oxidation of the elements
occur, the surface of the substrate acts cohesively during coating
leading to better wettability,
• Enough availability of oxygen during high dew points leads to internal
oxidation and thus, limiting the supply of silicon and manganese to
the substrate surface,
• The availability of oxygen being limited at the low dew point, the
surface oxide layer creates an obstruction towards the inward flux of
oxygen,
• For the steel with higher silicon (~ 1.5 wt%), the surface silicon
content remains very high on the substrate annealed at the lower dew
points and a combination of Si02 and Mn2Si04 deteriorates the
wetting as well as generates profuse bare spots,

• Elements those can form oxides more stable than Fe, can easily be
oxidized internally at high oxygen potential. As aluminium is
normally having a much lower equilibrium potential than silicon or
manganese during the usual temperature region of annealing, it
(aluminium) is expected to be more easily oxidized even, during low
oxygen potential.
However, for the steel grades in the present work, where silicon is partially
replaced by aluminium, some silicon and manganese would not be oxidized
internally and hence some enrichment of these elements on the surface will
occur during the annealing at a low dew point.
Therefore better coating occurs at higher dew point.
The steels (CMnSiAINb) with silicon level lower than that of conventional
(1.5% silicon) TRIP-aided steel (CMnSiNb) showed better wettability and
adherence property.
This is clear from the above observations that, amongst the two
compositions, better coatability was observed in CMnSiAINb. However, the
second one had also shown a good coatability at higher dew points. The
mechanical property requirements, as stated in the objective, were available
through this invention.

WE CLAIM :
1. A method of producing high strength-high uniform elongation cold-rolled
Transformation Induced Plasticity (TRIP) aided hot dip galvanized steel
sheets for automotive application as crash resistant components
comprising the steps of:
making liquid steels in a Vacuum Induction Furnace having compositions
in wt%.
Grade C-Mn-Si-A1-Nb
C- 0.18 to 0.20 Mn-1.4 to 1.55 Si - 0.5 to 06
S-not less than 0.01 P - not less than 0.02 A1-1.20 to 1.30
N- not less than 0.005 Nb-0.03 - 0.04
Grade C-Mn-Si-Nb
C-0.18 to 0.20 Mn-1.4 to 1.55 Si -1.55 to 1.65
S-not less than 0.01 P - not less than 0.02 Al-not less than 0.01
N-not less than 0.005 Nb- 0.03 - 0.04
casting the liquid steel in to ingots;
soaking the ingot at 1150°C to 1180°C and forging to 25mm to 30 mm
thick plates,

- hot rolling the forged plates to an average thickness of 3.5mm to 4mm at
FRT 880-920°C;
- air cooling the hot rolled plate,
- pickling to remove the surface scales and cold rolling by 70% to 75%,
- surface cleaning of the cold rolled sheets followed by two step heat
treatment (intercritical annealing and isothermal bainitic transformation) in
controlled atmospheres (and different dew points) and hot dip galvanising
(in Rhesca simulator).

2. The method as claimed in claim 1, wherein the preferred temperature for
soaking is 1150°C.
3. The method as claimed in claim 1, wherein the preferred thickness of the
plates for the step of soaking is 30mm.
4. The method as claimed in claim 1, wherein the preferred thickness of the
forged plate is 4mm.

5. The method as claimed in claim 1, wherein the steel possess > 850 MPa UTS
and uniform elongation > 20% and strain hardening coefficient (n) greater
than equal to 0.20.
6. The method as claimed in claim 1, wherein the steel possess improved
wettability (better hot dip galvaised coating).
7. The method as claimed in claim 1, wherein the experimental results
corroborated a method of prediction of the volume fraction retained austenite
in microstructure through application of multilayered perceptron (MLP)
artificial neural network method.

ABSTRACT

The current work includes a method of producing high strength- high
uniform elongation cold-rolled Transformation Induced Plasticity (TRIP)-
aided hot dip galvanized steel sheets (in laboratory) for automotive
application as crash resistant components comprising the steps of making
liquid steels in a Vacuum Induction Furnace having compositions in wt%
Grade C-Mn-Si-A1-Nb
C- 0.18 to 0.20, Mn- 1.4 to 1.55, Si- 0.5 to 0.6, S- 0.01, P- 0.02, A1- 1.20 to
1.30, N- 0.005 and Nb- 0.03-0.04
Grade C-Mn-Si-Nb
C- 0.18 to 0.20, Mn- 1.4 to 1.55, Si- 1.55 to 1.65, S- 0.01, P - 0,02, A1- 0.01,
N-0.005, Nb-0.03-0.04;
casting the liquid steel in to ingots; soaking the ingot at 1150 °C and forging
to 30 mm thick plates; hot rolling the forged plates to an average thickness
of 4mm at FRT 880-920°C; air cooling the hot rolled plate; pickling to
remove the surface scales and cold rolling by 70%; surface cleaning of the
cold rolled sheets followed by two step heat treatment (intercritical
annealing and isothermal bainitic transformation) in controlled atmospheres
(and different dew points) and hot dip galvanising (in Rhesca simulator); the
steels possessing > 850 MPa UTS, uniform elongation > 20% and strain
hardening coefficient (n) greater than equal to 0.20 and improved wettability
(better hot dip galvaised coating), the experimental results corroborating a
method of prediction of the volume fraction retained austenite in
microstructure through application of multilayered perception (MLP)
artificial neural network method.