Increasing the energy absorption of monolithic manganese boron steels in oxygen-free environment

Abstract

The heat-treatable steel 22MnB5 is used in hot stamping processes to produce high-strength body-in-white components. In this process, sheet blanks are conventionally heated in roller hearth furnaces and then hot-stamped, whereby strengths of 1,500 MPa can be achieved. Disadvantages of this process are the low plastic deformation of the material in hardened state and the poor energy efficiency of roller hearth furnaces. In a new approach, these disadvantages are eliminated by combining edge decarburisation with resistance heating. Due to a diffusion-controlled removal of the carbon in the edge layer of the blanks heated in an oxygen-free atmosphere, the energy absorption in bending tests was improved by 61 % compared to customary hot-stamped 22MnB5. Furthermore, with a subsequent resistance heating in an oxygen-free silane atmosphere, the sheet can be heated and coated. A hermetically sealed heating chamber was developed which allows to heat the blanks up to 950 °C without scale formation. The coating during heating further improves the corrosion properties of the component. With this approach, hot-stamped components with improved properties and coated in an energy-efficient resistance-heated process can be manufactured.

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Increasing the energy absorption of monolithic manganese boron steels
in oxygen-free environment
To cite this article: Bernd-Arno Behrens et al 2021 IOP Conf. Ser.: Mater. Sci. Eng. 1157 012021
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40th International Deep-Drawing Research Group Conference (IDDRG 2021)
IOP Conf. Series: Materials Science and Engineering 1157 (2021) 012021
IOP Publishing
doi:10.1088/1757-899X/1157/1/012021
1
Increasing the energy absorption of monolithic manganese
boron steels in oxygen-free environment
Bernd-Arno Behrens1, Sven Hübner1, Ulrich Holländer2, André Langohr2, Chris
Pfeffer1 and Lorenz Albracht1*
1 Institute of Forming Technology and Machines, An der Universität 2, 30823 Garbsen,
Germany
2 Institute of Materials Science, An der Universität 2, 30823 Garbsen, Germany
* Albracht@ifum.uni-hannover.de
Abstract. The heat-treatable steel 22MnB5 is used in hot stamping processes to produce high-
strength body-in-white components. In this process, sheet blanks are conventionally heated in
roller hearth furnaces and then hot-stamped, whereby strengths of 1,500 MPa can be achieved.
Disadvantages of this process are the low plastic deformation of the material in hardened state
and the poor energy efficiency of roller hearth furnaces. In a new approach, these disadvantages
are eliminated by combining edge decarburisation with resistance heating. Due to a diffusion-
controlled removal of the carbon in the edge layer of the blanks heated in an oxygen-free
atmosphere, the energy absorption in bending tests was improved by 61 % compared to
customary hot-stamped 22MnB5. Furthermore, with a subsequent resistance heating in an
oxygen-free silane atmosphere, the sheet can be heated and coated. A hermetically sealed heating
chamber was developed which allows to heat the blanks up to 950 °C without scale formation.
The coating during heating further improves the corrosion properties of the component. With
this approach, hot-stamped components with improved properties and coated in an energy-
efficient resistance-heated process can be manufactured.
1. Introduction
In the automotive industry, ultra-high-strength steels are used in safety-relevant areas of the
passenger compartment to ensure the greatest possible protection of passengers and simultaneously
reduce vehicle weight. Hot stamping is the established process for the production of such components
as it combines both the shaping and the heat treatment of a sheet metal work piece in one process step
[1]. Therefore, prior to the forming step, the blanks are heated up to a specific temperature in order to
austenitise the steel material and subsequently quenched in a water-cooled forming tool. In industrial
processes, the heating takes place in roller hearth furnaces [2].
The steel 22MnB5 with an aluminium-silicon coating (AlSi) has established itself in recent years as
a suitable material for hot stamping. In the hardened state, 22MnB5 can achieve tensile strengths of up
to approx. 1,500 MPa. However, in consequence of the hardening, the elongation at fracture decreases
to 5%. [3]. In order to increase the crash performance, respectively the energy absorption, the
mechanical properties can be specifically adjusted by applying tailored properties [4]. The approach of
these varying methods is to avoid the formation of martensite in specific areas of a component in order
to achieve a more ductile microstructure. In turn, this is accompanied by a significant loss of strength.
Roll-cladded material systems like TRIBOND®1200 and TRIBOND®1400 developed by thyssenkrupp

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Steel Europe combine a high-strength steel with a mild steel in a three-layer sandwich material. The
arrangement of the mild steel as surface layer leads to a beneficial combination of both, strength and
ductility, especially regarding bending load [5]. This study presents an approach by means of which a
comparable distribution of the mechanical properties across the cross-section of a monolithic sheet blank
can be achieved. Therefore, the surface of uncoated 22MnB5 is decarburised by a heat treatment in an
argon-hydrogen atmosphere.
In order to prevent scale formation in hot stamping processes, the material either has to be coated or
processed in an oxygen-free environment. The established AlSi coating primarily serves as scale
protection [6]. A major disavantage associated with this coating is the limited heating rate. In order to
prevent a melting of the coating, the blanks need to be heated up slowly, allowing the formation of
intermetallic FeAl phases with a higher melting temperature [7]. This, respectively the formation of
scale in general, represents one of the reasons why rapid heating processes, like resistance heating, have
not yet become established in industrial production. By resistance heating, electric power is converted
into heat by the current flow in the sheet and as a result of ohmic resistance. Due to the direct power
conversion in the material, up to 74 % of the energy can be saved compared to roller hearth furnaces.
Nevertheless, the hot-stamping temperature of 950 °C can be reached within seconds [8]. However, the
AlSi layer is unsuitable for this short process due to the duration of the formation of the intermetallic
phase and due to the fact that scale is generated on
uncoated sheets, as can be seen in Figure 1. Therefore,
the development of a process chamber and suitable sheet
coating for resistance heating in an oxygen-free
monosilane-doped nitrogen atmosphere is the content of
subproject A04 of the Collaborative Research Centre
(CRC) 1368 “Oxygen-free production” [9]. For this
purpose, an experimental chamber was developed in
which resistance heating can be conducted in an oxygen-free silane atmosphere. Within this chamber,
the blanks can be simultaneously braze-coated with the nickel-based alloy Ni700 (chemical composition
89% nickel + 11% phosphor), which is designed to bond with the sheet material within seconds and
provides corrosive protection during the subsequent transfer to the forming tool. This creates an
alternative process chain that allows to proceed the above-mentioned decarbonised material as well as
customary uncoated 22MnB5 in an energy-efficient manner without scale formation.
2. Methodology and Results
In this chapter, the procedure and results of the investigations are described. First, the heat treatment
of the surface decarburisation of uncoated 22MnB5 blanks and the plate-bending tests conducted are
described. Subsequently, the developed process chamber is presented in which uncoated 22MnB5
blanks can be heated by resistance heating in an oxygen-free silane atmosphere and can simultaneously
be coated with Ni700.
2.1. Decarburisation
The decarburisation of carbon steel can be performed by means of heat treatment in ambient air [10] or
in a protective atmosphere containing hydrogen [11]. The latter offers the great advantage, that a
simultaneous oxidation of the steel surface is avoided. A certain amount of water residue in the hydrogen
atmosphere, which is inevitably generated in open conveyor-belt furnaces, is necessary to ensure
decarburisation. The water reacts with the carbon to form gaseous carbon monoxide. For that, the carbon
atoms dissolved in the steel must migrate from the bulk to the surface, which is a diffusion-controlled
process. Therefore, process temperature and time are the dominant parameters for controlling the
decarburisation depth achievable.
Figure 1: Hot-stamped uncoated IFUM Bumper

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Investigating appropriate process parameters for the
edge decarburisation of 22MnB5 steel (thickness
1.5 mm), sheet specimens were heat-treated in a
conveyor-belt furnace (HTE 1200-200/80-1500,
Kohnle GmbH, Birkenfeld, Germany) using an
argon-based process gas mixture with 20 vol.-%
hydrogen ratio. Two different temperatures in the
heating zone, 860 and 960 °C, were combined with
two different dwell times, 25 min and 40 min
(controlled by the speed of the conveyor belt) for a
parameter study within application-suited limits for
continuous heat treatment of 22MnB5. The water
residue in the process gas measured by a dew point
sensor (DMT152, Vaisala Oyi, Finland) amounted to
(60±10) ppm during all measurements. The resulting
temperature curves of the samples that had been
instrumented with thermocouples for temperature detection are shown in Figure 2.
Sheet samples heat-treated in this way were prepared metallographically and examined under an optical
microscope. For this purpose, the metallographic cross sections were suitably etched with "Beraha II"
[12] beforehand to visualise carbon-containing microstructural constituents.
The micrographs displayed in Figure 3 show that the microstructure of the specimens heat-treated at
860 °C is finer-grained than that of the sheets treated at 960 °C. This is due to the higher degree of
recrystallisation, which is accompanied by more pronounced grain coarsening at higher temperatures.
Generally, the carbon-containing phases show up in the form of a pearlitic microstructure, dark grey
phases as well as row-like arrangements in the microstructure. The transition to the lower-carbon edge
layer, which is fine grained ferrite with only few fraction of perlite, is smooth. The transition area is
marked by dashed white lines in the micrographs. The decarburisation depth for the specimens heat-
treated at 860 °C is thus about 50 µm, while the specimens treated at 960 °C reached about 90 µm. With
respect to the different treatment times (25 min and 40 min dwell time in the annealing zone) at the
respective temperatures, no significant difference in the decarburisation depth was observed.
Corresponding to a conventional hot-stamping process, sheet specimens were heated up to 950 °C in a
chamber furnace (N161/S, Nabertherm). After a dwell time of 360 s, the specimens were quenched in a
water-cooled plate tool. Surface-decarburised and subsequently hot-stamped plates were also subjected
to metallographic examination to investigate the extent and effect of decarburisation on the hardened
microstructure. In this case, the transverse sections were first etched in ethanolic nitric acid, which
reveals the martensitic microstructure in the form of brown, marbled areas shown in Figure 4.
Figure 2: Temperature curves of
22MnB5
sheet specimens heat-treated in a conveyor-
belt
furnace in Ar+ 20 vol.-% H2 atmosphere
with
different furnace temperatures and dwell times
Figure 3: Micrographs of 22MnB5 sheet specimens after decarburisation in Ar/H2 atmosphere
.
Decarburisation depth is approximately at the dashed lines (
transition from a linear microstructure
with increased pearlitic structure to a fine grained ferrite structure with lower perlite contents).

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Relevant micrographs are shown in Figure 4. It can be seen that the heat treatment carried out in this
case under atmosphere has resulted in a peeling iron oxide layer on the sheet surfaces. This is followed
by a narrow region consisting of ferritic microstructure (light grey crystalline microstructure) which, as
expected, is much more pronounced in the specimen decarburised at 960 °C than in the sheet treated at
860 °C. A comparison of the extent of these edge zones with the results from Figure 4 shows that in
both cases a comparable part of the ferritic edge structure (approx. 30-40 µm) was obviously oxidized,
so that the decarburised edge thickness was to some extent reduced by the hot- stamping process.
Nevertheless, in both cases a ferritic surface layer remained, suggesting a more ductile edge region.
2.2. Plate-bending Test
Plate-bending tests have been conducted according to VDA 238-100 [13] in order to investigate the
effect of the decarburisation on energy absorption and bending angle. Additionally, customary
22MnB5+AS, TRIBOND®1200+AS and TRIBOND®1400+AS have been used as comparative
samples. After austenitising at 950 °C for 360 s, all samples were hot-stamped and subsequently cut to
60 mm x 60 mm samples by water jet. The samples were bent in a bending device without lubricants
and supported on pivoted rollers, until a drop in the force of 30 N was detected after the maximum force
had been reached. The force is measured with a load cell and the traverse path with an inductive path
transducer. For the analysis, the energy as well as the bending angle were calculated via the force-
displacement curve. TRIBOND®1200 and TRIBOND®1400 were tested in 0 ° and 90 ° rolling
direction, while the remaining samples were tested in 90 ° to rolling direction. The classification of the
specimens is shown in table 1.
Table 1: Process parameters of individual samples for plate bending test
Nomination Surface decarburization Rolling direction in [°] Quantity [-]
Temperature in [°C] Time in [min]
22MnB5
none
90
3
TRIBOND®1200 0
TRIBOND®1200 90
TRIBOND®1400 0
TRIBOND®1400 90
Sample 1 860 25
90 1
Sample 2 860 40
Sample 3 960 25
Sample 4 960 40
The bending energy determined and the bending angle can be seen in figure 5. In this comparison,
customary 22MnB5 achieves the lowest values with regard to a determined bending energy of 41 J and
a bending angle of 42 °. The highest bending energy with 97 J and a bending angle of 121 ° was detected
Figure 4:
Micrographs after hot
stamping: etching in ethanolic
nitric acid on martensitic
microstructure. Bright crystalline
surface-
layer structure is ferritic.
The dark grey coatings
on the
specimen surfaces indicate an
iron oxide layer obviously
formed when the
sheets were
reheated (in this case under
atmosphere) for the hot-
stamping
process.

40th International Deep-Drawing Research Group Conference (IDDRG 2021)
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for TRIBOND®1200 for a rolling direction of 0 °, with only slightly lower values for a rolling direction
of 90 °. A more pronounced anisotropy was determined for TRIBOND®1400 with a bending energy
between 70 J to 87 J. For the decarburised samples, the determined bending energy varies between 68 J
and 76 J. Compared to customary 22MnB5, the energy absorption can be increased by 44 % for sample
2 and even by 61 % for sample 4 and thus, achieve values in the range of a TRIBOND®1400. This
shows that surface decarburisation decisively improves the material behaviour with resprect to crash
potential under bending stresses.
Figure 5: Determined bending energy (a) and bending angle (b) resulting from conducted plate-
bending test
41
97 94 87
70 69 68 73 76
0
20
40
60
80
100
120
22MnB5
TRIBOND®1200
TRIBOND®1200
TRIBOND®1400
TRIBOND®1400
Sample 1
Sample 2
Sample 3
Sample 4
90 ° 0 ° 90 ° 0 ° 90 ° 90 ° 90 ° 90 ° 90 °
Bending energy [J]
42
121 118
96
81
68 70 74 74
0
20
40
60
80
100
120
140
22MnB5
TRIBOND®1200
TRIBOND®1200
TRIBOND®1400
TRIBOND®1400
Sample 1
Sample 2
Sample 3
Sample 4
90 ° 0 ° 90 ° 0 ° 90 ° 90 ° 90 ° 90 ° 90 °
Bending angle [°]
a)
b)

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2.3. Oxygen-free silane resistence heating
In order to heat uncoated 22MnB5 blanks, such as surface-decarburised 22MnB5 blanks, without
scaling, an experimental chamber was developed in CRC 1368 "oxygen-free production" in subsection
A04. Within an oxygen-free silane atmosphere, blanks can be heated by resistance heating and
simultaneously coated, in order to prevent the formation of scale during the subsequent transfer to the
forming tool. The experimental chamber is shown in figure 6. On the left, the chamber is open and the
main components, consisting of the oxygen and temperature measurement, the process gas flow, the
sheet with the different areas and the high temperature insulation for regulating the atmosphere and the
coating process can be seen. The pass of the sheets into the experimental chamber and the experimental
chamber itself are sealed with a smooth inorganic non-conductive high-temperature insulation to
compensate for the rapid temperature fluctuations. The gas is distributed into the experimental chamber
through a piping system. An oxygen-measurement system collects process gas directly above the metal
surface of the sheet and measures the oxygen concentration. For this purpose, an automated
measurement recording and evaluation system (developed in subproject S01 of the CRC) was
implemented on the basis of a commercial oxygen-measurement system (Mesa Industrie-Elektronik
GmbH, Marl, Germany) in order to record the residual oxygen content prevailing in the process chamber
in situ. On the right side in figure 6, the closed chamber is displayed during a heating process. The
electrodes, located outside on the left and right of the chamber, clamp the sheet and transmit the current
for heating. The viewing windows in the chamber are used for visual assessment of the coating process
and for temperature measurement by means of a pyrometer, which is mounted outside the chamber due
to the temperatures of up to 400 °C inside.
The process gases nitrogen and silane are used to suppress the formation of scale. The chamber is
first evacuated with nitrogen to remove the air from the chamber. At an oxygen concentration of
0.001 vol. %, silane is added and heating is started. During heating, the blank can be coated with a
plastic-coated nickel powder. The plastic ensures adhesion to the blank in advance and protects the
powder from oxidation. The plastic coating decomposes at a temperature of 200 °C and has completely
evaporated when the hot-stamping temperature is reached. The process window of the coating is in the
Figure 6: Structure of the process chamber for oxygen-free resistance heating

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range of 850 °C, where it starts to become liquid, and about 1000 °C. The properties can be adjusted
with the heating temperature and holding time of a few seconds.
Figure 7 shows a cross-section of a resistance-heated sheet metal coated on one side in an oxygen-
free silane atmosphere at a heating temperature of 973 °C and a holding time of 31 s. The 50 µm thick
Ni700 coating (grey) metallurgically
bonded to the 1.5 mm thick 22MnB5
sheet (light blue), exhibits a loadable
joint between the coating and the
parent metal. It can be seen that neither
in the upper area of the coating nor on
the bottom, scale was formed. This
shows that heating in an oxygen-free
silane atmosphere suppresses the
formation of scale and that the Ni700
coating protects the surface of the
blank from corrosion in the
environment after removal from the
chamber. Scale-free heating and
coating of uncoated 22MnB5 is thus
possible.
3. Conclusion
In this study, oxygen-free atmosphere was used in order to improve the mechanical properties of hot-
stamped components with respect to crash performance, and the hot-stamping process itself regarding
process time and energy consumption.
By means of surface-layer decarburisation, an increase in crash performance could be determined in
plate-bending tests. By decreasing the carbon content in the surface layers, the specimens achieved
higher bending angles and higher energy absorption. With the parameters used in this study, it was
possible to increase the bending energy by up to 61 % compared to customary 22MnB5. The energy
absorption of the surface-decarburised 22MnB5 samples could be increased to the range of a
TRIBOND®1400.
In order to process uncoated sheet blanks in hot stamping, a test chamber was developed. Within this
test chamber, the sheet blanks are heated by resistance heating in an oxygen-free silane atmosphere and
can simultaneously be coated to prevent the formation of scale during the subsequent transfer to the
forming tool. First results presented in this study show a homogeneous distribution of the applied Ni700
coating without any formation of scale.
Nevertheless, the results presented in this study need to be validated in further tests. With regard to
surface decarburisation, more parameter combinations need to be investigated. It is furthermore planned
to process the decarburised sheet in the developed test chamber, in order to prevent the formation of
scale. The resistance heating system is also being modified on the basis of the shown experimental
chamber. The focus is on shortening the cycle time as well as further developing coating powders and
powder application processes. In the next step, the electrodes are to be integrated in the process chamber
so that the entire sheet can be heated and coated without scaling under an oxygen-free silane atmosphere.
The coating powder is being further developed for hot stamping and corrosive protection. The behaviour
of the coating from 200 to 850 °C for hot stamping is in focus and will be described by means of a
model.
Acknowledgement
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-
ID 394563137 – SFB 1368
Figure 7: Cross-
section of a
resistance-heated
sample coated on
one side
under
oxygen-free
silane atmosphere
with a heating
temperature of
973 °C and a
holding time of
31 s

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