Niobium influence in Countinuous Casting

Prévia do material em texto

Some Process and Physical Metallurgy Aspects of Niobium Microalloyed Steels for 
Heavy Structural Beams 
 
 
Devesh K. Misra1, Sankaran Shanmugam1, Todd Mannering2, Dhiren Panda2 and Steven G. Jansto3 
 
1Center for Structural and Functional Materials and Department of Chemical Engineering, 
University of Louisiana at Lafayette, Lafayette, LA 70504-4130, USA (*dmisra@louisiana.edu) 
2Nucor-Yamato Steel, P.O.Box 1228, 5929 East State Highway 18,Blytheville, AR 72316, USA 
3Reference Metals, 1000 Old Pond Road, Bridgeville, PA 15017, USA 
 
Keywords: Microalloyed steels, precipitation, degenerated pearlite 
 
INTRODUCTION 
 
It is now widely accepted that the microalloyed (MA) steels are cost-effective and exhibit superior properties such as fatigue strength, 
low distortion, and residual stress in comparison to the quenched and tempered (Q and T) steels. In general, to obtain good toughness, 
the carbon content of the microalloyed steels is maintained at ~ 0.03-0.08 wt % and manganese at ~ 1-1.2 wt % [1-3]. The carbon and 
manganese primarily contribute to solid solution strengthening. Additional, strength is obtained by precipitation hardening through 
microalloying elements, Nb, Ti, and V individually or in combination [2, 3]. The microalloying elements also facilitate a fine-grained 
microstructure to be obtained. In recent years, Nb-microalloyed design has been used to produce hot rolled steels of yield strength 
580-770 MPa characterized by good impact toughness and superior edge formability [4]. They were characterized by fine grain size 
(2-5 µm), narrow grain size distribution, inherently ductile behavior, and microplasticity. 
 
In view of the recent demand for high strength structural beams in North America, the microalloying approach is being increasingly 
considered to process structural beams. In structural beams, the desired strength-toughness combination can be obtained using the 
microalloying approach in spite of low percentage of deformation experienced by beams in comparison to conventional hot rolled 
steels. In the present paper, we describe the microstructural features and impact toughness behavior as a function of cooling rate for 
Nb- and V-microalloyed steels of almost similar yield strength (57-61 ksi). 
 
STEEL COMPOSITION AND EXPERIMENTAL TESTING METHODS 
 
The chemical composition range of Nb- and V-microalloyed steels is presented in Table I. The composition range meets the ASTM 
specification. It may be noted that the niobium content required to obtain the desired yield strength of 55-60 ksi in structural beams 
was approximately one-third of the vanadium content. The processing conditions were similar for both Nb- and V-microalloyed steels 
with no intentional differences. A representative beam size is W24×103; the designated size means that the nominal depth of beam is 
24 inches, when the beam is lying in the h-position with the web horizontal, the width is close to 24 inch. The 103 refers to nominal 
weight in lbs/ft. 
Table I Chemical composition range of Nb- and V-microalloyed steels. 
 
 
 
 
 
 
 
 
 
 
 
Elements Nb-microalloyed steel (wt.%) V-microalloyed steel (wt.%) 
C 0.030-0.100 0.030-0.100 
Mn 0.500-1.500 0.500-1.500 
V 0.001 0.020-0.050 
Nb 0.020-0.050 0.001 
Si 0.15-0.25 0.15-0.25 
P 0.010-0.020 0.010-0.020 
S 0.015-0.025 0.015-0.025 
N 0.009-0.010 0.009-0.010 
1792006 New Developments in Long and Forged Products Proceedings
Mean grain size was estimated by the circular intercept method. Intercept lengths were determined and then converted to nominal 
grain size using the standard tables. Tensile tests were done according to ASTM E8 and ASTM A370 specifications and Charpy v-
notch impact test was carried out according to ASTM E23 and ASTM A673 standards. 
 
Small coupons were cut from the beams and mounted for metallographic examination. Standard grinding and polishing techniques 
were employed, and specimens were etched with 2% nital. Light microscopy and scanning electron microscopy (JEOL 6300F) 
imaging techniques were used to obtain low magnification images that revealed the overall microstructure. The amounts of different 
microstructural constituents were estimated with conventional point-counting techniques in association with a square point grid as 
described below. The metallographic measurements were made on at least 20 fields-of-view in order to obtain representative data for 
stereological analysis. 
 
Considering that our recent work [5] indicated that small differences in toughness of steels with similar yield strength can be explained 
in terms of stereological parameters, notably, contiguity ratio. Thus, it was decided to adopt stereological analysis as one of the 
approach to understand the underlying differences in toughness of steels. The parameters determined for stereological analysis were 
volume fraction, mean intercept length and contiguity ratio. 
 
The volume fraction of ferrite 
αVV was calculated using the systematic point count method. Ferrite grain size and pearlite colony size 
were estimated in terms of mean intercept length )L( α determined by the following expression [5]: 
 
 
 
α
α
α
×=
N
LV
L TV (1) 
 
 
where 
αVV is the volume fraction of ferrite phase, 
Nα is the number of ferrite grains intercepted by the test lines, 
LT is the line length of the test lines. 
The ferrite contiguity ratio Cα was calculated by equation 2 [5]: 
 
 
α−α−α
α−α
α−α−α
α−α
α +=+= )P(2)P(
)P(2
)S(2)S(
)S(2
C
LPL
L
VPV
V (2) 
 
 
where (PL) α-α is the number of point intersections per unit length of the test line with ferrite-ferrite, 
(PL) α-P is the number of point intersections per unit length of the test line with ferrite-pearlite boundaries, 
(SV)α-α is surface area per unit volume of ferrite-ferrite boundaries, 
(SV)α-P is the surface area per unit volume of ferrite-pearlite boundaries. 
Contiguity is defined as the fraction of the total interface area of phase that is shared by grains of the same phase. Thus, ferrite 
contiguity ratio is indicative of the amount of ferrite that is continuous, i.e., it tells how much ferrite is adjacent to the ferrite. It is 
calculated by finding the ratio of number of ferrite-ferrite grain boundaries to the total number of grain boundaries in the 
microstructure (equation 2). From equation 2, contiguity ratio will vary from 0 to 1. 
 
Transmission electron microscopy was carried out on thin foils of Nb- and V-microalloyed steels. These foils were prepared by cutting 
thin wafers from the steel samples, and grinding them to ~ 100 µm in thickness. Three millimeter discs were punched from the wafers 
and electropolished using a solution of 10% perchloric acid in acetic acid electrolyte. Foils were examined with a JEOL FEG 
TEM/STEM operated at 200 kV. 
 
RESULTS AND DISCUSSION 
 
Tensile and Impact Behavior 
Tensile property data obtained from the tensile test of Nb- and V-microalloyed steels are listed in Table II for conventionally/normally 
cooled beams. Both the steels exhibited similar yield strength, tensile strength, and percent elongation. A similar behavior was 
observed at other cooling rates. However, there was variation in toughness of the two steels, as schematically depicted in Figure 1. 
Figure 1 shows the variation of impact toughness of Nb- and V-microalloyed steels as a function of cooling rate. It may be noted that 
the steels generally experienced improvement in toughness with increase in cooling rate. However, the toughness improvement 
appeared to be greater for the Nb-microalloyed steel as compared to the V-microalloyed steel. 
180 2006 New Developments in Long and Forged Products Proceedings
Table II Representative room temperature tensile properties of Nb- and V-microalloyed steelsFigure 1 (a) Room temperature Charpy v-notch impact toughness and (b) % degenerated pearlite of Nb- and V-microalloyed steels. 
Microstructures of Nb- and V-Microalloyed Steels 
Representative scanning electron micrographs of Nb- and V-microalloyed steels are presented in Figures 2 and 3. The low and high 
magnification micrographs of Nb-microalloyed steels processed at conventional (low) and high cooling rates are presented in Figures 
2 (a, b) and 2(c, d), respectively. Similarly, the micrographs of V-microalloyed steels processed at conventional and high cooling rates 
are presented in Figures 3 (a, b) and 3(c, d), respectively. The primary microstructural constituents of Nb- and V-microalloyed steels 
processed at conventional and high cooling rates were polygonal ferrite, pearlite, and degenerated pearlite. It may, however, be noted 
that the fraction of degenerated pearlite was high for the steels subjected to relatively high cooling rate (Figure 1b and Table III) and at 
a given cooling rate, the Nb-microalloyed steel contained significantly higher amount of degenerated pearlite (Figures 2c, d) as 
compared to the V-microalloyed steels (Figures 3c,d). Degenerated pearlite is formed by the nucleation of cementite at 
ferrite/austenite interface followed by carbide-free ferrite layers enclosing the cementite particles in the transformation temperature 
between normal pearlite and upper bainite [6]. Similar to lamellar pearlite, degenerated pearlite is also formed by diffusion process 
and considering its morphology, the difference is attributed to the insufficient carbon diffusion to develop continuous lamellae [7]. It 
is reported that the interface between ferrite and cementite in degenerated pearlite is wider than the conventional pearlite, thus the 
ferrite grain boundary area of the controlled-rolled steels that contain degenerated pearlite is higher as compared to the conventionally 
processed steel [8]. Degenerated pearlite is a microstructural constituent believed to promote toughness in steel [8]. The average ferrite 
grain size of both the steels processed at conventional and high cooling rates was similar (~ 26-29 µm). The quantitative 
metallographic data for Nb- and V-microalloyed steels are summarized in Table III. 
The microstructures of Nb-and V-microalloyed steels processed at conventional (normal) cooling rate are presented in Figures 4 and 
5. The general microstructure and the dislocation substructure in ferrite of Nb-microalloyed steels are presented in Figures 4a and 4b, 
respectively. The representative low magnification TEM micrographs show large polygonal ferrite with high dislocation density. 
There were some grains that were virtually free of dislocations. Figures 5a and 5b show the general microstructure and dislocation 
density in ferrite, of V-microalloyed steels. Representative bright field TEM micrographs of Nb- and V-microalloyed steels subjected 
to relatively high cooling rate are presented in Figures 6 and 7. Two types of ferrite morphologies (polygonal ferrite and lath-type 
ferrite) were observed in both the steels. The microstructure of Nb-microalloyed steels showing regions of polygonal ferrite, lath-type 
ferrite grains and degenerated pearlite are presented in Figure 6. Similarly, the microstructures of V-microalloyed steels that contain 
polygonal ferrite, lath-type ferrite grain structure and degenerated pearlite are presented in Figure 7. At higher cooling rates, it is 
anticipated that the austenite transforms to fine ferrite crystals in the intermediate temperature range as compared to the conventional 
ferrite structure. In Figures 6 and 7, the ferrite grains in groups of parallel laths are termed as acicular ferrite or bainitic ferrite [9]. 
 
Precipitation in Nb- and V-Microalloyed Steels 
Figure 8(a, b) shows grain boundary precipitation and precipitation on dislocations in ferrite region of Nb-microalloyed steels, while 
Figure 9a shows precipitation in ferrite matrix together with the corresponding selected area diffraction (SAD) pattern in Figure 9b. 
Properties Nb-microalloyed steel V-microalloyed steel 
Yield Strength, ksi 57-60 58-61 
Tensile Strength, ksi 72-74 75-76 
% Elongation 23-26 23-25 
0
2
4
6
8
10
12
14
16
18
1 2 3
Increasiing cooling rate
D
eg
en
er
at
ed
 p
ea
rli
te
, %
Nb V
(Absent)
V
(Absent)
Nb 
Nb 
V 
b. 
0
50
100
150
200
250
1 2 3
Increase in Cooling Rate
Im
pa
ct
 T
ou
gh
ne
ss
, f
t-l
bs
Nb Nb
Nb
V
V
a. 
1812006 New Developments in Long and Forged Products Proceedings
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2 Representative low and high magnification scanning electron micrographs of Nb-microalloyed steel processed at (a, b) 
conventional and (c,d) high cooling rates. The micrographs (b) and (d) show degenerated pearlite. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3 Representative low and high magnification scanning electron micrographs of V-microalloyed steel processed at (a, b) 
conventional and (c,d) high cooling rates. The micrograph (d) shows degenerated pearlite. 
c 
b a 
c 
Degenerated 
Pearlite 
a 
Degenerated 
Pearlite 
b Degenerated 
Pearlite 
d 
Degenerated 
Pearlite 
d Degenerated 
Pearlite 
182 2006 New Developments in Long and Forged Products Proceedings
Table III Microstructural features of Nb- and V-microalloyed steels 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4 Bright field TEM micrographs of Nb-microalloyed steels processed at conventional (or normal) cooling rate showing (a) 
polygonal ferrite structure and (b) dislocation substructure in ferrite. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5 Bright field TEM micrographs of V-microalloyed steels processed at conventional (or normal) cooling rate showing (a) 
polygonal ferrite-pearlite structure and (b) dislocation substructure in ferrite. 
Nb-microalloyed steels V-microalloyed steels 
Properties Conventional 
cooling rate 
Higher 
cooling rate 
Conventional 
cooling rate 
Higher cooling 
rate 
Area fraction, (%) 82.4 ± 3.1 82.8 ± 0.02 83 ± 2.5 82.8 ± 3.8 
Ferrite 
Mean intercept length ( αL ), µm 27 ± 1.5 
27 ± 2.8 
 
28.9 ± 3 
 
25.9 ± 1 
 
 Contiguity ratio, (Cα) 
(at different depth from the 
surface) 
10 µm 
15 µm 
20 µm 
25 µm 
30 µm 
0.74 
 
 
0.80 
0.80 
0.80 
0.81 
0.80 
0.76 
 
 
 - 
- 
- 
- 
- 
0.77 
 
 
0.82 
0.81 
0.81 
0.82 
0.81 
0.74 
 
 
 - 
- 
- 
- 
- 
Area fraction, (%) 9.6 ± 3.1 1.2 ± 1.9 17 ± 2.3 8.2 ± 2.9 
Pearlite Mean intercept length ( αL ), µm 10.3 ± 0.7 10.2 ± 0.7 8.4 ± 0.9 8.9 ± 1 
Degenerated 
pearlite Area fraction, (%) 8 ± 0.9 16 ± 2 - 9 ± 1 
b
Dislocation 
substructure 
a 
Polygonal ferrite 
a 
Pearlite 
Polygonal 
ferrite 
b
Dislocation 
substructures 
1832006 New Developments in Long and Forged Products Proceedings
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6 Bright field transmission electron micrographs of Nb-microalloyed steels processed at relatively high cooling rate showing 
(a) polygonal ferrite structure (b) lath-type (acicular) ferrite structure, (c) degenerated pearlite. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7 Bright field transmission electron micrographs of V-microalloyed steels processed at relatively high cooling rate showing (a) 
polygonal ferrite structure, (b) lath-type (acicular) ferrite structure, (c) degenerated pearlite. 
 
The SAD pattern analysis indicated that the fine precipitates were MC type of cubic niobium carbides and the precipitates exhibited 
[100]α//[110]NbCBaker-Nutting orientation relationship with the ferrite matrix. Grain boundary precipitation and precipitation on 
dislocations in the ferrite region of V-microalloyed steels was also observed and is presented in Figure 10. Figure 11a shows 
precipitation in the ferrite matrix of V-microalloyed steels and the corresponding selected area diffraction (SAD) pattern is shown in 
Figure 11b. In a manner similar to Nb-microalloyed steel, the SAD pattern analysis indicated that the fine precipitates were MC type 
of cubic vanadium carbides and the precipitates exhibited a cube-cube [001]α//[001]VC Baker-Nutting orientation relationship with the 
ferrite matrix. The characteristics of precipitates in terms of mean particle size, mean inter-particle spacing and particle density in the 
ferrite matrix of both Nb- and V-microalloyed steels are summarized in Table IV. 
 
The above results suggest that Nb- and V-microalloyed steels experienced strain induced precipitation at grain boundaries, and 
dislocations, while the fine precipitates in ferrite formed during cooling. The precipitation of microalloying elements occurs during 
various stages of thermomechanical processing of steels. At soaking temperatures the microalloying elements, Nb and V, are taken 
into solution depending on the limitation imposed by the solubility product. For carbide and nitride forming elements, the solubility in 
austenite at any given temperature depends on C and N content of the steel. When the temperature is lowered during cooling, 
supersaturation of these solute elements increases and precipitation begins at favorable kinetic conditions. Deformation of austenite 
introduce large amount of lattice defects such as dislocations and vacancies that assists the diffusional process that control the 
precipitation kinetics. As a result, strain induced precipitation occurs at the prior austenite grain boundaries or defects. In summary, 
the Nb- and V-microalloyed steels exhibited similar precipitation behavior in ferrite and the size range was from ~ 5 -10 nm (Table 
IV). It is reported that the effective size range for precipitation hardening is ~5-20 nm [3, 5]. These fine precipitates exhibited Baker-
Nutting orientation relationship (Figures 9b and 11b) with the ferrite matrix of Nb- and V-microalloyed steels that may confirm that 
the precipitation occurred in ferrite. 
b 
b a c 
c a 
184 2006 New Developments in Long and Forged Products Proceedings
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8 Bright field TEM micrographs of Nb-microalloyed steels showing (a) grain boundary precipitation and (b) precipitation on 
dislocations. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 9 Bright field TEM micrograph of Nb-microalloyed steels showing (a) fine-scale precipitation in the ferrite matrix and (b) 
corresponding SAD pattern analysis for the precipitate and the matrix. 
 
Table IV Precipitate characteristics of Nb- and V-microalloyed steels. 
 
 
 
Nb-microalloyed steels V-microalloyed steels 
Properties Conventional 
cooling rate 
Higher 
cooling rate 
Conventional 
cooling rate 
Higher 
cooling rate 
Mean particle size (d), nm 5.25 ± 3.5 7.5 ± 4.5 10.3 ± 4 8 ± 4.3 
Mean inter-particle spacing, nm 
(50 measurements) 66 ± 37 50 ± 29 62 ± 31 44 ± 18 
Particle density, 
(No. of particles/~ 0.5 µm2) 280 320 210 375 
a b 
[110]NbC [100]α 
002α 
020α 
002 
111 
b 
Precipitates on 
dislocations
a 
Grain boundary 
precipitates 
0.5 µm 
1852006 New Developments in Long and Forged Products Proceedings
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 10 Bright field TEM micrographs of V-microalloyed steels showing (a) grain boundary precipitation and (b) precipitation on 
dislocations. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 11 Bright field TEM micrograph of V-microalloyed steels showing (a) fine-scale precipitation in the ferrite matrix and (b) 
corresponding SAD pattern analysis for the precipitate and the matrix. 
 
Toughness Behavior of Nb- and V-Microalloyed Steels 
In the present case, the microstructural parameters that are likely to influence toughness are ferrite grain size, degenerated pearlite, and 
acicular ferrite. A finer grain size and higher contiguity ratio is expected to impart higher toughness. From Table III, it may be noted 
that the ferrite grain size and ferrite contiguity ratio are similar for both the steels processed at conventional and relatively high 
cooling rates. However, there are significant differences in the area fraction of degenerated pearlite for the two steels (Figure 1b and 
Table III). It is reported that the cementite thickness in degenerated pearlite is finer as compared to the conventional pearlite, and 
hence the volume fraction of cementite and ferrite are different in the former as compared to the latter [10]. Coarse pearlite deforms 
inhomogeneously with strain localized in narrow slip bands, whereas fine degenerated pearlite experience uniform strain distribution 
during deformation. It is also shown that the steels containing degenerated pearlite with specific cementite thickness exhibits 
maximum ductility [10]. Thus, at present, we believe that the increase in the toughness of Nb- and V-microalloyed steels at relatively 
a b 
[001]VC [001]α 
020α 
200α 
020 
220 
a 
Grain boundary 
precipitates 
b 
Precipitates on 
dislocations 
186 2006 New Developments in Long and Forged Products Proceedings
high cooling rate is a consequence of higher fraction of degenerated pearlite and the presence of lath-type ferrite. While, the higher 
toughness of Nb-microalloyed steels in relation to V-microalloyed steels at conventional or normal cooling rate is a consequence of 
higher fraction of degenerated pearlite, with ferrite grain size being similar for the two steels. 
 
SUMMARY 
 
1. At conventional cooling rates employed in the mill, the microstructure of Nb- and V-microalloyed steels primarily contained 
polygonal ferrite-pearlite, while Nb-microalloyed steels contained significant fraction of degenerated pearlite. The Nb- and 
V-microalloyed steels processed at relatively higher cooling rate compared to conventional or normal cooling rate contained 
degenerated pearlite and lath-type (acicular) ferrite in addition to the primary ferrite-pearlite constituents. The fraction of 
degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels. 
2. Both Nb-and V-microalloyed steels exhibited precipitation at grain boundaries, dislocations and in the ferrite matrix. Fine 
scale (~ 5-10 nm) precipitation occurred in the ferrite matrix of both the steels. The SAD pattern analysis revealed that these 
fine precipitates were MC type of niobium and vanadium carbides in the respective steels and obeyed Baker-Nutting 
orientation relationship with the ferrite matrix. 
3. The Nb-and V-microalloyed steels experienced improvement in toughness with increase in cooling rates during processing. 
However, Nb-microalloyed steels appear to exhibit relatively higher toughness than the V-microalloyed steels during 
processing at conventional and high cooling rates. The increase in toughness of Nb-microalloyed steels is attributed to its 
higher fraction of degenerated pearlite in the steel. 
 
 
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1872006 New Developments in Long and Forged Products Proceedings
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