A. Grajcar - Thermo mechanical processing of high-magnese austenistic TWIP-steel.pdf

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ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING
Vol. VIII
2008
No. 4
Thermo-mechanical processing of high-manganese
austenitic TWIP-type steels
A. GRAJCAR, W. BOREK
Silesian University of Technology, Konarskiego 18a, 44-100 Gliwice, Poland
The high-manganese austenitic steels are an answer for new demands of automotive industry con-
cerning the safety of passengers by the use of materials absorbing high values of energy during collisions.
The chemical compositions of two high-manganese austenitic steels containing various Al and Si con-
centrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control
the grain growth under hot-working conditions. The influence of hot-working conditions on recrystalliza-
tion behaviour was investigated. On the basis of initial investigations realized by hot upsetting the
thermo-mechanical conditions resulting in a fine-grained structure were designed. The
V
-
H
curves and
identification of thermally activated processes controlling work-hardening by the use of the Gleeble
simulator were determined. It was found that the thermo-mechanical treatment conditions influence
a phase composition of the investigated steels after solution heat treatment.
Keywords:
hot-working, thermo-mechanical processing, TWIP steel, dynamic recrystallization,
H
martensite
1. Introduction
Higher and higher demands of automotive industry referring to fuel consumption and
emission of harmful exhaust gas limiting and especially improvement of car users’
safety can be met by steels with austenitic microstructure. In the last few years an in-
creased interest in high-manganese austenitic steels has been observed. High-manganese
austenitic steels consist of 15 to 30% of manganese, from 0.02 to 0.1% of carbon and
around 3% of aluminium and 3% of silicon. These steels achieve profitable group of
mechanical properties, i.e. UTS = 600–900 MPa, YS
0.2
= 250–450 MPa, UEl = 35–80%
which strongly depends on chemical composition, especially concentration of Mn [1–3].
Within a framework of numerous works the behaviour of high-manganese steels in
cold plastic deformation conditions is investigated [2–4]. It was found that processes
forming structure and mechanical properties of high-manganese steels depend on the
stacking fault energy of austenite, depended on a chemical composition of the alloy and
deformation temperature [1, 5]. Therefore, plastic deformation is realized in a wide
range of temperature from –200 °C to 400 °C [3–6]. According to many works it was
found that mechanical twinning is deciding mechanism about final properties of alloys
with SFE>60 mJm
–2
. Decreasing the SFE from 60 to 20 mJm
–2
has prevalent signifi-
cance on the course of mechanical twinning connected with TWIP effect (TWinning In-
duced Plasticity). With further decreasing the SFE below about 20 mJm
–2
essential
meaning for improving the ductility of the steel has TRIP effect (TRansformation In-
duced Plasticity).
30
A. G
RAJCAR
, W. B
OREK
Developing the technology of production of high-manganese austenitic steels re-
quires knowledge about their behaviour during hot-working. There is a shortage of
sufficient information in science publications. In the work [7], the influence of initial
grain size and deformation parameters on plasticity characteristics obtained in hot tor-
sion tests for 18Cr-8Ni and 18Cr-17Mn-0.5C steels was investigated. It was found that
Cr-Mn steel characterizes with much higher intensity of strain hardening than Cr-Ni
steel, which makes more difficulties during plastic deformation. Higher intensity of
strengthening of the steel with addition of manganese causes occurrence of maximal
flow stress for smaller
H
max
value. It gives opportunity for the refinement of structure
by dynamic recrystallization. This phenomenon was investigated by Hamada et. al. [4]
in 25Mn and 25Mn3Al steels in a temperature range from 900 to 1100 °C. It was
found that maximal flow stress at a temperature of 1100 °C for 25Mn steel occurs for
H
max
= 0.17.
Application of thermo-mechanical treatment consisting in immediate cooling of
products from a finishing temperature of hot-working in controlled conditions should
increase mechanical properties [8]. Thermo-mechanical treatment of Hadfield’s steel
was investigated by Król [9] where was found that dynamic recrystallization can occur
after deformation with 30% reduction at a temperature of 800 °C. Introduction of Nb
and Ti microadditions to steels could be the reason for additional strain hardening of
high-manganese steels and allows forming a fine-grained microstructure in successive
hot-working stages. This problem is a main subject of the work.
2. Experimental procedure
Investigations were carried out on two high-manganese austenitic Mn-Si-Al steels
containing Nb and Ti microadditions (Table 1). Melts were realized in the Balzers
VSG-50 inductive vacuum furnace. Ingots with a mass of 25 kg were submitted open
die forging on flats with a width of 220 mm and a thickness of 20 mm. Then, cylindri-
cal samples
10×12 mm were made. Preliminary tests consisted in hot upsetting in
a temperature of 900 and 1000 °C with 20, 40 and 60% reduction. Upsetting was car-
ried out using the PMS 50 eccentric press with a strain rate of about 30 s
–1
.
Table 1. Chemical composition of the investigated steels, mass fraction
Designation
C
Mn
Si
Al
P
S
27Mn-4Si-2Al-Nb-Ti 0.040
27.5
4.18
1.96
0.002 0.017
26Mn-3Si-3Al-Nb-Ti 0.065
26.0
3.08
2.87
0.004 0.013
Nb
0.033
0.034
Ti
0.009
0.009
N
0.0028
0.0028
On the basis of preliminary tests worked out the time-temperature conditions of
hot-working which should cause a formation of fine-grained recrystallized austenite
before cooling of samples from a temperature of final deformation. A selected se-
quence of deformations and time between particular deformations correspond to de-
signed hot-rolling of steel (Table 2). Thermo-mechanical treatment was realized using
the Gleeble 3800 thermo-mechanical simulator.
Thermo-mechanical processing of high-manganese austenitic TWIP-type steels
31
Metallographic examinations of samples were carried out using the LEICA MEF4A
light microscope. In order to reveal austenite grain boundaries, the samples etched in
a mixture of nitrous and hydrochloric acid in various proportions. Identification of the
phase composition of steels in the initial state and after thermo-mechanical treatment
achieved using the X’Pert PRO diffractometer with X’Celerator detector. The lamp
with Co anode was applied.
Table 2. Parameters of the thermo-mechanical treatment realized in the Gleeble simulator
Cooling
Cooling Deformation Cooling Deformation
t
isother
Final
Deformation II
I - II
II - III
III
III - IV
IV
at cooling
T
A
,
No
1
2
3
4
M
3
V
3
t
3
T
4
°C
M
1
V
1
t
1
T
2
M
2
V
2
t
2
T
3
M
4
850°C,
T
1
–1
°C/s
–1
°C/s
–1
°C/s
s
°C
s °C
s °C
s °C
s
s
–1
s
s
Water
1–5 1100 1100 0.29 7
4 12.5 1050 0.29 8
8 12.5 950 0.29 9 10 10 850 0.29 10 0-64
6 1100 1100 0.29 7
4 12.5 1050 0.29 8
8 12.5 950 0.29 9 10 10 -
-
-
-
Deformation I
T
A
– austenitizing temperature,
T
1
–T
4
– deformation temperatures,
1
–
4
– true strains,
V
1
–V
4
– cooling
rates between deformations,
t
1
–t
3
– times between deformations,
t
isother
– time of the isothermal holding of
specimens at a temperature of 850°C
3. Results
Melted steels possess diversified initial structures presented in Figures 1 and 2. The
27Mn-4Si-2Al-Nb-Ti steel is characterized with the austenitic structure with a content
of about 12% of
H
martensite, whereas 26Mn-3Si-3Al-Nb-Ti steel possesses a pure
austenitic structure. Both steels have comparable grain size in a range from 100 to 150
Pm.
Numerous annealing twins and non-metallic inclusions can be observed.
Application of plastic strain at a temperature of 1000 °C allowed identifying a mecha-
nism controlling the work-hardening of steel. The structures of the 27Mn-4Si-2Al-Nb-
Ti steel solution heat-treated from a temperature of 900 °C after deformation with 20,
40 and 60% reduction are shown in Figures 3–5. Effect of the dynamic recrystalliza-
tion is visible by refinement of structures after applying 40 and 60% reduction. Defor-
mation of the steel at 1000 °C with a reduction of 20% does not lead to initiate dy-
namic recrystallization. Whereas, applying deformation with a reduction of 40%,
causes dynamic recrystallization (Figure 6).
Work-hardening curves of steels are presented in Figure 7. It can be useful to esti-
mate force-energetic parameters of hot rolling. It was established that finish rolling
will be realized in four passes at a temperature range from 1100 to 850 °C. It is appar-
ent from Figure 7, that for applied parameters of plastic deformation, dynamic recrys-
tallization is a process controlling work-hardening in a whole temperature range of
hot-working. The steel containing 3% Si and 3% Al possesses a little higher value of
flow stress. Strain value
H
max
corresponding to maximum flow stress for particular
stages amounts from 0.19 to 0.27. Additionally, maximum stress moves towards
higher stress value for the 26Mn-3Si-3Al steel.
32
A. G
RAJCAR
, W. B
OREK
Fig. 1. Initial structure of 27Mn-4Si-2Al steel
containing
H
martensite in austenitic matrix
Fig. 2. Initial structure of austenitic
26Mn-3Si-3Al steel
Fig. 3. Structure of 27Mn-4Si-2Al steel solution
Fig. 4. Refined structure of 27Mn-4Si-2Al steel
heat-treated from a temperature of 900 °C after de- solution heat-treated from a temperature of 900 °C
formation with reduction 20%
after deformation with reduction 40%
Fig. 5. Refined structure of 27Mn-4Si-2Al steel
Fig. 6. Refined structure of 27Mn-4Si-2Al steel
solution-heat treated from a temperature of 900 °C solution heat-treated from a temperature of 1000 °C
after deformation with reduction 60%
after deformation with reduction 40%
Thermo-mechanical processing of high-manganese austenitic TWIP-type steels
33
Fig. 7. Changes of flow stress as a function of strain in successive deformation stages
Fig. 8. Structure development of 27Mn-4Si-2Al-Nb-Ti steel in successive stages of the plastic deforma-
tion; a) solution heat treatment from a temperature of 850 °C before final deformation,
b) solution heat treatment directly after final deformation, c) solution heat treatment after isothermal
holding steel for 16s at a temperature of 850 °C

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