双相钢中残余奥氏体对性能的影响及其热稳定化

用声发射法研究了一种低合金钢于双相区热处理所得残余奥氏体的马氏体相变，发现残余奥氏的热稳定化程度和稳定性均与奥氏体的颗粒尺寸有关。颗粒愈小，热稳定化程度愈高，且愈稳定，不存在马氏体核胚的极小奥氏体颗粒不能仅靠过冷来使其转变，形变能诱发试验钢中残余奥氏体转变，且增加钢的塑性，但只有奥氏体颗粒尺寸有合适的分布，其中小部分稳定性很高，才能使马氏体相变随应变增加而逐渐发生并延伸到大的应变，使延伸率明显增加     

淬火配分贝氏体钢不同位置残余奥氏体C、Mn元素表征及其稳定性

采用部分奥氏体化-两相区保温-淬火-配分(IQPB)热处理工艺,借助SEM、TEM、XRD研究了淬火配分贝氏体钢组织形貌及残余奥氏体特征,利用EPMA、EBSD、纳米压痕等表征了不同位置残余奥氏体中合金元素的分布情况,结合室温拉伸应力-应变曲线,研究了C、Mn元素对不同位置残余奥氏体稳定性的影响及其相变规律。结果表明,淬火贝氏体钢室温组织中残余奥氏体以块状和薄膜状形态存在。在拉伸形变过程中,发生TRIP效应,残余奥氏体体积减小,相变优先发生在铁素体晶界,最后发生在贝氏体板条之间,C、Mn元素对残余奥氏体有稳定作用,使残余奥氏体不易发生相变。拉伸断口处应力集中,残余奥氏体完全转变为马氏体,距离断口2和4 mm处,残余奥氏体体积分数分别为3.12%和5.03%。薄膜状残余奥氏体比块状残余奥氏体稳定性更强,并且111γ晶向的残余奥氏体不稳定,容易向马氏体转变。     

17-4PH不锈钢连续冷却转变及相变动力学

通过热模拟试验、X射线衍射、扫描电镜(SEM)、透射电镜(TEM)等方法研究了17-4PH不锈钢连续冷却转变行为、奥氏体稳定化以及马氏体相变动力学。研究发现,在0.05~5 ℃/s冷却速度范围内,17-4PH不锈钢只发生马氏体转变,Ms和Mf分别为116 ℃和35 ℃;在0.05~0.3 ℃/s冷速范围内出现奥氏体稳定化现象,当冷速≥0.5 ℃/s时奥氏体稳定化现象消失。在0.05~5 ℃/s冷速范围内,试验钢中都有少量的NbC析出;而在冷速≤0.08 ℃/s时,试验钢中可析出一定数量弥散的纳米ε-Cu颗粒。此外,根据试验数据构建了预测17-4PH不锈钢马氏体相变动力学的K-M方程。     

TRIP钢单双向应力状态下的材料行为

TRIP钢中残余奥氏体的马氏体相变是TRIP钢相变增塑的重要基础,与材料的宏观应力应变状态密切相关。为探索这一微观材料行为与宏观变形条件间的对应关系,该文对低碳Si-Mn TRIP600钢杯突变形顶部与过渡区的材料行为进行研究。研究结果表明,杯突过程中变形区顶部为等双拉平面应力状态,过渡区为单拉平面应力状态;TRIP钢的马氏体相变对应变条件非常敏感,随变形量的增加,无论变形区顶部还是过渡区的残余奥氏体体积分数,均呈近线性降低趋势,但随应变的增加则呈逐渐饱和趋势;与底部相比,顶部等双拉平面应力状态更利于残余奥氏体的马氏体相变,在近相同应变条件下,等双拉平面应力状态下的残余奥氏体含量明显低于单拉平面应力状态。Si-Mn TRIP600钢在6%应变条件下,等双拉平面应力状态与单拉平面应力状态的残余奥氏体含量相差约7%。     

Q-P-T钢热处理过程中的马氏体相变及其计算机仿真分析

通过Q-P-T钢微观组织EBSD图建立二维有限元模型,利用计算机对由TRIP效应产生的应力松弛对Q-P-T钢微观组织和马氏体相变的影响进行模拟研究。结果表明,应变较小时,马氏体相变基本不发生,随着应变的不断提高,应力松弛效应能显著延缓和抑制Q-P-T钢中残余奥氏体向马氏体的转变。     

元素Nb对C-Si-Mn-Cr-Mo双相钢相变规律的影响

用Gleeble-1500热模拟机测定了C-Si-Mn-Cr-Mo和C-Si-Mn-Cr-Mo-Nb两种试验钢连续冷却转变(CCT)曲线,分析了微合金元素Nb对试验钢相变规律和组织演变的影响.结果表明,Nb可显著抑制试验钢铁素体转变,随着冷却速度增加,Nb的抑制作用逐渐增强,铁素体晶粒尺寸明显细化,显微硬度明显增加.铁素体转变充分时,Nb可提高亚稳奥氏体稳定性和淬透性,但对马氏体和贝氏体产物的显微硬度影响不大.在铁素体转变量很少或未转变的情况下,Nb使得粒状贝氏体产物中马氏体-残余奥氏体(MA)岛数量明显增多,贝氏体显微硬度增加,但其增幅随着冷却速度增加逐渐减小.     

基于动态相变热轧TRIP钢塑性断裂的实验研究

通过热模拟压缩实验,研究了基于动态相变热轧C-Mn-Al-Si系TRIP钢塑性断裂后不同部位的组织。结果表明,实验钢组织中尺寸较大的残留奥氏体,在塑性过程中很快就会发生马氏体相变,从而导致孔洞或裂纹的起源或萌生;而且,裂纹的起源和扩展与组织的铁素体/残留奥氏体（马氏体）密切相关。减小残留奥氏体的晶粒尺寸,提高残留奥氏体的稳定性,不让其在形变初期快速转变为马氏体,有利于TRIP钢强度及塑性的提高。     

预相变马氏体对超级贝氏体钢组织热稳定性的影响

为明确超级贝氏体组织失稳机制以及探索提高超级贝氏体钢中残余奥氏体热稳定性的方法,通过预相变马氏体工艺,即在等温贝氏体相变前引入预相变马氏体,制备了中碳超级贝氏体钢。对比分析了回火前后中碳超级贝氏体钢显微组织和力学性能的变化,研究了预相变马氏体对中碳超级贝氏体钢中贝氏体组织及残余奥氏体热稳定性的影响。结果表明:预相变马氏体的存在能够细化贝氏体铁素体板条,提高残余奥氏体含量和热稳定性。预相变马氏体的引入及其对超级贝氏体组织的细化作用使得试验钢的屈服强度超过1 000 MPa,伸长率大于20%;300~600 ℃回火1 h后,高碳薄膜状残余奥氏体首先发生分解,形成细小的碳化物,然后贝氏体铁素体板条发生回复和再结晶,形成沿原板条方向的铁素体晶粒;600 ℃回火后试验钢的屈服强度仍与回火前相当,主要是预相变马氏体周围的薄膜状残余奥氏体未发生明显分解,能够抑制相邻贝氏体铁素体板条的回复。     

CrNi3Si2MoVNb钢淬火和碳再分配处理的相变行为

用膨胀法和高温激光共聚焦显微镜分别对CrNi3Si2MoVNb钢经淬火和碳再配分(QP,Quenching and Partitioning)工艺的相变及马氏体相变的组织不均匀性进行表征,讨论了组织不均匀性对QP工艺钢微观组织的影响。结果表明,CrNi3Si2MoVNb钢第一次淬火过程中先形成马氏体在某些区域的批次增加,将原奥氏体晶粒分割成尺寸不同的若干区域,尺寸较大的未转变奥氏体稳定性较差,最终淬火过程中容易发生马氏体转变成二次淬火马氏体,而尺寸较小的更易于成为残留奥氏体。     

非调质钢40MnV热变形后冷却相变动力学

为了实现非调质钢轴类件楔横轧热成形后控制冷却过程性能优化和节能减排,采用物理模拟手段对热变形后的非调质钢40MnV进行了连续冷却和等温冷却相变动力学研究。首先采用膨胀仪测定试验钢临界点Ac1、Ac3温度,然后利用Gleeble 3500热模拟机测定了40MnV钢的冷却过程相变动力学转变曲线,并分析了转变产物的显微组织。结果表明:冷却速度增大将减小铁素体的晶粒尺寸,同时也减小铁素体的体积分数;奥氏体晶粒尺寸和钒都对转变温度有影响,但在相变区快冷时奥氏体晶粒尺寸起主要作用,且晶粒尺寸减小将提高转变温度;相变区冷却速度为3℃/s时相变生成贝氏体,且其形态为典型的上贝氏体,而相变区冷却速度为6℃/s时对应转变的组织是马氏体;随着冷却速度的增加,析出过程中V(C,N)的数量增加,最大颗粒尺寸减小;等温冷却中V(C,N)析出物的形态为圆形和方形且数量很少,析出物的颗粒尺寸随转变温度的降低而减小。     

Mechanical Properties and Retained Austenite Stability of High Aluminum TRIP-aided Cold-rolled Steel Sheets

The mechanical properties, microstructure and retained austenite stability of CMnAlSi-TRIP steels were investigated in this paper. The steel sheets were hot-rolled, cold-rolled and heat treated by intercritical annealing and isothermal heat treatment. The microstructure, volume fraction of retained austenite and its carbon concentration were observed by Optical microscopy and X-ray diffraction. The mechanical properties were obtained through uniaxial tensile test. The results show that the CMnAlSi cold-rolled TRIP-aided steels have good combination of strength and ductility with proper isothermal heat treatment, the retained austenite stability determines incremental strain hardening exponent during strain-induced martensitic transformation, and affected by its volume fraction and carbon content. The retained austenite stability has a good correlation with the combination of strength and ductility.     

Effect of Retained Austenite on Properties of Austempered Ductile Iron

Austempered ductile iron (ADI) exhibits a favourable combination of strength and toughness, and has been used as a substitute for quench-tempered or carburise-quenched steel. A characteristic feature of bainite transformation of cast iron, as opposed to carbon steel, is that precipitation of carbide is suppressed by the high concentration of silicon. Thus, a favourable structure, consisting of bainitic ferrite and retained austenite without carbide, can be provided by the optimum austempering treatment. Such microstructure and the mechanical properties of the iron are significantly affected by the conditions of the austempering treatment and the chemical composition. In this study, several grades of ductile iron were austempered under various conditions. The relationship between the impact strength, the quantity of retained austenite and the isothermal transformation curve was investigated. The stability of the retained austenite is considered important, because ADI contains a large amount of retained austenite which contributes to the improvement of ductility and toughness and which may transform to martensite when held at low temperature or subjected to stress. In this study, the stability of the retained austenite at low temperatures was examined by holding or stressing to establish the relations between transformation and temperature, stress and strain.

When the austempering time is short, the untransformed austenite partially transforms to martensite during air cooling, due to the lower carbon content, resulting in lower impact strength. As the austempering time increases, the untransformed austenite is stabilised by carbon-enrichment and there is little transformation to martensite, resulting in a large amount of retained austenite and higher impact strength. When the austempering time becomes much longer, the carbon-enriched austenite decomposes, presumably to bainitic ferrite and carbide, decreasing impact strength. In increasing the silicon content, precipitation of carbide in bainite is suppressed and both the maximum impact value and the content of retained austenite increase. The decreasing rates after the maxima through an additional isothermal holding becomes smaller.

By holding at temperatures down to –40°C, the decrease in retained austenite and the increase in hardness are both small. The retained austenite is stable under stress lower than that required to cause plastic deformation. Compressive stress hinders the martensitic transformation, because the transformation is accompanied by volume expansion.     

Effect of Prior Cold Deformation on the Stability of Retained Austenite in GCr15 Bearing Steel

In this work, the effect of prior cold deformation on the stability of retained austenite in GCr15 bearing steel was investigated after quenching and tempering treatment. The thermal stability was evaluated by calculating thermal activation energy for decomposition of retained austenite using differential scanning calorimeter. The mechanical stability was investigated according to the strain-induced martensitic transformation behavior of retained austenite under the standard compression testing. It is found that the prior cold deformation not only accelerates the carbide dissolution during the austenitization process but also contributes to the carbon partitioning in the tempering stage due to the higher density of phase boundaries, which results in the improvement of the thermal stability of retained austenite. Due to the enhanced carbide dissolution, the higher carbon content in the prior austenite will intensify the isotropic strain of martensitic transformation. As a consequence, the film-like retained austenite is likely to form under a higher hydrostatic pressure and thus shows a higher mechanical stability. Additionally, it is noteworthy that the benefits of the prior cold deformation to the stability of retained austenite would be saturated when the cold deformation degree is larger than 40%.     

On the role of martensitic transformation on damage and cracking resistance in TRIP-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J_R curves and CTOD measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite.     

残余奥氏体增强低碳Q-P-T钢塑性的新效应

低碳Fe-0.25C-1.48Mn-1.20Si-1.51Ni-0.05Nb(质量分数,%)钢通过新型Q-P-T工艺处理后获得高的抗拉强度和良好延伸率的综合性能.对该低碳Q-P-T钢在拉伸过程中残余奥氏体含量的XRD测定和形变孪晶马氏体的TEM观测,证明了相变诱发塑性(TRIP)效应的存在.基于形变过程中马氏体和残余奥氏体中的平均位错密度测定和TEM的观察,验证了在中碳钢中最新发现的残余奥氏体吸收位错(DARA)新效应在低碳钢中同样存在,由此提出了DARA效应产生的条件,阐明了残余奥氏体增强高强度钢塑性的机制.     

几何特征对SiC颗粒增强Al基复合材料力学行为的影响

利用有限元方法建立三维模型分析SiC颗粒在不同形状、不同体积分数和不同尺寸时对Al基复合材料力学行为的影响,并进行拉伸试验研究.结果表明,颗粒形状比颗粒尺寸和颗粒体积分数对材料的应力、应变分布及材料韧性的影响大.颗粒尖角附近有严重的应力集中现象,在外力方向上颗粒端部附近的基体上的应变也有集中现象.随颗粒角度的减小,颗粒的应力很快增大而韧性减小,材料的弹性模量有增大的趋势.随颗粒体积分数的增大,颗粒的应力有减小趋势,材料的弹性模量呈增大趋势.颗粒尺寸较小时(平均尺寸为5 μm),颗粒尺寸对材料的应力及应变的影响小.     

中锰钢中原奥氏体晶粒尺寸对马氏体相变的影响

对中锰钢中原奥氏体晶粒尺寸对马氏体相变动力学的影响进行了分析。利用SEM、XRD、热膨胀相变仪和EBSD等检测手段,研究了不同原奥氏体晶粒尺寸下马氏体相变速率和马氏体板条组织的变化。通过不同温度的奥氏体化处理,分别得到了尺寸为(190±15)、(36±2)、(11±2)和(4.8±2) μm的原奥氏体晶粒。结果表明:随着原奥氏体晶粒尺寸的降低,马氏体相变开始温度由289 ℃降低到250 ℃,而马氏体的相变速率先升高后降低。分析表明,马氏体的相变速率与马氏体的形核点数量直接相关,而马氏体的形核点数量受原奥氏体晶粒尺寸大小和马氏体板条的宽长比影响。当原奥氏体晶粒尺寸小于5 μm时,马氏体板条的宽长比明显增加,马氏体形核点数目随过冷度增加而增加的速率明显降低,从而造成马氏体相变速率降低。     

Thermodynamic analysis of two-stage heat treatment in TRIP steels

In this work, we present a detailed thermodynamic analysis of the two-stage heat treatment (intercritical annealing (IA) and banite isothermal transformation (BIT)) necessary to stabilize retained austenite in transformation-induced plasticity (TRIP) assisted steels. Through a set of experiments on alloys with nominal composition Fe–0.32C–1.42Mn–1.56Si (wt.%), we monitored the evolution of the volume fraction of retained austenite at room temperature as a function of the IA and BIT temperatures. We also investigated the thermodynamic limit for the bainitic transformation during BIT under the displacive (partitionless) transformation assumption. The fraction of retained austenite at the end of the two-stage heat treatment was calculated by taking into account the corresponding start of the martensitic transformation (T_Ms). Comparisons with experiments suggest good qualitative agreement in the fraction of retained austenite when considering the effect of the IA temperature. On the other hand, the analysis of the effect of BIT on the amount of retained austenite showed qualitative disagreement with the observations. To further analyze this discrepancy, we utilize a modified thermodynamic analysis with empirical observations as input, and conclude that the assumption of thermodynamic equilibrium at IA is not valid at lower IA temperatures. Moreover, the unexpected high carbon enrichment in retained austenite indicates the importance of the kinetic effects. We conclude that the thermodynamic limit for the bainitic transformation can be used at least to provide a lower bound to the expected fraction of retained austenite under specific IA + BIT treatment schedules.     

Wear Performance of Cu-Alloyed Austempered Ductile Iron

An investigation was carried out to examine the influence of structural and mechanical properties on wear behavior of austempered ductile iron (ADI). Ductile iron (DI) samples were austenitized at 900 °C for 60 min and subsequently austempered for 60 min at three temperatures: 270, 330, and 380 °C. Microstructures of the as-cast DI and ADIs were characterized using optical and scanning microscopy, respectively. The structural parameters, volume fraction of austenite, carbon content of austenite, and ferrite particle size were determined using x-ray diffraction technique. Mechanical properties including Vicker’s hardness, 0.2% proof strength, ultimate tensile strength, ductility, and strain hardening coefficient were determined. Wear tests were carried out under dry sliding conditions using pin-on-disk machine with a linear speed of 2.4 m/s. Normal load and sliding distance were 45 N and 1.7 × 10⁴ m, respectively. ADI developed at higher austempering temperature has large amounts of austenite, which contribute toward improvement in the wear resistance through stress-induced martensitic transformation, and strain hardening of austenite. Wear rate was found to depend on 0.2% proof strength, ductility, austenite content, and its carbon content. Study of worn surfaces and nature of wear debris revealed that the fine ausferrite structure in ADIs undergoes oxidational wear, but the coarse ausferrite structure undergoes adhesion, delamination, and mild abrasion too.     

淬火温度对5Cr15MoV钢空冷淬火组织与性能的影响

通过1000~1200 ℃间隔50 ℃的系列加热温度对5Cr15MoV马氏体不锈钢进行空冷淬火试验,并采用光学显微镜、EBSD和洛氏硬度计对不同温度淬火后组织和硬度进行检测,研究了淬火温度对试验钢组织、晶粒尺寸、残留奥氏体含量以及硬度的影响。结果表明,试验钢淬火后组织为马氏体+未溶合金碳化物+残留奥氏体。随着淬火温度升高,马氏体板条尺寸增大,未溶碳化物量逐渐减少直至消失,残留奥氏体含量先增加后减少。试验钢的硬度变化趋势为先增加后显著降低,在淬火温度为1050 ℃达到最大值60.8 HRC。试验钢硬度主要是马氏体的含碳量、晶粒尺寸、残留奥氏体含量和碳化物含量综合作用的结果。