3 Fundamentals of press design

Fine blanking
not been soft annealed: The microstructure comprises ferrite and perlite.
The hard cementite plates have to be broken through when the blanking
punch penetrates the material. The result is tearing of the cut surface.
The illustration at the top left of Fig. 4.7.10 shows the C45 in a soft
annealed state, spheroidized: The microstructure comprises a ferrite
matrix with spheroidal cementite embedded in it. Here, the spheroidal
cementite grains are not divided during the blanking process, but
pressed into the soft ferrite matrix: The blanking process takes place
without tear formation.
With an increasing carbon and alloy content, the tensile strength of
the material in the non-annealed and soft-annealed state (Fig.4.7.11)
increases. C45 which is not soft-annealed has a hot forming microstructure
with a strength of around 700 N/mm 2 . Where C45 has an optimum
soft annealing microstructure, i.e. extra soft spheroidized, its tensile
strength lies at around 480 N/mm 2 , while the corresponding value
for the standard spheroidized quality more commonly used for fine
blanking is 540 N/mm 2 .
tensile strength R m
850
N
mm 2
[ ]
750
700
650
600
550
500
450
400
16Cr3
16Ni14
16MnCr5
25CrMo4
34Cr4
a:
normalized
steel
b:
steel with
90-100%
spheroidal
cementite
350
0 0.2 0.4 0.6 0.8 1 1.2 1.4
C content [%]
Fig. 4.7.11 Dependency of tensile strength upon the proportion of carbon/alloys in
unannealed a and annealed b steels
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
341