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 amiroslav.nemet@tuke.sk, bmaria.mihalikova@tuke.sk, cakovalcikova@imr.saske.sk,

danna.liskova@tuke.sk

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

[1] M. Mihalikov, M. Nmet, Increments of plastic strain and hardness HV10 of automotive steel sheets, Metalurgija, 51 4 (2012) 449-452.

[2] M. Mihalikov, M. Nmet, The hardness HV1 analysis of automotive steels sheets after a plastic deformation, Acta Metallurgica Slovaca,17 1 (2011) 26-31.

[3] M. Burak et.all, Influence of blasting on mechanical properties of steel sheet, Metalurgija, 43 2 (2004) 101-105.

[4] H. Hofmann, D. Mattissen, T.W., Schaumann, Advanced cold rolled steels for automotive applications, Steel Research International, 80 1 (2009), 22-28.

[5] E. Kormankov, K. Kotrasov, Finite element analysis of damage modeling of fiber reinforced laminate plate, Applied Mechanics and Materials, 617 (2014) 247-250.

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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