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Materials and Processes for the Third-generation Advanced High-strength SteelsWerkstoffe und Prozesse für AHSS-Stähle der dritten Generation

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The third-generation advanced high-strength steels (AHSS 3. Gen.) have been developed in order to meet the requirements of the automotive industry for weight reduction, improved fuel efficiency, and CO2 mitigation. The alloy design of AHSS 3. Gen., their microstructure characteristics, mechanical properties, potential applications as well as the need for thorough process parameter control are discussed in this paper. The cold-rolled AHSS 3. Gen. can be differentiated as quenching and partitioning (Q&P) steels and medium-Mn steels (MMnS). Specific to these materials are the occurrence of different phases with typical sizes on the nm-scale, the elemental partitioning between these phases and the impact of the metastable austenite phase on the mechanical properties by means of transformation-induced-plasticity (TRIP) as well as of twinning-induced-plasticity (TWIP) effects. The requirements for the process design and the new annealing concepts are emphasized. Furthermore, the industrial feasibility and challenges of production of cold-rolled high-strength steels are discussed. In particular, the development of new furnace technologies is introduced for modern steels with complex microstructures.
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Berg Huettenmaenn Monatsh
https://doi.org/10.1007/s00501-019-00904-y
© Austrian Society for Metallurgy of Metals (ASMET) and
Bergmännischer Verband Österreich (BVÖ) 2019
Materials and Processes for the Third-generation Advanced
High-strength Steels
Wolfgang Bleck1, Fritz Brühl2,YanMa
1, and Caesar Sasse2
1Steel Institute (IEHK), RWTH Aachen University, Aachen, Germany
2SMS group GmbH, Düsseldorf, Germany
Received August 27, 2019; accepted September 18, 2019
Abstract: The third-generation advanced high-strength
steels (AHSS 3. Gen.) have been developed in order
to meet the requirements of the automotive industry for
weight reduction, improved fuel efficiency, and CO2mitiga-
tion. The alloy design of AHSS 3. Gen., their microstructure
characteristics, mechanical properties, potential applica-
tions as well as the need for thorough process parameter
control are discussed in this paper. The cold-rolled AHSS
3. Gen. can be differentiated as quenching and partitioning
(Q&P) steels and medium-Mn steels (MMnS). Specific to
these materials are the occurrence of different phases with
typical sizes on the nm-scale, the elemental partitioning
between these phases and the impact of the metastable
austenite phase on the mechanical properties by means of
transformation-induced-plasticity (TRIP) as well as of twin-
ning-induced-plasticity (TWIP) effects. The requirements
for the process design and the new annealing concepts
are emphasized. Furthermore, the industrial feasibility
and challenges of production of cold-rolled high-strength
steels are discussed. In particular, the development of new
furnace technologies is introduced for modern steels with
complex microstructures.
Keywords: Advanced high-strength steels, Quenching and
partitioning (Q&P) steel, Medium-Mn steel (MMnS),
Multiphase structure, Nanostructure, Elemental
partitioning, Formability, Intelligent furnace concept
Werkstoffe und Prozesse für AHSS-Stähle der dritten
Generation
Zusammenfassung: Hochfeste Stähle der dritten Generati-
on (Advanced High-Strength Steels; AHSS 3. Gen.) wurden
W. Bleck ()
Steel Institute (IEHK),
RWTH Aachen University,
Aachen, Germany
bleck@iehk.rwth-aachen.de
entwickelt, um die Anforderungen der Automobilindustrie
an Gewichtsreduktion, verbesserte Kraftstoffeffizienz und
CO2-Einsparung zu erfüllen. Das Legierungsdesign von
AHSS 3. Gen., ihre Mikrostruktureigenschaften, ihre me-
chanischen Eigenschaftsprofile, mögliche Anwendungen
sowie die Notwendigkeit einer anspruchsvollen Prozesspa-
rameterkontrolle werden in diesem Beitrag diskutiert. Die
kaltgewalzten AHSS 3. Gen. lassen sich in Quenching und
Partitioning Stähle (Q&P) sowie mittel Mn Stähle (MMnS)
unterscheiden. Spezifisch für diese Werkstoffe sind das
Auftreten verschiedener Phasen mit typischen Gefüge-
strukturen im nm-Skalenbereich, das Element-Partitioning
zwischen diesen Phasen und der Einfluss der metastabilen
Austenit-Phase auf die mechanischen Eigenschaften durch
umwandlungsinduzierte Plastizität (TRIP) sowie durch zwil-
lingsinduzierte Plastizität (TWIP). Die Anforderungen an
die Prozessgestaltung und die neuen Glühkonzepte wer-
den hervorgehoben. Zusätzlich werden die industrielle
Umsetzbarkeit und die Herausforderungen bei der Herstel-
lung von kaltgewalzten hochfesten Stählen diskutiert. Der
Fokus liegt hierbei insbesondere auf der Entwicklung neu-
er Ofentechnologien für moderne Stähle mit komplexen
Mikrostrukturen.
Schlüsselwörter: Hochfeste Stähle, Quenching und
Partitioning Stahl, Mittel Mn Stahl, Mehrphasige
Mikrostruktur, Nanostrukturierung, Partitioning-Effekte,
Umformbarkeit, Intelligentes Ofendesign
1. Introduction
Advanced high-strength steels (AHSS) have been devel-
oped within the last decades to meet the growing demands
for weight reduction and improved crashworthiness prop-
erties in the automotive industry [1,2]. The formability and
strength balance of different steel concept is illustrated in
Fig. 1. Dual-phase (DP) and low-carbon transformation-in-
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Fig. 1: Formab ility and
strength balance of different
steel concepts
duced-plasticity (TRIP) steels are generally considered as
the first-generation AHSS (AHSS 1. Gen.). The combina-
tion of the ferritic matrix and second phases offers the
AHSS 1. Gen. high strength with reasonable formability.
The development of high-Mn steels (HMnS) with various
deformation modes like TRIP, twinning-induced plasticity
(TWIP), and microband-induced plasticity (MBIP) have re-
sulted in a better understanding of the strength-formability
balance. The austenitic HMnS are considered as the sec-
ond-generation AHSS (AHSS 2. Gen.). New concepts for
the third-generation AHSS (AHSS 3. Gen.) are currently
evaluated, and these are based on the high Mn content
in combination with nanostructures as well as local parti-
tioning phenomenon. Quenching and partitioning (Q&P)
steel and medium-Mn steel (MMnS) are the most promis-
ing candidates of the AHSS 3. Gen. The typical chemical
TAB LE 1
Alloying concepts of the selected advanced high-strength steels (AHSS) [3–5]
AHSS Steel designation CSi and/or Al Mn
1. Gen Low-C TRIP 0.10–0.30 1.0–2.0 1.0–2.0
2. Gen High-Mn steel 0.10–0.60 0–3.0 >14.0
3. Gen Q&P steel 0.10–0.30 1.0–2.0 1.5–3.0
3. Gen Medium-Mn steel 0.05–0.40 1.0–3.0 3.0–12.0
TAB LE 2
Typical phase constituents of the selected advanced high-strength steels (AHSS) [3–5]
AHSS Steel designation Austenite Ferrite Bainite Martensite
1. Gen Low-C TRIP 5–15 40–60 35–45
2. Gen High-Mn steel 100
3. Gen Q&P steel 5–20 0–20 60–95
3. Gen Medium-Mn steel 20–60 40–80
compositions and microstructures of three generations of
AHSS are summarized in Table 1and 2, respectively [35].
The microstructure design concepts of the AHSS 3. Gen.
aim at obtaining a considerable amount of retained austen-
ite (>20 vol%) in a martensitic/ferritic matrix. To achieve
a sophisticated multiphase structure, complex thermal
processing routes are employed. Q&P is a novel process
to produce martensitic steel with a certain amount of re-
tained austenite by controlling carbon partitioning [6,7].
Austenite-reverted-transformation (ART) annealing brings
new opportunities to produce an ultrafine-grained (UFG)
duplex ferrite-austenite microstructure in MMnS [8]. In the
latter case, the carbon and mainly manganese partitioning
plays an essential role in stabilizing austenite [9].
The combination of the unique characteristics of each
microstructural constituent contributes to the extraordi-
nary mechanical properties of the AHSS 3. Gen. Moreover,
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Fig. 2: Schematic illustration
of heat treatmentprofile of
quenchingand partitioning
process [3,7]
the TRIP effect or/and the TWIP effect in the metastable
austenite improves the strain-hardening behavior effec-
tively, leading to an excellent combination of high strength
and good formability. In this paper, the alloy design con-
cept, microstructure characteristics, mechanical properties
as well as the potential applications of the cold-roll ed AHSS
3. Gen. are summarized. The requirements for the process
design and the new annealing concepts are emphasized.
Furthermore, the industrial feasibility, opportunities, and
challenges of cold-rolled steel grades processed by the
Q&P are discussed.
2. Groups of the Third-generation Advanced
High-strength Steels
In 2003, Speer et al. [6] first proposed a novel pro-
cess—Q&P—to develop martensitic steel with a certain
amount of retained austenite. The austenite is stabilized
mainly by carbon partitioning between martensite and
austenite via a special Q&P process, which is shown in
Fig. 2[3,7]. For cold-rolled Q&Psteel, following austenitiza-
tion, quenching is interrupted at a temperature (quenching
temperature, QT) in between of the martensite start (Ms)
temperature and martensite finish (Mf) temperature. At
QT martensite transformation is not completed, resulting
in a mixture of quenched martensite and untransformed
austenite in the microstructure. Subsequently, an isother-
mal partitioning step is carried out at either the temperat ure
identical to the quenching temperature (one-step Q&P) or
elevated temperature (two-step Q&P). During the parti-
tioning treatment process, carbon atoms diffuse from
carbon-supersaturated martensite into the untransformed
austenite. Consequently, carbon-enriched austenite can
be retained when the steel is finally quenched to room
temperature.
Ultrafine-grained MMnS were first reported by Miller [8]
in 1972. Since the 2010s, the MMnS have attracted inten-
sive interest by materials scientists due to the excellent
combination of high strength and good ductility of MMnS.
A large amount of austenite can be stabilized by carbon
and mainly manganese partitioning in MMnS [9]. The heat
treatment cycle of MMnS consists of austenitization and
ART annealing [10]. Before ART annealing, the steel stripis
heated above A3temperature to achieve a fully austenitic
microstructure. Subsequently, the austenite transforms to
athermal α’-martensite by quenching to room temperature.
Alternatively, conventional rolling and ART-annealing pro-
cesses might be employed to manufacture this steel grade.
Hot rolling is conducted above A3temperature in combi-
nation with an austenitization treatment. During the cold
Fig. 3: Schematic illustration o f austenite-reverted-transformation
(ART) annealing profile of med ium-Mn steel (MMnS) [3]
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rolling process, some retained austenite from hot-rolled
MMnS might transform into martensite. Afterward, the
cold-rolled steel strip is reheated to the intercritical anneal-
ing region between Ac1 and Ac3, and is maintained at this
elevated temperature for some time, followed by cooling
to room temperature, as shown in Fig. 3[3].
3. Microstructure, Elemental Partitioning,
and Mechanical Properties
The AHSS 3. Gen. are characterized by nanostructures, el-
emental partitioning, and the utilization of the additional
deformation modes, i.e. the TRIP and TWIP effects. The
understanding of the sophisticated microstructure devel-
opment, on the one hand, requires the advanced charac-
terization techniques on the nanometer scale; on the other
hand, it promotes new opportunities to adjust microstruc-
ture and tailor mechanical properties by controlling phase
stability and elemental partitioning phenomenon.
3.1 Ultrafine-grained Microstructure
Both Q&P steels and MMnS manifest ultrafine multiphase
microstructures, as illustrated in Fig. 4. The grain sizes
of the different phases of the AHSS 3. Gen. are gener-
ally in the range of 1 µm or below. The microstructure
of Q&P steels consists of martensite, retained austenite,
and ferrite (under partial austenitization condition). The
Q&P process allows for creating a fine acicular microstruc-
ture of lath martensite interwoven with carbon enriched
retained austenite, as displayed in Fig. 4a. For MMnS,
austenite/ferrite duplex microstructures with either lamella
or equiaxed grain morphologies are obtained after ART an-
nealing, as illustrated in Fig. 4b, c [11 ]. In general, the hot-
rolled MMnS inherits the morphology of martensite and
possesses lamella morphology. In contrast, the cold-rolled
MMnS shows equiaxed grain morphology because of the
active recrystallization of martensite.
Fig. 4: Typicalstructures of
the 3rd-generation ad vanced
high-strengthsteels (AHSS
3. Gen.): (a)microstructure of
a quenchingand partitioning
(Q&P) steel, (b) microstruc-
ture of a hot-rolled (HR )
medium-Mn steel (MMnS),
and (c) microstructureof
a cold-rolled(CR) MMnS [11 ]
3.2 Elemental Partitioning Phenomenon
Local elemental partitioning phenomenon becomes ex-
tremely important in the AHSS 3. Gen. and provides new
chances to design multiphase steels with a large amount
of retained austenite.
Carbon partitioning between austenite and martensite
plays a prominent role in Q&P steels, which mainly takes
place during the partitioning step in the relatively low-
temperature range (300 ~ 500 °C). Fig. 5a shows an ex-
ample of elemental partitioning features in a Q&P steel
Fe–1.36C–3.3Si–2.6Mn (at%). It can be seen that there is an
obvious chemical gradient of carbon across the interface
of martensite and retained austenite. Austenite shows
strong enrichment of carbon atoms (approx. 5 at%), while
martensite is depleted of carbon (below 0.5 at%). Conse-
quently, the austenite is thermodynamically stabilized by
the enriched carbon atoms even at room temperature.
In contrast, manganese partitioning between austenite
and ferrite/martensite has a decisive effect on the stabilityof
austenite in MMnS and this phenomenon usually occurs at
a relatively high temperature (550 ~ 750°C) associated with
austenite reverted transformation. The element distribu-
tion in a MMnS Fe–0.3C–11.5Mn–5.8Al (at%) after intercrit-
ical annealing at 700°C is shown in Fig. 5b. It can be seen
that austenite is enriched of manganese and carbon, while
ferrite is enriched of aluminum. The manganese concen-
tration in austenite is approx. 14 at%, while the manganese
concentration in ferrite is approx. 7at%. The strong man-
ganese partitioning results in a duplex microstructure in
the MMnS with about 60 vol% retained austenite at room
temperature.
3.3 Mechanical Properties and the TRIP and
TWIP Effects
The typical tensile properties represented by the yield
strength (YS), ultimate tensile strength (UTS) and total
elongation (TEL) of the selected AHSS are listed in Table 3.
It can be seen that the AHSS 3. Gen. offer an extraordinary
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Fig. 5: Elemental partitioning
behaviors in the3rd-genera-
tion advancedhigh-strength
steels (AHSS 3. Gen.): (a)ele-
mentalpartitioning between
martensite and ferrite in
a quenchingand partitioning
(Q&P) steel; (b)elemental
partitioningbetween ferrite
and austenitein a medium-Mn
steel (MMnS)
combination of high strength and superior formability.
Besides, the tensile properties of AHSS 3. Gen. cover
a large spectrum, which are strongly dependent on the
heat treatment. The activation of additional deformation
mechanisms like TRIP and TWIP beside a dislocation glide
provides a high strain-hardening capacity and promotes an
excellent combination of high strength and good ductility.
Fig. 6a shows the engineering stress—engineering
strain curves of Q&P980 specimens [13]. The evolution
of austenite measured by in situ synchrotron X-ray diffrac-
tion during tensile tests is illustrated in Fig. 6b[13]. The
TAB LE 3
Typical tensile properties of the selected advanced high-strength steels (AHSS) [3, 12]
AHSS Steel designation YS (MPa) UTS (MPa) TEL (%)
1. Gen Low-C TRIP 350–750 600–980 15–30
2. Gen High-Mn steel 500–950 900–1200 20–50
3. Gen Q&P steel 600–1150 980–1300 8–22
3. Gen Medium-Mn steel 400–1150 780–1350 15–60
YS yield strength, UTS ultimate tensile strength, TEL total elongation
initial austenite fraction prior to the tensile test is around
0.10. During deformation, the austenite progressively
transforms into α’-martensite and its fraction declines
to 0.03–0.04 at the end of deformation. The TRIP effect
contributes to the enhanced strain-hardening rate and
improved ductility in martensitic steels.
In MMnS, the TRIP effect also plays a crucial role in the
enhancement of strain-hardening behavior and ductility.
Besides, the TWIP and TRIP effects can occur progressively
during the tensile tests, as illustrated in Fig. 7[14,15]. Dur-
ing mechanical deformation, primary twins are generated
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Fig. 6: Mechanical properties of a commercialquenching and partitioning (Q&P)steel: (a)Engineering stress—straincurve, and (b)fractionofretained
austenite as a function of strain. The test has been repeated with 3 samples, designated as 1,2and 3in the figure [13]
Fig. 7: Microstructure and
mechanical behavior of
a cold-rolled medium-Mn
steel (MMnS): (a) microstruc-
ture prior todeformation (top)
and after deformation (bot-
tom)[14], (b) engineering
stress—strain curve [15]
and followed by secondary twins. The twin intersections
are expected to be the nucleation sites for the following de-
formation induced α’-martensite transformation. The suc-
cession of the TWIP and TRIP effects improves the strain-
hardening capacity of the steel. As it can be seen in Fig. 7b,
the TWIP-TRIP MMnS exhibits an extraordinary combina-
tion of high strength and superior ductility [15].
4. Producing Modern Steel Grades with
Industrial Annealing and Galvanizing
Lines
The furnace technology in high-capacity annealing and gal-
vanizing lines has to be adapted to the various annealing
curves required for the different steel grades. Heating,
soaking, and cooling capacities have to be sufficient and,
in most cases, higher than those in conventional lines. Fur-
thermore, high-strength steel grades require a complex an-
nealing schedule, including reheating and a time window
for elemental partitioning. Some grades also need cooling
or reheating devices to reach the necessary temper ature for
galvanizing.
A comparison of typical annealing curves for different
high-strength steel grades in Fig. 8shows that processing
furnaces for cold strip must offer considerable flexibility
with regard to heat treatment. Equipped with the latest fur-
nace technology, the processing furnaces of SMS group are
capable of producing the latest high-strength steel grades
for the automotive industry. Especially the layout of the
furnace changes to fulfill the requirements of industrial pro-
duction of the modern grades in AHSS 3. Gen. Fig. 9gives
an overview about the most important components and
technical parameters of the third generation galvanizing
line. The detailed description of each section is given be-
low.
4.1 PrOBOX®-technology
The PrOBOX®-technology (pre-oxidation box) has become
widely accepted as the best process for hot-dip galvanizing
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Fig. 8: Annealing scheduleof
advanced high-strength steels
in industrialgalvanizing line
Fig. 9: Technologies in thethird generation galvanizingline
of high-strength steel grades with high silicon and man-
ganese contents. The coating problems incurred in con-
ventional hot-dip galvanizing processes are prevented by
a specific oxidation and reduction process.
4.2 High Soaking Temperature
To achieve the optimal microstructure and mechanical
properties, the high soaking temperature is usually re-
quested for the production of new generation AHSS. The
highly efficient radiant-tube furnaces are equipped with
powerful heating systems to reach the required tempera-
ture up to 900 °C.
4.3 Higher Cooling Capacity
Fast and homogenous cooling with high cooling rates is
possible with UFCplus (ultra-fast cooling). The advanced
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Fig. 10: Intelligent furnace co ncept with X-CAP
gas jet cooling technology allows increasing the hydrogen
content to 50%, ensuring a continuous high cooling rate
from 820/870 °C down to 180/400 °C. The patented hydro-
gen migration technology ensures a controlled migration
of the hydrogen into the adjacent furnace sections in order
to keep the total hydrogen consumption low.
4.4 Reheating and Partitioning
Q&P steels are processed by interrupted quenching in the
temperature range between Msand Mfwith a clearly de-
fined target temperature window of ±10–20K, followed by
subsequent reheating to partitioning treatment. Furnace
concepts for Q&P grades comprise high-speed cooling sys-
tems, subsequent induction reheating modules, partition-
ing space, and optional cooling capacities.
4.5 Intelligent Furnace Concept
The Intelligent Furnace (I-Furnace) optimizes theheat treat-
ment and production process. It smartly combines math-
ematical/physical models to control the furnace and to op-
timize production with an online strength measurement
system and a newly developed annealing microstructure
model to predict material properties after the heat treat-
ment.
4.6 X-CAP (X-ray Controlled Annealing Process)
SMS group, Drever International, and IMS Messsysteme
jointly developed X-CAP, enabling a quantitative identifi-
cation of crystal structures within the annealing process.
X-CAP complements the I-Furnace concept, as shown in
Fig. 10. The new online measuring system uses X-ray
diffraction to define the crystalline phase fraction, up-
stream of the rapid furnace cooling section. Besides,
continuously monitoring of the phase content in the fur-
nace allows directly a compensating process and material
changes, which otherwise would lead to product quality
deviations. By this technology, the microstructure can be
adjusted immediately in the processing steps in order to
guarantee the final mechanical properties. For instance,
the fraction of austenite in DP steel before rapid cooling
determines the amount of the secondary hard constituent,
i.e. martensite, formed in the subsequent cooling stages.
The secondary phase fraction substantially influences the
strength of the final product. As we mentioned before, con-
trolling the fraction and stability of austenite is essential
in the AHSS 3. Gen. to obtain the desired microstructure
and mechanical properties. Therefore, the online measure-
ment and adjustment of austenite become prominent in
the production of new generation AHSS.
A first X-CAP system was installed, inside a specially
developed protective housing, in the furnace of a hot-dip
galvanizing line at Tata Steel in Liège, Belgium, in the SE-
GAL plant in the summer of 2017. The measurement results
were compared with the final product strength. Various
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Fig. 11: Application of QP980 to au tomotive structure parts: (a) B-pillar
reinforcement left/right, (b) B-p illar inner, (c)side member front floor left,
and (d) door panel inner left/right[16]
tests proved that with X-CAP it is possible to improve ma-
terial quality significantly. X-CAP is able to really help save
coils and keep the characteristic material properties within
the required range, which is even challenging for experi-
enced operators.
5. Applications of the AHSS 3. Generation
The development and fundamental research on the AHSS
3. Gen. are still going on, in order to explore the relation-
ship among alloy design, process adjustment, microstruc-
ture control, and mechanical properties. Simultaneously,
a few attempts have been made to produce the Q&P steel
and MMnS on an industrial scale. For instance, the first
industrial production of cold-rolled Q&P steel was carried
out by Baosteel in 2009 [16]. In 2012, cold-rolled Q&P steel
with a tensile strength of 980 MPa was successfully com-
mercialized [16]. Some typical automotive parts produced
by Q&P steel are shown in Fig. 11 . Q&P steels with a good
combination of high strength and adequate ductility are
suitable for structure and safety parts in the automotive
industry.
6. Conclusions
The third-generation advanced high-strength steels have
been developed for an excellent balance of strength and
formability. The upgrade of technologies of modern an-
nealing lineswith accurate online control of processparam-
eters has stimulated new ideas for complex heat treatment
schedules in order to create mixtures of different phases
with ultrafine-grained morphologies and chemical gradi-
ents. The local enrichment of carbon and/or manganese in
austenite contributes to the enhanced stability of austen-
ite. Using the TRIP effect or/and the TWIP effect results in
improved formability of the new steel concepts.
Funding . W. Bleck and Y. Ma gratefully acknowledge the financial support of
the Deutsche Forschungsgemeins chaft (DFG) within the Collaborative Research
Center (SFB) 761 ‘Steel—ab initio’.
References
1. Bouaziz, O.; Zurob, H.; Huang, M.: Driving force and logic of devel-
opment of advanced high strength steels for automotive applica-
tions, Steel Res. Int., 84 (2013), pp 937–947
2. Matlock, D. K.; Speer,J. G.; Moor, E. D.; Gibbs, P. J.: Recent de-
velopments in advanced high strength sheet steels for automotive
applications: an overview, Jestech, 15 (2012), pp 1–12
3. Bleck, W.; Guo, X.; Ma, Y.: The TRIP effect and its application in cold
formable sheet steels, Steel Res. Int., 88 (2017), p 1700218
4. Liu, L.; He, B.; Huang, M.: The role of transformation-induced plas-
ticity in the development of advanced high strength steels, Adv.
Eng. Mater., 20 (2018), p 1701083
5. Bouaziz, O.; Allain, S.; Scott, C. P.; Cugy, P.; Barbier, D.: High man-
ganese austenitic twinning induced plasticity steels: A review of
the microstructure properties relationships, Curr. Opin. Solid State
Mater. Sci., 15 (2011), pp 141–168
6. Speer, J. G.; Matlock, D. K.; De Cooman, B. C.; Schroth, J. G.: Car-
bon partitioninginto austenite after martensite transformation, Acta
Mater., 51 (2003),pp 2611–2622
7. Edmonds, D. V.; He, K.; Rizzo, F. C.; De Cooman, B. C.; Matlock, D. K.;
Speer, J. G.: Quenching and partitioning martensite—A novel steel
heat treatment, Mater. Sci. and Eng. A, 438–440 (2006), pp 25–34
8. Miller, R.L.: Ultrafine-grained microstructures and mechanical prop-
erties of alloy steels, Metall. Mater. Trans. B, 3 (1972), pp 905–912
9. Lee, S.-J.; Lee, S.; De Cooman, B. C.: Mn partitioning during the
intercritical annealing of ultrafine-grained 6 % Mn transformation-
induced plasticity steel, Scr. Mater., 64 (2011), pp 649–652
10. Sun, J.; Wang, M.; Hui, W.; Dong, H.; Cao, W.: Enhanced work-
hardening behavior and mechanical properties in ultrafine-grained
steels with large-fractioned metastable austenite, Scr. Mater., 63
(2010), pp 815–818
11. Han, J.; Nam, J.-H.; Lee, Y.-K.: The mechanism of hydrogen em-
brittlement in intercritically annealed medium Mn TRIP steel, Acta
Mater., 113 (2016), pp 1–10
12. Keeler, S.; Kimchi, M. (ed.).: Advanced High-Strength Steel Applica-
tion Guidelines Version 5.0, WorldAutoSteel, World Steel Associa-
tion, Brussels, 2014
13. Hu, X.; Sun, X.; Hector, L. G.; Ren, Y.: Individual phase consti-
tutive properties of a TRIP-assisted QP980 steel from a combined
synchrotron X-ray diffraction and crystal plasticity approach, Acta
Mater., 132 (2017), pp 230–244
14. Lee, S.; De Cooman, B. C.: Annealing temperature dependence of
the tensile behavior of 10 pct Mn multi-phase TWIP-TRIP steel, Met-
all. Mater. Trans. A, 45 (2014), pp 6039–6052
15. Lee, S.; De Cooman, B. C.: Tensile behavior of intercritically an-
nealed 10 pct Mn multi-phase steel, Metall. Mater. Trans. A, 45
(2014), pp 709–716
16. Wang, L.; Speer, J. G.: Quenching and partitioning steel heat treat-
ment, Metall. Microstruct. Anal., 2 (2013), pp 268–281
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... The growth of austenite during ICA treatments is believed to be controlled by the partitioning of austenite stabilizing elements (here C and Mn) as it was indicated from the compositional changes of ferrite and austenite in the ICA range based on the thermodynamic calculations represented in Figure 3. Such partitioning phenomena that locally occur at the ferrite/austenite interfaces of medium-Mn steel under investigation were previously depicted using atom probe tomography (APT) that enables identifying the nano-scale spatial chemical variations [42]. The XRD measurements and SEM micrographs demonstrated the increase in austenite fractions and the gradual vanishing of the austenite elongated grain morphology as the ICA temperature increases. ...
... The initiation of the selective dissolution occurred most likely at the austenite/ferrite interface. Bleck et al. [42] found a local Mn segregation at the austenite/ferrite interface of intercritically annealed (700 °C) medium-Mn steel, which is similar in the chemical composition compared to the X6MnAl12-3 material in the current investigation. The Mn segregation leads to a bilateral potential gradient, which is the highest in direction to the ferrite, since the Mn gradient is more pronounced. ...
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Medium-Mn steels exhibit excellent mechanical properties and lower production costs compared to high-Mn steels, which makes them a potential material for future application in the automotive industry. Intercritical annealing (ICA) after cold rolling allows to control the stacking fault energy (SFE) of austenite, the fraction of ferrite and reverted austenite, and the element partitioning (especially Mn). Although Mn deteriorates the corrosion behavior of Fe-Mn-Al alloys, the influence of austenite fraction and element partitioning of Mn on the electrochemical corrosion behavior has not been investigated yet. Therefore, the electrochemical corrosion behavior in 0.1 M H2SO4 of X6MnAl12-3, which was intercritically annealed for 2 h at 550 {\deg}C, 600 {\deg}C and 700 {\deg}C, was investigated by potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and mass spectroscopy with inductively coupled plasma (ICP-MS). Additionally, specimens after 1 h and 24 h of immersion were examined via SEM to visualize the corrosion damage. The ICA specimens showed a selective dissolution of reverted austenite due to its micro-galvanic coupling with the adjacent ferrite. The severity of the micro-galvanic coupling can be reduced by decreasing the interface area as well as the chemical gradient of mainly Mn between ferrite and reverted austenite by ICA.
... Further, the simulated data are compared with the experimental Atom Probe Tomography (APT) data from Refs. [30,31]. The C and Mn's concentration-spikes can be seen clearly for a similarity between APT (Fig. 6) and simulation (Fig. 5b,d). ...
... The images are adopted from Refs. [30,31]. herewith is that the solute concentration-spikes in the simulation are located away from the interface, against that on γ-surface in experimental (compare Fig. 5b,d versus Fig. 6). ...
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Serration is a complex issue in medium-Mn steel, associated with a jerky metal flow and deformation banding. The problem has been well debated over the decades. In order to investigate the reason, a medium-Mn steel is prepared by hot-rolling in the austenitic domain, followed by air cooling. This results in martensitic microstructure due to high hardenability provided by adequate austenite stabilizers (C, Mn, Ni) in the steel. The intercritical annealing was performed for martensite to austenite reversion at 650 °C for different durations. From solute partitioning kinetics estimated by Dictra simulation, it appears that martensite released C-atoms, but the product austenite could not occupy them into octahedral under thermodynamic and kinetic constraints. This leads to an undissolved state of C, disowned by both martensite and austenite. With an enhanced diffusivity by intercritical annealing at 650 °C, those undissolved C-atoms without a bonding restriction repeatedly interact with sample dislocations, acquired from the rolling process and martensite. The interaction minimizes elastic energy by forming numerous C-dislocation aggregates into clusters, as the building block of Cottrell atmospheres. In metal plasticity theory, the Cottrell atmosphere usually dictates at room temperature. This work suggests it at an elevated temperature to enlighten hitherto unexplored mechanism of the dynamical C-dislocation interaction behind serrations. Additionally, austenite inhomogeneity by a lack of redistribution of Ni and Mn at 650 °C is found to be responsible for a staircase-wise propagation of the jerky metal flow.
... This matrix is attained by the novel heat treatment process route of carbon partitioning to achieve less retained austenite in the matrix unlike 2nd generation AHSS. [1] Resistance spot welding is a process of joining two overlapped metal sheets by passing electric current through the contacting welding electrodes, applying pressure to the sheets. The electric current converted into heat energy along with compressive force on both sides of the metal sheets overlapped lead to the formation of a weld spot (nugget) between them. ...
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Third generation advanced high strength steel (3rd gen AHSS) have been largely used in the automotive industry due to its outstanding mechanical properties. This alloy has the possibility of reducing the vehicle’s structural weight by using thinner sheets. Nevertheless, AHSS are more prone to liquid metal embrittlement cracks (LME) due to the zinc (Zn) coating on its surface. Resistance spot welding (RSW) is the most used fabrication process in automotive industry. Avoiding LME cracks during RSW of 3rd gen AHSS is a critical task in the industry which needs to be addressed. Therefore, in this work the critical parameters and its values are estimated for 3rd gen AHSS to produce a defect free weld by generating a weld lobe curve. 3rd generation AHSS type CR850Y1180T was welded with CR4 steel as a counterpart using the RSW process. A list of 202 different process conditions was planned by varying welding current from 4 to 12 kA, welding time from 100 to 1300 ms and electrode force from 2 to 6.5 kN based on a full factorial design of experiment. Quality analyses were made for each welded samples to identify defect and defect free welds (i.e. LME and other defective welds) using stereomicroscope and energy dispersive X-ray spectroscopy (EDS). Scatter plot was generated using MS Excel to plot the output of the welds. As a result, the defect and defect free weld output in the scatter plot are clustered in which weld lobe curve was estimated. Similarly, the 3D plot was made using plotly library in a jupyter notebook which consist of welding current, welding time and electrode force in a three-dimensional space. The resulting 3D plot shows the clear picture of clustering of the defect and defect free weld outputs in a plot. Based on the clustering in the 2D and 3D plot, the weld lobe curve could be prepared, and proper welding parameters were estimated to produce defect free weld in 3rd gen AHSS during RSW process.
... One effective way of reducing emissions of passenger cars is to lower vehicle weight. The greatest potential for savings can be found in the area of autobody design, where, for example, high-strength steels are now replacing conventional deep drawing sheet material [1,2]. One of the most efficient processes for the production of high-strength crash-relevant components is press hardening, in which forming and hardening are combined in a single process step (direct press hardening) [3]. ...
Article
It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.
... Even though the goal of significantly increasing both the strength and elongation characteristics can be met in these materials, they are hardly used in the automobile industry due to their high costs and challenges related to weldability, galvanizing, elevated wear on forming dies, increased springback, flange stretching, edge cracking and fatigue compared to other steels [16,17]. Recently, there has been increased funding and research for the development of the "3rd Generation" of advanced high-strength steels (AHSS) [5,8,[18][19][20][21][22][23]. The third generation of AHSS seeks to provide ductility and high strength without the joining problems and high costs associated with the previous generations [5,8]. ...
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The present work presents a theoretical and experimental study regarding the microstructure, phase transformations and mechanical properties of advanced high-strength steels (AHSS) of third generation produced by thermal cycles similar than those used in a continuous annealing and galvanizing (CAG) process. The evolution of microstructure and phase transformations were discussed from the behavior of intercritical continuous cooling transformation diagrams calculated with the software JMatPro, and further characterization of the steel by scanning electron microscopy, optical microscopy and dilatometry. Mechanical properties were estimated with a mathematical model obtained as a function of the alloying elements concentrations by multiple linear regression, and then compared to the experimental mechanical properties determined by uniaxial tensile tests. It was found that AHSS of third generation can be obtained by thermal cycles simulating CAG lines through modifications in chemistry of a commercial AISI-1015 steel, having an ultimate tensile strength of UTS = 1020–1080 MPa and an elongation to fracture of Ef = 21.5–25.3%, and microstructures consisting of a mixture of ferrite phase, bainite microconstituent and retained austenite/martensite islands. The determination coefficient obtained by multiple linear regression for UTS and Ef was R2 = 0.94 and R2 = 0.84, respectively. In addition, the percentage error for UTS and Ef was 2.45–7.87% and 1.18–16.27%, respectively. Therefore, the proposed model can be used with a good approximation for the prediction of mechanical properties of low-alloyed AHSS.
... During the last five decades, as an extension of HSLA steels, three generations of AHSS were developed for the purpose of lightweighting in the automotive industry (Figure 10a). Depending on the steel generation, there may be some challenges regarding formability and weldability [44]. The first generation of the AHSS family includes dual phase (DP), complex-phase (CP), martensitic (MS), and regular transformation-induced plasticity (TRIP). ...
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The automotive lightweighting trends, being driven by sustainability, cost, and performance, that create the enormous demand for modern lightweight materials and design concepts, are assessed as a part of the circular economy solutions in modern mobility and transportation. The current strategies that aim beyond the basic weight reduction and cover also the structural efficiency as well as the economic and environmental impact are explained with an essence of guidelines for materials selection with an eco-friendly approach, substitution rules, and a paradigm of the multi-material design. Particular attention is paid to the metallic alloys sector and progress in global R&D activities that cover the “lightweight steel”, conventional aluminum, and magnesium alloys, together with well-established technologies of components manufacturing and future-oriented solutions, and with both adjusting to a transition from internal combustion engines to electric vehicles. Moreover, opportunities and challenges that the lightweighting creates are discussed with strategies of achieving its goals through structural engineering, including the metal-matrix composites, laminates, sandwich structures, and bionic-inspired archetypes. The profound role of the aerospace and car-racing industries is emphasized as the key drivers of lightweighting in mainstream automotive industry.
... To improve the crashworthiness and promote the lightweight of automotive steels, the automotive industry increasingly uses advanced high-strength steels (AHSSs). The third generation of AHSSs has the superior mechanical properties exceeding the first generation of AHSSs and the lower cost compared with that of the second generation of AHSSs [1,2], which meet the requirements of automotive industry. Medium Mn steel is one of the most promising candidates of the third generation of AHSSs mainly due to the high volume fraction of retained austenite (f RA is above 30 vol%) with the moderate mechanical stability [3], resulting in the excellent combination of high ultimate tensile strength (UTS) and good total elongation after fracture (TEL) and thus the high product of strength and elongation (PSE is above 30 GPa⋅%) [4]. ...
Article
Controlling the amount of retained austenite and precipitate particles allows for tailoring of mechanical properties in medium Mn steel alloyed with Cu, Ni and Al. The austenite reversion and precipitation/dissolution occur simultaneously during aging treatment at 550 °C being lower than Ac1. The degree of austenite transformation depends on the diffusion of austenite stabilizing elements (Mn, Cu and Ni), and the dissolution of the precipitate particles promotes the diffusion. Small angle neutron scattering (SANS) measurements were performed at the China Spallation Neutron Source (CSNS) to investigate the precipitation/dissolution behaviors. A large number of precipitate particles form at the aging time of 1 h, and then the volume fraction of the particles continuously decreases, and the average particle size has little change as holding time increases, indicating the dissolution of the precipitate particles. Atom probe tomography and transmission electron microscopy were used to study the type, composition and distribution of the precipitate particles. The particles contain Cu and NiAl-type precipitates, which have BCC and B2 structures, respectively. The characterization and modeling of austenite reversion kinetics indicate that austenite transformation is mainly controlled by the diffusion of Cu, which is promoted by the dissolution of Cu particles. This work offers significant insights towards an austenite-precipitate cooperative design for controlling the mechanical properties of the steel.
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The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the variant pairing of the early-formed martensite. A mixed microstructure with tempered early-formed martensite and fresh later-formed martensite was achieved by a heat treatment with an isothermal hold below the martensite start temperature. In-situ high-energy X-ray diffraction showed no further transformation of austenite to ferrite/martensite during the isothermal hold. The tempered early-formed martensite was characterized with a combination of light optical microscopy and local tetragonality determination by electron backscatter diffraction. The characterization allowed qualitative as well as quantitative analysis of the tempered early-formed martensite with regard to the prior austenite grain boundaries (PAGB) and variant pairing. The early-formed martensite was shown to grow predominantly along the PAGBs and clustering was observed indicating an autocatalytic nucleation mechanism. The variant pairing of the early-formed martensite had a stronger plate character compared to the later-formed martensite.
Chapter
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Lightweight structural components made of advanced high‐strength steels (AHSSs) in the automotive industry can substantially reduce greenhouse gas emissions. The 3rd‐generation AHSSs, which consist of medium Mn steel, quenching and partitioning (Q&P) steel, and carbide‐free bainitic (CFB) steel, is the current research focus of the steel community. In particular, the retained austenite grains are the intrinsic components of the 3rd‐generation AHSSs. These retained austenite grains can demonstrate a transformation‐induced plasticity (TRIP) effect by transforming into martensite during mechanical loading, improving the strain‐hardening behavior of AHSSs. Consequently, intensive research has been carried out over the past 30 years to understand the role of the TRIP effect on the development of AHSSs. Therefore, this review article is aimed to provide a state‐of‐the‐art summary of recent progress on AHSSs with the TRIP effect. Specifically, the processing, the relationship between microstructure and mechanical properties, and the potential industrial applications of TRIP‐enabled AHSSs will be addressed in this review. More importantly, the mechanical stability of the retained austenite grains, which determines the overall performance of the TRIP effect in AHSSs, will be discussed by considering several governing factors. The review article provides a state‐of‐the‐art summary of recent progress on advanced high strength steels (AHSSs) with the transformation‐induced plasticity (TRIP) effect. Specifically, the process‐microstructure‐property relationships, the deformation mechanism, and the potential industrial applications of TRIP‐enabled AHSSs are addressed in this review.
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The Transformation-Induced-Plasticity (TRIP) effect is used for enhancing the formability of cold formable sheet steels. While the first observation of this phenomenon dates back to the 1930’s, the industrial usage of the TRIP steels started after 1950. First fully austenitic steels, later on multiphase steels have been developed with a meta-stable austenitic phase that can transform stress-assisted or strain-induced into ε- or α’-martensite during deformation. The historic development, the principles of the TRIP effect, and the different groups of steels using the TRIP effect are described. For the already commercialized TRIP steels, characteristic chemical compositions and microstructures are discussed; the requirements for the process design as well as new annealing concepts after cold rolling are explained.
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In the present study, the relationship between the microstructure and the mechanical properties of Fe-10 pct Mn-3 pct Al-2 pct Si-0.3 pct C multi-phase steel was investigated. The 10 pct Mn multi-phase steel exhibits a combination of high tensile strength and enhanced ductility resulting from deformation-twinning and strain-induced transformation occurring in succession. A pronounced intercritical annealing temperature dependence of the tensile behavior was observed. The annealing temperature dependence of the retained austenite volume fraction, composition, and the grain size was analyzed experimentally, and the effect of the microstructural parameters on the kinetics of mechanical twinning and strain-induced martensite formation was quantified. A dislocation density-based constitutive model was developed to predict the mechanical properties of 10 pct Mn multi-phase steel. The model also allows for the determination of the critical strain for dynamic strain aging effect.
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The major scientific and technological advances and breakthroughs of advanced high strength steels (AHSS) were achieved due to the strong demands of automotive industry. The development of AHSS began in the early 1980s with the aim of improving passenger safety and weight‐saving. The present paper presents the driving forces and logic of development of various AHSS for automotive applications since 1980s. The importance of crash performance, weight‐saving, formability, and rigidity are critically reviewed for the development of new steel grades for automotive applications. The logical sequences of the development of dual phase (DP) steel, transformation induced plasticity (TRIP) steels, tempered DP steels, complex phases (CP) steels, Ferrite‐Bainite steels, hot‐stamping technology, twinning induced plasticity (TWIP) steels, Quench and Partitioning (Q&P) steels, Medium Mn steels, and steels–polymer composites are presented.
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The exceptional elongation obtained during tensile testing of intercritically annealed 10 pct Mn steel, with a two phase ferrite–austenite microstructure at room temperature, was investigated. The austenite phase exhibited deformation-twinning and strain-induced transformation to martensite. These two plasticity-enhancing mechanisms occurred in succession, resulting in a high rate of work hardening and a total elongation of 65 pct for a tensile strength of 1443 MPa. A constitutive model for the tensile behavior of the 10 pct Mn steel was developed using the Kocks–Mecking hardening model.
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A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.
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The objective of this study was to investigate the mechanisms of hydrogen embrittlement (HE) in intercritically annealed medium Mn steel. For this purpose, both hot-rolled and cold-rolled Fe-7Mn-0.1C-0.5Si (wt.%) steels were annealed at 640 °C for 30 min. The annealed specimens had a dual-phase microstructure of retained austenite (γR) and ferrite (α) with different morphologies; a lath shape for the hot-rolled and annealed (HRA) specimen and a globular shape for the cold-rolled and annealed (CRA) specimen. Although the difference in microstructural morphology did not influence the H permeation, it significantly affected the HE behavior. The H charged HRA (HRAH) specimen was fractured by intergranular cracking occurring along the boundaries of prior γ grains by the H-enhanced decohesion (HEDE) mechanism. The intergranular cracking leaved both flat and rugged facets, which appeared at the prior γ grain boundaries without and with γR, respectively. The H-charged CRA (CRAH) specimen was fractured to leave both dimples filled with grains and empty dimples at the fractured surface. The dimples filled with grains were generated by intergranular cracking occurring along the boundaries of γR grains by the HEDE mechanism. The empty dimples were made by intragranular cracking occurring inside the α grains by the H-enhanced local plasticity (HELP) mechanism. The CRAH specimen exhibited a smaller elongation loss than the HRAH specimen because cracks were propagated by frequently changing their direction along the boundaries of nano-sized γR grains or into α grains.
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The partitioning of Mn to austenite formed during the intercritical annealing of an ultrafine-grained 6% Mn transformation-induced plasticity steel was investigated by means of transmission electron microscopy–energy-dispersive spectroscopy and dilatometry. The partitioning of Mn to ultrafine austenite grain was observed during annealing of cold-rolled martensite where no prior partitioning of alloying elements had occurred. The calculated volume contraction related to the austenite formation and the associated alloying element partitioning as verified by the Mn partitioning during the intercritical annealing was compared with the measured result.