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Numerical and Experimental Investigation on Tube
Hot Gas Forming Process for UHSS
To cite this article: Pengzhi Cheng et al 2018 J. Phys.: Conf. Ser. 1063 012172
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NUMISHEET2018
IOP Conf. Series: Journal of Physics: Conf. Series
1063 (2018)
1234567890
‘’“” 012172
IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
Numerical and Experimental Investigation on Tube Hot Gas
Forming Process for UHSS
Pengzhi Cheng1, Yulong Ge2, Yong Xia2, Qing Zhou1,2
1. Suzhou Automotive Research Institute, Tsinghua University, Suzhou, China,
215134
2. Department of Automotive Engineering, Tsinghua University, Beijing, China,
100083
E-mail: chengpengzhi@hyzzbj.com
Abstract. Ultra-high strength steels (UHSS) have been regarded as one of the most attractive
alternatives for lightweight parts in automotive industry. In this study, the tube hot gas forming
process is introduced to implement high temperatures forming for UHSS tubular components. In
the proposed process, the tube blank is first heated up to 900℃ for several minutes to achieve
homogeneous austenitization and then quickly transferred to a press for forming. Nitrogen gas
replaces? water in traditional hydro-forming process as media to apply internal pressure in a
heated tube. To enhance the energy absorption capability of the components, a one-step
quenching and partitioning (Q&P) process is conducted by the die cooling during deforming
simultaneously to increase the elongation and ductility of the material. Moreover, a numerical
investigation on a tube component fabricated from 22MnB5 steel is performed. The temperature
distribution and microstructure evolutions are studied, which demonstrates the forming quantity
advantages of tube gas forming process.
1. Introduction
The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has
drastically increased the demand for ultra-high strength steel components with a maximum tensile
strength up to 1900Mpa[1]. The benefits of press-hardening are evident both in the production phase
(lower press forces, improved part shape accuracy and fewer forming steps due to higher true strain)
and in the utilization phase (improved crash performance due to adapted component properties and
lower component mass at the same level of stiffness). Thus, hot stamping is common practice in
automotive manufacturing to produce parts with complex shapes and high strength. The process is
successfully used to produce body cover panels with merits of low-forming load and small springback. Comparing to these sheet components, tube profiles show better stiffness and energy absorption
for structure components such as chassis and B-pillows. Moreover, using tubes and profiles offers
further substantial potential for savings in terms of lightweight structural components.
Tube hydroforming (THF) is a material-forming process that uses a pressurized fluid in place of a
hard tool to plastically deform a given tubular material into a desired shape. Tube hot gas forming
process is an innovative forming technology whose concepts are based on tube hydroforming and hot
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NUMISHEET2018
IOP Conf. Series: Journal of Physics: Conf. Series
1063 (2018)
1234567890
‘’“” 012172
IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
stamping, combining both part shaping and heat treatment during the actual forming process. Such a
technology offers the advantages of easy formability in a hot state and final high-strength product for
complex tubular shapes[2]. L. Vadillo and M.T. Santos performed simulations of tube bulge tests and
tubes forming processes using dies at high temperatures of Ferritic stainless steel hat showed a 55%
expansion under 14 bars only[3]. H.K. Yi focused on aluminum alloy tubes forming process[4]. A rapid
heating system combining induction heating and additional heating elements was developed to obtain a
uniform temperature distribution on the tube. Forming parameters such as internal pressure, axial
feeding and heating conditions were optimized. Fraunhofer Institute for Machine Tools and Forming
Technology IWU introduced tube gas forming technology in the automotive industry using 22MnB5
manganese-boron alloyed steel[5]. Finite element analysis and experiments were performed to prove the
process capability and several technological limits were observed. Results indicate that the gas has an
influence on the thermal management of the process by causing a different cooling behavior of the part.
More examples were demonstrated by Prof. Dr.-Ing. Dirk Landgrebe and Frank Schieck[6]. To simplify
the apparatus and controlling scheme, T. Maeno developed a new gas forming process of ultra-high
strength steel hollow parts using air filled into sealed tubes and resistance heating to omit the subsequent
heat treatment. In this method, the air is heated by resistance in the sealed tube, where temperature is
the only controlled parameter. The dimensional accuracy of the formed part was improved[7].
As it is still not mature and not industrially established, the process and toolset design present many
challenges that require addressing. In this paper, a self-designed integrated device is demonstrated,
which includes a heating apparatus, conveying machine, pneumatic system, cooling and press equipment
and a complex shape 22MnB5 steel part.
2. Methodology
The tube hot gas process is a combination of press hardening and heat treatment, where both
formability in forming and ductility in crash performance are guaranteed for ultra-high strength steel
part. The thermal process guidance is closely driven by the process window for press-hardening of the
sheet metal components. The process flow is illustrated in Figure 1: (a) In the first, the process starts
with a heating step to austenite the tube blanks in a furnace, typically at 900℃, so that a better
formability and flow stress can be obtained; (b) then the blank is transferred into a die cavity quickly
for sthe forming process; (c) the die closes, a crush forming process is performed , and the Nitrogen
gas is filled into the tube. During the pressure increase, the tube is bulged up to the desired shape; (d)
the water is filled into the tunnel in the die and the formed part is quenched for martensite production;
(e) then the part transferred out at about 300℃ and maintained at room temperature for several
minutes so that a partitioning (Q&P) process can be obtain for an additional increase in ductility[8].
Fine
Four critical controlling temperatures in the process should be marked:
1. Heating temperature. The blank is heated up to 900℃ and held in the furnace for 30 minutes so
that it could be completely austenitized;
2. Bulging temperature. The temperature drops down during the transition and closing steps. It is
important to guarantee that it should be higher than 800℃ during bulging step. It dictates rapid
transiting and closing speeds;
3. Quenching temperature. In the bulging process, the blank is cooled quickly due to contact with
the dies within the cooling system and the quenching starts. So, the gas pressure loading time
should be as short as possible.
4. Partition temperature. The quenching should be stopped between the reheating and martensite
formation temperature and held the component should be transferred into another furnace for
partitioning at 350℃ for 30min.
To implement the process design, an integrated forming device is designed and manufactured.
Instead of controlling temperature and pressure simultaneously, a sequence controlling strategy is
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NUMISHEET2018
IOP Conf. Series: Journal of Physics: Conf. Series
1063 (2018)
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‘’“” 012172
IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
employed in this study, which monitors the blank temperature at key times and applies the loads
rapidly to satisfy the process and simplify the forming system. A customized integrating device is
presented in this paper, which includes a heating apparatus, conveying machine, pneumatic system,
cooling and press equipment. Two robots are employed to shorten the transferring time, and the
pneumatic system supplies a rapid gas pressure increment in 3 seconds up to 40 MPa, shown in
Figure 2. Two booster pumpers and a gas tank with maximum pressure of 45 MPa and content 50 L
are used to achieve this goal. A press sensor is located on the entrance to control the input pressure
on the tube blank. Although the gas is heated in the tube and leads to a further rise in pressure, there
is no side effects on the forming process. In addition, a hydraulic press has a total press force of
50,000 KN and can transmit a force up to 835 KN for sealing punches. The maximum stroke of the
axial cylinders is 300 mm. For our demonstration, a closing force of 2,000 KN is sufficient. The
forming tool set is shown in Figure 2.b.
Blank Temperature
Forming Pressure
(b) Transition
(c) Forming
Temperature(C)
Austenitization
40
800
(d) Quenching
600
(e) Partition
400
20
Martensitization
200
0
0
100
200
300
400
500
600
700
800
0
900
Time(s)
Figure 1. Sequence of hot gas forming of 22MnB5 steel tube
Turbocharger
x
Gauge
y
Silencer
Dryer
Component
Pump
a. Scheme of pneumatic system
b. Tool set
c. Induced heating
Figure 2 Scheme of tools
3
Pressure (MPa)
(a) Heating&Preservation
1000
NUMISHEET2018
IOP Conf. Series: Journal of Physics: Conf. Series
1063 (2018)
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IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
3. Component Specifications
In the present paper, the manufacture of a complex tubular component was studied, as illustrated in
Figure 3. The initial thickness of the blank is 1.6 mm. and the minimum and maximum diameters
𝐷𝑚𝑖𝑛 , 𝐷𝑚𝑎𝑥 are 47 mm and 60 mm respectively which means the maximum deformation ratio can be
calculated as (𝐷𝑚𝑎𝑥 − 𝐷𝑚𝑖𝑛 )⁄𝐷𝑚𝑖𝑛 to be 28%.It should be noted that there exists many local features
with small fillets in the range of 2 to 5 mm. The material of the tube blank is 22MnB5 steel, whose
stress strain curves under a range of temperatures at strain rate 0.1𝑠 −1 are shown in Figure 4.
Figure 3 Schematic of tubular component
Young's Module
2.2E+05
2500
1.8E+05
2000
Stress (MPa)
MPa
2.0E+05
1.6E+05
1.4E+05
0C austenite
100C austenite
200C austenite
300C austenite
400C austenite
500C austenite
600C austenite
700C austenite
800C austenite
900C austenite
1000C austenite
ferrite
pearlite
bainite
martensite
1500
1000
500
1.2E+05
0
200
400
600
800
0
0.00
1000
Temperature (C)
0.05
0.10
0.15
0.20
Strain
Figure 4 Stress strain curve under Temperatures of 22MnB5 and forming tool set
4. Process simulation
The simulation is performed based on LS-DYNA finite element codes [9]. The tube blank is built with
MAT_UHS_STEEL material model using shell element [10] which has five phases transformation and the
thermal properties are defined in MAT_THERMAL_ISOTROPIC_TD_LC model. Rigid shells with similar
thermal properties are used for the dies and punches. The thermal contact between blank and tools are
listed in
Table 1 using FORMING_SURFACE_TO_SURFACE
The process simulation is divided into three steps: gravity, forming & quenching and partitioning. The
tube blank stabilized within 1 second for transferring and then the gravity load is applied to attach the
lower die. The tool set closes and the punches move to the blank and seal the ends. In this case, the
movement of the square punch is 30 mm with considering to the end shrink during the bugling. On the
other hand, the circle punch moves 45mm after die closing for axial feeding in a large expanded area.
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Table 1 Contact coefficients
Thermal
Radiation
conductivity
factor
0.2SBC
0.025W/𝑚𝐾
Heat transfer coef
Friction coef
2000W/𝑚2 𝐾
0.46
IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
5. Result and discussion
The simulation of forming process takes 33 hours to complete using an Intel 3.4GHz CPU. The
thickness distribution is depicted in Figure 5. The maximum thinning ratio is about 11%. There are
two significant thinning areas which can be observed. The first one is the large bugling area (A) and
the second one (B) is the fillets on the right straight area. According to the thickness vs. time curves in
Figure 5, area A deforms along the right punch feed, whereas area B deforms until the pressure
reaches 36MPa. In addition, attaching capability between the blank and dies are quite satisfying. The
local features are bulged up to attaching the die.
a) thickness distribution
b) attaching capability to dies
Area A
Area B
1.60
Thickness (mm)
1.55
1.50
1.45
1.40
1.35
1.30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time (s)
c) thickness reducing in area A & B
Figure 5 Thickness distribution in simulation
During the quenching process, the temperature drops down in a rate of 27℃⁄𝑠 until the 350℃.
Because of the Partitioning process, the final component contains not only the desired martensite, but
also a small amount of ferrite and retained austenite. The ferrite is mainly located on the fillet area and
retained austenite are located on the square end, as evidenced in Figure 6.
Austenite
ferrite
Martensite
Figure 6 Microstructure distribution in simulations(Up: End of Bulging, Down: End of Q&P)
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NUMISHEET2018
IOP Conf. Series: Journal of Physics: Conf. Series
1063 (2018)
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‘’“” 012172
IOP Publishing
doi:10.1088/1742-6596/1063/1/012172
The experiment is performed and an experimental product is obtained, which is consistent to the
simulation, shown in Figure 7 . Then a specimen is cut in a hoop direction to test the strength the part.
It presents that the tensile strength reaches 1400MPa and 3% elongation is achieved which are quite
satisfying, shown in Figure 8.
Figure 7 Experimental product
In this paper, experiments and simulations are performed to analysing a novel tube hot gas forming
process of 22MnB5 ultra-high strength steel. The followed conclusion can be drawn:
(a) The simulation proves that tube hot gas forming technology can enhance the formability of the
material to obtain complex product free from defects;
(b) An equipment is designed and manufactured for the tube hot gas forming process;
(c) An experimental product is obtained which shows a good agreement to the simulation and a
satisfying strength and ductile is achieved.
1400
Stress(MPa)
1200
1000
800
600
400
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Strain
Figure 8 Strength test of final product
Acknowledgments
This research financially supported by Natural Science Foundation of Jiangsu Province (BK20160298)
Scholar.
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