Application of TiN to LC Nb V Steels

Prévia do material em texto

An Assessment of the Application of TiN
Technology to Controlled-Rolled, Low-Carbon
Nb - (V) Steels
R. L. Bodnar, Y. Shen, and W. Furdanowicz
Bethlehem Steel Corporation
Homer Research Laboratories, Building G
Bethlehem, PA 18016-7699
Tel. nos. 610-694-7878,2479, and 5680, respectively
ABSTRACT
In recent years, a small addition of titanium, e.g.,
0.012% Ti, has become a popular alloy-design method-
ology for ferrite grain refinement in as-rolled HSLA steels,
However, even with the requisite rapid post-solidification
cooling rate to ensure a fine dispersion of TiN particles for
pinning austenite grain boundaries, a small Ti addition
may not result in a strength enhancement in controlled-
rolled 0.08% C - 0.035% Nb - (V) steels. This paper
presents examples where a small Ti addition results in a
strength increase, a strength decrease, and no
strengthening, but in most cases results in an increase in
toughness in such steels. A nitrogen addition to a 0.08% C
- 0.035% Nb - (V) - 0.012% Ti - 0.005% N steel results in
a decrease in strength and toughness.
1. INTRODUCTION
The physical metallurgy of controlled-rolled Nb-V
plate steels is well known and is described elsewhere [e.g.,
1,2]. In recent years, the application of titanium nitride
technology to both Nb [3-21] and Nb-V [4,6,10,17,22-30]
plate steels has increased substantially. TiN technology
involves the addition of about 0.008 to 0.020% Ti to an
aluminum-killed steel. The Ti level is maintained within
this range to ensure that Ti and N combine and precipitate
as fine TiN particles after solidification and not as coarse
particles in the superheated liquid. Continuous casting (as
opposed to ingot casting) is necessary to achieve a suffi-
ciendy-fast post-solidification cooling rate (greater than
abou~t 25 OC/minute) required for the formation of a fine
dispersion of stable TiN particles. The nitrogen content in
these Ti-treated steels is normally maintained such that the
Ti:N ratio is hypostoichiometric (i.e., a Ti:N ratio less than
3.4: 1), but less than about 0.01 20% to minimize the forma-
tion of coarse TiN particles. A more in-depth discussion
of TiN technology can be found in the open literature [31-
35].
In Nb or Nb-V plate steels, a fine dispersion of TiN
4
particles provides the basis for a number of metallurgical
benefits. For example, TiN particles can refine the as-cast
structure, remove nitrogen from solution and minimize the
precipitation of the more detrimental AIN and nitrogen-
rich Nb(C,N) particles, and hence reduce the susceptibility,
to transverse cracking in continuously-cast slabs of Nb-
bearing HSLA steels [36-40]. TiN particles, by pinning
austenite grain boundaries, increase the austenite grain
coarsening temperature, restrict austenite grain growth
during welding, and improve the heat-affected-zone (HAZ)
toughness in plate steels [27,41-46]. A fine dispersion of
TiN particles also prevents the formation of excessively
large austenite grains during slab reheating, thereby
ensuring complete recrystallization in the early, light
(s1O’YO)roughing passes, i.e., minimizes strain-induced
grain boundary migration [6,47]. With the effective
application of TiN technology, a finer austenite grain size
may be achievable after finish rolling, which can transform
into finer ferrite, pearlite, and bainite microstructure.
In addition to the benefits of minimizing transverse
slab cracking and improving HAZ toughness, TiN tech-
nology has become a popular alloy-design methodology
for ferrite grain refinement in as-rolled HSLA steels.
However, even when a refined austenite grain size is
achieved with TiN technology, a small Ti addition may not
result in a strength enhancement in controlled rolled
0.08% C - 0.035% Nb - (V) steels. This paper provides an
assessment of the application of TiN technology to such
steels and relates as-rolled plate mechanical properties to
microalloying precipitation behavior and microstructure.
Specifically, it presents examples where a small Ti
addition results in a strength increase, a strength decrease,
and no strengthening, but in most cases results in an
increase in toughness. The effects of nitrogen and
accelerated cooling on such Ti-treated steels are also
discussed,
2. EXPERIMENTAL PROCEDURE
Three examples covering various groups of steels,
plate thicknesses, and processing are presented in this
paper. In each series of steels, the effects of alloying
elements and processing variables are examined. The pur-
poses of each series are listed as follows:
Series I: Effects of alloying elements (Ti, V and N) and
plate thickness (cooling rate);
Series II: Effects of alloying element (Ti) and finish
rolling temperature (FRT);
Series III: Effects of alloying element (Ti), FRT and
accelerated cooling (cooling rate).
2.1 Composition
The compositions of the ten steels investigated are
shown in Table L Each steel nominally contains 0.0870 C,
0.015% P, 0.00470 s, 0.27% Si, 0.045% Al, and
0.035% Nb. Although the manganese content among all
the steels ranges from about 1.20% to 1.45%, it is kept
0TH MWSP CONF. PROC., 1SS 1998 – 929
Table 1. Compositions of the Steels Investigated, wt.OA
.:.:, ...... .,
,
,. . . ,
0
0
0
...
0
0
0
0
0
0
0
The Series II steels were designed to examine the
effect of Ti (O or 0.015%) on a 1.30% Mn - 0.035’% Nb -
0.07% V - 0.0070% N base composition. Plates were
rolled to 9 mm in thickness with various FRT’s ranging
from 700 to 845”C. The Series III steels contain
1.45’% Mn, 0.035% Nb, 0.07% V and 0.0070% N. Here,
the effect of a 0.015% Ti addition is examined in 19 mm
thick plates produced with various FRT’s ranging from
735 to 845°C followed by either air cooling or accelerated
cooling.
2.2 Melting
All of the steels were vacuum-induction melted in
the laboratory using Arrnco iron and cast as 225 kg ingots,
11 Nb-v 100 9 79 700 to 845
Nb-V 150 19 78 700,725
III
Nb-V 150 19 78 730 to 835
Air Cooled
I
Air Cooled I
2.4 Grain Coarsening Studies
To understand the austenite grain coarsening
behavior of the steels employed for Series I and II listed in
Table I, an austenitizing series was conducted. Represen-
tative specimens from each of the steels, measuring
approximately 13 mm x 25 mm x 25 mm, were
austenitized at temperatures between 1040”C and 1370°C
for one-half hour and water quenched. The specimens
930 – 40TH MWSP CONF. PROC., 1SS 1998
~:~’j~
.,..,.,,’ .:.,.
—
Nb 0.07 1.17 0.015 0.0
la Nb-Ti 0.07 1.20 0.015 0.0
Nb-Ti-N 0.07 1.18 0.012 0.0
....................-..—-----.................... .. .....-..........-----....................................... ..... .......
Nb-v 0.07 1.30 0.015 0.0
Ib, Nb-V-Ti 0.08 1.33 0.016 0.0
Nb-V-Ti-N 0.06 1.26 0.014 0.0
Nb-V 0.08 1.30 0.017 0.0
11,
Nb-V-Ti 0.08 1.31 0.014 0.0
Nb-V 0.08 1.45 0.017 0.0
111
Nb-V-Ti 0.08 1.45 0.017 0.0
constant within a given series of steels. Therefore, man-
ganese content is not considered as a variable in this paper.
Table I divides the steels into three separate series.
Series I consists of two groups of steels (each con-
taining three compositions), designated as “Nb-steels”
(Series Ia) and “Nb-V steels” (Series Ib). The Series Ia
steels have a base composition of 1.20% Mn, 0.005% N
and no vanadium. Small amounts of Ti and N were added
to the base to show the effects of Ti and N. The first steel
(base) is labeled “Nb”. The second steel, designated Nb-
Ti, is the base composition with an addition of 0.015% Ti
to provide a near stoichiometric Ti:N ratio. The third steel,
designated “Nb-Ti-N’, is similar to the second steel with
the (exception of a higher nitrogen level of 0.015% to
provide a hypostoichiometric Ti:N ratio of 1.1. Some
excess nitrogen, after all of the available Ti reacts with N
to form TiN, is expected to precipitate with Nb asNb(C,N)
in the austenite to retard austenite recrystallization, and
some Nb(C,N) is expected to precipitate in the ferrite to
provide precipitation strengthening. The Series Ib steels
(Nb-Y type) have a base composition of 1.30% Mn,
0.07’% V and 0.005% N, and are similarly designated “Nb-
V“, “Nb-V-Tin, and “Nb-V-Ti-N”. All six steels in
Series I were rolled to 13 mm and 25 mm thick plates with
fixed finish rolling temperatures followed by air cooling.
,:,:,,: ,,.,,,>,:.,, ,,.:,.,:
~;~fif
,:..., ,., ,,, w., ..: .,... ... ... ...
-
4 0.28 0.049 0.037 <0.003 -- 0.0053 0
3 0.27 0.050 0.037 <0.003 0.019 0.0043 4.4
3 0.26 0.051 0.039 <0.003 ‘0.017 0.0150 1.1
...... ................._......--------................,_-............ ..---------------..._..+----------------------------
4 0.27 0.050 0.038 0.07 -- 0.0047 0
4 0.28 0.053 0.042 0.07 0.017 0.0046 3.7
4 0.27 0,049 0.039 0.07 0.017 0.0120 1.4
6 0.25 0.038 0.030 0.07 -- 0.0070 0
4 0.29 0.046 0.031 0.07 0.015 0.0070 2.1
4 0.27 0.035 0.035 0.07 -- 0.0070 0
4 0.27 0.035 0.035 0.07 0.012 0.0070 1.7
each measuring about 215 mm square x 500 mm long.
Based on previous work, the post-solidification cooling
rate for this ingot size and shape should approximate that
of a 250 mm thick continuously’-cast slab [35,41,42,48].
2.3 Rolling/Accelerated Cooling
The ingot rolling, plate rolling, and accelerated
cooling procedures are described in reference 30. The
starting billet thickness, final plate thickness, total reduc-
tion below 101O”C, FRT, and cooling practice employed
for each plate are summarized in Table II. The actual plate
thicknesses include 9, 13, 19, and 25 mm and the plates
were control rolled using a plate mill computer model to
simulate actual production processing [49].
Tablen. SommaryofRollingandCoolingPracticesEmployed
Nb l(x) 13 66 760
Nb-v ILK) 13 66 760
I
Nb 100 25 50 760
Nb-v 100 25 60 700
Air Cooled I
+
Air Cooled
Air Cooled
Air Cooled
in the ferritic matrix. Wafers were cut from tensile ends
4
were metallographically prepared and etched in an aqueous
solution saturated with picric acid plus 0.25 g sodium
dodecylbenzene sulfonate (40% by weight in water) at
70”C to delineate the prior austenite grain boundaries. The
average prior austenite grain size was determined using the
circular intercept method of ASTM El 12.
2.5 Mechanical Testing
Duplicate tensile specimens from the plates were
tested in the transverse orientation, i.e., normal to the
rolling direction. Flat tensile specimens were machined
from each of the 9 and 13 mm thick plates; 12.8 mm
diameter tensile specimens were machined from the mid-
thickness of the 19 and 25 mm thick plates.
Charpy V-notch (CVN) specimens were machined
from the mid-thickness of each of the plates in the trans-
verse orientation and two or more specimens were tested at
either -20 or -45°C. While 2/3-size CVN specimens were
machined from the 9 mm thick plates, full-size CVN speci-
mens were machined from the thicker plates. In selected
cases, full energy transition curves were determined.
2.6 Metallography
Tensile ends representing each plate were polished
on a longitudinal face and examined in the light
microscope in both the unetched and etched conditions. A
Leco 2005 image analyzer was used to determine the
volume fractions of oxides and sulfides in the unetched
condition. Klemm’s Reagent was used to distinguish
coarse TiN particles from the fine ferrite grains. A
solution of equal parts of 570 nital and 470 picral was the
etchant used to differentiate ferrite from pearlite. The
volume fractions of TiN and pearlite were determined by
point counting. The volume fractions of coarse grain
ferrite patches were determined based on the procedure
described in ASTMEl181. The ferrite grain size was
measured using the circular intercept method of
ASTM E112. Regardless of whether the ferrite grains are
uniform or duplex in size, an average ferrite grain size is
reported.
Both extraction replicas and thin foils were produced
from many of the plates. Dry, two-stage extraction
replicas were prepared using cellulose acetate tape from
some plates according to standard practice. The replicas,
supported by nickel grids, were examined in both a Philips
300 transmission electron microscope (TEM) and a VG
Scientific model HB501 scanning transmission electron
microscope (STEM), both operating at 100 kV. The VG
microscope is equipped with a Link Systems EDS unit and
windowless detector for identifying those elements present
in extracted precipitates. The chemical compositions of
particular types of precipitates represent average results
from at least ten different particles.
The thin foils were made to confirm precipitate size
and morphology, as well as establish their true distribution
.. . ..— . . . ..-
and mechanically/chemically thinned to a thickness of
about 0.07 mm. Discs measuring about 3 mm in diameter
were then punched from the wafers and electrochemically
thinned in a Struers Tenupol 2 twin-jet unit using a voltage
of 60 V, a current of 180 mA, and a solution of 85?10
glacial acetic acid/10% perchloric acid/5% glycerin main-
tained at about 9“C. The foils were examined in the TEM
at an operating voltage of 100 kV.
Selected CVN fractures were examined in an Amray
Turbo 1600 scanning electron microscope (SEM) opera-
ting at 20 kV. In selected cases, the CVN fractures were
plated with nickel, polished on a transverse plane which
intersected the fracture face, etched, and examined with a
light microscope.
3. RESULTS AND DISCUSSION
3.1 Series I. Effects of Ti and N in 13 and 25 mm Thick
Nb and Nb-V Steel Plates
Grain coarsening – The austenite grain coarsening
behavior for each of the Series Ia and Series Ib steels is
shown in Figure 1. In comparison with the base steels (Nb
“and Nb-V steels), the Ti-containing steels provide signifi-
cantly improved resistance to austenite grain growth. In
particular, the steels with both Ti and N additions exhibit
virtually no grain coarsening up to 1320”C. The improve-
ment in grain coarsening resistance due to the additions of
Ti and N is consistent with our previous work on C-Mn
and C-Mn-V steels [33,35]. The data further show that for
low slab reheating temperatures (up to about 1150°C),
there is no advantage of a Ti addition for grain growth
control. Within this temperature range, the austenite grain
sizes are essentially the same in the base, Ti-added and Ti-
N-added steels for both Nb and Nb-V steel types.
600
E
&
:400 *
N ‘“+”- Nb-Tl-N.-
OY
.~ 300 /
4
..”
“m (a) Ia Series (Nb) P
‘1~:[.TI(b) lb Series (~-v)
g 300
;
@ 200 Nb-V l
.:
J 100
e
:0 00 1060 1100 1160 1200 1260 1300 1360 1400
1/2 hr Austenitizing Temperature, ‘C
Fig. 1. Comparison of the one-half hour austenite grain
coarsening results for the (a) Ia series and (b) Ib
series steels.
0TH MWSP CONF. PROC., 1SS 1998 – 931
Metallography – Microcleanliness data for the
25 mm thick plates are summarized in Table III. In all
of coarse grain ferrite patches in these steels is shown in
Figure 3; in all cases, pearlite seems to concentrate in the
cases, the volume fractions of oxides and sulfides are
similar for each steel. In contrast, the volume fractions of
the coarse TiN particles (edge dimensions 1 to 5 pm),
which presumably form in the liquid, vary significantly
among the steels. As expected, the Nb-Ti-N and Nb-V-Ti-
N steels have more coarse TiN particles than the
corresponding Nb-Ti and Nb-V-Ti steels.
Table III. Microcleanliness Data (in vol.%) for
25 mm Thick Series I Plates
Steel % Oxides % Sulfides % TiN
Nb 0.008 0.049 --
Nb-Ti 0.005 0.047 0.001
Nb-Ti-N 0.008 0.049 0.009
All plate microstructure consist of a mixture of
ferrite and 6 to 9% pearlite. Figure 2 shows the average
ferrite grain size as a function of plate thicknessand com-
position. The use of TiN technology does not significantly
affect the ferrite grain size of the 13 mm thick plates;
however, it clearly refines the ferrite grain size of the
25 mm thick Nb-V plates. At the 25 mm thickness, the
nitrogen level appears to have little effect on the degree of
ferrite grain refinement achieved, despite the fact that the
high N steels appear to have a higher volume fraction of
TiN particles.
I9
Nb Nb-Tl Nb-Ti-N
Fig. 2.
25mm
e l
13mm
I
Nb-V Nb-V-Tl Nb-V-Ti-N
Comparison of the ferrite grain sizes for 13 and
25 mm controlled rolled plates of the Ia and Ib
series steels.
The volume percentage of coarse grain ferrite
patches could be determined with sufficient accuracy for
the 25 mm thick Nb-V, Nb-V-Ti, and Nb-V-Ti-N steel
plates. Typical microstructure illustrating the distribution
932 – 40TH MWSP CONF. PROC., 1SS 1998
coarse grain ferrite regions. The volume percentages of
coarse grain ferrite patches were determined to be 35, 23,
and 19% for the Nb-V, Nb-V-Ti, and Nb-V-Ti-N steel
plates, respectively. Thus, TiN technc)logy improves the
ferrite grain size uniformity in these control-rolled plates,
consistent with the literature [3,4,6, 13,41 ,47,48]. This is
important since coarse grain ferrite patches provide
localized areas of weakness for crack propagation.
Figure 4 is a light micrograph of a fractured transverse
impact specimen from a Nb-V steel plate, polished and
etched on a transverse through-thickness plane which
intersects the fracture face. This figure shows secondary
cracks running through coarse grain ferrite patches.
From examining thin foils and replicas in the TEM
and STEM, each of the 25 mm thick control-rolled Ti-
bearing plates were found to contain two types of particles,
viz., relatively large cuboidal (Ti,Nb)l-rich nitrides and
smaller round Nb- or Nb-V-rich carbonitride particles.
The cuboidal (Ti,Nb)-rich nitrides have a size ranging
from about 100 to 200 nm (O.1 to 0.2 ~m). Table IV sum-
marizes the average size and composition of these
particles. Note that these nitride particles are smaller than
the coarse TiN particles (1 to 5 ~m) observed in the light
microscope and reported in Table HI.
Electron micrographs comparing the size and disper-
sion of the (Ti,Nb)-rich nitrides in each of the steels are
shown in Figure 5. In general, the nitrides appear finer and
more numerous in the Nb-Ti-N and Nb-V-Ti-N steels
(Figures 5b and 5d) compared to the Nb-Ti and Nb-V-Ti
steels (Figures 5a and 5c), thus confirming the benefit of a
hypostoichiometric Ti :N ratio for these steels. Although
the volume fraction of these particles was not determined,
the presence of finer and more numerous nitride particles
is consistent with the better austenite grain size control,
and more uniform ferrite grain size found in the Ti-N
added steels. The nitride particles in the Nb-Ti steel
contain more Ti and less Nb than in the Nb-Ti-N steel. On
the other hand, there is no significant difference in nitride
composition between the Nb-V-Ti and Nb-V-Ti-N steels
(Table IV).
Table IV. Summary of the Fine Particle Sizes and Compositions in the
25 mm Thick Series I Plates
Partiek
No. of Avg. Avg. Pmtirle (.lompasition, wt.%
steel
Type
Partiefea Particle
Examined Si* nm Ti Nb v
Cuboidal
Nb-T1
10 20(3*55
H
84.8 *4.1 15.4 *4.1
Round 10 5.0+ 1.6 13.4+ 1.6 86.3 + 1.8 -
Cuboidal 15
Nt-Ti-N
111 142 67.6 t 100 32.4 + 9.9 -
Round 10 6.8 + 3.1 0.8 + 0.5 98.7 t 0.8 -
Cuboidal 10
Nb-V-Ti
117+36 78.2 ~ 7..3 16.9 i 6.3 4,6 t 2.0
Round 10 5.8 i 1.8 6.9 * 0.9 69.2 +4.3 23.9 +4.1
Nb-V-Ti - Cuboidal 10 97* 37 70.7 + 101 19.7 + 7.2 9,6 k 3.2
N Round 11 6.8 * 3.4 0.7 * 0.4 44.6 + 8.4 54.8 + 8.4
In all of the ste~ls examined, the cuboidal (Ti,Nb)-
rich nitrides are associated with discrete Nb-rich carbides
(presumably occurring by epitaxial deposition on the
Secondary Ni Plating Over Brittle
(a) Nb-V
(b) Nb-V-Ti
(c) Nb-V-Ti-N
-
Fig.3. Light micrographs compting tiemicrostmctmeof
the 25 mm controlled rolled plates of the Ib series
steels.
nitrides as suggested by the work of Houghton, et al.
[48,50]). An example of nitrides with these NbC “caps”
are identified by the arrows in Figures 5a and 5c. Typical
elemental X-ray maps of a (Ti,Nb)-rich nitride with a NbC
cap in the Nb-V-Ti steel are shown in Figure 6. In some
cases, NbC completely encapsulates the (Ti,Nb)-rich
nitri ales. This effect, illustrated in Figure 7, is best
observed by first subtracting out background radiation and
..
Crack Fracture Qegion\
Fig. 4.
-
Light micrograph of a fractured transverse impact
specimen of a Nb-V steel plate, polished and etched
on a transverse through-thickness plane which
intersects the fracture face, showing secondary
cracks running through coarse grain ferrite patches.
normalizing the Ti and Nb X-ray intensities to eliminate
any effects due to variations in particle thickness
(Figure 7C and 7d). Such co-precipitation of NbC on
(Ti,Nb)N has also been observed by others [13,48,50]. .
The smaller, round particles in the Ti-bearing Nb
steels are (Nb,Ti) carbonitrides. There is no significant
statistical difference in the size of these particles between
Nb-Ti and Nb-Ti-N steels; all particle sizes are about 5 to
7 nm. However, the fine carbonitride particles in the Nb-
Ti steel contain about 86’XO+2%]Nb and 13%+2% Ti,
while similar particles in the high N steel (i.e., Nb-Ti-N)
consist of almost all Nb and no Ti. In comparison, the
,similar round particles in the Nb-V-Ti and Nb-V-Ti-N
steels are (Nb,V,Ti) carbonitrides. The carbonitride
particles in the Nb-V-Ti steel contain primarily Nb
(69%+4%), V (24%+4Yo) and Ti (7%+1%). In
comparison, similar particles in the high N steel (i.e., Nb-
V-Ti-N) contain less Nb (45%+8%), more V (55%+8%),
and less than 10/0 of Ti. The particle sizes of these
carbonitrides are about the same as those for the Nb steels
(Series Is).
Mechanical properties – The mechanical properties
of the Nb, Nb-Ti, and Nb-Ti-N steels are plotted as a
fi.mction of plate thickness in Figure 8. For both
thicknesses, the small addition of Ti to the Nb steel (i.e.,
the Nb-Ti steel) improves strength and transverse CVN
‘All percentages are in weight Y. and the* numbers represent
one standard deviation.
40TH MWSP CONF. PROC., 1SS 1998 – 933
-.
Fig. 5. Electron micrographs of extraction replicas showing the typical dispersions of small Ti-rich nitride
particles in 25 mm thick controlled rolled plates of the Ti-containing steels used in Series I. The
arrows in (a) and (c) locate Nb-rich caps.
energy at -45°C. However, a further addition of N (i.e.,
the Nb-Ti-N steel) reduces the strength and CVN
toughness of the Nb-Ti steel at both 13 and 25 mm
thicknesses. The CVN energy of the Nb-Ti-N steel is even
lower than that of the base Nb steel, presumably due to the
relatively high volume fraction of TiN particles in this
steel.
The mechanical properties of the Nb-V, Nb-V-Ti,
and Nb-V-Ti-N steels are plotted as a function of plate
thickness in Figure 9. Although the FRT of the 13 and
25 mm plates are different (i.e., 760°C vs. 700”C,
respectively), the plot still provides a comparison in steel
composition. It is shown that a small Ti addition does not
consistently improve the strength or toughness of the base
Nb-V steel at either thickness. However, a high level of N
results in reduced strength and CVN toughness in the Nb-
V-Ti-N steel, consistent with the results of the Nb-series
steels reported in Figure 8.
At a plate thickness of 13 mm, all six steels in
Series I exhibit good toughness levels as a result of refined
‘ferrite grain sizes provided by a high degree of
deformation during rolling and a relatively high air-
cooling rate associated with these relatively thin plates.
Consequently, the benefit of grain refinement provided by
TiN technologydiminishes at a plate thickness of 13 mm.
In comparison, at a thickness of 25 mm, the degree of
deformation during rolling and the air cooling rate are
lower, and a small addition of Ti to either the Nb or Nb-V
steel significantly improves the toughness. The
improvement is presumably due to a finer, more uniform
ferrite grain size provided by TiN technology. The
toughness of the Ti and high N added steel is the lowest in
each series of compositions at both thicknesses. This
reduced level of toughness is probably due to the larger
volume fraction of TiN particles in these steels.
934-- 40TH MWSP CONF. PROC., 1SS 1998
-.
.
w
cc
nf
to
in
a
)
>
u
Fig. 6. X-ray maps showing NbC in association with a (Ti,Nb,V) nitride particle in the 25 mm thick
controlled rolled plate of the Nb-V-Ti-N steel used in Series Ib.
Series I summary – Transverse CVN energy at -45°C slightly increased or unchanged when Ti is added to the
s plotted against yield strength for each combination of base steel, the addition of Ti results in a significant
rnposition and plate thickness. Since similar trends were improvement in CVN energy. The improvement in CVN
ted for both plate thicknesses, only the strength/ energy for the Nb steel is about 46 J (Figure 10a) and 62 J
ghness balance for the 25 mm thick plates are presented for the Nb-V steel (Figure 10b). This increase in
Figure 10. In both cases, while the yield strength is toughness is largely attributed to a more uniform, refined
40TH MWSP CONF. PROC., 1SS 1998 – 935
Fig. 7. Normalized X-ray maps showing a (Ti, Nb,V) nitride particle in the 25 mm thick controlled rolled
plate of the Nb-V-Ti steel used in Series Ib. To remove the effects of particle thickness on perceived
concentrations, X-ray intensities for Ti (c) and Nb (d) are shown normalized by dividing the sum of
the Nb and Ti X-ray intensities.
ferrite grain size. Secondary causes of the toughness
improvement could be provided by a lower free nitrogen
level (due to the formation of (Ti,Nb)-rich nitrides) and a
reduction in the level of carbonitride strengthening (due to
a reduction in the available N and the precipitation of NbC
caps on (Ti,Nb)-rich nitrides). The minor change in
strength between comparable Ti-added steels and non-Ti
base compositions is probably due to the offsetting
metallurgical strengthening effects associated with the
presence of TiN particles, i.e., a more uniform, refined
ferrile grain size, and a reduction of Nb-rich carbonitride
precipitation due to TiN formation and the precipitation of
Nb-rich carbide caps on TiN particles.
936- 40TH MWSP CONF. PROC., 1SS 1998
When nitrogen is added to either the Nb-Ti or the
Nb-V-Ti steels, the strength is reduced and the toughness
is lowered considerably. The higher nitrogen level leads to
an increase in the volume fraction of both coarse and fine
(Ti,Nb)-rich nitrides, and possibly the free nitrogen level
in the Nb-Ti steel, both of which reduce toughness. The
increased number of nitride particles in the Nb-Ti-N and
Nb-V-Ti-N grades also prpvides more sites for the subse-
quent precipitation of NbC caps. This reduction in Nb-rich
carbonitride strengthening more than offsets the strength
increment due to grain refinement provided by TiN
technology. Based on the results of Series I, it appears that
there is an optimum amount of TiN required to ensure a
-.
 .-
Ib Series (lW-V)
40T
;- 2oo-
&Y
y 150-
@
i? 1oo-@
Iii
z 50”
8
Ia Series (Nb) K
~
““”-......OO
““-........
‘9A
1- nl
10 15 20 25 30
Plate Thickness, mm
Fig. 8. Summary of the mechanical properties of the Nb
steels used in Series Ia in the controlled rolled
condition. The legend identifies the grade.
good balance between strength and toughness. To mini-
mize the presence of detrimental coarse TiN particles, the
nitrc~gen level of subsequent compositions investigated
was reduced to approximately 0.0070%.
3.2 Series II. Effect of Ti in 9 mm Thick Plates
Produced at Various FRT’s
Grain coarsening – The austenite grain coarsening
response as a function of temperature for the Series II ~
steels is shown in Figure 11. In comparison with the Ti-,
free steel, the Ti-bearing steel provides improved
resistance to austenite grain growth. The Ti-bearing steel
experiences minimal austenite grain coarsening over the
temperature range of 1040 to 1320°C, As expected, the
results of these 0.0070% N steels are consistent with those
of the Nb-V-Ti and Nb-V-Ti-N steels in Series Ib, which
have very similar compositions and nitrogen levels of
0.0046% and 0.01 20%, respectively.
Metallography – Again the microstructure consists of
a mixture of ferrite and pearlite (7 to 1170). Mean ferrite
grain sizes for the 9 mm thick plates ranged between 3.5
and 5 pm, which are very similar to the ferrite grain sizes
observed in the 13 mm thick plates of Series 1. As shown
in Figure 12, the mean ferrite grain size is not significantly
influenced by either Ti content or FRT at this light thick-
ness. It appears that at thin gages (i.e., t S 13 mm), TiN
IiF==-s
..,.l.**.... E+ Nb-Vl. . .,.. . - + Nb-V-TiA“*”*” l-AD Nb-V-Ti-h
A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1- o,&
15 20 25
Plate Thickness, mm
Fig. 9. Summary of the mechanical properties of the Nb-V
steels used in Series Ib in the controlled rolled
condition. The legend identifies the grade.
technology does not have any significant beneficial effect
on ferrite grain size due to the relatively high degree of
deformation during rolling and high air-cooling rates
associated with thinner gage plates. Nevertheless, the Ti-
bearing steel tends to have a smaller volume fraction of
coarse grain ferrite patches, as reported for Series I.
Both the small, round (Nb,V)-rich and large cuboidal
(Ti,Nb)-rich precipitates were observed in the Nb-V-Ti
steel plate (FRT = 730”C) using transmission electron
microscopy. The small (Nb,V)-rich particles for these
steels measured 10.0+4.8 nm in diameter. The average
composition for these (Nb,V)-rich carbonitride precipitates
in the Nb-V-Ti steel is 61~7% Nb/37.5~7.5% V/
1.5&).4~o Tio The cuboidal (Ti,Nb)-rich nitrides observed
in the Nb-V-Ti steel measured 9 1A61 nm with an average
composition of 76A 107o Ti/20*8% Nb/4*3% V. As shown
previously, in many cases, NbC has precipitated on these
(Ti,Nb)-rich nitride particles.
Mechanical properties – The mechanical properties
for the Nb-V and Nb-V-Ti plates are plotted as a function
of finish rolling temperature in Figure 13. With decreasing
FRT (especially for FRT < 780°C), the yield and tensile
strength generally increase for both compositions. Since
the ferrite grain size is relatively constant for all FRT’s
H MWSP CONF. PROC., 1SS 1998 – 937
evaluated (Figure 12), this strength increment is likely the
result of warm working of the ferrite (i.e., substructure
10
strengthening) when the plate was finish rolled in the two-
phase region,
‘@t
Ti
A
. cries fHb~
l Nb
A Nb-Ti
n Nb-Ti-N
loo~ I I 1
400 460 600 660 600
0.2% Yield Strength, MPa
125} I , I I
A
/
N Ti
B
lb series !~ -v}
l Nb-V
A Nb-V-Ti
n Nb-V-Ti-N
;5 o 500 S50 600 650
0.2% Yield Strength, MPa
Fig. “1O. Strength/toughness balances for the 25 mm thick
controlled rolled (a) Nb and (b) Nb-V steels used
in Series I.
400 } f I I I I I I
.
i~
J 260 –
“: 200
G :
~ 150 – Bl :
.= ,’
g 100 : .’
m A A?-**
z 50; l #..=
.-.-=”.A A ‘--.&-----------
Nb-V
Nb-V-Tl
Lo
I I 1 I I 1
1000 1100 1200 1300 1400 1600
1/2 hr. Austanitlzing Temperature, “C
Fig. 11. Comparison of the one-half hour austenite grain
coarsening results for the steels used in Series II.
938-- 40TH MWSP CONF. PROC., 1SS 1998
.— . .. . ..
9
8
E
a7.
]6
‘5.-
:
4
{3
l!!
2
1
0
7
n+ Nb-VA Nb-V-Ti
f & ~ l lA*---------------- -------- . . . . . . . . ------
-------- -------- ------ AA.. -. .-. . . . . . . . .
1 725 750 775 800 825 [
Finish Rolling Temperature, ‘C
Fig. 12. Mean ferrite grain size as a fhnction of finish
rolling temperature for the 9 mm thick controlled
rolled plates of the steels used in Series II.
Also shown in Figure 13, a subtle difference in
strength exists between Ti-bearing and non-Ti steels at this
thin gage. Specifically, the Nb-V-Ti steel generally has a
slightly lower strength compared to its Ti-free counterpart.
Since the ferrite grain sizes are about the same for all
plates, the lower strength is probably due to a loss in
(Nb,V) carbonitride strengthening caused by the co-
precipitation of (Ti,Nb)N and NbC particles. This
phenomenon has been presented earlier for the Series I
steels (see Figures 6 and 7).
CVN energy at -20°C, which falls on the upper shelf
in all cases, increases with increasing FRT. For a given
FRT, the Nb-V steel exhibits a lower CVN energy at
-20”C than the Nb-V-Ti steel. The effect of Ti addition on
CVN toughness is better illustrated using full CVN
transition energy curves. Figure 14 shows the CVN
transition curves for plates finish rolled at 790”C.
Although there is a small difference in the strength levels
of these plate samples, the Ti-bearing steel clearly exhibits
better toughness than the Ti-free steel, as measured by
either upper shelf energy or 30 J transition temperature.
These mechanical property data are summarized in
the yield strength/toughness plot provided in Figure 15.
For a given yield strength, the Nb-V-Ti steel exhibits
superior CVN energy at -20”C compared to the Nb-V
steel. Also, the Nb-V-Ti steel exhibits a better balance of
yield strength and toughness. In comparison, the Nb-V
steel provides slightly higher yield and tensile strengths
and lower toughness for a given FRT.
Series II summary – A small Ti addition to a Nb-V
steel results in the precipitation of (Ti,Nb)N particles,
which are effective in restricting austenite grain growth at
a reheating temperature up to about 1320°C. However,
this benefit diminishes due to the relatively high degree of
deformation during rolling and the high cooling rates
associated with thinner gage plates. Nevertheless, the Ti-
bearing steel exhibits a superior balance of strength and
toughness compared to the Ti-free steel. This improved
balance of mechanical properties for the Ti-bearing steel is
The Ar, temperature, as detwrnined by the Ouchi, et
al. “[51] equation, is estimated to be approximately 780°C
possibly due to a subtle refinement in the mean ferrite
grain size and a reduction in the precipitation
strengthening increment. As reported for Series I, TiN
technology reduces the volume fraction of coarse grain
ferrite patches and improves the ferrite grain size
uniformity in control-rolled plates. Although not apparent
from the mean ferrite grain size measurements, the Nb-V-
Ti steel did appear to have a smaller volume fraction of
coarse grain ferrite patches. A subtle improvement in the
ferrite grain size uniformity helps explain the superior
strengtl-dtoughness balance of the Ti-bearing steel.
Notwithstanding the superior balance of mechanical
properties for the Nb-V-Ti steel, the Nb-V steel generally
exhibits slightly higher yield and tensile strengths for a
given FRT. The slightly lower strength in the Nb-V-Ti
steel occurs because of the formation of TiN particles and
the co-precipitation of NbC on (Ti,Nb)N particles, thus
decreasing the available Nb and N for (Nb,V) carbonitride
precipitation strengthening. At a relatively thin plate
thickness of 9 mm, the potential competing strengthening
mechanism of grain refinement is not at play since both
compositions exhibit similar ferrite grain sizes regardless
of whether Ti is added to the steel or not.
+ 50JU [
700 725 760 775 800 825 850
Finish Rolling Temperature, ‘C
Fig. 13. Summary of the mechanical properties of the
9 mm thick controlled rolled Series II steel plates
as a function of finish rolling temperature.
for these steels. This indicates that about half of the plates
investigated in this series were finished rolled below the
Ar~ temperature. When the FRT is increased in the tem-
perature range of between 700”C and 780”C, yield and
tensile strength decrease, while toughness increases.
These effects are largely attributed to the
strength/toughness trade-off associated with substructure
strengthening. Since the mean ferrite grain size is not
significantly affected by the range of FRT’s explored,
increasing the FRT in the range of 780°C to 845°C does
not significantly lower either yield or tensile strength.
8
8
4
0 -140 -120 -loo -80 -60 -40 -20 0 II
Test Temperature, “C
Fig. 14. Transverse (2/3-size) CVN energy transition
curves for the 9 mm thick Series II steel plates
finish rolled at 790°C. The legend indicates the
steel grade.
El+Nb-VA * Nb-V-Ti
-.
‘..
-.*
A % -*
-.
‘..
l.
-.
A
-“’4. *
‘..
A‘.
-— (1 4s0 460 600 520 640 560 680 6
0.2”A Yield Strength, MPa
Fig. 15. Strength/toughness balance for the 9 mm thick
controlled rolled Series II steel plates.
Since TiN technology has been shown to provide
only a small mechanical property benefit to thin gage
plates, we next examined thicker product with a nitrogen
content of 0.007070.
40TH MWSP CONF. PROC., 1SS 1998 – 939
(b) Nb-V /AC
(c) Nb-V-Ti /Air (d) Nb-V-Ti /AC
-1
.
Fig. 16. Representative light micrographs from the quarter-thickness of selected 19 mm thick controlled
rolled plates of the Nb-V and Nb-V-Ti steels used in Series III in the (a,c) air cooled (FRT = 700”C)
and (b,d) accelerated cooled (FRT = 830”C) conditions.
3.3 Series III. Effects of Ti, FRT and Accelerated Cool-
ing in 19 mm Thick Plates
Metallography – The microstructure of the 19 mm
thick plates generally coarsen slightly on moving from the
surface to mid-thickness location. Light micrographs at
the quarter-thickness location of selected plates in both the
air-cooled and accelerated-cooled conditions are presented
in Figure 16. The air-cooled plates (FRT = 700°C) exhibit
a mixture of polygonal ferrite, deformed ferrite, and
pearl,ite (Figures 16a and 16c). In the air-cooled condition,
the Nb-V steel plate (Figure 16a) contains a coarser
average ferrite grain size and a higher percentage of coarse
grain ferrite patches than the Nb-V-Ti steel plate
(Figure 16c), reflecting the known beneficial effects of a
‘finer and more uniform microstructure provided by TiN
technology. With accelerated cooling, the microstructure
‘changes to a mixture of ferrite and martensite with traces
,of bainite (photomicrograph not shown). When the FRT is
increased to 830°C followed by accelerated cooling, a
refined microstructure with more bainite and less martens-
ite is obtained, Figures 16b and 16d. In contrast to the air-
cooling process, accelerated cooling leads to a more
refined and very uniform microstructure for ~ the Nb-V
and Nb-V-Ti steels.
Mechanical properties – The mechanical properties
for the 19 mm Nb-V and Nb-V-Ti steel plates are
presented as a function of FRT in Figure 17. Both steel
grades behave similarly with respect to yield and tensile
940 – 40TH MWSP CONF. PROC., 1SS 1998
(a) Nb-V /Air
... .. ...-,.,., ..-.,. .
strength. In the air-cooled condition, yield” and tensile
strength increase with decreasing FRT due to an increase
in substructure strengthening. For a given FRT in the air-
the Ti-bearing and Ti-free compositions. Since plate mill
productivity increases with increasing FRT, a rolling
practice with a FRT of 830”C offers the best processing
cooled condition, the Nb-V-Ti steel exhibits slightly
higher strength than the Nb-V steel due to the ferrite grain
refinement provided by the Ti addition (Figures 16a and
c). With accelerated cooling, there is no significant
differencein strength between the two steels. As the FRT
increases for the accelerated-cooled plates, the yield
strength decreases gradually, while the tensile strength
remlains fairly constant.
n
l
~
l
l = .l nAl.
I
~ Nb-V-TilAi
1n Nb-V-Ti/AC
+ 50
J !
700 725 750 775 800 825 850
Fig. 17.
Finish Roiling Temperature, “C
Summary of the mechanical properties of the
19 mm thick Series III steel plates as a function of
finish rolling temperature. The legend identifies
the grade/cooling practice.
The CVN specimens tested at -20°C are in the upper
shelf regime for all conditions. Figure 17 shows that the
CVN energy for the air-cooled plates increases slightly
with increasing FRT from 700”C to 725°C due to a
strengthhoughness trade-off. The Nb-V-Ti air-cooled
plates exhibit a somewhat higher CVN energy than their
Nb-V counterparts, primarily due to the additional ferrite
grain refinement and reduced coarse grain ferrite patches
provided by the Ti addition.
In the case of the accelerated-cooled plates, the FRT
has no major effect on the CVN energy (tested at -20”C)
over the entire FRT range examined. All samples exhibit a
CVN energy roughly in the range of 200 to 250 J, and
there is no significant difference in CVN energy between
40
.-
option. Figures 18a and 18b compare the transverse CVN
energy transition curves for the Nb-V and Nb-V-Ti plates
in air-cooled (FRT = 700”C) and accelerated-cooled (FRT
= 830”C) conditions, respectively. Although the upper
shelf energies among grades and processing are very
similar, both accelerated-cooled plates have lower 30 J
transition temperatures than the two air-cooled plates.
Splits [52] were observed on the fracture faces of the
CVN specimens from the air-cooled plates. As shown in
Figure 19 for plates finish rolled at 700”C and air cooled,
the number of splits increases with decreasing test temper-
ature and both the Nb-V and Nb-V-Ti steels exhibit a
similar behavior. Two types of splits are present for each
composition. One type consists of very deep splits, about
1 to 1.5 mm, occurring between -20”C and -70°C for the
Nb-V-Ti steel and between -20°C and - 10O°C for the Nb-
V steel. Morrison and Mintz [53] found that the presence
of one or two deep splits can lower the impact transition
temperature by 20 to 30°C. The other type of split is
shallower, observed at lower temperatures. and is believed
to have no major effect on toughness [13].
~ m
200
150- n
l
1oo-
Nb-V ~ ~ Nb-V-Ti
50-
:160 -140 -120 -loo 40 40 40 -20
Test Temperature, ‘C
1oo- Nb-V-Ti 9 ~ Nb-V
50- \
0
-160 -140 -120 -100 -60 -60 -40 -20
Fig. 18.
Test Temperature, “C
Comparison of the transverse CVN energy
transition curves for the 19 mm thick Series-lll
steel plates in the (a) air cooled (FRT = 700”C)
and (b) accelerated cooled (FRT = 830”C)
conditions.
TH MWSP CONF. PROC., 1SS 1998 – 941
fraction af coarse ferrite grain patches in the Nb-V steel
n
n
Nb-V
l
l
—
I I I
.!40 -120 -loo
I I I I I
-80 -60 -40 -20 0
Test Temperature, “C
Fig. 19. Comparison of the number of splits on the CVN
fracture faces from the 19 mm thick controlled
rolled (FRT = 700”C) and air cooled Series III
steel plates as a fimction of test temperature.
The lower 30 J transition tempertiture for the Nb-V
steel plate compared to the Nb-V-Ti steel plate
(Figure 18a) is attributed to the deeper splits in the tem-
perature regime of -70 to - 100”C. The greater propensity
for the Nb-V steel to exhibit deeper splits in this tempera-
ture range is attributed to its coarser ferrite grains and
higher percentage of coarse grain ferrite patches [54-56].
According to Speich and Dabkowski [56], these coarse
ferrite regions are even coarser in the plane of the plate
than in the transverse orientation. As a result, the through-
thickness transition temperature would be expected to be
higher than that of the transverse orientation. Thus, when
an impact specimen is broken, the transverse stresses [57]
developed lead to splitting-type fractures in the rolling
plane because cleavage in this direction (through the
coarse-grained region) is easier than the propagation of a
completely ductile fracture in the transverse direction. In
contrast, Kejian and Baker [13] observed a lower impact
transition temperature and more deeper splits in a control-
rolled O.109IoC - 0.50%Ni - 0.017YoNb - 0.01 %Ti steel
plate than a similar steel plate without a Ti addition. No
explanation was offered for the more intense splitting in
the Ti-treated steel.
In the FRT = 830”C and accelerated cooled condi-
tion, splitting was not observed in the CVN specimens.
However, there is a subtle difference in transition region
toughness between the Nb-V and Nb-V-Ti steels. As a
result of TiN technology, there is presumably reduced
(Nb,V) carbonitride precipitation strengthening and
perhaps a subtle refinement in microstructure in the Nb-V-
Ti steel which provides a slightly lower transition tempera-
ture.
Series III summary – In the air-cooled condition, a
small Ti addition leads to a slight increase in strength and
toughness. This effect is attributed to ferrite grain refine-
ment provided by the Ti addition. The higher volume
942-- 40TH MWSP CONF. PROC., 1SS 1998
results in more deep splits in the CVN specimens thW
those found in the Nb-V-Ti steel specimens. The use of
accelerated cooling produces a more refined and uniform
microstructure than that of the air-cooling process. Hence,
the benefit of a Ti-addition is not as apparent compared to
the air-cooled plates. This phenomenon is similar to that
reported in the thinner gage plates, where a higher cooling
rate provided adequate grain refinement even without the
Ti-addition. Nevertheless, the Nb-V-Ti steel in the accel-
erated-cooled condition provides a slight improvement in
the 30 J transition temperature toughness. This effect may
be due to a subtle refinement in the microstructure
(although not apparent by light microscopy) and a
reduction in the (Nb,V) carbonitride precipitation strength-
ening increment due to the use of TiN technology.
4. CONCLUSIONS
The application of TiN technology in three series of
controlled-rolled 0.08~0 C - 0.03590 Nb - (V) steel plates
has been assessed in terms of microstructure and mechan-
ical properties. From this work, the following conclusions
are drawn.
1. Although an addition of about 0.015% Ti to a simu-
lated continuously-cast steel effectively restricts austenite
grain growth during reheating, ferrite grain refinement
may not be achieved. This is particularly true for lighter
gage air-cooled plates (< 13 mm) and accelerated cooled
plates (19 mm), where the cooling rate after rolling is high
enough to limit ferrite grain growth.
2. TiN technology is effective in providing a fine and
homogeneous ferrite grain size, with its effectiveness
increasing with increasing plate thickness.
3. Both Nb and V can be present in TiN nitride particles,
e.g., (Ti,Nb,V)N, and NbC can precipitate on these parti-
cles. As a result, less Nb, V, and N are available for
Nb,V(C,N) precipitation strengthening of the ferrite.
4. Depending on the degree of ferrite grain refinement
and loss of Nb,V(C,N) precipitation strengthening
associated with a small Ti addition, strength can either
increase, remain the same, or decrease. On the other hand,
toughness generally increases when a small Ti addition is
made.
5. Although a combined addition of Ti and N can raise
the austenite grain coarsening temperature and reduce the
volume fraction of coarse grain ferrite patches, it can
increase the volume fraction of TiN particles (especially
the coarse particles
increase the degree
ening. The latter
toughness.
which serve as inclusions) and may
of Nb,V(C,N) precipitation strength-
two effects lead to a reduction in
6. The optimum nitrogen range for effective useof TiN
technology in the present steels appears to be between
*
about 40 ‘and 100 ppm, with an aim of about 70 ppm. Nb-
V microalloyed steels with 0.015% Ti and 0.0070% N
7. J. G. Williams, C. R. Killmore, and G. R. Harris,
“Recrystallization Behavior of Fine Grained Nb-Ti
4
exhibit a superior balance of strength and toughness com-
pared to similar Ti-free steels.
7. Due to the effective refinement in microstructure
provided by accelerated cooling in 19 mm thick plates, a
small addition of Ti did not have a significant effect on
tensile or upper shelf CVN impact properties. Neverthe-
less, a small addition of Ti is still recommended for such
processing due to titanium’s well-known benefits of mini-
mizing transverse slab cracking and improving HAZ
toughness.
ACKNOWLEDGMENTS
Special thanks go to S. S. Hansen and P. P.
Podgurski for their technical input and B. L. Bramfitt for
his critical review of this manuscript. The assistance of R:
J. August, J. W. Burkit, J. L. Clarke, K. S. Follweiler, K. #
E. Downey, J. C. Hlubik, J. R. Kilpatrick, T. R. Knauss, S.
J. Lawrence, H. B. Leuckel, R. R. Lichty, F. J. Marsh, R.
L. Perry, R. E. Steigerwalt, and G. E. Weiss in this work is
deeply appreciated.
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.-
H MWSP CONF. PROC., 1SS 1998 – 945
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