FIELD OF THE INVENTION
The present invention relates to a method of joining uncoated cold rolled
Transformation Induced Plasticity (TRIP) aided steel for automotive structures.
More particularly the invention relates to a process for resistance spot welding of
cold rolled and heat treated uncoated transformation induced plasticity (TRIP)
aided steel of different grades.
BACKGROUND OF THE INVENTION
Prior Indian Patent application 633/KOL/2010 by the same applicant which is
herein incorporated by way of reference disclosed a method for producing cold
rolled high strength and high uniform elongation TRIP aided steel with improved
wettability (coatability during hot dip galvanizing) while processed through a
Rhesca simulator. Necessary references on patents relevant to fabrication of
TRIP aided steel are enunciated in the aforesaid patent application to
substantiate the process of innovation. However, the prior art of record was
silent about weldability parameters of the TRIP aided steel. Weldability of TRIP
aided steel is a big challenge, and sometimes impossible to implement
[Publication (Journal/ Proceedings) reference: Stefan Melzer and Jaap Moerman,
Proceedings of the International Conference on Microstructure and Texture in

Steels and Other Materials, Springer, ISBN 978-1-84882-453-9 (2008) 267-
284.].
While resistance spot welding (RSW) route is attempted for integration of
advanced high strength steel (AHSS), such as TRIP aided steel, however
specifying optimal welding conditions is still a big challenge for such grades
having relatively rich chemistries and complex microstructure- mechanical
property relationships.
Conventionally, TRIP aided steels contain about 0.15- 0.20 wt% carbon, 1.5
wt% manganese and 1.5 wt% silicon. The TRIP effect arises from deformation
induced transformation of retained austenite to martensite and thus, the
amount and stability of retained austenite being in the microstructure plays the
most crucial role. Silicon enhances the volume fraction and stability of the
retained austenite by suppressing cementite formation during the isothermal
bainitic transformation [Publication (Journal/Proceedings) reference: O.
Matsumura, Y.Sakuma and H. Takechi, Transactions ISD 21 (1987) 570-579.].
Formation of a cementite nucleus in the presence of Si requires the diffusion
controlled ejection of Si at the transformation front, causing a build up in the
concentration of Si around the cementite nucleus. This locally increases the

activity of carbon resulting in lowering of the flux of carbon and hence inhibits
the development of cementite embryos
[Publication (Journal/ Proceedings) reference: W. S. Owen, Transactions of the
American Society for Metals, 46(1954) 812- 829.]. Inhibition of carbide
formation allows austenite to inherit a large amount of carbon, which decreases
martensite start temperature (Ms) to lower value and thus a considerable
amount of metastable austenite is retained at room temperature [Publication
(Journal/ Proceedings) reference: E. Girault, P. Jacques, Ph.Harlet, K. Mols, J.
Van Humbeeck, A. Aernoudt, and F. Delannay, Materials Characterization
40(1998) 111-118.].
However, the conventional C-Mn-Si TRIP aided steel with high silicon (1.5 wt%)
suffers from poor wettability during hot dip galvanizing carried out after
continuous annealing (intercritical annealing IA and isothermal bainitic
transformation IBT). This problem is due to the presence of complex Si/Mn
oxides on the strip surface formed [Publication (Journal/ Proceedings) reference:
J. Maki, J. Mahieu, B. C. De Cooman, and S. Claessens, Materials Science and
Technology January 2003 Vol. 19]. It is reported that silicon in concentration
higher than 0.5 wt% is detrimental to coatability [Publication (Journal/
Proceedings) reference: B. Mintz, International Materials Review 46 (2001) 169-
197.]. High Si leads to poor weldability as well [Publication (Book)

reference: ASM Handbook, Vol. 6, Welding, Brazing and Soldering, ISBN 0-
87170-383-3 (Dec. 1993) 405-407.]. To avoid this problem silicon can be
partially replaced by aluminium without giving any harmful effect on coatability.
As presence of silicon results in higher retained austenite in microstructure,
aluminium helps in retarding cementite formation having been insoluble in
cementite [Publication (Journal/ Proceedings) reference: 1. J. Maki, J. Mahieu, B.
C. De Cooman, and S. Claesseur, Materials Science and Technology, Vol. 19
(Jan. 2003) 125-131, 2. W. Bleck, A. Frehn, and J. Ohlert, Proceedings of the
International Symposium on Niobium, Orlando (2001) 727-752. 3. J. Gao and M.
Ichikawa, Proceedings of the International Conference on Advanced High
Strength Sheet Steels for Automotive Applications, AIST, Winter Park, Colorado
(2004) 107-116]. Aluminium also facilitates coatability by forming an inhibition
layer on the steel surface, which in turn prevents the formation of Si/Mn oxides
and bare spots. However, Aluminium being a weak solid solution strengthening
element, complete replacement of silicon by aluminium cannot offer the strength
level that is achieved in conventional Si alloyed steel. Chen et al. [Publication
(Journal/ Proceedings) reference: Chen et al., SE AISI Q (April 1991) 44-49.]
have shown that Si level should be more than 0.8 wt% to obtain reasonable
amount of retained austenite. Development of various alternative compositions

have been proposed by various researches. Addition of P [Publication (Journal/
Proceedings) reference: B. C. De Cooman, Current Opinion in Solid State and
Material Science 8 (2004) 285-303.] or microalloying elements for enhancing the
strength level are attempted for CMnSiAl steels [Publication (Journal/
Proceedings) reference: l.D.Krizan, B. C De Cooman and J. Antonissen,
Proceedings of the International Conference on Advanced High Strength Sheet
Steels for Automotive Applications, AIST, Winter Park, Colorado (2004) 205-216.
2. K. Hulka, W. Bleck and K. Papamantellos, Proceedings of the 41st Mechanical
Working and Steel Processing Conference, Iron and Steel Society, Baltimore
(1999) 67-88].
Better coatability during galvanizing of TRIP aided steel necessitates major alloy
engineering. It has invited renewed challenge for welding of TRIP aided steel
[Publication (Journal/ Proceedings) reference: M. I. Khan, M. L. Kuntz, E. Biro
and Y. Zhou, Materials Transactions, Vol. 49, No. 7 (2008) 1629 to 1637]. It has
two folds spectra. Once for the bare steel and the other for the coated material
(galvanized or galvannealed steel, well-prescribed for autobodies to mitigate
galvanic corrosion).
At the outset, the invention is aimed at improved weldability of uncoated TRIP
aided steel, suitable for improved coatability. The referred previous patent
application [633/KOL/10] depicts the

improvement of wettability for some specific cases (processed through Rhesca
simulator).
There are several inventions (Patent Nos.US 2007/ 0020478 Al, US 2006/
0140814 Al, US 7,294,412 B2) where processes as well as properties for TRIP
aided steels are enumerated.
Resistance spot welding is based on resistive heating process (Joule's formula),
H = I2 Rt [H: heat, I: current, R: resistivity, t: time]. The resistance comes from
the cumulative resistance from bulk resistance (electrode & sheet) and contact
resistance (electrode & sheet and sheet & sheet). Thermoelectric properties are
also responsible for interface resistance [A. G. Thakur et al., International
Journal of Applied Engineering Research, Dindigul, Vol. 1, No. 3, 2010 pp. 483-
490].
In this process contacting metal surfaces are joined by the heat obtained from
resistance to electric current flow. Work-pieces are held together under pressure
exerted by electrodes. A wide range of sheet thicknesses is possible to get
welded starting from 0.5 mm up to 10 mm. Two electrodes made up of copper
alloy in to specific shapes are used with a predetermined pressure to concentrate
welding current into a small "spot" and to simultaneously clamp the sheets
together. Flow of a large current under pressure through the spot melts the

metal and form the weld. An appreciably high energy is delivered in spot
resistance welding in a very short time (in the range of milliseconds). It results in
welding within a concentrated region without excessive heating to the other part
of the job. It is an intricate route for integration of metals, which requires
specific optimal welding conditions based on experimental data. Usually,
determination of weldability lobe (weld time vs. weld current relationship) is a
prerequisite for resistance spot welding which present a clear cut understanding
about the range of process parameters between achieving a minimum nugget
diameter and expulsion. Therefore, it is crucial to judiciously choose the welding
parameters in tune with the material chemistry and its thickness, and electrode
geometry. Adequate energy is required to melt the metal and to have a good
weld. On the contrary, application of too high current leads to expulsion. From
the point of view of alloy engineering, as referred, it is important to note that
resistivity of steel increases along with addition of alloying elements having
higher resistivity than iron (resistivity of iron: 9.7 x 10-8 ohm m). Resistivity of
some common elements usually added to the steels of the genre of interest,
have been collated below [Table 1].

Table: 1. Electrical resistivity of different alloying elements purposely added to
TRIP aided steel/ used for coating

Theoretically, resistivity of material is enhanced with higher alloying additions.
For such cases, the weldability lobe is narrowed due to shifting of expulsion
current towards lower range [Publication (Journal/ Proceedings) reference: T.
Herai et al, Resistance spot welding of high strength steel sheets, IIW Doc. Ill,
664-80].
The prior art teach the following technical observations:

The weldability of TRIP aided steel faces severe complexities. After solidification,
welds become very hard and can show a brittle behavior. The hardness of the
heat affected zone (HAZ) attain to the tune of 500 HV and more [Publication
(Journal/ Proceedings) reference: Cretteur Laurent et al., Steel Research, ISSN
0177-4832, 2002, vol. 73, pp 314-319], and cold cracking phenomenon is very
prone to occur. During resistance spot welding, especially, the interface between
the plates can act like a notch and promote fracture of the weld. It becomes very
severe during coach peeling/ shear tensile fracture which usually produces
partially interfacial fracture (inferior in terms of ductility) as opposed to full
button peel off - plug type fracture (superior) [Publication (Journal/ Proceedings)
reference: Yun Peng et al., Materials Science Forum Vols. 638-642 (2010) pp
3591-3596].
Some more relevant observations could be brought together about spot
resistance welding of TRIP like advanced high strength steels as follows
[Publication (Report) reference: Hatsuhiko OIKAWA et al., Resistance spot
weldability of high strength steel (HSS) sheets for automobiles, Nippon steel
technical report No. 95, January 2007]:

I. Heat generation in spot welding being higher due to high electric
resistance, welding current range shifts to the lower current side,
II. Achieving appropriate nugget diameter is extremely difficult as fully
contact diameter cannot be obtained between sheets when electrode
force is low by the effect of spring back for steels of this genre. The
nugget diameter is either low or underdeveloped for most of the cases,
III. Vickers hardness at welded zone increases swiftly with increase of
martensite formation and hardness linked with ascending carbon
equivalent,
IV. In AHSS, though interfacial failure occurs predominantly, the load bearing
capacity of the welds is quite high. Therefore, facture mode alone is not a
good indicator of weld performance,
V. The hardness of the weld nuggets having been very high, the interface
between the sheets acts as a notch resulting in brittle, interfacial fracture
for most of the cases,
VI. Introduction of post heating current to reduce the cooling rate, leads to
lessening of hardness of martensite for resistance spot welding.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process for resistance spot
welding of cold rolled and heat treated uncoated transformation induced
plasticity (TRIP) aided steel.
Another object of the invention to propose a process for resistance spot welding
of cold rolled and heat treated uncoated transformation induced plasticity (TRIP)
aided steel, which is adaptable for TRIP-aided steels of different grades each
having unique chemical compositions of starting materials.
A still another object of the invention to propose a process for resistance spot
welding of cold rolled and heat treated uncoated transformation induced
plasticity (TRIP) aided steel, in which the produced steel is heat-treated in two-
steps for example, intercritical annealing (IA), and isothermal bainite
transformation (IBT).
A further object of the invention to propose a process for resistance spot welding
of cold rolled and heat treated uncoated transformation induced plasticity (TRIP)
aided steel, in which the uncoated TRIP-aided steels have thickness between
1.2-1.5mm and ultimate tensile strength between 800 MPa to 1000 MPa.

SUMMARY OF THE INVENTION
According to this invention there is provided in a first aspect a process for
resistance spot welding of cold-rolled heat-treated (uncoated) transformation
induced plasticity (TRIP) aided steel, comprising the steps of :-
• making liquid steels in a vacuum induction furnace;
• casting the liquid steel in to ingots;
• soaking the ingot at 1150°C and forging to 30 mm thick plates,
• hot rolling of the forged plates to an average thickness of (~)4 mm with
finish rolling temperature 880-920°C;
• normal air cooling of hot rolled plate;
• surfacing to remove the surface scales;
• cold rolling to 1.2 mm - 1.5 mm after an average reduction of ~75%
from hot rolled strips;
• surface cleaning of the cold rolled sheets;
• two step heat treatment (intercritical annealing and isothermal bainitic
transformation) in two adjacent salt bath furnaces (allowing shortest
possible shifting time from IA to IBT bath),
Wherein the process parameters for welding are: current and time during the
welding cycle ranges from 8-8.5 KA, and 200 to 250 ms, and that during post-
welding tempering cycle ranges from 4-4.5 KA, and 400-450 ms respectively.

In a preferred embodiment of the invention, there is provided a process for
resistance-spot welding of cold-rolled heat treated incoated transformation
induced plasticity (TRIP) aided Steel from a liquid Steel composition, wherein the
composition of the liquid steel in wt% is
C-0.18 to 0.20 Mn-1.4 to 1.55 Si-0.5 to 0.6
S-0.01 max p - 0.06 to 0.07 AI-1.20 to 1.30
N-0.005 max,
wherein the nugget diameter for a sample thickness of 1.3 mm at applied load of 13.5
KN is 4.82 mm, and wherein the mechanical properties of the steel grade consists of
UTS, uniform elongation, total elongation, strain-hardening exponent (n) respectively
at 800-810 MPa, 20-26%, 28-35%, 0.28 to 0.31.
In a second aspect of the invention, there is provided a process for resistance-spot
welding of Cold-rolled heat treated uncoated transformation induced plasticity (TRIP)
aided Steel from a liquid steel composition, wherein the composition of the liquid steel
in wt% is
C-0.18 to 0.20 Mn-1.4 to 1.55 Si- 0.5 to 0.6
S-0.01 max P - 0.02 max AI-1.20 to 1.30
N-0.005 max Nb- 0.03-0.04,
wherein the nugget diameter for a sample thickness of 1.2 mm at applied load of 15.1
KN is 4.40 mm, and wherein UTS, uniform elongation, total elongation, strain
hardening exponent (n) respectively are 840-860 MPa, 20-23%, 25-28%, and 0-24-
0.27.

In a third aspect of the invention, there is provided a process for resistance-spot
welding of cold rolled heat treated uncoated transformation induced plasticity
(TRIP) aided steel from a liquid steel composition, wherein the composition of
the liquid Steel in wt% is
C-0.18to0.20 Mn-l.4to l.55 Si-1.55 to 1.65
S-0.01 max P- 0.02 max AI-0.01 maxd
N-0.005 max Nb- 0.03-0.04,
wherein the nugget diameter for a sample thickness of 1.5 mm at an applied
load of 20.23 KN is 4.92 mm, and wherein the UTS, uniform elongation, total
elongation, and strain hardening exponent (n) respectively are 990-1010 MPa,
17-20%, 23-25%, and 0.21 to 0.24.
The present invention addresses the gap area on welding behavior of the TRIP
aided steel, considering the howling hindrance of weldability of steels of this
genre.
The present invention addresses to the three new compositions of TRIP aided
steel developed for this work. The present invention depicts a common range of
processing parameter to achieve acceptable welds for the aforementioned three
grades of TRIP aided steels for a thickness range of 1.2 - 1.5 mm and with
different strength levels between 800 MPa to 1000 MPa.

The present invention is not directly related to coated product. Here, cold rolled
samples from three low carbon- low alloy heats (CMnSiAlP, CMnSiAINb and
CMnSiNb) [prepared in a vacuum induction furnace] were heat treated in salt
bath heat treatment furnaces for intercritical annealing followed by isothermal
bainitic transformation to achieve target mechanical properties. As a part of
weldability assessment, these heattreated samples were spot welded.
For the present invention, cold rolling followed by two stage heat treatment
route (very popular for automotive industries for achieving TRIP aided steel with
required thickness), as described in the preceding paragraph, is implemented.
Two different chemical compositions (Grade 1 CMnSiAlP and Grade 2
CMnSiAINb), suitable for better wettability were selected. For evaluation of
performance of weldability with respect to alloy engineering, a third composition
(Grade 3, CMnSiNb, bit modified conventional TRIP aided steel) was also
selected for necessary comparison. As depicted in the referred previous patent
application [633/KOL/10], transformation behaviour was studied through
Gleeblel500D simulator. ThermocalC was used as well to evaluate the
transformation behavior in equilibrium condition and to draw To line [Publication
(Book) reference: H. K. D. H. Bhadeshia, Bainite in Steels, Second Edition, IoM
Communications, London (2001).]; ANN tool was applied to predict retained

austenite in the microstructure at the room temperature after cold rolling and
two stage heat treatment (IA: intercritical annealing, IBT: isothermal bainitic
transformation). Accordingly, suitable heat treatment cycles were determined for
achieving a good structure-property relationship having adequate retained
austenite for TRIP-effect, high ultimate tensile strength (UTS) /yield strength
(YS) ratio, high n(strain hardening exponent)- value and uniform elongation,
prerequisite for good TRIP-effect.
According to the invention, the carbon equivalent (Ceq) was calculated using the
equation;
Ceq = C+ Mn/20+Si/30+2P+4S — Eq. (1)
[Publication (Report) reference: KAWASAKI STEEL TECHNICAL REPORT No. 48
March 2003].
The proposed invention has been developed to solve the difficulties of achieving
weldability of TRIP aided steel having three different alloying arrangements and
property levels. The proposed invention also intended to trial-and-error method
to determine the optimal conditions through adequate number of experiments.
The welding current, welding time and welding force were selected as input
variables, and the shear strength, size of nuggets, mode of fracture and breaking

load under shear tensile strength and hardness profile were selected as output
variables.
It has also been endeavoured within the purview of the invention for betterment
of the weldments to introduce dual cycles (postweld tempering) for higher
ductility.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 Tensile shear test specimens of spot welded sheets (ASTM D1002)
Fig. 2 Initial welding cycle (schematic) without post weld tempering
Fig. 3 Modified welding cycle (schematic) combined with post weld
tempering
Fig. 4 Failure mode under shear tensile fracture for (a) Gr. 1 CMnSiAlP (b) Gr. 2
CMnSiAINb and (c) Gr. 3 CMnSiNb after the final cycle
Fig. 5 Nital etched mosaic microstructure for different samples used for
measurement of nugget diameter
Fig. 6 (a) microstructure of weld zone for Gr. 1 CMnSiAlP
Fig. 6 (b) microstructure of HAZ for Gr. 1 CMnSiAlP
Fig. 7 (a) microstructure of weld zone for Gr. 2 CMnSiAINb
Fig. 7 (b) microstructure of HAZ for Gr. 2 CMnSiAINb

Fig. 8 (a) microstructure of weld zone for Gr. 3 CMnSiNb
Fig. 8 (b) microstructure of HAZ for Gr. 3 CMnSiNb
Fig. 9 Hardness profile Gr. 1 CMnSiAlP
Fig. 10 Hardness profile Gr. 2 CMnSiAINb
Fig. 11 Hardness profile for Gr. 3 CMnSiNb
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
According to the present invention, there is provided a process for resistance
spot welding of cold rolled and heat treated uncoated Transformation Induced
Plasticity (TRIP) aided steel. The inventive process is applicable for TRIP-aided
steel having different chemical compositions. Liquid steel with different chemical
compositions after steel making is heat-treated in two steps for example,
intercritical annealing and isothermal bainitic transformation. The details about
the steels and primary processing routes are stated below making liquid steels in
a vacuum induction furnace having compositions in wt%. Grade 1; CMnSiAlP
C- 0.18 to 0.20 Mn-1.4 to 1.55 Si- 0.5 to 0.6 S- 0.01 max P - 0.06 to 0.07
Al- 1.20 to 1.30 N- 0.005 max (carbon equivalent for the sample drawn from
the lot and, henceforth, Referred to CMnSiAlP for all cases = 0.47) Grade 2: C
Mn Si Al Nb C- 0.18 to 0.20 Mn- 1.4 to 1.55 Si- 0.5 to 0.6 S- 0.01 max
P - 0.02 max Al-1.20 to 1.30 N- 0.005 max Nb- 0.03-0.04 (carbon equivalent

for the sample drawn from the lot and, henceforth, referred to CMnSiAlNb for all
cases =0.38) Grade 3: CMnSiNb C- 0.18 to 0.20 Mn- 1.4 to 1.55 Si- 1.55 to
1.65 S- 0.01 max P - 0.02 max Al- 0.0 ax N- 0.005 max Nb- 0.03-0.04 (carbon
equivalent for the sample drawn from the lot and, henceforth, referred to
CMnSiNb for all cases = 0.41) casting the liquid steel in to ingots; soaking the
ingot at 1150 °C and forging to 30 mm thick plates, hot rolling of the forged
plates to an average thickness of (~)4 mm with finish rolling temperature 880-
920°C; normal air cooling of hot rolled plate, surfacing to remove the surface
scales, cold rolling to 1.2 mm - 1.5 mm after an average reduction of ~75%
from hot rolled strips, surface cleaning of the cold rolled sheets, two step heat
treatment (intercritical annealing and isothermal bainitic transformation) in two
adjacent salt bath furnaces (allowing shortest possible shifting time from IA to
IBT bath), oil quenching to attain room temperature from IBT temperature
range.The mechanical properties of the three steel grades are presented below
(Table 2).


An inverter based 90 kVA medium frequency DC spot welding machine was used
for welding. Welding was carried out using truncated conical shape water cooled
Cu-Cr alloy electrode of 6 mm face diameter. The sheets were cleaned by
acetone prior to the welding operation. Weld current and weld time were varied
at a given electrode force of 3.8 kN. The effective current and voltage at the
time of welding was measured by a Miyachi weld checker. The tensile shear test
specimens of spot welded sheets were prepared according to ASTM D1002 as
shown in Fig. 1.
It was found from a number of experimental trials that formation of a precise
weldability lobe for resistance spot welding of these grades are extremely
difficult as it is reported that in case of resistance spot welding of TRIP assisted
grades for this work, the richer chemistry resulted in a very narrow operating
window.
Therefore, performance of RSW is measured based on compliance of nugget
diameter achieved and actual load against the calculated values. In accordance
with the Kawasaki Steel Technical Report [No. 48, March 2003], the acceptable
diameter (d) of the nugget formed should be 4Vt (t being the thickness of the
strip), i.e. d = 4Vt Eq. 2
The required TSS (tensile shear strength) load (calculated), P = 2.6 x d x t x UTS —
-Eq. 3

The calculated nugget diameters and loads with the help of Eq. 2 and Eq. 3 are
collated in Table 3.
Table 3. Calculated nugget diameters and loads for the samples used for the
invention

Again, as evident from Table 3, for the initial cycle (as depicted schematically in
Fig.2 and detailed in Table 4), it was difficult to achieve a clear weldability lobe.
Even while actual load complied to the calculated value, the mode of failure was
not most ideal (i.e. plug failure). However, suggestive parameters (current and
time) could be sensed as highlighted in Table 4. For all the cases, the welded
samples were subjected to tensile load in Instron 8801 with a cross head speed
of 1m/minute.

All the three new grades were found weldable in terms of compliance to
acceptable nugget diameter and required TSS load. Mode of failure (Fig. 4a, Fig.
4b) and nugget sizes were found superior for CMnSiAlP and CMnSiAINb (both
containing lower silicon) grades to the CMnSiNb grade (containing higher silicon,
comparable to the conventional TRIP aided steel grade) (Fig. 4c). For Grade 1
CMnSiAlP, a clear cut plug type of failure was observed. For Grade 2 CMnSiAINb,
a partial plug type of failure was observed. For Grade 3 CMnSiNb, the mode of
failure was interfacial. However, it is evident from the above that all the three
grades (developed for the invention with new chemistry) could be welded
through the modified welding parameters
Metallography study for transverse microspecimen of the spot welds was carried
out (selective cases) in Leica DM 6000M microscope. Nugget diameters were
measured examining the transverse microstructure of the welded parts after
tensile test. Fig. 5 shows nital etched mosaic microstructures for different
samples used for measurement of nugget diameters.
The microstructure of weld zone revealed martensitic matrix for the specimen
examined (Fig. 6a, Fig. 7a and Fig. 8a). The microstructure of the heat affected
zone (HAZ) revealed a mixed structure (Fig. 6b, Fig. 7b and Fig. 8b).

Micro-hardness along the diagonal section of weld nugget was measured to
capture the variation from base metal to weld metal in Leco Microhardness
Tester LM 247 AT.
Hardness profile for samples from all three grades significantly revealed that the
hardness values of the weld as well as HAZ, for all cases, are below 500 HV (Fig.
9, Fig. 10 and Fig. 11) [ resonating with the Publication (Journal/ Proceedings)
reference: Cretteur Laurent et al., Steel Research, ISSN 0177-4832, 2002, vol.
73, pp 314-319].

WE CLAIM
1. A process for resistance spot welding of cold-rolled heat-treated
(uncoated) transformation induced plasticity (TRIP) aided steel,
comprising the steps of :-
• making liquid steels in a vacuum induction furnace;
• casting the liquid steel in to ingots;
• soaking the ingot at 1150°C and forging to 30 mm thick plates,
• hot rolling of the forged plates to an average thickness of (~)4 mm with
finish rolling temperature 880-920°C;
• normal air cooling of hot rolled plate;
• surfacing to remove the surface scales;
• cold rolling to 1.2 mm - 1.5 mm after an average reduction of ~5%
from hot rolled strips;
• surface cleaning of the cold rolled sheets;
• two step heat treatment (intercritical annealing and isothermal bainitic
transformation) in two adjacent salt bath furnaces (allowing shortest
possible shifting time from IA to IBT bath),
Wherein the process parameters for welding are : current and time during
the welding cycle ranges from 8-85 KA, and 200 to 250 ms, and that
during post-welding tempering cycle ranges from 4-4.5 KA, and 400-450
ms respectively.

2. The process as claimed in claim 1, wherein the composition of the liquid
steel in wt% is
C-0.18 to 0.20 Mn- 1.4 to 1.55 Si-0.5 to 0.6
S-0.01 max p - 0.06 to 0.07 AI-1.20 to 1.30
N-0.005 max,
wherein the nugget diameter for a sample thickness of 1.3 mm at applied
load of 13.5 KN is 4.82 mm, and wherein the mechanical properties of the
steel grade consists of UTS, uniform elongation, total elongation, strain-
hardening exponent (n) respectively at 800-810 MPa, 20-26%, 28-35%,
0.28 to 0.31.
3. The process as claimed in claim 1, wherein the composition of the liquid
steel in wt% is
C-0.18to0.20 Mn-1.4to1.55 Si-0.5 to 0.6
S-0.01 max P - 0.02 max AI-1.20 to 1.30
N-0.005 max Nb- 0.03-0.04
wherein the nugget diameter for a sample thickness of 1.2 mm at applied
load of 15.1 KN is 4.40 mm, and wherein UTS, uniform elongation, total
elongation, strain-hardening exponent (n) respectively are 840-860 MPa,
20-23%, 25-28%, and 0-24-0.27.

4. The process as claimed in claim 1, wherein the composition of the liquid
Steel in wt% is
C-0.18 to 0.20 Mn-1.4 to 1.55 Si-1.55 to 1.65
S-0.01 max P-0.02 max AI-0.01 max
N-0.005 max Nb- 0.03-0.04
wherein the nugget diameter for a sample thickness of 1.5 mm at an
applied load of 20.23 KN is 4.92 m, and wherein the UTS, uniform
elongation, total elongation, and strain hardening exponent (n)
respectively are 990-1010 MPa, 17-20%, 23-25%, and 0.21 to 0.24.

The invention relates to a process for resistance spot welding of cold-rolled
heat-treated (uncoated) transformation induced plasticity (TRIP) aided
steel, comprising the steps of :- making liquid steels in a vacuum
induction furnace; casting the liquid steel in to ingots; soaking the ingot at
1150°C and forging to 30 mm thick plates, hot rolling of the forged plates
to an average thickness of (~)4 mm with finish rolling temperature 880-
920°C; normal air cooling of hot rolled plate; surfacing to remove the
surface scales; cold rolling to 1.2 mm - 1.5 mm after an average
reduction of ~75% from hot rolled strips; surface cleaning of the cold
rolled sheets; two step heat treatment (intercritical annealing and
isothermal bainitic transformation) in two adjacent salt bath furnaces
(allowing shortest possible shifting time from IA to IBT bath), Wherein the
process parameters for welding are : current and time during the welding
cycle ranges from 8-85 KA, and 200 to 250 ms, and that during post-
welding tempering cycle ranges from 4-4.5 KA, and 400-450 ms
respectively.