Scripta Materialia 262 (2025) 116636

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Scripta Materialia
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Significant austenite decomposition during slow heating of cold-rolled

medium Mn steel with a high fraction of pre-existing austenite
Saeed Sadeghpour a,* , Roohallah Surki Aliabad a , Shubo Wang b , Vahid Javaheri a ,
Harishchandra Singh b, Mahesh Somani a , Dong-Woo Suh c , Pentti Karjalainen a ,
Jukka Komi a
a
Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, 90014 Oulu, Finland
b
Nano and Molecular Systems Research Unit, University of Oulu, 90014 Oulu, Finland
c
Graduate Institute of Ferrous & Eco-materials Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea

A R T I C L E I N F O A B S T R A C T

Keywords: In-depth analysis of phase transformations during continuous heating (2.4 ◦ C/s) of 50 % cold-rolled Fe-6Mn-2Al-
Medium Mn steel 1Si-0.4C (wt.%) steel with 32 % pre-existing austenite was conducted using in-situ high-energy synchrotron X-
Austenite decomposition ray diffraction and microstructural characterisation. Significant austenite decomposition was found to occur in
In situ synchrotron XRD
two stages between 226 and 530 ◦ C and 530–600 ◦ C, involving bainitic transformation and cementite precipi­
Cementite
Bainite
tation, respectively, reducing the austenite fraction to 17.5 %. Lattice parameter changes suggested C enrichment
in austenite during bainite formation and C and Mn depletion during precipitation. The austenite fraction
increased around 650 ◦ C, yielding 8 % new austenite by 680 ◦ C in ~40 s and 7 % during isothermal holding at
680 ◦ C for 600 s. A further 3 % decrease during cooling, presumably due to pearlite formation, resulted in a final
austenite fraction of 29.5 %, slightly below the initial level but further enriched in C and Mn. These findings
provide insights into austenite evolution during thermal processing of cold-rolled medium Mn steels.

Medium Mn steels (MMSs) have attracted substantial interest from produces more stable RA than at 80 ◦ C/s. Using in-situ high-energy
the automotive industry and the steel research community owing to X-ray diffraction, Mueller et al. [9] observed that in Fe-0.19C-4.39Mn
their potential to achieve a wide range of mechanical properties [1]. steel, existing austenite (2 %) disappeared during heating at 0.76 ◦ C/s
MMSs typically undergo intercritical annealing treatment (IAT) of hot to 375 ◦ C accompanied by the appearance of cementite.
and/or cold rolled sheets, producing an ultrafine microstructure of The initial microstructure also significantly affects phase trans­
ferrite and retained austenite (RA) [2–4]. The mechanical properties of formations during IAT. The IAT of a hot-rolled material differs from that
MMSs are largely determined by the characteristics of RA, including of a cold-rolled (CR) microstructure, where austenite formation and
volume fraction, morphology, stability, size, and distribution, which can ferrite recrystallisation coincide, resulting in an equiaxed final micro­
be controlled through parameters such as IAT temperature, duration, structure [10]. Pre-existing austenite from prior IAT can accelerate
and heating/cooling rates [5]. austenite formation and cementite dissolution during subsequent IAT,
While research has focused on austenite formation and stabilisation achieving optimal properties faster [11–14]. Moreover, pre-existing
through Mn partitioning during IAT [3,4], as well as phase evolutions austenite fractions above 10 % enhance the final mechanical proper­
during subsequent cooling [6], studies on microstructural evolution ties after IAT [15].
during continuous heating up to the intercritical region remain limited. Paying attention to microstructural evolution during heating to IAT,
Jing et al. [7] implied that in Fe-0.2C-7.81Mn-1.38Al (in wt.% and also we report for the first time a significant austenite phase decomposition
onwards) steel, slow heating rates (e.g., 0.1 ◦ C/s) allow more Mn par­ during heating of a CR MMS containing a substantial fraction of pre-
titioning, leading to RA with heterogeneous Mn content and stability, existing austenite. Present findings provide new insights for optimis­
which enhances yield strength and elongation compared to faster ing MMS thermal processes.
heating (10 ◦ C/s). Kozlowska et al. [8] found a two-stage IAT at 3 ◦ C/s The studied MMS (Fe-6Mn-2Al-1Si-0.4C) was cast in a vacuum

* Corresponding author.
E-mail address: saeed.sadeghpour@oulu.fi (S. Sadeghpour).

https://doi.org/10.1016/j.scriptamat.2025.116636
Received 19 December 2024; Received in revised form 13 February 2025; Accepted 3 March 2025
Available online 13 March 2025
1359-6462/© 2025 The Authors. Published by Elsevier Inc. on behalf of Acta Materialia Inc. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
S. Sadeghpour et al. Scripta Materialia 262 (2025) 116636

induction furnace, homogenised at 1200 ◦ C for 2 h, hot-rolled to 4 mm Fig. 3 shows changes in austenite fraction and LP along with ferrite
thickness, air-cooled, intercritically annealed at 680 ◦ C for 30 min and LP during the heat treatment. During heating, the austenite fraction
then CR to a final thickness of 2 mm. decreases to 17.5 % at 600 ◦ C before increasing to 26 % at 680 ◦ C. This
Phase evolutions during continuous heating were monitored using consumption occurs in two distinct stages, characterised by different
in-situ high-energy synchrotron X-ray diffraction (HE-SXRD) at the rates of austenite decomposition. Up to 226 ◦ C, the austenite fraction
Brockhouse High Energy Wiggler Beamline, Canadian Light Source. The remains constant and both austenite and ferrite LPs show linear thermal
CR specimen (1 × 1 × 20 mm3) was heated at an average rate of 2.4 ◦ C/s expansion (Fig. 3a and 3b).
to 680 ◦ C, held for 10 min, and cooled to room temperature. HE-SXRD Then, in the first stage of austenite evolution (after 42 s), the
patterns, as shown in Fig. 1 and Supplement 1, were obtained using a austenite fraction decreases from 32.2 % at 226 ◦ C to 27.0 % at 530 ◦ C.
70 keV monochromatic X-ray beam (wavelength 0.1779 Å) and a During this stage, ferrite LP steadily increases, while austenite LP in­
sample-to-detector distance of 1044 mm. The acquisition time for each creases slightly faster than expected from thermal expansion (Fig. 3a).
diffraction pattern was optimised to 0.3 s. Radial integration of 2D This expansion could result from a change in stress state and/or chem­
diffraction patterns was done using GSAS-II software, and phase frac­ ical composition, i.e., from C enrichment. The decomposition acceler­
tions and lattice parameters were obtained through Rietveld refinement ates above 530 ◦ C, with 9 % austenite decomposed in 32 s as the
in GSAS. temperature reaches 600 ◦ C. During this stage, austenite LP remains
To study the microstructures, samples of the CR material were heat- unchanged, whereas ferrite LP continues increasing (Fig. 3a and 3b).
treated under corresponding conditions in a Gleeble 3800 thermo­ The observed drop in austenite fraction from 32.2 % to 17.5 % be­
mechanical simulator. Microstructural examinations were conducted tween 226 and 600 ◦ C likely results from bainitic transformation or/and
using a Zeiss Sigma field emission scanning electron microscope (FE- carbide precipitation, while LP changes could stem from changes in
SEM) equipped with an electron backscatter diffraction (EBSD) detector alloying elements concentrations in existing phases. HE-SXRD patterns
and a 200-kV JEOL JEM-2200FS scanning transmission electron mi­ (Fig. 4) show no cementite peaks below 530 ◦ C but detected them
croscope (STEM). beyond this temperature. Mueller et al. [9] and Hu et al. [16] reported
The microstructure of the hot-rolled and air-cooled material con­ cementite formation around 254 ◦ C in Fe-0.19C-4.39Mn steel, with
tained 2 % austenite. After IAT at 680 ◦ C for 30 min, the microstructure austenite fully decomposed (only 2.3 % present initially) by 368 ◦ C
comprised 49 % ferrite, 43 % RA and 8 % fresh martensite, with both during heating at 0.76 ◦ C/s.
austenite and martensite enriched in C and Mn (Supplement 2). Fig. 2 Since austenite LP increases between 226 and 530 ◦ C, and cementite
(a1) shows the initial CR microstructure. The phase map (a1) and cor­ peaks appear only at 530 ◦ C and beyond, austenite decomposition in this
responding HE-SXRD pattern at room temperature (a3) reveal a high temperature range cannot result from cementite formation. The increase
austenite fraction, even after a 50 % thickness reduction during cold in austenite LP above 226 ◦ C, beyond expected thermal expansion, likely
rolling. HE-SXRD results indicated 32 % RA, while EBSD detected only results from C partitioning or the generation of type II internal stresses
21 %, likely due to very thin (<100 nm) RA films in the heavily rolled [17]. For simplicity, C partitioning is assumed to be the primary cause.
microstructure, as shown in Fig. 2(a2), not captured by EBSD. The LP of austenite (LPγ) depends on chemical composition [18]:
Fig. 2b presents the IAT sample’s microstructures (b1 and b2) and
LPg = 3.572 + 0.033C + 0.0012Mn + 0.00157Si + 0.0056Al (1)
corresponding HE-SXRD pattern (b3). After cold rolling and IAT at 680
◦
C for 10 min, an ultrafine-grained microstructure of equiaxed ferrite
where concentrations are in wt.% and LP in Å.
and austenite (~300 nm grain size) was obtained. The austenite fraction
Starting with an initial room temperature LP of 3.608 Å, austenite
measured by EBSD (28.8 %) closely matches the HE-SXRD results (29.5
was enriched in C and Mn after the first IAT, as expected. ThermoCalc
%), likely due to the low dislocation density and larger equiaxed
estimates C and Mn contents of 0.7 % and 9.0 %, respectively (Supple­
austenite grains. To understand microstructure evolution during the
ment 2). Extrapolating the thermal expansion line (close to Allain et al.
heat treatment, the phase fractions and lattice parameters (LPs) were
[19] value of 2.526 10− 5 K− 1) yields an LP of 3.640 Å at 530 ◦ C with
analysed in two stages: 1) Heating to 680 ◦ C (intercritical annealing
unchanged composition (Fig. 3a). However, the measured LP at 530 ◦ C
temperature) and 2) Holding at 680 ◦ C for 10 min, followed by cooling
is ~3.648 Å, indicating 0.008 Å expansion, corresponding to ~0.18 % C
to room temperature.
increase. Fresh martensite (8 vol.%) containing 0.7 % C could partition
1) Heating to intercritical annealing temperature:
to austenite (about 26 % austenite present) during heating. Calculation
shows that 0.08×0.7 % =0.056 % C could partition to 26 % austenite,
allowing up to 0.22 % C enrichment. This could increase LP by
0.033×0.22 ≈ 0.007 Å, aligning very well with the measured increase in
LP. Bainitic ferrite has an LP practically equal to that of ferrite, so ferrite
LP changes only with thermal expansion.
Santofimia et al. [20] showed that carbon partitioning may lead to
austenite/ferrite interface migration for a short period, increasing
austenite volume. However, in-situ HE-SXRD results indicate that
although austenite LP increased with temperature up to 530 ◦ C, the
austenite fraction decreased. Based on the
time-temperature-transformation diagram from JMatPro (Supplement
3), bainitic transformation is possible in this temperature range. Bainite
transformation enriches austenite in C in addition to partitioning from
martensite/ferrite [17], further enlarging LP.
Exploring the microstructure quenched from 250 ◦ C reveals a small
bainitic ferrite fraction (Fig. 4), confirming bainitic transformation
initiation. The micrograph of the sample quenched from 480 ◦ C shows
Fig. 1. Contour plots for the evolutions of integrated HE-SXRD patterns as a significantly more bainitic ferrite, indicating continued bainitic trans­
function of time/temperature (t/T) during IAT. White and black arrows depict formation with temperature. Morawiec et al. [21] showed that higher
the (hkl) diffraction peaks of FCC and BCC structures, respectively. The colour Mn content in MMSs containing 3–5 % Mn slows bainitic transformation
bar represents the peak intensities after normalisation. Red circles highlight the by reducing the chemical driving force. However, pre-existing
interrupted heat treatments conducted in a Gleeble for microstructure analysis.

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S. Sadeghpour et al. Scripta Materialia 262 (2025) 116636

Fig. 2. EBSD (1) and STEM (2) micrographs and corresponding room temperature HE-SXRD patterns (3) of the a) cold-rolled (CR) specimen and b) specimen after
IAT at 680 ◦ C for 10 min. Austenite is shown in green colour.

Fig. 3. Evolution of austenite fraction and the lattice parameters of austenite and ferrite during the a) whole process as a function of temperature, and during b)
heating up to 680 ◦ C, c) holding at 680 ◦ C for 600 s, and d) cooling down from 680 ◦ C, as a function of time.

martensite significantly accelerates this transformation, likely through consumed due to bainite formation during reheating at 10 ◦ C/s from a
nucleation at dislocations from martensite formation or at quenching temperature of 360 ◦ C to a partitioning temperature of 400
martensite-austenite interfaces [18,22,23]. Toji et al. [18] showed that ◦
C. Microstructural heterogeneity in MMSs resulting from Mn parti­
in steel with 30 % pre-existing martensite, substantial bainite formed tioning [24,25] may also promote local bainitic transformation during
after partitioning at 300 ◦ C for 10 s, compared to minimal bainite in IAT. Kim et al. [26] showed that Mn-depleted austenitic regions enable
martensite-free steel even after 2100 s at the same temperature. Using rapid bainitic ferrite nucleation. Furthermore, a high density of dislo­
HE-SXRD, Huyghe et al. [17] observed that 13.9 % austenite was cations introduced by warm deformation, as shown by Huang et al. [27],

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S. Sadeghpour et al. Scripta Materialia 262 (2025) 116636

Fig. 4. One-dimensional HE-SXRD patterns of the sample at RT, 250 ◦ C, 480 ◦ C, 530 ◦ C, 600 ◦ C and 680 ◦ C along with corresponding FE-SEM micrographs. Bainite is
shown by arrows at 250 and 480 ◦ C.

can also accelerate bainitic transformation, while deformation also stages of new austenite formation but ceases in the later stage as the
promotes the pearlite reaction [24]. concentration level existing at 500–530 ◦ C is reached. Partial cementite
FE-SEM images (Fig. 4) revealed no cementite in the microstructure dissolution would release Mn, but residual particles persist even after
of the initial sample or those water-quenched from 250 ◦ C and 480 ◦ C. At 600 s holding at 680 ◦ C, so the old martensite is likely a source of Mn.
530 ◦ C, very small particles begin to nucleate at austenite/ferrite Thus, it may be assumed that the depleted austenite will be enriched,
boundaries. By 600 ◦ C, a significant fraction of cementite precipitates and new austenite likely forms from strain-induced martensite (11 %
within austenite (Fig. 4). Thermodynamic calculations (Supplement 2) from cold rolling) and old martensite (8 % existing), already enriched in
suggest that the cementite fraction could increase to 10 % and remain C and Mn during the first IAT. The 8 % new austenite means that less
stable up to 685 ◦ C, consistent with HE-SXRD measurements indicating a than half of the enriched martensite reversed within 40 s between 650
9 % austenite volume reduction between 530 and 600 ◦ C. Thus, this and 680 ◦ C, leaving the remaining martensite available for further
austenite decomposition could be a result of cementite precipitation. austenite formation during holding.
This also decreases LP as C and Mn are bound in (Fe,Mn)3C particles. 2) Intercritical annealing and cooling:
While thermal expansion and C/Mn depletion occur simultaneously, the Stage 2 corresponds to isothermal IAT during holding at 680 ◦ C for
LP remains nearly constant over this temperature interval. 600 s. Fig. 3c shows the austenite fraction increased by only 7 vol.%
Between 600–650 ◦ C, austenite LP increases while its fraction stays during this stage, indicating austenite formation starts at a high rate
constant. Partial austenite decomposition by cementite precipitation during the final heating and then slows down during holding. Fig. 3c
and formation of new austenite might occur simultaneously, balancing suggests that austenite formation could continue beyond 10 min if a
the austenite volume fraction. The increase in LP reflects Mn enrichment higher RA fraction is desired.
in both the depleted and newly formed austenite. However, the LP is still It has been suggested that a small fraction of pre-existing austenite
smaller than it would have been without the previous depletion caused can act as a site for rapid new austenite formation during IAT by
by cementite precipitation (it remains below the LP line during cooling enabling growth without nucleation [11–14]. However, excessive (over
until 680 ◦ C in Fig. 3a). Microstructural observations, consistent with 10 %) pre-existing austenite can impede this process [15]. In the present
previous works [28–30], show ferrite recrystallisation beyond ≈600 ◦ C experiment, quite a high amount of 32 % of austenite was present before
and partial austenite recrystallisation at even higher temperatures. heating, while at least 15 % of new austenite formed within 640 s.
Above 650 ◦ C (after 174 s), austenite formation exceeds decompo­ During holding, the austenite LP stayed practically constant with
sition, increasing its fraction by 8 % during slow heating to 680 ◦ C over minimal compositional changes due to the constant temperature
40 s. During this increment, the austenite LP also increases but at a (Fig. 3c). This implies that new austenite forms in this stage from the
decreasing rate until reaching a constant level at 680 ◦ C. The increase in still-existing enriched martensite. Cementite is reported to persist in the
LP suggests a continued Mn enrichment of austenite during the initial microstructure even after substantial austenite growth because the

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S. Sadeghpour et al. Scripta Materialia 262 (2025) 116636

elevated Mn content in cementite can retard its dissolution during IAT Supplementary materials
[9,24,31]. This is consistent with Fig. 4, which shows numerous
cementite particles remaining after 10 min of annealing at 680 ◦ C. Supplementary material associated with this article can be found, in
Importantly, these particles bind part of C and Mn, meaning further the online version, at doi:10.1016/j.scriptamat.2025.116636.
austenite enrichment depends on the cementite dissolution rate.
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