c. v. White

Kettering University

G. Krauss and D.K. MatlockColorado School of Mines

INTRODUCTION

The traditional steel

production processconsisted of the

refining and teeming

of liquid steel into

large ingots. These in-

gots required exten-

sive high temperature

soaking and hot roll-

ing to produce slabs,

blooms and billets that

were subsequently hot

rolled to finished

product shapes. Although some ingot

casting capacity still exists, continuous

casting directly to slabs, blooms and

billets has largely displaced ingot cast-

ing. Continuous casting has grown to

more than 80 percent of steel produc-

tion in North America, and to more than85 percent in the United Kingdom.1, 2

The popularity of continuous casting

has been driven by improvements in

surface quality, greater uniformity in

composition (due to the elimination of

top-to-bottom ingot segregation) , en-

ergy savings, high yields and increased

production efficiency. The history and

evolution of continuous caster design

and technology have been described in

detail by IMng.2A concern exists that smaller as-cast

sections of continuously cast steel prod-

ucts may not receive sufficient hot work

to produce microstructures and prop-

erties characteristic of more highly de-

formed steel products. This is especially

a concern in bar steels that are forged to

complex shapes. This article will review

the literature related to the effects of hot

reduction on the properties and perfor-

mance of continuously cast bar steels.

Diminished mechanical properties and

fatigue performance have been reported

in strand-cast steels subjected to limited

reduction. Properties representative of

wrought steel eventually are attained

with increasing hot reduction. The

amount of hot reduction required to

achieve the transition from cast to

wrought structure and properties is of

great interest, especially in small con-

tinuously cast billets that may receive

little hot reduction to finished bar sizes.

The causes of reduced performance

as a function of limited hot work have

been difficult to unravel. Performance

depends on a multitude of factors: the

homogenizing effects of hot work; steel-

making, which establishes the cleanli-

ness and inclusion content of the prod-

uct; caster design, which establishes the

size and geometry of the as-cast product;

the nature ofhotwork or roll pass design

of the hot rolling stands used to produce

bars of various sizes; and, ultimately, the

microstructure of the heat-treated bar or

forgings, such as ferrite-pearlite or high

strength tempered martensite.Thus, the amount of hot work re-

quired to produce wrought propertiesis caster- and mill-specific. A universal

criterion for acceptability, such as a

single value of the commonly used hot

reduction ratio, based on reduction of

SEPTEMBER 1998 g 73

Figure 3 The histogram shows the frequency of inclusions with various shape factors as afunction of the hot reduction ratio.9 JO The shape factor is defined in the text.

and a central zone with equiaxed den-

drites with random orientations. As dis-

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-

mines thermal gradients and solidifica-

tion velocity. Electromagnetic stirring is

used to promote the size of the equiaxed

zone, minimize centerline porosity and

reduce the size of the columnar zone.

The scale of the dendritic structure

determines the distribution of inter-

dendrite chemical segregation and

porosity due to solidification shrinkage

or gas evolution. These factors may

be detrimental to mechanical proper-

ties and performance. The fine-scale

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 evidence for

interdendritic 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 acid and sodium

tridecylbenzene. The austenite grain

structure and the ferrite-pearlite micro-

structure formed on cooling of this steel

were superimposed on the chemical

variations. When tested in torsional fa-

tigue, all specimens showed wrought

behavior and fatigue crack initiation at

inclusions within the ferritic compo-

nent of the microstructure.

Thus, in this experiment, microstruc-

ture and inclusions controlled fatigue

performance. The coarser effects of so-

lidification, such as columnar dendrite

crystal orientation and porosity, were

dissipated by a reduction ratio of 7: 1.

Residual effects of dendritic segrega-

tion may contribute to ferritelpearlite

banding beyond association with as-cast

structures! Also, residual chemical seg-

regation effects from the corners of square

billets eventually may contribute to

distribution of porosity often is charac-

terized by secondary dendrite arm spac-

ing, which is directly related to heat

transfer during solidification.

Figure 5 shows secondary dendrite

arm spacing as a function of the dis-

tance from the chill surface for a num-ber of casters. 14 The larger the section

size and further the distance from the

surface, the coarser the dendrite spac-

ing and associated micro segregation

and porosity.

M.CROSEGREGA T.ON

Center macroporosity and interdendrite

microporosity are healed by hot work,

~

1- Chill zone

Columnar zone

3.~ .Low carbon steels22 .

2 .Stainless steels (type 304) .

...A

1.8 ~ .. .-14 ..

! 1.0I = .6 ..

I 2

= -.2

o -.6..I -1.0

-2.4 -1.6 -0.8 -0.0 0.8 1.6 2.4

Log x (mm) Figure 6 A picric acid-sodiumtridecylbenzene etchJO depicts effectsassociated with residual interdendriticmicrosegregation in bars of 10V45 steelsubjected to hot reduction ratios of (a)7:1, (b) 10:1, (c) 27:1 and (d) 49:1.

Equiaxed zone

Figure 5 The secondary dendrite armspacingfor a number of dijJerent castersis shown as a function of distance fromthe chill surface. !4

Figure 4 The diagram shows various

solidification zones in a continuously

cast billet.12

SEPTEMBER 1998 a 75

nonsyrnmetrical distortion during finalheat treatment of bars and forgings.15. 16

with discontinuities caused by speci-

men machining.17

Solidification porosity increases with

greater distance from the surface of as-

cast billets. As the dendritic structure

coarsens, the interior cooling rate de-

creases, the last liquid steel solidifies and

final shrinkage develops. Density mea-

surements, as a function of distance from

the surfaces of 140-mm-square cast bil-

lets, show porosity is low in columnar

dendrite solidification zones and in-

creases to high levels close to and at billet

centerlines.18 Brada has shown that cor-

ners of square billets, where columnar

dendrite zones from orthogonal surface

impinge, also are locations of high po-

rosity in continuous cast billets.19

Figure 9 shows macroetched trans-

verse sections of electromagnetically

stirred and unstirred continuously

cast billets of 4140 steel,171n the unstirred

billet, the columnar dendrite zone occu-

pies most of the cross section, and mac-

roscopic porosity is present at the

centerline of the billet. Electromagnetic

SOLIDIFICATION POROSITY

Porosity is a major consequence of so-

lidification and has been shown to cause

diminished fatigue performance. For

example, Figures 7 and 8 show fatigue

crack initiation at porosity in as-cast

continuously cast 152-mm-square bil-lets and hardened 4140 steeV 7 Figure 7

illustrates fatigue fracture surfaces de-

veloped by axial fatigue testing of

specimens removed from the columnar

dendritic zone. Meanwhile, Figure 8

illustrates fatigue in the equiaxed

zone. The interdendritic morphology

of the porosity is revealed, especially in

the second example; the former re-

vealed better fatigue life. Specimens

removed from bars of the same billet

subjected to hot reduction ratios of

3.3: lor greater showed marked im-

provement in fatigue resistance. Fatigue

initiation of hardened specimens from

hot worked bars largely was associated

stirring produces an extensive equiaxed

dendritic zone and eliminates macro-

scopic centerline shrinkage. A light-

etching band, because of solute deple-

tion, marks the initiation of electromag-

netic stirring.

In addition to stirring, porosity and

the extent of the equiaxed central zone

of billets also are affected by superheat.

High superheat in unstirred billets in-

creases the extent of columnar solidifi-

cation by retarding the nucleation ofequiaxed dendrites:Q, 21 High superheats

also have been found to increase

interdendrite segregation and internal

porosity, requiring higher reduction

ratios for the elimination of porosity.21

Electromagnetic stirring offsets the ef-

fect of high superheats, but does not

compensate completely for the in-

creased size of columnar solidification

zones at higher superheat temperatures.

Hot work eliminates porosity, but

the amount required to produce full

density in bars is a function of the meth-

ods by which the hot work is applied.

~ ~10J.lrn

I Figure 7 Porosity is a major conseqltence of solidification. The micrographs show theporosity at the fatigue crack initiation in a hardened 4140 steel specimen taken from thecolumnar dendritic zone of an as-cast sti"ed billet.ll

76 H SEPTEMBER 1998

etching effects associated with chemicalsegregation are still apparent.

Figure 12, taken from the work of Fett

and Ross, shows strength and ductility

parameters as a function of the reduc-

tion ratio for normalized specimens of

1040 steel,24 As noted previously, ulti-

mate strength and hardness are not

degraded in as-cast structures, but elon-

gation and reduction of area are re-

duced by the effects of solidification.

Figure 13, from the same study, shows

dependence of impact energy on the

reduction ratio, similar to that of the

ductility parameters. Both the impact

and ductility measures of fracture resis-

tance reach constant values at the low

reduction ratio of 3:1. Morris et al.

show similar sets of data for tensile

properties, but greater variation in im-

pact properties in specimens is obtained

from billets subjected to low reduction

ratios of between 3:1 and 7:1.18

The studies of fatigue in as-cast and

hot rolled continuously cast billets show

that specimens from as-cast billets fail

for one of two reasons: coarse as-cast

columnar grain structure or porosity

incorporated during solidification.17

After relatively small amounts of hot

reduction, the effects of residual solidi-

fication no longer playa significant role

in the initiation of fatigue cracks. In-

stead, fatigue initiates at inclusions or

surface defects, which are determined

by steel cleanliness, specimen prepara-

tion and microstructure.

For example, Figure 10 shows poros-

ity is eliminated by relatively low reduc-

tion ratios, and rolling schedules with

heavy passes increase density more

readily than those with light passes.22The

heavy passes were accomplished with

larger diameter cylindrical rolls. A study

of the elimination of porosity in continu-

ously cast slabs also emphasized that a

combination of rolling reduction and

rolling shape factor (as related to radius

of the work rolls) must be considered for

the elimination of porosity.23

Another study was conducted on the

effectiveness of various deformation

processes on consolidating central

looseness and voids in bars produced

from strand-cast billets. Morris et al.

found that hammer forging restored full

density at a nominal reduction ratio of

3:1.18 But, when continuous 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 bil-let. 17 No macroscopic evidence ofporos-

ity is visible in the bar produced from an

electromagnetically stirred billet, but

MINIMUM HOT REDUCTION

RATIOS

Table I lists a number of investigations

that have been designed to evaluate the

transition from cast to wrought me-

chanical behavior and structure in con-

tinuously cast steels. The criteria, as

identified by each set of authors, dif-

fered for an effective transition. The

criteria included an elimination of

porosity, mechanical behavior charac-

teristic of products produced from in-

got cast steels or mechanical behavior

that attained a stable high level as a

function of increasing hot reduction.

A number of the investigations of

mechanical properties determined by

tensile testing show yield and tensile

strength are relatively insensitive to

microstructural components intro-

duced by solidification or hot work ofthe as-cast structures. 17, 18,21,24 How-

ever, the properties that measure duc-

tility, such as reduction of area or total

elongation, are sensitive to solidifica-

tion structures and improve with in-

creasing hot reduction.

The ductility parameters are deter-

mined by ductile fracture mechanisms

related to pore and inclusion distribu-

tions, and are adversely affected by

those distributions in as-cast specimens.0

Heavy pass~ ~C')-

E()

...

s 01

I

.02

/

.031 15

Rolling Ratio

2 2.5 3

Figure 10 Density differences betweencore and surface zones of cast plates, shownas a function of reduction ratio and rollingseverity, indicate that porosity is eliminatedby relatively low reduction ratios!2

SEPTEMBER 1998 a 77

the Advanced Steel Processing and Prod-

ucts Research Center, a National Sci-

ence Foundation Industry-University Co-

operative Research Center at the

Colorado School of Mines. ~

superheat and electromagnetic stirring.

Deternlination of the success of these

efforts must then be made on a case-by-

case basis. The mechanical properties

must be evaluated and demonstrate that

the properties are adequate for the pro-

posed application.This article has reviewed a number

of technical papers on the effect of hot

reduction on the properties of continu-

ously cast steel. Two of the most com-

prehensive papers regarding mill proc-

essing were commissioned by theEuropean Commission.18, 21 Producers

and users of steels should perform more

work to be published and evaluated.

This work would enable the establish-

ment of clear limits of the minimum

amounts of hot reduction for specific

steel compositions, steelmaking condi-

tions and product applications.

ACKNOWLEDGMENTS

C.V. White acknowledges the support of

American Iron and Steel Institute, Deere

& Company, Chaparral Texas Steel Re-

cycling, Caterpillar Inc. and Inland Steel

Company. This support made possible

a sabbatical leave from GMI Engineer-

ing and Management Institute (now

Kettering University) at the Colorado

School of Mines. The program on the

effects of the hot reduction ratio on

continuous cast steels is supported by

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SEPTEMBER 1998 la 79