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

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 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.

Full text

Originalarbeit
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 [3–5].
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’.
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