Boron Bearing Ti Nb FS Steels

Prévia do material em texto

Influence of Phosphorus Content
on the Recrystallization Behavior
and Mechanical Properties of
Boron-bearing Ti-Nb Fully
Stabilized Steels
D. P. Hoydick and T. M. Osman
Product Technology Division
U. S. Steel Research
Monroeville, PA 15146
ABSRACT
Laboratory processed boron-bearing,
titanium-niobium fully stabilized steels
with varying phosphorus levels were
studied in order to investigate differences
in annealing response and resultant
mechanical properties of these steels.
The results of isothermal annealing
experiments and batch annealing
simulations show an anomalous effect of
phosphorus on the recrystallization
behavior of the boron-bearing, titanium-
niobium fully stabilized steels
investigated in this study. The results
appear to be consistent with commercial
experience.
1. INTRODUCTION
Boron is often added to high
strength, fully stabilized (FS) steels (i.e. an
interstitially free steel) to improve the
resistance to secondary work embrittlement ~
(sWE)”*. SWE is a particular concern in
rephosphorized FS steels as previous workl’2
has shown that phosphorus segregates to
grain boundaries during processing and
causes an embrittling effect.
Mechanistically, it is believed that boron
additions improve the resistance to SWE by
either lowering phosphorus segregation by
competing with phosphorus for grain
boundary sites or just improving the
4
cohesive strength of the boundaries
independently with its presence3-5.
As previously citedl’2, boron
additions drastically improve SWE
behavior; however, the addition of boron
has some detrimental effects also. Other
mechanical properties, particularly the
ductility/formability parameters such as
total elongation, uniform elongation, and r~-
value, are degraded by the additional
alloying. For example, an almost linear
decrease in r~-value was found with boron
additions to a titanium (Ti)-stabilized steel
by J. Haga et als. In this study, r~ values
dropped from 2.0 to below 1.6 with a boron
addition of 0.0024 weight percent. A
systematic study of the effects of variations
in phosphorus content on r~ values in boron-
bearing, FS steels, however, has yet to be
documented.
Other negative effects of boron in
FS steels involve factors related to
processing. According to Baetens et al.7,
hot mill loads have been found to be
relatively high in boron bearing FS steels. In
their work, a titanium-niobium (Ti-Nb) FS
steel with a boron addition was found to
exhibit a hot rolling deformation resistance
of 600 MPa at 800 “C (1472 ‘F) as
compared to a 500 MPa resistance for a
similar non-boron-bearing grade.
Consequently, higher finishing temperatures
may be required when processing these
materials.
Additionally, ferrite
recrystallization temperatures have been
found to be high in boron-bearing FS steels
in comparison to non-boron-bearing grades,
The recrystallization temperature of one
particular Ti-Nb FS steel was shown to
increase from 750 ‘C (1380 ‘F) to 850 ‘C
(1560 “F) with a boron addition8. While
there are many examples in the literature of
such changes, more work appears necessary
to provide a definitive explanation of the
role of boron in microstmctural
development of FS steels. Also, it is not
0TH MWSP CONF. PROC., 1SS 1998 – 195
through an identical processing schedule,
clear if any synergistic effects exist between
boron and other alloying elements in these
steels.
Some of the above phenomena may
result from the fact that boron has been
shown to have a strong influence on the ‘y-
ct transformation7’9’ 10. Several authors have
shown that boron additions to Ti, Nb, and/or
Ti-Nb stabilized FS steels can result in hot
band structures of that are either quasi-
polygonal or even bainitic. It may therefore
not be surprising that the structure and
property development through cold rolling
and annealing may be different from that
observed in non-boron-bearing fully
stabilized steels.
The commercial implications of the
effects of boron on processing, particularly
with respect to recrystallization behavior,
were highlighted during recent mill trials.
In these trials, a boron-bearing fully
stabilized grade was found to be only
partially recrystallized in practice using
conventional commercial batch annealing
cycles (hot spot 732 “C, cold spot 677 ‘C).
The problem materials had the following
compositions:
Trial 1: 0.034 weight percent
0.037 weight percent
0.013 weight percent P,
0.0009 weight percent B
Trial 2: 0.038 weight percent
0.037 weight percent
0.006 weight percent P,
0.0007 weight percent B
These grades are Ti-Nb
materials with boron additions
Nb,
Ti,
and
Nb,
Ti,
and
FS
and
phosphorus levels in the 0.005 to 0.015
weight percent range. This problem with
annealing was unexpected since previous
experience with boron-bearing Ti-Nb steels
containing higher phosphorus levels (>0.025
weight percent) had resulted in a fully
recrystallized structure when processed
196 – 40TH MWSP CONF. PROC., 1SS 1998
Also, a similar high phosphorus (>0.025
weight percent) Ti-Nb grade without any
boron has been processed without incident.
A summary of these occurrences are shown
in Table I. The table lists the boron and
phosphorus contents of the respective
grades and also indicates whether or not the
steel was fully recrystallized after annealing
in identical commercial batch annealing
cycles. These occurrences were the
impetus for the current work to investigate
the influence of phosphorus content on the
recrystallization behavior and resultant
properties of boron-bearing fully stabilized
steel.
Table I. Summary of Commercial
Experience With Different Boron and
Phosphorus Containing Ti-Nb FS Grades.
Grade B level P level Recrystalliza-
tion Problem
1 0.0006- 0.ooo5- yes
0.0012 0.015
2 0.0006- 0.025 - no
0.0012 0.04
3 <0.0002 0.025 - no
0.04
2. EXPERIMENTAL PROCEDURE
Four laboratory heats were
produced via vacuum induction melting
with the compositions shown in Table II,
The intent was to produce boron-bearing,
Ti-Nb steels containing a typical boron
content seen in commercial grades while
varying phosphorus contents. This would
allow for a determination of the influence of
phosphorus on the recrystallization behavior
and structural development. As seen in
Table II, the phosphorus content varied from
0.004 to 0.039 weight percent. The boron
content was similar for steels A, C, and D,
but was higher for steel B. It is noted,
however, that the boron content for steel B
ositi
0
0
0
0
Salt pot anneals were performed utilizing an
alkali salt bath and recrystallization
behavior was monitored using a
combination of hardness measurements and
optical microscopy. Batch annealing
simulations were performed according to the
hot spot (732”C) and cold spot (677”C)
thermal cycles as shown in Figure 1.
quenched in order to examine
recrystallization kinetics. Initially the steels
were annealed at 815 “C (1500 ‘W) for
various times and hardness values were
measured as a function of soak time at
temperature to monitor the degree of
softening. The results are shown in Figure
2. For each material, the curves are seen to
Table II Nominal Comp
Steel c Mn P s
A 0.0043 0.19 0.004 0.011
B 0.0027 0.19 0.016 0.011
c 0.0028 0.19 0.030 0.011
D 0.0037 0.18 0.039 0.011,~
still lies within the typical compositional
range used commercially (0.00 12 weight
percent maximum). The compositions of
the alloying elements deemed to be
important in stabilization reactions,
including N, C, S, Ti, Nb, and Mn, appeared
to be reasonably uniform throughout the
steels, although some variation in carbon
levels existed. All of the steels were fully
stabilized based ‘upon stoichiometric
calculations.
After induction melting, each ingot
was heat treated at 1260 “C (2300 ~ for 3.5
hours and then slabbed on a laboratory hot
rollingfacility. The initial hot reduction
employed a finishing temperature of 982 “C
(1800 T) followed by air-cooling and
resulted in 3.8 cm (1.5”) thick slab. The
slabs were then sectioned, reheated to 1260
‘C (2300 ‘F) and held for 45 minutes, and
re-rolled to a final hot band thickness using
a finishing temperature of 913 “C (1675 “F).
The samples were cooled to 649 ‘C (1200
‘F) and then placed in a coiling simulation
furnace (28 “C/hour cooling cycle). The
specimens were subsequently cold-rolled 70
percent to a final nominal thickness of 0.76
mm (0.030 in.).
Recrystallization temperatures were
determined isothermally using a salt pot.
40TH
ons of Steels (in weight percent)
Si Ti Nb N B
.024 0.043 0.040 0.0031 0.0004
.024 0.040 0.048 0.0029 0.0012
.025 0.040 0.040 0.0032 0.0006
.022 0.041 0.049 0.0032
_ 0.0005 I
Mechanical testing was performed on the
batch annealed (hot spot cycle) materials in
accordance with ASTM E8 and r-values
were determined using MODUL-r testing.
800
700
E
- 500
~ 400
~ 300
a
g 200
100
0
0 510152025303540
Time (hours)
Figure 1. Thermal profiles for batch
annealing simulations.
3. RESULTS
3.1 Isothermal annealing studies
After cold rolling, the series of
steels was annealed isothermally at various
temperatures and times and then water
MWSP CONF. PROC., 1SS 1998 – 197
level at times greater than 120 seconds.
Optical microscopy was then used to
determine the point of complete
recrystallization (i.e. elimination of
‘pancake’ grain structure induced by cold
working). The sequence in terms of
increasing time for recrystallization at 815
‘C ( 1500 “F) was found to be: steel A, D, C,
and B. The recrystallization behavior was
also investigated using another approach.
Each steel underwent a 60 second heat
treatment at various temperatures between
760 “C (1400 ‘F) and 871 ‘C (1600 “F). The
results of hardness testing on the water
quenched materials is seen in Figure 3.
Defining the recrystallization temperature as
the minimum temperature in which the
material will recrystallize in 60 seconds, the
recrystallization temperatures were
determined via optical microscopy as shown
in Table III.
-0- Steel A
100 .. . . .
~ Steel B
e
~ Steel C
- Steel D
90 ~ I I d
40tttttH
o 50 100 150 200 250 300
Time (sec.)
Figure 2. Hardness versus time curves for
materials at the 815 ‘C ( 1500°F) isotherm.
198- 40TH MWSP CONF. PROC., 1SS 1998
4 1 1 I
760 780 800 820 840 860 880
Temperature (“C)
Figure 3. Hardness versus temperature
curves for a constant 60 second heat
treatment.
Table III. Recrystallization
Temperatures for Each of the Steels.
Material Recrystallization
Temperature, ‘C (“F)
A 799 (1470)
B 854 (1570)
c 832 (1530)
D 832 (1530)
3.2 Mechanical Properties
The mechanical properties of the
materials undergoing the hot spot batch
annealing cycle were also measured. A
summary of these properties is shown in
Table IV. As expected, with an increase in
phosphorus level, an appreciable increase in
strength was observed along with a
corresponding decrease in r~ value.
Mec
T
Elon
(
4
4
3
4
than that for Steel A. It should be noted that
O 0.008 0.016 0.024 0.032 0.04
PllOs#loruE Odx?nt (weight perumt)
Figure 4. Recrystallization Temperature as
a function of phosphorus level.
the peak in Figure 4 corresponds to the
recrystallization temperature found for steel
B, which has a much higher boron content
(0.0012 weight percent versus about 0.0005
weight percent) than the other three steels.
*The slight difference is most likely due to a
lower amount of cold reduction in the current
study (70% versus 75% in previous work) and
the slightly larger amount of solute Nb in the
current study.
Table IV As-Batch Annealed
Steel Yield Tensile
Strength, Strength,
MPa (ksi) MPa (ksi)
A 131 (19.0) 318 (46.1)
B 147 (21.3) 337 (48.9)
c 152 (22.0) 349 (50.7)
D 155 (22.5) 354 (51.4)
4. DISCUSSION
4.1 Recrystallization Behavior
As mentioned earlier, the addition
of boron to both Ti and Ti-Nb fidly
stabilized steels has been found to result in
higher recrystallization temperatures than
similar non-boron bearing materials. In the
previous studys, however, the corresponding
phosphorus level was not reported. In the
present work, the phosphorus level is shown
to have a significant effect on the
recrystallization behavior of boron-bearing,
Ti-Nb FS steels.
40TH M
hanical Properties (Hot Spot Anneal)
otal N-value r~-value
gation
%) (5Nlt)
2.5 0.283 1.81
0.2 0.275 1.66
7.9 0.270 1.68
2.3 0.268 1.59
As the phosphorus level increases in
these boron-bearing Ti-Nb fully stabilized
steels, the recrystallization temperature
initially increases, reaches a peak, and then
decreases. This trend in recrystallization
temperature with respect to phosphorus
content observed in the isothermal
annealing studies is summarized in Figure 4.
Steel A, exhibiting the lowest
recrystallization temperature of the steels
studied, represents a boron-bearing FS steel
with essentially no phosphorus (i.e., even
less than normal residual levels). In
isothermal annealing, the time to complete
recrystallization was comparable* to that
previously observed for a non-boron-bearing
Ti-Nb FS steel*l, suggesting a limited effect
of the boron alone on recrystallization
kinetics. In contrast, the recrystallization
temperatures for Steels C and D, which have
comparable boron contents to Steel A but
varying phosphorus contents, are greater
WSP CONF. PROC., 1SS 1998 – 199
98
One can argue that the higher
recrystallization temperature may be due
solely to the higher boron content; however,
Haga et al.s observed an increase in
recrystallization temperature of only 10 ‘C
when increasing the boron content from
0.005 weight percent to 0.012 weight
percent in a titanium-only FS steel with a
similar phosphorus content (0.01 weight
percent). In this work, a remarkable
increase in recrystallization finishing
temperature was observed with increasing
boron content up to 5 ppm but only gradual
increases resulted with further boron
additions. The large change in
recrystallization temperature observed in the
current titanium-niobium FS steels coupled
with the commercial experience mentioned
earlier suggests that the effect of
phosphorus should also be considered.
The maximum recrystallization
temperature in the laboratory study was
found for the phosphorus level of 0.016
weight percent, which is comparable to the
phosphorus level of the steels that would not
recrystallize during commercial trials (see
Table I). In addition, recrystallization
readily occurred in commercial production
at higher phosphorus contents, apparently
agreeing with the results in Figure 4
showing a decrease in recrystallization
temperature with higher phosphorus levels.
This suggests that the recrystallization
process in boron-bearing, Ti-Nb FS steels is
related to both the boron and phosphorus
contents.
The results from this study and from
commercial experience therefore suggest
that the effect of a given boron addition (e.g.
0.0005 weight percent B) on the
recrystallization behavior of Ti-Nb FS steels
is influenced by phosphorus level. The fact
that a phosphorus addition influences
recrystallization behavior is not surprising
as phosphorus additions to non-boron-
bearing FS steels have been shown to hinder
recrystallization. However, an anomalous
behavior with increasing phosphorus
200 – 40TH MWSP CONF. PROC., 1SS 19
content is observed here. As revealed by
both the commercial experience (Table I)
and laboratory isothermal experiments,
there appears to be range of phosphorus
approximately between 0.005 to 0.02 weight
percent where the recrystallization process
is most sluggish in boron bearing Ti-Nb FS
steels. Both lower and higher phosphorus
contents exhibit more reasonablerecrystallization behavior.
In an attempt to provide an
explanation for the trends with respect to
phosphorus content observed above, a
metallographic study was conducted.
Optical microscopy and transmission
electron microscopy were employed to
provide microstructural observations of
these steels at different stages of processing.
Both hot band samples as well as cold rolled
and annealed samples were investigated.
4.2 Microstructural Development
The hot bands were examined via
optical microscopy with the microstructure
for steels A through D shown in Figure 5.
All of the structures appeared to be quasi-
polygonal in nature with characteristically
jagged grain boundaries. The general
features of these structures appear to be in
agreement with some of the microstructure
observed by previous investigators in other
boron-bearing fully stabilized stee17. These
results suggest that the nature of the et - y
transformation is altered by the additional
alloying. Also, it must be noted that the
grain size varies appreciably, as steel A
exhibits the coarsest structure, steel B the
finest, and steels C and D being
intermediate.
It is generally believed in the theory
of recrystallization that after an identical
cold rolling and annealing treatment, an
initially fine grained material will
recrystallize more rapidly than a coarse
grained material. Thus, in this study, based
upon hot band grain size alone, material B Although not definitively
40
would be expected to have the lowest
recrystallization temperature, material A the
highest. In actuality, the opposite behavior
was observed with material A and B exhibit
recrystallization temperatures of 799°C and
854°C, respectively. Therefore, it is clear
that some other microstructural
feature/features is contributing to the
recrystallization behaviors observed here.
A limited study was also conducted
on the precipitate distribution in the hot
band. In terms of precipitate type and
distributions, extraction replicas were
evaluated using energy dispersive
spectroscopy (EDS) on a transmission
electron microscope (TEM) and polished
samples were also analyzed using energy
dispersive spectroscopy on a scanning
electron microscope (SEM). Using these
techniques, only particles larger than one
micron could be analyzed for the individual
steels. The results showed that for such
particles, similar precipitate distributions
were found among all steels, with titanium
sulfides and titanium carbo-sulfides
dominating the structures. Of the particles
greater than one micron, no borides or
phosphides were observed.
Differences in the microstructure of
cold rolled and annealed samples were also
observed. While all of the steels exhibited
equiaxed, polygonal grains with slightly
smaller grain sizes with increased alloying,
TEM imaging revealed differences in the
precipitate distributions. Figure 6 shows
bright field images of precipitates in
recrystallized samples of steels A, B, and D
from the isothermal annealing experiments.
In relative terms, the images show coarse,
finely distributed particles in steel A;
coarse, sparsely distributed particles in steel
B; and fine, finely distributed particles in
steel D. It can be speculated, although not
experimentally determined, that the higher
carbon levels in samples A and D may
contribute to their higher precipitate volume
fractions.
determined during this study, a potential
explanation for the observed differences in
recrystallization behavior may be related to
the effects of boron and phosphorus in each
steel. As discussed earlier, phosphorus and
boron tend to segregate to the grain
boundaries. Such segregation may result in
solute drag during recrystallization,
reducing boundary mobility and retarding
the recrystallization process. Based solely
upon a solute drag mechanism, as the
amount of phosphorus and boron increases
in the steel, it is expected that the
recrystallization temperature would
increase. Similarly, for a given boron
addition, an increase in phosphorus content
should result in more sluggish
recrystallization kinetics. These
assumptions hold true as long as all of the
phosphorus and boron remains in solution.
If precipitation of boron or phosphorus
occurs, however, solute is removed from
solution, decreasing the solute drag effect,
and leading to a lower recrystallization
temperature. Based upon these ideas, one
mechanism that could contribute to the
behavior reflected in Figure 4 and Table I
involves the precipitation of FeTiP particles.
These precipitates have been previously
observed12-]4 in similar Ti and Ti-Nb high
strength FS steels. For a given boron level,
if the phosphorus level is continually
increased, a critical phosphorus level may
be reached where FeTiP particles begin to
form. Assuming that the resulting
precipitate distribution is such that boundary
pinning is not a factor]4, the removal of
phosphorus from solution should increase
boundary mobility by decreasing the amount
of solute drag. This could therefore
contribute to the decrease in
recrystallization temperatures seen at the
higher phosphorus contents in Figure 4.
It must be noted that more work is
necessary to determine if this mechanism or
another mechanism is operative to describe
the annealing behavior of the current steel
TH MWSP CONF. PROC., 1SS 1998 – 201
Figure 5. Hot band microstructure for
steels A, B, C, and D.
202 – 40TH MWSP CONF. PROC., 1SS 1998
0TH
grades. Clearly, a more definitive study on
the nature and distribution of precipitates in
the materials both in the hot band and after
annealing is needed to elucidate the
mechanism for the anomalous annealing
behavior observed in this investigation.
5. CONCLUSION
1) The results of laboratory isothermal
annealing experiments on boron-bearing Ti-
Nb steels show an anomalous behavior with
phosphorus content. The recrystallization
kinetics are most sluggish at approximately
0.005 to 0.02 weight percent phosphorus
whereas at both higher and lower
phosphorus contents, the recrystallization
behavior is more reasonable. These results
appear to be consistent with commercial
experience.
2) Hot band structures are quasi-polygonal
in nature with the grain size varying
appreciably with alloy content.
Additionally, variations in the precipitate
distribution in cold rolled and annealed
samples were found although the nature of
the distribution differences were not
determined.
3) Although not definitively determined in
this study, it appears that the anomalous
recrystallization behavior of the boron-
bearing Ti-Nb FS steels may be due to
differences in precipitate distributions and
solute contents.
4) The mechanical properties scaled as
expected from previous work. The yield
strength and the tensile strength increased
while the r~-values decreased with
increasing phosphorus contents.
ACKNOWLEDGEMENTS
The authors would like to
acknowledge the assistance of F. Bakewell,
K.Ruediger, G. Walters, T. Collier, C.
4
Diable, A. Thompson, C. Scelsi, F.
Shannon, T. Lewertowicz, and J. Conroy of
U. S. Steel Research.
The material in this paper is
intended for general information only. Any
use of this materiaJ in relation to any
specific application should be based on
independent examination and verification of
its unrestricted availability for such use, and
a determination of suitability for the
application by professionally qualified
personnel. No license under any USX
Corporation patents or other proprietary
interest is implied by the publication of this
paper. Those making use of or relying upon
the material assume all risks and liability
arising from such use or reliance.
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04 – 40TH MWSP CONF. PROC., 1SS 1998
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