钢中贝氏体相变的论争及前景

简单地阐述了钢中贝氏体相变理论研究的2个学派学术论争的情况，并对某些观点应用自然辨证法的哲学理论进行了分析和纠正，指出，珠光体分解与贝氏体相变有着本质的区别，贝氏体组织是整合系统，不是混合物．应当把“过冷奥氏体→珠光体、贝氏体、马氏体”转变系列作为一个整合系统采研究．贝氏体相变是属于扩散型和切变型相变之间的中间过渡型相变的总体认识，是统一认识的基本条件．贝氏体相变具有扩散一切变整合机制．     

钢中贝氏体相变理论研究的新进展

总结了近年来内蒙古科技大学在贝氏体相变领域的主要研究成果.指出过渡性是贝氏体相变的主要特征;提出了贝氏体和贝氏体相变的新定义;认为以往的热力学计算不准.贝氏体铁索体的相变驱动力约为905 J/tool;提出了切变一扩散整合机制,贝氏体相变的晶核是单相BF,不是共析分解,贝氏体铁索体(BF)在贫碳区形核,是贫碳的Pa的无扩散相变,不是切变过程,而是以界面替换原子热激活跃迁方式形核长大;钢中贝氏体碳化物(Bc)在γ/a相界面上形核,向奥氏体和铁索体中长大,最终被铁索体包围,是以原子热激活跃迁方式进行的扩散型相变.     

钢中贝氏体相变热力学

应用综合理论分析的方法研究了钢中贝氏体相变热力学.指出以往的KRC,LFG模型不适于贝氏体相变驱动力的计算.在进行相变热力学分析的基础上,依据γ→αB γ1→BF γ1转变机制设计了新的计算模型,并估算了Bs温度下相变阻力为105 J/mol.指出相变不仅与驱动力有关,而且取决于原子扩散能力.在贝氏体上部温度区,可以依靠界面扩散进行台阶长大;在460 ℃以下的某一段温度,可能以原子热激活跃迁无扩散机制进行贝氏体铁素体形核-长大过程;在Ms点稍上一段温度,可能以切变方式进行.     

贝氏体相变理论研究新进展

贝氏体相变理论研究具有重要理论意义和应用价值.综合阐述了贝氏体相变学术论争的焦点问题.通过大量试验事实和深入的理论分析,批驳了两派的错误观点.指出了贝氏体的正确的定义;贝氏体相变的过渡性特征,贝氏体相变与共析分解和马氏体相变存在原则区别.贝氏体浮凸为帐篷形,非N型,不具备切变特征.从热力学、动力学、晶体学角度阐述了贝氏体相变非切变,也非扩散过程.提出了贝氏体铁素体(BF)、贝氏体碳化物(BC)形核长大新机制.贝氏体相变过程中原子移动的特点是界面控制,原子非协同热激活跃迁机制,并且与实际相符合.     

块状转变与钢中贝氏体相变的亲缘关系——再论贝氏体相变的切变扩散整合机制

应用JEM-2010高分辨电镜和Quanta-400型环境扫描电镜,运用试验与综合分析的方法,研究了纯铁的块状转变和钢中的贝氏体相变,通过对相变的形核、长大,贝氏体亚单元和组织的形成的综合研究和分析,认为贝氏体相变与块状转变存在亲缘关系.依靠随机涨落,形成贫碳区,贝氏体铁素体在贫碳的奥氏体中形核.Fe原子和替换原子通过热激活跃迁、界面扩散或切变等方式,重复产生亚单元.在亚单元边界处,富碳奥氏体析出碳化物或成为残留奥氏体.贝氏体相变机制具有过渡性,即切变扩散整合机制.     

过冷奥氏体转变时原子位移规律

研究过冷奥氏体转变时原子的位移方式具有重要理论价值.原子位移方式不同是区别相变机制的依据之一.过冷奥氏体转变为珠光体、贝氏体、马氏体等组织,最基本的区别是原子位移方式的不同,从高温到低温,原子位移方式是逐渐演化的.研究表明,三个相变中原子位移方式分别是:(1)高温区的共析分解是扩散型相变,原子每次位移距离为一个原子间距.界面扩散是主要位移方式;(2)贝氏体相变是过渡性相变,贝氏体铁素体的形成是无扩散过程,而贝氏体碳化物的形成是扩散过程.铁原子和替换原子进行热激活跃迁位移.原子位移距离小于一个原子间距,各原子位移矢量不等,界面控制.(3)马氏体相变是无扩散的,所有原子进行集体协同位移,原子每次移动距离远远小于一个原子间距.切变机制不正确.在晶格改组过程中,为调整应变能一般需要形成位错、孪晶或层错等亚结构.     

珠光体到贝氏体的演化

综合以往的科学研究结果,论述了从珠光体分解到贝氏体相变是1个逐渐演化的过程.批驳了扩散学派所说的贝氏体是共析分解产物,所谓贝氏体是珠光体转变的延续的观点.指出了珠光体与贝氏体在扩散、形核、动力学、组织形貌、亚结构等方面的区别.     

Fe－C合金贝氏体相变机制

贝氏体在奥氏体贫碳区马氏体式切变相变机制为：γ→γ＇（贫碳）＋γ１（富碳）→ｘ＇（与ｙ’同成分）＋γ１→ＢＦ＋γ１．整个相变过程受碳扩散控制．热力学分析表明，贝氏体切变形成具有热力学可能性，贝氏体相变具有阶段性特点．     

合金钢贝氏体切变机制热力学分析

对合金钢贝氏体在贫碳奥氏体区相变机制进行了热力学分析．结果表明，贝氏体相变驱动力随贫碳奥氏体区合碳量的减小而增加，并能克服其相交阻力，贝氏体在贫碳奥氏体区马氏体式切变形成具有热力学可能性．     

β黄铜中贝氏体的形态及亚结构

用透射电镜高温原位观察了β黄铜中贝氏体的应力诱发生长形态,并根据晶体学分析否定了这些形态为感应形核,发现了剪切应力场存在的证据．在初生贝氏体中发现存在层错亚结构．结果支持贝氏体的切变机制．     

TEM and HREM study on the fine structure and the interfacial structure of bainite

The fine structure of bainite,the morphology and distribution of carbide in steels and the morphology of bainite in Cu-Zn-Al alloys have been investigated with TEM.The interfacial structure ledges and interfacial crystal lattice images of Cu-Zn-Al alloys have also been investigated with HREM.The addition of alloying microelements can fine the structure of bainitic ferrite markedly.The bainitic ferrite is composed of subunits or subchunks.The carbides differ in morphologies and are distributed in between laths,inside the plates and on the boundaries of subunits.There are abundant fine structures in bainitic ferrite.In the primary bainite of Cu-Zn-Al alloy there are interfacial structure ledges,the height of which is about 3 -40 nm,equal to 15-200 atomic layers.The phase transformation mechanism of bainite is discussed.     

Refinement of packet size in low carbon bainitic steel by special thermo-mechanical control process

The packet size of bainitic steel can be refined by a special relaxation-precipitation-control phase transformation (RPC) technology, When processed by RPC process, the low carbon bainitic steel composes of two kinds of main intermediate transformation phases. One is ultra-fine lath-like bainitic ferrite and the lath is less than 1 tam in width and about 6 tam in length; the alignment of laths forms a refined packet, and the size of packets is about 5-7 ~am in length and about 3-4 tam in width, The other is acicular structure. The morphology and distribution of these acicular structures are influenced by relaxation process, the thin and short acicular structures cut the prior austenite grain and refine the bainitic packet size, For the optimum relaxation time, the packet size can be refined to the finest. The mechanical properties are influenced by relaxation time and the 800 MPa grade low carbon bainitic steel with excellent toughness can be obtained by RPC process,     

TEM study of transformation units and the growth mechanism of bainite

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Effect of zirconium addition on the austenite grain coarsening behavior and mechanical properties of 900 MPa low carbon bainite steel

The ultra-free bainitic microstructure of a 900 MPa low carbon bainitic Cu-Ni-Mo-B steel was obtained by a newly developed relaxation precipitation control (RPC) phase transformation processing.In a pan-cake like prior-anstenite grain,the microstructure consisted of lath bainite,a little of abnormal granular bainite,and acicular ferrite.The effect of zirconium carbonitrides on the austenite grain coarsening behavior was studied by transmission electron microscopy (TEM).The results show that,the lath is narrower with increasing cooling rate.The ratio of all kinds of bainitic microstructure is proper with the intermediate cooling rate;and Zr-containing precipitates distribute uniformly,which restrains austenite grain growing in heat-affected welding zone.     

残余奥氏体及其机械稳定性与钢强韧性关系的研究

钢中残余奥氏体发生应变诱发马氏体相变的百分量与拉伸应变量的对数呈线性关系,用直线斜率的倒数Ks值可表示残余奥氏体机械稳定性的大小。提高钢中残余奥氏体量及其机械稳定性是改善钢强韧性的有效途径之一。残余奥氏体发生应变诱发马氏体相变吸收能量是提高钢韧性的主要原因。在低碳贝氏体钢中,以M—A岛形式存在的残余奥氏体因受岛中马氏体的强化作用使屈服强度保持较高水平,而残余奥氏体在拉伸的均匀塑性变形阶段因应变诱发相变形成的马氏体使钢的抗拉强度提高。     

Influence of isothermal bainitic processing on the mechanical properties and microstructure characterization of TRIP steel

The mechanical properties of transformation induced plasticity (TRIP) steel are strongly affected by the conditions of iso-thermal bainitic processing. The multiphase microstructure of TRIP steel under different conditions of isothermal bainitic processing was investigated using OM, SEM, XRD and TEM. The volume fraction of retained anstenite and the carbon content in anstenite were determined quantitatively using X-ray diffraction patterns. The relationship between mechanical properties and isothermal bainitic processing parameters was investigated. The stability of retained anstenite was analyzed by the volume fraction of retained austenite and the carbon content in retained anstenite. The experimental results show that the multiphase microstructure consists of ferrite,bainite and metastable retained austenite. To obtain good mechanical properties, the optimal conditions of isothermal bainitic tem-perature and holding time are 410-430℃ and 180-240 s, respectively. After isothermal bainitic processing under the optimal condi-tions, the corresponding volume fraction of retained anstenite is 5vol%-15vol%, which can provide enough retained austenite and plastic stability for austenite with high carbon content.     

Combined Reconstructive and Displacive Mechanisms Involved in Bainitic Transformation

TEM observation and analysis have been conducted on bainitie transformation with and without the influence of externally applied tensile sress for alloyed steel 35MV7. Recrystallizafion was found to occur within the bainitic structures transformed at 450 ℃ in cases of both with and without the application of external stress, and coupling between the reconstructive and displacive mechanisms is expected, due to the relatively high holding temperature and high dislocation density yielded with the displacive mechanism. RecrystaUization was not observed within the bainitic structures transformed at lower temperature of 350 ℃, both with and without the application of stress; However, for the stressed specimen, the structure with very fine subgrains was found to be preserved and not reconstructed thermically, due to the low temperature and short holding time.     

Microstructure evolution during reheating of a Nb-bearing steel II. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unrelaxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.     

Effect of cooling parameters on the microstructure and properties of Mobearing and Cr-bearing steels

To develop low-cost low carbon bainitic steel,Mo-bearing and Cr-bearing steels were melted in a vacuum induction furnace and were researched by thermal simulation and hot rolling at the laboratory.As the cooling rate increases from 0.2 to 50℃/s,the transformation temperatures of two steels lie between 650 and 400℃,and the final microstructures of them change from quasi-polygonal ferrite and granular bainite to lath bainite.Compared with cooling in air or by interrupted cooling,Mo-bearing and Cr-bearing steel plates cooled by sprayed water boast higher strength and superior toughness,for large-size islands are responsible for the poor mechanical properties.Compared to Mo,Cr is effective to isolate the bainitic reaction in low carbon steel,and the bainitic microstructure can also be obtained in Cr-bearing steel cooled at a wide range of cooling rate.