15 Simulation of Multi-step Tube Hot Gas Forming Process …
189
quenching results in a phase transformation from austenite to martensite and significant increase of the final strength of the part. A cooling rate of at least 27 K/s is
essential for preventing bainitic phase transformation according to the continuous
time temperature transformation (TTT) diagram. To achieve a full martensitic, an
increase of the minimum cooling rate required for martensite transformation up to
35 K/s is necessary [16]. Thus, components with homogeneous mechanical properties at a final high tensile strength level of above 1500 MPa and a sufficient remaining
ductility of 6–8% uniform elongation as well.
15.4 Simulation of Indirect HGMF
The simulations are performed using LS-DYNA R10.0 finite element codes, which
has several features that are useful to FE modeling hot forming [17, 18]:
Heat conduction, convection and radiation transfer of tube blank before tool to
part contact.
Contact conductance between tools and parts during forming, which is a pressure
dependent;
Phase transform and temperature dependency are considered in UHSS material
model.
The middle layer of tube is modeled as Belytschko-Tsay shell elements with the
size of 5 mm. The dies are modeled as rigid bodies. The thickness of the tube is
considered in the simulation while that of the dies are not considered. In three cold
forming processes, MAT_PIECEWISE_LINEAR PLASITICY is used for the tube
with flow curve at 1 /s. The friction coefficients between the tube and dies is set to
be 0.15.
Fig. 15.8 Thinning contour after end expansion
189
quenching results in a phase transformation from austenite to martensite and significant increase of the final strength of the part. A cooling rate of at least 27 K/s is
essential for preventing bainitic phase transformation according to the continuous
time temperature transformation (TTT) diagram. To achieve a full martensitic, an
increase of the minimum cooling rate required for martensite transformation up to
35 K/s is necessary [16]. Thus, components with homogeneous mechanical properties at a final high tensile strength level of above 1500 MPa and a sufficient remaining
ductility of 6–8% uniform elongation as well.
15.4 Simulation of Indirect HGMF
The simulations are performed using LS-DYNA R10.0 finite element codes, which
has several features that are useful to FE modeling hot forming [17, 18]:
Heat conduction, convection and radiation transfer of tube blank before tool to
part contact.
Contact conductance between tools and parts during forming, which is a pressure
dependent;
Phase transform and temperature dependency are considered in UHSS material
model.
The middle layer of tube is modeled as Belytschko-Tsay shell elements with the
size of 5 mm. The dies are modeled as rigid bodies. The thickness of the tube is
considered in the simulation while that of the dies are not considered. In three cold
forming processes, MAT_PIECEWISE_LINEAR PLASITICY is used for the tube
with flow curve at 1 /s. The friction coefficients between the tube and dies is set to
be 0.15.
Fig. 15.8 Thinning contour after end expansion