铁素体马氏体双相钢动态加载下的应变硬化行为

利用拉伸Split Hopkinson bar实验装置,对双相钢进行不同应变率下的动态拉伸实验,并应用Johnson-Cook本构模型,对双相钢的动态拉伸性能及应变硬化行为进行分析。结果表明,双相钢具有明显的应变率敏感性;与静态加载类似,在动态加载条件下,双相钢具有较高的初始应变硬化率,随着应变的增加,应变硬化率先迅速下降,而后趋于平缓。     

低碳钢在Ae3温度之上的形变诱导铁素体 (一种马氏体)的相变研究

通过对低碳钢Q235的单向压缩实验,研究了应变、应变速率和变形温度（高于奥氏体铁素体平衡转变温度Ae3）对形变诱导铁素体相变的影响．通过光学显微镜、扫描电子显微镜和X射线衍射仪研究了热变形试样的微观组织结构,利用纳米压痕仪测定了形变诱导铁素体和先共析铁素体的纳米压痕硬度和弹性模量．结果表明,形变诱导铁素体相变可以在Ae3温度之上发生且应变速率和应变越大,相变越容易．在名义应变ε=80％,应变速率ε=20s^-1的条件下形变诱导铁素体相变上限温度为945℃（Ae3＋98℃）．同时发现一个有趣的现象是,在870-920℃区间内变形时,随变形温度下降,应力上升;而在830-870℃区间变形时,随变形温度的下降,整体应力反而下降．与先共析铁素体X射线衍射峰比较,形变诱导铁素体X射线衍射峰明显向小角度方向漂移,形变诱导铁素体的纳米压痕硬度和弹性模量亦明显大于先共析铁素体．实验表明,这种形变诱导铁素体本质上是一种马氏体．     

Simulation of stored energy and orientation gradients in cold-rolled interstitial free steels

The plane strain compression of polycrystalline interstitial free steels is simulated using the crystal plasticity finite element method. In order to capture the interaction between neighboring grains, a simplified quasi 3-D geometric mesh is used in the simulation. The analysis shows that the orientation gradients depend not only on the initial orientation of the grain, but also on the neighboring grains. The simulation shows that the distribution of orientation in the deformed grains can be classified into three types based on the main texture components. The simulations provide a method to quantitatively evaluate the orientation-dependent stored energy in cold-rolled IF steels. The accumulation of stored energy at the specific grain boundaries is also a function of initial orientations as well as neighboring grains.     

Effect of prior hot rolling on the microstructures and mechanical properties of duplex stainless steel containing tempered martensite and ferrite

A duplex structure of δ-ferrite and lath martensite with interlath retained austenite film is developed in this study by modifying the alloy addition. The presence of δ-ferrite can further strengthen the grain refinement of austenite during hot rolling. As a result, the amount of retained austenite is enhanced. Tempered martensite embrittlement occurs due to the decomposition of retained austenite, and grain refinement can in fact ameliorate the tempered martensite embrittlement by delaying the onset of the embrittlement to a higher temperature. The combined effect of uniform and small grains as well as a large amount of retained austenite provides a further increase in the mechanical properties. After identical tempering treatments, all mechanical properties measured in the as-rolled condition were found to be higher than those of direct quenching without rolling. After hot rolling, the increase in the hardness and tensile strength was not accompanied by a drop in the ductility and toughness.     

Formation of martensite during deformation in steel of the transition class

Conculusions The amount of deformation martensite and the level of strengthening for steel 23Kh15N7M2 depend on the deformation method; during upsetting with a small level of deformation the intensity of the -transformation and the level of steel strengthening is greater than with stretching.Institute of Problems of Casting, Academy of Sciences of the Ukrainian SSR. V. I. Lenin Donetsk Metallurgical Plant. Translated from Metallovendenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 61–62, January, 1984.     

超细晶铁素体基体Q&P钢恒应力下的变形行为

采用预淬火热处理工艺得到了超细晶Q&P钢,通过力保载程序控制试验研究了试验钢在某一恒定应力作用下的组织演变和变形行为。结果表明,随着保载应力不断增大,应变量逐渐增加,应变随保载时间的延长略微增加;当加载应力低于屈服强度时,随保载时间延长,应变无变化;加载应力高于屈服强度时,变形量剧烈增加,在变形过程中应变诱导的残留奥氏体向马氏体转变;同时随着应变量不断增加,残留奥氏体的稳定性呈先降低再升高趋势。     

低碳钢铁素体区热变形行为

利用单道次压缩变形试验,对含微量Cr、Mn、Ti的低碳钢在不同变形温度及应变速率条件下的热变形行为,特别是铁素体区的热变形行为进行了研究.结果表明,峰值应力在铁素体区比在奥氏体区随变形温度的降低而提高的速率更快;在750～825℃铁素体区与950～1050℃奥氏体区,峰值应力的大小基本相当;根据流变应力曲线,确定了试验用低碳钢铁素体区的热变形激活能和热变形方程.     

IF钢铁素体区轧制的变形抗力模型

利用Gleeble热模拟试验机对IF钢铁素体区轧制的变形抗力进行了试验研究。通过实测铁素体区不同变形温度、应变速率、变形程度和变形抗力的关系,建立了变形抗力的数学模型。通过对模型进行回归分析,证明该模型具有良好的曲线拟合特性,可为IF钢铁素体区轧制力能参数计算提供准确的数学模型。     

Simulation of deformation twins and deformation texture in an AZ31 Mg alloy under uniaxial compression

To investigate deformation twins and the evolution of deformation texture during plastic deformation, uniaxial compression tests on a hot-rolled AZ31 Mg alloy were carried out at 200 °C. Cylindrical specimens were then compressed in both the rolling and the normal directions. The findings revealed that texture evolution, work hardening and macroscopic anisotropy are strongly dependent on the loading direction. Electron backscattered diffraction analysis was used to examine the orientation of parent grains and twin bands in the AZ31 Mg alloy under uniaxial compression. A viscoplastic self-consistent model (VPSC) was theoretically employed to calculate the relative activities of slip and twin systems in polycrystalline hexagonal aggregates under uniaxial compression. Each deformed grain exhibited an independent number and type of twin variants under uniaxial compression. Neutron diffraction was used to measure the macroscopic texture of the AZ31 Mg alloy. The VPSC model was used to simulate texture evolution, work hardening and macroscopic anisotropy during the uniaxial compression. A modified predominant twin reorientation (PTR) scheme was suggested to explain the gradual increase in twin volume in deformed grains.     

The effect of prior ferrite formation on bainite and martensite transformation kinetics in advanced high-strength steels

Most advanced high-strength steel products contain complex phases, including ferrite, bainite and martensite, which form successively during elaboration. It is essential to understand the effect of prior ferrite transformation on the subsequent bainite and martensite transformation kinetics to achieve precise control of the final microstructure. Nevertheless, the effect of the interface between the prior formed ferrite and the residual austenite (α/γ), together with the related chemical heterogeneity at the interface, on the subsequent phase transformations has been studied only rarely, and remains unclear. This study pays particular attention to the effect of the α/γ interface and its related concentration gradients on bainite and martensite transformation. It is shown that the interface and its related concentration gradients can play a very significant role on the subsequent bainite or martensite transformation kinetics: it retards bainite transformation whereas it accelerates martensite transformation. It is revealed from microprobe wavelength-dispersive spectrometry analysis and model calculations that there are both manganese and carbon gradients in front of the α/γ interface at the end of the ferrite transformation holding. The subsequent bainite transformation kinetics is controlled by the competition between the acceleration effect of the interface boundary itself and the retardation effect of the higher alloying concentration near the interface. Martensite transformation should initiate at the pre-existing dislocations in the center of the residual austenite grains, where the C and Mn contents are the lowest. A simple martensite transformation kinetics model taking into account C heterogeneity is proposed that can describe well the martensite transformation kinetics following the prior ferrite transformation.     

铁素体区形变对含铌微合金高强度钢组织及硬度的影响

采用Gleeble-3800热模拟试验机,研究铁素体区变形对含铌微合金高强度(HSLA)钢组织及显微硬度的影响。结果表明:相比于最终变形在Ar3附近(840℃)时获得近等轴状铁素体+退化珠光体组织,最终变形在铁素体区(550～650℃)时获得部分拉长的多边形铁素体+弥散分布的退化珠光体组织。此外,铁素体区最终变形析出的NbC与晶界析出的Fe3C析出相更加细小且弥散。与高温下最终变形相比,铁素体区最终变形获得的组织显微硬度增量达30 HV。通过强化机理分析表明,位错强化与析出相Fe3C和NbC的沉淀强化作用是铁素体区变形组织硬度提高的主要原因,退化珠光体和渗碳体的细化对硬度的提高也有一定量的贡献。     

Stabilisation of martensite due to shear deformation via variant reorientation in polycrystalline NiTi

The effect of deformation via martensite reorientation on the transformation behaviour of polycrystalline near-equiatomic NiTi was studied. Differential scanning calorimetry measurement showed that both the critical temperature and the endothermic heat of the first reverse transformation of the deformed martensite were increased as compared to an undeformed specimen, indicating a stabilisation effect to the reoriented martensite. Thermal dilatation measurement revealed a two-way memory effect in subsequent transformation cycles after the deformation, suggesting that an anisotropic internal stress field had been established. X-ray diffraction measurement showed a systematic line broadening on the major diffraction peaks with increasing amount of deformation. These experimental observations are discussed in terms of the changes in the internal elastic and irreversible energies associated with the reorientation deformation in a polycrystalline matrix.