INFLUENCE OF PRECIPITATION ON THE GRAIN GROWTH MECHANISM
DURING THE WELDING OF INTERSTITIAL FREE (IF) STEELS-II
E. Bayraktar (1), D. Kaplan (2) and J.P. Chevalier (3)
Supmeca/LISMMA-Paris, School of Mechanical and Manufacturing Engineering, France
(2)
ARCELOR Group, Paris-France
(3)
Chair of Industrial Materials, CNAM, case 321, 292 rue Saint Martin, 75141 Paris 03 -France,
(1)
ABSTRACT
Abnormal grain-growth in the Heat Affected Zone (HAZ) during the welding of Interstitial Free (IF) steels has been reported in
our previous study [1]. Observations in the welded joints indicate the presence of very large grains near the fusion line and
these are oriented along the directions of the heat flow. This phenomenon takes place in certain IF steels at a given distance
from the fusion line and corresponds to regions where the maximum attained temperature is slightly over Ac3 (α→γ
transformation temperature). In the grain-growth zone, the measured ferrite grain size for different steel grades varies from
150µm up to 500µm for the welding conditions [1, 2]. This Abnormal grain-growth was interpreted by using two parameters,
the thermal gradient (G), which must constitute the “driving force of it”, and the displacement rate of the Ar3 isotherm (R) in
terms of competition between nucleation and growth in a thermal gradient. However, it is natural to think that the growth of the
ferrite grain can occur only if its development is done easily within a matrix of great purity, low in precipitates.
Thus, this stage of the research aims understanding the private role and/or the effect of each precipitate individually on this
phenomenon that will be focal point of this paper
Introduction
Interstitial free steels (IFS - with a carbon percentage of 15-20ppm) are generally built up because of a decarburization
treatment in a vacuum installation type called “RH-OB” (vacuum treatment of liquid metal with the oxygen blowing together in
secondary metallurgy). Thus, a possible way for improving the textures was to decrease to the lowest values of interstitial
elements in solution in steels suitable for a very high deep drawing quality (hyper deep drawable steels in car industry). On the
other hand, IF steels are the products of the several sequential processing operations: the steelmaking and vacuum
degassing, casting to slabs, reheating, hot rolling to hot band or strip, coiling, cold rolling and continuous annealing. Each
processing step influences the microstructural evolution of cold rolled and annealed sheet of high formability. IF steels develop
very strong concentration of orientation texture (111) <110> parallels to the sheet surface, thus taking to the high anisotropy
(Lankford) ratio « r ≥ 2 » and to the very well deep drawing behaviour. For this, four principal factors are supposed playing an
important role; these are the absence of the carbon in solid solution, the influence of the different elements, the grain size and
the influence of the precipitates (their size and particularly their quantity). It is necessary to diminish the quantity of interstitial
elements in solution of matrix essentially the carbon and nitrogen, in order to obtain exceptional ductility and deep drawing
proprieties. In IF steels, these interstitial elements are stabilised by means of the addition of some micro alloying elements
such as the titanium and the niobium which can trap the nitrogen, the sulphur and the carbon very easily under the stable
precipitated forms during the hot rolling. These precipitates have too much important hardening effect diminishing their quantity
and increasing their size in IF steels. Owing to the weak percentages of C and N realised, the percentages of Ti and/or Nb to
add are therefore very weak in this grade of the steels. Additionally, these added (stabilizing) elements in IF steels decrease
recrystallisation rate and the annealing temperatures [2-4]. Owing to the total trapping of C under the form of TiC and/or NbC
in IF steels, they are hence insensitive to the ageing and thus can be the products directly on the rapid annealing lines that do
not dispose of the over ageing section without being necessary accomplish a post-annealing of the over ageing in compact
coils. However, an incomplete trapping of C in the IF-TiNb grades can bring the bake hardening properties without damage of
the "r" value. Moreover, the annealing of recrystallisation aims to give to the steel an optimal recrystallised grain size for
obtaining of a reasonable level of yield strength, and for the deep drawing quality steels, to develop the favourable
crystallographic orientations for obtaining a high anisotropy ratio « r ». Utilisation of very low carbon IF steels stabilized with Ti
and/or Nb, allows then to obtain the hyper-deep drawable steels in continuous annealing with significantly higher mechanical
properties [1-11]. The grades of IF steels are classified in different categories: IF-Ti, IF-TiNb and/or IF-TiNb, IF-TiNb-B, IF-B,
IF-Ti-B, etc. As shown in the former paper [1], the titanium in IF-Ti grades is found in large quantity with regard to that is
required for the stochiometric conditions in order to improve the deep drawing proprieties. On the other hand, an addition of
the niobium traps the available carbon. The co-addition of Nb with Ti can retard very easily the recrystallisation temperature.
Moreover, an addition of Boron element is an effective way for preventing abnormal grain growth in IFS during the welding
operations because Boron and Boron precipitation make the Intergranular ferrite plate in a coarse grain.
Thus, abnormal grain-growth in the Heat Affected Zone (HAZ) during the welding of Interstitial Free (IF) steels has been
discussed in details in our former study [1]. This phenomenon takes place in certain IF steels at a given distance from the
fusion line and related to regions where the maximum attained temperature is slightly over α→γ transformation temperature,
Ac3. In that paper, the phenomenon of abnormal grain growth was primarily interpreted in relation to the thermal gradient,
which must constitute the “driving force of it”. However, it is natural to feel that the abnormal grain growth can occur only if its
development is completed easily within a matrix of great purity and/or very low precipitates. In bear of this claim, one observes
indeed a contrary that the phenomenon of abnormal grain growth does not occur during the welding of steel grades, of which
-3
carbon contains is slightly larger than that of IFS (example: C > 15-20.10 %), and under the same conditions of very intense
thermal gradient (resistance spot welding - RSW, GTAW, LASER welding, etc…). Thus, this stage of the research requires
understanding the private role and/or the effect of each precipitate individually on this phenomenon, in order to answer the
following questions:
- What is the initial state of precipitation of the various grades of IFS examined here?
- How this state of precipitation can be modified during the welding and in which condition play a role on the phenomenon of
abnormal grain growth?
Experimental
Different industrial grades of IF steels have been studied. Table 1 presents the compositions of these different steels whose
thickness varies between 0.7 and 0.9 mm. Grain growth in the HAZ has been studied using the following processing
conditions:
- Gas Tungsten Arc Welding (GTAW) has been used with a voltage of 10V, an intensity of 65A, a welding rate of 40cm/min
and argon gas shielding (12 l/min). Considering the coefficient of thermal efficiency of the process (η≈ 50%), the
corresponding linear energy varies from 0.75 to 0.95kJ/cm.
- Resistance spot welding experiments were carried out with a voltage of 1V, an intensity of 6000A and a welding time of 30
periods (50 Hz). The corresponding energy varies from 1.5 to 8.5 kJ. Spot diameter was approximately 4mm.
- Grain size was measured using optical metallographic microscopy.
- Because of considerable anisotropy of the ferrite grain size, the grains have been measured individually in the longitudinal,
dαL and transverse, dαT directions of the grain in order to derive a mean value in each direction.
- The calculations of precipitates in the different grades of IFS have been carried out by using a special PC program developed
by IRSID (ARCELOR research).
Steel
grade
IF Ti)
(0.8 mm)
IF Ti
(0.9 mm)
IF-TiNb
(0.8mm)
IF TiNbB
(0.7mm)
IF TiB
(0.67 mm)
C
Mn
P
S
Si
Al
Ni
N
Cr
Cu
Nb
V
Ti
B
Ceq
1.4
205
13
10
4
38
33
3.5
20
13
2
3
107
44
5
195
6
9
7
32
20
3.8
15
9
1
2
90
44
3.3
140
13
6.6
20
7
21
20
7
18
18
36
3.1
158
7
5.9
11
49
17
3.4
19
10
18
26
0.6
37
2.7
159
8
7
5
37
17
2.4
16
4
75
10
35
-3
Table 1 Chemical compositions of different grades of IF steels (10 wt %).
Initial State of precipitation (before welding) in interstitial free steels, IFS
Many direct observations have been carried out by the researchers on the IFS grades [1-9] but many of them were under the
hot deformation (rolling) conditions and only a few studies were published related to the real welding operations. First, not
having been able, in the framework of this paper, to make direct observations of precipitates in Transmission Electronic
Microscopy (TEM), for the sake of the simplicity, the state of precipitation has been estimated by means of calculations of
products of solubility. After then, for each grade, the evolution of the state of the precipitate fraction volume in HAZ of was
determined as a function of the temperature reached during welding operation.
In a general way, the state of precipitation in equilibrium can be estimated by the product of solubility:
Log (%M) (%C, N) = A - B/T
where, M and C are the percentages (wt % or at %) of the elements liable to for form the compound,
A and B are coefficients used in this calculation and T is the solution temperature (in Kelvin).
T is the temperature of handing-over in solution (in Kelvin).
By detailing the various grades of IFS studied here, the precipitates were taken into account separately for each grade such as
IF-Ti, IF-TiNb, IF-TiB, etc. in this analysis.
Thus, the objective of this study thus was to search the factors (steel grades and welding conditions) which make clear the role
of each precipitate qualitatively on the abnormal grain growth phenomenon. A typical study that is a special interest in
manufacturing of body in car industry was then undertaken on TIG (GTAW) and the Resistance spot welding (RSW) of the
different grades of IFS and a special application of tailored blanks was given on the grade of IFTiNb.
By detailing the various grades of IFS studied; the precipitates taken into account in the analysis are as follows:
IF-Ti:
One meets primarily in this type of grades:
- Titanium nitrides (TiN)
- Titanium sulphides (TiS), of which a part is most probably transformed into carbon-disulphides Ti4C2S2 at the stage of coiling
in the presence of free carbon.
- Titanium carbides (TiC) of 10-20 nm (Intragranular) or 50 nm (Intergranular) precipitating during hot rolling and mainly at the
stage of coiling.
IF-TiNb:
The precipitates of the Nb in the form of unstable sulphides for the considered contents, only the type of compounds of carbonnitrides can be formed, facilitated by possible substitution of nitrogen and carbon, titanium and niobium. These compounds can
thus be described in the general form: (TiαNbβ) (NXC1-x). Following calculations thus consider the formation:
of TiN
of TiS
of NbC, precipitate at the stage of coiling.
IF-TiNbB:
One can consider that the precipitates to be taken into account are the same ones as those in the steel grades of IF-TiNb, in
so far as boron cannot form any precipitates with the nitrogen or the carbon, which are already fixed by other elements such as
titanium.
By preoccupation with a simplicity, calculations have been carried out by supposing a co precipitation of the elements, by the
fact that the different precipitates are not soluble the ones in the others. Table 2 specifies the used values of the expressions
of the products of solubility in the austenite, as well as the values of the enthalpy of the different compounds at ambient
temperature.
Precipitates A
B
field of calculation
TiN
5.19 15490
austenite
TiS
8.2 17640
austenite
Tic
5.02 10500
austenite
NbC
2.81 7019
Ferrite
a)
Carbide Sulphur Nitride
ΔG
ΔH
kJ/mole
kJ/mole
-149 to 168 -134 to -152
NbC
TiS
TiN
-160
-201
TiC
-200
-338
-235
-297
b)
Table 2: Coefficients values used of the products of solubility in austenite used for calculations of the precipitates a) and free
enthalpy values of the different simple compounds at ambient temperature suitable to form in studied steels b)
For the studied compositions, Table 3 shows the quantities of the different elements precipitated at ambient temperature. It will
1
be noted that titanium is in situation of over – stochiometric values compared to the nitrogen in the grades of IF-Ti and IF-TiB.
The nitrogen is thus entirely precipitated in the form of TiN. The excess of titanium also can fix the carbon in these same
1
The excess of titanium in solution allows increasing very favourably the deep drawing capacity of steel by delaying of the restoring
phenomenon of work hardened structures.
grades. This case is valid only in the grades containing low titanium content (IF-TiNb, IF-TiNbB, etc) that only a part of carbon
is precipitated in the form of NbC.
Moderate Precipitate
Ti
Nb
S
C
N
IF-Ti
TiN
11.97
10 1.4 3.5
(0.8mm)
TiS
15
TiC
5.6
IF-Ti
TiN
13
9
5 3.8
(0.9mm)
TiS
13.4
Tic
20
TiN
10.26 18 5.18 2.32 3
IF-TiNb
(0.8mm)
TiS
7.74
NbC
TiN
IF-TiNbB
11.63 13.32 5.9 1.38 3.4
(0.7mm)
TiS
8.81
1.72
TiC
5.52
NbC
IF-TiB
TiN
8.9
7
2.6 2.6
(0.67mm)
TiS
10.5
TiC
10.4
-3
Table 3: Percentage (10 wt %) of precipitate of each element at ambient temperature in the IFS as received conditions
Evolution of the precipitates in HAZ during welding
By following the example of Easterling [12], we will make the assumption that the evolution of the initial precipitates can be
evaluated by means of the products of solubility. This step does not take naturally account the kinetic problems, because, the
very high heating rates in welding delay the handing-over in solution towards higher temperatures. This type of approach
however makes it possible to obtain default values of the real temperatures of handing-over in solid solution.
Table 4 calculated temperatures of handing-over in complete solution of the precipitates evoked above. According to studied
grades, these temperatures are towards 1360-1520°C (TiN), 1140-1300°C (TiS), 800-960°C (TiC), 570-720°C (NbC) By order
of increasing stability, one finds well the classical development as: carbides< sulphurs< nitrides.
IF-Ti
IF-Ti
IF-Ti1289 IF-TiNb IF-TiNbB
IF-TiB
Precipitate (0.7mm) (0.8mm) (0.9mm) (0.8mm) (0.7mm) (0.67mm)
°C
°C
°C
°C
°C
°C
TiN
1428
1525
1516
1365
1403
1466
TiS
1274
1306
1274
1139
1164
1264
Tic
890
914
960
865
800
935
NbC
566
592
725
669
-
Table 4: Temperature of handing-over in solution to balance of the various compounds formed in studied steels IF
Moreover, we also evaluated the handing-over in progressive solid solution of the various compounds by calculating, from the
various products of solubility and of the chemical compositions, the precipitated fraction depending on the temperature
reached in HAZ. The results are shown in Figure 1 presenting the examples of such a calculation for the grades of IF-Ti and
IF-TiNb respectively.
Figure 2 gives also the evolution of the stability of different compounds in the grade of IF-TiNb and also the estimation of the
solid solution as a function of the temperature.
1600
1600
IF-Ti (1289)
wt %(10-3)
C : 5, S : 9, N : 3,8, Nb : 1, Ti :
1500
TiN
Θ(°C)
1400
1400
1300
1300
1200
1200
1100
IF-TiNb
wt %(10-3)
C : 3,3, S : 6,6, N : 3, Nb : 18, Ti : 18
1500
°C
TiN
1100
TiS
TiS
γ
1000
1000
900
900
α
800
800
TiC
700
700
600
600
500
500
NbC
400
400
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0
0.08
0.01
0.02
0.03
0.04
precipitate frac, volume (%)
a)
0.05
0.06
0.07
0.08
precipitate frac, volume (%)
b)
Figure 1: Evolution of the state of the precipitated fraction in HAZ of a grade of IF-Ti a) and IF-TiNb b) depending on the
reached temperature during welding
log[M][N,S,C]
-0,5
IF-TiNb
IF-TiNb
wt %(10-3)
C : 3,3, S : 6,6, N : 3
Nb : 18, Ti : 18
-1,5
-2,5
-3,5
δ
liquide
Austenite
ferrite
-4,5
NbC
-5,5
TiS
TiN
-6,5
-7,5
-8,5
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
1/T (10-4)
Figure 2 Evolution of the stability of different compounds in the grade of IF-TiNb as a function of the temperature.
-3
In the case of this steel (C= 5.10 %, N=3.8, S= 9, Ti=90), this type of presentation verifies the different stages of the handingover in solid solution: initially the titanium carbides, the most present precipitates, are partially decomposed before the (γ Æ α)
transformation and completely disappeared even in the HAZ at low temperature (950-1000°C). This is only valid from the
higher temperatures (1100°C) that the titanium sulphides themselves are also given in solid solution, in another words, this
stage becomes total towards 1300°C. In the same way, TiN also begins to decompose at this temperature.
From now on, the interaction level of these various phenomena will be examined with the grain growth mechanism that
reported in the former paper [1].
Interactions between the phenomenon of dissolution of the precipitates and the growing of the grain
Many studies [11, 6 - 11] related to the influence of the state of precipitation on the grain growth phenomenon in IFS during the
welding. This interaction is typically described by a formalism more or less close to that proposed by Zener [12, 13], which
expresses the fact that a population of fine and many number of dispersed particles, can break down the grain growth since
the displacement of the grain boundaries through these particles is accompanied by a local increase in free Enthalpy. This
description cannot be applied directly in the case of welding of IFS considered here, because it relates mostly to the interaction
of the ferrite grain growth in an austenitic phase with the precipitates. However, as detailed in the former research paper [1]
that the interpretation suggested here is based not on the austenite grain growth but absolutely on the mobility of the interface
(γ Æ α) in the presence of a very considerable thermal gradient. The problem could thus be formulated as follows: which is
the state of precipitation “met” during the displacement of the interface towards the weld metal bead?
As seen in the same paper [1], the first ferrite germ was born in the joint welded towards 950°C in the cooling stage. With this
temperature, the niobium carbides are completely dissolved and the precipitates of TiC are partially given in solution.
In other words, the interface (γ Æ α) moves gradually to the zones:
-Having undergone a cycle of austenitisation at more and more important temperatures
-Having undergone a cooling at more and more low temperatures (transformation point (γ Æ α) is lowered with the maximum
temperature.
On this last point, it is reasonable to think that the precipitation is much reduced during the cooling phase, taking into account
the very fast cooling rates. Even in the case of cooling rate of Δt 300 = 100s, many researches have shown [11, 12, 13, 14]
that the fraction of precipitates was reduced during the cooling rate. In other words, the state of precipitation “met” by the
transformation interface of α/γ is without doubt very close to that existing at the maximum temperature of the welding cycle. It
means that the progression of the interface occurs with in a structure containing less and less precipitates (progressively the
disappearance of TiS, Ti4C2S2, TiN….) and this moves more and more towards the weld metal bead. Under this aspect, one
will observe that the grade of IF-TiNb, which is most sensitive to the grain growth during the welding, presents a less quantity
2
of precipitates TiN and TiS, and handing-over in solid solution with most precociously .
Thus, in order to illustrate the possible role of the state of precipitation on the grain growth during the welding and to separate
its role from that of the thermal gradient, some complementary tests have been carried out that will be now described:
700
Influence initial state of precipitation on the growing of the grain, without presence of a thermal gradient.
Some certain cycles of reheating with a maximum temperature of 950°C have been carried out (temperature likely to reveal
the ferrite grain growth) on a steel grade of IF-Ti (thickness = 0.7mm) in various metallurgical states:
- The first series of the test have been carried out as received conditions
3
- The second series of the test have been carried out after a preliminary heating to 950 or 1150°C
These treatment tests have been carried out on the dilatometer, i.e. without thermal gradient within the test-tubes heated in a
homogeneous way. The microstructures obtained are given in the Figure 2. In the case of a thermal cycle starting from as
received condition (initial state with precipitates of TiN, TiS, TiC) the obtained grain size is fine, of the order of 10 µm. It was
seen previously that, in this temperature range, only and only the precipitates of the titanium carbides undergo a likely
modification. The temperature of 950°C corresponds to dissolution of these precipitates (total solid solution: 890°C).
When the initial state of precipitation is modified (for example, preliminary cycle at 950°C), the grain size was increased
considerably (225 µm), which indicates that the titanium carbides play an important role on the grain growth.
After a first-preliminary cycle at 1150°C, (which in addition to the titanium carbides, makes disappear partially the precipitates
of TiS, of which the temperature for arriving to the complete dissolution is 1251°C), the grain size is now 250 µm, which
indicates that the contribution of these last precipitates to the resistance to the grain growth of the is relatively weak. As well
known, the precipitates of the TiN and TiS, formed at high temperature, are coarser than the others (size > 1µm) and naturally,
they have a very weak effect.
a)
b)
c)
Figure 2: Influence of initial state of precipitation on the grain size simulation in dilatometer, steel IF-Ti, t=0.7mm.
a) One thermal cycle θmax=950°C dα = 10µm,
b) Two successive thermal cycle θmax = 950+950°C, dα = 225µm
c) Two successive thermal cycle θmax =1150+950°C dα = 250µm
2
At 1100°C, for example, the TiS practically (entirely) disappeared in the grade of IF-TiNb while it exist still in other grades
Thermal cycles are considered as the following:
- One cycle: Vheating= 200°C/s, θM=950°C, dwell time=1s, Vcooling=500°C/s.
st
nd
- Double cycle: 1 one Vheating= 200°C/s, θM=950°C, dwell time=1s, Vcooling=500°C/s, 2 one Vheating= 200°C/s, θM=950/1150°C, dwell time=1s,
Vcooling = 500°C/s.
3
Influence initial state of precipitation on the growing of the grain, in presence of a thermal gradient.
In order to understand the influence of the initial state of the precipitation on the grain growth in presence of an important
thermal gradient, the following experiments have been carried out on several steel grades of IF-Ti and IF-TiNb: In the first time,
one carries out a GTAW remelting which makes it possible to understand the grain growth obtained in HAZ of the welded steel
sheets having an initial state of precipitates characteristic of industrial sheets.
One carries out then a second GTAW remelting, so as to make crossing the first weld metal bead. This type of the test thus
makes it possible to obtain a zone, in which the thermal cycle associated the HAZ of the second weld metal bead is applied in
a state where the precipitates had to be largely decomposed by the first welding cycle. The grain size can be appreciated in
this zone of intersection of the two weld bead, and be compared with the measured grain size formerly. Figure 4 indicates a
typical microstructure observed in the zone of intersection (crossing zone).
2
nd
2 fusion
(Welding line)
nd
fusion
st
1 fusion
st
1 fusion
(Welding line)
Figure 4: Evolution of the grain size in HAZ at the intersection in the case of two GTAW remelting carried out on the sheet
steel IF-TiNb, E= 1kJ/cm with a thickness of t=0.8mm.
All the tests carried out in this study verify a very significant grain growth (passing here for example 400 µm in the case of a
single remelting, with 800 µm in the zone of intersection), which correctly explains the importance of the initial state of
precipitation.
One can thus sum up in the Table 5 the whole of the tests carried out on steel grade of IF-Ti (t = 0.7mm), with and without
thermal gradient, in the following way:
Initial State of precipitation
Without thermal gradient With thermal gradient
(case of welding)
Complete precipitation
10 μm
400 μm
(as received condition )
(equiaxed grain)
(elongated grain)
After preliminary dissolution
225 μm
800 μm
of the TiC
(equiaxed grain)
(elongated grain)
Table 14 : Ferrite grain size observed in HAZ or at 950°C (initial temperature of the grain growth) depending on the initial state
of precipitation and the thermal gradient for the steel grade of IF-Ti, t=0.7mm.
Results
All the tests carried out in this study make it possible to separate and quantify the influence of the state of precipitation and the
thermal gradient on the grain growth mechanism in IFS during the welding operations:
- The thermal gradient has a dominating influence on the grain growth phenomenon. This aspect is awarded in the structure as
“directed” or elongated aspect that is a typical characteristic phenomenon occurred in HAZ. It can be modified by the
metallurgy (influence of the composition on the transformation temperatures, etc) and/or by the welding parameters.
- The initial state of precipitation, particularly the titanium carbides (which are the precipitates most and those likely to handingover in solid solution in a temperature range corresponding precisely to that of the appearance of the coarse grain). They play
an important role on the displacement of the interface (γ Æ α) under the effect of a gradient that will be facilitated if those are
very few. Moreover, the effect of precipitation is reinforced in the case of a situation with thermal gradient.
Acknowledgments
This project is supported by ARCELOR auto applications. The authors are indebted to the research managers of ARCELOR
Research for their valuable help for this project which is going on.
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