Effect of cementite morphology on pearlitic wire drawing deformation

A comparative study was conducted on the effects of lamellar cementites and globular cementites on the cold drawing process and the mechanical properties of pearlitic wire steel, with the help of metallographic microscope, scanning electron microscope, transmission electron microscope, tensile tester and hardness tester. The lamellar cementites showed the deformation capacity to some extent during the cold drawing process. As the drawing strain increased, the pearlitic wire with globular cementites evolved into the fibrous form gradually and no obvious defects were found in the microstructure. The globular cementites turned to the drawing direction without any deformation of itself during the deformation process. And micro- cracks occurred in the cementite/ferrite interface due to stress concentration caused by pinning dislocations in spherical cementites. The strength and hardness of both pearlitic wires gradually increased as the drawing strain rose. And the pearlitic wire with lamellar cementites had a higher drawing hardening rate. The ferrite <110> texture formed in both pearlitic wires during the cold drawing process. Compared with the globular pearlite, the pearlitic wire with lamellar cementites had higher ferrite <110> texture intensity. And the difference of their ferrite <110> texture intensity became bigger and bigger as the drawing strain increased.     

剧烈塑性变形对珠光体钢丝奥氏体转变的影响

 用金相显微镜、扫描电镜、透射电镜、X射线衍射仪、同步热分析仪研究了冷拉拔形变对珠光体钢丝奥氏体化热处理过程和组织的影响。经过冷拉拔剧烈塑性变形后,珠光体组织呈纤维状,其片层沿拉拔轴向排列,厚度剧烈减薄,铁素体<110>丝织构强度达到饱和。经剧烈塑性变形的珠光体在奥氏体化转变时,奥氏体形核更早、更密集,且沿轴向生长;在完成等温转变后,其原奥氏体晶粒、珠光体团尺寸均明显细化,其铁素体<110>丝织构一定程度遗传到了相变后的组织中,但沿拉拔轴向排列的片层形貌特点并未得到遗传。     

82B高速线材拉拔过程中显微组织的演变分析

借助扫描电子显微镜(SEM)观察分析研究了82B线材在拉拔过程中显微组织和芯部马氏体的演变规律。分析认为:由于索氏体属于细片状珠光体,拉拔变形时,承受滑移的铁素体相不易引起应力集中;渗碳体相为细薄片层形态,也能够发生塑性变形,经过多道次的拉拔,索氏体基体变形量较大,没有出现任何裂纹。而芯部由偏析形成的马氏体,其塑性变形能力明显低于索氏体,基本无变形,在基本拉应力和附加拉应力的共同作用下,在早期拉拔时就在芯部出现裂纹,在后期拉拔过程中,裂纹沿着拉拔方向不断扩展。为减少断丝,可采用加大吹风量、扩大连铸坯、控制钢水过热度、增加电磁搅拌等工艺,来提高索氏体含量和细化渗碳体片层及预防马氏体的形成,以提高拉拔性能。     

Delamination of Pearlitic Steel Wires: The Defining Role of Prior-Drawing Microstructure

This article reports the occasional (< 10 pct of the actual production) delamination of pearlitic wires subjected to a drawing strain of ~ 2.5. The original wire rods which exhibited post-drawing delamination had noticeably lower axial alignment of the pearlite: 22 ± 5 pct vs 34 ± 4 pct in the nondelaminated wires. Although all wires had similar through-thickness texture and stress gradients, delaminated wires had stronger gradients in composition and higher hardness across the ferrite–cementite interface. Carbide dissolution and formation of supersaturated ferrite were clearly correlated with delamination, which could be effectively mitigated by controlled laboratory annealing at 673 K. Direct observations on samples subjected to simple shear revealed significant differences in shear localizations. These were controlled by pearlite morphology and interlamellar spacing. Prior-drawing microstructure of coarse misaligned pearlite thus emerged as a critical factor in the wire drawing-induced delamination of the pearlitic wires.     

Fracture Characteristics of Fully Pearlitic Steel Wire in Tension and Torsion

The fracture characteristics of fully pearlitic steel wires with fine and randomly oriented lamellae have been investigated after tension and torsion,respectively.It is found that the predominant fracture mode under small predeformation is dimple.The analysis of the colony size and the lamellar structure near the fracture surface indicates that each dimple roots from one colony.A simulation of tensile deformation with several pearlitic colonies based on the real scanning electron microscopy(SEM)observation shows that the plastic deformation concentrates and the stress traxiality is larger at the boundaries of colonies.It demonstrates the microcracks initialize at colony boundaries.Thus,the colony size is a significant factor for fracture behaviors under small pre-deformation.On the other hand,the fracture surface is investigated after large pre-deformation via torsion.The results show that fracture characteristics vary with radius from dimples,elongated dimples to the fibrous structure.It indicates that the fracture characteristics are dependent on the pre-deformation.The fracture mode under large pre-deformation becomes an anisotropic fibrous structure instead of dimples.     

Strain-Induced Dissolution of Cementite in Cold-Drawn Pearlitic Steel Wires

Metallurgical and Materials Transactions A - Strain-induced dissolution of cementite is one of the most important microstructural changes undergone by a pearlitic steel wire during cold drawing,...     

The micro structure of high carbon low alloy steel for easy drawing

For better processing performance of high carbon low alloy steel wire rod,an investigation about the influence of cementite lamellar spacing on wire ’easy drawing’ performance is completed.It is pointed out that too thin cementite lamellar spacing(<80 um) reduces the strain hardening level of wire drawing, and reduce the torsion performance of drawn wire at same time.For the wire or wire rod from industrial production,compared with the micro-structure with troostite,the micro-structure with sorbite or sorbite mixed with pearlite is more suitable to the drawing process with high reduction ratio.     

Microstructures and Mechanical Properties of as-Drawn and Laboratory Annealed Pearlitic Steel Wires

Near eutectoid fully pearlitic wire rod (5.5 mm diameter) was taken through six stages of wire drawing (drawing strains of 0 to 2.47). The as-drawn (AD) wires were further laboratory annealed (LA) to re-austenitize and reform the pearlite. AD and LA grades, for respective wire diameters, had similar pearlite microstructure: interlamellar spacing (λ) and pearlite alignment with the wire axis. However, LA grade had lower hardness (for both phases) and slightly lower fiber texture and residual stresses in ferrite. Surprisingly, essentially identical tensile yield strengths in AD and LA wires, measured at equivalent spacing, were found. The work hardened AD had, as expected, higher torsional yield strengths and lower tensile and torsional ductilities than LA. In both wires, stronger pearlite alignment gave significantly increased torsional ductility.     

Evolution of the Defect Structure of Pearlitic Steel in Cold Deformation

A method of microdiffraction analysis is developed in order to determine the defect structure of pearlitic steels during cold plastic deformation. The total reduction in the drawing of C86D steel coil is shown to affect the azimuthal blurring of the subreflexes in the microdiffraction pattern and the distortion of the plates in the pearlite colony.     

Investigation of Orientation Gradients in Pearlite in Hypoeutectoid Steel by use of Orientation Imaging Microscopy

The microstructure of pearlite in hypoeutectoid steel was investigated using high resolution orientation imaging microscopy. Systematic orientation gradients were observed along the longitudinal direction within the pearlitic ferrite lamellae. Corresponding orientation gradients seemed to occur also in the cementite within the same pearlite colony. The orientation gradients in the ferrite strongly correlated with the local topographical curvature of the pearlitic colony. The orientation relationship between the ferrite and the cementite lamellae was constant even in areas with strong orientation gradients.     

Optimization of mechanical properties of high-carbon pearlitic steels with Si and V additions

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the mechanical properties of pearlitic steels being developed for wire rod production. Mechanical test results demonstrate that the alloy additions are beneficial to the mechanical properties of the steels, especially the tensile strength. Silicon strengthens pearlite mainly by solid-solution strengthening of the ferrite phase. Vanadium increases the strength of pearlite mainly by precipitation strengthening of the pearlitic ferrite. When added separately, these elements produce relatively greater strengthening at higher transformation temperatures. When added in combination the behavior is different, and substantial strength increments are produced at all transformation temperatures studied (550 °C to 650 °C). The addition of silicon and vanadium to very-high-carbon steels (>0.8 wt pct C) also suppresses the formation of a network of continuous grain-boundary cementite, so that these hypereutectoid materials have high strength coupled with adequate ductility for cold drawing. A wire-drawing trial showed that total drawing reductions in area of 90 pct could be obtained, leading to final tensile strengths of up to 2540 MPa in 3.3-mm-diameter wires.     

Effects of microstructural factors on quasi-static and dynamic deformation behaviors of Ti-6Al-4V alloys with widmanstätten structures

The effects of microstructural factors on the quasi-static tensile and dynamic torsional deformation behaviors in Ti-6Al-4V alloys with Widmanstätten structures were investigated in this study. Dynamic torsional tests were conducted using a torsional Kolsky bar for five Widmanstätten structures, in which microstructural parameters such as colony size and α lamellar spacing were varied by heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Under dynamic torsional loading, maximum shear stress was largely dependent on colony size, whereas shear strain at the maximum shear stress point was on colony size as well as α lamellar spacing. Adiabatic shear bands were found in the deformed area of the fractured torsional specimens, and their width was smallest in the structure whose colony size and α lamellar spacing were both large. The possibility of the adiabatic shear band formation was quantitatively analyzed in relation to microstructural factors. It was the highest in the coarse Widmanstätten structure, which was confirmed by the theoretical critical shear strain (υ _c ) condition for the adiabatic shear band formation.     

影响77MnA盘条拉拔性能组织缺陷分析

作为拉拔用材的77MnA盘条正常显微组织是索氏体和少量珠光体,而在盘条生产过程中出现的马氏体、屈氏体、网状碳化物、较多粗片状珠光体、表面缺陷和拉拔中产生的硬化层组织都会导致盘条在拉拔过程中断裂。     

Macro- and Micromechanics of Pearlitic-Steel Deformation in Multistage Wire Production

Ninefold drawing of pearlitic steel wire is investigated. On the basis of multiscale computer models, the behavior of pearlite colonies at the surface of the wire and in its central layer is analyzed. The key factors are the orientation of the cementite lamellae relative to the drawing axis, the interlamellae distance, and the shape of the cementite inclusions. On the basis of finite-element models, the laws governing the reorientation of the pearlite colonies, change in shape and size of the cementite lamellae, and localization of the deformation in the ferrite are determined. The model results are verified by means of metallographic results and industrial experiments.     

Behavior of pearlite of various morphologies during cyclic tension

The structural evolution of hypereutectoid U10 steel with a pearlitic structure of various types (fine lamellar or partly spheroidized pearlite) is studied during fatigue loading. The fracture of these structures is considered using fractography data. The specific features of the structural transformations and the changes in the dislocation structure of the U10 steel are revealed during cyclic tension in the high-cycle fatigue region at a significant distance (10 mm) from a fatigue fracture surface. Substantial structural changes are shown to occur in U10 steel with various pearlitic structures during high-cycle fatigue tests (tension at a stress amplitude in a cycle lower than the macroscopic yield strength). These are the fragmentation, partial dissolution, and spheroidization of cementite lamellae and the polygonization of the ferrite component. The relationship between the type of fracture surface and the type of structure formed upon fatigue loading is found.     

Microscopic Evaluation of Strain Distribution in Granular Materials during Shear

The evolution of local strains during shear of particles of a granular material is presented in this paper. A cylindrical specimen composed of 6.5-mm spherical plastic particles was loaded under an axisymmetric triaxial loading condition. Computed tomography (CT) was used to acquire three-dimensional images of the specimen at three shearing stages. The high-resolution CT images were used to identify the 3D coordinates of 400 particles. Nine strain components (normal, shear, and rotation), rotation angles, and local dilatancy angles for particle groups were calculated, and their frequency distribution histograms are presented and discussed. It was found that there is no preferred shear direction, and the standard deviation values for shear strain components (εxy, εxz, and εyz) were almost equal for the specific test shearing stage. Shear strains as high as 25.6% were recorded for some particle groups. Furthermore, granular particle groups rotated in the 3D space with almost equal amounts of rotation strains when loaded under axisymmetric triaxial condition. Rotation strain values are very close to the corresponding shear strains. Compared to particle sliding, rotation plays a major role in the shearing resistance of granular materials. The cumulative vertical rotation angles can be as high as 38° and the horizontal rotation angles have values as high as 60°. The statistical distributions of the local dilatancy angle (ψ1) of particle groups were calculated and found to be increasing as shearing continues. The “global” dilatancy angle value is very close to the mean local ψ1 during the first stage of shearing (i.e, when global εz = ?7.3%)     

Evolution of Microstructure and Texture during Hot Torsion of Ferritic Stainless Steels Stabilized by Nb and Ti

The study was carried out to understand the mechanisms occurring during dynamic recrystallization of hot deformed 11% chromium stabilized ferritic stainless steels and to compare the behaviour induced by various types of stabilization. The experimental temperatures ranged from 800 to 1150°C and strain rates from 10^?2 to 15 s^?1. The development of the textures and microstructures was analysed using EBSD maps. It was observed that continuous dynamic recrystallization occurs in all materials starting at the onset of straining. Niobium has a more pronounced influence on hardening than titanium during hot deformation. The D2 component was found as the major texture component at the steady state for the torsion tests conducted along the negative shear direction. It was likely to be formed by the combination of straining and growth of the grains exhibiting both low stored energy and low rotation rate of the crystallographic axes. The texture evolution was the main reason for the flow stress behavior during the hot torsion tests. A complementary study was carried out to understand the texture formation occurring during the direct (negative shear direction) and reversed (positive shear direction) hot torsion. After reversion of the shear direction, i.e. during positive shear, the above major texture component is gradually changed into the D1 component. Using the method of Continuum Mechanics of Textured Polycrystals, the stress evolution is explained by the volume fraction changes of each component at various strains, associated with their respective Taylor factors. This simplified approach leads to a good agreement with experimental results.     

Deformation Behavior of Cementite in Deformed High Carbon Steel Observed by X-ray Diffraction with Synchrotron Radiation

The deformation behavior of cementite in drawn pearlitic steel and spheroidal cementite steel, which have hypereutectoid composition, was investigated by X-ray diffraction using synchrotron radiation. A detailed analysis of diffraction peak profiles reveals that the deformation behavior strongly depends on the shape of cementite in steel. The unit cell volume of the cementite in the drawn pearlitic steel compressively and elastically deforms by 1.5 to 2 pct of the initial volume at the early stage of drawing, whereas that in the drawn spheroidal cementite steel is compressed by 1 pct of the initial volume even at a large true strain. The cementite in the drawn pearlitic steel fragments into small pieces with increasing the true strain, and these pieces change to amorphous cementite. The dislocation densities of the cementite in the drawn pearlitic steel and in the drawn spheroidal cementite steel are estimated to be ~10¹³/m² before drawing and ~10¹⁴/m² after drawing. Although the large strain is induced in the cementite by drawing, the maximum strain energy in the cementite is too small to contribute to the dissolution of the cementite.

     

φ5.35mm-2100MPa桥梁用锌铝钢丝的工艺与组织性能

 为了制备更高强度级别的桥梁缆索用热镀锌铝钢丝，通过对高碳钢盘条的成分设计、低损伤拉拔技术以及热浸镀锌铝等工艺过程的优化，成功试制出?5.35 mm-2 100 MPa级桥梁缆索用热镀锌铝钢丝。设计的SWRS92Si盘条主要成分为（质量分数）：C 0.90%～0.95%，Si 0.8%～1.1%，Cr 0.20%～0.30%。试制结果表明，?13 mm-SWRS92Si盘条经铅浴处理后，珠光体层片平均尺寸从120下降至90 nm，盘条强度上升约260 MPa。经冷拉拔与热镀锌铝后，层片宽度约40 nm的珠光体层片未明显球化，可制备出2 100 MPa级直径5.35 mm的桥梁缆索用热镀锌铝钢丝。钢丝的平均抗拉强度为2 128 MPa，平均断后伸长率为5.4%，扭转圈数平均值为22圈，断口均为平断口，镀层较均匀致密；其他性能指标均优于交通部标准JT/T 1104—2016《桥梁用热镀锌铝合金钢丝》中的要求。     

热拔工艺对高氮无镍奥氏体不锈钢线材开裂的影响

在开发一种高氮无镍奥氏体不锈钢线材时,热拔线材表面出现了大量的裂纹。通过金相、扫描电子显微分析和能谱检测,对其开裂原因进行研究。发现线材的表面裂纹基本垂直于拉拔方向,裂纹在表面产生,之后向线材内部发展。线材表层存在着大量的富Cr和N的氮化物析出,沿晶析出的析出相,会导致孔洞形成,这些孔洞相连,形成微裂纹,最终导致沿晶开裂。通过加长加热区、提高拉拔速率保证了线材表面温度,避免了表面裂纹的产生,获得了质量满足要求的高氮无镍奥氏体不锈钢线材,并最终给出了建议的热拔工艺。