A Study On Microstructural Characterization And Mechanical Properties Of Cold Drawing Pearlitic Steel Wires For Bridge Cable

Microstructures of two typical （domestic and foreign） commercial pearlitic steelrods （SWRH82B） before cold drawing have been characterized systematacially bymeans of field emission gun scanning electron microscopy （FEG-SEM） equipped withback scattered electron imaging （BSEI） and electron back scattered diffraction （EBSD）,X-ray diffraction （XRD） and transmission electron microscopy （TEM）. Thedisadvantages of domestic rods which were quenched by Stelmor line （DP rod） havebeen expounded. Microstructural and texture evolution of steel wires during colddrawing process have also been systematically analyzed by the above methods. Therelationship between microstructural parameters and mechanical properties of colddrawing steel wires were determined, and the strengthening mechanisms of colddrawing pearlitic steel wires were discussed. Besides, a new torsion deformationmethod was designed to evaluate the stability of mechanical properties of final hotgalvanizing wires.The investigation has yielded the following results:（1） BSEI is an effective andpractical technique for directly characterizing the pearlite colony and the ferrite andcementite lamellae in specimens prepared without etching in metallographic etchant. Tosome extent, BSEI method is more powerful than SEI technique for characterizingundeformed and not severely deformed pearlitic steels. Combing BSEI and EBSDtechniques, information of pearlite structure including pearlite colony, pearliteinterlamellar spacing and pearlite nodule could be quantified by SEM technique throughone time sample preparation.（2） The differences in microstructures and mechanicalproperties of commercial DP rods and DLP rods （quenched in salt bath） have beenexamined. The micro hardness of DP rods is higher because the pearlite interlamellarspacing of DP rods with high degree of microalloying is thinner than that in DLP.However, the existence of a great amount of MnS inclusions distributed inside and thedecarburization at the surface layer result in reducing the strength of DP rod. The themicrostructure of DP rod is nodular pearlite which has more than one colony exists ineach nodule, so the colony size is very small. Thus, the cementite lamellae of DP rodare shorter and curvier. As a result, the plasticity is relatively worse than DLP rod. TheEBSD results show that more low angle boundaries （LABs） exist inside DP rods. Wethought that the existence of LABs can be ascribed to thermal stress arising from inhomogeneous heat transfer during the isothermal transformation and thermal stresscausing by different physical properties among austenite, ferrite and cementite. Thesecond thermal stress is intrinsic and inescapable. The first thermal stress is related toquenching substance of the isothermal pearlite transformation. The higher heat capacityof the quenching substance, the lower amount of LABs will be. Thus, DLP has lessLABs than DP because the heat capacity of salt bath is better than wind.（3） Theevolution of microstructures and texture of pearlitic wires during cold drawing havebeen analyzed. A metallographic fibrous texture of cementite lamellae and a <110>fiber crystallographic texture of ferrite matrix begin to form and develop gradually withincreasing strain from low to medium. The micro parameters, pearlite interlamellarspacing （ILS） and thickness of cementite plates （Tθ） determined in transverse sectionand longitudinal section, decrease gradually with increasing strain. Both ILS and Tθare isotropic in original steel rod, and those isotropic features of ILS and Tθ are notchanging with increasing strain. Localized shear bands （LSBs） with shear directionparalleling to the traces of （001） or （112|-） slip planes of ferrite were observed inlongitudinal section at low strain stage. The occurring of LABs is caused by slippingboth of ferrite and cementite lamellae inside the soft colonies which are encircled byhard colonies. The colonies with cementite lamellae initially perpendicular to thedrawing direction become to rotate into the drawing direction through localized sheardeformation. Both of the metallographic fibrous texture and the <110> fibercrystallographic texture have little contribution to the strength of codl drawing wiresbecause the microhardness of transverse section and longitudinal section are alwayssimilar to each other with increasing drawing strain. The strength and hardness of colddrawing pearlitic wires depend mainly on the pearlite interlamellar spacing.（4）Strengthening mechanisms of pearlitic steel wires during cold drawing from low tomedium strain were discussed. It is found that the strengthening mechanisms could beexplained by boundary strengthening model. Boundary strengthening model: therelationship between the yield/flow stress and the pearlite interlamellar spacing issubject to Hall-Petch function. An empirical formula is derived by fitting our data andthe data in literature:σ_ys=323.4+1563.9exp（ε/4）.（5） A new torsional deformationtest has been designed for evaluating the stability of the mechanical properities of thefinal wires （hot dip galvanizing wires）. The results show that the yield strength of thefinal wires decrease significantly after one revolution of torsion, which means the finalwires are very sensitive to torsional defromation. We thought that ferrite lamellae are very easy to slip during tensile test because that movable dislocations increase aftertorsional deformation. Moreover, slip deformation alos occurs in the cementite lamellaeand some lamellae broke into particles. As a result, the yield stress decreases becausethe effect of boundary strengthening has been reduced. Dislocation density in thesurface layer is much higher than that in the center layer because of higher shear stressin the surface layer, thus work hardening effect in the surface layer is stronger than thatin the center region. Therefore, the micro hardness in the surface layer is higher than thecenter region.