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Hot deformation studies on 2.7% Si steel using gleeble thermo-mechanical simulator

Authors:
  • Steel Authority of India Ltd (SAIL), Ranchi

Abstract and Figures

1173-1423 K) and at strain rates of 0.1, 1, 10 and 100/s using Thermo-mechanical 0 Simulator (Gleeble-350 C System) on a 2.7% Si electrical steel to understand the hot workability issues associated with this steel during hot rolling. The flow curves obtained revealed dynamic recovery as the predominant softening mechanism at majority of hot deformation conditions except at lower temperature and high strain rate where work hardening was observed. However, the work hardening was not very prominent due to ferrite structure throughout the hot deformation temperature range established by Thermo-Calc software. Small amount of cementite (pearlite) transformed from austenite along prior ferrite grain was observed due to presence of carbon in excess of 0.02. Strain rate sensitivity varied within a narrow range of 0.18-0.21 with rising tendency with an increase in temperature.
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251
Journal of Metallurgy and Materials Science, Vol. 58, No. 4, October-December 2016, pp. 251-258
Printed in India, © NML, ISSN 0972-4257
Hot deformation studies on 2.7% Si steel using
gleeble thermo-mechanical simulator
KUMAR ANIKET ANAND*, KARTIK NAGESWARAN,
VINOD KUMAR and ATUL SAXENA
Ranchi, Jharkhand-834 002, India
Abstract :
(1173-1423 K) and at strain rates of 0.1, 1, 10 and 100/s using Thermo-mechanical
0
Simulator (Gleeble-350 C System) on a 2.7% Si electrical steel to understand the hot
workability issues associated with this steel during hot rolling. The flow curves obtained
revealed dynamic recovery as the predominant softening mechanism at majority of hot
deformation conditions except at lower temperature and high strain rate where work
hardening was observed. However, the work hardening was not very prominent due to
ferrite structure throughout the hot deformation temperature range established by
Thermo-Calc software. Small amount of cementite (pearlite) transformed from austenite
along prior ferrite grain was observed due to presence of carbon in excess of 0.02. Strain
rate sensitivity varied within a narrow range of 0.18 - 0.21 with rising tendency with an
increase in temperature.
Keywords : Silicon steel, Electrical steel, Hot deformation, Hot rolling.
INTRODUCTION
Electrical steels, also called lamination steel or silicon electrical steel, are an important
category of iron-silicon alloys containing 1-5 wt % silicon and very low carbon (less than 0.005
[1, 2]
wt %) . Silicon significantly increases the electrical resistivity of the steel, which decreases
the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the
[3]
core loss .
Electrical steels contain generally unavoidable Mn and S after the metallurgical process.
The Mn and S would even be used as alloy elements in oriented electrical steels. The Mn and
S in steels would precipitate as secondary phase in the form of dispersive MnS particles
during the production processes and these particles have very important influences on the
steel properties. It is generally believed, that the precipitation and coarsening of MnS
particles in high grade non-oriented electrical steel could not only reduce obviously the core
[4]
loss but also weaken the magnetic aging effects during the service time of the steel sheets .
The core loss of conventional oriented electrical steels would be reduced by means of high Si
content. However, large amounts of dispersive MnS particles as inhibitors are also
introduced in conventional oriented electrical steels to control the matrix structure and the
abnormal grain growth during the final secondary recrystallization annealing in order to
obtain strong Goss texture.
However, due to high Si in such steel (2.7% Si), hot workability is adversely affected.
This is reflected in form of elongated structure with high dislocation density. Presence of
second phase (austenite), if any, also leads to differential flow leading to cracking thus
showing poor workability. In view of this, efforts were made to study such behavior
occurring during its processing stages by performing uni-axial compression tests on
2.7% Si steel.
R&D Centre for Iron & Steel, Steel Authority of India Ltd.,
Uni-axial hot compression tests were conducted at different temperatures
*Corresponding Authors Email :
atul-saxena@sail-rdcis.com
aniketanand@sail-rdcis.com, nkartik@sail-rdcis.com, vkumar@sail-rdcis.com,
252 J. MET. MATER SC., Vol. 58, No. 4, 2016
KUMAR ANIKET ANAND, KARTIK NAGESWARAN, VINOD KUMAR and ATUL SAXENA
EXPERIMENTAL DETAILS
The chemical composition of material used was ascertained by using Optical Emission
Spectroscope (Model-THERMO, ARL 3460) is given in Table 1. The Fe-Si binary phase
®
diagram determined with the help of Thermocalc 3.1 for this composition is shown in Fig. 1.
Table 1 : Composition of the 2.7% Si steel, in wt%
C Si Mn P S Al
0.010 2.72 0.054 0.010 0.029 0.011
Fig. 1 : Phase Diagram as determined on ThermoCalc 3.1®
Cylindrical specimens of 15mm height and 10mm diameter were cut from the slab for hot
uniaxial compression testing on thermomechanical simulator Gleeble 3500 (Dynamic
Systems, Inc.). The hot deformation schematic is shown in Fig. 2. Uniaxial Compression tests
were done at temperatures ranging from 1173 K to 1423 K, with a 50 K interval, so as to cover
Deformation at various strain rates
Deformation
temperature
Temp. (°C)
Time (sec)
Air
Fig. 2 : Hot Deformation Schematic
the nose of gamma phase (Fig. 1). The strain rates were also varied in multiples of 10 from 0.1/s
to 100/s covering the entire processing range. All the samples were heated to deformation
temperatures of 1173 K -1423 K for 1 minute and deformed to a total strain of 0.8. After the
compression, the samples were air cooled in the test chamber itself. MoS was used as lubricant
between the anvil and specimen, and graphite (up to 1273 K)/tantalum foil (>1273 K) of thickness
0.1 mm was used to avoid sticking of the anvil to the specimen surface. Hot deformation software
0
(HDS) was used to run the thermo-mechanical programming in Gleeble 350 C.
J. MET. MATER SC., Vol. 58, No. 4, 2016 253
HOT DEFORMATION STUDIES ON 2.7% SI STEEL USING GLEEBLE THERMO-MECHANICAL...
After hot deformation, samples were cut in the transverse direction and were copper mounted
using hot mounting technique. They were then polished using emery papers and etched using
6% Nital solution for 15 seconds before Microstructural examination under Optical
Microscope and Scanning Electron Microscope (SEM).
RESULTS AND DISCUSSION
Flow stress of any material is a function of temperature, strain rate at a particular strain. The
general relationship between flow stress and strain rate at constant strain and temperature is
[5]
represented by Equation 1 :
m
σ = C (ε) | ...(1)
ε, T
Where, σ is flow stress, ε is strain, ε is strain rate, T is temperature and m is strain rate
sensitivity. The strain rate sensitivity, m can be determined from the slope of the plot of log σ vs
log ε. It represents thevariation in flow stress with strain rate at particular temperature of
deformation. The variation in flow stress is large, the material is said to be strain rate sensitive
and it needs to be deformed with great care. On the other hand, if the strain rate sensitivity is
small, the material is supposed to be strain rate insensitive and can be deformed with comfort.
Uni-axial hot compression test is the most common method to study the flow behavior of
material under the influence of temperature and strain rate. Accordingly, tests were carried out
to study the influence of deformation conditions (within the broad range of hot rolling of such
steels) on flow behavior and ultimately to arrive at the safe rolling window.
The results obtained from uni-axial hot compression test were analyzed and correlated with
microstructure examined through optical and scanning electron microscopy.
Effect of Deformation on Microstructure
Microstructural examination of samples deformed at different temperatures and strain rate
were carried to study the influence of hot deformation on microstructural evolution, in general,
and to identify any softening process, like dynamic recrystallization or recovery occurring
during the deformation. This is important in view of the fact that high Si steels are prone to
work hardening leading to high rolling load during processing. Optical micrographs of
samples deformed at 1223 K and 1423 K and at two different strain rates of 0.1/s and 10/s are
shown in Fig. 3. Elongated grains are observed at 1223 K and a strain rate of 0.1/s whereas
Fig. 3 : Optical Micrographs of Hot-Deformed Samples: a) 1223 K, 0.1/s, b) 1223 K, 10/sec,
c) 1423 K, 0.1/s, d) 1423 K, 10/sec
254 J. MET. MATER SC., Vol. 58, No. , 20164
KUMAR ANIKET ANAND, KARTIK NAGESWARAN, VINOD KUMAR and ATUL SAXENA
some recovery observed at higher strain rate of 10/s at the same temperature. Extent of dynamic
recovery improved with the increase in deformation temperature to 1423 K. At 1423 K, extensive
dynamic recovery along with a small amount of recrystallization are observed at a low strain rate
of 0.1/s. But, at higher strain rate of 10/s, highly recrystallized equiaxed grains are observed.
Microstructural examination was also carried out with the help of scanning electron
microscopy to identify dislocated/ work hardened grains. A typical SEM micrograph in Fig. 4
shows both highly strained and unstrained grains simultaneously reflecting that deformation in
a
c
b
Fig. 4 : SEM Image showing: a) Strained grains, b) Unstrained grains and c) Second-phase
this steel took place preferentially in some grains due to preferred orientation. Presence of sub-
grain boundaries in some grains are a good indication of non-recrystallized grain. Further,
formation of second phase was observed along ferrite grain boundaries. Elemental analysis
confirmed enrichment of C in this phase resembling austenite. It has been reported that
presence of more than 0.020 wt% carbon in 3% Si iron passes through a small two-phase a+ g
[6]
region . The volume fraction of austenite will depend upon amount of carbon present in the
steel. In the present steel, it is expected to be about 4% maximum. Since the volume fraction of
austenite is very low, it will be present along the prior ferrite grain boundaries. On cooling, the
austenite along ferrite grain boundaries will transform to cementite (pearlite) as has been
observed and reported.
It is to state that Si steel forms a gamma (austenite) loop. If silicon is low, austenite phase may
form as second phase in an otherwise single ferrite phase. Formation of austenite along ferrite
grain boundaries will pose problems during processing due to differential deformation and
poor hot workability.
Precipitates were also anlyzed to confirm formation of MnS in such steel which is the key to
texture development at a later stage of processing. Fig. 5 shows EDAX analysis of precipitate
which shows presence of MnS.
Fig. : 5 EDAX Analysis Showing Presence of MnS Precipitate
Effect of Temperature on Flow Stress
A series of flow stress-strain curves obtained from isothermal compression of 2.7% Si steel at
different deformation temperatures at constant strain rates are shown in Fig. 6. It is observed
that the flow stress decreases with increasing deformation temperature during the isothermal
Fig. 6 : Effect of Deformation Temperature on Flow Stress at Different Strain Rates
HOT DEFORMATION STUDIES ON 2.7% SI STEEL USING GLEEBLE THERMO-MECHANICAL...
J. MET. MATER SC., Vol. 58, No. 4, 2016 255
compression at all strain rates. Dynamic recovery was observed at all temperatures (except
1173 K) when deformed at a strain rate of 0.1/s. At lower temperature of 1173 K, work
hardening was observed continuously throughout the deformation with no sign of recovery.
Work hardening was also observed at other temperatures of 1223 K through 1323 K towards
higher deformation in excess of 0.4 strain. Similar trend in flow behavior was observed when
deformed at a strain rate of 1/s also.
However, flow behavior changed at a strain rate of 10/s where we observed serrations in flow
stress inthe temperature range of 1223 K to 1373 K. The serrations were of both positive and
negative nature alternatively. This may be due to precipitate - dislocation interactions in the
presence of MnS precipitate confirmed by EDAX analysis (Fig. 5). However, the actual
mechanism for such serrated flow is under investigation. It is to note that there are three
mechanisms concerning serrated flow. The first one is the dynamic interaction between solute
[7] [8]
atoms and dislocations . The second one is larger groups of dislocations move together . The
[9]
third one is shearing of precipitates by dislocations . Although there is no agreement in the
theory of serrated flow, motion of dislocations is necessary condition for it and dislocation -
precipitate interaction has a strong possibility in the presence of precipitate, like MnS.
For strain rate of 100/s, we observe continuous work hardening which may be due to very less
time available for dynamic recovery.
Effect of Strain Rate on Flow Stress
Fig. 7 shows the effect of strain rate on flow behavior at temperatures of 1223 K and 1423 K. It
is observed that as the strain rate increases, flow stress increases owing to enhanced
dislocation-dislocation interactions. The optical micrographs in Fig. 3 shows recrystallization
Fig. 7 : Effect of strain rate on flow stress at deformation temperatures of 1223 K and 1423 K
at strain rate of 10/s at 1423 K while at strain rate of 0.1/s only a small amount of recovery is
observed . Similarly at 1223 K more recovery was observed at higher strain rate even though
the time available for recovery was more at lower strain rates.
Fig. 8 shows effect of temperature on flow stress at a typical strain of 0.4 at different strain
rates. It can be observed that flow stress increases with decrease in temperature and with the
increase of strain rate. The increase in flow stress is gradual at lower strain rates but becomes
steep as strain rate increases with lowering of temperature. This results in increase of rolling
load due to which extreme care should be taken during processing of these electrical steels.
KUMAR ANIKET ANAND, KARTIK NAGESWARAN, VINOD KUMAR and ATUL SAXENA
256 J. MET. MATER SC., Vol. 58, No. , 20164
Strain Rate Sensitivity
The strain rate sensitivity describes the dynamic material behavior, and is, therefore, an
important material property to be determined. The strain rate sensitivity is defined by its
relation to the plastic flow curve, and not to the engineering stress-strain curve. Strain-rate
sensitivity index is referred to as 'm' and defined as:
(At constant temperature and strain) ...(2)
[5]
Where, σ is flow stress and is strain rate . As can be seen in Fig. 9, strain rate sensitivity
increases with temperature in general but forms a plateau in the temperature range of 1223 K to
1323 K.
ε
Fig. 8 : Effect of temperature on flow stress at 0.4 strain for all strain rates
m = δ
δ log ε
log σ
Fig. 9 : Variation of Strain Rate Sensitivity with Temperature
0.160
0.180
0.200
0.220
1150 1250 1350 1450
Temperature (K)
Strain = 0.2
Strain Rate Sensivity
CONCLUSIONS
Flow stress decreased with an increase in deformation temperature and decrease in strain
rate.
Extent of dynamic recovery increased with increase in deformation temperature and
strain rate.
HOT DEFORMATION STUDIES ON 2.7% SI STEEL USING GLEEBLE THERMO-MECHANICAL...
J. MET. MATER SC., Vol. 58, No. 4, 2016 257
Recrystallization was observed at 1423 K which increased considerably with increase in
strain rate from 0.1/s to 10/s.
Preferential deformation was observed in some grains due to difference in the orientations
of grains.
Small amount of cementite (pearlite) transformed from austenite along prior ferrite grain
was observed due to presence of carbon in excess of 0.02.
The strain sensitivity of the material increased gradually from 0.19 to 0.21 with an
increase in temperature from 1173 K to 1423 K.
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258 J. MET. MATER SC., Vol. 58, No. , 20164
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Chapter
Summary This document is part of Subvolume I1 ‘Magnetic Alloys for Technical Applications. Soft Magnetic Alloys, Invar and Elinvar Alloys’ of Volume 19 ‘Magnetic Properties of Metals’ of Landolt-Börnstein - Group III Condensed Matter.
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Studies were made into the process behind the excessive grain growth which is observed in continuous cast slabs of both regular and high permeability oriented 3 Pct Si-Fe during reheating from 1230 °C to 1400 °C. These large grains are undesirable because of the greater difficulty incurred in obtaining the suitably uniform and fine primary grain size desired prior to the final high temperature anneal during which the (110) [001] texture is developed. It was found that the driving force for the growth is the subgrain structure which develops due to the strains of solidification and cooling during continuous casting; however, the temperature at which growth initiates is related to the austenite-ferrite phase relationship. The grain growth begins when the austenite which forms during slab reheating decomposes to form highly perfect ferrite which then grows by consuming the strained preexisting (as-cast) ferrite matrix. Data summarizing studies into the energy storage and recrystallization processes which occur with the use of slab breakdown (or prerolling) prior to reheating from 1230° to 1400 °C are also discussed.
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  • D H Plantinga
  • W S T Maathuis
Sleeswyk A.W., James M.R., Plantinga D.H. and Maathuis W.S.T., (1983), Acta Metall., 31, pp. 1715.
  • Z G Wang
  • W Liu
  • Y B Xu
  • T Y Zhang
  • Y Zhang
Wang Z.G., Liu W., Xu Y.B., Zhang T.Y. and Zhang Y. (1994), Scripta Metall. Mater., 31, pp. 1513.