过共析珠光体钢在冷轧过程中的组织演变

用SEM、TEM和XRD等方法研究了过共析珠光体钢在冷轧变形过程中的组织演变情况.结果表明:变形珠光体组织不均匀,主要由不规则弯曲片层、带有剪切带的粗大片层以及精细片层组成,随着变形量的增加,精细片层比例增大,渗碳体片层严重塑性变形的同时发生部分溶解,铁素体点阵常数也随之增大.     

珠光体组织的形变、裂纹形核与扩展微观过程的动态研究

用扫描电镜对含碳量为0.15,0.47和0.79wt-％的三种碳素钢进行了拉伸试验,观察了珠光体组织形变与断裂的动态过程。结果表明,15和45号钢形变主要在铁素体中进行,珠光体可发生剪切形变与旋转等现象。裂纹主要在夹杂物界面、珠光体与铁素体两相界面上产生,最后的断裂遵循延性断裂机制;对于完全是片层珠光体组织的T8钢,渗碳体片层表现出一定程度的塑性变形能力,并随片层间距的减小变形能力增强。对于细片层组织,应变分布比较均匀,变形范围比较广泛,渗碳体片层可产生滑移和弯曲,断裂方式表现出延性断裂特征;在粗珠光体中,应变分布很不均匀,变形主要集中在强烈剪切变形带中进行,裂纹形核主要是渗碳体片层沿剪切形变带开裂的结果,断裂方式表现出脆性断裂特征。用位错塞积模型讨论了珠光体组织的脆性开裂现象。     

AN IN S ITU STUDY OF CRACK NUCLEATION AND PROPAGATION IN PEARLITE DURING DEFORMATION

用扫描电镜对含碳量为0.15,0.47和0.79wt-％的三种碳素钢进行了拉伸试验,观察了珠光体组织形变与断裂的动态过程。结果表明,15和45号钢形变主要在铁素体中进行,珠光体可发生剪切形变与旋转等现象。裂纹主要在夹杂物界面、珠光体与铁素体两相界面上产生,最后的断裂遵循延性断裂机制;对于完全是片层珠光体组织的T8钢,渗碳体片层表现出一定程度的塑性变形能力,并随片层间距的减小变形能力增强。对于细片层组织,应变分布比较均匀,变形范围比较广泛,渗碳体片层可产生滑移和弯曲,断裂方式表现出延性断裂特征;在粗珠光体中,应变分布很不均匀,变形主要集中在强烈剪切变形带中进行,裂纹形核主要是渗碳体片层沿剪切形变带开裂的结果,断裂方式表现出脆性断裂特征。 用位错塞积模型讨论了珠光体组织的脆性开裂现象。     

等通道角挤压变形中高碳珠光体钢的组织演变

在650℃对原始组织为层片状珠光体的T8钢进行Bc路径的多道次等通道角挤压(ECAP)变形,借助SEM、TEM和AFM研究了变形过程中渗碳体片层的演变情况.结果表明:渗碳体片层表现出强烈的塑性变形能力,主要以剪切断裂、扭折、弯曲等变形方式来适应剧烈的塑性变形;四道次温变形后得到晶粒尺度均在亚微米量级的超微细复相组织(α+θ);球化完全的渗碳体颗粒粒径呈双峰分布,从三维的角度表征渗碳体的组织演变及其特性与从二维角度观察分析的结果相吻合.     

高应变速率下共析珠光体钢的微观组织演变

利用分离式霍普金森压杆(SHPB)对原始组织为层片状珠光体的高碳钢进行了高应变速率变形实验,借助SEM、TEM对变形后珠光体的微观组织演变规律进行了分析研究。结果表明:在高应变速率变形条件下,共析珠光体钢发生绝热剪切现象,原始珠光体组织中存在着绝热剪切带,绝热剪切带的宽度约为1.5μm,绝热剪切带外部区域存在着高密度位错、位错胞以及亚晶,而绝热剪切带内部区域则在动态回复和动态再结晶的作用下演变成晶粒尺度均在亚微米量级的超微细复相组织(铁素体+渗碳体)。     

冷变形过共析珠光体钢丝的微观组织演化

观察并研究了过共析珠光体钢丝在冷拉过程中的显微组织变化,并通过背散射电子衍射技术(EBSD)测试了变形过程中(应变量为0、0.54、1.58、2.29)过共析珠光体钢丝织构的变化。结果表明,过共析钢中渗碳体表现出良好的变形能力。随着应变量的增加,渗碳体片层经弯曲、剪切、拉伸等变形逐渐转向拉拔方向,最后形成平行于拉拔方向的纤维状组织。冷变形还引起了渗碳体部分溶解,定量测试表明渗碳体的溶解量随着应变量的增加而增加。由于溶解后的碳原子进入到铁素体晶格中,使得铁素体晶格点阵常数变大,其衍射峰随应变量的增加而产生微小左移。拉拔后的织构取向以平行于拉拔方向的110丝织构为主,且织构强度随着拉拔应变量增加而逐渐增强。     

不同组织共析钢室温拉伸变形的塑性失稳

通过不同热处理或热机械处理工艺获得了共析钢的4种不同组织,即2种片层间距的珠光体,球化珠光体以及超细晶(α+θ)复相组织。利用室温单轴拉伸实验、SEM及TEM分析了组织状态对共析钢力学性能和塑性失稳行为的影响,结果表明:细化片层间距可以改善片层珠光体组织的协调变形能力并提高强度,均匀伸长率有所降低的同时总伸长率提高。细片层珠光体的拉伸变形失稳符合塑性失稳判据,但不同片层间距的片层珠光体拉伸断裂均属于准解理断裂。与片层珠光体组织相比,球化珠光体和超细晶(α+θ)复相组织的协调变形能力强,虽然在均匀塑性变形阶段均存在少量渗碳体粒子开裂的情况,但拉伸变形失稳均符合塑性失稳判据,拉伸断裂为韧性断裂。与片层珠光体和球化珠光体相比,超细晶(α+θ)复相组织具有较好的强度-塑性配合。     

轧制工艺对GCr-15球化退火的影响

通过热模拟的方法研究轧制工艺对GCr15球化退火的影响。结果表明，轧制工艺对GCr15轴承钢组织有显著影响。相对于常规轧制（CR）试样，控轧控冷（CRC）试样珠光体发生退化且片层细小．渗碳体呈短棒状或颗粒状，部分渗碳体片产生扭折甚至断裂。亚温球化退火中，CRC试样可获得细小均匀的球化组织，其球化效果明显优于CR试样，且显著地缩短球化退火时间。     

ECAP温变形制备亚微晶钢中渗碳体变形特征

采用透射和扫描电子显微镜研究了ECAP变形中渗碳体的变形特征。结果表明:片状渗碳体相主要以弯曲变形、剪切变形、拉伸变形、扭折变形和剪断变形5种方式协调ECAP剧烈塑性变形,这表明在本实验ECAP纯剪切变形条件下钢中硬而脆的渗碳体相具有良好的塑性变形能力。渗碳体片层的变形方式主要是取决于渗碳体所处的应力状态,而渗碳体所处的应力状态是与渗碳体的片层取向和片层厚度密切相关,其中渗碳体的片层厚度起决定性作用。     

Si-Mn合金弹簧钢带控轧控冷工艺研究

研究了不同的热轧生产工艺对Si-Mn合金弹簧钢带组织和性能的影响,得出要改善其冷轧加工性 能,关键是控制热轧钢带组织中的珠光体球团尺寸和珠光体片层间距,同时回归得出了冷轧最大变形量与珠光体含量、铁素体状态参数间的定量关系。     

Quantitative three-dimensional characterization of pearlite spheroidization

We investigated the pearlite spheroidization of a 0.8 mass% C–Fe steel under 700 °C static annealing conditions using a combination of computer-aided three-dimensional (3-D) tomography and electron back-scattered diffraction. The holes present in naturally grown cementite lamellae cause shape instability and induce shape evolution of the lamellar structure during spheroidization. 3-D visualization demonstrated that the intrinsic holes play an important role in the initiation and development of pearlite spheroidization. The hole coalescence and expansion causes the break-up up of large cementite lamellae into several long narrow ribbons. Furthermore, the growth mechanism of inter-hole coalescence is related to the ratio of half the inter-hole distance on a cementite lamella to the thickness of that lamella. The driving force for hole growth is either the difference in surface energy or the curvature between the hole edges and the adjacent flat surface of the lamella. The morphologies of cementite ribbons depend on the hole expansion position on cementite lamella, and can change their shape to cylinders or small spheres by Rayleigh’s perturbation process after prolonged spheroidization.     

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic SteelWithout Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide-pearlite steel has been investigated. Particular attention has been given to a quantitative characterization of changes in the ferrite grains. Electron back-scattered diffraction and transmission electron microscopy observations show that the ultrafine ferrite (~388 nm) can be produced through low equivalent strain cold rolling without severe plastic deformation (SPD) and annealing. The average grain size of ferrite depends on the volume fraction, shape and distribution of granular carbides as well as interlamellar spacing of pearlite. A general explanation of granular carbides-assisted grain refinement is that the embedded carbides between natural barrier will significantly facilitate dislocation nucleation during cold rolling. Dislocation reaction occurs more drastically and quickly near these granular carbides. Such reactions promote the formation of high-angle grain boundaries. The formation of ultrafine ferrite grains and subgrains in steel after cold rolling to ε=1.4 strain makes the strength and ductility increased simultaneously compared with ε=0.6 cold-rolled steel. The results suggest a new material design strategy to obtain ultrafine-grained structure via the granular carbides assistance.     

轧后热处理对珠光体钢轨相变组织及硬度的影响

通过在Gleeble-3500热模拟试验机上对珠光体钢轨进行双道次热压缩试验,得到试验钢在轧后不同热处理工艺下的显微组织及硬度,分析热变形后不同冷却速率、等温时间和等温温度对珠光体片层与硬度的影响及其机制。结果表明,1℃/s连冷、1℃/s欠速淬火等温转变后快冷(1℃/s-580℃-30 s)、3℃/s冷却淬火等温转变60 s后快冷(3℃/s-580℃-60 s)、5℃/s高冷速淬火620℃等温转变后快冷(5℃/s-620℃-60 s)试验钢得到珠光体+少量铁素体。而3℃/s连冷、3℃/s冷却淬火等温转变30 s后快冷(3℃/s-580℃-30 s)试验钢因等温时间不足出现了马氏体或贝氏体组织。相比于1℃/s连冷,1℃/s欠速淬火等温转变后快冷对减小珠光体的片层间距以及提高硬度有着积极的作用。延长等温时间后得到的3℃/s冷却淬火等温转变60 s后快冷(3℃/s-580℃-60 s)试验钢的珠光体层间距最细,达到73.19 nm,其片层取向多样,部分渗碳体片断裂,硬度提升幅度不大与类珠光体组织的含量增加有关。1℃/s连冷试验钢的珠光体片层最粗大,硬度最低归因于析出相NbC的过分长大以及断裂...     

低碳钢压缩变形时的形变不均匀性及其对铁素体转变的影响

宏观的形变不均匀性会导致相变和再结晶的差异 ,从而造成最终组织和性能的变化。本文观察并测量了单向压缩的低碳钢形变强化相变、铁素体热、冷形变时样品中心与边缘的铁素体形貌、珠光体或渗碳体分布和织构的差异。分析了不同区域相变及再结晶时的组织演变特点。讨论了形变储存能、渗碳体粒子及形变温度对铁素体相变及再结晶的影响。     

Three Distinct Deformation Behaviors of Cementite Lamellae in a Cold-Drawn Pearlitic Wire

High-resolution transmission electron microscopy is used to investigate the deformation behaviors of cementite lamellae in the heavily cold-drawn piano wires. Three distinct morphologies of cementite are observed, namely, complete lamella, partly-broken lamella and nearly-disappeared lamella. For the complete cementite lamella, it remains a single-crystalline structure. For the partly-broken cementite lamella, polycrystalline structure and neck-down region appear to release the residual strain. The lattice expansion of ferrite takes place in two perpendicular directions indicating that the carbon atoms dissolve from cementite into ferrite lattices. An orientation relationship is found between ferrite and cementite phases in the cold-drawn pearlitic wire.     

温轧温度对中碳钢组织与力学性能的影响

采用透射电镜(TEM)、扫描电镜(SEM)、室温拉伸等手段,研究了650~730 ℃温轧温度对0.46%C中碳钢的组织演变及力学性能的影响。结果表明,经90%的轧制变形,试验钢铁素体晶内引入大量位错,渗碳体片层产生应力集中导致层片状渗碳体弯曲、扭折、碎化为颗粒状。随着温轧温度的降低,位错增殖明显,渗碳体球化率增加,分布越来越均匀,抗拉强度和伸长率整体上升。当温轧温度为650 ℃时,渗碳体球化最好,抗拉强度877 MPa,断后伸长率16.0%,综合力学性能最好。拉伸断口结果表明,随着温轧温度的降低,试验钢的断裂机制由韧-脆混合断裂转变为韧性断裂,塑性提高。     

Magnetic properties of cementite and the coercive force of carbon steels after plastic deformation and annealing

Magnetic hysteresis properties of cementite obtained by the method of mechanical alloying have been studied. It is shown that the strongly deformed cementite is in a low-coercivity state, and the cementite annealed at 500°C is in a high-coercivity state. The need to allow for the contribution of the coercivity of cementite to the coercive force of high-carbon steel is shown. Taking into account this point of view, the behavior of the coercive force depending on the degree of cold plastic deformation by drawing is explained for a number of carbon steels with a structure of fine platelike and globular cementite.     

Graphitization in high carbon commercial steels

Graphitization kinetics in two commercial high carbon steels, AISI types 1075 and 1095, are studied by conducting a series of isothermal annealing treatments in the temperature range of 560 to 680‡ C for periods of time ranging from 20 to 500 h. The samples selected were collected along the processing route in a commercial production line dedicated to the fabrication of thin strip. The structures studied were those of hot rolling (consisting of fine pearlite), cold rolling (spheroidized carbides within a deformed ferritic matrix), and subcritical annealing (spheroidized carbides in undeformed ferrite). The samples obtained from hot rolled coils do not graphitize, whereas those cold rolled graphitize at a rate that depends on the type of steel and degree of deformation. No graphite was found in samples from the lower carbon steel, which were subcritically annealed, although they were observed in specimens from the other steel, which were cold rolled to a reduction of 50% prior to the subcritical annealing.     

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

The formation of fine ferrite grains by the asymmetric rolling of low carbon steel and their mechanical properties were studied. Super-cooled low carbon austenite was deformed by asymmetric rolling at 750 °C with a roll size ratio of 1.5 and immediately cooled at various cooling rates ranging from 3 °C/s to 15 °C/s. Fine ferrite grains (∼2 μm) were formed after asymmetric rolling, preferentially at the prior austenite grain boundaries. The volume fraction of the fine ferrite grains increased with increasing rolling reduction. A ferrite plus pearlite microstructure was obtained at smaller strains and slower cooling rates. However, after heavy deformation, a fine ferrite grain structure with carbide particles dispersed at the ferrite grain boundaries was obtained and the pearlite structure was not observed even after very slow cooling, which implies that most of the ferrite grains were formed dynamically, i.e. during deformation. The yield strength of the asymmetrically rolled steel plates increased with increasing deformation; however, the yield ratio also increased with increasing rolling reduction. The best combination of strength and yield ratio was obtained by using a low level of deformation and a high cooling rate, in which case a portion of the untransformed austenite transformed to martensite.     

碳在变形铁基体中的非平衡固溶

综合文献报道及在研制超高碳钢中所观察到的实验现象,讨论了珠光体在变形中发生渗碳体溶解以及碳在变形铁基体中非平衡固溶的机理,并着重讨论了在高温变形下的情况。