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EuroWire – May 2012
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Technical article
steel. In addition, the B steel exhibited a
larger pearlite transformation region.
Stress-strain curves and tensile properties
of the hot rolled rods are given in
Figure 4
and
Table 2
.
The Base and B steels exhibit very similar
stress-strain behaviours albeit that the
B steel exhibits a yield point elongation
(YPE) whereas the Base steel exhibits
continuous (ie smooth, “round-house”)
yielding.
The occurrence of YPE might be somewhat
unexpected as the alloy was designed
to have nitrogen tied up to boron and
the YPE should hence not result from
“free” nitrogen strain aging. The behaviour
hence presumably relates to carbon
strain aging.
It should be recognised that the rods
were straightened at room temperature
following hot rolling, and non uniform
strain during straightening may have
led to removal of YPE in some cases.
Similar tensile strengths and elongations
were obtained in the Base and B steel.
The High B steel exhibited lower strength
values; smooth yielding is observed at
lower strengths compared to the other
steels and an ultimate tensile strength
value lower by about 25 MPa was
obtained. This strength difference cannot
be ascribed to carbon as samples with the
same carbon content were selected for
testing. A higher tensile elongation was
exhibited by the High B steel.
It is interesting to note that reduced
tensile strength with boron alloying is
in agreement with earlier work on low1
and high7 carbon steels and is also in
agreement with increased hardenability
observed in the dilatometry study.
Increased pearlite transformation kinetics
may lead to increased lamellar spacing
and/or coarser pearlite. One might also
argue that the reduced strength level
may be related to reduced solid solution
strengthening. It should however be
recognised that the B alloy does not
exhibit strength reduction compared to
the Base.
It has been suggested previously that the
strength reduction relates to an alloying
effect on the austenite to ferrite1 or
pearlite
11
transformation.
Mechanical properties following wire
drawing to 2.5mm diameter are given in
Figure 5a
and
Table 3
.
In the drawn condition, the B steel exhibits
the lowest tensile strength and elongation,
the High B steel exhibits the highest
tensile strength and higher elongation
compared to the B steel.
The Base steel exhibits similar uniform and
total elongation compared to the High
B steel albeit at a lower tensile strength.
It should be recognised that failures
occurred at the tensile grips which likely
influenced the total elongation values.
Tensile properties obtained after patenting
at 2.5mm diameter are given in
Figure 5b
and
Table 3
.
UTS, MPa
UE, %
TE, %
Base
952
9.4
13.7
B
951
8.2
13.9
High B
926
11.2
16.6
▲
▲
Figure 3
:
Transformation start (squares) and finish (triangles) temperatures for different constant cooling speeds.
Filled symbols: base alloy and open symbols: B steel
Temperature, °C
Time, s
▼
▼
Table 2
:
Tensile properties of the hot-rolled rods
▼
▼
Figure 4
:
Stress-strain curves of the hot-rolled rods
Engineering stress, MPa
Engineering strain, %
Base
High B
High B
Base