International Journal of Automotive Technology, Vol. 16, No. 5, pp. 849−863 (2015)

DOI 10.1007/s12239−015−0087−1

Copyright © 2015 KSAE/ 086−15

pISSN 1229−9138/ eISSN 1976−3832

849

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN

AUTOMOTIVE INDUSTRY

T. TRZEPIECI SKI1), A. BAZAN1) and H. G. LEMU2)*

1)Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology, Al. Powstañców Warszawy 12, 35-959 Rzeszów, Poland

2)Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway

(Received 22 July 2014; Revised 24 September 2014; Accepted 14 November 2014)

ABSTRACT−In this paper the results of experimental tests aimed to determine the friction coefficient in sheet metal forming

operations for various sheet metal materials and at different operative conditions are presented. The research has considered

the frictional characterization of three kinds of drawing quality steels that are commonly used in automotive industry. These

are a drawing quality steel (DQ), a deep drawing quality steel (DDQ), and extra deep drawing quality steel (EDDQ). For

measurement of the sheet surface topography, a 3D stylus instrument Alicona InfiniteFocus was used. To determine the

friction coefficient three tribological tests, i.e. a strip drawing test, a draw bead test and a pin-on-disc tribometer, have been

conducted. The experimental results have ascertained several relationships showing the effect of sheet metal surface

roughness, lubricant conditions and sheet orientation on the value of friction coefficient in sheet metal forming processes. The

results further showed that the surface topography and sample orientation in the rolling direction of the sheet are significant

factors that influence the friction coefficient. It was found that the tested steel sheets, selected from automotive industry

applications, exhibit anisotropic resistance to the friction corresponding to the measured orientation in relation to the rolling

direction of the sheet.

KEY WORDS : Friction, Friction anisotropy, Strip drawing test, Coefficient of friction, Draw-bead test, Sheet metal forming

1. INTRODUCTION

The developments in the automotive industry within the

last decades has demanded the actual vehicle designs to be

innovative and robust (Kim et al., 2014) in order to

achieve, among others, short product development period,

very high safety standards of the body panels and the

overall reduced weight (Park and Dang, 2011). Such

vehicles require stronger and lighter materials (Bae and

Huh, 2012; Cho et al., 2013). One of the new market

requirements in the design of body panels for vehicles and

various other industries is the development of sheet metal

forming technologies that can optimize the performance of

the panels. In this respect, deep drawing process of a sheet

metal is widely used in the automotive industry (Guillon et

al., 2001; Wiklunda et al., 2004). Resistance to friction is

one of the main factors limiting the sheet metal forming

process (Darendeiler et al., 2002). Friction in sheet metal

forming depends on several parameters such as the contact

pressure, sliding velocity, sheet metal and tool surface

roughness, tool and blank material, lubrication, etc.

Moreover, resistance to friction depends on physicochemical

factors acting on the contact surface, dynamics of loads and

temperature (Matuszak, 2000). Friction between the sheet

and tool during the forming process is influenced not only

by the surface topography, but also by several parameters

such as lubricant, surface chemistry, contact pressure, and

sliding speed (Wilhborg and Gunnarsson, 2000). Friction

on contact surface brakes free metal flow that results in

deformed metal of distinguish zones with different

deformation extent. Friction in the deep drawing process,

in particular, increases the inequality of deformation and

loading force value and thus worsens the quality of the

drawpiece surface (Choi et al., 2013). As a result,

clarification of friction is essential for modelling and

analysis of the sheet metal forming processes.

Several kinds of friction tests have been developed to

determine coefficient of friction in different regions of

formed drawpiece (Bay et al., 2008). Among others, the

most often used test methods are the strip drawing test,

drawbead simulator friction test and bending under tension

test. Strip drawing test is one of these techniques and

allows different tool geometry configurations: flat/flat, flat/

cylinder and cylinder/cylinder (Guillon et al., 2001; Roizard

et al., 2009). In this test, the macroscopic coefficient of

friction is easily measured and usually used to characterise

the lubricated contact between tool and metal sheet. The

bending under tension (BUT) test simulates the die radius

Né

*Corresponding author. e-mail: hirpa.g.lemu@uis.no

850 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

conditions in a stamping tool (Lemu and Trzepieci ski,

2013).

The results from the BUT test show that the surface

topography is a significant factor for the frictional behaviour

of uncoated steel sheets (Wiklunda et al., 2004). (Han,

1997) performed both drawing test and stretching under

bending test by changing punch corner radii and investigated

the friction characteristics affected by punch corner radii.

(Azushima et al., 1991) developed a sheet drawing apparatus

characterized by a transparent quartz die and the increase of

friction coefficient at increased drawing speed was

interpreted in terms of the measured volume of the surface

pockets trapping lubricant. (Fratini et al., 2006) evaluated

the coefficient of friction by considering parameters such

as the variation of the contact area during metal flow using

dedicated fixture that was designed and set-up in

accordance with the one proposed by (Wilson, 1978). The

draw bead simulator (DBS) friction test was developed by

(Nine, 1978) to model friction in region of stamping tool

where the metal passes through the drawbead. It is

common in sheet metal forming simulation to use an

equivalent drawbead instead of using the real geometry of

the drawbead (Larrson, 2009).

The nature of interactions between asperities in the near-

surface material in a lubricated contact depends on the

surface roughness and texture, including the lay of waviness

(Ajayi et al., 2009). For anisotropic surface texture, the

interaction between asperities in dry and lubricated

contacts is expected to be non-uniform in all directions.

This condition may produce a directional effect on the

measured friction. This phenomenon may be determined

using the pin-on-disc test (Ajayi et al., 2009; Trzepieciñski

and Lemu, 2012). (Kim et al., 2012) developed the

pressure dependence on the anisotropic friction model

using the draw-bend friction test.

The objective of this article is multi-factoral frictional

study of three grades of deep drawing steel sheets that are

commonly used in automotive industry. To determine the

friction coefficient three tribological tests: a pin-on-disc

tribometer, a strip drawing test and a draw bead test have

been conducted. The several relations between the value of

the friction coefficient and specimen orientation, normal

load, friction measurement orientation in relation to the

rolling direction of the sheet, surface roughness of tools

and lubricant conditions are studied. Furthermore, the

estimation of the changes in the surface roughness of the

steel sheet was done.

2. MATERIALS

In this research conducted to characterise friction in sheet

metal forming for the automotive industry, three kinds of

low carbon steel sheets, namely, a drawing quality steel

(DQ), a deep drawing quality steel (DDQ), and extra deep

drawing quality steel (EDDQ) were examined. A tensile

test in the universal testing machine was carried out to

determine the mechanical properties. The mechanical

properties determined in this test (as given in Table 1) are

yield stress σy, ultimate strength σu, ultimate elongation εu,

anisotropy coefficient r, strain hardening coefficient C and

strain hardening exponent n. The samples for the tensile

tests were cut in two orthogonal directions: along the

rolling direction (0o) and transverse to the rolling direction

(90o).

The measurement of surface roughness parameters was

carried out using the Alicona InfiniteFocus instrument. The

né

Table 1. Mechanical properties of the tested sheets.

MaterialThickness

(mm)Orientation

(o)

Mechanical properties

σy (MPa) σu (MPa) εu C (MPa) n r

DQ 10 193 351 0.36 554 0.166 1.212

90 193 353 0.34 563 0.174 1.410

DDQ 0.80 196 336 0.42 557 0.192 1.464

90 198 311 0.41 526 0.177 1.902

EDDQ 10 151 282 0.44 494 0.221 1.630

90 153 287 0.42 487 0.211 1.638

Table 2. Surface roughness parameters of the tested sheets.

Material

Surface roughness parameters

Sa Sq Sp Sv Ssk Sku Sz Sds* Str Sbi Sci Svi

(µm) (µm) (µm) (µm) (µm) (Peaks/mm2)

DQ 0.302 0.376 2.37 1.39 0.267 3.48 3.26 749 0.029 0.241 1.64 0.110

DDQ 0.340 0.423 2.48 1.62 0.298 3.34 3.3 697 0.036 0.243 1.67 0.094

EDDQ 0.362 0.41 2.98 2.08 0.338 3.67 3.51 685 0.041 0.255 1.58 0.112

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 851

main standard 3D parameters determined by this

measurement are the roughness average Sa, the root mean

square roughness parameter Sq, the highest peak of the

surface Sp, the maximum pit depth Sv, the surface

skewness Ssk, the surface kurtosis Sku, the 10-point peak-

valley surface roughness Sz, the density of summits Sds,

the texture aspect ratio of the surface Str, the surface

bearing index Sbi, the core fluid retention index Sci and the

valley fluid retention index Svi. The values of these

measured parameters are given in Table 2. The results of

surface roughness measurements clearly show that the used

steel sheets have narrow range of Sa parameter value. It

seems that this allows determination of the effect of

mechanical properties of the sheets on the value of friction

coefficient. The surface topography views of the steel

sheets are shown in Figure 1, where each sample has an

area of 1.0849 mm × 1.0849 mm. The values of roughness

parameters presented in the table are average values from

the three measurements in three places on the sample.

Since the tested sheets are characterised by a similar value

of Sa parameter, the decisive significance of this parameter

may have directionality of surface topography, which was

previousely studied by (Ajayi et al., 2009; Masuko et al.,

2005; Xiao et al., 2004).

3. TEST METHODS

3.1. Tribometer

The friction properties of the low carbon steel sheets used

in the experiments were determined by using the pin-on-

disk tribometer T01-M, strip drawing test and draw bead

simulator (DBS). Thed friction tests were carried out for

two friction conditions: dry and lubricated. Dry friction

was realized by degreasing the surface of the specimens by

using acetone while machine oil L-AN 46 of 44 mm2 s-1

viscosity at 40oC was used for lubricated conditions. The

oil was distributed uniformly on the surface of the samples

using a teflon shaft. This method of application of grease

was used in all tests discussed in the paper. T01-M pin-on-

disk machine is intended for the determination of friction

and wear of engineering materials (Antoszewski et al.,

2008). This machine makes it possible to determine the

wear resistance and friction coefficient for a pair of

materials sliding against each other, depending on sliding

velocity and applied load. The experiment was carried out

in accordance with the ASTM G 99 and DIN 50324

standards described in the machine service manual (Pin-on-

disc tribometer, 1999).

The tribosystem consists of the stationary ball pressed at

the required load FN against the disk rotating at a defined

speed n (Figure 2). The operating parameters defined for

the pin-on-disk CSM® tribometer tester were as follows:

normal load 6 N, 9 N and 12 N; linear slipping speed

1 mm/s and specimen angular velocity 0.045 rad/s. The

ball in bearing steel 100Cr6 has a diameter of 6 mm.

The ball has an isotropic surface finish and texture.

During the tests, the friction force and the slip distance for

the first disk revolution were measured where a computer

program was used to register and control the values of the

friction force as a function of time. The value of friction

coefficient value was determined based on the values of

both normal FN and friction FT forces from the formulae:

μ = FT/FN (1)

3.2. Strip drawing test

Figure 1. Views of surface of steel sheets: (a) DQ, (b) DDQ and (c) EDDQ.

Figure 2. View of measuring station.

852 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

The strip drawing test and DBS test were previousely

carried out by special friction simulator (Lemu and

Trzepieciñski, 2013) built based on Nine’s concept (1978).

Friction strip drawing tests were conducted by using

friction simulator where the sheet strip was placed between

two fixed cylindrical rolls with equal radii (Figure 3). The

tests were done in such a way that a strip of the sheet was

clamped with specified force between two cylindrical rolls

of equal radii of 20 mm. Values of both forces, the

clamping force FC and the pulling force FP, were constantly

recorded using electric resistance strain gauge technique, 8-

channel universal amplifier of HBM's QuantumX data

acquisition system and a computer (PC). Samples were

prepared as strips having 20 mm width and about 200 mm

length, cut along rolling direction and transverse direction

of the sheet. Four sets of rolls (Figure 4) with different value

of a roughness average Ra measured along generating line of

rolls have been used in the friction tests. The rolls were

made of cold-worked tool steel X165CrV12 heat treated to

58 HRC.

To determine the effect of surface topography of the

tools on the frictional behaviour, four sets of rolls with

different grinding lay were used (Figure 5). All rolls were

made by similar turning setup, such as the feed rate and

cutting speed, to guarantee that the roll roughness measured

along pitch-surface generator are identical. In order to

distinguish four kinds of rolls Ra parameter was found

convenient to use. The tests were performed under the

following conditions:

• Roughness average Ra of rolls denoted as Raw: 0.32;

0.63; 1.25 and 2.5 μm,

• Sliding velocity: 0.001 m/s, which is relatively high

compared with the industrial values,

• Clamping force whose value was increased gradually up

to about 2 kN.

Stereometric parameters of the surface roughness of

rollers are shown in Table 2. In order to distinguish the

change in friction conditions associated with the surface

roughness variation of the rollers, the value of parameter

Ra: 0.32, 0.63, 1.25, 2.5 μm were adopted. To calculate the

friction coefficient from formulae (2) under dry condition,

the pulling FP and clamping FC forces were recorded while

the sheet was drawn for a distance of about 3 mm. Next the

clamping force value was increased simultaneously by

rotating the set bolt (Figure 3 (b)). Typical load-time

dependence is also presented in Figure 6 where the transient

region has been rejected from considerations.

μ = FP/FC (2)

The value of the friction coefficient in the strip drawing

test and using the drawbead simulator was determined as

the mean value for the set of three trials. Samples for

friction test were cut from the same sheet metal. Since the

Figure 3. View (a) and scheme (b) of measuring device: 1 – frame; 2 – working rolls; 3 – load cells; 4 – specimen; 5 –

blocked pin; 6 – grip of testing machine; 7, 8 – tension members; 9 – set bolt.

Figure 4. Four sets of rolls for the strip drawing test.

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 853

friction was measured for each of the clamping force levels

(steady region in Figure 6), the three independent distribu-

tions of friction coefficient changes with time were

obtained (about 180-220 results). The friction coefficient of

the test was then determined as an average value of the

above-mentioned registered number of changes of the

values. After determining the average value of the test, the

value of the average coefficient of the friction of the three

trials was set.

3.3. Draw Bead Simulator Test

The drawbead simulator tests were carried out using the

aforementioned modified device (Trzepieciñski and Lemu,

2014). In the drawbead simulator, the sheet metal was

pulled to flow between three cylindrical rolls of equal radii

of 20 mm (Figure 7). The test material was cut along the

rolling direction and transverse direction into 200 mm long

and 20 mm wide strips. The surface roughness of the rolls

is the same as that used for strip drawing tests.

The clearance c (Figure 8) between working rolls was

adjusted and maintained at th + th/2 (where th is the sheet

thickness). The main purpose of this clearance is to prevent

locking of the sheet between the rolls, especially during use

of fixed rolls. The high wrap angle may produce a very

high slide resistance and may result in over-increasing of

plastic tension of the sheet and thus fracture. Furthermore,

the test was carried out for middle roll penetrations p

(Figure 8) of 14 mm.

The pulling and clamping forces were controlled using

load cells. To determine the coefficient of friction, it was

found necessary to carry out two tests with two samples.

One specimen was pulled between cylindrical rolls

supported by bearings and then the measured pulling force

(denoted as Droll) and clamping force (Croll) give the bending

and unbending resistance of the sheet under “frictionless”

conditions respectively. The sheet was displaced between

the rotating rolls so that the friction between the sheet and

rolls is minimized, while the second specimen was pulled

between the fixed rolls. Friction opposes the sliding of the

sheet over the fixed rolls. The combined loads required to

slide and to bend/unbend the sheet with the fixed rolls is

then given by the measured pulling force (Dfix) and

clamping force (Cfix). During both tests the supporting roll

was kept free to rotate.

It has been argued in a previous study (Nanayakkara and

Hodgson, 2006) that the wrap angle corresponding to the

actual engagement of the strip with the roller or bead was

not taken into account in the derivation done by (Nine,

1978). Furthermore, (Green, 2001) stated that the tangent-

to-tangent bead wrap assumption becomes approximately

valid only at very deep penetrations. This supports the

argument that the validity of the equation derived by (Nine,

1978) is limited to deep penetrations. In this experimental

Figure 5. Surface topography of the rolls with Raw

parameter: (a) 0.32 μm; (b) 0.63 μm; (c) 1.25 μm and (d)

2.5 μm.

Figure 6. Load-time curves.

Figure 7. Measurement system used for friction testing

1 – frame; 2 – front roll; 3 – middle roll; 4 – back roll;

5 – specimen; 6 – supporting roll; 7 and 8 – tension

members; 9 and 10 – extensometers; 11 – fixing pin; 12 –

ear nut.

854 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

setup, all force values were constantly recorded using

electric resistance strain gauge technique, 2-channel

universal amplifier of data acquisition system and a

computer. When the wrap angle is not equal to 180o the

friction coefficient is calculated from (Nanayakkara et al.,

2004):

(3)

where Θ is the quarter contact angle of actual engagement

of the strip over the bead, and a value of Θ = π/2 confirms

a full penetration.

4. RESULTS AND DISCUSSION

4.1. Tribotester

To confirm that steel sheets are characterized by the

anisotropy of tribological properties, friction anisotropy on

a given surface has to be clearly distinguished from friction

anisotropy for different perpendicular orientations between

the pin and the surface. As shown in Figure 9, changes of

friction coefficient value exhibit two maxima for a rotation

through 360o. These points correspond to the measurdn of

friction coefficient value transverse to the rolling direction.

The representative value of friction coefficient should be

determined for first sample revolution. Together with a

number of sample revolutions, the sheet surface wears and

the value of friction coefficient increases. A local plastic

deformation and wear are the sources of increased noise of

friction coefficient, which was also observed by (Ajayi et

al., 2009). After four revolutions of the sample, the

difference between the friction coefficient value measured

at along and transverse to the rolling direction is clearly

decreasing. The noise in the friction measurement system

increases with the number of revolutions so frictional

anisotropy may be then an error. The noise is proportional

to the yield strength of material which was reported by

(Ajayi et al., 2009). The maximal value of friction

coefficient existed every 180o that coincides to the point s

where the ball specimen slides perpendicular to the rolling

direction of the sheet.

The anisotropic friction model was implemented by

specifying different friction coefficients in two orthogonal

directions on the contact surface that coincide with the

defined slip directions. In recent studies (Trzepieciñski,

2013) it was found that the elliptic function is accurate to

describe the anisotropic friction model. The polar plots in

Figure 10 show variation of the friction coefficient through

the first sample revolution. All tested materials exhibit two

maxima on one evolution of the sample. The same effect

was observed for all tested sheets. The highest effect of

lubrication was observed for DQ steel sheet, the lowest for

μDfix Droll–

Cfix

---------------------- θsin

2θ----------=

Figure 8. Geometrical parameters of drawbead simulator

test.

Figure 9. Friction coefficient as a function of disc

revolutions from the rolling direction for DQ sheet, normal

load 12 N, lubricated condition.

Figure 10. Variation of friction coefficient during first

revolution of the sample for: (a) DQ, (b) DDQ and (c)

EDDQ steel sheets; load force 9 N.

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 855

the EDDQ steel sheet.

Figure 11 shows the variation of the calculated friction

coefficient value observed for the first revolution of the

sample. The figure shows that the value of friction

coefficient is different for all materials. On the other hand,

as given in Table 2, the tested sheets have similar value of

the Ra parameter of the surface roughness. This difference

may coincide with mechanical properties of the sheets and

different material deformation at the contact interface. With

increasing ultimate elongation of the sheets, which

indicates their higher plasticity, the friction coefficient

value increases. The value of the friction coefficient

increases with the ball load for both dry and lubricated

sliding conditions. In the case of dry friction conditions, the

range between minimum and maximum value of friction

coefficient increases with the ball load. As it was

Figure 11. Variation of friction coefficient determined in (a) dry friction and in (b) lubricated conditions.

Figure 12. Friction coefficient vs. clamping force of rolls in dry friction. Plots for Raw of rolls: (a) 0.32 μm; (b) 0.63 μm; (c)

1.25 μm and (d) 2.5 μm.

856 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

mentioned earlier, the effect of surface wear and noise in

the friction measurement system was emphasized. During

the lubricated sliding test the deviation of the friction

coefficient value was smaller compared to the test results in

dry friction conditions. In the case of dry friction conditions

conducted at the highest ball load, which is 12 N, the value

of friction coefficient considerably increased. Lubrication

allows the range of friction coefficient value variation to

decrease along the friction track.

Increasing the deep-drawability of the sheets (from DQ

through DDQ to EDDQ) for the same pressure increases

the value of friction coefficient value. Increased plastic

properties of the sheet cause that a tester ball penetrates

more into the sheet material and the surface area of contact

increases. The increasing drawability of the sheets can be

related with the increase of ultimate eleongation εu.

The measured friction coefficient under lubrication

conditions is due to shearing of the fluid film and elasto-

plastic deformations of the asperities. As has been reported

by (Ajayi et al., 2009), no frictional anisotropy was

observed for material that does not plastically deform (e.g.,

DLC-coated flat).

4.2. Strip Drawing

According to the present experimental results, the value of

the friction coefficient decreases as the clamping force

increases for both lubricated and dry conditions (Figure

12). In general, the value of friction coefficient decreases

with increasing clamping force of rolls for both lubricated

and dry friction conditions. This may be as a result of the

dependence of friction on the clamping (normal) force

where beyond certain load the relationship between friction

force and clamping force is nonlinear and the value of

friction coefficient is not constant. In other words, it

changes with normal load.

The nonlinear dependence between clamping force and

the value of friction coefficient was previousely studied by

the authors and the results are reported in literature

(Trzepieci ski and Lemu, 2012) where genetic algorithm

and artificial neural networks were applied to forecast the

friction values.

Different results were obtained in the tribometer test

(Figure 11), with increasing clamping force an increase in

the coefficient of friction value was observed. In the

tribometer test exists much smaller contact area and the

increase of the pin pressure causes rapid expansion of the

area of contact. In the strip drawing test sheet contact area

is distributed over the width of the sample (20 mm) and

load is transfered by higer surface than in the tribometer

test. With the increasing deep drawability of the sheets the

friction coefficient value also increases. The highest values

of friction coefficient were observed for EDDQ steel sheet

which may be explained by the highest value of surface

roughness represented by the basic roughness parameter

Sa. During pin-on-disk test, strain hardening of the sheet

material occurs because of plastic deformations of the

né

Figure 13. Mean value of friction coefficient vs. material type.

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 857

asperities and the increased frictional resistance. The real

area of contact depends on different flattening and

roughening mechanisms of the deforming asperities. Further,

the tested sheets have different surface roughness and

materials properties that make the analysis work difficult.

Figure 13 shows the comparison of mean values of the

friction coefficients determined by the tests. The variation

of friction coefficient as a function of orientation,

lubrication condition and material type are plotted. As

depicted by line 1 in the figure, the value of the friction

coefficient in dry friction conditions is lowest for the DDQ

sheet and highest for EDDQ sheet. This trend is in general

observed for both orientations. However, in the case of

orientation 0o for the lubricated conditions, the above-

mentioned relation is different where the friction coefficient

value increases with sheet plasticity specified by ultimate

elongation εu value. The value of friction coefficient for

both dry friction and lubricated conditions decreases as the

surface roughness (Raw parameter) of the rolls increases

(line 2 in Figure 13). The reduction of friction coefficient

value as a result of lubricant application is smaller in the

case of sample orientation at 0o compared to sample

orientation at 90o.

The lubricant reduced the coefficient of friction and the

intensity of its action is similar for all used rolls. In case of

samples cut transverse to the rolling direction (90o), the

value of friction coefficient is larger. This can be explained

by two facts. Firstly, as it has been found in pin-on-disc

test, the tested sheets exhibit anisotropic frictional

behaviour. Secondly, the different mechanical properties of

the sheet in both rolling and transverse to the rolling

directions may lead to different character of plastic

smoothing of asperities. The friction coefficient value

measured on samples cut transverse to the rolling direction

is higher than for samples cut along the rolling direction of

the sheet.

It is evident that rough surfaces enable lubricant

adhesion to the sheet material. This effect appears to be due

to the manner in which the lubricant is captured by

roughness on the surface of the sheet. To examine

effectiveness of the sheet lubrication the L-index was

introduced, defined as follows:

(4)

As shown in Figure 14, the L-index value has a non-

linear relation with the value of roughness parameter Raw

of rolls. In the case of sample orientation at 0o, the value of

the L-index increases with increasing value of roughness

parameter Raw of rolls. For sample orientation at 90o, on the

other hand, a continuous decreasing trend of the L-index

value is observed. As it was stated earlier, in the case of

higher surface roughness of contact bodies, high volume of

lubricant may be trapped in the surface pits and

consequently the effectiveness of lubrication is higher. But,

as it is shown in Figure 13, sample orientation (surface

texture) has also effect on the effectiveness of lubrication.

4.3. Analysis of Surface Roughness Change

The shape of the contact surface and the roughness of the

bodies in contact affect the contact type and the stress state

in the surface layer as well as the value of the required

tangential force to overcome friction. The increase in the

real contact area occurs gradually from the initial peak

surface roughness contact to full contact after balancing

normal force and the force required to deform roughness.

The change of the contact surface is accompanied by a

continuous change in the geometry of the contact. The

estimation of the changes in the surface roughness was

done for DQ steel sheet destined for difficult drawpieces. It

was assumed that the measured surface topography is

representative for all area of the sample.

The views of sheet surface after the friction process with

roller clamping force of rollers of 0.8 kN and 1.2 kN,

which correspond to normal pressure of 240 and 360 MPa,

are shown in Figures 15 ~ 16, respectively. For the roll

roughness of Raw = 0.63 μm the dominant phenomenon

during friction was the smoothing of roughness peaks

during the test. For both clamping forces the profile height

L index–μdry μoil–

μdry

--------------------=

Figure 14. Effectiveness of lubrication (L-index) as a

function of Raw of rolls oriented at (a) 0o and (b) 90o to the

rolling direction of the sheets.

858 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

was similar. With increasing value of the surface roughness

of the rollers, clear directional scratches on the surface of

the sheet was observed. This is as a result of the movement

of the counter-specimen surface irregularities across the

sheet surface. However, no significant increase in resistance

to friction has been detected. This may be explained by

reduction of the real contact area during friction tests

performed with rollers of higher roughness Raw.

After friction tests with the reference object, i.e.

roughness of the sheet metal surface which is not subjected

to friction, a comparative analysis of the sheet metal surface

was conducted in Allicona InfiniteFocus optical 3D micro

coordinate system. It was assumed that the measured surface

roughness of the sheet before friction is representative for

Figure 15. Change of surface topography of DQ steel sheet tested under a 0.8 kN clamping force of rollers and for Raw of

rolls parameter value: (a) 0.63 μm; (b) 1.25 μm and (c) 2.5 μm.

Figure 16. Change of surface topography of DDQ steel sheet tested under a 0.8 kN clamping force of rollers and for Raw of

rolls parameter value: (a) 0.63 μm; (b) 1.25 μm and (c) 2.5 μm.

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 859

the entire surface of the sheet. The results of changes in the

surface topography are shown in Table 3.

In all the analysed cases, friction has reduced the sheet

surface roughness. In the case of rolls with roughness Raw

= 0.63 μm and Raw = 1.25 μm the biggest change in the

roughness volume occurs over a reference surface (peaks

flattening). For rollers roughness Raw = 2.5 μm the greatest

deviation of the test surface in relation to the reference

surface is below the reference surface, which is due to

interaction of high roughness of the tool. With increasing

value of the Raw parameter the penetration of the tool

surface increases causing the increase of both the lower and

upper test surface deviations in relation to the reference

surface.

4.4. Draw Bead Test

The results demonstrate the expected relationship that the

value of the friction coefficient of the tested sheets without

lubrication is higher than in the case of lubricated sheets.

The lubricant reduced the coefficient of friction, but

intensity of its action was dependent on the sheet surface

topography and the value of Raw of rolls. The strips

oriented along the rolling direction demonstrated higher

value of the fiction coefficient compared to the samples cut

transverse to the rolling direction.

For all sheets cut along the rolling direction increasing

Raw of rolls causes that the value of friction coefficient

increases for both lubricated and dry friction conditions

(Figure 17 (a)). In the case of transverse orientation, the

local minimum of the friction coefficient value for Raw of

rolls equals to 1.25 μm is visible (Figure 17 (b)). This

effect is visible for both friction conditions, so it can be

explained by the arising favourable mutual interactions

between sheet metal – tool roughness. As it has been

shown in Figure 17 (b) the high surface roughness of rolls

does not necessarily mean the increase of resistance to

friction. For both friction conditions the values of the

friction coefficient are the highest for EDDQ steel sheet,

and lowest for DQ steel sheet. It may be related with the

value of the strain hardening coefficient whose value

decreases in a similar sequence (for EDDQ, DDQ and DQ

steel sheets). During the DBS test the sheet metal strongly

resists deformation and is subjected to strain hardening

phenomenon. Furthermore, it is stated by (Masters et al.,

2013) that increasing the amount of plastic deformation

increases the surface roughness and friction behaviour

changes as a result of plastic deformation during metal

forming. The influence of initial (original) surface topography

of the sheet is not very significant because during the test

the sheet is subjected to series of local bending,

Table 3. Results of comparative analysis of surfaces before and after friction process.

Clamping force FC,

(kN)

Ra of rollers

Raw (μm)

Deviation of surface after friction process from reference surface, (μm) Volume of positive

deviations, (μm3)Volume of negative

deviations, (μm3)Min Max Average

0.8

0.63 -3.687 5.2309 -0.0309 127783 113020

1.25 -5.5176 4.009 -0.0153 144368 121157

2.5 -6.6996 4.4872 -0.3287 153700 248358

1.2

0.63 -3.7411 5.4508 -0.0064 129493 104458

1.25 -5.122 4.0808 -0.024 155633 135419

2.5 -11.705 6.1676 -0.2785 165294 240747

Figure 17. Friction coefficient value versus Raw of rolls for (a) orientation 0o and (b) 90o, values measured at depth

penetration of 12 mm.

860 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

straightening and reverse bending deformation on the

sheet. So, the topography is changed as an influence of

plastic deformation of the sheet and flattening of asperities

as a result of the frictional contact between the sheet and

the tool interface.

The draw beads action essentially influences the

springback phenomenon (Figure 18). The highest deviation

of the sheet profile from the drawing direction was

observed for DQ steel sheet. The higher degree of sheet

deep-drawability is responsible for lower value of

springback. Similar dependence was observed for the

samples cut transverse to the rolling direction of the sheet.

Lower values of sheet profile deviation from the drawing

direction were observed in the case of existing higher

resistance to friction.

The influence of Raw of rolls on the value of the friction

coefficient has been shown in Figure 19. The relations are

not linear because the relation between the penetration

depth and contact area are nonlinear. The increase of bead

penetration causes the increase of the friction coefficient

value. Similar results have been found by (Firat et al.,

2009) who indicated that the value of the drawbead restraint

force increases gradually with increasing penetration level.

As it was mentioned earlier the EDDQ sheet demonstrated

the highest value of the friction coefficient. There is no

evident relation between the sample orientation and the

value of friction coefficient.

In order to reduce the friction and minimize sheet

failure, lubricants are typically applied to portions of the

workpiece that undergoes severe contact with dies. When

lubricant is applied to the workpiece, the frictional

resistance of the sheet material decreases and the strain

uniformity of the sheet increases – it means that application

of any lubricant should result in the reduction of the value

of the friction coefficient. Comparison of the values of the

friction coefficient determined in dry friction μdry and

lubricated conditions μoil demonstrates nearly linear relation

(Figure 20).

The higher slope of the inclination angle of the trend line

for orientation 0o compared with that of orientation 90o

indicates that the lubricant effectiveness for sample along

the rolling direction is higher than in the case of the

transverse direction. For the sample in the rolling direction,

the degree of the reduction of the friction coefficient value

by the lubricant application is similar for all Raw of rolls

used in the test. As shown in the figure, for sample

orientation 0o, the inclination angle of the trend line for

lubricated and dry conditions are identical (i.e., μoil = μdry).

Figure 18. Springback phenomenon of the tested steel

sheets cut along rolling direction.

Figure 19. Friction coefficient curves determined for orientation at 90o for different Raw of rolls: (a) 0.32 μm; (b) 0.63 μm;

(c) 1.25 μm and (d) 2.5 μm.

FRICTIONAL CHARACTERISTICS OF STEEL SHEETS USED IN AUTOMOTIVE INDUSTRY 861

5. IMPLICATIONS FOR THE AUTOMOTIVE INDUSTRY

As previousely indicated, the friction forces on the contact

surface have a significant impact on the sheet forming

process. The nature of the effect of the friction and

conditions associated with the mechanical properties of

formed sheet metal can be changed by suitable design of

tools. During the forming process of drawpieces, there

exist zones of diverse stress and strain state, displacement

velocity and friction conditions. In the case of complicated

product shapes, sometimes several processes exist during

forming a sheet to obtain the final shape. The most widely

used processes are bending, stretching and deep drawing.

Deep drawing is widely used for production of shell panels

and for car body panels in the automotive industry. In this

process, the occurrence of different strain state is caused by

the action of tools and technological factors that change the

frictional conditions.

The complex shape of tools for the production of car

bodies and constructional components makes it necessary to

use many technological tests and modeling the phenomenon

of the friction in the particular places of stampings. As it is

shown in the paper, according to the modeled area of the

drawpiece, the different values of the coefficient of friction

are determined. Rolled sheets exhibit strong anisotropy of

mechanical properties and different surface topography.

The basic test that allows to determine the anisotropy of the

frictional resistance is tribotester. While forming the sheet

metal, the friction force between the blankholder and the

die has to be sufficiently high in order to obtain required

plastic flow and to avoid wrinkling.

Many tribological problems of sheet metal forming of

car bodies, especially deep drawing, lead to several

experimental friction tests that has a huge impact on the

resulting product. For instance, (Choi et al., 2011) reported

that defects such as fracture and wrinkle occur when hot

stamping is performed using a conventional drawing or

forming method. The implemented modelling approaches,

on the other hand, contribute to better characterization of

steel sheets used for car bodies. The strip drawing test

modelled the friction between the punch and flat surfaces of

the drawpiece, while the drawbead test simulates resistance to

friction during deformation of the sheet in the drawbead

region. The draw bead generates a stable tensile force

opposite to the sheet drawing direction by introducing a

series of local bending, straightening and reverse bending

deformations on the sheet.

6. CONCLUSION

The study of the frictional characteristics of selected steel

sheets used in automotive industry is conducted and

presented in this paper. The research work focused on

determination and comparison of friction coefficients

obtained by three tribological tests: a strip drawing test, a

draw bead test and a pin-on-disc tribometer. The

experimental results allow to draw the following conclusions:

(1) The production method of sheet metals by rolling

process causes form of directional topography that

results in diversification of friction coefficient value

according to the orientation to the rolling direction of

the sheet metal. Furthermore, the plastic properties of

the sheets (e.g. deep-drawability) influence the value of

friction coefficient. The friction coefficient value

increases with the increasing ultimate elongation mu of

the sheets, which indicates their higher plasticity.

During the lubricated sliding tribometer test the

deviation of the friction coefficient value was smaller

compared to the tests realized in dry friction

conditions. Lubrication allows to minimize the effect

of directionality of sheet surface topography on the

friction anisotropy by decreasing the range of friction

coefficient value variation along the friction track.

(2) The friction coefficient value measured on samples cut

transversely to the rolling direction is higher than for

samples cut along the rolling direction of the sheet.

Furthermore, the reduction of friction coefficient value

for lubricated contact is smaller in the case of sample

oriented at 0o compared to sample oriented at 90o to the

rolling direction. Considering the influence of material

properties of the sheet, it is found that the value of

friction coefficient is lowest for the DDQ sheet and

highest for EDDQ sheet. However, this general

conclusion is slightly different for orientation 0o in the

lubrication conditions where the friction coefficient

value tends to increase with the increase of sheet

plasticity specified by ultimate elongation μu value.

The value of friction coefficient for both analysed

lubrication conditions decreases as the Ra surface

roughness parameter value of the sheet of the rolls

increases. The value of L-index characterized lubricant

effectiveness has a non-linear relation with the value of

Figure 20. Relation between the friction coefficients

determined in dry friction conditions (μdry) and in lubricated

conditions (μoil).

862 T. TRZEPIECI SKI, A. BAZAN and H.G. LEMUNé

roughness parameter Raw of rolls. In the case of sample

orientation at 0o, the value of the L-index increases with

increasing the value of roughness parameter Raw of

rolls. For sample orientation at 90o, on the other hand, a

continuous decreasing trend of the L-index value is

observed. In all the analyzed cases, the friction process

has reduced the sheet surface roughness by peaks

flattening.

(3) The results of draw bead simulator tests demonstrate

the expected relationship that the value of friction

coefficient of the sheets tested without lubrication is

higher than in the case of lubricated sheets. The

lubricant reduced the coefficient of friction, but intensity

of its action depends on sheet surface topography and

value of Raw of rolls. In case of Raw values of 1.25 μm

and 2.5 μm, the strips oriented in the rolling direction

demonstrated higher value of fiction coefficient

compared to the samples cut transverse to the rolling

direction. The different interdependence exists for Raw

values of 0.32 μm and 0.63 μm. For sheet strips cut

according to the rolling direction together with

increasing of Raw of rolls the value of friction

coefficient increases for both lubrication and no

lubrication conditions. For both fiction conditions the

value of fiction coefficient are the highest for EDDQ

steel sheet, and decrease for DDQ and DQ steel sheet,

respectively. This may be related with value of strain

hardening coefficient whose value decreases in the

aforementioned order of the sheet grades.

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