M. MIHALIKOVÁ et al.: STATIC AND DYNAMIC TENSILE CHARACTERISTICS OF S420 AND IF STEEL SHEETS
543–546
STATIC AND DYNAMIC TENSILE CHARACTERISTICS OF
S420 AND IF STEEL SHEETS
STATI^NE IN DINAMI^NE NATEZNE LASTNOSTI PLO^EVINE IZ
S420 IN IF JEKLA
Mária Mihaliková1, Vladimír Girman2, Anna Li{ková3
1Technical University of Ko{ice, Faculty of Metallurgy, Department of Materials Science, Letná 9, 042 00 Ko{ice, Slovakia
2P. J. [afarik University in Ko{ice, Faculty of Science, Department of Condensed Matter Physics, 042 00 Ko{ice, Slovakia
3Technical University of Ko{ice, Faculty of Metallurgy, Department of Materials Science, Letná 9, 042 00 Ko{ice, Slovakia
maria.mihalikova@tuke.sk
Prejem rokopisa – received: 2015-06-22; sprejem za objavo – accepted for publication: 2015-07-27
doi:10.17222/mit.2015.125
Two automotive steels were investigated; the Interstitial Free Steel (IF) HSLA (High Strength Low Alloy) and the S420
micro-alloyed steel. The properties of these materials were determined by static 10–3 s–1 and dynamic 103 s–1 rate stress
experiments. The plastic properties were determined after static and dynamic tests. The aim of examination of substructures was
to determine the distribution of dislocations for various types of stress. The hardness of all the tested materials was higher at a
slow rate of deformation. The greater strain hardening of the materials was confirmed by the dislocation distributions.
Keywords: IF steel, micro alloyed steel (S420), dynamic tensile test, hardness (HV1), dislocation structure
Preiskovani sta bili dve jekli iz avtomobilske industrije: jeklo brez intersticij (IF) HSLA (visko trdnostno nizko legirano jeklo)
in mikro-legirano jeklo S420. Namen {tudije je bil dolo~iti spremembe lastnosti teh materialov pri stati~ni hitrosti 10–3 s–1 in
dinami~ni hitrosti 103 s–1 obremenjevanja. Plasti~nost je bila dolo~ena na vzorcih po stati~nih in dinami~nih preizkusih. Namen
preiskave podstruktur je bil dolo~iti razporeditev dislokacij pri razli~nih vrstah obremenjevanja. Skladno z izmerjenimi
vrednostmi in s podatki iz literature je bila trdota vseh preizku{enih materialov vi{ja pri manj{i hitrosti deformacije. Ve~je
napetostno utrjevanje materialov je bilo potrjeno z razporeditvijo dislokacij.
Klju~ne besede: IF jeklo, mikro legirano jeklo (S420), stati~ni natezni preskus, dinami~ni natezni preskus, trdota (HV1),
dislokacijska mikrostruktura
1 INTRODUCTION
Strain rate, as a modifier of internal structure, is a
significant external factor that influences the material
behaviour in the forming process. In practice, an
understanding of the behaviour of steel under extreme
loading conditions is essential for the accurate prediction
of material response when a material is subjected to a
combination of severe load scenarios such as in colli-
sions. Presently sheets of different qualities are used in
the automotive industry. Therefore it is necessary to
create research and development for innovation capabi-
lities which could facilitate the rapid development of
materials and their cost reduction. The strain rate influ-
ences the strength properties through the internal
structure and thus affects the material function.1–3 Special
attention in research is paid to progressive IF Steels and
Micro-Alloyed Steels. Interstitial Free Steel (IF) contains
only a small amount of carbon (C <0.005%) and has very
good deep-ductility, as result of its low yield strength
(YS= 100–310 MPa). On the other hand, good deep-
ductility requires higher ultimate tensile strength (UTS
=140–450 MPa). The material ability to plastically
deform without breaking is determined by the ratio
Re/Rm.4–6 Micro-Alloyed Steels have a fine-grained
ferrite-pearlite microstructure with small quantities
(max. 0.15%) of precipitates of Al, Ti, Nb and V bound
to C and N.7 The micro-alloying effects are related to the
solubility of carbides (TiC, NbC, NC), nitrides (TiN,
AlN) and carbonitrides (Ti (C, N)) in austenite and
ferrite.7,8 An increase of strength can be obtained by
grain refinement and precipitation hardening. High
strength steels are considered as steels with a nominal
yield stress equal to or above 420 MPa. The mechanical
properties of the Micro-Alloyed Steels are largely a
result of a microstructure which depends on the chemical
composition and the processing method.9,10
2 EXPERIMENTAL MATERIALS AND
METHODS
Two steel materials, IF Steel and S420 steel with
chemical compositions presented in Table 1 were
investigated. Tensile testing at the specified strain rates
was performed with a Zwick 1387 servo-hydraulic
machine with a load capacity of 1000 kN (accuracy
±0.005 % of load capacity). In Figure 1 the size and
shape of the test bars is depicted. Static tensile testing
was carried out according to the EN ISO 6892-1 standard
at three traverse speed loads.11 The strain rate was
calculated according to Equation (1):12
Materiali in tehnologije / Materials and technology 50 (2016) 4, 543–546 543
UDK 67.017:620.17:669.15:622.023.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(4)543(2016)




= =
t
v
L0
(1)
Where: 
 – relative deformation, t – duration of the
deformation, L0 – working length of the test bar, v –
speed of the load
The dynamic tests were performed according to ISO
26203-1 and ISO 26203-2 standards on the rotary
hammer RSO13,14 with the data evaluated using the Scope
4 program. Figures 2 and 3 show the effect of strain rate
on yield stress and ultimate tensile strength of IF and
S420 steels sheets. In Figure 4 the elongation of both
steels (IF and S420) by quasi-static and dynamic con-
ditions is given. The S420 steel demonstrated an increase
of 48 % tensile strength at a strain rate of 100 s–2
showing its better formability by sheet metal forming.
2.1 Substructure
The substructure evolution was investigated on
samples formed at a static strain rate of 8.33.10–3 s–1 and
dynamic strain rates of (600, 2000, 3000 and 4000) s –1.
Samples were imaged in a JEOL 2100F transmission
electron microscope at 300 kV with STEM detector.15
Mechanically ground thin plates of 0.1 mm thickness
were then punched out as discs with a diameter of 3 mm.
The discs were then electrolytically polished using a
double jet device (TenuPol5) in a solution of acetic acid
and perchloric acid to obtain specimens for TEM inve-
stigation15 of dislocations. In Figure 5 planar structures
are mainly observed. Such stacking faults and planar
dislocation structures (regular dislocation pile-ups,
planar tangled bundles) were often observed. After
dynamic strain of e = 4000 s–1, the dislocations were still
mainly present as more evenly distributed planar
structures (Figure 6), with stacking faults less often
observed.16 Figure 7 shows the S420 steel after static
strain with numerous dislocation arrangements close to a
M. MIHALIKOVÁ et al.: STATIC AND DYNAMIC TENSILE CHARACTERISTICS OF S420 AND IF STEEL SHEETS
544 Materiali in tehnologije / Materials and technology 50 (2016) 4, 543–546
Figure 4: Influence of loading rate on fracture elongation of IF and
S420 steel sheets
Slika 4: Vpliv hitrosti deformacije na raztezek pri prelomu IF in S420
jeklenih plo{~
Figure 2: Dependence of yield stress with strain rate of IF and S420
steel sheets
Slika 2: Odvisnost med hitrostjo deformacije jeklenih plo{~ IF in
S420 ter mejo plasti~nosti
Figure 3: UTS dependence on strain rate of IF and S420 steel sheets
Slika 3: Odvisnost hitrosti deformacije jeklenih plo{~ IF in S420 ter
raztr`no trdnostjo
Figure 1: Size of the transverse test bars
Slika 1: Velikost testnih pre~nih preizku{ancev
Table 1: Chemical composition of IF and S420 steel (in mass fractions, w/%)
Tabela 1: Kemi~na sestava IF in S420 jekla (v masnih odstotkih, w/%)
Material C S N Mn P Si Al Nb V Ti
IF 0.0013 0.0105 0.0017 0.82 0.011 0.006 0.055 0.001 0.002 0.04
S420 0.12 0.002 - 1.44 0.009 0.05 0.046 0.035 0.2 0.016
grain boundary, whereas the dislocations in the middle of
the same grain were much fewer. After dynamic strain

 = 3000 s–1, the dislocation density was much higher
than that of the static condition, with the dislocations
distributed homogeneously (Figure 8).
3 RESULTS AND DISCUSSION
The experimental results indicate that the strain rate
affects the basic mechanical properties of tested steels.
The change of properties is greater by at higher strain
rates (Figure 2) and potentially lead to a change in
deformation properties (Figure 4).16–19 The dependence
of the strength properties on the strain rates for the steels
tested in the range from 10–3 to 103 s–1 is described by
parametric Equations (2) and (3):20
R R Ae e  ln


 







= + ⋅ (2)
R R Bm m  ln


 







= + ⋅ (3)
Where:
Re
 and Rm
 are the yield stress and ultimate tensile
strength at a given strain rate 
.
Re
 and Rm
 are the yield stress and ultimate tensile
strength at a static deformation rate (10–3 s–1).
The parameters A and B are material constants and
express the steel sensitivity to strain rate. With higher A
and B parameters, the steel is more sensitive to strain
rate, presenting less obstruction to dislocation motion. A
and B values of 24.1 and 20.4, respectively, were deter-
mined for the IF steel and 16.61 and 24.9, respectively,
for the S420 steel.20
The greater increase of the strength properties by
dynamic stress than by static stress could be explained
by increasing lattice resistance to the movement of dislo-
cations. The assumption is if the deformation is dynamic
M. MIHALIKOVÁ et al.: STATIC AND DYNAMIC TENSILE CHARACTERISTICS OF S420 AND IF STEEL SHEETS
Materiali in tehnologije / Materials and technology 50 (2016) 4, 543–546 545
Figure 7: Dislocation structure of S420 steel in the static condition
(
 = 8.33·10–4 s–1)
Slika 7: Struktura dislokacij pri stati~nem stanju jekla S420
(
 = 8,33· 10–4 s–1)
Figure 5: Dislocation structure of IF steel in the static condition
(
 = 8.33·10–4 s–1)
Slika 5: Struktura dislokacij v stati~nem stanju IF jekla
(
 = 8.33·10–4 s–1)
Figure 6: Dislocation structure of IF steel in the dynamic condition
(
 = 4000 s–1)
Slika 6: Struktura dislokacij pri dinami~nem stanju IF jekla
(
 = 4000 s–1)
Figure 8: Dislocation structure of S420 steel in the dynamic condition
(
 = 3000 s–1)
Slika 8: Struktura dislokacij pri dinami~nem stanju jekla S420
(
 = 3000 s–1)
there is not sufficient time for it to pass through the
best-oriented lattice planes and slip planes yielding
higher critical shear stresses and greater stress is required
for deformation.21,22
4 CONCLUSION
The experimental results and calculations support the
following conclusions:
• The strength properties of tested steels increase with
strain rate.
• The dynamic yield strength is increased substantially
with respect to quasi static strain. In an uninterrupted
tensile test at a strain rate of 1 s–1 the yield strength
(YS = 0.2 %) is increased by 16 % for IF steel and by
6 % for S420. With a strain rate 10 s–1, the yield
strength was increased by 98 % for IF steel and 49 %
for S420. The tensile strength also increased with
increased strain rate.
• IF steel with the coarse-grained ferritic structure was
more sensitive to strain rate. For yield strength the
sensitivity coefficient A = 24.1.
• S420 steel with the fine-grained ferritic – pearlitic
structure and precipitates had a lower sensitivity to
the strain rate and the sensitivity coefficient for yield
stress is A = 16.6.
• The dynamic response can be associated with the
regrouping of dislocations.
• It is concluded that the IF steel had fewer barriers to
the movement of dislocations than the S420 steel.
The effect of strain rate reflects the resistance of
lattice against the motion of dislocations and was
more pronounced in dynamic load conditions.
Acknowledgement
This study was supported by the Grant Agency of
Slovak Republic, grant project VEGA 1/0549/14.
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M. MIHALIKOVÁ et al.: STATIC AND DYNAMIC TENSILE CHARACTERISTICS OF S420 AND IF STEEL SHEETS
546 Materiali in tehnologije / Materials and technology 50 (2016) 4, 543–546