The relationship between microstructures and ductility parameters, including reduction of area, elongation to failure, occurrence of delamination, and number of turns to failure in torsion, in hypereutectoid pearlitic steel wires was investigated. The transformed steel wires at 620 °C were successively dry-drawn to drawing strains from 0.40 to 2.38. To examine the effects of hot-dip galvanizing conditions, post-deformation annealing was performed on cold drawn steel wires (ε = 0.99, 1.59, and 2.38) with a different heating time of 30–3600 s at 500 °C in a salt bath. In cold drawn wires, elongation to failure dropped due to the formation of dislocation substructures, decreased slowly due to the increase of dislocation density, and saturated with drawing strain. During annealing, elongation to failure increased due to recovery, and saturated with annealing time. The variation of elongation to failure in cold drawn and annealed steel wires would depend on the distribution of dislocations in lamellar ferrite. The orientation of lamellar cementite and the shape of cementite particles would become an effective factor controlling number of turns to failure in torsion of cold drawn and annealed steel wires. The orientation and shape of lamellar cementite would become microstructural features controlling reduction of area of cold drawn and annealed steel wires. The density of dislocations contributed to reduction of area to some extent.

The effects of microstructural features on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires were investigated. The patented wire rods were successively dry drawn to ε = 0.79 (54.7%) ~ 2.38 (90.7%). To examine the effects of hot-dip galvanizing conditions on torsional ductility, steel wires with ε = 1.95 were annealed at 500 °C for 30 s for ~1 h in a salt bath. In cold drawn wires, the number of turns to failure increased steadily, showing the maximum peak, and then decreased with drawing strain. During the post-deformation annealing at 500 °C, torsional ductility of steel wires decreased with annealing time, except for the rapid drop due to the occurrence of delamination for 10 s annealing. The decrease of the number of turns to failure would be attributed to the microstructural evolutions, accompanying the spheroidization and growth of cementite particles and the recovery of ferrite in cold drawn steel wires. From the relationship between microstructural evolution and torsional ductility, it was found that among microstructural features, the shape and orientation of lamellar cementite showed the significant effect on torsional ductility of cold drawn and annealed hyper-eutectoid steel wires.

The microstructural evolution and corresponding mechanical properties of 5052 Al alloys with different deformation temperatures and post-deformation annealing conditions were investigated. The warm-rolled alloy showed higher strength and elongation than the cold-rolled and cryo-rolled alloys. The improved strength and ductility of warm rolled alloys is attributed to the formation of fine precipitates and a higher degree of recovery during rolling. The formation of precipitates and the occurrence of dynamic recovery during the warm-rolling process were confirmed by the absence of the first two peaks in DSC curves of warm-rolled alloys. In particular, it was found that the application of cryo-rolling combined with warm-rolling at 448K increased tensile strength and yield strength without a decrease of ductility. This notable increase of strength is attributed to the increased dislocation density during cryo-rolling and the subsequent formation of fine precipitates at dislocations during warm-rolling. The contribution of fine precipitates and fully recovered microstructures, during cryo-rolling, warm-rolling, and subsequent annealing enhances the effective combination of strength and elongation. The ultra-fine grained 5052 Al alloy with high strength (405 MPa) and ductility (11.4% elongation) could be achieved by a combination of cryo-rolling with warm-rolling (448 K) and subsequent static annealing at 448 K.

The effect of annealing on the microstructural evolution of the matrix in low carbon manganese steels with different initial microstructures, including martensite, bainite and a mixture of ferrite and pearlite, was investigated in conjunction with the charge in mechanical properties. During annealing, steels with a deformed ferrite + pearlite microstructure underwent spheroidization of the lamellar cementite in the pearlite region and recovery or recrystallization in the ferrite region. Martensitic steels and bainitic steels experienced recovery and recrystallization of the matrix and the precipitation of cementite particles during annealing. Although the martensitic steels showed the highest strength up to the annealing temperature of 600 oC, the large driving force from dislocations accumulated during cryogenic deformation, as well as dislocations inherited from the martensitic transformation, accelerated the softening rate during annealing. A small amount of equi-axed grains produced during annealing at 550 oC did not effectively improve ductility. The rapid softening and the improvement in ductility in martensitic steels and bainitic steels annealed at 600 oC could be attributed to the disappearance of elongated substructures, rather than the coarsening of cementite particles during annealing. The bainitic steel showed higher thermal stability and a better combination of strength and ductility than the other microstructures, such as the martensitic microstructure and a mixture of ferrite and pearlite. (Received April 16, 2018; Accepted July 2, 2018)

This study investigated the effects of manufacturing conditions, such as patenting temperature and drawing strains, on the tensile fracture behaviors, especially on reduction of area (RA), of hyper-eutectoid steel wire. Steel wires with the chemical compositions of Fe-0.92C-1.3Si-0.6Mn-0.3Cr (wt%), were austenitized at 900 degrees C for 3 min and isothermally transformed at 580 degrees C and 620 degrees C for 3 rain. Patented steel wires were cold drawn from 4.90 mm to 1.49 mm in diameter with an 18% reduction per pass. The cup-and-cone fracture surfaces of the tensile specimens consisted of a fibrous zone (crack initiation zone), radial marks (crack propagation zone), and shear lips. Four types of fracture mode were found in the tensile fractured surfaces of the patented specimens: dimple (fibrous zone), shear cracking (fibrous zone), boundary fracture (radial marks), and cleavage-type fracture (radial marks). Increasing transformation temperature contributed to an increase in the area fraction and size of the shear cracking and cleavage-type fracture. The increase in RA with the reduction in transformation temperature was attributed to a reduction in the pearlite block size as well as refinement of the interlamellar spacing. Meanwhile, as drawing strain increased, the area fraction of the fibrous zone in the tensile fractured surface decreased until a drawing strain of 1.79 and then increased, while RA increased up to a strain of 1.79 and decreased. It was found that the area fraction of the fibrous zone in the tensile fractured surface is closely related to the difference in RA in hyper-eutectoid steel wires.

The effects of alloying elements, especially Cr and Si, and transformation temperature on the microstructures and mechanical properties of hyper-eutectoid steels were investigated. Increases in Si and Cr changed the morphology of cementite in upper bainite, while the morphology of pearlite seemed to be unchanged by the amount of alloying elements, except for variation in interlamellar spacing. Increasing Si and Cr content also caused increases in tensile strength and a reduction of area of the steels containing pearlitic microstructure, while decreasing tensile strength and reduction of area in steels transformed into upper bainitic microstructure. The tensile strength of steels consisting of pearlite, upper bainite and a mixture of pearlite and upper bainite, can be expressed with the equation sigma = (sigma(po) + k(p) . lambda(-1/2)(p)) . V-p (sigma(ubo) + k(ub) .lambda(-1/2)(ub)).V-ub. The values calculated using the above equation coincided well with the measured tensile strength of hyper-eutectoid steels containing Cr and Si.

The formation of abnormal structures and their effects on reduction of area (RA) were investigated in eutectoid steels transformed at different temperatures ranging from 560 °C-650 °C. The occurrence of abnormal structures, such as upper bainite, degenerate pearlite, free ferrite, and grain boundary cementite, was confirmed. The volume fraction of upper bainite and degenerate pearlite decreased on increasing the transformation temperature, while the amount of free ferrite increased. As the transformation temperature increased, RA increased, reached a maximum, and then decreased, while the tensile strength continuously decreased. The crack formations during the tensile test could be classified into three types: tearing, shear cracking, and void formation/ coalescence. The decrease of the ductility at low transformation temperatures was attributed to the increased amount of upper bainite and degenerate pearlite, since the formation of cracks occurred by tearing interfaces or by void formation at abnormal structures during the tensile test. Meanwhile, the decrease in RA at high transformation temperatures was attributed to the occurrence of shear cracking rather than the presence of abnormal structures.

Effects of manufacturing conditions, such as austenitizing temperature, patenting temperature and carbon content in steels, on mechanical properties, especially on reduction of area (RA), of hyper-eutectoid steel wires were investigated. RA increased and then decreased with transformation temperature. This was attributed to the presence of abnormal structures in steels transformed at low transformation temperatures and the occurrence of shear cracking during tensile testing of steels transformed at high transformation temperatures. The increase of austenitizing temperature resulted in the increased austenite grain size and consequently the decrease of RA. The decrease of RA with increasing the carbon content in steels was attributed to the increased fraction of cleavage fracture in tensile fractured surfaces.

Pure Fe-C pearlite was heat-treated and selectively etched to extract [0 0 1]- and [1 0 0]-oriented single crystalline cementite sheets. The elastic properties of the shaped cementite were measured in a simple, in situ bending test system set up inside the scanning electron microscope using a micronewton-range force sensor. The Young's modulus experimentally measured from a single crystal sheet was lower than the value obtained from theoretical calculation. (c) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The effects of drawing strains and post-deformation annealing conditions on the aging behavior and the occurrence of delamination in cold drawn hyper-eutectoid steel wires were studied. At low annealing temperatures the increased tensile strength for a short annealing time would be attributed to age hardening, which came from the decomposition of the unstable cementite in deformed pearlite. The decrease of tensile strength at the high annealing temperature was due to age softening, which would be attributed to the decreased carbon content in lamellar ferrite through the spheroidization or the re-precipitation of cementite, and recovery or recrystallization of ferrite. The extent for the occurrence of delamination during annealing expanded to the high temperature region with the increased amount of deformation in steel wires. At high annealing temperatures, the decreased carbon content dissolved in lamellar ferrite due to age softening would result in the decrease of tensile strength and suppress the occurrence of delamination. The total magnitude of carbon content dissolved in lamellar ferrite either by the partial dissolution of lamellar cementite during wire drawing or by the partial decomposition of lamellar cementite during post-deformation annealing would control the occurrence of delamination in cold drawn steel wires.

Strain aging and hardening behaviors of a 304 stainless steel containing deformation-induced martensite were investigated by examining mechanical properties and microstructural evolution for different aging temperature and time. Introduced age hardening mechanisms of a cold rolled 304 stainless steel were the additional formation of α′-martensite, hardening of α′-martensite, and hardening of deformed austenite. The increased amount of α′-martensite at an aging temperature of 450 °C confirmed the additional formation of α′-martensite as a hardening mechanism in a cold rolled 304 stainless steel. Additionally, the increased hardness in both α′-martensite and austenite phases with aging temperature proved that hardening of both α′-martensite and austenite phases would be effective as hardening mechanisms in cold rolled and aged 304 stainless steels. The results suggested that among hardening mechanisms, hardening of an α′-martensite phase, including the diffusion of interstitial solute carbon atoms to dislocations and the precipitation of fine carbide particles would become a major hardening mechanism during aging of cold rolled 304 stainless steels.

The ultrafine-grained Al 6061 alloy, which was fabricated by the combination of cryogenic rolling with warm rolling, achieved high ultimate tensile strength of 420 MPa. Compared with the results by other severe plastic deformation methods, the strengthening effect by the combination of cryogenic rolling with warm rolling was found significantly effective. This notable increase of tensile strength was achieved by the formation of finer precipitates during warm rolling. The presence fine precipitates of diameter below 100 nm, in ultrafine-grained matrix, were confirmed with TEM and STEM. The estimated precipitation strengthening by the fine precipitates was approximately 100 MPa. Based on the results, it was found that cryogenic rolling combined with warm rolling would be effective in increasing strength.

The effects of deformation strains and annealing temperatures on microstructures and mechanical properties of martensitic steels were examined. The amount of cold deformation was changed as 30%, 50% and 60%, and annealing temperatures varied from 500°C to 600°C. In samples cold rolled 30%, the dominant microstructure for an annealing at 500°C was dislocation substructures with uniformly distributed rod-shaped carbide particles. For an annealing at 600°C, the microstructure consisted of equiaxed ultrafine grains, spherical carbide particles and elongated dislocation substructures. A proper annealing temperature for martensitic steels received 30% reduction, showing a good combination of a high strength, 1230MPa, and an adequate total elongation. 9.4%, was found as 500°C.

The effects of deformation strain and aging temperature on strain aging behavior in a 304 stainless steel were investigated. Age hardening of a cold-rolled 304 stainless steel was significantly influenced by the amount of α′-martensite as well as aging temperature. The similar variation of tensile strength to the amount of α′-martensite with aging temperature indicated that tensile strength of an aged steel is significantly influenced by the amount of α′-martensite formed during cold deformation. The increase of strength during aging in a 304 stainless steel cold-rolled with 40 % reduction is attributed to the additional formation of α′-martensite at 450 °C as well as to the increase of hardness in both the α′-martensite and austenite phases.

The effects of deformation temperatures and post-deformation annealing on mechanical properties, in conjunction with microstructural evolution in the 5052 Al alloy, were investigated. The combination of cryogenic-rolling with warm-rolling effectively increased tensile strength and yield strength without the decrease of ductility through the formation of ultra-fine grains with dynamic recovery in the 5052 Al alloy. And static annealing, as a post-heat treatment, enhanced the ductility. Therefore, ultra-fine grained 5052 Al alloy with high strength and a moderate level of ductility could be made by the combination of cryogenic-rolling with warm-rolling and the additional static annealing process.

The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

The evolution of microstructure and mechanical properties of cold rolled and warm rolled aluminium 5052 alloys were investigated. The work-hardening behaviour was closely related to the evolution of microstructure. Special emphasis was put on the relation between mechanical properties and substructure development during warm rolling. A warm rolled Al 5052 alloy showed higher strength and higher work-hardening rate than a cold rolled Al 5052 alloy. The increased strength and work-hardening rate in a warm rolled Al 5052 alloy would be attributed to the presence of fine precipitates and the higher amount of microbands. The contribution of precipitates and inhomogeneous microstructures to work-hardening was discussed. In the intermediate strain region, well defined double wall dislocation microband and inhomogeneous microstructures were observed.

The effects of alloying elements and initial interlamellar spacing on tensile strength and the occurrence of delamination in cold-drawn hyper-eutectoid steel wires were investigated under equivalent drawing conditions. The initial interlamellar spacing showed little influence on the occurrence of delamination. The addition of Cr effectively increased attainable tensile strength, since the added Cr not only increased tensile strength but also delayed delamination. The addition of Ni also increased attainable tensile strength, since the effect of the added Ni on delaying delamination prevailed over the tendency to decrease tensile strength and work hardening. The addition of Cr increased the attainable tensile strength more effectively than the addition of Ni in cold-drawn hyper-eutectoid steel wires, although the added Ni markedly delayed delamination-associated strain.

The effects of post-deformation annealing temperature and time on mechanical properties and the occurrence of delamination in cold drawn steel wires were investigated. For the low temperature annealing, the carbon dissolution into lamellar ferrite due to strain aging would enhance not only the increase of strength but also the occurrence of delamination. However, as annealing temperatures and annealing time increase further, age softening, including such as the break-up and the spheroidization of lamellar cementite, starts to operate. Consequently, the reduction of carbon content dissolved in lamellar ferrite would result in the decrease of tensile strength and suppress the occurrence of delamination. The voids formed at the surface of globular cementite particle, which were produced during post-deformation annealing at high temperatures for a long time, would act as one of the origins for delamination during torsion.