c. v. White
Kettering
University
G. Krauss and D.K. Matlock
Colorado School of Mines
reduction. Properties representative of
INTRODUCTION
The traditional steel
production
consisted
wrought steel eventually are attained
with increasing hot reduction. The
process
of the
amount of hot reduction required to
refining and teeming
of liquid steel into
achieve the transition from cast to
wrought structure and properties is of
large ingots. Thesein-
great interest, especially in small con-
gots required exten-
tinuously cast billets that may receive
sive high temperature
soaking and hot roll-
little hot reduction to finished bar sizes.
The causesof reduced performance
as a function of limited hot work have
ing to produce slabs,
blooms and billets that
were subsequentlyhot
been difficult to unravel. Performance
depends on a multitude of factors: the
homogenizing effectsof hot work; steel-
rolled to finished
product shapes. Although some ingot
and technology have been described in
making, which establishes the cleanli-
casting capacity still exists, continuous
casting directly to slabs, blooms and
detail by IMng.2
A concern exists that smaller as-cast
ness and inclusion content of the product; caster design,which establishesthe
billets has largely displaced ingot casting. Continuous casting has grown to
sectionsof continuously caststeelprod-
sizeand geometryof the as-castproduct;
ucts may not receive sufficient hot work
more than 80 percent of steel production in North America, and to more than
85 percent in the United Kingdom.1,2
to produce microstructures and properties characteristic of more highly deformed steelproducts. This is especially
a concern in bar steelsthat are forged to
the nature ofhotwork or roll passdesign
of the hot rolling standsused to produce
bars of various sizes;and, ultimately, the
microstructure of the heat-treatedbar or
The popularity of continuous casting
has been driven by improvements in
surface quality, greater uniformity in
forgings, such as ferrite-pearlite or high
complex shapes.This article will review
the literature related to the effectsof hot
strength tempered martensite.
Thus, the amount of hot work re-
composition (due to the elimination of
reduction on the properties and perfor-
top-to-bottom ingot segregation), en-
mance of continuously cast bar steels.
Diminished mechanical properties and
quired to produce wrought properties
is caster- and mill-specific. A universal
criterion for acceptability, such as a
fatigueperformance havebeenreported
in strand-caststeelssubjectedto limited
single value of the commonly used hot
reduction ratio, based on reduction of
ergy savings,high yields and increased
production efficiency. The history and
evolution of continuous caster design
SEPTEMBER 1998 g
73
~
and recrystallization of the austenite grain
structure is achieved by the same method.
Despite this, microsegregation persists
even after extensive hot deformation.
Figure
6 presents
interdendritic
evidence
for
microsegregation,
al-
though with increasingly finer spacing,
in bars of 10V45 steel for reduction
ratios
between
7:1 and 49:1.10 The
chemical segregation was revealed by
etching with picric
tridecylbenzene.
acid and sodium
The austenite
structure and the ferrite-pearlite
grain
micro-
structure formed on cooling of this steel
were superimposed
on the chemical
variations. When tested in torsional fatigue, all specimens showed wrought
Figure 3 The histogram shows the frequency of inclusions with various shapefactors as a
function of the hot reduction ratio.9 JOThe shape factor is defined in the text.
behavior and fatigue crack initiation
inclusions
within
the ferritic
at
compo-
nent of the microstructure.
and a central zone with equiaxed den-
distribution of porosity often is charac-
drites with random orientations. As dis-
terized by secondarydendrite arm spacing, which is directly related to heat
transfer during solidification.
ture and inclusions
Figure 5 shows secondary dendrite
arm spacing as a function of the dis-
crystal orientation
cussed here, extensive porosity may exist
in the central equiaxed zone. The relative
areas of the columnar and equiaxed zones
depend on caster speed, which deter-
Thus, in this experiment, microstrucperformance.
lidification,
controlled
fatigue
The coarser effects of so-
such as columnar dendrite
and porosity, were
dissipated by a reduction ratio of 7: 1.
tion velocity. Electromagnetic stirring is
tance from the chill surface for a number of casters.14The larger the section
used to promote the size of the equiaxed
size and further the distance from the
banding beyond association with as-cast
zone, minimize centerline porosity and
surface, the coarser the dendrite spac-
structures! Also, residual chemical seg-
reduce the size of the columnar zone.
ing and associated micro segregation
and porosity.
regation effects from the corners of square
mines thermal gradients and solidifica-
The scale of the dendritic structure
determines
dendrite
the distribution
chemical
or gas evolution.
be detrimental
tion may contribute
billets
eventually
to ferritelpearlite
may contribute
to
of inter-
segregation
porosity due to solidification
Residual effects of dendritic segrega-
and
shrinkage
These factors may
M.CROSEGREGA
T.ON
Center macroporosity and interdendrite
microporosity
are healed by hot work,
to mechanical proper-
ties and performance.
The fine-scale
3.~
2.
2
.Low
carbon
.Stainless
steels
steels
(type
304)
.
...A
.-14
I !=
1-
Chill
~
1.8
1.0
.6
Columnarzone
..
2
=
-.2
o
..I
-.6
-1.0
-2.4
Equiaxed zone
4
solidification
cast billet.12
The diagram
..
zone
I
Figure
..
shows various
zones in a continuously
-1.6
Log
x
-0.8
-0.0
0.8
1.6
2.4
(mm)
Figure 5 The secondary dendrite arm
spacingfor a number of dijJerent casters
is shown as a function of distance from
the chill surface. !4
Figure 6 A picric acid-sodium
tridecylbenzene etchJOdepicts effects
associated with residual interdendritic
microsegregation in bars of 10V45 steel
subjected to hot reduction ratios of (a)
7:1, (b) 10:1, (c) 27:1 and (d) 49:1.
SEPTEMBER 1998 a
75
nonsyrnmetrical distortion during final
heat treatment of bars and forgings.15.
16
with discontinuities caused by speci-
stirring produces an extensive equiaxed
men machining.17
dendritic
Solidification porosity increaseswith
zone and eliminates macro-
scopic centerline
shrinkage.
A light-
SOLIDIFICATION
POROSITY
Porosity is a major consequenceof so-
greater distance from the surface of ascast billets. As the dendritic structure
etching band, because of solute deple-
lidification and hasbeen shown to cause
diminished fatigue performance. For
coarsens, the interior cooling rate decreases,the last liquid steelsolidifies and
final shrinkage develops. Density mea-
netic stirring.
example, Figures 7 and 8 show fatigue
crack initiation at porosity in as-cast
tion, marks the initiation of electromagIn addition to stirring, porosity and
the extent of the equiaxed central zone
surements,asa function of distancefrom
of billets also are affected by superheat.
continuously cast 152-mm-square billets and hardened 4140 steeV 7Figure 7
the surfacesof 140-mm-squarecast bil-
High superheat in unstirred
lets, show porosity is low in columnar
creases the extent of columnar solidifi-
illustrates fatigue fracture surfaces de-
dendrite solidification zones and in-
veloped by axial fatigue testing of
creasesto high levelscloseto and at billet
cation by retarding the nucleation of
equiaxed dendrites:Q, 21High superheats
specimensremoved from the columnar
dendritic zone. Meanwhile, Figure 8
illustrates fatigue in the equiaxed
zone. The interdendritic morphology
centerlines.18Brada has shown that corners of square billets, where columnar
dendrite zonesfrom orthogonal surface
of the porosity is revealed, especially in
the second example; the former re-
impinge, also are locations of high porosity in continuous cast billets.19
also have been found
interdendrite
porosity,
billets in-
to increase
segregation and internal
requiring
higher
ratios for the elimination
Electromagnetic
reduction
of porosity.21
stirring offsets the ef-
Figure 9 shows macroetched transverse sections of electromagnetically
fect of high superheats, but does not
vealed better fatigue life. Specimens
removed from bars of the same billet
stirred and unstirred
creased size of columnar solidification
subjected to hot reduction ratios of
3.3: lor greater showed marked improvement in fatigue resistance.Fatigue
castbillets of 4140 steel,171n
theunstirred
billet, the columnar dendrite zone occu-
zones at higher superheat temperatures.
pies most of the cross section, and mac-
the amount required
initiation of hardened specimens from
hot worked bars largely was associated
roscopic porosity is present at the
centerline of the billet. Electromagnetic
density in bars is a function of the meth-
~
I Figure
continuously
~10J.lrn
7 Porosity is a major conseqltence of solidification. The micrographs show the
porosity at the fatigue crack initiation in a hardened 4140 steel specimen taken from the
columnar dendritic zone of an as-cast sti"ed billet.ll
76 H
SEPTEMBER 1998
compensate
Hot work
completely
eliminates
for the in-
porosity,
but
to produce
full
ods by which the hot work is applied.
For example, Figure 10 shows porosity is eliminated by relatively low reduc-
etchingeffectsassociated
with chemical
segregation
are still apparent.
tion ratios, and rolling schedules with
heavy passes increase
density more
Figure 12, taken from the work of Fett
and Ross, shows strength and ductility
parameters as a function of the reduc-
MINIMUM
HOT REDUCTION
tion ratio for normalized
specimens of
with
RATIOS
Table I lists a number of investigations
mate strength and hardness are not
larger diameter cylindrical rolls. A study
that have been designed to evaluatethe
degraded in as-cast structures, but elon-
of the elimination of porosity in continu-
gation and reduction
ously cast slabs also emphasized that a
transition from cast to wrought mechanical behavior and structure in con-
combination
and
tinuously cast steels. The criteria, as
rolling shape factor (as related to radius
identified by each set of authors, dif-
dependence
of the work rolls) must be considered for
fered for an effective transition. The
reduction
the elimination of porosity.23
criteria included an elimination of
porosity, mechanical behavior charac-
ductility
readily than those with light passes.22The
heavy passes were accomplished
of rolling
reduction
Another study was conducted on the
effectiveness
processes
of various
deformation
on consolidating
central
looseness and voids in bars produced
from strand-cast billets. Morris
et al.
1040 steel,24 As noted previously, ulti-
of area are re-
duced by the effects of solidification.
Figure 13, from the same study, shows
of impact energy on the
ratio, similar to that of the
parameters.
Both the impact
and ductility measures of fracture resis-
teristic of products produced from ingot cast steels or mechanical behavior
tance reach constant values at the low
that attained a stable high level as a
function of increasing hot reduction.
show similar
reduction
ratio of 3:1. Morris
properties,
et al.
sets of data for tensile
but greater variation in im-
found that hammer forging restored full
A number of the investigations of
pact properties in specimens is obtained
density at a nominal reduction ratio of
mechanical properties determined by
from billets subjected to low reduction
3:1.18 But, when continuous
tensile testing show yield and tensile
ratios of between 3:1 and 7:1.18
strength are relatively insensitive to
microstructural components intro-
hot rolled continuously cast billets show
duced by solidification or hot work of
the as-cast structures. 17,18,21,24
How-
for one of two reasons: coarse as-cast
ever, the properties that measure ductility, such as reduction of area or total
incorporated
forging,
groove rolling and flat rolling were used
for hot deformation,
reduction ratios of
at least 5:1 were required.
Figure 11, from the study by Schultz et
al., shows in continuously cast 4140 bars
subjected to a reduction
ratio of 3:3,
some residual center porosity persists in
the bar produced from an unstirred billet. 17No macroscopic evidence ofporosity is visible in the bar produced from an
electromagnetically
stirred
billet,
but
0
Heavy pass~
C')-
~
The studies of fatigue in as-cast and
that specimens from as-cast billets fail
columnar
grain structure
during
or porosity
solidification.17
elongation, are sensitive to solidification structures and improve with in-
After relatively
creasing hot reduction.
The ductility parameters are deter-
fication no longer playa significant role
mined by ductile fracture mechanisms
related to pore and inclusion distribu-
stead, fatigue initiates at inclusions
tions, and are adversely affected by
thosedistributions in as-castspecimens.
by steel cleanliness, specimen prepara-
small amounts of hot
reduction, the effects of residual solidiin the initiation
of fatigue cracks. Inor
surface defects, which are determined
tion and microstructure.
E
()
...
s
01
I
.02
/
.03
1
Rolling
15
2
2.5
3
Ratio
Figure 10 Density differences between
core and surface zones of cast plates, shown
as a function of reduction ratio and rolling
severity, indicate that porosity is eliminated
by relatively low reduction ratios!2
SEPTEMBER
1998 a
77
superheat and electromagnetic stirring.
the Advanced Steel Processing and Prod-
14. A.W.
Cramb,
"Secondary
in Continuous
Deternlination
of the success of these
ucts Research Center, a National Sci-
of Near
efforts must then be made on a case-by-
ence Foundation Industry-University Co-
673-682.
case basis. The mechanical properties
operative
must be evaluated and demonstrate that
Colorado School of Mines. ~
Net
Shape
15. S. Gunnarson,
the properties are adequate for the proposed application.
This article has reviewed a number
of technical papers on the effect of hot
reduction on the properties of continu-
Research
Center
Cast Bar Steels in North America,"
Bar Applications
Group/American
The Institute
of Materials,
3. W .C. Leslie, "Inclusions
Trans.
oftbe
5. D. Schauwinhold,
ASM, Materials
plications
steel compositions,
tions and product applications.
Creation
7. J.D. StoverandR.V.
8. N .Islam,
4140
et al., "Effect
on Torsional
Microalloyed
at the Colorado
ratio on
G.R. Speich
Fraction
cast steels is supported by
Reduction
of Strand
Working
Cast AISI
and
Steel
ISS, Vol. XXVII,
1989, pp.
of Hot Reduction
and Bar
Fatigue
of a Strand-Cast
and
W.A.
Working
Thickness Ductility
Spitzig,
"Effect
of Volume
on Through-
and Impact Energy ofHigh Strength
4340 Plate Steels," Metall.
TransactionsA,
Vo113A,
1982, pp 2,239-2,257.
12. Y. Tomita,
4340
tionsA,
Berlin,
Wheels,"
Hartereip. 216.
and Shape Changes
J.
Grosch
Hardened
4140
"Effect of Hot Rolling
mtrahigh
Reduction
and Fracture
Strength
Steel,"
on Shape
Toughness
Metall.
of A/SI
Transac-
1986.
Clean
Steel,
Springer
J.
AWT, 1989, pp. 285-303.
Reduction
Steel," 34th
Steel Processing
Mechanical
Proceedings,
and
Working
ISS, Vol. XXX,
1992, pp. 309-319.
S.P. Ryalls and B.A. Wade, "Optimiza-
tiun of the defonnation
erties,"
process for continuously
the most appropriate
Final Report, Contract
D5.5/88),
European
Structure
School
prop-
No. 7210.EB/804
(DI-
1994.
ofContinuouslyCastAISI
and Axial Fatigue,"
of Mines,
Steel,"
Observed
Size-Alloying
Proceedings
England,
Ratio on
M.S. Thesis, Colorado
1993.
"Aberrations
ship ofDendrite
cast
materi:j!
Commission,
19. G. Brada, "Characterization
in the Relation-
Elements
for Low Alloy
of the International
Confer-
The Metals Society, London,
1979, pp. 21-29.
21. F. Fattorini
and
Reduction
B. Grifoni,
Ratio
"Effect
on the Quality
of the Rolling
and Properties
Rolled
Engineering
Steel Bars Deriving
Billets
and Blooms
via C. C. Route,"
Contract
No. 7210-EB/402
Commission,
22. J-C. Brunet,
from
of
Cast
Final Report,
(1.7.1985-30.6.1988),
1990.
"Reduction
ing: How Important
Ratios In Continuous
Cast-
Are They? ," Metal Progress,
Vol.
XX, 1985, pp. 45-53.
23. N. Okumura
tinuous
Verlag,
et al., "Hot
Cast Slabs,"
24. G.A. FettandJ.W.
Impact
Rolling
Conditions
Properties
of Improved
Performance
Vol. 28,1991,
pp. 1-14.
Quality
"Evaluatiun
and Breach,
ASME, MD
On the Minimill
1984, pp. 29-31.
of Continuous
for Seamless Tube Production,"
ing, Gordun
on Proper-
and Design,
25. R.H. McCreery , "Effects of Reduction
Progress,
Ratio
of Strand Cast Steel,"
Material
ties, Product
Steel," Metal
in Cun-
pp. 217-228.
Ross, "The Effect of Reduction
on the Mechanical
26. H.B. Emerick,
Vol. 19A, 1988, pp. 1,555-1,561.
Calcium
by
and
and
ISS, 1988, Vol. VVXI,
and Shape of Sulfide Inclusions
13. T. Ototani,
continuous
Fatigue
Proceedings,
of Sulfide Inclusions
effects of the hot reduction
of Hot Working
Steel," 30thMechanical
pp. 83-94.
School of Mines. The program on the
casting of
pp.
Ratio on the Axial Fatigue of Cuntinuously-Cast
European
et al., "Effects
Diameter
II.
Editors,
ence on Solidification,
99-113.
Steel Processing
Kettering University)
Bear-
1989, pp. 392-396.
Mechanical
Proceedings,
cycling, Caterpillar Inc. and Inland Steel
(now
With
ImpToves
in the continuous
and Materials,
& Company, Chaparral Texas Steel Re-
Institute
"Air-MeltedSteel
SME, Paper No.871208.
Steel," 31st
10. J. Dyek
American Iron and Steel Institute, Deere
ing and Management
Investment
SAE, Warrendale,
Content Further
"Developments
steel," Metals
Processing
Casting
1988,
Vol. 46,1991,
Einsatzharten,
20. D.J. Hurtuk,
Kolarikll,
Inclusion
ing Fatigue Life,"
ACKNOWLEDGMENTS
a sabbatical leave from GMI Engineer-
Through
Improvement,"
Ratios on the Torsional
Company. This support made possible
1982 ,
PA, 1992, Paper No.921678.
9. B. Rittgers
C.V. White acknowledges the support of
and Ap-
Cast Steel,"
and Steel Conference,
"Value
and Continuous
for specific
steelmaking condi-
Properties
from Continuously
TMS,
4130 Steel and the Effects of Hot-Reductiun
6. R.L. Widner,
Ultra-Low
Crown
"Dimension
18. P.W. Morris,
Steels,
pp. 3/3-3/12.
This work would enable the establishamounts of hot reduction
Steel
billets to provide
"Characteristic
4th Internationallron
and users of steels should perform more
ofCarbon
Park, OH, 1980.
ofProducts
Products,
17. E.J. Schultz et al., "The Effect of Hot-RoU
and
Properties,"
ISS, ISS, Vol. 2, 1983, pp. 1-24.
essing were commissioned
by the
European Commission.18, 21 Producers
ment of clear limits of the minimum
and Steel
1993, pp. 1-20.
and Mechanical
prehensive papers regarding mill proc-
and evaluated.
Iron
Institute,
1995, pp. 3-4.
2. W .R.lrving, "Continuous
Casting of Steel, " Book 584,
Wunning,
Spacings
"Effect of Strand Casting on Distortion
Carburized
Carburizing,"
I. G. Millar , "Strand
Arm
Steel Sections,"
TechniscbeMitteilungen,
References
4. L.E. Samuels, OpticalMicroscopy
to be published
of
16. H. Mallender,
ously cast steel. Two of the most com-
work
at the
Dendrite
Cast, Thin
Cast Steel
Continuous
Cast-
1962, pp. 197-208.
SEPTEMBER 1998 la
79