ORIGINAL RESEARCH ARTICLE
Time-Dependen E olu ion o Volume F ac ion
and S abili y o Re ained Aus eni e in a Ho -Rolled
and In e c i ically Annealed Al-Alloyed Medium-Mn
S eel
ADAM SKOWRONEK , ADAM GRAJCAR , CARLOS GARCIA-MATEO ,
JOSE
´A. JIME
´NEZ , and ROUMEN H. PETROV
The de elopmen o supe io mechanical p ope ies in medium-Mn equi es he op imiza ion o
mic os uc u al pa ame e s such as e ained aus eni e (RA) s abili y, olume ac ion, and
mo phology. The p esen wo k explo es he possibili y o using a con inuous annealing
app oach ins ead o con en ional ba ch annealing o pe o m an in e c i ical annealing (IA)
ea men in a ho - olled s ip o an Al-alloyed 5Mn s eel. Dila ome ic s udies we e pe o med
a a empe a u e o 680 ºC wi h soaking imes anging om 1 o 300 min o ollow he
mic os uc u al changes as a unc ion o ime. The mic os uc u es hus ob ained we e
ho oughly cha ac e ized by means o X- ay diff ac ion, SEM and TEM, TEM-EDS
mic oanalysis and EBSD phase and o ien a ion maps. I was obse ed ha wi h inc easing
soaking imes, he olume ac ion o e ained aus eni e g adually inc eases, albei a he cos o
i s s abili y. The compa ison o ma ensi e s a empe a u es (M
s
) based on he chemical
composi ion o aus eni e a 680 ºC wi h ha expe imen ally ob ained a highe p ocess
empe a u e e ealed he effec o he g ain size on he educ ion o RA s abili y o longe
p ocess imes. Acco dingly, mechanical es s esul s showed ha he yield s ess, ensile s eng h
and ha dness dec ease wi h an inc ease in he IA soaking ime.
h ps://doi.o g/10.1007/s11661-023-06998-x
The Au ho (s) 2023
I. INTRODUCTION
STEELS con aining e ained aus eni e (RA) a e
becoming inc easingly a ac i e o he au omo i e
sec o , as new ehicle sa e y and uel economy s anda ds
manda e he use o alloys ha combine high duc ili y
and high s eng h.
[1]
Such s eng h-duc ili y combina-
ion may be ob ained by s ain-induced ma ensi ic
ans o ma ion (SIMT) o e ained aus eni e, which
enhances s ain ha dening.
[2,3]
The use o mul iphase
s eels con aining RA allows dec easing ehicle mass
since complica ed pa s cha ac e ized by simila s eng h
and s iffness may be p oduced om hinne bu s onge
me al shee s. Mo eo e , i some ac ion o RA emains
a e he o ming ope a ions, i may be used o any
subsequen e en p o iding be e ene gy abso p ion
p ope ies.
[4,5]
Re ained aus eni e s abili y is a key ac o
con olling he kine ics o SIMT and, he e o e, he
mechanical beha io o mul iphase s eels. I depends on
nume ous ma e ial and s uc u al ac o s as: chemical
composi ion,
[6]
mo phology,
[7]
g ain size,
[8]
s eng h o
su ounding phases,
[9]
and e en c ys allog aphic ex-
u e.
[10]
The mos ecen g oup o alloys specifically
designed o ha e high ac ions o s able RA in he
mic os uc u e a e he medium-Mn s eels, which a e he
nex s ep de elopmen a e he high manganese s eels.
The la e ones, despi e con aining ully aus eni ic
mic os uc u e which p o ided he excellen combina-
ion o p ope ies, u ned ou o be oo expensi e o be
used in he indus y. Medium-Mn s eels ypically
con ain 3 o 12 w pc o Mn wi h a ca bon con en o
up o 0.2 w pc .
[7]
The addi ion o Mn is in ended o
imp o e he s abili y o he aus eni e by educing he M
s
empe a u e a a lowe ma e ial cos .
ADAM SKOWRONEK and ADAM GRAJCAR a e wi h he
Depa men o Enginee ing Ma e ials and Bioma e ials, Facul y o
Mechanical Enginee ing, Silesian Uni e si y o Technology,
Kona skiego 18a S ., Gliwice 44-100, Poland. Con ac e-mail:
[email p o ec ed] CARLOS GARCIA-MATEO and JOSE
´A.
JIME
´NEZ a e wi h he Na ional Cen e o Me allu gical Resea ch
CENIM-CSIC, A . De G ego io del Amo 8, Mad id 28040, Spain.
ROUMEN H. PETROV is wi h he Resea ch G oup Ma e ials Science
and Technology, Depa men o Elec omechanical, Sys ems and
Me al Enginee ing, Ghen UE
´ni e si y, Tech Lane Ghen Science
pa k-Campus A Zwijnaa de, Technologiepa k 46, Ghen 9052,
Belgium and also wi h he Depa men o Ma e ials Science and
Enginee ing, Del Uni e si y o Technology, 3mE-TUDel , Building
34, Mekelweg 2, Del 2628 CD, The Ne he lands.
Manusc ip submi ed Oc obe 27, 2022; accep ed Janua y 31, 2023.
A icle published online Feb ua y 20, 2023
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, APRIL 2023—1443
Cu en ly, he IA hea ea men allows ob aining he
la ges ac ion o s able RA in he mic os uc u e o he
s eel.
[7]
This enables o imp o e hei plas ic p ope ies
significan ly in compa ison o he momechanically p o-
cessed s eels.
[11,12]
IA consis s o a single aus eni iza ion
in he a+c egion, whe e a wide ange o phase a io
and chemical elemen s edis ibu ion may be ob ained.
The wo mos impo an pa ame e s o he p ocess a e
aus eni iza ion empe a u e and soaking ime. Bo h
pa ame e s con ol he aus eni e ac ion and he
diffusion ange o C and Mn, which a e a he same
ime he wo mos impo an elemen s in defining he
s abili y o he RA.
[13]
In gene al, an inc ease o he
aus eni iza ion empe a u e inc eases he aus eni e ac-
ion and accele a es i s o ma ion.
[14,15]
Howe e , he e
comes a poin when he o e all C and Mn concen a ion
in s eel and he diffusion a e canno keep up wi h he
apid inc ease in he RA ac ion, esul ing in a poo
s abili y, and a o ing ma ensi ic ans o ma ion du -
ing cooling, which de e io a es duc ili y. The e o e,
many o he IA op imiza ion a emp s ha e ocused on
long- ime ba ch annealing a mode a e empe a u es
be ween 580 C and 650 C, depending on he chemical
composi ion.
[16–19]
This hea ea men equi es a lo o ime and ene gy,
and he e o e, effo s should be made o op imize he use
o in e c i ically ea ed medium-Mn s eels in he mo e
ene gy efficien con inuous annealing line. Fo his
eason, i is necessa y o in es iga e he mic os uc u al
e olu ion o medium-Mn s eels o e a wide ange o
sho p ocessing imes, bu a ele a ed empe a u es, o
de e mine whe he such a combina ion o p ocessing T/
allows good mechanical p ope ies o be main ained.
Fu he mo e, he use o highe Al con en in
medium-Mn-con aining s eels no only widens bu also
inc eases he in e c i ical in e al owa ds highe em-
pe a u es,
[20]
hus p omo ing an inc ease o he ans-
o ma ion and diffusion a e.
[14]
This no el app oach,
combining he inc ease o he p ocessing empe a u e
wi h an app op ia e chemical design o he s eel, can
u he sho en he du a ion o he IA ea men .
Addi ionally, an inc eased con en o Al in med-
ium-Mn s eels widens he in e c i ical ange owa ds
highe empe a u es,
[20]
p omo ing inc eased ans o -
ma ion a e and as e diffusion.
[14]
Such no el
app oaches combining he inc eased p ocess empe a-
u e and p ope chemical design o he s eel may u he
sho en he IA p ocess. The e o e, he aim o his wo k
is o in es iga e he changes and dependencies in RA
ac ion, i s s abili y, and he co esponding mechanical
p ope ies o Al- ich 5 pc Mn s eel as a unc ion o he
soaking ime.
II. EXPERIMENTAL PROCEDURE
A. Ma e ial and Hea T ea men
The chemical composi ion o he Al-alloyed med-
ium-Mn s eel used in his in es iga ion is gi en in
Table I. Chemical composi ion was de e mined by
op ical emission spec oscopy (OES) a e ho olling
and quenching a 2/3 o he shee hickness, 4.5 mm, as
he mos ep esen a i e a ea, no influenced by possible
mic oseg ega ion. The addi ion o 1.6 w . pc Al, whose
main ole in he s eel is o p e en ca bide p ecipi a ion,
addi ionally causes a widening o he in e c i ical
empe a u e ange om 648 C(A
C1
) up o 924 C
(A
C3
).
[14]
Fu he de ails in he alloy design and p o-
duc ion p ocess s eps a e a ailable in a p e ious
wo k.
[14]
The fi s s age o in es iga ions conce ning
he influence o soaking empe a u e showed ha he
op imal mic os uc u e condi ion con aining 35 pc o
s able RA we e ob ained a 680 C o 60 min o p ocess
du a ion.
[14]
This empe a u e was he e o e adop ed in
he p esen s udy, whe e he a en ion is ocused on he
effec o he soaking ime be ween 1 o 300 min in o de
o ully desc ibe he changes and dependencies aking
place in he ma e ial.
The kine ics o aus eni e o ma ion du ing IA, and
ma ensi ic ans o ma ion upon subsequen cooling o
oom empe a u e, we e s udied by dila ome y using
cylind ical samples o 4 mm diame e and 10 mm leng h
in a high esolu ion dila ome e , Bah 805A. This de ice
enables o s udy phase ans o ma ion by moni o ing
he ela i e change in leng h (RCL) o he sample using
a Linea Va iable Diffe en ial T ansduce (LVDT) in
con ac wi h used silica push ods. The dila ome e uses
an induc ion coil and helium blown sys em o hea and
cool he sample. The empe a u e is con olled h ough-
ou he expe imen wi h an S- ype he mocouple welded
a mid leng h o he sample su ace. The dila ome ic
da a we e analyzed acco ding o ASTM A1033-04.
The ini ial ma ensi ic mic os uc u e p esen in he
ho - olled pla e, was subjec ed o he mal ea men s
consis ing o hea ing a 3 C/s up o an IA empe a u e
o 680 C, hen holding o 1, 5, 15, 60, 120, 300 min
and finally cooling o oom empe a u e a 60 C/s. The
e olu ion o e i e and aus eni e ac ion a he IA
empe a u e as a unc ion o he soaking ime was
calcula ed om dila ome ic esul s using a me hod
p oposed in p e ious s udy.
[14]
B. Mic os uc u al Cha ac e iza ion
The mic os uc u es ob ained a e he diffe en p o-
posed IA ea men s we e e alua ed by means o
scanning elec on mic oscopy (SEM), ansmission elec-
on mic oscopy (TEM), and X- ay diff ac ion (XRD).
Specimens o SEM and XRD we e p epa ed ollowing
s anda d me allog aphic p ocedu es ha included
mechanical g inding wi h SiC pape up o 2000 g id
and polishing wi h 3 and 1 lm g ain size diamond pas e.
A final polish using a 35 nm colloidal silica suspension
Table I. Chemical Composi ion o he S eel Ho -Rolled Band
Used o This In es iga ion
Chemical elemen , w pc
C Mn Al Si Mo Fe-balance
0.16 4.7 1.6 0.20 0.20 93.12
1444—VOLUME 54A, APRIL 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A
(OPS) was included o ensu e a fla su ace wi h minimal
de o ma ion. Fo SEM obse a ions a 3 pc Ni al
solu ion was used o e ch he polished specimens o ~4s
a oom empe a u e o de elop he mic os uc u al
ea u es. SEM obse a ions we e pe o med wi h a FEI
Quan a FEG 450 SEM equipped wi h an ene gy-dispe -
si e X- ay spec ome e (EDS) and an elec on backsca -
e diff ac ion (EBSD) de ec o . As he s uc u e p esen
in he ma e ial exhibi s la h-like mo phology, he la h
hickness was measu ed ins ead o he g ain diame e o
desc ibe he mic os uc u e and o unde s and ma e ial
p ope ies such as ha dness and s ain–s ess ela ionship.
RA la h hickness was de e mined by a leas 200 linea
in e cep s on SEM mic og aphs o ob ain eliable s a is-
ics. EBSD measu emen s we e pe o med wi h SEM
ope a ing a 20 kV and a p obe cu en o ~2.5 nA. All
samples we e il ed a 70º and obse ed a a wo king
dis ance o 15 mm o maximize backsca e ed elec ons.
EBSD mapping has been ca ied ou in a hexagonal scan
g id on an a ea o abou 40 940 lm
2
a s ep sizes o
70 nm. The OIM-TSLda a Analysis so wa e .7.31
has been used o da a p ocessing. A wo-s ep clean-up
p ocedu e was pe o med o emo e he bad da a poin .
In a fi s s ep, i was es ablished a g ain ole ance angle o
5and a minimum g ain size g ea e han 2 pixels. Finally,
in he second s ep, all poin s wi h CI (confidence
index) <0.1 we e emo ed om he measu emen as
no being obus .
The olume ac ion o RA was de e mined om he
XRD pa e ns eco ded wi h Co adia ion (kKa
1
=
0.1789 nm) in a Panaly ical X’Pe P o MPD diff ac-
ome e ) equipped wi h a PIXcel 3D-de ec o in he
diffe en hea ea ed ma e ials and in he uni o m
de o ma ion a ea o he samples es ed in ension o
ailu e. Con en ional h o 2hscans we e pe o med o e
a2h ange o 45 o 105 deg wi h a s ep size o 0.026 deg
and scan speed o 0.6 s/s ep. The collec ed diff ac ion
pa e ns we e analyzed using he e sion 3.0 o he High
Sco e Plus so wa e (Panaly ical). The RA ac ion as
well as i s la ice pa ame e we e de e mined using he
Rie eld efinemen me hod and he c ys allog aphic
in o ma ion o e i e and aus eni e.
[21]
The C concen-
a ion in RA was calcula ed om he aus eni e la ice
pa ame e using he ollowing exp ession.
[22]
ac¼3:556 þ0:0453xCþ0:00095xMn þ0:0056xAl;½1
whe e acis in A
˚,xC,xMn and xAl a e elemen s
concen a ion in aus eni e (w pc ).
The samples o XRD measu emen s o he RA
ac ion a e uniaxial ensile de o ma ion we e cu om
uni o m de o ma ion a eas o he ensile samples.
TEM obse a ions we e pe o med in an S/TEM
TITAN 80–300 mic oscope ope a ing a 200 kV in
lamellas o app ox. 8 98lm
2
cu along he olling
di ec ion wi h ocused ion beam (FIB). Semiquan i a i e
mic oanalyses by ene gy-dispe si e X- ay spec oscopy
(EDS) using TEM we e pe o med o e alua e changes
in Mn and Al concen a ions in RA as a unc ion o he
soaking ime. Fo his goal, a leas wen y measu e-
men s we e done in andom selec ed a eas o diffe en
g ains. F om hese esul s he heo e ical M
s
empe a u es we e de e mined using he ollowing
equa ions.
[23–25]
Ms¼561 474 C33 Mn 21 Mo ½2
Ms¼545601:2ð1Exp 0:868CðÞÞ34:4Mn
13:7Si15:4Mo1:4Al
½3
Ms¼273 545:8e1:365C34:4Mn 7:5Si þ30
AlÞ
½4
C. Mechanical P ope ies
The dependence o he mechanical p ope ies wi h
soaking ime was e alua ed a oom empe a u e by
means o ha dness and ensile es s. Vicke s ha dness
measu emen s we e pe o med wi h a mic oha dness
es e FB-700 a a load o 1 kg . Ten measu emen s o
each sample we e aken and he mean ha dness de e -
mined a e disca ding he wo ex eme alues. Sub-sized
fla ensile samples wi h a gauge leng h o 40, 6 mm wide
and 2.5 mm hick we e used. They we e machined om
he cen al pa o ho - olled shee along he olling
di ec ion. These samples we e subjec ed o IA ea -
men s a 680 ºC in a Nabe he m LT15/12/P330 muffle
u nace unde a gon a mosphe e wi h he same holding
ime used in dila ome y es s. The ensile p ope ies
we e de e mined acco ding o ASTM E8 s anda d om
s a ic ensile es pe o med a a nominal s ain a e o
10
–3
s
1
on a uni e sal es ing machine fi ed wi h an
ex ensome e .
III. RESULTS AND DISCUSSION
A. Fo ma ion and Chemical S abili y o RA
The ela i e change in leng h (RCL) measu ed by
dila ome y du ing he iso he mal IA ea men a 680º
C es s clea ly show a p og essi e con ac ion o he
sample wi h inc easing holding ime, as obse ed in
Figu e 1(a), which is mainly due o he aus eni e- e-
e ed- ans o ma ion (ART) o ma ensi e, and o a
lesse ex en o he dissolu ion o cemen i e pa icles in o
his phase. Medium-Mn s eels a e some imes suscep ible
o seg ega ion o his elemen du ing solidifica ion and
consequen ly he RCL measu ed du ing he IA may
change om sample o sample.
[26]
As he a e age
kine ics o aus eni e- e e ed- ans o ma ion measu ed
in se e al dila ome ic samples shows high consis ency,
i was deduced ha he ma e ial used o his s udy does
no p esen s ong mic oseg ega ion o alloying ele-
men s. F om same Figu e 1(a), i is possible o deduce
ha RCL dec eased p og essi ely and 50 and 90 pc o
he e-aus eni iza ion ans o ma ion was eached
a e ~4 min and ~50 min, espec i ely. This figu e also
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, APRIL 2023—1445
shows a pla eau in he RCL cu e a e ~160 min
associa ed wi h he comple ion o he ans o ma ion,
which is mo e han 3 imes longe ha he ime needed
o an IA a 720 C as epo ed in a p e ious s udy.
[14]
This esul shows he g ea impo ance o annealing
empe a u e and ime in he kine ics o aus eni e
o ma ion du ing IA ea men s.
The aus eni e ac ion should inc ease as he anneal-
ing empe a u e and/o ime inc ease, and his phase
mus be e ained a oom empe a u e o ensu e an
excellen combina ion o s eng h and duc ili y. As
shown in Figu e 1(b) i is obse ed ha ega dless o he
holding ime used, when cooling o oom empe a u e,
he dila ome ic cu es emain linea and no ma ensi ic
ans o ma ion is de ec ed in any case. The
de ec able diffe ences in he slopes o he abo e men-
ioned cu es a e explained by he diffe ences in he
he mal expansion coefficien s o he diffe en
mic os uc u es ob ained (diffe en aus eni e/ e i e
a ios) du ing he IA ea men s. Acco ding o he
me hod p oposed in a p e ious wo k,
[14]
he heo e ical
ac ion o high empe a u e aus eni e has been calcu-
la ed om he dila ome ic cu es (Figu e 2(a)) and
compa ed o he expe imen al da a (XRD). A good
ag eemen be ween he esul s by bo h me hods is
obse ed. This figu e shows he e en ion o a consid-
e able amoun o aus eni e (abou 16 pc ) in he
mic os uc u e a e jus 1 min o annealing a 680 ºC.
The a e o ans o ma ion o aus eni e inc eases apidly
du ing he fi s 15 min (33 pc ac ion), and hen
dec eases d as ically, needing ~160 min o each a final
ac ion o ~41 pc .
As an icipa ed, he s abili y o RA is closely ela ed o
i s ca bon and manganese con en , which has been
de e mined om he aus eni e la ice pa ame e using
Eq. [1] and by dispe si e ene gy spec ome y (EDS)
measu emen s in TEM, espec i ely. The measu ed
change in concen a ion o bo h elemen s as a unc ion
o he soaking ime is shown in Figu e 2(b). The ca bon
concen a ion in RA (Figu e 2(b)) is he highes a e
Fig. 1—Dila ome ic esul s showing: (a) he e olu ion o he ela i e change in leng h (RCL) as a unc ion o he soaking ime; g een, ed, and
iole do s ma k he soaking ime equi ed o comple e 50, 90 and 100 pc , espec i ely, o he ans o ma ion and (b) dila ome ic cu e, RCL,
ob ained on cooling o RT a e he diffe en IA ea men s (Colo figu e online).
Fig. 2—Influence o he soaking ime on he e olu ion o : (a)
heo e ical and expe imen al aus eni e ac ion calcula ed om
dila ome ic cu es and XRD da a, espec i ely; (b) Mn and C
con en s in aus eni e de e mined om EDS spec a and aus eni e
la ice pa ame e , espec i ely; and (c)M
s
empe a u e acco ding o
Eqs. [2–4] (Colo figu e online).
1446—VOLUME 54A, APRIL 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A
1 min o annealing (0.74 w pc ) and dec eases o 0.39 w
pc a e 5 h, ollowing he aus eni e o ma ion eco ded
by dila ome y. The C concen a ion in he aus eni e
dec eases apidly in he fi s 15 min o iso he mal
soaking as he g ea es amoun o aus eni e o ms
apidly up o ha poin . In case o he Mn concen a ion
(Figu e 2(b)), he e is a sha p inc ease in he ea ly s age
o IA om 4.8 o 6.5 pc a e jus 1 min. Such in ense
en ichmen in a sho soaking ime was measu ed also
by o he esea che s.
[27,28]
As he soaking ime inc eases,
he de ec ed en ichmen a e becomes mo e sluggish,
eaching a Mn concen a ion o ~8 w pc a e 5 h o
IA ea men . The diffe en beha io o C and Mn
pa i ioning can be explained by he diffe ences in hei
diffusion a es, which a 680 ºC o he o me is much
as e (~10
–10
m
2
s
1
in e i e and ~10
–13
m
2
s
1
in
aus eni e)
[29]
han in he la e (~10
–16
m
2
s
1
in e i e
and ~10
–19
m
2
s
1
in aus eni e),
[29]
he e o e, enabling
he C concen a ion o ollow closely he RA ac ion
changes,
[30]
while changes in Mn a e mo e sluggish.
[31]
Mn concen a ion in aus eni e inc eases sligh ly e en
a e 5 h o annealing a 680 C, while ca bon achie es a
maximum alue and s a s o d op a e only 1 min.
The de ec ed a ia ion o C and Mn in e ained
aus eni e mus eflec in i s s abili y agains ma ensi ic
ans o ma ion. Figu e 2(c) shows he e olu ion o he
M
s
calcula ed using Eqs. [2–4] and using RA composi-
ion gi en in Figu e 2(b). I is obse ed ha , al hough
he M
s
alues diffe depending on he o mula used, he
o e all endency is almos iden ical. Despi e he sho
soaking ime, a e only 1 min o soaking ime aus eni e
exhibi s he highes chemical s abili y, lowes M
s
em-
pe a u e, due o he e y as en ichmen in ca bon
(~0.74 pc ) and manganese (~6.5 pc ) o i s ela i ely
low ac ion (16 pc ). Du ing he fi s 15 min o
annealing he M
s
empe a u e inc eases significan ly
which is expec ed o cause a d op o he he mal s abili y
o he e ained aus eni e. The eason is ha despi e he
appa en inc ease in manganese con en o 7.4 pc , he
s abili y canno be main ained because o as dec easing
C concen a ion in an inc easing RA ac ion (32 pc ).
A e p olonged soaking imes he men ioned in e play
o as diffusing C and slow diffusing Mn allows o
main ain e y simila s abili y o he e ained aus eni e,
M
s
empe a u e, up o 5 h.
The M
s
empe a u e changes p esen ed in Figu e 2(c)
we e calcula ed only on he basis o changes in he
chemical composi ion o aus eni e. I should be emem-
be ed ha RA s abili y and he M
s
empe a u e a e also
influenced by o he ac o s besides chemical composi ion
which we e men ioned in he in oduc ion sec ion. Fo
his eason, ano he IA a ian wi h iden ical p ocess
pa ame e s bu a highe IA empe a u e, 760 C, was
es ed using dila ome y o eco d ac ual changes in M
s
empe a u e. The calcula ed aus eni e ac ion a 760 C
inc eases o 38 and 69 pc a e 2 s and 1 h o soaking,
espec i ely, a e which i emains almos cons an . A
ele a ed empe a u e, all p ocesses aking place du ing
soaking a e accele a ed and aus eni e ac ion is
highe .
[14]
The changes in aus eni e ac ion a e simila
as a 680 C, howe e , o sho e soaking ime. The M
s
empe a u e again is he lowes o he lowes RA ac ion
despi e e y sho soaking ime (2 second) and inc eases
wi h soaking du a ion. Wha is impo an he e, e en a e
he e i e–aus eni e equilib ium is ob ained a e 60 min
o soaking ea men , is ha he M
s
empe a u e ends o
con inuously inc ease (Figu e 3) wi h soaking ime,
which diffe s om he obse ed endency a 680 Co
annealing empe a u e (Figu e 2(c)). As measu ed, he C
concen a ion ollows closely changes in aus eni e ac-
ion and e y quickly eaches equilib ium, whe eas Mn
diffuses o aus eni e con inuously (Figu e 2(b)). The e-
o e, o cons an ac ion o aus eni e in he mic os uc-
u e, he M
s
empe a u e should also be cons an o g ow
sligh ly i we conside only he influence o chemical
composi ion o aus eni e. The g ow h endency o expe -
imen al M
s
shows he influence o ac o s o he han
chemical composi ion as g ain size o ma ix s eng h, on
aus eni e s abili y.
The local edis ibu ion o Mn in samples soaked a
680 C has also been de e mined om EDS da a
ob ained using a spec ome e a ached o an SEM
and TEM. Bo h, TEM elemen al line p ofiles and
SEM–EDS mappings in Figu e 4show simila e olu ion
o he Mn con en wi h soaking ime. Clea and
significan changes in manganese concen a ions in
e i e and aus eni e egions we e eco ded. Manganese
and ca bon edis ibu ed o he aus eni e, while we e
deple ed in e i e (Figu e 4). Aus eni e manganese
en ichmen is ai ly uni o m in a e 1 min o soaking
(Figu e 4(a)). By inc easing he soaking ime o
300 min, u he inc ease i s deg ee o en ichmen
al hough i became mo e he e ogeneous, deno ed by
peaks o he Mn concen a ion p ofiles close o he
e i e/aus eni e in e aces,
[32]
see Figu e 4(b). This en-
dency is e en s onge i he RA is adjacen o coa se
e i ic g ains and is caused by significan ly as e Mn
diffusion in e i e han in aus eni e.
[33]
A apid deple-
ion o e en la ge e i e g ains in o manganese and
ca bon occu s. Howe e , hese elemen s accumula e
close o he aus eni e g ain bounda y, which is con-
fi med by o he esea che s.
[34]
B. Mic os uc u e E olu ion
The o ma ion o aus eni e du ing IA o ho - olled
medium manganese s eels wi h ma ensi ic ini ial
mic os uc u e occu s in an aus eni e e e se
Fig. 3—The changes in aus eni e ac ion (A) and co esponding
expe imen al M
s
a 760 C o diffe en soaking ime de e mined
om he dila ome ic cu es (Colo figu e online).
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, APRIL 2023—1447

ans o ma ion (ART) mode by he g ow h o in e la h
aus eni e nuclea ed along ma ensi e la h bounda ies.
[35]
The newly o med phase he e o e inhe i s he la h-like
mo phology p esen in he pa en ma ensi e
(Figu e 5(a)) leading o a duplex mic os uc u e wi h
aus eni e and e i e lamella (Figu e 5(b)-(d)). As
desc ibed p e iously, he aus eni e has an ini ial apid
g ow h and a e 1 min o annealing a ela i ely high
olume ac ion o hin la h-like e ained aus eni e
dis ibu ed be ween coa se empe ed ma ensi e la hs is
ob ained, as shown in Figu e 5(b). On he o he hand,
his mic os uc u e also included a significan amoun o
ca bide pa icles p ecipi a ed in he cen al a ea and a
he la h and g ain bounda ies o empe ed ma ensi e,
whe e he C and Mn a oms end o seg ega e.
[36]
The
supe sa u a ed C in ma ensi e in s a ing
mic os uc u es pa i ioned in o he in e c i ical aus en-
i e and also p ecipi a ed as ca bide pa icles du ing he
fi s s age o he IA ea men (Figu es 5(b) and (c)).
Figu e 6shows some ca bide pa icles a highe
magnifica ion and he diff ac ion pa e n om an a ea
con aining one o hese ca bide pa icles, which is
compa ible wi h cemen i e. These cemen i e pa icles
a e almos comple ely dissol ed a e 15 min o IA and
he olume ac ion o RA inc eased up o 33 pc .
Al hough he amoun o aus eni e has doubled as he
soaking ime inc eases o 15 min, he ca bon con en o
his phase has only been educed by 0.26 pc due o he
dissolu ion o he cemen i e pa icles. The a e o
aus eni e o ma ion o soaking imes longe han
60 min d ama ically dec eased. Fo soaking imes
be ween 120 and 300 min, he RA con en emains
Fig. 4—EDS esul s: TEM mic og aphs wi h Mn elemen al line ob ained in sample annealed o (a) 1 min and (b) 300 min, (c) SEM
mic og aphs o a sampled annealed o 300 min oge he wi h (d) Mn and (e) C mappings ob ained in he same zone (Colo figu e online).
1448—VOLUME 54A, APRIL 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A
p ac ically cons an . Mo eo e , his longe holding ime
esul s in a clea agmen a ion o hick e i ic la hs,
which leads o a homogeneous wo-phase s uc u e
obse ed in Figu e 5(d). This figu e shows ha inc eas-
ing he soaking ime esul s in an inc ease o he mean
la h hickness o RA and a widening o he la h
hickness dis ibu ion (Figu e 5(e) and ( )). The mos
significan inc ease in he la h hickness (abou 40 pc )
occu s wi hin he fi s 15 min o IA, wi h an a e age la h
hickness inc easing om 107 ±30 o 150 ±55 nm,
when he soaking ime is inc eased om 1 o a e
15 min, espec i ely. Figu e 4(e) shows an almos linea
inc ease o he aus eni e la h hickness o longe
soaking imes, eaching a alue o 240 ±90 nm a e
5 h. Wang and Van de Zwaag showed ha dec easing
he RA size inc eases he numbe o ma ensi e nuclei
needed o ob ain a gi en ma ensi e ac ion and hus
inc eases i s s abili y.
[37]
I is caused by RA bounda ies
ac ing as obs acles o ma ensi e g ow h. In gene al, all
p oduced mic os uc u es show a e y fine mic os uc-
u e, which should s abilize RA much mo e han
sugges ed om he calcula ed alues o M
s
Fig. 5—SEM images o he mic os uc u e p esen in (a) ini ial sample, a e soaking a 680 ºC o (b) 1 min, (c) 15 min, (d) 300 min, and
e olu ion wi h soaking ime o (e) a e age la h hickness and ( ) equency o hickness edis ibu ion; RA – e ained aus eni e; M – ma ensi e;
M
T
– empe ed ma ensi e; F – e i e; h– ca bides (Colo figu e online).
Fig. 6—TEM mic og aphs o he sample ea ed o 1 min (a) b igh field image; (b) diff ac ion pa e n om he a ea ma ked by a da k blue
ci cle (Colo figu e online).
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, APRIL 2023—1449
(Figu e 2(c)). The inc ease in RA la h hickness and
ma ix empe ing wi h soaking p og ess, howe e , a e
ac o s dec easing he ela i e RA s abili y sligh ly,
which was e idenced by expe imen al M
s
da a measu ed
o he ea men a 760 C (Figu e 3). The RA la h
hickness e olu ion may also explain why he a e age
en ichmen o RA in Mn is so high jus a e 1 min o
IA. The Mn en iches as hin laye s, whe e he diffusion
dis ance in RA is sho (Figu e 4(a)). Fu he en ich-
men is inhibi ed by limi ed diffusion a e in hickening
la hs (Figu e 5(e)), hence, he inc ease in he a e age Mn
concen a ion slows down significan ly as soaking ime
inc eases (Figu e 2(b)). I should be no ed ha all
samples wi h up o 120 min o IA con ain some ac ion
o RA la hs wi h a hickness o £50 nm (Figu e 5( )),
while o he 300 min sample he smalles g ains
a e >100 nm hick. This may p o e ha new aus eni e
g ains nuclea e almos du ing he en i e applied soaking
ime, which esul s in a educ ion and agmen a ion o
coa se e i ic g ains as obse ed in he SEM images
(Figu e 5). The RA la h hickness dis ibu ion widens
wi h inc easing soaking ime, which should posi i ely
affec he kine ics o SIMT and should allow an inc ease
in he plas ici y o he ma e ial by means o a g adual
ans o ma ion o RA since he la ge g ains, less
mechanically s able, will p omp ly ans o m o ma en-
si e a he beginning o plas ic de o ma ion, while he
fine g ains, mo e mechanically s able, will ans o m as
u he plas ic de o ma ion is applied.
EBSD analysis (Figu e 7) indica es ha aus eni e
g ains nuclea e fi s a bounda ies o ma ensi ic la hs.
Wi h con inued soaking, he aus eni e g ows along he
ma ensi ic la hs and s a s o su ound hem and ge s
hicke as he RA ac ion inc eases (Figu es 7(b)-( )).
EBSD maps o samples ea ed o 1 and 5 min show
an unde es ima ed ac ion o RA since he e y
hin <50 o 70 nm aus eni e la hs o med du ing he
sho p ocess du a ion (Figu es 5(b) and ( )) a e no
esol ed by EBSD, since he su ounding phase in e -
e es wi h hei diff ac ion pa e ns leading o low IQ
alues a hese loca ions. The g ain miso ien a ion
(Figu e 8(a)) dis ibu ion o he ini ial sample co e-
sponds o ha o a ypical ma ensi ic mic os uc-
u e.
[38,39]
The e a e isible peaks o low angle (5 deg >)
and high angle (>55 deg) bounda ies ela ed o
inc eased disloca ion densi y
[40]
and mu ual o ien a ion
o ma ensi ic la hs, espec i ely.
[38,39]
Wi h an inc ease
in aus eniza ion ime, he a o emen ioned peaks ade
ou (Figu e 8(a)), while a as inc ease in he numbe o
g ains wi h a miso ien a ion o ~45 deg is isible
(Figu e 8(b)), which ep esen he a/co ien a ion ela-
ionship.
[38]
Based on he EBSD phase map (Figu e 7)
and miso ien a ion esul s (Figu e 8(b)) i can be
assumed ha each single e i ic g ain is su ounded
by RA wi h an a e age o 60 pc a e 5 min o soaking
and almos 80 pc a e 5 h, ensu ing a e y homoge-
neous mic os uc u e. This is a diffe en ela ionship
han in he case o IA a e cold olling, whe e bo h he
e i e and aus eni e g ains a e o en loca ed nex o each
o he in a andom manne .
[7,36]
This endency may
influence he diffe en mechanical beha io o bo h
ypes o medium-Mn s eels.
Fig. 7—EBSD phase maps o samples soaked o diffe en imes (Colo figu e online).
1450—VOLUME 54A, APRIL 2023 METALLURGICAL AND MATERIALS TRANSACTIONS A
C. Mechanical P ope ies
In a s eel, each phase p esen in he mic os uc u e
con ibu es o he o e all mechanical p ope ies o he
agg ega e. Thus, hei ha dness and ensile p ope ies
a e de e mined by con ibu ion o he ma ix ( e i e,
baini e, ma ensi e o a combina ion o hem) and he
olume ac ion, shape, and mechanical s abili y o
aus eni e. All hese mic os uc u al pa ame e s can be
modified in a ho - olled band pla e by adap ing he ime
and/o empe a u e used o he IA ea men , making
possible he ailo ing o he mechanical p ope ies o
hese ma e ials. The ha dness o hea ea ed samples
(Figu e 9) dec eases wi h p olonged aus eni iza ion ime
om 318 ±10 HV1 a e 1 min o 279 ±8 HV1 a e
5 h, which is ela ed o ma ensi e empe ing and
inc ease o he RA ac ion. The same endency is also
obse ed in he epo ed YS alues, which dec ease om
859 ±16 MPa o 591 ±11 MPa o he same ime
ange as abo e (Figu e 10). Since he ha dness and yield
s ess o he empe ed ma ensi e is highe han ha o
aus eni e, he a e age ha dness alues o he s eel will
dec ease as he olume ac ion o RA inc eases. The
dec ease o he UTS is much smalle han o he YS
( om 938 ±18 o 890 ±20 MPa) due o in ense SIMT
in he samples soaked o mo e han 15 min. Such a
esul was an icipa ed as he p olonged soaking causes
an inc ease in he ac ion o RA wi h lowe s abili y,
whose ans o ma ion p omo es an enhanced s ain
ha dening.
[2]
None o he samples exhibi discon inuous
yielding due o he specific la h-like mo phology. Bo h
uni o m elonga ion (UEl) and o al elonga ion (TEl)
(Figu e 10) o he samples ini ially inc ease wi h inc eas-
ing IA ime demons a ing he highes UEl alue o
19.5 ±0.5 pc ollowing a 15 min ea men and he
highes TEl o 28.3 ±0.6 pc ollowing a 60 min
ea men . Thei duc ili y dec eases sligh ly o final
alues o 16 ±0.4 pc o UEl and 26.5 ±0.8 pc o
TEl a e a ea men o 5 h.
Fo samples ea ed wi hin 1 and 5 min he o e all
s ain ha dening is low due o ema kably high s abili y
and co esponding low M
s
empe a u es, and hin la hs
(<110 ±30 nm) o RA. Also, he RA in hose samples
is su ounded by empe ed ma ensi e, while in he es
o he samples he ma ix is composed o e i e. The
high s eng h o he empe ed ma ensi e ma ix u he
enhances RA s abili y.
[9]
The e o e he influence o his
ac o should be he s onges in he sample ea ed o
only 1 min. High RA s abili y in his sample is con-
fi med by he p esence o 12 pc o his phase in he UEl
egion a e ensile es (Figu e 11). I means ha RA
exhibi ing high s abili y does no ans o m e en unde
Fig. 10—Tensile es esul s o samples annealed o diffe en imes
(Colo figu e online).
Fig. 9—Ha dness (HV1) esul s o samples annealed o diffe en
imes.
Fig. 8—EBSD esul s, (a) dis ibu ion o g ain bounda ies miso ien a ion; and (b) ac ion o g ain bounda ies wi h miso ien a ion o ~45
(Colo figu e online).
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 54A, APRIL 2023—1451