ferritic rolling to produce deep-drawable hot strips of steel - metal 2013

FERRITIC ROLLING TO PRODUCE DEEP-DRAWABLE HOT STRIPS
OF STEEL
Radko Kaspar
Andreas Tomitz
a) MAX-PLANCK-INSTITUT FÜR EISENFORSCHUNG GmbH, Düsseldorf, Germany
b) HOESCH HOHENLIMBURG GmbH, Hohenlimburg, Germany
Abstract
The aspired good deep-drawability (high r- and n-values, Δr . 0) is basically achieved by a
definite anisotropic flow mechanism. Such necessary anisotropy can be ensured in a deepdrawable
steel sheet by a preferential {111}||ND-texture. In a conventional production, a hot
rolling in austenite and a cold rolling at room temperature together with a subsequent recrystallization
annealing are applied for such texture development in the final cold strip. As a cost saving
replacement for this, a thin-gauge hot strip with a required deep-drawability can be employed.
As a promising realization of cost saving thin-gauge deep-drawable hot strips of steel, a ferritic
rolling can be implemented. In this new practice the finishing is shifted down into the temperature
region of ferrite. To optimize the process parameters, extensive laboratory tests on IF steel
were carried out by using the hot deformation simulator WUMSI. By the measurements of the
texture development as well as by the computing of r-values, the texture formation could be
optimized achieving a deep-drawability in hot strips comparable to that of a cold strip after a
conventional austenitic rolling.
1. DEEP-DRAWABILITY OF STEEL
A good cold workability (low proof strength, high uniform elongation) additionally to a
sufficient strength (after cold forming) and, particularly, a good deep-drawability are the properties
that are of a large importance for many flat products of steel. The aspired good deepdrawability
can be realized by a favorable anisotropic material flow during the deep-drawing
process.
For the anisotropic flow behavior of a polycrystalline material the distribution of the orientations
of individual grains plays a decisive
role and is determining for the r-values 1,2) .
Outgoing from a statistic disorderly
distributed grain orientations, r-value
increases with increasing fraction of
{111}-oriented grains and decreasing
amount of those with {100} orientation
parallel to sheet plane, Fig. 1 3) 3.0
parallel to sheet plane
2.0
.
{111} {100}
2. DEEP-DRAWABLE STRIPS
PRODUCED BY CONVENTIONAL
ROLLING
To realize the requirements for deepdrawable
steels with a pronounced {111}texture
by a conventional route, hot rolling
is traditionally carried out in the austenite
range followed by cold rolling with a
r -value [-]
m
1.0
0.0
orientation:
rm-value theoretical:
{111}
2.9
{554}
0.1 1 10 100 1000
Volume ratio of {111} and {100} grains (I {111} / I {100} )
2.8
{211}
2.3
{110}
1.8
{100}
Figure 1. Effect of texture on the mean r-value r m.
I{111} and I{100} are the texture intensities of the
corresponding orientations
0.1

sufficient deformation (70 - 80 %) and a
subsequent batch or continuous annealing for
recrystallization. A cold strip is a final product
in such case. In order to reduce production
costs there is a tendency to achieve a sufficient
deep-drawability without cold rolling, that
means already with a hot strip as a final
product. Fast developments of the rolling
technique make it possible to produce so-called
thin-gauge hot strips with minimum
thicknesses that are nowadays within the range
of those of cold strips, Fig. 2 4) . Such
production requires unavoidably lowering
finishing temperatures because of large heat
losses of thin hot strips. But, the austenitic
rolling with low finishing temperatures is not
easy to perform because of high transformation
temperatures of extra low carbon steels (ELC),
ultra low carbon steels (ULC) and low carbon
interstitial free (IF) steels with manganese
content less than 0.2 %. The reheating
temperatures above 1250 °C would be
necessary for thicknesses of 2 - 5 mm. Hot
strips thinner than 1.8 mm are not producible at
all by a conventional “austenitic” hot rolling.
3. DEEP-DRAWABLE THIN-GAUGE
HOT STRIPS PRODUCED BY FERRITIC
ROLLING
The difference in the processing routes of
a conventional austenitic rolling and a novel
ferritic rolling is apparent from Fig. 3.
Consider-able cost reductions may be
achieved in different way. The most evident
is a reduced reheating temperature in the
ferritic rolling practice, which gives also the
potential for an increased throughput of the
furnace. Lower reheating temperatures for
ferritic rolling (between 950 and 1050 °C)
result to a reduced AlN dissolution
(enhancing ferrite recrystallization kinetics)
and a smaller initial austenite grain size.
This low rolling temperature practice leads
also to an im-proved hot rolled product
Flow stress, MPa
Minimum hot strip thickness, mm
1.5
1.0
0.5
0.0
1.6
0.9
60% of
cold strip
production
0.7
1995 1998 future
Figure 2. Minimum thicknesses of hot strip as a
potential substitution of cold strip
a)
roughing
�
b)
finishing
260
220
180
140
100
60
ELC
IF
���� - transformation ELC
���� - transformation IF
700 800 900 1000 1100 1200
Deformation temperature, °C
Figure 4. Flow stress for � = 0.8 of ELC and IF steel
over the range of deformation temperatures (strain
-1
rate 10 s )
quality with less surface defects, improved flatness of the hot strips due to reduced internal
stresses owing to the fact that steel strips are already transformed prior to cooling on the run-out
table.
Temperature
�����
�
C Mn
coiling
Time Time
Figure 3. Comparison of conventional austenitic (a)
and ferritic (b) rolling
�����
�
roughing
coiling
�
finishing

Fortunately, moderate rolling loads in the
finishing mill enable the application of
ferritic rolling even on existing mills. As
shown in Fig. 4 5) , the flow stresses - and so
the rolling loads - of the IF steel are lower in
the temperature range between 870 and
700 °C than those at 950 °C in the
conventional austenitic temperature region.
At lower temperatures higher flow stresses
of ELC steel are measured presumably due
to dynamic strain aging in these steel grades.
Two different groups of ferritic rolled deepdrawable
thin-gauge hot strips can be
produced, Fig. 5:
“Soft” hot strip: In this product group
the coiling condition must guarantee a
complete recrystallization in the coil
(becoming soft), Fig. 5a. For this, the
finishing and coiling temperatures must
be appropriate high.
`HardA hot strip annealed: By further
lowering finishing temperatures in this
processing, compared to the production
of soft hot strip, thinner hot strips can
be produced (< 1 mm). Such hot strip
does not recrystallize in coil (becoming
hard) and must additionally be recrystallized
by annealing, Fig. 5b.
Temperature
a) ����� b)
finishing � annealing
coiling
Time
Time
�����
�
Figure 5. Production of „soft“ hot strip (a) and
„hard“ hot strip additionally annealed (b)
�
� -fibre
� 1
� 2
{111}
{111}
{111}
{ 11 0}
{001}
{112}
� -fibre
Figure 6. Eulerian space showing the position of �-
and �-fibres
(a)
4. TEXTURE DESCRIPTION
For the description of texture development the method of grain orientation distribution (ODF)
is the most applied way. 6) In a so-called Eulerian space each point corresponds to one orientation,
defined by three Eulerian angles Φ, ν1 and ν2, Fig. 6. For practical purposes the key textures in
steel sheets can be followed by the focusing
only some special lines in this space,
so-called fibres, in the Eulerian space. For
deep-drawable steel sheets two fibres are
important: α-fibre ( || RD) and γ-fibre
({111} || ND). In this way the pole densities can
be expressed only along these two fibres in the
form of two 2D-diagrams.
The texture development in simulated warm
(ferritic) rolled strips was the main objective of
this work. The study was focused on the
potential final products mentioned above: a
“soft” hot strip and a “hard” hot strip
additionally annealed.
Finishing temperature, °C
840
800
760
720
partially
recrystallized
completely
recrystallized
680
hardly
recrystallized
not realizable
640
600 620 640 660 680 700 720 740
Coiling temperature, °C
Figure 7. The region of a complete ferrite
recrystallization of the IF steel presented in the
coordinate system of finishing temperature and
coiling temperature

5. MATERIAL AND EXPERIMENTAL
TECHNIQUE
The investigations were done on an IF-steel as a
typical representative of deep-drawable steels. The
chemical composition was as follow (in mass %):
C: 0.002%, Si: 0.007%, Mn: 0.097%, P: 0.010%,
S: 0.004%, N: 0.003%, Al: 0.042%, Ti: 0.038%,
Nb: 0.007%. The laboratory tests were done on the
hot deformation simulator WUMSI 7) by using the
plane strain hot compression test as a simulation
of rolling. The texture was measured on a Siemens
D500 texture goniometer. The r-values were
computed from the texture measurements by an
“ANIS-MPI” program of the University Birmingham.
8)
Texture intensity
Texture intensity
10
8
6
4
2
0
� - fibre : ||RD
0 15 30 45 60 75 90
�
- fibre: {111} ||ND
{001} {112} {111} {110}
710°C
710°C
810°C
760°C
austenite
940°C
10
60 75 90
� [°] � 1 [°]
Figure 8. Deformation texture for different finishing
temperatures
6. RESULTS AND DISCUSSION
For the design of the rolling schedules in ferrite region the determination of the range of the γα-transformation
temperatures as well as the knowledge of the recrystall-ization behavior of
ferrite is indispensable. Fig. 7 shows the range of coiling temperatures in which ferrite can
recrystallize completely in coil in the production of “soft” hot strips. Coiling bellow this temperatures
leads to a “hard” hot strip that has to be additionally annealed to achieve a designed
texture.
6.1. Deep-drawable "soft" hot strip
In the development of texture during the production of deep-drawable hot strip, the deformation
texture just after finishing is decisive for the quality of the recrystallized texture after coiling.
Generally, the deformation texture should involve a sufficient intensity of γ-fibre including
typically some component of {001} as well. After finishing with ε = 4 x 0.3 the deformation
texture was measured for various finishing
temperatures in ferrite and compared with
that after finishing in austenite, Fig. 8. The
high finishing temperature in ferrite
(810 °C) leads to an unfavorable rolling
texture with a poor coverage of {111} and
� - fibre : ||RD
{001} {112} {111}
10
after finishing
8 at 710 °C
� - fibre: {111} ||ND
{110}
10
8
the maximum amount of grains with α-fibre
near to{001} component. By reducing
6
6
finishing temperature (760 °C or lower) a
more distinctive γ-fibre components with a
4
4
strongest coverage of α-fibre in the range of 2
2
{112} can be observed. In contrast to ferritic
rolled specimens, there is nearly irregular 0
after coiling at 670 °C
0
texture with random oriented grains after the
γ-α-transformation of austenitic rolled steel,
as also reported in.
0 15 30 45 60 75 90 60 75 90
� [°]
� [°]
1
Figure 9. Texture development in the simulated “soft”
hot strip showing the change of the deformation
texture just after finishing into a recrystallized texture
after coiling (see arrows)
9)
Fig. 9 displays the texture development of
a favorable deformation texture (a low
8
6
4
2
0

finishing temperature of 710 °C) due to the recrystallization
in coil. The formation of a typical annealing texture with a
strong γ-fibre orientation and a reduced coverage of {001}
component of α-fibre can be observed. There is a striking
increase in the component of γ-fibre.
As observed, the lowering finishing temperature improves
the deformation texture with an increasing amount of {111}
oriented grains which supports the formation of a sharper
{111} recrystallized texture after coiling. This is reflected by
a significant increase in r-values (as computed from the
texture measurements) with decreasing finishing temperature,
Fig. 10.
6.2. Deep-drawable “hard” hot strip
By a further lowering of finishing temperature thinner hot
strips with a more favorable rolling texture can be produced.
r-value [-]
2.0
1.5
1.0
0.5
ferrite deformation: � = 1.2
finishing at 710°C
760°C
860°C
0
0(r) 0 45 (r 45)
90 (r 90)
Angle to rolling direction [°]
Figure 10. Effect of finishing
temperature on the r-values of
simulated “soft” hot strips
Nevertheless, the coiling temperature becomes too low for a complete recrystall-ization of the
warm deformed material in the coil and, therefore, an additional recrystallization annealing is
necessary by using batch or continuous processing.
The texture development during a batch annealing of specimens finished at 660 °C with two
different coiling temperatures is given in Fig. 11. Whereas the higher coiling temperature of
550 °C leads to a rather low γ-fibre coverage, the lower coiling temperature of 400 °C brings
about a significant improvement in texture with a high level of γ-fibre showing a maximum at
the {111} component. This is supposed to reflect the recovery processes during the coiling
at higher temperatures that reduce the stored energy surplus of {111}-oriented grains and so
diminish their amount after recrystallization.
The r-value distributions after the simulation both of the possible annealing processes (batch
and continuous), are displayed in Fig 12. The
distribution of r-values as a function of the
angle to RD shows considerably higher values,
especially in RD (r 0), in comparison to those of
“soft” hot strips. So the form of the curves is
more similar to that of cold strips. The mean rvalues
> 1.5, Δr < 0.6 as well as other
mechanical properties (elongation A > 50 %,
0.2%-proof strength Rp0.2

G By reducing finishing temperature of ferritic rolling
a more favorable initial rolling texture can be
generated as a pre-condition for a beneficial final
recrystallized texture.
G A minimum coiling temperature (670 °C for the IFsteel
tested) must be met if producing deepdrawable
“soft” hot strips directly after coiling.
G By a further lowering of finishing and coiling
temperature (to achieve even lower hot strip
thicknesses) “hard” (not recrystallized) hot strips
must additionally be annealed after coiling to
guarantee a complete recrystall-ization. Lower
coiling temperatures are more desirable for these
products.
r-value [-]
3.0
2.5
2.0
1.5
1.0
0.5
batch annealing
at 700 °C
continuous annealing
at 780 °C
0
0(r) 0 45 (r 45)
90 (r 90)
Angle to rolling direction [°]
Figure 12. r-values after different
simulated annealing procedures
applied on the “hard” hot strip
(finishing at 660°C, coiling at
400°C)
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