J. Mater. Sci. Technol., 2011, 27(4), 369-376.
Semi-hot Stamping as an Improved Process of Hot Stamping
Malek Naderi1) , Mostafa Ketabchi1) , Mahmoud Abbasi1)† and Wolfgang Bleck2)
1) Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran
2) Department of Ferrous Metallurgy, RWTH-Aachen University, Aachen, Germany
[Manuscript received December 5, 2010, in revised form February 11, 2011]
Reducing the forming load, deletion of springback, increasing the formability of sheets as well as producing
high strength parts are the main reasons to apply hot stamping process. Hot stamping process and 22MnB5
steels are the state of the art process and grades, respectively; however novel processes and steel grades
are under considerations. In the current research, behavior of the steel grade MSW1200 blanks under semi
and fully hot stamping processes was characterized. During semi-hot stamping process, the blank was firstly
heated to a temperature of about 650◦ C and then formed and quenched in the die assembly, simultaneously.
Microstructure and mechanical properties of semi and fully hot stamped blanks were studied and the results
were compared with those of normally water/air quenched blanks. The hot stamped blanks attained the
strength values as high as water quenched blanks. The highest ductility and consequently, the best formability
were achieved for the blank which had been semi-hot stamped. It was concluded that for the mentioned steel,
semi-hot stamping process could be considered as an improved thermo-mechanical process which not only
guaranteed a high formability, but also led to ultra high strength values.
KEY WORDS: Semi-hot stamping; Hot stamping; MSW1200 steel; Mechanical properties
1. Introduction
To optimize the fuel consumption of cars, the
weight reduction of cars is intensively required in
the automobile industry. Hence, application of high
strength steel sheets in automobile bodies would be a
good solution to reduce the weight of cars[1] . However, the use of high strength steel sheets usually
leads to some disadvantages like high impact on the
tools, reduced formability and the increased tendency
to springback[2] . To improve the formability characteristic and reduce or avoid the springback of high
strength steel sheets, hot stamping process has been
used widely. During this process, not only the flow
stress is largely reduced by preheating the sheets,
but also due to simultaneous occurrence of forming
and quenching, forming load and springback decrease
and formability increases[3] . Hot stamping is de† Corresponding author.
Tel.: +98 21 64542949; Fax:
+98 21 66405846; E-mail address: m.abbasi@aut.ac.ir; abbaci.m@gmail.com (M. Abbasi).
fined as the process in which the blank is heated to
the temperature of the austenite stabilization region,
i.e. about 900◦ C, for definite time and then formed
and quenched simultaneously[4] . Many attempts have
been done on the hot stamping process of steel sheets
especially on quenchable boron-alloyed steels. It is reported that boron enhances hardenability and retards
and postpones thermally activated transformations,
e.g. ferrite and pearlite transformations[3] . Merklein
and Lechler[2] used 22MnB5 steel as material and
studied its hot stamping process. They showed that
hot stamping process can result in production of components with tensile strength more than 1500 MPa
due to changes in microstructure. Naderi et al.[5] analyzed the microstructural and mechanical properties
of different B-bearing steels after being hot stamped.
Microstructures of the hot stamped blanks were composed of martensite. The resulted microstructures
provided the yield strengths of 650–1370 MPa and the
tensile strengths of 850–2000 MPa. The disadvantage
of the hot stamping is remarkable oxidation on the
surface of products[6] . The sheets are heated at
370
M. Naderi et al.: J. Mater. Sci. Technol., 2011, 27(4), 369–376
Table 1 Chemical composition of the investigated steel (mass%)
C
0.14
Si
0.12
Mn
1.71
Cr
0.55
about 900◦ C to attain the martensitic transformation
induced by the die quenching, and the oxide scales
are generated at this temperature. The oxidation becomes remarkable due to contact with air after taking
the sheet out of the furnace. The shot blasting is used
to remove the oxide scales of stamped products or
the aluminum and zinc coated sheets are employed to
prevent the oxidation during the hot stamping. But
the shot blasting and coating are costly. Mori and
Ito[7] evaluated the effect of the oxidation preventive
oil on oxidation of quenchable steel sheets in a hot
hat-shaped bending experiment. They found that the
oxidation was prevented by using coating. Schieβl
et al.[8] analyzed corrosion behavior of coated and
non-coated steels after being hot stamped and notified
that high temperature austenization could damage
the coatings. Innovation of a stamping process that
brings advantages of hot and cold stamping processes
all together not only will result in low forming load
and minimizing of springback but also may preserve
the coating properties properly and save time and
cost. Semi-hot stamping process is a reply to this
request. During the semi-hot stamping, the blank is
heated for a while at the temperature of about 650◦ C
and subsequently it is deformed and quenched, simultaneously. So, study on the effects of semi-hot stamping process on different properties of sheet metals is
worthy.
Mori et al.[9] used a resistance heating to elevate
the temperature of sheet during forming and studied
the effects of warm stamping on the springback of ultra high tensile strength steel sheets. The springback
in hat-shaped bending of the high tensile strength
steel sheets was eliminated by heating the sheets.
They found that the optimum heating temperature
is around 600◦ C due to the small springback and oxidation and the increase in hardness. Schieβl et al.[8]
studied the corrosion behavior and mechanical properties of 22MnB5, CP-W800 and MSW1200 steels after being hot stamped and semi-hot stamped. Prior
to hot and semi-hot stamping operations, the blanks
were heated to 950 and 650◦ C temperatures, respectively, and then they were cooled in a closed-die. The
material 22MnB5 reached component tensile strength
levels over 1500 MPa at elongations of 5%–8% while
with non-boron alloyed steels tensile strengths of only
maximum 850 MPa were obtained at elongation of
about 8%–12%.
In the current research, more details on semi-hot
stamping process along with MSW1200 steel grade
characterizations were investigated. MSW1200 is developed for cold forming and thus has high strength in
the as-delivered condition. For better comparison and
understanding, cold and hot stamping processes using
Ni
0.06
Al
0.02
Ti
0.002
Ceq
0.258
Fig. 1 Microstructure of as-received MSW1200 steel. It
consists of about 70% ferrite as well as 30% fine
spheroidized pearlite
water or nitrogen cooled punch were also performed.
2. Experimental
2.1 As-delivered condition
The industrially processed un-coated MSW1200
steel sheet with a thickness of 1.5 mm was provided.
Chemical composition of the studied steel as well as its
corresponding carbon equivalent is given in Table 1.
Carbon equivalent of investigated steel was calculated
according to the equation presented by Patchett[10] for
carbon steels. As-received microstructure of the steel
is shown in Fig. 1.
As it is seen in Fig. 1, most of the as-received microstructure consisted of ferrite phase, i.e. about 70%.
The yield and tensile strengths were about 400 MPa
and 640 MPa, respectively and the total elongation
value (A25 ) was about 26%.
2.2 Methods
The effect of three processes, i.e. cold stamping
followed by quench hardening, semi-hot stamping and
hot stamping on microstructure and mechanical properties of the studied steel was investigated. Thermal
schedules for each process as well as the resulted microstructure are given in Table 2. Geometry of blanks
and description of tools including press, die, cooling
system, and temperature recording tools in addition
to the condition of stamping are reported in literature
[11]. The mold assembly included water or nitrogen
cooled punch and a non-cooled die.
Mechanical properties of the deformed parts were
determined by Vickers hardness test (HV0.8 ) and
standard tensile test, as reported in literature [11].
Surface hardness maps in addition to lateral hardness
profiles were obtained to quantify and qualify material
properties.
M. Naderi et al.: J. Mater. Sci. Technol., 2011, 27(4), 369–376
371
Table 2 Thermal schedules and the resulted microstructures after various stamping processes
Cold stamping+
Quench hardening
Semi-hot stamping
Hot stamping
Heating
Soaking time
temperature/◦ C condition/min
950
10
650
950
5
10
10
Cooling
Phase fraction
Martensite Bainite Ferrite Pearlite
Air coolant
–
–
70
30
Water coolant
85
–
15
–
Water cooled punch
–
–
68
32
Water cooled punch
–
–
62
38
Water cooled punch
42
55
3
–
Nitrogen cooled punch
60.5
36.5
3
–
The CCT diagram in Fig. 2, indicates that the
martensite start temperature (M s) is about 400◦ C
and cooling rates higher than 40◦ C/s result in a fully
martensitic microstructure.
2.4 Temperature and force evaluation
Fig. 2 The CCT diagram of the studied steel
2.3 CCT diagram
The continuous cooling transformation (CCT) diagram, Fig. 2, was determined by means of dilatometry tests, metallographic investigations and hardness
measurements. The circled numbers indicate the values of final hardness, given in the Vickers HV10 scale.
The temperature evolution of blanks, die and
punch during pointed processes was obtained using
thermocouples set in proper positions of the blank
and the tools. Representations of non-cooled die
and cooling system designed for cooled punch as well
as schematic demonstration of appropriate placement
of Pt/Pt-Rh10% thermocouples for monitoring and
recording the temperature evolution of the tools and
blanks during the stamping processes, are provided in
Fig. 3.
The temperature and the force evolutions of the
blank during hot stamping process after austenization at 950◦ C for 10 min and in the condition of using
water as coolant, is represented in Fig. 4. As it is observed, the cooling rate in the range of initial deformation temperature (825-200◦ C) was about 158◦ C/s.
Fig. 3 (a) Die assembly including cooling system in punch, (b) position of Pt/Pt-Rh10% thermocouples for
recording temperature evolution of tools and blanks during stamping processes
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3. Results and Discussion
3.1 Microstructural investigation
Fig. 4 Temperature and the force evolutions during hot
stamping process of MSW1200 steel. Blank was
austenized at 950◦ C for 10 min (HS-hot stamping
& WCP- water cooled punch)
The temperature of the die, which was non-cooled,
rose to maximum 100◦ C while this enhancement for
the nitrogen cooled punch was about 70◦ C. The forming rate was 40 mm/s and deformation took about 1
s to finish. Sudden increment of force at the 14th second (Fig. 4), which increases to about 30 kN, relates
to initiation of the deformation step.
The microstructure of blank after being cold
stamped and subsequently quenched in air consisted
mostly of ferrite while quenching in water caused
mostly martensitic microstructure (Fig. 5). It is definitely due to the role of coolant. Air, as a coolant,
which does not bring the possibility of obtaining critical cooling rate, results in the formation of a ferrite phase, while a cooling rate more than the critical
value, in the case of using water, causes martensite
formation.
As it is observed in Fig. 6, hot stamped blanks obtained a martensitic-bainitic microstructure. When
blanks are cooled from the austenization temperature (950◦ C) using water or nitrogen cooled punch,
since cooling curve does not cross ferrite and pearlite
phase region in the CCT diagram, the microstructure
includes the non-equilibrium phases such as martensite and bainite. More martensite content, which resulted after hot stamping by the application of nitrogen cooled punch, was due to a higher cooling rate
(Table 2).
Fig. 5 Microstructure of MSW1200 steel after quench hardening: (a) 950◦ C, 10 min+air quench; (b) 950◦ C,
10 min+water quench
Fig. 6 Microstructure of MSW1200 steel after hot stamping: (a) 950◦ C, 10 min+hot stamping, WCP, (b) 950◦ C,
10 min+hot stamping, NCP (WCP stands for water cooled punch- NCP stands for nitrogen cooled punch)
M. Naderi et al.: J. Mater. Sci. Technol., 2011, 27(4), 369–376
373
Fig. 7 Microstructure of MSW1200 steel after semi-hot stamping: (a) 650◦ C, 5 min + semi-hot stamping,
(b) 650◦ C, 10 min + semi-hot stamping
The most noticeable microstructure belonged to
semi-hot stamped blanks. It consisted of ferrite and
pearlite having the morphology as shown in Fig. 7.
Before applying the stamping process, the blanks
were heated to a temperature of about 650◦ C. It is
predicted that low heating temperature of semi-hot
stamping process does not result in occurrence of any
phase transformation. Therefore, minor discrepancies
in volume fractions of ferrite and pearlite proportions
in the microstructures of semi-hot stamped blanks
with respect to as-received ones (Table 2) might be
related to minor image analysis errors during evaluation. Quantitative evaluations of the microstructures
were conducted by using “ImageJ” program, which
was an image processor and analyzer developed at the
National Institute of Health, USA. The kind of microstructure constituted in semi-hot stamped blanks
(Fig. 7) could be related to inherent characteristic
of substances to decrease their interior energy. At
constant volume fraction of pearlite phase, as nodule size decreases, surface energy of whole substance
increases[12] . As it was observed in Fig. 1, as-received
microstructure of MSW1200 steel mostly included ferrite as well as fine pearlite nodules. Heating the
blank to a temperature of about 650◦ C for a sufficient time brings the possibility of coalescence of very
fine pearlite nodules (Fig. 7) in order to decrease the
surface energy. So, large pearlite nodules, as it is observed in Fig. 7(b), form in the microstructure. The
presence of fine pearlite nodules in the microstructure
of Fig. 7(a) indicates that five minutes is not adequate to complete the coarsening of the fine pearlite
nodules. Fast cooling due to the employment of water
cooled punch, during semi-hot stamping, resulted in
non-equilibrated ferrite grains with non-ordered grain
boundaries in both samples, especially for the ten
minutes heated specimen (Fig. 7).
3.2 Hardness profiles
Hardness profiles were obtained by using an instrument which not only measured the hardness va-
Fig. 8 Lateral hardness profiles of MSW1200 steel along
the half of the stamped blank
lues linearly, but also was able to determine the hardness values for a predefined region. More information about this instrument can be found in literature
[11]. Lateral hardness profiles of blanks after being
hot and semi-hot stamped are represented in Fig. 8.
Hardness values of the hot stamped blanks (samples A
and B) due to the presence of martensite and bainite
phases were higher than those for semi-hot stamped
blanks (samples C and D) which consisted of ferrite
and pearlite phases.
One important point in Fig. 8 is the improvement
in the homogeneity of hardness profiles by enhancing the heating time. It is observed in Fig. 8 that,
the non-homogeneity of hardness in sample A is the
most. A sufficient heating time for samples B and
C resulted in a more homogeneous microstructure,
and correspondingly, more homogeneity of hardness in
semi-hot and hot stamped blanks. Unsuccessful austenization treatment yields a non-uniform distribution
of carbon in the primary austenite and, as a result, in
the obtained martensite[13] . An increase in the carbon
content of a local region of primary austenite increases
the distortion of the body centered tetragonal (BCT)
lattice during martensite formation, and decreases
the movement of dislocations in the region and as
a result, the hardness values increase[14] . Therefore,
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Fig. 9 Surface hardness map of MSW1200 steel: (a) 950◦ C, 10 min+hot stamping, WCP: (b) 950◦ C, 10 min+hot
stamping, NCP: (NCP- nitrogen cooled punch, WCP- water cooled punch)
Fig. 10 Surface hardness map of MSW1200 steel: (a) 650◦ C, 5 min+semi-hot stamping, WCP; (b) 650◦ C,
10 min+semi-hot stamping, WCP; (WCP: water cooled punch)
proper heat treatment prior to the semi-hot and hot
stamping processes of steel with non-uniform initial
microstructure should be an essential condition to improve the hardness uniformity; although, due to the
presence of various phases with different hardness values beside each other, the hardness non-uniformity is
inevitable.
Surface hardness profiles of hot stamped and semihot stamped blanks are shown in Figs. 9 and 10,
respectively. As it comes from Figs. 9 and 10, the
hot stamped blanks have hardness values greater than
semi-hot stamped ones. Increasing the cooling rate,
employment of nitrogen as coolant instead of water
during hot stamping process, resulted in higher hardness values (Fig. 9). The higher hardness limits represented in Fig. 10(a) with respect to the hardness
limits in Fig. 10(b) might be originated from the development of internal stresses in the microstructure
due to little time of heating and correspondingly nonuniform initial microstructure[15] .
The surface hardness maps can be a good substitution for metallographic analysis. By using this
technique, hardness distribution and similarly phase
distributions can be well studied. The advantages of
this technique over lateral hardness profiles, metallographic investigations as well as more details about
quantitative and qualitative measurement of different
phases are presented in literature [4].
Fig. 11 Flow of true stress in terms of true strain for
MSW1200 steel in different stamping conditions
(WQ: water quenched, AQ: air quenched, WCP:
water cooled punch, NCP: nitrogen cooled punch)
3.3 Mechanical properties
True stress-strain curves of MSW1200 steel for different stamping conditions were determined (Fig. 11).
Yield strength (YS), ultimate tensile strength (UTS)
and total elongation (A25 ) values related to different
blanking conditions are presented in Table 3. The
highest strength values were related to the samples
which had been cold stamped and quenched in water
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Table 3 Mechanical properties of MSW1200 steel grade after various types of stamping
Cold stamping +
quench hardening
Semi-hot stamping
Heating temperature
/◦ C
950
Soaking time
/min
10
650
Hot stamping
950
5
10
10
As-received
–
–
Cooling condition
Air coolant
Water coolant
Water cooled punch
Water cooled punch
Water cooled punch
Nitrogen cooled punch
–
YS
/MPa
540
1110
448
400
916
936
400
UTS
/MPa
810
1430
650
930
1300
1330
640
A25
/%
14
4
25.6
20
5.5
2.5
26
as well as the hot stamped samples; however the samples which had been cold stamped and quenched in air
along with the semi-hot stamped samples represented
the highest ductility values. These were related to
the microstructures of the studied samples. As it was
pointed before (Table 2), the microstructure of the
former sample contained mostly martensite, while the
latter consisted of ferrite-pearlite phases.
Five minutes isothermal heating at 650◦ C prior to
stamping did not affect the as-delivered properties of
the blanks, however yield strength showed some increment. The sample kept for 10 min at temperature of
about 650◦ C before stamping, obtained compromised
values of high tensile strength and ductility. High
values of both tensile strength and ductility of a part
bring the possibility of absorbing energy during the
crash without being torn or fractured[16] . It should
be paid attention that in spite of the benefits pointed
for semi-hot stamping process, one drawback of this
process is the relatively low yield strength value of
stamped blank. Dent resistance decreases as the yield
strength lowers[14] .
Mechanical properties reported for semi-hot
stamped blanks originate from their microstructure.
The microstructure of the sample heated for 5 min
at about 650◦ C included fine pearlite nodules distributed in a matrix of ferrite phase, while that of
the sample kept 10 min, due to coalescence of fine
pearlite nodules, consisted of coarse and elongated
pearlite nodules beside the ferrite phase. The former microstructure involves the possibility of easy
movement of dislocations during the forming process
and, consequently, results in high ductility and low
work hardening characteristics, while the latter causes
impediment of dislocations movement and increases
work hardening characteristic due to the presence of
different phases and non-ordered grain boundaries[17] .
It can be concluded that heating the MSW1200 steel
for 10 min at a temperature of about 650◦ C prior to
stamping guarantees high tensile strength and ductility values due to the formation of special microstructure as it was observed before (Fig. 7(b)).
The ability of a material to have both a good ductility or formability and a high strength is best quantified with the UTS×A25 value that is known as the
formability index value[18] . This value for the mentioned steel in different stamping conditions is repre-
Fig. 12 Formability index values of the blanks deformed
at different stamping conditions
Fig. 13 Comparison between mechanical properties of hot
and semi-hot stamped blanks of MSW1200 steel
and various types of industrially well-known steel
grades
sented in Fig. 12.
As it is observed in Fig. 12, the highest formability index value is related to the condition D which
shows a good combination of high ductility and tensile
strength characteristics together. So it seems that the
studied steel grade and the mentioned heat treatment
is a good choice to produce industrial parts.
It is useful to summarize the results in Fig. 13,
to illustrate mechanical evolution of MSW1200 steel
after hot and semi-hot stamping processes along with
the various types of industrially well-known steel
grades. According to Fig. 13[19] , which represents
the common mechanical properties of some advanced
steels, it is observable that the semi-hot stamped
blanks represent an acceptable combination of tensile
strength and total elongation as TRIP steels, while
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M. Naderi et al.: J. Mater. Sci. Technol., 2011, 27(4), 369–376
hot stamped blanks exhibit mechanical properties like
Martensitic (MART) steels. It can also be observed
in Fig. 13 that MSW1200 steel, after being semi-hot
stamped, shows better and more confidential properties than steels pointed out in Fig. 13, namely HSLA,
Dual phase (DP) and complex phase (CP) steels. The
semi-hot stamping process can be introduced as an
improved thermo-mechanical process. Due to heating temperature below 720◦ C, the A1 arrest line of
equilibrium diagram of Fe-C, and fast cooling during
forming not only guaranties a great deal of elongation
and ductility, but also results in high strength values.
At the end, more investigation into semi-hot stamping
process of suitable steels for production of industrial
parts is recommended.
4. Conclusions
The effect of three different stamping processes
on microstructure and mechanical properties of
MSW1200 steel was studied. The processes included
hot stamping, semi-hot stamping and traditional cold
stamping followed by quenching. It was concluded
that:
(1) Although deletion of springback and lowering
the forming load are the main reasons which encourage conducting hot and semi-hot stamping processes,
but thermal parameters have a significant effect on
achievable properties of stamped parts. In this regard,
parts with high strength (semi-hot stamped parts)
and ultra-high strength values (hot stamped parts)
are obtainable.
(2) Isothermal heating of the studied steel at about
650◦ C for 10 min, prior to semi-hot stamping process,
resulted in a good combination of high tensile strength
and ductility values.
(3) Employing water or nitrogen as coolants with
different cooling severities has no noticeable effect
on microstructure and mechanical properties of hot
stamped blank of MSW1200 steel. So, using water as
coolant during hot stamping process, due to ease of
use and low cost, is recommended.
(4) Surface hardness mapping technique would be
a reasonable and reliable method to quantify and
qualify material characteristics.
(5) Semi-hot stamped blanks of MSW1200 can
sometimes represent an acceptable combination of
tensile strength and total elongation as normal TRIP
steels and better than conventional DP and CP steels.
This sometimes makes the mentioned steel grade and
the process as an acceptable choice to produce industrial parts.
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