MICROSTRUCTURE EVOLUTION OF HYPEREUTECTOID STEELS DURING WARM DEFORMATION I. Formation of Equiaxial Ferrite and Effects of Al

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 结合SEM, TEM和EBSD等方法研究了过共析钢温变形过程中的组织演变规律, 重点讨论了铁素体的等轴化演变过程, 同时考察了合金元素Al的影响. 结果表明: 过共析钢温变形经历片层渗碳体的熔断球化、铁素体的等轴化以及渗碳体的溶解再析出等过程. 温变形初期, 片层状渗碳体缺陷处局部熔断的同时其近邻铁素体内产生大量位错, 并通过动态回复过程形成亚晶; 继续变形过程中, 由于渗碳体粒子钉扎, 亚晶发生转动, 导致大角度晶界的形成, 即铁素体通过连续动态再结晶过程实现等轴化.Al的加入细化了铁素体晶粒尺寸, 提高了等轴状铁素体晶粒大角度晶界的比例.     

晶界非平衡偏聚与晶体缺陷构型的关系

对Ｆｅ－３０％Ｎｉ合金和超低碳贝氏体钢在高温进行不同方式变形和保温,通过变形、回复、与再结晶,获得秒同的晶体缺陷数量与构型（主要是位错分布形态）。用硼径迹显微照相技术研究了 空冷过程中硼向晶界的非平衡偏聚及与不同形态晶体缺陷的关系。结果表明,在再结晶新晶粒冷却时晶界偏聚明显。晶界附近会出现较显著的贫硼现象,而变形并回复后的原始晶粒中此现象不明显,高温回复阶段形成的多边形化亚晶界（位错墙）对冷却时硼     

IF钢退火过程中再结晶行为的EBSD表征

借助EBSD等技术研究了从冷轧到退火过程中IF钢中铁素体再结晶晶粒的取向演变。研究结果表明,从冷轧到退火过程中,铁素体晶粒取向向着平行于法向的[111]晶粒演变,而平行于法向的[100]晶粒逐渐消失;在冷轧变形过程中,铁素体晶粒的晶体取向决定着发生滑移变形的难易程度,与[100]晶粒相比,[111]晶粒更易于发生滑移变形,并在晶粒内部积累大量的位错,储存了大量的应变能,在随后的退火过程中,应变能较高的[111]晶粒优先形核并长大,优先发生再结晶,而应变能较低的[100]晶粒的再结晶受到阻碍。随着退火温度的升高,γ织构([111]//ND)明显增强,其织构组分(111)[112]尤为明显。     

中碳钢过冷奥氏体形变过程中碳的分布与扩散

利用热模拟压缩变形实验以及SEM、XRD和热磁法,研究了中碳钢过冷奥氏体变形时组织演变过程中碳原子的分布与扩散.结果表明,动态相变过程中碳的有效扩散系数与等温过程相比明显增大,相变完成时间显著缩短.在随后的片层状珠光体的球化过程中,相界以及形变过程中产生的高密度位错和空位等缺陷促进了间隙碳原子的扩散,使得球化动力学过程与等温退火相比显著缩短.渗碳体的溶解和铁素体中碳的过饱和现象得到证实,其中过饱和碳原子高度聚集在铁素体晶界和位错核心处,而不是均匀地分布在铁素体点阵的间隙位置.     

Plastic flow characteristics of ultrafine grained low carbon steel during tensile deformation

In order to explain steady-state plastic deformation, i.e. the absence of strain hardening in ultrafine grained low carbon steel during tensile deformation, steel of different ferrite grain sizes was prepared by intense plastic straining followed by static annealing and then tensile-tested at room temperature. A comparison between the ferrite grain size of ultrafine grained steel and the dislocation cell size of coarse grained steel formed during tensile deformation revealed that uniform dislocation distribution with high density and cell formation were unlikely to occur in this ultrafine grained steel. This is ascribed to the fact that the ultrafine grain size is comparable to or smaller than the cell size at the corresponding stress level. In addition, from a consideration of dynamic recovery, it was found that the characteristic time for trapped lattice dislocations to spread into the grain boundaries was so fast that the accumulation of lattice dislocation causing strain hardening could not occur under this ultrafine grain size condition. Therefore, the extremely low strain hardening rate of ultrafine grained low carbon steel during tensile deformation is attributed to the combined effects of the two main factors described above.     

Dislocation Source and Pile-up in a Twinning-induced Plasticity Steel at High-Cycle Fatigue

Dislocation behaviour of a twinning-induced plasticity(TWIP) steel subjected to high-cycle fatigue tests is investigated in the present study. Grain boundaries are the important sources of dislocation generation during fatigue tests, contributing to the increase in dislocation density. Continuous emission of dislocations from grain boundaries is observed in many grains. Inclusions can sustain large dislocation pile-ups at the inclusion interfaces, leading to a high stress concentration and therefore acting as potential sites of microcrack nucleation. In contrast, annealing twin boundaries are relatively weak boundaries for dislocation pile-ups. When the number of dislocations in a pile-up is large, dislocations can crossover twin boundaries and glide inside the annealing twins. The stress concentration at the twin boundary is relatively low so that twin boundaries could not act as the sites for microcrack initiation.     

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.     

Mechanism of Stress Relief Cracking in a Granular Bainitic Steel

Coarse-grained heat-affected zone(CGHAZ) of a low alloyed,granular bainitic steel T24 was simulated in a Gleeble apparatus.The stress relief of the CGHAZ was analyzed by annealing the samples.The morphology and behavior of the microstructure near the grain boundaries during stress relief were investigated by means of focused ion beam,in situ tensile testing,transmission electron microscopy,scanning electron microscopy and electron back-scatter diffraction.It was observed that there were large martensite/austenite islands distributed along the grain boundaries of CGHAZ.During stress relief at elevated temperature,the retained austenite at the grain boundaries decomposed into M_3C carbides and a ferrite forming softening zone.Together with the stress relief,piled up of dislocations occurred within the ferrite in the area adjacent to the ferrite/M_3C interface,which resulted in high level of stress accumulation and caused crack initiation along the grain boundaries.These results indicate that the stress relief induced the grain boundary crack is controlled by other mechanisms rather than the creep-like grain boundary sliding.     

退火时间对冷轧中碳Cr-Mo钢微观组织的影响

通过SEM观察和EBSD技术研究了冷轧中碳Cr-Mo钢在600 ℃退火不同时间后的组织演变规律。结果表明,随退火时间的延长,渗碳体颗粒分布越弥散,并且球化越来越明显,变形后的铁素体条带转变为铁素体等轴晶粒。晶粒内部渗碳体颗粒较小,晶界处渗碳体颗粒较大,部分呈连续分布,且晶粒尺寸不断增大,但在退火时间小于240 min时,晶粒长大不明显,进一步延长保温时间后,晶粒尺寸增长变快。当保温时间从15 min延长到120 min和480 min时,铁素体平均晶粒尺寸从0.670 μm长大到0.732 μm和2.000 μm。在退火0~120、120~240、240~360和360~480 min时段的晶粒长大速率分别为55.7%、9.7%、74.3%和42.9%,其中0~15 min时晶粒尺寸的增长比例高达42.6%。在退火0~120 min时段,晶粒长大速率相对较大,从节约能源的角度考虑,退火时间可以定在120 min。     

A new low carbon steel microstructure: Ultrafine ferrite grains with homogeneously distributed fine cementite particles

Low carbon steels containing carbon less than 0.2 wt.% are the most widely used ferrous alloys in structural application. These steels consist of ferrite of large volume fraction with pearlite as the remainder and exhibit a strength of ∼400 MPa. To date, considerable effort has been devoted to enhancing the strength of these steels. However, existing methods of improving their strength are limited by the counter effect of loss of ductility and toughness. To overcome this deficiency, a new low carbon steel microstructure and its processing route are reported in this study. The steel with the new microstructure-submicrometer scale equiaxed ferrite grains with fine cementite particles distributed uniformly—was manufactured by imposing severe plastic deformation to introduce ultrafine ferrite grains and subsequent static annealing for uniform precipitation of nanosized cementite particles. The strength of the steel with the new microstructure increased nearly 100%, over 800 MPa, without significant loss of ductility.     

TEM STUDY OF AUSTENITE FORMATION IN A LOW CARBON STEEL

<正> 低碳铁素体加马氏体双相钢的强度和塑性取决于马氏体的数量和分布,这与亚温区加热过程中奥氏体形成的特点有关。由于奥氏体的形成是在高温下进行,速度快,难于直接获得有关形核的信息。本文研究低碳钢中奥氏体形成的部位,以及预先冷轧的影响。 试验用低碳1.5Mn钢,其化学成分为(wt-％):C 0.08,Mn 1.45,si 0.21,Al 0.045,N 0.005。经高频感应电炉熔炼成45kg的钢锭后,热轧成2.5mm厚的板材。在真空炉中经过1200℃均匀化退火24h。然后,在盐炉中900℃加热15min后空冷,获得铁素体加珠光体型的正火原始组织。另将一部分正火的坯料进行厚度压下量     

低碳钢形变强化相变的特征

介绍了低碳钢形变强化相变的基本概念及主要特征．系统的研究工作证实了变形显著地加速了低碳钢过冷奥氏体向铁素体的相变过程．形变强化相变是一个以形核为主导的过程直到相变完成以前，形核始终存在于新相与原奥氏体相界面的高应变区．由于几何空间与成分条件上受到一定的限制，长大及各向异性都不太明显，铁素体晶粒超细化．实验工作还证实了转变动力学呈现明显的3个阶段，它们分别与相变铁素体在原奥氏体晶界上的形核，在铁素体／奥氏体相界前沿高畸变区的形核，及被铁素体晶粒所包围的残存奥氏体上的相变形核等过程相对应。     

温度对低碳钢微观组织与高温力学性能的影响

利用场发射扫描电镜(FE-SEM)、电子背散射衍射技术(EBSD)与电子万能试验机对低碳钢不同温度下的微观组织与高温力学性能进行了详细的研究与讨论。结果表明,无论室温拉伸还是高温拉伸,位于晶界上的碳化物(Fe₃C)颗粒是诱发低碳钢裂纹的主要因素。与室温拉伸性能相比,提高加热温度,抗拉强度明显下降,伸长率显著增加。在高温下,随着温度的提高,抗拉强度线性下降,而伸长率先降低而后趋于稳定。在520 ℃拉伸过程中,低碳钢中产生了大量的滑移带,诱发了动态回复。提高温度至720 ℃时,珠光体组织发生球化,形变铁素体晶粒内出现等轴状小晶粒,即发生了动态再结晶;经EBSD分析,形变铁素体晶粒间取向差较大,而其发生再结晶的等轴小晶粒间取向差较小。     

Grain refinement mechanism during equal-channel angular pressing of a low-carbon steel

The grain refinement mechanism during equal-channel angular pressing of a plain low-carbon steel was explored by a careful analysis of the slip systems operating at each pass of repetitive pressing. The steel was subjected to one to eight passes of pressing, in which a single passage yielded an effective strain of ∼1, at 623 K. At the initial stage of pressing, submicrometer-order ferrite grains enclosed by serrated and low-angled boundaries were formed. Transmission electron microscopy examination revealed that these boundaries resulted from interaction between the slip systems that are typical in body-centered cubic structures. Further pressings mainly resulted in rotation of ultrafine subgrains rather than grain refinement, providing the formation of high-angle grain boundaries. Since the serrated boundaries restrict dislocation movement, the rotation of subgrains with the serrated boundaries is more favorable for accommodating further deformation than intragranular strain, and therefore boundaries become high-angled.     

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate ε carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.     

Mn-Si-Cr系中碳钢在过冷奥氏体状态下变形时的显微组织演变

将Mn-Si-Cr系中碳钢在过冷奥氏体状态下进行低速率变形, 变形促进先共析铁素体转变, 但未见层状珠光体形成. 铁素体在奥氏体晶界和晶内形核, 以近似等轴状长大、交联, 并分割奥氏体, 形成富碳奥氏体区. 随着变形量的增大, 铁素体可在富碳奥氏体区内部继续形核长大并交联, 导致富碳奥氏体区不断被分割且碳浓度升高, 当碳浓度足够高时, 一次析出球状碳化物可在富碳奥氏体区边界处形成, 尺寸为0.5-1 μm; 变形过程中铁素体的动态回复和再结晶导致碳原子从Cottrell气团中逸出, 在铁素体内部形成几十纳米的二次析出球状碳化物.     

Zr-Ti复合脱氧对低合金高强度钢CGHAZ组织和韧性的影响

对Zr-Ti复合脱氧低合金高强度30mm厚钢板采用气体保护焊进行多层多道焊接,利用光学显微镜,扫描电子显微镜及能谱分析对焊接热影响区组织进行了观察与分析．结果表明,该区域存在大量微米、亚微米级的复合铬、钛氧化物夹杂,呈细小弥散分布,没有发现条状MnS夹杂物的存在．夹杂物粒子在晶界和晶内析出,钉扎奥氏体品界,有效抑制了奥氏体晶粒长大,晶粒粗化不明显．在粗晶区奥氏体晶粒内部优先形成的针状铁素体,能有效分割奥氏体晶粒,细化组织．焊接热影响区粗晶区的力学性能测试表明,Zr-Ti复合脱氧技术使焊件具有良好的低温冲击韧性．     

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.     

Effects of the annealing temperature and time on the microstructural evolution and corresponding the mechanical properties of cold-drawn steel wires

The effects of the annealing temperature and annealing time on the microstructural evolution and corresponding mechanical properties of cold-drawn high carbon steel wires were investigated. During the annealing of cold-drawn steel wires, the increment of the tensile strength at low temperatures found to be due to age hardening, while the decrease in the tensile strength at high temperatures was attributed to age softening, involving the spheroidization of lamellar cementite and recovery of lamellar ferrite. To investigate the mechanisms of strain ageing, a thermal analysis using DSC was performed. The mechanisms for the first and second stages were found to be the diffusion of carbon atoms to dislocations in the lamellar ferrite and the decomposition of lamellar cementite. The third peak of the DSC curves was controlled by the re-precipitation of cementite or by the spheroidization of lamellar cementite.     

Stages and Fracture Mechanisms of Lamellar Pearlite of 100-m-Long Differentially Hardened Rails Under Long-Term Operation Conditions

Using the methods of transmission electron microscopy, the carbide phase evolution in surface layers of the differentially quenched rails is studied after the passed tonnage of 691.8 million tons at the depth up to 10 mm along the central axis and fillet of rail head. The action of two mutual supplement mechanisms of steel carbide phase transformation in surface layers at rail operation is established: (1) cutting mechanism of cementite particles with the following departure in the volume of ferrite grains or plates (in pearlite structure); (2) cutting mechanism and following dissolution of cementite particles, transfer of carbon atoms on dislocations (in Cottrell atmospheres and dislocation cores), transfer of carbon atoms by moving dislocations into ferrite grains volume (or plates) with the following repeated formation of nanosized cementite particles. The first mechanism is accompanied by the change in linear sizes and morphology of carbide particles. Cementite element composition change is not essential. Carbide structure change can take place during the second mechanism.