Fe－Ni合金贝氏体长大原位观察

利用透射电镜观察了Ｆｅ—３０ｗｔ—％Ｎｉ合金贝氏体铁素体／奥氏体界面结构，并利用透射电镜高温样品台对贝氏体铁素体台阶长大进行了原位动态观测。结果表明，铁素体呈台阶长大形态，在铁素体／奥氏体宽面上存在Ｂｕｒｇｅｒｓ矢是为［１１１］ｂ型的刃型错配位错，铁素体台阶的增厚速率与按Ｚｅｎｅｒ—Ｈｉｌｌｅｒｔ方程计算结果相符。为关于贝氏体生长台阶长大机制提供实验证据。     

低碳钢中晶界铁素体/原奥氏体界面对贝氏体转变的影响

采用电子背散射衍射 (EBSD) 研究了低碳Fe--C--Mn--Si钢中晶界铁素体/原奥氏体界面对贝氏体形核的影响. 通过两阶段等温热 处理, 获得了晶界铁素体和贝氏体的混合组织. 结合金相观察和取向测量, 发现晶界铁素体与贝氏铁素体之间的界面分为两种, 一种界面不清晰, 一种界面清晰. 分析表明, 在晶界铁素体/贝氏体界面不清晰一侧, 晶界铁素体与原奥氏体保持取向关系, 贝氏体在这类界面形 核生长, 且取向与晶界铁素体保持一致; 在晶界铁素体/贝氏体界面清晰一侧, 晶界铁素体与原奥氏体无取向关系, 且贝氏体与晶界铁素体之间取向差较大.     

关于60Si2MnA钢奥氏体中温转变的研究

60Si2MnA钢过冷奥氏体的中温转变产物为贝氏体铁素体和残余奥氏体,贝氏体铁索体的形成与自催化效应密切相关。在回火过程中,贝氏体中的奥氏体以扩散转变方式分解为铁素体和渗碳体。从而说明奥氏体的等温转变与回火转变是本质不同的转变。     

Analysis of transformation stasis during the isothermal bainitic ferrite formation in Fe–C–Mn and Fe–C–Mn–Si alloys

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation has been investigated in a series of Fe–C–Mn and Fe–C–Mn–Si alloys. The Gibbs energy balance (GEB) approach is applied to model the transformation stasis phenomenon, and theoretical predictions are compared with experimental observations. The good agreement over several alloy systems demonstrates that the transformation stasis is caused by diffusion of alloying elements into the migrating austenite/bainitic ferrite interfaces. It is found that the occurrence of transformation stasis in the Fe–C–Mn and Fe–C–Mn–Si alloys depends on the concentration of Mn, while the addition of Si has a negligible effect on transformation stasis. The GEB model clearly outperforms the diffusionless T₀ model.     

钢中贝氏体长大方式的TEM研究

利用ＴＥＭ考察了钢中贝氏体铁素体／奥氏体相界面台阶结构及奥氏体精细结构。用ＴＥＭ温台发现在贝氏体增厚过程中新形成的贝氏体中存在奥氏体预存孪晶；在贝氏体铁素体／奥氏体相界面存在台阶结构，但台阶阶面可对应于母相奥氏体中孪晶面或层错面，表明台阶的阶面为共格的滑移界面。因而贝氏体铁素体／奥氏体相界面可通过界面台阶沿面缺陷进行保守滑移，贝氏体长大具有滑移切变特征。根据实验结果提出了贝氏体二次层错切变模型及组织形貌示意图。     

Nb对中低碳超级贝氏体钢组织与性能的影响

针对无Nb和0.05wt%Nb两种中低碳钢,研究了Nb对0.25wt%C超级贝氏体钢组织与性能的影响。结果表明,对两种试验钢采用轧后先快冷后缓冷的等温工艺,均可获得贝氏体组织。300 ℃等温8 h,含Nb钢贝氏体含量达到80%,屈服强度提高12% (109 MPa),冲击吸收能量达到52 J。通过Thermal-Calc软件计算并结合TEM观察发现,含Nb钢中Nb元素与Mo等元素形成复杂碳化物,析出的细小碳化物钉扎板条边界,细化贝氏体板条,抑制板条合并与粗化,提高板条的稳定性。等温8 h后,含Nb钢的贝氏体铁素体板条尺寸在150~200 nm之间。利用背散射电子和EBSD分析发现,Nb元素通过促进碳化物的析出,降低过冷奥氏体稳定性,促进贝氏体转变,抑制马氏体转变,细化残留奥氏体,提高了组织的均匀性和稳定性,是性能提高的主要机制。     

Carbon supersaturation of ferrite in a nanocrystalline bainitic steel

The extremely slow transformation kinetics of a nanocrystalline bainitic steel allows the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography as the bainite transformation progresses at 200 °C. A high level of carbon, well above that expected from para-equilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.     

下贝氏体中碳化物的析出

通过3组实验证实了下贝氏体碳化物是在α/γ界面的奥氏体一侧析出,并向奥氏体中生长,同时也在贝氏体亚单元之间析出.在形态上,碳化物存在于铁素体内,也可跨越α/γ或α/α界面;在整个相变过程中,铁素体与碳化物的长大呈相互竞争机制,碳化物从α/γ界面的γ一侧析出,但铁素体的长大速度远高于碳化物,二者长大的结果是铁素体将碳化物包围,导致碳化物似乎是由铁素体中析出的假象;用热力学及台阶理论对实验结果做了分析,在实验证据及理论分析的基础上,提出了下贝氏体碳化物的析出及长大模型;下贝氏体碳化物是由富碳残余奥氏体中析出而不是由碳过饱和的铁素体中析出,其实质是确定新形成的贝氏体铁素体中是否含有过饱和碳,这直接涉及贝氏体相变机制的类型.     

Modeling austenite decomposition into ferrite at different cooling rate in low-carbon steel with cellular automaton method

The austenite decomposition into ferrite during continuous cooling in low-carbon steel has been investigated with a two-dimensional cellular automaton (CA) approach. In this model, the growth of ferrite grain is controlled by both carbon diffusion and γ–α interface dynamics. In order to predict the growth kinetics of ferrite grain, the coupled carbon diffusion behavior in untransformed austenite and γ–α interface dynamics are numerically resolved. The simulation provides an insight into the carbon diffusion process in retained austenite and microstructure evolution during the transformation. The predicted ferrite growth kinetics and average grain size at different cooling rates are compared with experimental results in the literature and the simulated results show that the final grain size and newly formed ferrite fraction vary with cooling rate. The γ–α interface is stable in the studied cooling rate range (up to 58 °C s⁻¹) in this work, so the simulated morphology of ferrite grain is almost equiaxed, which is not influenced by the anisotropy of the hexagonal mesh in this CA model.     

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm³. The presence of aluminium and phosphorus further stabilizes the austenite grains.     

Development of a kinetic model for bainitic isothermal transformation in transformation-induced plasticity steels

In this work, we modify existing models to simulate the kinetics of bainitic transformation during the bainitic isothermal transformation (BIT) stage of a typical two-stage heat treatment – BIT is preceded by an intercritical annealing treatment – for TRIP steels. This effort is motivated by experiments performed in a conventional TRIP steel alloy (Fe–0.32C–1.42Mn–1.56Si) that suggest that thermodynamics alone are not sufficient to predict the amount of retained austenite after BIT. The model implemented in this work considers the non-homogeneous distribution of carbon – resulting from finite carbon diffusion rates – within the retained austenite during bainitic transformation. This non-homogeneous distribution is responsible for average austenite carbon enrichments beyond the so-called T₀ line, the temperature at which the chemical driving force for the bainitic transformation is exhausted. In order to attain good agreement with experiments, the existence of carbon-rich austenite films adjacent to bainitic ferrite plates is posited. The presence of this austenite film is motivated by earlier experimental work published by other groups in the past decade. The model is compared with experimental results and good qualitative agreement is found.     

Kinetic transition during the growth of proeutectoid ferrite in Fe-C-Mn-Si quaternary steel

The kinetics of ferrite growth in Fe-0.1C-1.5Mn-0.94Si (mass pct) quaternary steel is investigated through the characterization of isothermal growth behavior, the thermodynamic prediction of kinetic boundary and the diffusional growth simulations using DICTRA. The change in microstructural evolution from slow growth to fast one is consistent with the calculated change of interface condition from the partitioning local equilibrium (PLE) to the negligible partitioning local equilibrium (NPLE). Compared with the DICTRA simulation, the observed growth kinetics of ferrite are between the calculated ones assuming local equilibrium (LE) and paraequilibrium (PE) criterions. At temperatures below the PLE/NPLE kinetic boundary, the observed growth behavior can be reasonably described by kinetic transition from PE to NPLE condition as isothermal time elapses, taking into account the critical velocity of interface at which trans-interface diffusion of subsitutional element permits the transition from PE to NPLE growth.     

钢中贝氏体预相变过程的研究

用扫描俄歇显微探针（ＡＥＳ）分析了６５Ｓｉ２Ｍｎ钢中温转变孕育期内碳原子的成分变化。结果表明，在中温转变孕育期等温时，奥氏体晶界附近及晶内均形成可稳定存在的贫碳区。贝氏体预相变的实质是碳原子向晶界及其他晶体缺陷扩散偏聚而形成贫碳区的过程。中温转变孕育期内形成贫碳区具有热力学及动力学可能性。     

Development of ferrite/bainite bands and study of bainite transformation retardation in HSLA steel during continuous cooling

The development of banded structure in HSLA steel during continuous cooling has been systematically studied using dilatometry and microstructural observations. At low cooling rates (<10 °C/min), the microstructure contains alternating bands of ferrite and a mixture of pearlite and bainite. At higher cooling rate (>10 °C/min), the formation of pearlite is suppressed, and the corresponding microstructure consists of banded ferrite and bainite. Dilatometric analysis indicates that as ferrite and pearlite transformation at low rates (or ferrite transformation at higher rates) completes, Manganese (Mn) and Carbon (C) concentration in austenite can retard the bainitic transformation and result in the transformation stagnancy phenomenon. The magnitude of retardation and the bainite morphology are affected by Mn and C concentration. The increased cooling rate decreases Mn and C content in bainite, and then the length of the stagnant stage decreases and bainite morphology changes from lower bainite to acicular ferrite.     

Phase-Field Modeling of Austenite-to-Ferrite Transformation in Fe-C-Mn Ternary AlloysEI北大核心CSCD

The effect of Mn on the austenite-to-ferrite transformation has been widely studied by both physical models and experiments due to its technological importance for alloy design in steel industries. In recent years, an increasing interest of this issue is moved onto the effect of alloying element on the migrating interface during the austenite-to-ferrite transformation. For ternary Fe-C-Mn alloys, the interfacial condition is more complicated than that of binary Fe-C alloys in view of the large difference in the diffusivity between the interstitial and substitutional alloying elements. Generally speaking, there are two main concepts, i.e. the paraequilibrium model and the local-equilibrium model, which have been proposed to describe the phase transformation kinetics in ternary Fe-C-Mn alloys based on different assumptions about the diffusion of the substitutional elements. And many modeling attempts have been made to study the effect of Mn on the migration kinetics by using these theories. In this work, a multi-phase-field (MPF) model coupling with a Gibbs-energy dissipation model was developed to simulate the isothermal austenite-to-ferrite transformation in ternary Fe-C-Mn alloys. This model has considered the Mn diffusion inside the migrating interface in a physical manner and takes its effect on the transformation kinetics into account. Comparison simulations were made to analyze the difference in the transformation kinetics and ferrite morphologies with and without considering the energy dissipation at the moving interface. It shows that the incomplete transformation phenomenon does occur due to the Mn diffusion inside interface. The modified MPF model was then used to study the effect of Mn contents on the microstructures and kinetics of the phase transformations. It is found that the ferrite growth along the austenite/austenite boundaries is faster than that in the perpendicular direction. This difference is intensified with increasing the Mn concentration, which hence leads to the ferrite morphology changed from elliptical to flat alike. It also produces a slower transformation kinetics and a larger degree of the incomplete transformation when increasing the Mn concentration.     

15SiMn钢奥氏体等温相变的元胞自动机模拟

建立了15SiMn钢奥氏体一铁素体相变的二维元胞自动机介观模型.模拟结果显示,对于某一给定温度,铁索体的长大速度在相变过程中逐渐降低.当生长速度接近0时,奥氏体相中相界面处平衡碳的质量分数为0.523 95%.此外,模拟结果还显示,由于奥氏体晶界上的碳原子扩散和奥氏体-铁素体相界面移动都较奥氏体晶内更快,所以奥氏体晶界上铁素体晶粒形貌为椭圆形.     

应力对钢中贝氏体相变的影响

外加应力使贝氏体相变形核率增大，等温孕育期缩短，即使所加应力远低于母相的屈服强度．由于钢中γ→α γ′的形核驱动力较大(约为kJ／mol数量级)，贝氏体相变的嘭胀应变能很小，过小的外加应力对形核率的影响甚微．考虑在外加应力的影响下，会使界面能量有所下降，也可能发生碳原子的再分布，偏聚在晶界或其它缺陷，甚至碳化物析出都会显著地增大形核率和缩短孕育期，有待进一步实验给予证明．无应力下，贝氏体相变动力学可以用Avrami的等温相变方程来表述；应力下则符合应力下铁索体及珠光体相变的动力学模型(经修改的Avrami方程)．形变奥氏体促发贝氏体相变，但随后会发生奥氏体的力学稳定化，其机制可能和马氏体相变时的奥氏体力学稳定化不完全相同，仅形变形成的位错阻碍贝氏体以一定位向长大，使相变动力学迟缓．贝氏体相变时奥氏体力学稳定化的模型有待建立。     

Time-dependent synchrotron X-ray diffraction on the austenite decomposition kinetics in SAE 52100 bearing steel at elevated temperatures under tensile stress

We have studied the decomposition kinetics of the metastable austenite phase present in quenched-and-tempered SAE 52100 steel by in situ high-energy synchrotron X-ray diffraction experiments at elevated temperatures of 200–235 °C under a constant tensile stress. We have observed a continuous decomposition of austenite into ferrite and cementite. The decomposition kinetics is controlled by the long-range diffusion of carbon atoms into the austenite ahead of the moving austenite/ferrite interface. The presence of a tensile stress of 295 MPa favours the carbon diffusion in the remaining austenite, so that the activation energy for the overall process decreases from 138–148 to 82–104 kJ mol^?1. Before the austenite starts to decompose, a significant amount of carbon atoms partition from the surrounding martensite phase into the metastable austenite grains. This carbon partitioning takes place simultaneously with the carbide precipitation due to the over-tempering of the martensite phase. As the austenite decomposition proceeds gradually at a constant temperature and stress, the elastic strain in the remaining austenite grains continuously decreases. Consequently, the remaining austenite grains act as a reinforcement of the ferritic matrix at longer isothermal holding times. The texture evolution in the constituent phases reflects both significant grain rotations and crystal orientation relationships between the parent austenite phase and the newly formed ferritic grains.     

Kinetics of the abnormal austenite–ferrite transformation behaviour in substitutional Fe-based alloys

The kinetics of the recently discovered abnormal austenite (γ)–ferrite (α) transformation of substitutional Fe–Co and Fe–Mn alloys were measured by dilatometry and compared with the, also measured, corresponding normal transformation behaviour. A phase transformation model, involving site saturation, interface-controlled continuous growth and incorporating an impingement correction, has been employed to extract the migration velocity of the γ/α interface. It was found that the normal transformation process could be well described assuming a constant nucleus density and interface migration velocity. The thus assessed misfit-accommodation, deformation Gibbs energy is of the same order of magnitude as the chemical Gibbs energy change driving the transformation. A large austenite grain size was shown to be the precondition for the occurrence of abnormal kinetics. The abnormal transformation process involves the occurrence of additional peaks in the transformation rate for the first stage of the transformation. An autocatalytic type of nucleation was successfully incorporated in the above model to describe the occurrence of the repeated nucleation during the abnormal transformation.     

硼在超低碳钢铁素体相变中的行为

用显微组织与硼的自射线径迹显微照相（ＰＴＡ）定位对照方法,研究了超低碳钢铁素体相变中微量硼的行为。发现在等温铁素体相变发生前,硼已偏聚在奥氏体晶界上。沿晶界长出的小块铁素体中硼浓度明显高于奥氏体,但随铁素体长大,其硼含量逐渐与母相持平。在铁素体／奥氏体,铁素体／铁素体边界上没有形成明显的硼偏聚。这些现象表明,铁素体长大不受硼在奥氏体中扩散的控制。