Effect of Carbon Content on Burring and
Tapping in Ultra High Strength TRIP Sheet Steels
Taro KOSEN and Hanako KOSEN
Nagano National College of Technology
Advanced course and Dept. of Mechanical Engineering
716 Tokuma, Nagano, Nagano 381-8550 Japan
taro@nagano-nct.ac.jp
Abstract— Effect of thermal drilling condition on burring of
(0.1-0.4)C-1.5Si-1.5Mn (mass%) ultra high strength TRIPaided sheet steels with polygonal ferrite matrix (TDP steels)
was investigated for automotive applications. The combined
rotational and downward force of the thermal drilling tool
bit created friction heat. The height of the bushing was
roughly 3 to 4 times the initial sheet thickness. The bushings
are ideal for thread applications, as the strength of threads
was significantly increased. We found that the burring and
tapping contributed to the improvement of the tensile
strength of 980 MPa class TRIP steel.
A flowdrill worked so well on burring of the TDP steels,
and make it possible that nuts are not used. As for tapping
of the following burring of TDP steel (thickness: 1.2 mm)
using M6 short type flowdrill. Nutless became possible by
machining center.
Keywords— TRIP Sheet Steel; Burring; Tapping; Thermal
Drilling
I.
percentage, and carbon was in the range of 0.1 to 0.4 mass%.
Hereafter, these steels after heat treatment [2] are named TDP1
toTDP4.
For comparison with TDP steel, the ferrite martensite
dual-phase sheet steel (MDP steel) tempered at 400°C for
1 000 s that did not contain retained austenite (γ R ), as
shown in Fig. 1(b), was also prepared.
Tensile testing was performed on an Instron type of tensile
testing machine at a crosshead speed of 1 mm/min (strain rate:
2.8×10－4/s), using JIS-13B type tensile specimens.
Table 1. Chemical composition (mass%) of steels used.
steel
TDP1
TDP2
TDP3
TDP4
MDP
C
0.10
0.20
0.29
0.40
0.14
Si
1.49
1.51
1.46
1.49
0.21
Mn
1.50
1.51
1.50
1.50
1.74
P
0.015
0.015
0.014
0.015
0.010
INTRODUCTION
Increasingly, weight reduction for fuel efficiency due to
environmental pressures, and the improvement of crash safety,
to defend the driver are requirements in the car industry. As
TRIP [1]-aided steels possess excellent press formability, a
great deal of research has been undertaken attempting to apply
these steels to member parts, seat rails and automotive
underbody parts such as lower arms. Research on the
improvement of warm forming [2] and microstructure control [3]
on stretch-flangeability of sheet steel has been reported,
however there have been few investigations of the effect of
drilling condition on burring TRIP-aided sheet steel. [4]
In this investigation, effect of thermal drilling condition on
burring of ultra high strength TRIP-aided sheet steels with
polygonal ferrite matrix (TDP steels) is studied.
II.
EXPERIMENTAL PROCEDURE
Table 1 shows the chemical composition of cold-rolled
sheet steels (thickness: 1.2 mm) used in this study, and Fig. 1
shows the heat treatment diagram. TRIP-aided sheet steel
with polygonal ferrite matrix (TDP steel) austempered at
400°C for 1 000 s in salt bath after intercritical annealing at
780°C for 1 200 s, as shown in Fig. 1(a), were prepared. The
TDP steels contained similar silicon and manganese
Fig. 1. Heat treatment diagram
S
0.0012
0.0011
0.0012
0.0012
0.0030
Burring test was performed by a machining center at
cutting feedrate of F=10 mm/min and rotational speed of
n=3500 rpm, using plate specimens (150×50 mm) and
experimental apparatus in Fig. 2. A bushing was produced by
flowdrill of M6 short type (diameter: D=5.3 mm). Burring and
tapping tests were measured z-axis loading meter (S)
corresponds to thrust and spindle loading meter (T)
corresponds to torque, respectively.
specimen
flowdrill
vise
retained austenite (fγ0) increases and its carbon concentration
(Cγ0) increases, as listed in Table 2.
The relation between loading meter of burring and
processing time (t) is shown in Fig. 4 (TDP2 steel, F=10
mm/min and n=3 500 rpm). Thrust (S) decreases with
increasing the processing time of burring, but it becomes
maximum thrust (Smax), and rises afterwards. Torque (T)
becomes maximum torque (Tmax), and it decreases afterwards.
We got a similar tendency in all specimens.
The following tapping outside and the cutting model are
shown in Fig. 5 (TDP2 steel, F=20mm/min, n=20 rpm). The
relation between loading meters of tapping and processing
time (t) are shown in Fig. 6(a) TDP2 and Fig. 6(b) TDP4.
(F=20mm/min, n=20 rpm and M6×1). Thrust (S) of tapping is
almost constant. Only Torque (T) becomes maximum torque
(Tmax) as well as Fig. 4 and it decreases afterwards. We got a
similar tendency in specimens except for TDP4 steel. TDP4
steel couldn’t conduct tapping and big torque was worked (Fig.
6(b)). When the flowdrill of M6 short type was used, the
50 mm
(a)
Fig. 2. Experimental apparatus for burring.
The amount of retained austenite was quantified by X-ray
diffractometry using Mo-Kα radiation. The initial volume
fraction of retained austenite (f γ0 ) was quantified on the
basis of the integrated intensity of (200)α, (211)α, (200)γ, (220)γ,
and (311)γ diffraction peaks, termed the five-peak method. [5]
The retained austenite lattice constant (aγ0) was measured from
(220)γ diffraction peak using Cr-Kα radiation. Substituting the
measured aγ0 value (nm) into the following equation, carbon
concentration of the retained austenite (Cγ0, mass%) was
calculated. [6]
Cγ 0  (aγ 0  0.35467) / 4.67 103
(b)
(1)
Hardness was measured with the dynamic ultra microVickers hardness tester and was evaluated with a Vickers
hardness (HV).
III.
RESULTS AND DISCUSSION
Figure 3 shows scanning electron micrographs of the
TDP2 steel (a), and that of the MDP steel (b). The
metallurgical characteristics of these steels are listed in Table
2. From the micrographs, it is clear that a network-like second
phase lies mainly on the polygonal ferrite (αf) grain boundaries
in the TDP steels, similarly to the secondary microstructure in
the MDP steel. And the second phase consists of the bainite
(αb) islands and the retained austenite (γR) particles, near or
apart from the bainite islands.
Retained austenite characteristics and tensile properties
of the steels are listed in Table 2. Tensile strength (TS) of the
TDP steels is in a range of 651 to 1 103 MPa, which tends to
increase with carbon content. Total elongation (TEl) of TDP
steel is larger than that of MDP steel. With an increasing
amount of carbon content of the steels, the volume fraction of
Fig. 3. Scanning electron micrographs of (a) TDP2 and (b) MDP
steels, in which “αf”, “αb” , “γR” and “αm” represent ferrite matrix,
bainite island, retained austenite particle and martensite, respectively
Table 2. Retained austenite characteristics and tensile properties of steels used.
fγ0
Cγ0
TS×TEl
YS
TS
TEl
steel
(mass%)
(MPa)
(MPa)
(%)
(GPa%)
TDP1
0.049
1.31
429
651
37.2
24.2
TDP2
0.090
1.38
526
825
36.0
29.7
TDP3
0.132
1.41
562
895
32.2
28.8
TDP4
0.170
1.45
728
1103
32.8
36.2
MDP
-
-
593
783
13.1
10.3
fγ0: initial volume fraction of retained austenite, Cγ0: carbon concentration
in retained austenite, YS: yield stress, TS: tensile strength, TEl: total
elongation and TS×TEl: strength-ductility balance.
T
S
S, T (%)
60
50
40
30
20
10
0
-10
Smax
Tmax
0
10
20
30
40
t (s)
50
60
70
Fig. 4. Effect of time (t) on thrust (S) and torque (T) , in which Smax
and Tmax represent maximum thrust and maximum torque,
respectively (TDP2 steel, F=10 mm/min and n=3 500 rpm).
5 mm
Fig. 5. Cut model after tapping (TDP2 steel, F=20 mm/min and
n=20 rpm).
90
(a)
70
50
S
T
30
max
face, voids that occur in punching is not seen (Fig. 7(a)). At
location of 0.3mm from the end face, large plastic flow can be
observed.
Figure 8 shows schematic diagram of cross-section, in
which diamond represents indentation of Vickers hardness.
Vickers hardness test (load: 0.98 N, holding time: 5 s) was
conducted at 0.3 mm intervals from burring edge.
Figure 9 shows the variation in Vickers hardness (HV) at
burring section. HV of lower burring area indicates higher
value than that of upper burring area. Therefore we can realize
that work hardening has occurred. Also, comparing MDP steel
with TDP1 to TDP4 steels, HV of the bushings in all
measurement locations has been increased with the increase of
carbon content. From this it seems that carbon content is a
major impact on work hardening and strain-induced
transformation.
Figure 10 shows hardness increment (ΔHV=HVmax ―HV0)
of TDP and MDP steels. We decided the initial hardness HV0
and the average of ⑨ to ⑫ HV is hardness on deforming
(HVmax) after transformation. ΔHV became higher with
increasing of carbon content in comparison from TDP1 to
TDP4. We consider that it was affected great on straininduced transformation by increasing total carbon
concentration (fγ0×Cγ0). It is the multiplication of initial carbon
concentration (Cγ0) and initial volume fraction (fγ0) of γR by
carbon content. In addition, comparing MDP steel with TDP
steels, we found that ΔHV of TDP steel is relatively large. We
consider that MDP steel generated work hardening by heat
generation on burring, and TDP steel is affected strain-induced
transformation.
Figure 11 shows relation between Vickers hardness HV and
total carbon concentration (fγ0×Cγ0). Comparing TDP1 to
TDP4 steel, ΔHV became higher with increasing of carbon
content (Fig. 11).
T
90
(a)
(b)
70
S
50
drilling
S, T (%)
10
T
max
30
T
10
-10
90
100
110
120
130
140
t (s)
(b)
Fig. 6. Effect of time (t) on thrust (S) and torque (T) ((a) TDP2
steel, (b) TDP4 steel, F=20 mm/min and n=20 rpm).
tapping after burring made it possible, using machining center
(MC). As a result, the steels of TDP1 to TDP3 made tapping
possible. In changing amount of carbon adding in a range of
0.1 to 0.4 mass%, the influence in tapping was observed.
Figure 7 shows the scanning micrographs of cross section
of TDP2 steel after burring. Figure 7(a) is end face and
Fig.7(b) is location of 0.3mm from the end face. Around end
5μm
Fig. 7. Scanning electron micrograph after burring (TDP2 steel,
(a) end face, (b) location of 0.3 mm from the end face).
ΔHV
400
350
300
250
200
150
100
50
0
TDP1 TDP2 TDP3 TDP4 MDP
steel
Fig .10. Hardness increment (ΔHV)
700
600
Fig 8. Schematic diagram of cross-section.
700
(a) TDP1
500
300
100
0
0
200
(b) TDP2
600
500
TDP4
HV0
300
200
400
3
1
0.05
2
⊿HV
0.1 0.15 0.2 0.25
fγ0×Cγ0 (mass%)
0.3
Fig 11. Relation between Vickers hardness (HV) and total
carbon concentration ( fγ0×Cγ0)
400
IV.
300
SUMMARY
300
1) The flowdrill worked so well on burring of the TRIPaided sheet steels with polygonal ferrite matrix (TDP steels).
As for tapping of the following burring of TDP steels
(thickness: 1.2 mm) using M6 short type flowdrill, the burring
became possible by machining center.
2) (0.1-0.3)C-1.5Si-1.5Mn (mass%) TDP steels (TDP1 to
TDP3 steels) made tapping possible. 0.4C-1.5Si-1.5Mn
(mass%) steel (TDP4 steel) was impossible of tapping.
200
REFERENCE
200
600
HV
HVmax
500
400
HV
600
(c) TDP3
500
400
600
(d) TDP4
[1]
500
[2]
400
[3]
300
[4]
200
600
[5]
[6]
13.
(e) MDP
500
V. F. Zackay, E. R. Parker, D. Fahr and R. Busch: Trans. Am. Soc. Met.,
60 (1967), p. 252.
K. Sugimoto, A. Nagasaka, M. Kobayashi and S. Hashimoto: ISIJ
International, 39 (1999), p. 56.
A. Nagasaka, Y. Kubota, K. Sugimoto, A. Mio, T. Hojo, K. Makii, M.
Kawajiri and M. Kitayama: ISIJ International, 50 (2010), p. 1441.
A. Nagasaka, S. Hasebe, T. Matsushima, K. Sugimoto and T. Murakami:
Journal of Iron Steel Research, International, 18 (2011), p. 442.
H.Maruyama: J. Jpn. Soc. Heat Treat., 17 (1977), p. 198.
Z. Nishiyama: Martensite Transformation, Maruzen, Tokyo, (1979), p.
Authors
Photo
400
Photo
Photo
Photo
Hanako
KOSEN
Ziro
KOSEN
Saburo
KOSEN
300
200
1
2
3 4
5 6
7 8 9 10 11 12
No.
Fig. 9. Variation in Vickers hardness (HV) at burring section.
Taro
KOSEN