Deep Drawing Steels

7/28/2019 Deep Drawing Steels

1/11

13/01/20

G. Ubertalli - 01/2011

The tensile test


stress strain curve

of a deep-drawing

steel.


2/11

13/01/20

STRAIN HARDENING

When a metallic material overcomes the yield strength and it enters

in the plastic field, the lattice planes slip as a consequence of the

dislocations movement. After a first ste of lanes sli , the number of

the dislocations increase and they start to interfere each other (jog)

inducing a pile up.

In such a condition, the load necessary to continue to deform the

material increases, as evidenced in the stress-strain curve.

Therefore, when a metallic material is deformed, it manifests a

higher strength.

This phenomenon is called:

STRAIN HARDENING


STRAIN HARDENING COEFFICIENT nThe behaviour in the plastic zone can be described in a simplified way from the

Hollomon equation:

= n

The n coefficient ranges from 0 to 1.

If it is 0, the equation is:

= K

So deformation without strain hardening (brittle crack).

,

= K

In this case there is a straight line with a rate equal to K (infinite toughness).



3/11

13/01/20

RELATIONSHIP BETWEEN AND nP = A

At Pmax dP=0=dA + dA d/ = -dA/A (eq.1)

V=AL= cost dV=0=dAL + AdL dA/A = -dL/L = -d (eq.2)

Thus combinin e .1 and e .2 it can be obtained:

d/d = In case of plastic strain the equation is:

= Kn

and substituting:

n-1 = nAnd then:

n = This mathematical relationship indicates that it is possible tohave a material with high strain , if such material has a high n.


ERIKSEN TEST

a



4/11

13/01/20

THE COEFFICIENT OF ANISOTROPY r1

3In the tensile test is normallydetected the

1strain. If

2and

3

are also measured , the ratio 2/

3

= = =

.

constant for every material and is

indicated with the rletter; it is called

COEFFICIENT OF ANISOTROPY

0b

b

2


0a

a

Typical values of cold drawn steels

STEEL Rp02(MPa)

Rm

(MPa)

E (%) r

avg.

n

avg.

E.I. L.D.R.

FeP01 220280 300410 2833 1.01.2 0.180.20 9.510 2.192.23

FeP04 180210 270350 3842 1.51.9 0.200.22 10.511 2.232.31

FeP06 160200 270340 4044 1.92.2 0.220.24 1112.2 2.312.44

High 275500 400650 1530 0.91.1 0.150.19 79 1.82.2

strength



5/11

13/01/20

DEEP DRAWING AND HIGH STRENGTH STEELS

A deep drawing steel must have high toughness and ductility properties (toguarantee the deformability in the die) and good strength properties (to ensure

high properties in use). Generally if there is an increase of strength properties a

decrease in deformability is observed; instead in this case the increase or

mechanical properties in directly connected to deforming process. These

materials have therefore the double advantage to be particularly deformable

during deep drawing process and increase their mechanical properties when are

in use.

Traditionally and for a long time were used for this purpose (and actually too) low

carbon and Ti steel: the ferritic matrix of these steels can guarantee a good

deformability, while the deep drawing can induce the strain hardening.

In automotive company, the aim is the weight reduction to reduce fuel

consumption. For this reason the market has imposed the use of different

materials, which adopt alternative strengthening mechanism, maintaining good

deformability. As a consequence, reduced depth can be used without reducing the

in use affordability.


MILD STEEL. Traditional not alloyed steel with low carbon (C


6/11

13/01/20

INTERSTITIAL FREE. Traditional not alloyed steel with low carbon and Tihigher than 0,3%. The titanium has high attitude for carbide and nitride formation. The

C and N content in solid reduction is therefore reduced, favouring the ferrite existence

and the cold forming operations.

The increase of mechanical properties in a consequence of the strain hardening.

e

X500 in orig. X3000 in orig


BAKE HARDENING. Not alloyed steel coil with low C percentage produced

adopting parameters (chemical composition, roll ing temperature, annealing) toreduce carbides precipitation at room temperature and to increase it at the

cataphoresis process temperature. High deformability in cold forming and an

increase of mechanical properties during the cook of painting, induced from a fine

carbide precipitation.

Steel BH 220 after heat treatment at 180 C for 20 min.

A fine presence of carbides in ferrite is detected.

X500 in orig. X3000 in orig



7/11

13/01/20

MICROALLOYED or HSLA: Steel coil with low C percentage and Vand Nb (min. 0.015%) as alloying elements. These elements form fine

precipitates at grain boundary (carbides and nitrides), induce fine grains

formation and an enhancing of the strength.

FeE355

Steel FeE355 high amount of carbides (Nb) and a very fine grain size.

X500 in orig. X3000 in orig.


DUAL PHASE: Steel with ferrite (good deformability) and almost 15% ofmartensite phase.

Fe600DP

Fig. 69 microstructure of a Dual Phase steel;

coarse martensite area. In Fig. 70 the

mar ens e s omogeneous y s r u e .

The Fig. 71 is a magnification of Fig. 70.

X500 in orig.Fig.69

X500 in orig. X3000 in orig.Fig 70 Fig 71G. Ubertalli - 01/2011


8/11

13/01/20

TRIP: Coil steel with ferritic/bainitic/austenitic structure (it guarantee good cold

forming). During deformation a percentage of energy is used to transform a certain

amount of metastable austenite in martensite; this phase enhance the mechanical

properties while the residual austenite acts as energy absorber in case of violent

deformation (impact).

Fe800 TRIP F

In the microstructure it is difficult

to distinguish the austenite from

ferrite phase.G. Ubertalli - 01/2011

MULTIPHASE: High strength steel with ferritic / bainitic /martensitic structure(high martensite content).

Fe800MP

MARTENSITIC: Steel characterized with very high martensite percentage.

.

Fe1300MS



9/11

13/01/20

SHEET METAL FORMING

A schematic section of a typical stamping die. The sheet

contacts only the punch or the die at any point. Membrane

stresses stretch the sheet over the tools.


SHEET METAL FORMING

A

C C

In order to analysed the deformability of the sheet of a

metallic material, a network mesh is painted on

undeformed surface. If a circle is painted, after

deformation three possible conditions can be obtained:

,

the plane of the foil. In this case the circle is deformed as in Fig. a.

A schematic section of a typicalstamping die.

A: Punch - B: Die

C: Blank-Holder - D: Blank

BDONE DIRECTION DEFOMATION, if the sheet is deformed in one

direction only (Fig. b).

DRAWING, if the sheet have a positive (in a direction) and a negative (in

a perpendicular direction) deformation (Fig. c).

e1>0 e1>0 e1>0a b c If d and d are res ectivel the diame-

e2>0 e2=0 e20, while the

sign of e2 depends on the deformationtype.



10/11

13/01/20

SHEET METAL FORMING


A schematic plot of the window of safestraining for simple paths the forming-

limit diagram. With an isotropic material,

the limits for 2>1 mirror those in the

region where 1>2.

Strain signatures along lines A to

J and A to E in a rectangular pan.

NAKAZIMA TEST

The Nakazima test proposes to induce, on

a metallic material to be deformed, all the

possible types of deformations (from

stretching to deep drawing). A punch with

a spherical shape is adopted to deform

a

e1>0

e2>0

Original

circle Deformed

a

samples of controlled geometry until the

necking appearance or until the starting

crack of material. The first sample has

square geometry and the blank-holder on

the four sides.

The result is evidenced in image a. The

other sample have one dimension that is

progressively reduced, to increase the

c

bc r c e

e1>0

e2=0

c

deep drawing effect. The different samplesand results are reported in Figs. (a-e).

The image e evidences a deep drawing

effect of 100%. The circles evidence the

deformation in the different samples.

e

de1>0

e2


11/11

13/01/20

1,5

2

2,5

e1

CURVE LIMITE DI FORMABILITA'

SAFE STRAINING CURVESample a

Sample b

Sample c

Sample d

Sample e

SAFE STRAINING CURVE

In figure are reported all the coupled

values obtained in many samples afterthe Nakazima tests. (e1 VS e2 ).

The line that connects the upper points is

0

0,5

-0,5 -0,3 -0,1 0,1 0,3 0,5

e2 EXPERIMENTAL POINTS

TEST OF NAKAZIMA

called SAFE STRAINING CURVE.

The right side of the diagram is the

stretching zone (e2 > 0), while the left

side is the deep drawing zone (e2 < 0).

The safe strainin curve SSC de end from the thickness are a,

characteristic of every material and are well known or can be easily

detected.

They are very useful to designer to choose the more appropriate material

for the different component, also from the point of view of the

deformability.


Deep Drawing Steels

Documents

Transcript of Deep Drawing Steels