Changing the Hardness Automotive Steels at Different Strain Rate
Miroslav Nmet1, a*, Mria Mihalikov1, b , Alexandra Kovalkova2, c,
Anna Likov1,d
1Technical University of Koice, Faculty of Metallurgy, Department of Materials Science, Park
Komenskho 11, 040 01 KOICE, Slovakia 2Institute of Material Research, Slovak Academy of Sciences, Watsonova 47, 040 01 KOICE,
Slovakia [email protected], [email protected], [email protected],
Keywords: hardness HV1, strain rate, IF steel, micro-alloyed steel
Abstract. Currently, the automotive industry used sheets of different qualities. The most common
include IF (Interstitial Free) steel and micro-alloyed steel. Use the sheet quality depends on the
point of application in the production car. Testing and product testing is a standard part of the
process of innovation and production itself. Testing of automotive steels under dynamic conditions
is increasingly important. Changing the hardness HV 1 was performed on the fractured bars on the
static and dynamic loading conditions. Tests were made on steel IF and S 460.
Introduction
The automotive industry is constantly growing and it is therefore necessary to build research,
development and innovation capacities that facilitate faster product development and cost reduction.
Testing and product testing is a standard part of the process of innovation and production itself.
Normal practice is known that the material and machine parts behave differently when slow and
with fast stress. With the change of strain rate studied automotive steels varied course of hardness
HV 1 along the length of the test bars [1-3].
The most common automotive steel are IF steel and micro-alloyed steel. Interstitial free steel IF
(interstitial free) constitute one of the main groups of steels applied in the automotive industry Fig.
1 interstitial free steels contain very small amounts of carbon (C < 0.00 5%). This results in lower
values HV1 of yield strength and conversely higher values HV1 plasticity. These are the reasons
why they are used for deep drawing example as fenders and roof of the car. Microalloyed steels are
now frequently used automotive structural material Fig. 2.
Fig. 1 Microstructure IF steel Fig. 2 Microstructure S 460 steel
Microalloyed steels are consolidated by a combination of precipitation and grain size. The content
of alloying elements in a range of 0.01 to 0.1 % and most often is alloyed with manganese. Other
alloying elements are Cr, Ni, Mo, W, Ti, and others. Microalloyed steels are not to alloy steels,
Key Engineering Materials Vol. 635 (2015) pp 41-44 Submitted: 23.09.2014Online available since 2014/Dec/11 at www.scientific.net Revised: 26.09.2014 (2015) Trans Tech Publications, Switzerland Accepted: 16.10.2014doi:10.4028/www.scientific.net/KEM.635.41
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although their properties are achieved by adding a small amount of alloying elements. Due to the
good ductility are used to shape complex components and particularly for dynamically loaded
components chassis vehicles [4, 5].
Experimental materials and methods
As an experimental material was used two cars steel IF steel and microalloyed steel S 460.
In static conditions, the tests carried out according to EN ISO 6892-1. In dynamic conditions to
achieve the desired strain rate used rotary hammer RSO. The principle of the test shock hammer
RSO is based on a large rotating flywheel momentum. After spinning flywheel rotating at the
desired speed will be output from the flywheel blasting jaws that capture the member and the
kinetic energy transferred to the end of the body, which tore.
The hardness value is an important characteristic of the material and hardness test is a simple
and inexpensive for obtain mechanical properties. In our case we used the Vickers hardness test,
where the stent is used as a penetrating diamond pyramid with the top angle of 136 o.
Microhardness test was performed Vickers hardness testing machine 432 SVD. Hardness HV1
was measured every 0.5 mm of the fracture line, up to a distance of 5 mm of the fracture. The line
of stitches is shown in Fig. 3. After measuring the hardness indentations were observed and
measured on confocal microscopy PLU Neox 3D Sensofar Fig. 4.
Fig. 3 Line injections HV1 from fracture Fig. 4 Injection observed in confocal microscopy
Hardness HV1 IF steel and micro-alloyed steel S 460 was measured on samples after static tensile
test carried out on a tensile testing machine ZWICK 1387 (strain rate 8.33. 10-3
.s-1
). The dynamic
tests were made on a rotary hammer RSO (the strain rate of 600, 2500 and 4000 s-1
). The measured
values of hardness HV1 for IF steel are in Table 1 and for steel S 460 in Table 2. A graphical
presentation of the results of the IF steel is in Fig. 5 for the steel S 460 in Fig. 6. The course of
hardness HV1 copied deformation distribution Fig. 5 and Fig. 6 with increasing distance from the
fracture zone HV1 decreased. The highest hardness in the case of IF steel was measured at a rate of
8. 33 10-3
s-1
it was 190 HV1. The lowest hardness was measured at a rate of 4000 s-1
it was 120
HV1. 45 HV1 was across all test bars IF steel. The highest hardness was measured for steel S 460 at
a rate of 2000 s-1
it was 289 HV1 and the lowest rate at 8.33 10-3
s-1
was 239 HV1. The hardness of
the dispersion over the length of the test bars S 460 were about 20 HV1.
42 Material Engineering Practice IX
Table 1 Hardness HV1 IF steel
distance from the
fracture [mm] steel / steel / steel / steel /
IF / 8.33 .10-3
.s-1
IF / 600 s-1
IF / 2500 s-1
IF / 4000 s-1
HV1 HV1 HV1 HV1
0.5 154.8 189 169 163
1 170.3 184 170 152
1.5 168.7 171 167 145
2 159.6 173 166 126
2.5 156.8 168 163 130
3 154.4 173 154 127
3.5 146.7 163 151 121
4 144.8 160 149 118
4.5 143.2 160 144 116
5 143.8 154 140 120
Table 2 Hardness HV1 S 460 steel
distance from
the fracture
[mm]
steel / steel / steel / steel /
S 460 / 8.33 .10-3
s-1
S 460 / 600 s-1
S 460 / 2500 s-1
S 460 / 4000 s-1
HV1 HV1 HV1 HV1
0.5 284 276 298 268
1 284 276 284 281
1.5 272 267 271 277
2 280 268 269 267
2.5 274 267 266 275
3 279 270 258 271
3.5 264 258 258 268
4 263 256 257 257
4.5 251 251 259 255
5 239 257 252 249
Fig. 5 Dependence of hardness HV1 on the distance from fracture IF steel
Key Engineering Materials Vol. 635 43
Fig. 6 Dependence of hardness HV1 on the distance from fracture S 460 steel
Summary
Relative increase in hardness due to hardening plastic deformation is mainly a result of its size. The
most significant increase has a relatively IF steel, steel S 460 lowest Assess the impact of strain rate
on the relative increase in hardness (hardening) would be possible only in the case of achieving the
same degree of plastic deformation. The measured values of hardness HV1 at different speeds
deformation related to the structure of the material. From literary knowledge is obvious that, IF
steel is formed by coarse-grained ferrite. Deformation at elevated speeds to avoid accumulation of
dislocations and thus reinforced materials was therefore determined hardness HV1 the lowest and
the highest variance. In the case of steel S 460 has a structure with fine-grained ferrite and pearlite,
therefore, has not been identified as a significant difference between the static and dynamic load,
which is the result of accumulation of dislocations, and the subsequent hardening already at the
static loading conditions. Scattering hardness HV 1 was for steel S 460 significantly smaller.
Acknowledgments
This work was supported by grant agency VEGA 1/0549/14.
References
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44 Material Engineering Practice IX
Material Engineering Practice IX 10.4028/www.scientific.net/KEM.635 Changing the Hardness Automotive Steels at Different Strain Rate 10.4028/www.scientific.net/KEM.635.41
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