L. A. Dobrzanski, W. Borek - Hot working of advanced high-manganese austenitic steels.pdf

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Hot-working of advanced
high-manganese austenitic steels
L.A. Dobrzański*, W. Borek
Institute of Engineering Materials and Biomaterials, Silesian University
of Technology, ul. Konarskiego 18a, 44-100 Gliwice, Poland
* Corresponding author: E-mail address: leszek.dobrzanski@polsl.pl
Abstract
Purpose:
The work consisted in investigation of newly elaborated high-manganese austenitic
steels with Nb and Ti microadditions in variable conditions of hot-working.
Design/methodology/approach:
The force-energetic parameters of hot-working were
determined in continuous and multi-stage compression test performed in temperature range of
850 to 1100°C using the Gleeble 3800 thermomechanical simulator. Evaluation of processes
controlling work-hardening were identified by microstructure observations of the specimens
compresses to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The
microstructure evolution in successive stages of deformation was determined in metallographic
investigations using light, scanning and electron microscopy as well as X-ray diffraction.
Findings:
The investigated steels are characterized by high values of flow stresses from 230 to
450 MPa. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn
steels and slightly higher compared to Fe-(15-25)Mn alloys. Increase of flow stress along with
decrease of compression temperature is accompanied by translation of
ε
max
strain in the
direction of higher deformation. Results of the multi-stage compression proved that applying
the true strain 4x0.29 gives the possibility to refine the austenite microstructure as a result of
dynamic recrystallization. In case of applying the lower deformations 4x0.23 and 4x0.19, the
process controlling work hardening is dynamic recovery and a deciding influence on a gradual
microstructure refinement has statical recrystallization. The steel 27Mn-4Si-2Al-Nb-Ti has
austenite microstructure with annealing twins and some fraction of
ε
martensite plates in the
initial state. After the grain refinement due to recrystallization, the steel is characterized by
uniform structure of
γ
phase without
ε
martensite plates.
Research limitations/implications:
To determine in detail the microstructure evolution during
industrial rolling, the hot-working schedule should take into account real number of passes
and higher strain rates.
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Volume 1 2011
Practical implications:
The obtained microstructure – hot-working relationships can be useful
in the determination of power-force parameters of hot-rolling and to design a rolling schedule
for high-manganese steel sheets with fine-grained austenitic structures.
Originality/value:
The hot-deformation resistance and microstructure evolution in various
conditions of hot-working for the new-developed high-manganese austenitic steels were
investigated.
Keywords:
High-manganese
steel;
Hot-working;
TRIP/TWIP
steels;
Dynamic
recrystallization; Static recrystallization; Grain refinement
Reference to this paper should be given in the following way:
L.A. Dobrzański, W. Borek, Hot-working of advanced high-manganese austenitic steels, in
L.A. Dobrzański (ed.) Effect of casting, plastic forming or surface technologies on the structure
and properties of the selected engineering materials, Open Access Library, Volume 1, 2011,
pp. 55-88.
1. Introduction
The beginning of XXI century has brought a development of new groups of steels to be
applied for sheets in automotive industry. From the aspect of materials, this development has
been accelerated by strong competition with non-metal aluminium and magnesium alloys as
well as with composite polymers, which meaning is successively increasing. From the aspect
of ecology, an essential factor it is to limit the amount of exhaust gas emitted into the
environment. It’s strictly connected to the fuel consumption, mainly dependant on car weight
and its aerodynamics. Taking into consideration increased quantity of accessories used in
modern cars, decreasing car’s weight can be achieved solely by optimization of sections of
sheets used for bearing and reinforcing elements as well as for body panelling parts of a car.
Application of sheets with lower thickness preserving proper tautness requires using sheets
with higher mechanical properties, however keeping adequate formability. Steels of IF and BH
type with moderate mechanical properties and high susceptibility to deep drawing were
elaborated for elements of body panelling [1]. The highest application possibilities belong to
DP-type steels with ferritic – martensitic microstructure. Their mechanical properties can be
formed in a wide range, controlling participation of martensite arranged in ferritic matrix.
Sheets made of these steels are widely used for bearing and reinforcing elements [2].
56
L.A. Dobrzański, W. Borek
Effect of casting, plastic forming or surface technologies on the structure
and properties of the selected engineering materials
In comparison to steels with ferritic microstructure they are characterized by high value of
hardening exponent n, what decides about their strong strain hardening during sheet-metal
forming [3].
Nowadays, apart from limiting fuel consumption, special pressure is placed on increasing
safety of car’s passengers. 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. Therefore, steels that are used for these parts should be characterized
by high product of UTS and UEl, proving the ability of energy absorption. Among the wide
variety of recently developed steels, high-manganese austenitic TRIP/TWIP steels with low
stacking faulty energy (SFE) are particularly promising, especially when mechanical twinning
occurs [3-6]. Beneficial combination of high strength and ductile properties of these steels
depends on structural processes taking place during cold plastic deformation, which are a
derivative of stacking fault energy (SFE) of austenite, dependent, in turn on the chemical
composition of steel and deformation temperature [1-4, 7-10]. In case, when SFE is equal from
12 to 20 mJm
-2
, partial transformation of austenite into martensite occurs, making use of TRIP
effect (TRansformation Induced Plasticity) [1-4, 8]. Values of SFE from 20 to 60 mJm
-2
determine intense mechanical twinning connected to TWIP effect (TWinning Induced
Plasticity) [5-10]. The steels cover a very wide carbon concentration in a range from about 0.03
to 1 wt.%, 15-30% Mn, 0-4% Si, 0-8% Al.
The best conditions for obtaining the total elongation up to 80%, due to a gradual increase
of mechanical twins, acting as obstacles for dislocation glide, occur when the carbon
concentration is in the range of 0.4-0.8% and manganese from 17 to 22% [5, 11, 12]. However,
high carbon content may lead to formation of M
3
C and M
23
C
6
-type carbides, which
precipitating on austenite grain boundaries negatively affect the strength and toughness of the
steel [11]. Moreover, in the Fe-(17-22) Mn-(0.4-1) C steel grades, besides the formation of
deformation twins during straining, a technologically undesirable jerky flow, which presents
the features of dynamic strain aging and PLC (Portevin-LeChatelier) effect is observed [6].
Because of these reasons, Frommeyer et al. [1-4] proposed a group of high-manganese steels
with carbon content, less than 0.1%. Lower hardening due to decreased carbon concentration
was compensated by Si and Al additions, which together with Mn decide about SFE of the
alloy and the main deformation mechanism. In a case of Mn
≥
25%, the mechanical properties
are mainly dependent on TWIP effect [1, 3, 13] and for Mn
≤
20%, a process influencing a
mechanical properties level is strain-induced martensitic transformation of austenite [2-4]. For
Hot-working of advanced high-manganese austenitic steels
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Volume 1 2011
the latter the initial structure is consisted of
γ
phase as a matrix, some fraction of
ε
martensite
and sometimes ferrite [2-4].
Results of our earlier investigations [14-17] indicate that
ε
martensite plates can appear in
the initial structure of the (0.04-0.05) C-25Mn-4Si-2Al alloys as a result of Nb and Ti
microadditions. The amount of C combined in precipitated carbonitrides reduces its content in
the solid solution, thus decreasing the SFE of austenite and resulting in a presence of
ε
martensite despite high manganese concentration in the investigated steels. It was found [16-
18] that the fraction of
ε
martensite plates is also dependent on a grain size of the
γ
phase and
hot-working conditions. It was also observed that the fraction of mechanical twins within the
austenite grains corresponds to the initial grain size, and at the same time affects the
mechanical properties [5].
The hot-working behaviour of high-manganese steels is of primary importance for
elaborating manufacturing methods consisted of hot rolling and successive cooling to room
temperature. However, their hot work hardening and microstructural evolution controlled by
thermally activated processes removing it, did not draw much attention compared to cold-
working behaviour. Niewielski [19] compared the flow resistance of the 0.5C-17Mn-16Cr
austenitic steel with conventional stainless steel of 18-8 type. He observed that the hardening
intensity of Cr-Mn steel is much higher than in case of Cr-Ni steel. The difference in a course
of work-hardening comes from a different ability of dislocations to splitting and association
during straining.
High strain hardening rate is a result of the ability of manganese austenite for dislocation
dissociation in the initial deformation stage [19, 20]. The reason for high hardening intensity of
Cr-Mn steel are much higher flow stress values compared to Cr-Ni steel, however at lower
deformation value of
ε
max
corresponding to maximal flow stress. For example, the yield stress
of the Cr-Mn steel hot-twisted at a temperature of 1100°C with a strain rate of 1s
-1
is equal to
134 MPa for the value
ε
max
= 0.18 and increases to 280 MPa for
ε
max
= 0.23 with decreasing the
deformation temperature to 900°C [19]. Cabanas et al. [21] investigated the retarding effect of
Mn content up to 20 wt.% on the grain boundary migration and dynamic recrystallization in
binary Fe-Mn alloys. The influence of Al addition on the flow behaviour of 0.1C-25Mn-(0-8)
Al alloys was the aim of investigations undertaken by Hamada et al. [22, 23]. They found that
flow resistance is slightly higher for the 25Mn3Al than for the 25Mn steel. Moreover, they
observed that the flow stress of the austenitic alloys containing Al up to 6% is much higher
compared to the steel containing 8% Al with a duplex austenitic-ferritic structure [22].
58
L.A. Dobrzański, W. Borek
Effect of casting, plastic forming or surface technologies on the structure
and properties of the selected engineering materials
Investigation results by Hamada et al. [22, 23], Sabet et al. [24] on 0.13C-29Mn-2.4Al steel
and Kliber et al. [25] on (0.6-1)C-(17-20)Mn steels confirmed the high work hardening rate of
high-manganese alloys in the deformation range lower than
ε
max
, likewise for Cr-Mn steels
investigated by Niewielski [19].
For manufacturing methods elaborating, it is especially important that relatively low values
of
ε
max
give the opportunities to refine austenitic structures in successive stages of hot-working.
Unfortunately, the flow resistance of high-Mn steels is usually investigated under conditions of
continuous compression or torsion [20-25]. To determine the softening kinetics, the double- or
triple-deformation tests are rarely carried out [22, 25]. Hot-rolling of sheets consists of many
passes characterized by the changing amount of deformation and strain rate from pass to pass.
This means that the flow stresses should be determined during multi-stage straining testing and
for various deformation values. In earlier investigations [14-16, 26, 27] we characterized the
force-energetic parameters of hot-working of new-developed low-carbon high-Mn-Si-Al steels
in continuous and four-stage compression tests. The aim of the paper is to describe in details
the microstructure evolution and phase composition of 0.04C-27Mn-4Si-2Al-Nb-Ti steel
subjected to four-stage compression with various amount of deformation.
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 prepared in the Balzers VSG-50
vacuum induction furnace. After homogenization at 1200°C for 4 h to remove the segregation
of Mn, ingots with a mass of 25 kg were submitted for open die forging on flats with a width of
220 mm and a thickness of 20 mm. Then, cylindrical machined samples
∅10x12
mm were
made. In order to determine the influence of temperature on a steel grain growth, samples were
solution heat-treated in water from the austenitizing temperature in a range from 900 to 1100°C
(Fig. 1). Determination of processes controlling work hardening was carried out in continuous
axisymetrical compression test using the DSI Gleeble 3800 thermomechanical simulator, used
as laboratory equipment of the Institute for Ferrous Metallurgy in Gliwice [28, 29]. The stress
– strain were defined in a temperature range from 850 to 1050°C with a strain rate of 10 s
-1
.
In order to determine
σ-ε
curves, the four-stage compression tests were carried out. The
temperatures of the successive deformations were 1100, 1050, 950 and 850°C. The details of
Hot-working of advanced high-manganese austenitic steels
59

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