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Tensile Fracture Behavior of Progressively-Drawn Pearlitic Steels Jesús Toribio *, Francisco-Javier Ayaso, Beatriz González, Juan-Carlos Matos, Diego Vergara and Miguel Lorenzo Fracture & Structural Integrity Research Group, University of Salamanca, E.P.S., Campus Viriato, Avda. Requejo 33, Zamora 49022, Spain; [email protected] (F.-J.A.); [email protected] (B.G.); [email protected] (J.-C.M.); [email protected] (D.V.); [email protected] (M.L.) * Correspondence: [email protected]; Tel.: +34-980-545-000 (ext. 3673); Fax: +34-980-545-002 Academic Editor: Isabel Gutierrez Received: 31 March 2016; Accepted: 10 May 2016; Published: 17 May 2016

Abstract: In this paper a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different cold-drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). To this end, samples of the different wires were tested up to fracture by means of standard tension tests, and later, all of the fracture surfaces were analyzed by scanning electron microscopy (SEM). Micro-fracture maps (MFMs) were assembled to characterize the different fractographic modes and to study their evolution with the level of cumulative plastic strain during cold drawing. Keywords: cold drawing; pearlitic steel; steel wires; mechanical properties; tensile fracture; fractographic analysis; anisotropic behavior

1. Introduction Pearlitic steels are widely used in a high variety of applications in engineering, e.g., pre-stressing steel wires [1,2], railway rails [3–5], rock bolts [6], steel cord wires (tire reinforcement) [7], music wires [8], etc. The wide use of these types of steels is due to their excellent mechanical properties induced in the raw material during the mechanical conforming process, e.g., hot rolling for complex shapes or wire drawing in the case of cylindrical bars. As a result of the plastic strains undergone by the material during the latter process, the yield stress is high enough [9] for considering it as a high strength steel. The today importance of these steels in industry can be quantified in terms of the world production of drawn wires of pearlitic steels which can be estimated in 25 million tons per year [10]. In addition, the interest of the scientific community on these research areas is also very high, as indicated by the number of research papers published up to date in multiple journals indexed in the SCOPUS database (Figure 1). Consequently, from a pure scientific point of view, increasing the knowledge about pearlitic steel wires is a quite interesting issue. In a common commercial wire drawing chain, wires undergo a progressive reduction of their cross-sectional area during several steps (usually 6–8 passes) [11]. As a consequence of material hardening due to huge plastic strains in the wire, key microstructural changes are caused in the steels as was reported in many studies [12–19]. The analyses of such changes revealed a progressive increment of the microstructural anisotropy as the drawing degree is increased. These microstructural changes also affect the macroscopic mechanical behavior of the material [20–22], as in their fatigue and fracture behavior [23–27]. This paper goes further in the research, so that a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different real cold drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). Metals 2016, 6, 114; doi:10.3390/met6050114

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Number of papers

chains, including each drawing step from the initial hot‐rolled bar (not cold‐drawn at all) to the final  commercial product (pre‐stressing steel wire).  8 10

3

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Pearlitic Steel

Cold Drawing

Wire Drawing

Pearlite

Keyword

 

Figure 1. Number of research papers indexed in Scopus related with the following keywords: pearlitic  Figure 1. Number of research papers indexed in Scopus related with the following keywords: pearlitic steel, cold drawing, wire drawing, pearlite. (Data collected on February 2016).  steel, cold drawing, wire drawing, pearlite. (Data collected on February 2016).

For this purpose, standard tension tests up to final fracture were carried out for each one of the  For this purpose, standard tension tests up to final fracture were carried out for each one of wires corresponding to each step of the five commercial wire drawing chains considered in this study.  the wires corresponding to each step of the five commercial wire drawing chains considered in this On one hand, results of testing allow one to obtain the mechanical properties—material yield strength  study. On one hand, results of testing allow one to obtain the mechanical properties—material yield (σY)  and  ultimate  tensile  strength  (UTS,  σR)—of  each  one  of  the  wires  tested  from  the  material  strength (σY ) and ultimate tensile strength (UTS, σR )—of each one of the wires tested from the material constitutive law (stress‐strain curve). This mechanical characterization presents a high interest not  constitutive law (stress-strain curve). This mechanical characterization presents a high interest not only only from the scientific point of view, but also from the educational viewpoint [28]. On the other hand,  from the scientific point of view, but also from the educational viewpoint [28]. On the other hand, the the  quantitative  fractographic  analysis  (post  mortem)  of  the  fracture  surfaces  of  tested  samples,  quantitative fractographic analysis (post mortem) of the fracture surfaces of tested samples, obtained obtained by scanning electron microscopy (SEM), allows one to reveal the microscopic fracture maps  by scanning electron microscopy (SEM), allows one to reveal the microscopic fracture maps (MFMs) (MFMs) and the fracture micro‐mechanisms. From these analyses, the evolution of both mechanical  and the fracture micro-mechanisms. From these analyses, the evolution of both mechanical properties properties and fracture behavior with drawing will be revealed for different wire drawing processes,  and fracture behavior with drawing will be revealed for different wire drawing processes, thereby thereby achieving a better understanding of the mechanical behavior of these key types of steels.  achieving a better understanding of the mechanical behavior of these key types of steels. 2. Experimental Program  2. Experimental Program Cylindrical samples of the wires obtained at the end of each drawing step of the five commercial  Cylindrical samples of the wires obtained at the end of each drawing step of the five commercial manufacturing processes were used for testing. Each one of the families of progressively‐drawn steels  manufacturing processes were used for testing. Each one of the families of progressively-drawn steels was  labelled  with  a  capital  letter  (A,  B,  C,  D  and  E).  In  addition,  to  identify  each  particular  steel  was labelled with a capital letter (A, B, C, D and E). In addition, to identify each particular steel (member of the family), a number was added representing the number of drawing steps undergone  (member of the family), a number was added representing the number of drawing steps undergone by by the specific wire. Thus, as a way of example, steel E4 corresponds to a wire obtained after the  the specific wire. Thus, as a way of example, steel E4 corresponds to a wire obtained after the fourth fourth drawing step of family E. According to Table 1, the chemical composition of the five families  drawing step of family E. According to Table 1, the chemical composition of the five families of steels of steels is similar with slight variations.  is similar with slight variations. Table 1. Chemical composition of the five families of steels.  Table 1. Chemical composition of the five families of steels.

Element 

Element

% C  %C % Mn  % Mn % Si  % Si % P  %P % S  %S % Al  % Al % Cr % Cr  %V % V 

A A 0.800  0.800 0.690  0.690 0.230  0.230 0.012  0.012 0.009  0.009 0.004  0.004 0.265 0.265  0.060 0.060 

Family of Steels B C D E B C 0.789  0.790  D0.795  E 0.789  0.789 0.698  0.790 0.670  0.795 0.624  0.7890.681  0.698 0.226  0.670 0.200  0.624 0.224  0.6810.210  0.226 0.200 0.224 0.210 0.011  0.009  0.011  0.010  0.011 0.009 0.011 0.010 0.005  0.009 0.009  0.008 0.008  0.0080.008  0.005 0.003  0.003 0.003  0.003 0.003  0.0030.003  0.003 0.071 0.071  0.187 0.187  0.164 0.164  0.2180.218  0.078 0.053 0.064 0.078  0.053  0.064  0.0610.061  Family of Steels

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Each one of the raw materials for each family undergoes different straining paths during the manufacturing process by cold drawing. Family A is forced to pass through six drawing dies, whereas the wire drawing of the other families (B, C, D and E) is divided into seven drawing steps. Table 2 summarizes each one of the five cold drawing procedures (straining paths or yielding histories) in terms of the wire diameter at the end of each drawing step. Table 2. Diameter of the wires at the end of each drawing step for the five families. Wire Diameter (mm)

Drawing Step 0 1 2 3 4 5 6 7

Family A

Family B

Family C

Family D

Family E

12.11 10.80 9.81 8.94 8.22 7.56 6.98 -

12.10 11.23 10.45 9.68 9.02 8.54 8.18 7.00

10.44 9.52 8.49 7.68 6.95 6.36 5.86 5.03

8.56 7.78 6.82 6.17 5.61 5.08 4.63 3.97

11.03 9.90 8.95 8.21 7.49 6.80 6.26 5.04

The cumulative plastic strain εP represents the drawing degree [23], and is defined as follows: εP “ 2ln

φ0 φi

(1)

where φ0 is the hot-rolled bar diameter (not cold drawn at all) and φi is the diameter of a wire undergoing i drawing steps. Results for the different drawn wires are given in Table 3. Table 3. Cumulative plastic strain of the progressively drawn steels. εP

Drawing Step 0 1 2 3 4 5 6 7

Family A

Family B

Family C

Family D

Family E

0 0.229 0.421 0.607 0.775 0.942 1.102 -

0 0.149 0.293 0.446 0.588 0.697 0.800 1.095

0 0.184 0.414 0.614 0.814 0.991 1.155 1.460

0 0.191 0.454 0.655 0.845 1.044 1.229 1.537

0 0.216 0.418 0.591 0.774 0.967 1.133 1.566

Standard tension tests were performed up to final fracture. Three tests were made for each drawing step (thus, as many as 117 standard tension tests were performed in the mechanical characterization, a huge collection of statistical quantitative data). 3. Microstructure of the Progressively-Drawn Wires The microstructure of the progressively-drawn wires is given in Figures 2–5 (the vertical side of longitudinal sections always corresponds to the wire axis). They show a progressive slenderizing of the pearlitic colonies and an increase of packing closeness (with decrease of pearlite interlamellar spacing). In addition, a progressive orientation (in the drawing direction) of both pearlitic colonies (first microstructural level) and ferrite/cementite lamellae (second microstructural level) can be observed. All these observations are fully consistent with previous research in similar steels [12–15].

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(B0) 

(B4)

(B1) 

(B5)

(B2) 

(B6)

(B3) 

(B7)

Figure 2. Metallographic analysis of the longitudinal section of family B for diverse drawing steps.  Figure 2. Metallographic analysis of the longitudinal section of family B for diverse drawing steps.

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(B0) 

(B4)

(B1) 

(B5)

(B2) 

(B6)

(B3) 

(B7)

Figure 3. Metallographic analysis of the transverse section of family B for diverse drawing steps.  Figure 3. Metallographic analysis of the transverse section of family B for diverse drawing steps.

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(E0) 

(E4)

(E1) 

(E5)

(E2) 

(E6)

(E3) 

(E7)

Figure 4. Metallographic analysis of the longitudinal section of family E for diverse drawing steps.  Figure 4. Metallographic analysis of the longitudinal section of family E for diverse drawing steps.

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(E0) 

(E4)

(E1) 

(E5)

(E2) 

(E6)

(E3) 

(E7)

Figure 5. Metallographic analysis of the transverse section of family E for diverse drawing steps.  Figure 5. Metallographic analysis of the transverse section of family E for diverse drawing steps.

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Metals 2016, 6, 114  4. Mechanical Behavior of the Progressively-Drawn Wires

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Conventional standard tension tests up to final fracture under a constant displacement rate of 4. Mechanical Behavior of the Progressively‐Drawn Wires  2 mm/minConventional standard tension tests up to final fracture under a constant displacement rate of 2  were carried out using cylindrical samples of 300 mm length for each one of the drawing steps and the five families of steels used in the experimental program. Such a length was selected mm/min were carried out using cylindrical samples of 300 mm length for each one of the drawing  following the recommendation of a previous study [29]. Figure 6 shows the obtained master curves steps and the five families of steels used in the experimental program. Such a length was selected  (stressfollowing the recommendation of a previous study [29]. Figure 6 shows the obtained master curves  vs. strain curves) as results of testing for each drawing step of the considered steels. Furthermore, (stress  strain  curves)  results  of  testing properties for  each  drawing  step  of  the  considered  steels.  Table 4 showsvs. the values of theas main mechanical of the previous steels. Furthermore, Table 4 shows the values of the main mechanical properties of the previous steels.  2.0

2.0

1.5

1.5

1.0

(GPa)

(GPa)

 

A6 A5 A4 A3 A2 A1 A0

0.5

B7 B6 B5 B4 B3 B2 B1 B0

1.0

0.5

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08





(a)   

 

2.0

2.0

1.5 (GPa)

1.5 (GPa)

 

(b) 

C7 C6 C5 C4 C3 C2 C1 C0

1.0

0.5

D7 D6 D5 D4 D3 D2 D1 D0

1.0

0.5

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08





(c)   

(d)  2.0

(GPa)

1.5

E7 E6 E5 E4 E3 E2 E1 E0

1.0

0.5

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08



 

(e) 

Figure 6. Stress-strain curves of the progressively-drawn steels: (a) family A with six drawing degrees; (b) family B with seven drawing degrees; (c) family C with seven drawing degrees; (d) family D with seven drawing degrees; and (e) family E with seven drawing degrees.

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Table 4. Main mechanical properties for each drawing step of the considered families. Steel

E (GPa)

σY (GPa)

σR (GPa)

εR

1.27 1.29 1.45 1.52 1.58 1.65 1.84

0.076 0.018 0.030 0.028 0.026 0.022 0.054

1.27 1.34 1.37 1.43 1.49 1.55 1.58 1.84

0.066 0.057 0.061 0.052 0.042 0.038 0.035 0.052

1.23 1.27 1.36 1.42 1.50 1.58 1.64 1.91

0.066 0.061 0.046 0.047 0.043 0.045 0.042 0.051

1.23 1.32 1.42 1.49 1.55 1.63 1.69 1.88

0.072 0.056 0.038 0.045 0.048 0.051 0.047 0.057

1.23 1.28 1.36 1.41 1.50 1.60 1.62 1.83

0.068 0.056 0.049 0.055 0.049 0.048 0.043 0.059

Family A A0 A1 A2 A3 A4 A5 A6

194 201 187 190 190 195 207

0.72 1.10 1.12 1.18 1.26 1.33 1.57 Family B

B0 B1 B2 B3 B4 B5 B6 B7

202 204 204 203 203 201 201 205

0.72 0.84 0.88 0.95 1.01 1.09 1.12 1.58 Family C

C0 C1 C2 C3 C4 C5 C6 C7

203 199 201 204 204 204 204 208

0.69 0.78 0.90 0.97 1.06 1.14 1.23 1.65 Family D

D0 D1 D2 D3 D4 D5 D6 D7

194 192 189 194 200 202 202 206

0.68 0.84 0.99 1.00 1.07 1.16 1.25 1.65 Family E

E0 E1 E2 E3 E4 E5 E6 E7

199 192 194 192 196 199 200 208

0.72 0.83 0.91 0.93 1.02 1.13 1.16 1.49

E: Young modulus; σY : yield strength; σR : ultimate tensile strength (UTS); εR : strain at maximum load.

From such curves (Figure 6), the mechanical properties can be determined and, thus, the evolution of yield strength σY (Figure 7a) and ultimate tensile strength σR (Figure 7b) with the drawing degree is revealed in terms of cumulative plastic strain (εP ).

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From  such  curves  (Figure  6),  the  mechanical  properties  can  be  determined  and,  thus,  the  10 of 18  10 of 18 evolution  of  yield  strength  σY  (Figure  7a)  and  ultimate  tensile  strength  σR  (Figure  7b)  with  the  P).  drawing degree is revealed in terms of cumulative plastic strain (ε From  such  curves  (Figure  6),  the  mechanical  properties  can  be  determined  and,  thus,  the 

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evolution  of  yield  strength  σY  (Figure  7a)  and  ultimate  tensile  strength  σR  (Figure  7b)  with  the  drawing degree is revealed in terms of cumulative plastic strain (εP). 

(a) 

(b) 

Figure 7. Mechanical properties evolution with cold drawing, for each one of the five pearlitic families  Figure 7. Mechanical (a)  properties evolution with cold drawing, for each one of the (b) five pearlitic families of steels: (a) yield strength; (b) ultimate tensile strength (UTS).  of steels: (a) yield strength; (b) ultimate tensile strength (UTS). Figure 7. Mechanical properties evolution with cold drawing, for each one of the five pearlitic families  As Figure 7a,b clearly show, a quasi‐linear growing trend does exist in the yield strength and  of steels: (a) yield strength; (b) ultimate tensile strength (UTS). 

As Figure 7a,b clearly show, a quasi-linear growing trend does exist in the yield strength and UTS UTS with cumulative plastic strain during drawing. Therefore, a linear dependence seems to exist  with cumulative strainand  during linear to  dependence seems to exist between between  both  plastic parameters  the drawing. drawing  Therefore, degree.  In a order  reveal  such  a  dependence,  a  As Figure 7a,b clearly show, a quasi‐linear growing trend does exist in the yield strength and  mathematical fitting (Figure 8) was applied to the data included in Figure 7a,b. The last step of wire  both parameters and the drawing degree. In order to reveal such a dependence, a mathematical UTS with cumulative plastic strain during drawing. Therefore, a linear dependence seems to exist  drawing was not considered, since manufacturing companies used to apply a special heat treatment  fitting (Figure 8) was applied and  to the data included in Figure 7a,b. to  Thereveal  last step of a  wire drawing was between  both  parameters  the  drawing  degree.  In  order  such  dependence,  a  notafter the last drawing step for relieving the residual stresses induced by manufacturing. Thus, these  considered, since manufacturing companies used to apply a special heat treatment after the last mathematical fitting (Figure 8) was applied to the data included in Figure 7a,b. The last step of wire  equations mathematically represent the increment of yield strength and the UTS of pearlitic steels  drawing was not considered, since manufacturing companies used to apply a special heat treatment  drawing step for relieving the residual stresses induced by manufacturing. Thus, these equations due  to  the  strain  hardening  mechanism.  could  useful  for  obtaining  simple  after the last drawing step for relieving the residual stresses induced by manufacturing. Thus, these  mathematically represent the increment ofThese  yield equations  strength and thebe  UTS of pearlitic steelsa due to the estimation of the material strength after a given straining process.  equations mathematically represent the increment of yield strength and the UTS of pearlitic steels  strain hardening mechanism. These equations could be useful for obtaining a simple estimation of the due  to strength the  strain  hardening  mechanism.  These  equations  could  be  useful  for  obtaining  a  simple  material after a given straining process. estimation of the material strength after a given straining process. 

  Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS)    with cumulative plastic strain of cold‐drawn wires.  Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS)  Figure 8. Mathematical fitting of the evolution of the mechanical properties (yield strength and UTS) with cumulative plastic strain of cold-drawn wires.

5. Fractographic Analysis  with cumulative plastic strain of cold‐drawn wires. 

Fracture surfaces were observed by means of SEM. The fracture surface obtained after tensile  5. Fractographic Analysis  testing of each drawing step of steel families B and E are shown in Figures 9 and 10, respectively. For  5. Fractographic Analysis each set of three tests (of the total amount of 117) a representative fracture surface was considered in  Fracture surfaces were observed by means of SEM. The fracture surface obtained after tensile  Fracture surfaces were observed of SEM. The fracture surface obtained tensile the  fractographic  analysis.  According by to means the  MFMs,  the  slightly‐drawn  steels  exhibit  an after isotropic  testing of each drawing step of steel families B and E are shown in Figures 9 and 10, respectively. For  testing of each drawing step of steel families B and E are shown in Figures 9 and 10 respectively. fracture  behavior  and  a  smooth  fracture  surface  at  the  macroscopic  scale.  As  the  drawing  degree  each set of three tests (of the total amount of 117) a representative fracture surface was considered in  Forthe  each set of three analysis.  tests (of the total amount 117) a the  representative fracture surface was fractographic  According  to  the ofMFMs,  slightly‐drawn  steels  exhibit  an considered isotropic  in the fractographic analysis. According tosurface  the MFMs, the slightly-drawn exhibit an isotropic fracture  behavior  and  a  smooth  fracture  at  the  macroscopic  scale. steels As  the  drawing  degree  fracture behavior and a smooth fracture surface at the macroscopic scale. As the drawing degree increases the mechanical behavior is modified (σY and σR increase, Figure 8, and the fracture behavior,

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too, showing a more irregular fractography with numerous peaks and valleys (strength anisotropy) as a consequence of the progressive microstructural orientation after manufacture. Metals 2016, 6, 114  11 of 18 

  (B0) 

(B4)

(B1) 

(B5)

(B2) 

(B6)

(B3) 

(B7)

 

 

  Figure 9. Evolution with drawing of MFMs of drawn wires corresponding to the family B.  Figure 9. Evolution with drawing of MFMs of drawn wires corresponding to the family B.

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  (E0) 

(E4)

(E1) 

(E5)

(E2) 

(E6)

(E3) 

(E7)

 

 

 

Figure 10. Evolution with drawing of MFMs of drawn wires corresponding to the family E.  Figure 10. Evolution with drawing of MFMs of drawn wires corresponding to the family E.

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In all steels, fracture initiates in a central fibrous region formed by micro-void coalescence (MVC), Metals 2016, 6, 114  13 of 18  this domain representing the fracture process zone (FPZ). In addition to the central zone of the wire all  steels,  fracture  initiates  in appears a  central  region  formed  micro‐void  coalescence  with In  a fibrous aspect, ductile fracture infibrous  the form of MVC at theby  periphery of all the wires (MVC), this domain representing the fracture process zone (FPZ). In addition to the central zone of  tested as an external ring (shear lip). The micro-void size at the central zone is higher than that the wire with a fibrous aspect, ductile fracture appears in the form of MVC at the periphery of all the  observed in the external ring. In moderately-drawn wires, an intermediate zone between the external wires tested as an external ring (shear lip). The micro‐void size at the central zone is higher than that  ring and the central zone was identified with a mixed fractography of MVC and cleavage (C). Radial observed in the external ring. In moderately‐drawn wires, an intermediate zone between the external  marks in the direction from the central zone to the external ring are observed. Therefore, fracture was ring and the central zone was identified with a mixed fractography of MVC and cleavage (C). Radial  initiated at the wire centre (central fibrous zone by MVC) and later it propagated in radial direction marks in the direction from the central zone to the external ring are observed. Therefore, fracture was  towards the wire periphery. As the drawing degree increases, the area covered by brittle fracture initiated at the wire centre (central fibrous zone by MVC) and later it propagated in radial direction  (cleavage zone) decreases, appearing MVC instead of C. Thus, in heavily drawn wires the observed towards  the wire  periphery. As  the  degree  increases,  the  area  covered  by  brittle fracture  MFM is constituted in the majority ofdrawing  cases only by MVC. (cleavage zone) decreases, appearing MVC instead of C. Thus, in heavily drawn wires the observed  Attention should be paid to certain exceptions to the previously depicted common trend. MFM is constituted in the majority of cases only by MVC.  For instance, the fibrous zone in the specimen E0 (Figure 10) is placed close to the external ring Attention should be paid to certain exceptions to the previously depicted common trend. For  (peripheral fracture origin). Probably this behavior is caused by surface damage generated in the instance,  the  in  the  specimen  E0  (Figure  10)  is  placed  close  to  the  external  ring  hot-rolled steelfibrous  during zone  storage. (peripheral fracture origin). Probably this behavior is caused by surface damage generated in the hot‐ MFMs were assembled from the SEM micrographs. Later, a quantitative fractographic analysis of rolled steel during storage.  the fracture surfaces was carried out by a commercial image analysis software, paying special attention to theMFMs were assembled from the SEM micrographs. Later, a quantitative fractographic analysis  following fracture features (Figure 11): of  the  fracture  surfaces  was  carried  out  by  a  commercial  image  analysis  software,  paying  special  ‚attention to the following fracture features (Figure 11):  Total fracture surface (SF ). ‚ Surface of the FPZ (SFPZ ). Total fracture surface (S F).  ‚ Radius of the FPZ (r ). Surface of the FPZ (SFPZ FPZ).  Radius of the FPZ (r FPZcrown ).  ‚ Surface of the external formed by MVC (SEC ). ‚ Surface of the external crown formed by MVC (S EC).  Mean or average depth (xm ), maximum depth (xmax ) and minimum depth (xmin ) of the external  Mean or average depth (x crown formed by MVC. m), maximum depth (xmax) and minimum depth (xmin) of the external  crown formed by MVC. 

xmin

SFPZ

SEC

rFPZ

R

xmax

 

Figure  11. 11.  Scheme  fracture  surfaces surfaces  in in  the the  quantitative quantitative  Figure Scheme of  of the  the parameters  parameters used  used for  for defining  defining the  the fracture fractographic analysis.  fractographic analysis.

An  indicator  of  the  steel  ductility  (maximum  plastic  strain  undergone  by  the  material  up  to  An indicator of the steel ductility (maximum plastic strain undergone by the material up to fracture) is the reduction of the cross sectional area Z. This parameter can be obtained from the initial  fracture) is the reduction of the cross sectional area Z. This parameter can be obtained from the initial area of the specimens (S0) and the final area after fracture (SF) as follows:  area of the specimens (S0 ) and the final area after fracture (SF ) as follows: ˆ  S0  SF˙ Z(%) S0 ´ (2) S   100   Zp%q “  S0 F  ˆ 100 (2) S0 The value of the areas of the FPZ (SFPZ) and the external crown (SEC) were measured in terms of  the  whole  fracture  area  SF.  Accordingly,  the  radius  of  the  FPZ  (rFPZ) and  the average  depth of  the  external  ring  (xm)  were  obtained  in  terms  of  the  radius  R  of  the  fractured  area  by  means  of  the  following equations: 

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The value of the areas of the FPZ (SFPZ ) and the crown (SEC ) were measured in terms  S external SFPZ (%)   FPZ   100   of the whole fracture area SF . Accordingly, the radius of the FPZ (rFPZ ) and the average depth(3) of  SF  the external ring (xm ) were obtained in terms of the radius R of the fractured area by means of the following equations: ˆ SEC ˙ SEC (%)  SFPZ (4)   100   SFPZ p%q “  SF  ˆ 100 (3) SF ˆ ˙ SEC  r  ˆ 100 SEC p%q “ (4) rFPZ (%)   SFPZ (5) F   100   R ¯ ´r (5) rFPZ p%q “ FPZ ˆ 100 R  x  ¯ ´ xm (%)  xmm   100   (6) (6) xm p%q “  R ˆ 100 R 6. Discussion 6. Discussion  The evolution of the reduction of the cross sectional area Z with cumulative plastic strain caused The evolution of the reduction of the cross sectional area Z with cumulative plastic strain caused  by wire drawing for the five drawn steels is shown in Figure 12. Two trends with the drawing degree by wire drawing for the five drawn steels is shown in Figure 12. Two trends with the drawing degree  are observed. On one hand, Z increases with the drawing degree for slightly drawn steels. On the are observed. On one hand, Z increases with the drawing degree for slightly drawn steels. On the  other hand, for heavily-drawn steels the trend is the opposite, i.e., Z decreases with the drawing degree. other  hand,  for  trend  the  drawing opposite, even i.e.,  Z  with  drawing  Surprisingly, forheavily‐drawn  the steel type A,steels  Z stillthe  rises withis cold in decreases  the last step, justthe  where such degree. Surprisingly, for the steel type A, Z still rises with cold drawing even in the last step, just  a variable decreases in the other steels. This behavior can be explained on the basis of cumulative where such a variable decreases in the other steels. This behavior can be explained on the basis of  plastic strain εP and the microstructural orientation inside the steels. cumulative plastic strain εP and the microstructural orientation inside the steels. 

  Figure 12. Evolution of the reduction of the cross sectional area with cold drawing. Figure 12. Evolution of the reduction of the cross sectional area with cold drawing. 

Mainly, two key consequences appear at the microstructural level in steels as they are drawn: (i)  Mainly, two key consequences appear at the microstructural level in steels as they are the  accumulation  of  plastic  of strains  and  increment  of  material  strength strength (cold  work),  and  (ii)  a  drawn: (i) the accumulation plastic strains and increment of material (cold work), and microstructure orientation in the wire axial direction. In hot rolled bar (not cold drawn at all), pearlitic  (ii) a microstructure orientation in the wire axial direction. In hot rolled bar (not cold drawn at all), colonies  (micro‐composed  of  alternated  ferrite ferrite and  and cementite  lamellae)  are  randomly  pearlitic colonies (micro-composed of alternated cementite lamellae) are randomlyoriented.  oriented. However, as the  drawing degree  increases,  the  pearlitic  colonies are  progressively  oriented  in cold the  However, as the drawing degree increases, the pearlitic colonies are progressively oriented in the cold drawing direction [13] and the ferrite/cementite lamellae do the same [15], both microstructural  drawing direction [13] and the ferrite/cementite lamellae do the same [15], both microstructural levels levels becoming quasi‐aligned with the wire axis or cold drawing direction in the commercial pre‐ becoming quasi-aligned with the wire axis or cold drawing direction in the commercial pre-stressing stressing  wires  (final  stages  of  manufacture).  The plastic cumulative  strain  at  the  end  of  the  steel wiressteel  (final stages of manufacture). The cumulative strainplastic  at the end of the drawing chain, drawing chain, i.e., that necessary for the full reorientation in axial direction of both microstructural  i.e., that necessary for the full reorientation in axial direction of both microstructural levels (hierarchical levels (hierarchical structures of pearlitic colonies and ferrite/cementite lamellae) ranges between 1.1  structures of pearlitic colonies and ferrite/cementite lamellae) ranges between 1.1 and 1.6 for the and 1.6 for the whole set of steel families analyzed in the present paper. This interval fully agrees  whole set of steel families analyzed in the present paper. This interval fully agrees with the value with  value  of  reported  literature in  the  scientific  [30,31],  of plastic the  cumulative  plastic for strain  of 1.5,the  reported in 1.5,  the scientific [30,31],literature  of the cumulative strain necessary the necessary for the aforementioned completion of reorientation. aforementioned completion of reorientation. Once  the  microstructure  (colonies  and  lamellae)  are  completely  oriented  in  the  drawing  direction  [13,15],  the  percentage  of  reduction  of  the  cross  sectional  area  Z  begins  to  decrease, 

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Once the microstructure (colonies and lamellae) are completely oriented in the drawing direction [13,15], the percentage of reduction of the cross sectional area Z begins to decrease, Metals 2016, 6, 114  15 of 18  Metals 2016, 6, 114  15 of 18  independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z reduction is linked with the strong decrease of the interlamellar spacing caused during the independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z  independently of the plastic strain increment (e.g., step 6 to 7 in family types B, C, D and E). The Z  microstructure re-orientation at the final stages of drawing [14] and the dislocation density rises in the reduction  reduction  is  is  linked  linked  with  with  the  the  strong  strong  decrease  decrease  of  of  the  the  interlamellar  interlamellar  spacing  spacing  caused  caused  during  during  the  the  ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path for microstructure re‐orientation at the final stages of drawing [14] and the dislocation density rises in  microstructure re‐orientation at the final stages of drawing [14] and the dislocation density rises in  dislocation movement during strain hardening of the progressively drawn steels. the ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path  the ferrite lamellae, thus increasing the microstructural packing closeness and reducing the free path  The FPZ (fibrous zone) was characterized by means of its size SFPZ represented in Figure 13, for dislocation movement during strain hardening of the progressively drawn steels.  for dislocation movement during strain hardening of the progressively drawn steels.  and by its characteristic length rFPZ shown in Figure 14. As a common trend, in all analyzed steel The FPZ (fibrous zone) was characterized by means of its size S The FPZ (fibrous zone) was characterized by means of its size SFPZ FPZ represented in Figure 13, and   represented in Figure 13, and  families the two parameters decrease with cold drawing. Figure 15 shows the evolution with the by its characteristic length r by its characteristic length rFPZ FPZ shown in Figure 14. As a common trend, in all analyzed steel families   shown in Figure 14. As a common trend, in all analyzed steel families  drawing degree of the area of the external crown SEC . It increases up to the second step of the cold the  the two  two parameters  parameters decrease  decrease with  with cold  cold drawing.  drawing. Figure  Figure 15  15 shows  shows the  the evolution  evolution with  with the  the drawing  drawing  drawing process and then continuously decreases as εP rises (with certain exceptions in the last degree of the area of the external crown S EC . It increases up to the second step of the cold drawing  degree of the area of the external crown SEC. It increases up to the second step of the cold drawing  step). With regard to its average depth (Figure 16), similar results were obtained for the different process and then continuously decreases as ε process and then continuously decreases as εPP rises (with certain exceptions in the last step). With   rises (with certain exceptions in the last step). With  steel families. regard to its average depth (Figure 16), similar results were obtained for the different steel families.  regard to its average depth (Figure 16), similar results were obtained for the different steel families.  50 50 A A B B C C D D E E

FPZ

S FPZ (%)

40 40 30 30 20 20 10 10 00 0.0 0.0

0.2 0.2

0.4 0.4

0.6 0.6

0.8 0.8 p

p

1.0 1.0

1.2 1.2

1.4 1.4

1.6 1.6

 

Figure 13. Evolution of the area of the FPZ with the drawing degree.  Figure 13. Evolution of the area of the FPZ with the drawing degree. Figure 13. Evolution of the area of the FPZ with the drawing degree. 

50 50 A A B B C C D D E E

FPZ

r (%) FPZ

40 40 30 30 20 20 10 10

00 0.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 1.2 1.2 1.4 1.4 1.6 1.6 p

p

 

Figure 14. Evolution of radius of the FPZ with the drawing degree. Figure 14. Evolution of radius of the FPZ with the drawing degree.  Figure 14. Evolution of radius of the FPZ with the drawing degree. 

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   Figure 15. Evolution of the area of the external crown with the drawing degree.  Figure 15. Evolution of the area of the external crown with the drawing degree. Figure 15. Evolution of the area of the external crown with the drawing degree.  100 100 AA BB CC DD EE

60 60

m

(%) xxm (%)

80 80

40 40 20 20 00 0.0 1.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.2 1.2 1.4 1.4 1.6 1.6 p

p

  

Figure 16. Evolution of the average depth of the external crown with the drawing degree.  Figure 16. Evolution of the average depth of the external crown with the drawing degree. Figure 16. Evolution of the average depth of the external crown with the drawing degree. 

7. Conclusions  7. Conclusions 7. Conclusions  The fracture surfaces of samples corresponding to very diverse steels, with different cumulative  The fracture surfaces of samples corresponding to very diverse steels, with different cumulative The fracture surfaces of samples corresponding to very diverse steels, with different cumulative  plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis  plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis plastic strain, were studied by means of scanning electronic microscope (SEM) and image analysis  techniques. In all steels a similar fracture behavior was observed:  techniques. In all steels a similar fracture behavior was observed: techniques. In all steels a similar fracture behavior was observed:   The  fracture  initiates  at  zone  of  each  wire  which  the  fracture  The fracture fracture  initiates  at  the  the  central  central  zone  each  wire  in  in  fracture  microscopic  microscopic  ‚ The initiates at the central zone of eachof  wire in which thewhich  fracturethe  microscopic topography topography  may  be  classified  as  micro‐void  coalescence  (MVC)  with  fibrous  aspect  and  topography  may  be  classified  as  micro‐void  coalescence  (MVC)  with  fibrous  and  may be classified as micro-void coalescence (MVC) with fibrous aspect and propagatesaspect  in a radial propagates in a radial direction through the intermediate zone up to reaching the external ring.  propagates in a radial direction through the intermediate zone up to reaching the external ring.  direction through the intermediate zone up to reaching the external ring.  The  intermediate  zone  shows  fractography  by  cleavage  and  MVC.  The  fracture  surface  by  The intermediate intermediate zone zone shows shows aa  a fractography fractography by by cleavage cleavage and and MVC. MVC. The The fracture fracture surface surface by by  ‚ The cleavage  diminishes  in  favor  of  MVC  in  the  last  steps  of  the  manufacturing  process  by  cold  cleavage  diminishes  in  favor  of  MVC  in  the  last  steps  of  the  manufacturing  process  by  cold  cleavage diminishes in favor of MVC in the last steps of the manufacturing process by cold drawing, so that the fracture process becomes more ductile as the drawing degree increases.  drawing, so that the fracture process becomes more ductile as the drawing degree increases.  drawing, so that the fracture process becomes more ductile as the drawing degree increases.  Another important characteristic of such an intermediate zone is the presence of radial cracks  Another important characteristic of such an intermediate zone is the presence of radial cracks  ‚ Another important characteristic of such an intermediate zone is the presence of radial cracks oriented quasi‐parallel to the drawing axis or cold drawing direction, embryos of anisotropic  oriented quasi‐parallel to the drawing axis or cold drawing direction, embryos of anisotropic  oriented quasi-parallel to the drawing axis or cold drawing direction, embryos of anisotropic fracture behavior as a consequence of manufacture‐induced microstructural orientation.  fracture behavior as a consequence of manufacture‐induced microstructural orientation.  fracture behavior as a consequence of manufacture-induced microstructural orientation.  With regard to the percentage values of the different parts in the fracture surface, the evolution  With regard to the percentage values of the different parts in the fracture surface, the evolution  With regard to the percentage values of of  the different parts in the fracture surface, the evolution ‚ with  with cold  cold drawing  drawing of  of the  the central  central zone  zone  of the  the fibrous  fibrous aspect  aspect (fracture  (fracture process  process zone  zone or  or FPZ)  FPZ) is  is  with cold drawing of the central zone of the fibrous aspect (fracture process zone or FPZ) is similar similar in all analyzed steels and the same happens in the matter of the external ring (shear lip).  similar in all analyzed steels and the same happens in the matter of the external ring (shear lip).  in all analyzed steels and the same happens in the matter of the external ring (shear lip).

Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish  Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish  Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002‐01831), Ministry for Education and  Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002‐01831), Ministry for Education and  Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Science  Grant  BIA2005‐08965),  Ministry  Science  and  Grant  Science (MEC;  (MEC;  Grant for BIA2005‐08965),  Ministry for  for  Science  and Innovation  Innovation (MICINN;  (MICINN;  Grant  BIA2008‐06810),  Institutions: Ministry Science and Technology (MICYT; Grant MAT2002-01831), Ministry forBIA2008‐06810),  Education and Ministry for Economy and Competitiveness (MINECO; Grant BIA2011‐27870), Junta de Castilla y León (JCyL;  Ministry for Economy and Competitiveness (MINECO; Grant BIA2011‐27870), Junta de Castilla y León (JCyL; 

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Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011-27870), Junta de Castilla y León (JCyL; Grants SA067A05, SA111A07 and SA039A08), and the steel supplied by EMESA TREFILERÍA (La Coruña, Galicia, Spain) and TREFILERÍAS QUIJANO (Los Corrales de Buelna, Santander, Spain). Author Contributions: J.T. conceived and designed the experimental procedure; F.J.A., B.G., J.C.M., D.V. and M.L. performed the tests; all the authors analyzed the results and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: FPZ SEM MFM MVC

Fracture Process Zone Scanning Electron Microscopy Micro-Fracture Map Micro-Void Coalescence

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