Challenges in Forming Advanced High Strength Steels · PDF fileChallenges in Forming Advanced High Strength Steels ... Advanced High Strength Steels ... AHSS are performance-based

285

Challenges in Forming Advanced High Strength Steels

E. Billur, M.S. and Prof. Dr-Ing T. Altan

Engineering Research Center for Net Shape Manufacturing (ERC/NSM) www.ercnsm.org

Abstract

Advanced High Strength Steels (AHSS) offer advantages for weight reduction and

increase in crashworthiness and safety. However, there are several issues to be addressed

in forming AHSS. This paper discusses the challenges encountered in forming AHSS and

summarizes some of the results of the R&D conducted to improve the application of

these materials.

1 Introduction

Advanced High Strength Steels (AHSS) are multi-phase steels which contain martensite,

bainite and/or retained austenite /1/. This microstructure enables high yield (min. 300

MPa) and tensile (min. 500 MPa) strength /2/. Although the formability of AHSS is

improved compared to conventional HSS, they are still much less formable than mild

steels. Fig. 1 depicts how the total elongation (i.e., formability) decreases with increasing

strength. This paper mainly discusses DP (Dual Phase) and TRIP (Transformation-

Induced Plasticity) materials.

Fig. 1. Total Elongation

(%EL) vs. Ultimate

Tensile Strength

(UTS) “Banana

Curve” of

automotive steels

/3/.

286 Challenges in Forming AHSS Billur and Altan

There are several challenges in forming AHSS. These are mainly,

Due to multi-phase structure and complex manufacturing processes:

(1) determining the material properties with accuracy requires new testing

methods,

(2) batch-to-batch inconsistency is common.

Due to their high strength and low formability:

(3) early fractures are observed in several forming operations, requiring

investigation of fracture,

(4) larger press capacities are required for forming or blanking,

(5) tools wear out quickly. Lubricants, tool materials and coatings require

careful selection,

(6) larger springback (leading to dimensional inaccuracy) is an important issue

requiring additional development.

2 Material Properties

2.1 Flow Stress

Flow stress curves are often fit to Hollomon’s equation nk . In this equation, the

strain hardening exponent (n) is also used as an indication for uniform elongation.

However, in AHSS, the strain hardening characteristic is not constant. This is mainly

because of the multi-phase microstructure and the phase transformations during

deformation. The variation in strain hardening characteristics can be illustrated by

plotting the instantaneous n-value vs. corresponding strain (Fig. 2). These observations

indicate that Hollomon’s equation may not be valid for AHSS, since there is no constant

value of n. Furthermore, extrapolation of flow stress data may not be accurate for analysis

or simulation purposes /4/.

Billur and Altan Challenges in Forming AHSS 287

Fig. 2. Variation of

instantaneous n-

value with

engineering

strain for HSLA,

DP and TRIP

materials with

350 MPa Yield

Strength /2/.

As seen in Fig. 3, the engineering strain-stress curves are almost flat around UTS, making

it hard to determine the engineering strain where the local necking starts /5/. It is also

stated that in AHSS fracture may occur with minimal necking /6/.

Fig. 3. Engineering

strain-stress

curves of several

DP and TRIP

steels,

determined by

tensile test /5/

Flow stress curves are essential for simulation and analysis purposes. Usually, these are

determined using tensile test. However, data obtained in a tensile test is for relatively

small strains and therefore must be extrapolated. Bulge test, on the other hand, can give

more reliable strain-stress data, and eliminate the need of extrapolation. Fig. 4 is a

comparison of flow stress curves determined by tensile and bulge tests. Note that, the

maximum strain in tensile test is around 0.15, and in bulge test is 0.5 /5/.


z

(a) (b)

Fig. 4. Comparison of flow stress curves of DP and TRIP steels, determined using (a) tensile test

and (b) bulge test /5/.

2.2 Elastic Modulus Variation

Elastic modulus includes both loading (Young’s) and unloading (apparent) modulus.

Usually, it is assumed that both are same and constant for a material at given temperature.

However, recent studies indicated that the loading and unloading moduli for AHSS are

different and unloading modulus decreases with plastic strain /7,8/.

140

145

150

155

160

165

170

175

180

185

190

195

200

205

210

215

220

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12

Ap

pa

ren

t M

od

ulu

s (

GP

a)

True Strain (mm/mm)

Material - DP780

Unloading

Loading

Fig. 5. Variation of

unloading

(apparent)

modulus by

plastic strain,

determined by

tensile test, using

DP780 /9/

Recent studies done at ERC also illustrated that the elastic modulus changes with

increasing plastic strain, as shown in Fig. 5 /9/. These results indicate that it is necessary

to consider the unloading modulus variation for accurate springback predictions. The data

obtained by tensile test is limited to small strains compared to actual stamping operations.

Therefore, there is a need for a method to experimentally determine the variation of

unloading modulus over a larger strain range /4/.


2.3 Inconsistency of Mechanical Properties

AHSS are performance-based steel grades. They are named and marketed according to

metallurgical type (DP, TRIP, etc.) and their strength. For example, DP 600 stands for a

Dual Phase steel with a minimum tensile strength of 600 MPa. Steel producers may

achieve the required minimum by various chemistries and manufacturing processes. Fig.

6 shows how several materials can maintain the minimum strength requirement although

their chemical compositions may not be constant /10/. Since different mills may have

different methods of production, even though their tensile properties match, other

material parameters such as elongation and weldability may vary /11/.

(a) (b)

TRIP 800DP 980

Fig. 6. Flow stress curves of (a) DP800 and (b) DP980 materials from different suppliers /10/

A recent study done at Ford showed that the material properties, such as yield and tensile

strengths, total elongation and n-value may vary significantly from supplier to supplier

and/or batch to batch. Results have shown that the yield strength may vary from 312 MPa

to 443 MPa and tensile strength may vary from 591 MPa to 692 MPa, Fig. 7 /12/.

600 610 620 630 640 650 660 670 680 6900

20

40

60

80

Ultimate Tensile Strength (UTS

) [MPa]

Fre

quency (

n)

320 340 360 380 400 420 4400

20

40

60

80

100

Yield Strength (YS

) [MPa]

Fre

quency (

n)

(a) (b)

YUTS

Fig. 7. Distribution of yield and tensile strength distributions (in MPa) for DP590 GI material /12/


Due to the performance based grading, sometimes mills may deliver steels with

properties that exceed standard requirements. A stamping company may require an AHSS

with minimum strength of 900 MPa. The steel supplier may have a batch that has 1200

MPa strength. Technically, the supplier meets the minimum requirement. However, the

formability of a stronger metal will be different (see Fig. 1) /13/.

3 Formability

It is well known that AHSS grades have different failure mechanisms compared to mild

steels and HSLA steels. This is mainly caused by local failures which are observed more

common in forming AHSS, due to multi-phase structure and phase changes during

deformation. These local failures do not necessarily correlate with n-value, R-value or

total elongation /14/. Therefore it is essential to test these materials under various stress

and strain states, such as 1) stretching, 2) bending, 3) stretch bending, 4) deep drawing

and 5) flanging /4/. Fig. 8 shows stampings with different stress states /14/.

(a) (b) (c)

Fig. 8. Shapes with different stress states: a) significant stretching, b) moderate stretching and

bending, c) high hole expansion and tight bending /14/.

3.1 Stretching

Stretchability is the increase in length-of-line without fracture /15/. To evaluate it,

limiting dome height (LDH) and hydraulic bulge tests are commonly used. Hydraulic

bulge test is more reliable, since there is no solid punch and therefore friction does not

affect the results /4/. Fig. 9 and 10 give a comparative idea about stretchability of several

grades of AHSS. Results shown confirm that a) banana curve’s validity (i.e., less

formability with increased strength, Fig. 1) and b) inconsistency of material properties

(see Fig. 9-b).


(a) (b)

Hig

he

r St

retc

hab

ility

Fig. 9. Comparison of stretchability from two studies: a) Compares several strength grades /15/ and

b) compares several 780 grade steels /14/

DC06 DP600 DP800 DP1000 DP1200 DP1400

Fig. 10. Limiting dome height samples for several DP steels and a draw quality steel /16/.

3.2 Bending

During bending, the outer portion of the material is subjected to tensile stress, while the

inner is subjected to compressive stress. Fracture occurs when the tensile stress at the

outer fiber exceeds a critical value. This stress depends on the bend radius, bending

angle, sheet thickness and the flow stress /4/. The local strain at the outer fiber may be

higher than the tensile elongation. Yan has reported that in DP980 steel the total

elongation was measured 16% in tensile test, while the elongation in bent part was 40%

/6/. Bendability is often measured by the r/t (bending radius / sheet thickness) ratio.

Smaller r/t ratio implies better bendability, as shown in Fig. 11.


Fig. 11. Bendability of several

AHSS grades. Lower

r/t values stands for

better formability /2/

3.3 Stretch Bending

Stretch bending failure refers to fracture in the bending region under tensile stress, Fig.

12. This type of fracture may not be predicted by conventional FLC (Forming Limit

Curve), since the material may fail before the strains reach the predicted forming limit

/17/. Wu et al came up with the concept of BFLC (Bending-modified FLC). This method

can predict failure height in ASB (Angular Stretch Bend – as shown in Fig. 13) test more

precisely than the conventional FLC /18/.

(a) (b)

Fig. 12. Stretch bending failures: a) DP780 underbody structural part, b) DP 980 b-pillar inner /17/.


(a) (b)

Fig. 13. a) Schematic of the ASB (Angular Stretch Bending) test, b) comparison of achievable

height of several steels with respect to r/t ratio /19/.

3.4 Deep Drawing

Drawability of a material can be defined by Limiting Draw Ratio (LDR) and determined

cup draw tests. LDR is the ratio of the largest blank diameter that can be drawn to a

circular cup. As expected from Fig. 1, drawability (LDR) decreases as the strength of

material increases. Fig. 14 compares several grades of AHSS to a mild steel grade /2, 16/.

DC06 DP600 DP800 DP1000 DP1200 DP1400

(a) (b)

Fig. 14. Comparison of a) LDR of AHSS with mild steel /2/ b) achievable cup height /16/.

In deep drawing AHSS, sidewall curls or local fractures are observed. Current research to

solve these problems is focused on (1) optimizing draw bead designs (Fig. 15-a), (2)

controlling active draw beads (Fig. 15-b) to optimize the metal flow and (3) optimizing

blankholder pressure, including multi-point cushion systems (Fig. 16) /15, 20/. Multi-

point cushions can be optimized to form different materials (Aluminum alloys, HSS and

AHSS) at different thicknesses using the same dies as shown in Fig. 17 /21,22/.


(a) (b)

Fig. 15. a) Conventional draw bead design, compared to design recommended for AHSS drawing,

b) an active draw bead may eliminate the sidewall curls /15/

(a) (b)

Fig. 16. For a given

geometry:

a) Optimized

cushion pin forces

(Ftotal = 530 kN),

b) good part with 68

mm draw depth /20/

Al 6111-T4, t=1 mm

BH210, t=0.8mm

DP500, t=0.8mm

(a) (b)

Fig. 17. Multi-point cushion pins used to form a tailgate inner: a) locations of pins, b) several parts

formed successfully from Al, HSS and AHSS /21,22/.


3.5 Flanging / Edge Stretching

After stamping, the excess material is trimmed off before operations such as flanging or

hemming are performed. During these post-forming operations, tensile stresses occur in

the trimmed edges, resulting in edge cracks /4/. Several studies have shown that, edge

cracks cannot be predicted by FLC and they are related to sheared edge quality /2, 16,

23/.

The ability of edge stretching without failure can be measured by hole expansion tests as

depicted in (Fig. 18-a). The conical punch is pushed through a hole with an initial

diameter of d0, until the crack is observed (Fig. 18-b). The diameter at crack (df) is

measured. Hole expansion ratio %HE is the ratio of final diameter (df) to initial (d0).

(a) (b)

Fig. 18. Hole expansion

test, a) geometry

with a conical

punch, b) cracked

part /4/

As shown in Fig. 19-a, AHSS have lower hole expansion ratio, compared to milder

grades. This number gets even lower with worse edge conditions or worn tools (Fig. 19-

b).

(a) (b)

Fig. 19. a) Hole expansion of several

grades /3/ and b) effect of

tool condition (SSAB)


A recent study has shown that, several heat treatment methods such as quenching and

partitioning (Q&P) or quenching and tempering (Q&T) can increase the hole expansion

ratio without sacrificing the tensile strength /24/.

Fig. 20. Measured hole expansion ratio

as a function of tensile strength

/24/.

4 Presses

Selection of a press for a stamping operation requires the knowledge of (1) maximum

load required for the operation, (2) energy requirement for a single stroke /25/ and (3)

reverse load, which is a case in blanking operations /26/.

4.1 Press Load and Energy

As pointed by Keeler and Ulnitz /15/, higher strength increases both the load (tonnage)

and energy required for stamping. While the flow stress of a material determines the

forming load, the area under a flow stress curve determines the forming energy (i.e.,

forming load x ram displacement).

Fig. 21. Comparison of DP and HSLA materials, in terms of load and energy requirement /15/.


The idea shown in Fig. 21 was demonstrated experimentally by drawing and embossing

several steels, including mild steel, HSLA and DP steels. As shown in Fig. 22, forming a

part using DP600 material requires about two times of force and energy required to form

using mild steel /2/.

Yield Strength (MPa) Yield Strength (MPa)

(a) (b)

Fig. 22. Experimental (a) press force and (b) required energy measurements for mild steel, HSLA

and DP grade steels /2/

4.2 Reverse Load in Blanking

In blanking operations, press load is built gradually and elastically deflects the press and

the tools. When the force generated is enough to fracture the part, a sudden release of

stored energy will be observed. This causes the press to generate reverse loads, as

depicted in Fig. 23. During this stage (also known as snap-through) the press components

designed to have tensile stresses will be in compression /27/.

Standard presses are designed to withstand reverse loads around 10-20% of the nominal

press load. For example, a 400 ton press can withstand 40-80 tons of reverse load /26/.

Harder materials will require more blanking (forward) force and will cause more reverse

loads. Keeping the punches in good shape, using stepped punches, reducing the blanking

speed and using hydraulic dampening devices are some ways to reduce the snap-through

forces /28/.


Fig. 23. Load-time curve for a typical

blanking operation, showing

reverse loads /27/.

5 Tribology

Due to the high strength of AHSS, more forming loads are required. In many cases,

forming loads are further increased on purpose, to reduce springback. This increases the

contact pressure, which increases the risk of observing all kinds of tool failures /29/.

5.1 Lubrication and Friction

Due to friction and high cold working, temperatures of tools are higher in forming AHSS.

There are studies showing that 90-120oC temperatures are common in production

conditions /30, 31/. Higher contact pressures and higher temperatures in the die-sheet

interface are both detrimental for lubricants. Lubricant manufacturers recommend HSP

(High Solids Polymer) lubricants or synthetic lubricants with Extreme Pressure (EP)

additives. In a recent study at ERC/NSM, 6 lubricants were compared using circular cup

drawing test (Fig. 24) and lubricants with EP additives were found to perform better with

DP590 GA material /31/.

Contact area with die

Fig. 24. Temperature

distribution in a cup

drawing test (BHF = 30

ton, COF = 0.05) /31/.


Steel sheet materials are usually coated with zinc (galvanized) to increase the corrosion

resistance. Several studies have shown that, galvannealed steels have less friction and

tendency to galling over hot dip galvanized steels and uncoated steels. This can be

explained by the higher hardness of the coating /32/.

5.2 Galling and Tool Wear

It is well known that an increase in contact pressure at the die/sheet interface will reduce

tool life - due to tool wear or galling. In order to select correct materials, coatings and

lubricants, it is essential to know the tool failure mechanisms and how to avoid them.

There is no standard test for tool wear for sheet metal forming. Nonetheless, researchers

have modified some standard tests to emulate sheet metal forming processes better. These

tests include slider-on-sheet, strip reduction test, draw-bead simulator and part forming

tests /29, 33/.

(a) (b)

Fig. 25. Strip drawing test:

a) schematic, b)

galling on a die

insert /33/.

5.3 Tool Materials, Treatments and Coatings

A variety of cast iron and steel grades are used for manufacturing dies in stamping

industry. The cost of these materials may vary considerably. However, with appropriate

surface treatments, coatings and lubricants, a cost-effective die material may outperform

the expensive ones. Therefore, in selecting die materials, a systematic evaluation of tool

materials, coatings and heat treatments are required, considering the cost and tool life as

parameters /29/.


0

10,000

20,000

30,000

40,000

50,000

Vancron 40

Calmax + Nitr. +

PVD CrN

Sleipner + Nitr.

Weartec Vanadis 6

Sleipner D2

Too

l Lif

e /

Nu

mb

er

of g

oo

d p

arts

sta

mp

ed

Tool Material and Coatings

DP600

Fig. 26. Tool life comparison

for channel forming

test, conducted with

DP600 (after /34/)

0

10,000

20,000

30,000

40,000

50,000

60,000

AISI D2 + CVD

TiC

Carmo + Nitr +

PVD CrN

AISI D2 Vanadis 4E

AISI D2 + PVD AlTiN

AISI M2 AISI M4 AISI M4 + CVD

TiC

AISI M2 + PVD CrN

AISI M4 + Hard

Cr

Too

l Lif

e /

Nu

mb

er o

f go

od

par

ts s

tam

ped

Tool Material and Coatings

DP980

Fig. 27. Tool life comparison

for part forming test,

conducted with

DP980 (after /35/)

6 Springback

After forming a part, when the tool loads are removed, the material undergoes an elastic

recovery. This is called springback and causes shape deviation from the design intended

geometry /6/. Four modes of springback are commonly observed: 1) bending (angular

change), 2) membrane (wall curl– Fig. 28-b), 3) hybrid and 4) twisting /4/.

AHSS are characterized with higher strength and higher strain hardening compared to

mild grades. Therefore, when AHSS steels are formed to a strain level the observed

springback is higher than that of mild steel as illustrated in Fig. 28-a /4/.


(a)HSLA350/450DP350/600

(c)

(b)

Fig. 28. Springback: a) Schematic illustrating elastic recovery, b) types of springback and c)

experimental results comparing AHSS to HSS /2/.

To compensate the springback, several countermeasures can be taken. These are: 1)

overforming (Fig. 29), 2) locally deforming/bottoming or 3) stretching by higher bead

forces. To design these processes, prediction of springback is essential. However,

modelling the springback of AHSS is a challenge due to: 1) flow stress equations do not

fit (see section 2.1), 2) unloading elastic modulus is not constant (section 2.2) and 3)

more Bauschinger effect is observed, compared to mild steels /4/.

Fig. 29. Overforming can be

a method to

compensate

springback /6/

7 Summary and Conclusions

Forming AHSS involves several challenges, mainly due to its higher strength, lower

formability and inconsistency of material properties. In this paper, current state of

research and development was summarized to overcome these problems; namely,

determination of formability, press loads, tribological conditions and springback.


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Challenges in Forming Advanced High Strength Steels · PDF fileChallenges in Forming Advanced High Strength Steels ... Advanced High Strength Steels ... AHSS are performance-based