Drawing-Induced Evolution of Inclusions in Cold-Drawn Pearlitic Steel
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metals
Article
Drawing-Induced Evolution of Inclusions in Cold-Drawn
Pearlitic Steel
Jesús Toribio * , Francisco-Javier Ayaso and Rocío Rodríguez
Fracture & Structural Integrity Research Group (FSIRG), Campus Viriato, University of Salamanca (USAL) E.P.S.,
Avda. Requejo 33, 49022 Zamora, Spain; fja@usal.es (F.-J.A.); rociorg@usal.es (R.R.)
* Correspondence: toribio@usal.es; Tel.: +34-677566723
Abstract: This article focuses on the analysis of the evolution of inclusions present in eutectoid
pearlitic steel subjected to a real cold drawing process. To this end, wires belonging to different stages
of the manufacture chain were studied, starting from an initial hot rolled bar (not cold drawn at
all). In addition to the information obtained through visual inspection, a quantitative analysis of
the microdefects generated by these inclusions was carried out. The analysis was performed using
materialographic techniques, scanning electron microscopy (SEM) and the image analysis program
(AnaliSYS 3.1® ).
Keywords: pearlitic steel; inclusions; cold drawing
1. Introduction
The inclusions present in steel have been the subject of many scientific studies, focusing
mainly on their composition, size and morphology [1–4]. Some of the studies deal with
Citation: Toribio, J.; Ayaso, F.-J.; the spatial distribution and shape of inclusions using mathematical modelling, using the
Rodríguez, R. Drawing-Induced method called shape control [5]. In pearlitic steels, a multitude of inclusions has been
Evolution of Inclusions in Cold- observed, among which oxides, silicates, manganese sulfides, carbides and also, to a lesser
Drawn Pearlitic Steel. Metals 2021, 11,
extent, nitrides stand out [6]. As for the progressively drawn pearlitic steels, subject of
1272. https://doi.org/10.3390/
study in the present work, the influence of the inclusions on the anisotropic behavior in
met11081272
fracture has been recently analyzed [7].
Some researchers have studied the influence of inclusions on the initiation of cracks
Academic Editor: Ildiko Peter
generated by fatigue in pearlitic steels, analyzing the morphology and distribution of
these inclusions [8–11]. The scientific literature includes numerous studies focused on the
Received: 30 June 2021
Accepted: 9 August 2021
quantification of morphology and its subsequent classification, using dispersive energy
Published: 12 August 2021
spectroscopy techniques [12,13]. The most recent research deals with the analysis of the
different types of inclusions and their influence on the mechanical properties of steel [14–17].
Publisher’s Note: MDPI stays neutral
It is worth mentioning the studies focused on improving the deformability of fragile
with regard to jurisdictional claims in
inclusions in order to ensure a good mechanical performance of the steel during the
published maps and institutional affil- drawing process [18,19], or the modification of the process itself, hot rolling, to obtain
iations. inclusions that improve the final mechanical properties of the steel [20].
The cold drawing process causes microstructural changes in the steel, such as the
reorientation of the perlite colonies and the lamellae that make them up (ferrite and
cementite) in the direction of the longitudinal wire axis, a progressive decrease in the
Copyright: © 2021 by the authors.
interlaminar spacing and an increase in the slenderness of the colonies [21,22].
Licensee MDPI, Basel, Switzerland.
In pearlitic steel, the greatest addition of alloying substances occurs to promote or
This article is an open access article
enhance the nucleation of perlite. Some studies try to analyze, by means of isothermal
distributed under the terms and methods, the influence of inclusions on the nucleation of intergranular ferrite in hypoeu-
conditions of the Creative Commons tectoid steels [23]. The nucleation of non-metallic inclusions of ferrite in steels is different
Attribution (CC BY) license (https:// depending on the chemical composition. Experimentally, for medium carbon steels, it was
creativecommons.org/licenses/by/ observed that the following compounds are not intergranular ferrite enhancers: SiO2 , MnO-
4.0/). SiO2 , Al2 O3 , TiN and MnS. On the contrary, the inclusions of chemical compositions such
Metals 2021, 11, 1272. https://doi.org/10.3390/met11081272 https://www.mdpi.com/journal/metals
Metals 2021, 11, 1272 2 of 13
as MnS and Al2 O3 , in steels with a presence of V and N, are enhancers of the nucleation of
the intergranular ferrite [24]. The inclusions chemically analyzed in the pearlitic steel wires
therefore belong to the group of endogenous or second phase inclusions, which have their
origin in alloying substances added to enhance the nucleation of perlite.
The critical size of the inclusion for the creation of microcracks has been well stud-
ied [25–27] but generally cannot be concluded at an exact size, as it depends on the type
of steel and the characteristics of the inclusion. The critical size of the inclusion has been
quantified for tool steels [26], steels AISI 4140 [5], ADF1 [27] (steel produced by Dalian
Iron and the CO group based on the chemical composition of commercial steel 42CrMo
decreasing the content of O, N, P, S and H, and increasing the content of V and Ti) and
for steels used as mechanical springs [28,29]. As for its size, the measurements of the
inclusions studied in pearlitic steel yield a size distribution between 1 and 10 µm. Most
of the inclusions analyzed in this study are mainly made up of oxides and sulfides. The
variation in the elements in the compounds will regulate the characteristics and mechanical
behavior of the inclusion [5,26,27].
This article studies the presence and evolution of inclusions throughout a real process
of drawing consisting of seven stages, analyzing five of them: the initial one (hot-rolled steel,
without drawing), the final one (commercial wire drawing steel) and three intermediate
stages of the manufacturing chain. Special attention is paid to the generation and evolution
of the microcracks that occurs around the inclusions when the pearlitic steel wire goes
through the different dies of drawing; such microcracks can have an influence on the
steel’s behavior in fracture [7]. The study of the inclusions was carried out from the
characterization of the same through the analysis of their chemical composition, as well as
a quantitative morphological study.
2. Materials and Methods
In the following study, an eutectoid pearlitic steel belonging to a real wire drawing
chain, formed by seven steps of cold drawing, was used. It was analyzed from the initial
hot rolled bar (wire which has not been subjected to any step of the drawing process) to the
final commercial prestressing wire. The nomenclature used to identify the analyzed wires
consists of a letter, which indicates the family of origin of the wire and a representative
number of the drawing step to which the wire belongs, 0 being the number for the initial
wire rod, 7 for the final product, and numbers from 1 to 6 to identify the wires corresponding
to the intermediate steps. The chemical composition of steel family E is shown in Table 1.
Table 1. Chemical composition (wt.%) of steels of family E (the balance is Fe).
C Mn Si P S Al Cr V
0.79 0.68 0.21 0.01 0.01 0.003 0.22 0.06
For the study of the evolution of the inclusions in progressively drawn pearlitic
steel, the following wires were used: initial hot rolled bar and the wires from the second,
fourth, sixth and seventh steps (E0, E2, E4, E6 and E7, respectively) of the cold drawing
process. This family of steels has been characterized in previous studies by the Fracture and
Structural Integrity Research Group (FSIRG) of the University of Salamanca (USAL) [30],
extracting from these the following data indicated in Table 2: the corresponding diameter
(D), the plastic deformation [7] that acquires the material at the exit of each corresponding
die/stage of the cold drawing process (εP cum ), the Young’s modulus (E), the yield stress
(σY ) and the ultimate tensile stress (σR ) for each of the wires under study. Figure 1 shows
the true stress vs. true deformation curves (σ−ε) obtained in these studies for all the wires
analyzed in the present work.
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13
Table 2. Dimensions and mechanical properties of the steels analyzed.
Table 2. Dimensions and mechanical properties of the steels analyzed.
Table 2. Dimensions and mechanical properties of the steels analyzed.
Steel D (mm)
εPεcum E(GPa)
σσY Y(GPa)
(GPa)
σσR(GPa)
R(GPa)
P cum
Steel D (mm) P E(GPa)
Steel
E0 D (mm)
11.03 0.00ε cum 199 E (GPa) σ Y (GPa)
0.72 (GPa)
σ R1.23
E0 11.03 0.00 199 0.72 1.23
E2
E0 8.95
11.03 0.42 0.00 194 199 0.910.72 1.36
1.23
E2 8.95 0.42 194 0.91 1.36
E2
E4 8.95
7.49 0.78 0.42 196 194 1.020.91 1.36
1.50
E4E4 7.497.49 0.78 0.78 196 196 1.02 1.02 1.50
1.50
E6 6.26 1.13 200 1.16 1.62
E6E6 6.266.26 1.13 1.13 200 200 1.16 1.16 1.62
1.62
E7 5.04 1.57 208 208 1.49 1.83
E7E7 5.045.04 1.57 1.57 208 1.49 1.49 1.83
1.83
2.0
2.0
1.6
1.6
(GPa)
1.2
σσ(GPa)
1.2
E7
E7
0.8
0.8 E6
E6
E4
E4
0.4
0.4 E2
E2
E0
E0
0.0
0.00.00 0.02
0.00 0.02 0.04
0.04 0.06
0.06 0.08
0.08 0.10
0.10
εε
Figure1.1.True
Figure Truestress
stressvs.
vs.true
truedeformation
deformation(σ(σ−ε)
−ε)ofofsteels
of steelsfrom
fromfamily
familyE.
E.
Figure 1. True stress vs. true deformation (σ−ε) steels from family E.
Thecollection
The collectionofofthe
thesamples
sampleswas wascarried
carried outout with
with thethe previous
previous realization
realization of longi-
of longitu-
The collection of the samples was carried out with the previous realization of longi-
tudinal
dinal cutscuts in the
in the wires
wires belonging
belonging to the
to the different
different stagesstages of drawing
of the the drawing
chainchain selected,
selected, as
tudinal cuts in the wires belonging to the different stages of the drawing chain selected,
indicated in the
as indicated following
in the followingFigure 2a.2a.
Figure This is so
This that
is so the
that micrographs
the micrographsofofthe
thepresent
presentwork
work
as indicated in the following Figure 2a. This is so that the micrographs of the present work
will
willalways
alwaysbe beoriented
orientedwith
withtheir
theirvertical
verticalside
sidefollowing
followingthe thecold
colddrawing
drawingdirection
direction(or
(or
will always be oriented with their vertical side following the cold drawing direction (or
the
thelongitudinal
longitudinalwirewireaxis).
axis). The
Thesections
sectionsobtained
obtainedfrom fromthethelongitudinal cuts(≈
longitudinalcuts (≈15
15mmmm
the longitudinal wire axis). The sections obtained from the longitudinal cuts (≈ 15 mm
long)
long)were
wererigorously
rigorouslyprepared
preparedtotoachieve
achievethethecorrect
correctmaterialographic
materialographicobservation
observationininthe
the
long) were rigorously prepared to achieve the correct materialographic observation in the
scanning
scanningelectron
electronmicroscope
microscope(SEM).
(SEM).InInorder
orderto tocarry
carryoutoutthe
themicrographs
micrographsfor forthe
thestudy,
study,
scanning electron microscope (SEM). In order to carry out the micrographs for the study,
these
thesesections
sectionswere
wereembedded
embeddedininaaphenolic
phenolicresin,
resin,thermally
thermallyhardened
hardenedby bymeans
meansofofaahothot
these sections were embedded in a phenolic resin, thermally hardened by means of a hot
compression
compressionassembly,
assembly,asasshown
shownininFigure
Figure2b.2b.
compression assembly, as shown in Figure 2b.
(a) (b)
(a) (b)
Figure2.2.Preparation
Figure Preparationofofpearlitic
pearliticsteel
steelsamples:
samples:(a)
(a)longitudinal
longitudinalcutting,
cutting,(b)
(b)mounted
mounted in resin.
Figure 2. Preparation of pearlitic steel samples: (a) longitudinal cutting, (b) mounted in resin.
Thenext
The nextstepstepisisthe
theexecution
executionofofthethemechanical
mechanicalpreparation,
preparation,grinding
grindingand andpolishing
polishing
The next step is the execution of the mechanical preparation, grinding and polishing
ofthe
of thesamples.
samples.This This step
step aims
aims to to achieve
achieve a a specular
specular surface
surface on on which
which a a chemical
chemical attackattack
can
of the samples. This step aims to achieve a specular surface on which a chemical attack
can be performed
be performed that
thatthat reveals
reveals the thethe microstructure
microstructure of
of the the corresponding
corresponding steel.steel.
With With grind-
grinding,
can be performed reveals microstructure of the corresponding steel. With grind-
ing, which
which is the isfirst
thestep
firstofstep
the ofprocess,
the process,
a flat asurface
flat surface
is is achieved
achieved (as (asasflat
flat as possible);
possible); for
for this
ing, which is the first step of the process, a flat surface is achieved (as flat as possible); for
this purpose,
purpose, a turning
a turning speed speed
of 300 of 300wasrpm was using
used, water
using water as lubricant, applying a
this purpose, a turning speed ofrpm
300 rpm used,
was used, using as lubricant,
water applying
as lubricant, a force
applying a
Metals 2021, 11, x FOR PEER REVIEW 4 of 13
Metals 2021, 11, 1272 4 of 13
force of 30 N for 30 s. For the next phase of polishing, three different polishing cloths were
of 30 with
used, N for9,303 s.and
For1the
μmnext phase powder
diamond of polishing, three different
and applying forcespolishing
of 30 N cloths
for thewere
first used,
two
cloths and 20 N for the third one. For all cases, the speed used was 150 rpm and thecloths
with 9, 3 and 1 µm diamond powder and applying forces of 30 N for the first two time
anda20
was N for the
function third
of the one.obtained
results For all cases,
in eachtheone.
speed
Afterused
thewas 150 rpmpreparation
mechanical and the time of was
the
a function of the results obtained in each one. After the mechanical preparation
samples, a chemical attack with nitric acid (HNO3) to 3% of solution in pure commercial of the
samples,
ethanol a chemical
was performed. attack with
Once thenitric acidwere
samples (HNO 3 ) to 3% of
obtained, solution
these were in pure commercial
analysed with the
scanning electron microscope (SEM) using a JEOL JSM-5610 LV (Jeol Ltd., Tokyo,with
ethanol was performed. Once the samples were obtained, these were analysed the
Japan)
scanning electron microscope (SEM) using a JEOL JSM-5610 LV (Jeol Ltd.,
in which a multitude of inclusions were visualized in this process; micrographs of said Tokyo, Japan)
in which awere
inclusions multitude
made to ofvarious
inclusions were
scales for visualized
further study.in this process; micrographs of said
inclusions were made to various scales for further study.
3. Results
3. Results
The inclusions examined in this study were analyzed using an X-ray dispersion en-
The inclusions examined in this study were analyzed using an X-ray dispersion energy
ergy
analysis unitunit
analysis (EDX,(EDX,OxfordOxford Instruments,
Instruments, mod. mod. 6587,
6587, High High Wycombe,
Wycombe, UK)UK) coupled
coupled to
to the
the
SEM.SEM. TheThe analysis
analysis by EDX
by EDX in the inSEM
the SEM doesdoes not determine
not determine exactly exactly the statistical
the statistical com-
composition
position of the compound that forms the inclusions; however,
of the compound that forms the inclusions; however, it identifies the elements that compose it identifies the elements
that
them,compose them, and
and provides provides an information
an approximate approximateofinformation
the percentage of the percentage
of each element ofthat
eachis
element that is present
present in the analyzed inclusion.in the analyzed inclusion.
Among
Amongthese theseinclusions,
inclusions,secondsecondphase phaseororendogenous
endogenousparticles
particleswere werefoundfoundsuchsuchas as
MnS
MnS(matte
(matte appearance,
appearance, with with irregular
irregularshapes),
shapes),SiO SiO
2
2 (oxides) and Al2O3 (bright appear-
(oxides) and Al O
2 3 (bright appearance
ance
and andwithwith moremoreregularregular shapes).
shapes). The analysis
The analysis by SEMby SEM detected
detected mainly mainly
thesethese
three three
types
types
of particles, although others were also found in a smaller proportion and areare
of particles, although others were also found in a smaller proportion and usedusedfor
for
thethe formation
formation ofof newphases
new phasesduring
duringthe thesolidification
solidificationof of steel
steel asas nucleating
nucleating agents.
agents. The The
compounds
compoundsfound foundand andanalyzed
analyzedwere weremainly
mainlythe thefollowing:
following:Ti Ti22OO3,3 Mn
, Mnsilicates
silicates(MnOSiO
(MnOSiO2,2 ,
MnSiO
MnSiO 3), ),
3 aluminum
aluminum silicates
silicates (SiO 2, AlO
(SiO 2 , AlO3), TiN, V and
3 ), TiN, V andCa. Ca.
As a As summary,
a summary,it canitbecan con-be
cluded
concluded that that
the inclusions
the inclusions in the steels
in the steels examined
examined arearemainly
mainly divided
dividedinto intothree
threedistinct
distinct
groups
groupsbasedbasedon ontheir
theirchemical
chemicalcomposition:
composition:sulfides, sulfides,oxides
oxidesandandsilicates.
silicates.
Once the chemical
Once the chemical analysis analysis of the inclusions was carried out, ititwas
inclusions was carried out, wasdecided
decidedtoto clas-
classify
sify them according to the response to deformation during
them according to the response to deformation during the drawing process [6], classifying the drawing process [6], clas-
sifying
them into them intosoft
hard, hard,
and soft and mix.
mix.
InInthe
thefirst
first group
group identified —hard-typeinclusions—oxides
identified—hard-type inclusions— oxidescan canbebe found
found mainly;
mainly; be-
because
cause ofoftheir theirnature
nature these
these areare usually
usually of of
highhigh hardness
hardness and and brittleness;
brittleness; suchsuch character-
characteristics
prevent
istics them them
prevent from fully
fromadapting to the surrounding
fully adapting pearlitic matrix
to the surrounding pearlitic during
matrix theduring
continuous the
process of drawing, creating a small discontinuity in the inclusion-perlite
continuous process of drawing, creating a small discontinuity in the inclusion-perlite in- interface (micro-
cavity).(microcavity).
terface As an example, As consider
an example, Figure 3, which
consider shows
Figure 3, awhich
complex shows oxide embedded
a complex in a
oxide
pearlitic matrix
embedded in a corresponding
pearlitic matrixtocorresponding
an E7 steel, commercial to an E7cold drawn
steel, pearlitic cold
commercial steel that
drawn has
passed through
pearlitic steel thatseven dies of through
has passed wire drawing. seven During the cold
dies of wire drawing,
drawing. the pearlitic
During the coldcolonies
draw-
compress
ing, the oxide,
the pearlitic colonieswithout
compressbreaking it, creating
the oxide, without a small
breakinginclusion-matrix
it, creating a small microcavity
inclu-
during the plastic forming process.
sion-matrix microcavity during the plastic forming process.
Figure
Figure3.3.Inclusion
Inclusionofof
hard-type, with
hard-type, complex
with oxide
complex composition,
oxide present
composition, in the
present commercial
in the pre-pre-
commercial
stressed wire E7.
stressed wire E7.
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Inthe
In
In thesecond
the secondgroup
second groupof
group ofinclusions
of inclusionsidentified,
inclusions identified,are
identified, are those
are thoseformed
those formed mainly
formed mainly by
mainly by manganese
by manganese
manganese
sulfide.
sulfide. Manganese sulfides are capable of withstanding greater
sulfide. Manganese sulfides are capable of withstanding greater plastic deformation than
Manganese sulfides are capable of withstanding greater plastic
plastic deformation
deformation than
than
inclusions
inclusions composed
inclusions composed
composed of of oxides;
of oxides; however,
oxides; however,
however, in in many
in many cases
many cases they
cases they break
they break
break as as a consequence
as aa consequence
consequence of of
of
the
the cold
thecold drawing
colddrawing process,
drawingprocess,
process,and and it
andititis is possible
ispossible
possibleto to distinguish
todistinguish
distinguishthe the
thepartsparts
partsin in
inwhichwhich
whichthe the embed-
theembedded
embedded
ded inclusion
inclusion
inclusion has has fragmented.
has fragmented.
fragmented. Figure
Figure Figure
44 shows4 shows
shows a soft-type
aa softtype
softtype inclusion
inclusion
inclusion consistingconsisting
consisting mainly
mainly mainly
of
of man-of
man-
manganese
ganese
ganese sulfide.
sulfide.
sulfide. The The inclusion
The inclusion
inclusion and itsand
and its its pearlitic
pearlitic
pearlitic matrix matrix
matrix have
have gone have
gone gone through
through
through two diestwo
two dies of dies
of wire
wire
of wire
drawing drawing
(E2 (E2
steel). It steel).
can beIt can be
observed observed
that the that the
inclusion inclusion
adapts adapts
quite
drawing (E2 steel). It can be observed that the inclusion adapts quite well to the defor- wellquite
to well
the to the
defor-
deformation
mation
mation acquired acquired
acquired by
by its by its
its matrix matrix
matrix during during
during thethe cold the cold
cold drawing drawing
drawing and and that and that
that itit has it has
has fragmented fragmented
fragmented duringduring
during the aforementioned
the process.
the aforementioned
aforementioned process.
process.
Figure
Figure4.
Figure 4.Inclusion
4. Inclusionof
Inclusion ofsoft-type,
of soft-type,with
soft-type, withmain
with maincomposition
main compositionof
composition ofofmanganese
manganese
manganesesulfide,
sulfide, belonging
belonging
sulfide, to
belonging the
totothe E2
E2E2
the
steel
steel(second
steel (seconddrawing
(second drawingstep).
drawing step).
step).
ItIt
Itshould
shouldbe
should benoted
be notedthat
noted that in in the
the study
study carried
carried out,
out, aa particular
particular typetype ofof inclusions
inclusions werewere
analyzed,
analyzed, which
analyzed, whichare are formed
areformed simultaneously
formedsimultaneously
simultaneously bybythethe
by twotwo
the main
two main
types
main types of
of constituents
of constituents
types dis-
discussed
constituents dis-
cussed
above: above:
cussed oxides oxides
above: and
oxides and
and manganese
manganese sulfides.sulfides.
manganese These
These inclusions
These inclusions
sulfides. were called
inclusions were
were called
called mix-type
mix-type (see Figure
mix-type (see5)
(see
Figure
and in 5) and
them itin
is them
possible it is
to possible to
differentiate differentiate
an inner an
part inner
formed
Figure 5) and in them it is possible to differentiate an inner part formed by a complex part
by a formed
complex by a
oxide complex
and an
oxide
outer and
oxide part,an
and an outer
that
outer part,
part, that
surrounds surrounds
this
that first one, this
surrounds first
first one,
composed
this composed
mainly
one, mainly
mainly by
by manganese
composed by manganese
sulfides.
manganeseThe
sulfides.
mechanical
sulfides. The mechanical
Thebehavior
mechanical that behavior that
that these
these inclusions
behavior inclusions
theseexhibit, exhibit,
during
inclusions the during
exhibit, cold
during the
the cold
drawing cold drawing
process,
drawing is
similar to
process,
process, isthat
is similarof the
similar to soft-type
to that
that of theinclusions,
of the soft-type because the
soft-type inclusions,
inclusions, outer part
because
because the of
the outer
outer mix-type
thepart
part of theinclusions
of the mix-type
mix-type
(that in direct
inclusions
inclusions (thatcontact
(that in
in direct
directwith the surrounding
contact
contact with
with the pearlitic matrix)
the surrounding
surrounding pearlitic
pearlitic soft one
is matrix)
the
matrix) is
is the(oxides
the soft on(ox-
soft one
one the
(ox-
inside,
ides on sulfides
the on
inside, the outside).
sulfides on
ides on the inside, sulfides on the outside). the outside).
Figure
Figure5.
Figure 5.Inclusion
5. Inclusionof
Inclusion ofmix-type
of mix-typeclass,
mix-type class,with
class, withcomplex
with complex oxide
oxide composition
composition inin the
the inner
inner central
central zone
zone and
and
and manganese
manganese
manganese sulfide
sulfide
sulfide in
in the the
the outer
inouterouter zone,
zone,zone, present
present
present in
in the the
the initial
ininitial wirewire
initial wire rod
rod E0.
rod E0. E0.
The
Theclassification
The classificationof
classification of inclusions
inclusions inin hard,
hard, soft
soft and
and mix
mix obey
obeyto
obey totheir
to theirmechanical
their mechanicalbehavior
mechanical behavior
behavior
during
duringthe
during thedrawing
the drawingprocess,
drawing process,aa process
process, process thanks
thanks to to which
which the the ferrite
ferrite and
and cementite
cementite lamellas
lamellas
that
thatmake
that makeup
make up the
the perlite
perlite colonies
colonies are
are re-oriented
re-oriented andand stretched
stretched [21].
[21]. The
The microstructural
microstructural
reorganization
reorganizationof
reorganization ofthe
of thedrawing
the drawingprocess
drawing processproduces,
produces,in in the
the soft-type
soft-type inclusions,
inclusions, large
large strains
strains
along
along the
along the longitudinal
the longitudinal wire
longitudinalwire axis
wireaxis (direction
axis(direction of the
(directionofofthe drawing
the drawing
drawing process); making
process);
process); it
making
making able to
to in-
it able
it able to
in-
crease
increase
crease its slenderness
itsits slenderness
slenderness by
bybyan order
anan orderof
order ofofmagnitude.
magnitude.In
magnitude. InInthe
thehard-type
the hard-typeinclusions,
hard-type inclusions,no
inclusions, novariation
no variation
variation
in
in their
their morphology
morphology and andsize
size was
was observed;
observed; however,
however,microflaws
microflaws or or microcavities
microcavities around
aroundMetals 2021, 11, 1272 6 of 13
Metals 2021, 11, x FOR PEER REVIEW 6 of 13
Metals 2021, 11, x FOR PEER REVIEW 6 of 13
in their morphology and size was observed; however, microflaws or microcavities around
them
themwerewereobserved
observedininthe
thewires
wiresbelonging
belongingtotothe
thelast
laststeps
stepsofofthe
thedrawing
drawingprocess
process(see
(see
Figure
Figure 4)
them were [7]. To
4) [7]. quantify
observed the present
in thethe
To quantify wires percentage
belonging
present of the
to theof
percentage different
last
thesteps types of inclusions
of thetypes
different drawing in steel,
process
of inclusions (see
in
measurements
Figure
steel, 4) [7]. were
measurements made
wereofmade
To quantify a sample
the presentcomposed
percentage
of a sample of 840
composed ofofthem,
of the 840 ofextracting
different types
them, theinclusions
of following
extracting the fol-in
results
steel, (Figure
lowing results6).
measurements
(Figurewere
6). made of a sample composed of 840 of them, extracting the fol-
lowing results (Figure 6).
Figure 6. Proportional quantitative results of the different types of inclusions present in steel E.
Figure 6. Proportional quantitative results of the different types of inclusions present in steel E.
Figure 6. Proportional quantitative results of the different types of inclusions present in steel E.
Oncethe
Once themicrographs
micrographswere wereobtained
obtainedand andselected
selectedaccording
accordingtotothe thetype
typeofofinclusion,
inclusion,
they Once
were the micrographs
measured using were
the obtained
image and
analysis selected
program according
AnaliSYS.
they were measured using the image analysis program AnaliSYS. 3.1 . In order to to3.1
®the type
®. In of
order inclusion,
tostudy
study
they
the were
thevariation measured
variationininthe theshapeusing the
shapefactor image
factorofofthe analysis
theinclusionsprogram
inclusionsthroughoutAnaliSYS.
throughoutthe 3.1
thedrawing®. In order to study
drawingprocess,
process,the the
the variation
morphology
morphology ofofin
the the
the shape factor
inclusion
inclusion has of the
hasbeen
been inclusions throughout
approximated
approximated totoananellipse.the
ellipse.InIndrawing
thisway,
this way,process,
thethemajor the
major
morphology
axis
axis (2a), of the
(2a),parallel
parallel to inclusion
to the drawing
the hasdirection,
been approximated
direction, represented
represented toin
an ellipse.
inFigure
Figure7,7, In
wasthismeasured
was way,
measuredthe in
major
the
in
axis (2a), parallel to the drawing direction, represented in Figure 7, was measured in the
inclusions.
the inclusions.
inclusions.
Figure 7. Approximation of the inclusion and its microcavity to an ellipse.
Figure Approximation
7. 7.
Figure ofof
Approximation the inclusion
the and
inclusion itsits
and microcavity to to
microcavity anan
ellipse.
ellipse.
For the measurement of this ellipse axis in the different micrographs it proceeded to
For
Forthethemeasurement
measurement ofofthis ellipse
thisplusellipse axis in in the
the different micrographs itit proceeded
proceeded
measure inclusion’s surface theaxis
microcavity, different micrographs
or microdamage, generated by to it
tomeasure
measurethe the inclusion’s
inclusion’s surface surface plus plus the
the microcavity,
microcavity, or
or microdamage,
microdamage, generated
generated bybyit it
during the cold drawing process. With this surface and the measurement of the major axis
during the cold drawing process. With this surface and the measurement of the major axis
during
of the cold
the ellipse, 2a,drawing
the minor process.
axis is With
obtained this surface
using the and the measurement
following equation: of the major axis
of the ellipse, 2a, the minor axis is obtained using the following equation:
of the ellipse, 2a, the minor axis is obtained using the following equation:
S= π⋅a⋅b (1)
S=π ·a·π⋅a⋅b
S= b (1)(1)
where a and b are the semi-axis of the ellipse and S is the inclusion surface or, in the case
where
of
where a and
non-matching,
a and b are
b are thethe
the semi-axis
surface
semi-axis of
ofofthe
thethe ellipse
inclusion
ellipse andand SSthe
is is
thethe inclusion
microdamage
inclusion surfaceor,or,
generated
surface inin the
around
the case
case it.
of
ofThe non-matching,
study of thisthe
non-matching, the
work surface
surfacefocuses of the
on the
of the inclusion
surfaceand
inclusion and
of the the microdamage
the microdefect
microdamage generated
generated
generated around
or around the it.it.
TheThe study
inclusion
study of of
duethisthis
towork work
the drawing focuses
focuses onon
process.
thethe surface
Once
surface ofof
this the
data
the microdefect
is obtained,
microdefect generated
it oror
is possible
generated around
to
aroundcalculate
thethe
inclusion
the shape
inclusion due duetoto
factor of
the thethedrawing
drawinginclusions process.
process.(a/b), Once
making
Once this
this the
datadatais is
mean obtained, it
is is
and theitstandard
obtained, possible toto
deviation
possible calculate
of the
calculate
thethe shape
different
shape factor
shape
factor of
offactors
the the inclusions
obtained(a/b),
inclusions in(a/b), makingthe
themaking
different the mean
steps
mean ofandandthe
the thestandard
standard
plastic deviation
conformation
deviation ofof
process.
thethe
different
differentForshapeshape
the inclusionsfactors
factors obtained obtained
in which in
inthere the different steps
was a decohesion
the different of the
steps of theofplastic plastic
the pearliticconformation
matrix due
conformation process.
to the
process.
process Forthe
For ofthe inclusions
inclusions
drawing, inin
the which
which
surface there
there waswas
of microcavity a decohesion
a decohesion
generated ofof thethe pearlitic
pearlitic
was measured matrix
matrix(S),duedue toto
which thethe
in-
process
process
cludes of
theof drawing,
drawing,
inclusion the
theitself. surface
surface In of ofinclusions
microcavity
microcavity
those generatedgenerated
in which was was measured
themeasured (S),
(S), which
perlite–inclusion which re-
includes
interface in-
thecludes
mains the inclusion
inclusion
united itself.
(without Initself.
those In those inclusions
inclusions
detachment fromin which inthe
which
the pearlitic the perlite–inclusion
perlite–inclusion
matrix during interface
drawing), interface
remains
only the re-
mains(without
united
surface united
of the proper(without
detachment detachment
inclusion fromwas from thematrix
the measured,
pearlitic pearlitic
sinceduring matrix during
drawing),
it coincides with the drawing),
only of only
the surface
value the
of
the sur-
surface of the proper inclusion was measured, since it coincides
face of the resulting microdamage itself. The shape factor average values for the different with the value of the sur-
face of the resulting microdamage itself. The shape factor average values for the differentable to compare these measures, a statistical treatment of the data obtained was carried
Metals 2021, 11, x FOR PEER REVIEW 7 of 13
out, the standard deviation, not being greater than 2 in any case.
In the micrographs belonging to the initial hot rolled bar steel E0, it is possible to
observe inclusions with deformations prior to drawing that can be attributed to the pro-
inclusions
cess (hard, soft
of hot rolling [14]and mix) steel
original are shown in the following
itself. Figure 8 shows anFigures 8 toof10.
evolution theInshape
orderfactor
to be
Metals 2021, 11, 1272 able to compare these measures, a statistical treatment of the data obtained was carried7 of 13
of the hard-type inclusions and the microdamage that they promote. The hard-type inclu-
out, the
sions, standard
being moredeviation, not being
resistant than greater matrix
the pearlitic than 2 in any
that case.
surrounds them, increase their
In the micrographs belonging to the initial hot
slenderness a bit during cold drawing due to the generation of rolled barmicroflaws
steel E0, itoriented
is possible to
in the
observe
direction inclusions
of inclusionwith deformations
the longitudinal prior
wire axis (cold to drawing that can be attributed to the pro-
the proper was measured, sincedrawing direction).
it coincides with the value of the surface of the
cess of hot rolling [14] original steel itself. Figure 8 shows an evolution of the shape factor
resulting
of the hard-type inclusions and the microdamage that theyvalues
microdamage itself. The shape factor average for The
promote. the different
hard-type inclusions
inclu-
(hard, soft E0
mix) E2shownE4 in theE6 E7
10being more resistant than the pearlitic matrix that surrounds them,beincrease
sions, and are following Figures 8–10. In order to able to their
compare
these measures, a statistical treatment of the data obtained was carried
slenderness a bit during cold drawing due to the generation of microflaws oriented in the out, the standard
deviation,
8 ofnot
direction thebeing greaterwire
longitudinal thanaxis
2 in(cold
any case.
drawing direction).
6 E0 E2 E4 E6 E7
10
a/b
4
8
2
6
a/b
0
-0.5
4 0 0.5 1 1.5 2
P
ε CUM
2
Figure 8. Shape factor evolution for hard-type inclusions.
0
-0.5soft-type
The 0 inclusions
0.5 and1the microcavities
1.5 2 generated by them behave in the same
P
ε
way as the hard ones, increasing CUM their slenderness throughout the process; however, this
increase is more accentuated in the soft ones. Figure 9 shows much greater increases in the
Figure
shape 8. Shape
8. Shape
Figure factor factor
factor
than evolution
evolution
in the for
for hard-type
hard-type
hard inclusions. inclusions.
inclusions.
The soft-type inclusions and the microcavities generated by them behave in the same
E0 E2 E4 E6 E7
way15as the hard ones, increasing their slenderness throughout the process; however, this
increase is more accentuated in the soft ones. Figure 9 shows much greater increases in the
shape factor than in the hard inclusions.
10
E0 E2 E4 E6 E7
15
a/b
5
10
a/b
Metals 2021, 11, x FOR PEER REVIEW 8 of 13
0
-0.5 0 0.5 1 1.5 2
5 P
ε CUM
result which could be expected since the inclusion in contact with the pearlitic matrix is a
Figure
Figure9.9.Shape
soft-type Shape
of factor
factorevolution
inclusion. evolutionforfor
soft-type inclusions.
soft-type inclusions.
0
-0.5
For the case0 of mix-type
0.5 1
inclusions 1.5 2 a similar behavior to the two types of
(Figure 10)
E0 E2 εPE4 E6 E7
20
inclusions above mentioned CUM is observed, that is: a progressive increase in their slenderness
due to the drawing process. This increase is very similar to that of soft-type inclusions, a
Figure 9. Shape factor evolution for soft-type inclusions.
15
For the case of mix-type inclusions (Figure 10) a similar behavior to the two types of
a/b
inclusions above mentioned is observed, that is: a progressive increase in their slenderness
10
due to the drawing process. This increase is very similar to that of soft-type inclusions, a
5
0
-0.5 0 0.5 1 1.5 2
P
ε CUM
Figure10.
Figure Shapefactor
10.Shape factorevolution
evolutionforfor mix-type
mix-type inclusions.
inclusions.
4. Discussion
The distribution of the inclusions is a parameter that is of vital importance in order
to understand the mechanical and fracture behavior of steels. The following diagrams ofMetals 2021, 11, 1272 8 of 13
In the micrographs belonging to the initial hot rolled bar steel E0, it is possible
to observe inclusions with deformations prior to drawing that can be attributed to the
process of hot rolling [14] original steel itself. Figure 8 shows an evolution of the shape
factor of the hard-type inclusions and the microdamage that they promote. The hard-type
inclusions, being more resistant than the pearlitic matrix that surrounds them, increase
their slenderness a bit during cold drawing due to the generation of microflaws oriented in
the direction of the longitudinal wire axis (cold drawing direction).
The soft-type inclusions and the microcavities generated by them behave in the same
way as the hard ones, increasing their slenderness throughout the process; however, this
increase is more accentuated in the soft ones. Figure 9 shows much greater increases in the
shape factor than in the hard inclusions.
For the case of mix-type inclusions (Figure 10) a similar behavior to the two types of
inclusions above mentioned is observed, that is: a progressive increase in their slenderness
due to the drawing process. This increase is very similar to that of soft-type inclusions, a
result which could be expected since the inclusion in contact with the pearlitic matrix is a
soft-type of inclusion.
4. Discussion
The distribution of the inclusions is a parameter that is of vital importance in order
to understand the mechanical and fracture behavior of steels. The following diagrams of
Figures 11–13 show the evolution of the microdamage generated by the inclusions during
the drawing process. It can be observed, regardless of the proper nature of the inclusions
(hard, soft and mix), how microflaws or microcracks are generated that will be, or not,
filled later by the pearlitic matrix surrounding the passage through to several drawing
dies. It can be observed how, as the process of drawing progresses, the surrounding
pearlitic matrix detaches from the inclusion, generating a kind of microflaw around the
considered inclusion; these microcavities are oriented in the direction of the drawing
process. Consequently, it can be concluded that they are a consequence of the drawing
process. The major axis average values (2a, Figure 7) of the measured ellipse can be
considered as a quantitative measure of the generated microdamage during cold drawing
and due to the presence of the inclusions, i.e., a microcrack oriented in the longitudinal
wire axis with a length 2a. The average value is lower (2a = 6.17 µm) for the initial hot rolled
steel (E0, not cold drawn at all), and greater (2a = 17.67 µm) for those steels corresponding
to the last steps of the cold drawing process.
The hard inclusions (Figure 11) are not able to undergo the same plastic deformation
as the pearlitic matrix that surrounds them during the drawing process. The difference in
mechanical behavior between the pearlitic matrix and the inclusion will cause a decohesion
of the pearlitic matrix, generating a microflaw, also called microdamage, around it [6,7]. As
the drawing process goes ahead, these microdefects tend to deform in the direction of the
longitudinal axis of the wire. However, the constraint suffered by the neighboring pearlitic
colonies causes the closure of the afore-said colonies. The microdamage generated cause a
discontinuity in the material that can subsequently influence the fracture behavior of the
steel [7], because they act in a similar way to stress concentrators. Oxides, carbides and
nitrides are part of the inclusions that shown this behavior during the cold drawing process.
On the other hand, there are soft-type inclusions (Figure 12) which are able to deform
plastically in a similar way to the pearlitic matrix that surrounds them during cold drawing;
these evolve along all the stretching dies, deforming a lot according to the longitudinal
axis of the steel wire (drawing direction). This type of inclusions, once deformed, tend to
fragment, generating cavities that will simultaneously be filled by the compression of the
surrounding pearlitic matrix when passing through successive drawing dies. Manganese
sulfides are one of the most representative compounds of this type of inclusions. Most
inclusions will generate microdefects of greater entity the larger the size of that inclusion.
The microflaws generated by the soft-type inclusions are surrounded by a pearlitic matrixMetals 2021, 11, 1272 9 of 13
Metals 2021, 11, x FOR PEER REVIEW 9 of 13
with a large microdamage, that microdamage being of a smaller size than the one generated
by other inclusions.
(a)
(b)
(c)
Figure
Figure 11. 11. Micrographs
Micrographs and of
and outline outline of the evolution
the evolution of hard-type
of hard-type inclusions
inclusions during
during the the drawing
drawing
process belonging to the steels E2 (a), E4 (b)
process belonging to the steels E2(a), E4(b) and E7(c). and E7 (c).
On theMix-type inclusions
other hand, there areare made up
soft-type of two typologies
inclusions (Figure 12)of which
inclusions, oneto
are able softer and one
deform
plastically in a similar way to the pearlitic matrix that surrounds them during cold draw- and
harder. Both typologies, within the same inclusion, are clearly different by tonality
shape; the hard-type inclusion being darker and with a rounded morphology. A multitude
ing; these evolve along all the stretching dies, deforming a lot according to the longitudi-
of mix-type inclusions was observed in the steels studied. In the following diagram—and
nal axis of the steel wire (drawing direction). This type of inclusions, once deformed, tend
micrographs (Figure 13)—it is shown how the hardest inclusion appears in the central area
to fragment, generating cavities that will simultaneously be filled by the compression of
of the mix-type inclusion, while its periphery is formed by a softer inclusion (of lighter
the surrounding pearlitic matrix when passing through successive drawing dies. Manga-
tonality). The microcavity generated from this type of inclusion has a slenderness similar
nese sulfides are one of the most representative compounds of this type of inclusions.
to that generated by the soft-type inclusion. The width of this microdamage is greater than
Most inclusions
that createdwill
bygenerate
soft-typemicrodefects of greater
inclusion, since entity of
the inclusion thehard-type
larger the size ofin
existing that
thein-
core of
clusion.the
The microflaws generated by the soft-type inclusions are surrounded by a pearlitic
mix-type inclusions is barely deformed transversely throughout the drawing process.
matrix with a large microdamage, that microdamage being of a smaller size than the one
generated by other inclusions.Metals 2021, 11, 1272 10 of 13
The soft outer part of the mix-type inclusion is deformed during the first steps of the
drawing process in the direction of the conformation process itself (longitudinal axis of
the wire). As this process grows, the soft part of the inclusion can become fragmented
forming empty spaces between the different fragments created during drawing. Once
the soft part of the mix-type inclusion is fragmented, due to the constraint of the colonies
during drawing, this area can be filled with the surrounding pearlitic matrix thanks to
the constricting action that occurs during drawing. This filling can leave the fragments
of the inclusion isolated; this is shown in the micrographs as independent inclusions, but
Metals 2021, 11, x FOR PEER REVIEW within the same microcavity. As a final result of the process, mix-type inclusions
10 generate
of 13
discontinuities within the pearlitic matrix since the padding is not strictly complete at the
edges of the fragments.
(a)
(b)
(c)
FigureFigure 12. Micrographs
12. Micrographs and outline
and outline of the evolution
of the evolution of the inclusions
of the inclusions of the during
of the soft-type soft-typethe
during the
drawing process belonging to the steels E2 (a), E4 (b) and
drawing process belonging to the steels E2(a), E4(b) and E6 (c). E6 (c).
Mix-type inclusions are made up of two typologies of inclusions, one softer and one
harder. Both typologies, within the same inclusion, are clearly different by tonality and
shape; the hard-type inclusion being darker and with a rounded morphology. A multitude
of mix-type inclusions was observed in the steels studied. In the following diagram —and
micrographs (Figure 13)—it is shown how the hardest inclusion appears in the central
area of the mix-type inclusion, while its periphery is formed by a softer inclusion (of lighter
tonality). The microcavity generated from this type of inclusion has a slenderness similar
to that generated by the soft-type inclusion. The width of this microdamage is greater thanconstricting action that occurs during drawing. This filling can leave the fragments of the
inclusion isolated; this is shown in the micrographs as independent inclusions, but within
the same microcavity. As a final result of the process, mix-type inclusions generate dis-
Metals 2021, 11, 1272 continuities within the pearlitic matrix since the padding is not strictly complete at the11 of 13
edges of the fragments.
(a)
(b)
(c)
Figure
Figure 13. 13. Micrographs
Micrographs andofoutline
and outline of the evolution
the evolution of the inclusions
of the inclusions of the mix-type mix-type
of the during during the
the
drawingdrawing process belonging
process belonging to steelstoE2(a),
steelsE4(b)
E2 (a),
andE4 E6(c).
(b) and E6 (c).
5. Conclusions
5. Conclusions
In this article, a qualitative and quantitative analysis of the inclusions existing in a
In this article, a qualitative and quantitative analysis of the inclusions existing in a
pearlitic eutectoid steel was performed, paying special attention to the morphological
pearliticchanges
eutectoid steel
in the was performed,
microdefects paying
generated special
by the attention
inclusions to the
during the drawing
morphological
process that
changessuffers
in the the
microdefects generated by the
wire during its manufacturing. inclusions during the drawing process that
suffers the wire
The during its manufacturing.
inclusions present in the analyzed pearlitic steel show as main chemical composi-
Thetions:
inclusions
complex present inmanganese
oxides, the analyzed pearlitic
sulfides, steel show
carbides as main chemical compo-
and nitrides.
sitions: complex oxides, manganese sulfides, carbides and nitrides.
The classification of the inclusions was carried out, on the other hand, based on
Thetheir
classification of the inclusions
behavior against was carried
the surrounding out,matrix
pearlitic on theduring
other hand, based on
the drawing their This
process.
behavior classification consists of the following: soft-type inclusions (mainly MnS type),clas-
against the surrounding pearlitic matrix during the drawing process. This hard-type
sification consists (Fe
inclusions of complex
the following:
oxides)soft-type inclusions
and mix-type (mainly
inclusions (Mn MnS type),
sulfides andhard-type
iron oxides) in
which a hard-type inclusion is usually surrounded by a soft-type inclusion.
As for the percentages of distributions of the analyzed inclusions, it can be concluded
that there is a greater proportion of soft-type inclusions (68%), while the remaining percent-
age of inclusions found is distributed as follows: hard-type (15%) and mix-type (17%).Metals 2021, 11, 1272 12 of 13
The existing inclusions in the steel, as it passes through the various wire drawing
dies, generate a series of microflaws elongated in the longitudinal direction of the steel
wire. These microflaws or microdamage show an increase in their slenderness (parallel
to the direction of the plastic conformation process, or longitudinal axis of the wire) as a
consequence of the drawing process itself; such increase being more pronounced in the case
of soft-type inclusions. In the transverse direction to the steel wire, the material becomes
compacted by diametral compression of the wire when it passes through the different cold
drawing dies.
The volume of the microdamage generated around an inclusion during cold drawing
is a function, fundamentally, of the size of the inclusion from which it originates, of the
chemical composition of the inclusion and of the number of dies of drawing which has
exceeded the corresponding inclusion (and its wire).
The presence of inclusions within the microstructure of pearlitic steel, as well as the
microdamage that occur around them as an exclusive consequence of the drawing process,
can be key to understanding the mechanical response of the material and also its fracture
behavior.
Author Contributions: J.T. conceived and designed the research; F.-J.A. and R.R. performed the
microscopy analysis, J.T., F.-J.A. and R.R. analyzed the data and wrote the paper. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by the following Spanish Institutions: Ministry for Science
and Technology (MICYT; Grant MAT2002-01831), Ministry for Education and Science (MEC; Grant
BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for
Economy and Competitiveness (MINECO; Grant BIA2011-27870) and Junta de Castilla y León (JCyL;
Grants SA067A05, SA111A07, SA039A08 and SA132G18).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest. With regard to the research funds,
the different institutions providing financial support for the scientific research had no role in the
design of the study; in the collection, analyses, or interpretation of data; in the writing of the present
manuscript; and in the decision to publish the results.
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