Post on 13-Mar-2022
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
INFLUENCE OF THERMO-MECHANICAL TREATMENTS ON STRUCTURE AND
MECHANICAL PROPERTIES OF HIGH-MN STEEL
Leszek Adam DOBRZAŃSKI, Wojciech BOREK, Janusz MAZURKIEWICZ
Division of Materials Processing Technology, Management and Computer Techniques in Materials Science,
Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego 18A,
44-100 Gliwice, Poland, wojciech.borek@polsl.pl,
Abstract
The aim of this paper is to determine the high-manganese austenite propensity to twinning induced by the
cold working and its effect on structure and mechanical properties, and especially the strain energy per unit
volume of ten new-developed high-manganese Fe – Mn – (Al, Si) investigated steel, containing about 24,5 %
of manganese, 1% of silicon, 3 % of aluminium and microadditions Nb and Ti. with various structures after
their heat- and thermo-mechanical treatments. The new-developed high-manganese Fe – Mn – (Al, Si) steel
provide an extensive potential for automotive industries through exhibiting the twinning induced plasticity
(TWIP) mechanisms. TWIP steel not only show excellent strength, but also have excellent formability due to
twinning, thereby leading to excellent combination of strength, ductility, and formability over conventional
dual phase steels or transformation induced plasticity TRIP steels. Results obtained for high-manganese
austenitic steel with the properly formed structure and properties in the thermo-mechanical processes
indicate the possibility and purposefulness of their employment for constructional elements of vehicles,
especially of the passenger cars to take advantage of the significant growth of their strain energy per unit
volume which guarantee reserve of plasticity in the zones of controlled energy absorption during possible
collision resulting from activation of twinning induced by the cold working as the fracture counteraction factor,
which may result in significant growth of the passive safety of these vehicles' passengers.
Keywords:
High manganese steel, TWIP steel, strengthening mechanisms, mechanical properties, twinning
1. INTRODUCTION
Constructional solutions and steels used in the frontal part of a vehicle are the most significant due to the
possibility of accident occurrence. The goal of structural elements such as frontal frame side members,
bumpers and the others is to take over the energy of an impact. Proper selection of chemical composition
and manufacturing technologies, guarantee obtaining the structure allowing for connections a favorable
strength and plastic properties of steel. In the last three decades new groups of high-manganese steels have
been developed [1-7]. One, I think, most important from group of high manganese steels are TWIP-tape
steels which contains carbon from 0.02 to 0.65% and manganese from 20 to 35% and with diverse
concentrations of Si and Al, introduced primarily to lower the density of about 7.3g/cm3 for this group of
steels. Such steels possess very good plastic properties as a result of an intense curve of twinning during
plastic deformation (TWIP effect, Twinning Induced Plasticity). An advantageous array of mechanical
properties achieved by such steels, i.e. Rm~800-1000MPa, Rp0.2=250-550MPa, un=35-90% is strongly
dependent on chemical composition, and especially a concentration of Mn. The role of silicone and
aluminium consists of solid solution hardening of steel, while carbon is an element stabilising austenite.
Apart from high strength and plastic properties, they possess an especially high ability to absorb energy in
the conditions of plastic deformation at high speeds and formation of parts with a complex shape [1-14].
Thermo-mechanical treatment applied to refine microstructure of austenitic steels used in the automotive
industry has to be done in controlled conditions. Appling too large deformation, or too long isothermal holding
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
times after the last deformation results in excessive grain refinement of the structure up to about 2 mm, what
has an impact on increasing strength properties in particular increasing the offset yield point by about 150-200
MPa and tensile strength increase to 1000 MPa [1-7, 15-23]. Whereas too large value for the average grain
diameter of about 70 mm can improve plastic properties, especially elongations which can achieve value about
80-90% at relatively low strength properties. Therefore the main objective of the application of thermo-plastic
deformation is the selection of the processing parameters to achieve optimal value for the average grain
diameter at which the product of tensile strength and elongation reaches optimal value. This will allow
increasing strain energy per unit volume of structural components of vehicles during traffic collision [1-3, 15-23].
2. MATERIALS AND EXPERIMENTAL PROCEDURE
Examinations were carried out on new-developed high-manganese steel TWIP-type steel -
X8MnSiAlNbTi25-1-3 containing 24,5 % Mn, 1 % Si, 3 % Al, and with microadditions Nb and Ti. The
chemical compositions of steel were shown in Table 1. For the investigated melts Nb and Ti microadditions
were added in order to refine the structure and achieve precipitation hardening. Steel is characterized by
high metallurgical purity, associated with low concentrations of S and P contaminants and gases. Melt were
modified with rare earth elements.
Table 1.
Chemical composition of new-developed high-manganese TWIP-type steel, mass fraction
Steel designation Chemical composition, mass fraction
C Mn Si Al Nb Ti P max S max Ce La Nd
X8MnSiAlNbTi25-1-3 0,08 24,60 0,91 3,10 0,040 0,024 0,002 0,003 0,005 0,001 0,002
Based on the results obtained during thermo-mechanical treatment carried out in continuous axisymetrical
compression test and multi-stage compression tests using the Gleeble 3800 thermo-mechanical simulator at
the Institute of Engineering Materials and Biomaterials, Silesian University of Technology [15-23] allowed to
work out a schedule of three different variants of hot-rolling of high-manganese austenitic steel (Fig. 1),
where the processes eliminating the consequences of strain hardening were, respectively, dynamic
recovery, dynamic, metadynamic and static recrystallisation.
Fig. 1. Schematic and parameters of the multi-stage hot-rolling carried out on single-cage reversing hot-
rolling mill line for semi-industrial simulation (LPS) module B at the Institute of Ferrous Metallurgy in Gliwice.
TA - austenitizing temperature, t - time of the isothermal holding of specimens at a temperature of last
deformation - 850°C
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
In the next step after four-stage hot-deformations specimens for static tensile tests were cut down in the
direction of hot-rolling of high-manganese austenitic sheets. Static tests were performed on tensile testing
machine ZWICK Z100 in order to investigate mechanical properties, especially strain energy per unit volume
of high-manganese austenitic steel, with various structures after their thermo-mechanical treatment.
Metallographic examinations of samples were carried out using the LEICA MEF4A light microscope. In order
to reveal austenite structure, the samples etched in a mixture of nitrous and hydrochloric acid in various
proportions. The structure of the investigated steel was also characterised using the SUPRA 25 scanning
electron microscope and the high resolution S/TEM TITAN 80-300 transmission electron microscope working
at accelerating voltage of 300 kV. TEM observations were carried out on thin foils. The specimens were
ground down to foils with a maximum thickness of 80 µm before 3 mm diameter discs were punched from
the specimens. The disks were further thinned by ion milling method with the Precision Ion Polishing System
(PIPS™), using the ion milling device (model 691) supplied by Gatan until one or more holes appeared. The
ion milling was done with argon ions, accelerated by voltage of 15 kV.
3. RESULTS AND DISCUSSION
After four-stage hot-rolling according to scheme shown in figure 1, static tensile tests were performed in
order to investigate mechanical properties, especially strain energy per unit volume of high-manganese
steel, with various structures after their thermo-mechanical treatment. After the solution heat treatment of
X8MnSiAlNbTi25-1-3 steel were noticed that from the end temperature of hot-rolling with a strain 0.23, the
structure of steel is represented by dynamically recovered austenite grains elongated in the direction of
rolling containing numerous twins (Fig. 2a). In addition, twins are visible occurring especially in large-sized
grains. Fine, recrystallized grains are arranged dynamically at the boundaries of dynamically recovered
austenite grains elongated in the direction of rolling. The dynamically recovered grains are refined twice less
than the specimens deformed in a Gleeble 3800 simulator [15-23] as a result of too rapid give up the heat
from very thin sheets (about 3-4mm) of high-manganese steel to the environment and thus it creates a lot of
problems to control of temperature of individual rolling possess. Isothermal holding for 30s after hot-rolling
with final pass reduction 20% (equal true strain 0.23), causes that the fraction of statically and
metadynamically recrystallized grains occurs in about 30% and is considerably lower than for specimens
after hot-working in the Gleeble 3800 simulator. They are arranged mainly at the boundaries of the elongated
grains of the statically recovered austenite, and are often situated also on the boundaries of twins. Creation
of refined structure has no influence on phase composition of the investigated steel and should increase
mechanical properties during cold plastic deformation.
a) b) c)
Fig. 2. Austenitic structures of high manganese X8MnSiAlNbTi25-1-3 TWIP-type steel; a) obtained after
four-stage hot-rolling with a true strain equal 4x0.23 and natural air-cooling after final deformation at
temperature 850°C, according to Variant No. II of thermo-mechanical treatment (Fig 1); b) and c) shows
structures with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain 4x0.23
b) according to Variant No. II, c) according to Variant No. III of thermo-mechanical treatment (Fig 1) and after
static tensile tests
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
a)
b)
c)
d)
Fig. 3. Twinned austenite area in the X8MnSiAlNbTi25-1-3 steel in the heated state with a true strain equal
4x0.23, with water cooling after isothermal holding 30s at temperature 850°C, according to variant No. III of
thermo-mechanical treatment (Fig. 1), and a following strain test until elongation of 30%: a) bright field, b)
dark field from the ( 200 ) plain Fe, c) diffraction pattern, d) solution of the diffraction pattern in fig. c
This group of steels can be applied on constructional elements of the car body which should transfer loads
during front or side impact collisions. On figures 2b, 2c and 3 are presented austenitic structures of high
manganese steel with mechanical and micro twins and slip bands obtained after hot-rolling with a true strain
0.23 according to thermo-mechanical treatment variant No. II or III and after static tensile tests. The
substantial contribution of mechanical twinning into the plastic deformation of steel in this variant causes a
substantial increase in plastic properties and continuous increase in the temporary strain hardening
coefficient. An increase in the parallel elongation of steel is closely linked to the intensive progress of
mechanical twinning in the places where local narrowing in the specimen occurs. The twin boundaries
formed are strong barriers for the dislocation movement; hence further plastic deformation is situated in
areas with a smaller local flow stress.
Fig. 4. Influence of various parameters of thermo-mechanical treatment on mechanical properties
of high manganese X8MnSiAlNbTi25-1-3 TWIP-type steel: yield strength Rp0.2, tensile strength Rm
and total elongation um
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
On the figure 4 are presented results mechanical properties of new developed high-manganese TWIP-type
steel, with various structures after various variants of thermo-mechanical treatments. Mechanical twinning
induced by the cold working of the high-manganese austenitic TWIP steels has a significant effect on
forming their structure and mechanical properties (Figs 2b, 2c and 3). Mechanism of twinning induced by the
cold working of the high-manganese austenitic steel results in growth of the strain energy per unit volume
after the successive cold deformation. On figure 5 is presented representative tensile curve of the TWIP-type
steel with designated strain energy per unit volume after the cold deformation equal 263 MJ/m3. To increase
strain energy per unit volume the temperature of plastic deformation should be decrease below ambient
temperature. Energy increase can be also achieved by increasing the strain rate cold plastic deformation, for
instance for TWIP type steel acceleration of deformation to 500 s-1 causes that energy per unit volume
increases by approximately 200 MJ/m3 in comparison with energy determined in static condition. The high-
manganese austenitic steels with the properly formed structure and properties and especially with the big
strain energy per unit volume yield the possibility to be used for the constructional elements of cards
affecting advantageously the passive safety of the vehicles' passengers.
Fig. 5. Representative tensile curve of investigated X8MnSiAlNbTi25-1-3 TWIP-type steel with designated
strain energy per unit volume after the cold deformation in static conditions
4. CONCLUSIONS
High-manganese Fe – Mn – (Al, Si) steels provide an extensive potential for automotive industries
through exhibiting the twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP)
mechanisms.
It was concluded on the basis of the own investigations [15-23] that mechanical properties of high-
manganese austenitic steels in the condition after supersaturation state and on the basis of data from the
literature that the examined steel subjected to thermo-mechanical processing are characterised by a yield
point higher by approx. 200-250MPa, which is a result of precipitation strengthening of such steels by the
dispersive particles of Nb and Ti carbonitrides and is also connected with thermo-mechanical treatment
employed.
Twinning mechanism induced with cold plastic deformation, forming the structure of the investigated steel
after prior thermo-mechanical processing with varied share of dynamic, metadynamic and static
mechanisms removing the consequences of strain hardening, has a substantial effect on increasing the
strain energy per unit volume and strength and plastic properties of steel in the conditions of cold plastic
deformation.
Nov 19th – 21st 2014, Pilsen, Czech Republic, EU
ACKNOWLEDGEMENTS
Project was founded by the National Science Centre based on the decision number
DEC-2012/05/B/ST8/00149.
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