热冲压硼钢非等温奥氏体相变动力学模型

 利用Gleeble-3500热模拟试验机对热冲压硼钢进行了线膨胀试验,得到了不同加热速度下硼钢奥氏体化动力学曲线;研究了加热速度对相变过程的影响,结果显示,加热速度越高,获得相同体积分数的奥氏体所需的时间越短,且奥氏体开始转变温度升高,当加热温度介于[Ac1]和[Ac3]时,奥氏体体积分数转变速度随着加热温度的升高先增大后减小。根据奥氏体形核长大理论,考虑加热速度的影响,建立了非等温条件下热冲压硼钢的统一奥氏体相变动力学模型。利用遗传算法确定并优化了硼钢奥氏体化动力学模型中的材料常数,所得材料常数确定的相变模型能够较好的描述硼钢连续加热过程线膨胀曲线,并能够较好地预测硼钢在不同加热速度下的奥氏体体积分数。     

拉伸应变下中碳贝氏体钢中残留奥氏体转变及其稳定性

摘要：采用分阶段拉伸应变试验,研究中碳贝氏体钢在拉伸过程中TRIP效应的影响,探究残留奥氏体转变量与应变量之间的关系。试验结果表明,拉伸变形初期残留奥氏体转变较快（0%到3.0%应变量(体积分数)对应残留奥氏体转变量为13.72%）,主要为分布在贝氏体束与束之间的块状残留奥氏体（尺寸为微米级）发生相变;拉伸变形后期残留奥氏体转变较慢（10.0%到18.5%应变量对应残留奥氏体转变量为7.78%）,主要为分布在贝氏体铁素体板条与板条之间的薄膜状残留奥氏体（尺寸为纳米级或者亚微米级）发生相变。归因为块状残留奥氏体稳定性低,在外部应力作用下容易转变为硬而脆的马氏体组织,而薄膜状/条状残留奥氏体稳定性高,在外部应力的作用下不易发生相变,TRIP效应的发生使得钢种强度和塑性同时得到提高。通过3种模型（Hollomon模型,C-J模型,修正C-J模型）描述试验钢的加工硬化行为,将整个拉伸过程可分为3个阶段,并结合微观组织变化,详细解析了每个阶段所对应的主要加工硬化机制。     

Modelling the Kinetics of Austenite Decomposition in Nb‐Ti‐V Microalloyed Steel after Hot Strip Rolling

In hot strip rolling of Nb‐microalloyed steels the austenite transformation usually starts in the hot run‐out table (HRT) under a high cooling rate and finishes while the strip is coiled, when the cooling rate is much slower. Thus the transformation takes place during a two‐step‐cooling regime. This paper presents a mathematical model able to predict such transformation. The experimental alloy used here was a Nb‐V‐Ti microalloyed steel. The model employs an Avrami type equation associated with the additivity rule. The time constant b in the Avrami equation was expressed as a function of parameters A and B whereas the time exponent n was best modelled as a constant of value 1. Experiments have shown that the constants A and B are linearly dependent. An artificial neural network (ANN) model was used to predict B. The ANN model takes into account the following process variables: austenite deformation applied above the non‐recrystallization temperature (Tnr), deformation applied below Tnr, cooling rate in the HRT, and coiling temperature. From the results it was proposed that the transformation is governed by early site saturation, diffusion being the only operating mechanism. This conclusion was based on the experiment value found for n and on the need of a single temperature dependent parameter, either A or B, to predict the course of the overall austenite transformation.     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.     

热轧带钢组织性能预报模型及应用

 基于物理冶金理论,研究了热轧带钢过程中的奥氏体晶粒长大模型、奥氏体再结晶模型、奥氏体相变模型以及力学性能模型。奥氏体再结晶模型中,通过研究位错密度的变化来描述由于再结晶不完全造成的变形抗力的变化。奥氏体相变模型中,通过碳扩散理论描述了奥氏体-铁素体相界面随冷却过程的变化规律。基于热轧带钢过程中的冶金物理模型,开发热轧带钢组织性能预报系统。系统包括4个模块,分别用于计算板坯在加热炉、粗轧精轧、层流冷却和卷取完成各阶段的组织和力学性能参数,生产工艺是该系统的重要输入参数。利用该系统对某钢厂实际生产过程的组织性能进行预报,预报的力学性能和现场实测值有较好的一致性。     

Modeling of Incubation Time for Austenite to Ferrite Phase Transformation

 The giant stress-impedance (GSI) effect in as-cast and DC current annealed Co71.8Fe4.9Nb0.8Si7.5B15 amorphous glass-covered wires is presented. The SI ratio of the as-cast sample exhibits negative GSI effect. For the sample annealed by 60 mA DC current, the SI ratio first increases with applied tensile stress, then decreases with the applied tensile stress. The maximum ΔZ/Z ratio of 304% is obtained. Frequency dependence in the range from 1 to 110 MHz of the GSI effect is investigated. Experimental results show that the real part R and the imaginary part X of impedance play an important role at high frequency and low frequency, respectively. At 1 MHz, the maximum ΔX/X of 1 448% is obtained. At 110 MHz, the maximum ΔR/R ratio of 648% is obtained.     

An integrated computer model with applications for austenite-to-ferrite transformation during hot deformation of Nb-microalloyed steels

This work presents an austenite decomposition model, based on the thermodynamics of the system and diffusion-controlled nucleation theory, to predict the evolution of microstructure during hot working of niobium-microalloyed steels. The differences in microstructural development of hotdeformed microalloyed steel in the single-phase austenite and two-phase (austenite + ferrite) regions have been effectively described using an integrated computer modeling process. The complete model presented here takes into account the kinetics of recrystallization, recrystallized austenite grain size, precipitation, phase transformation, and the resulting ferrite structure. After considering existing austenite decomposition models, we decided that the method adopted in the present work relies on isothermal transformation kinetics and the principle-of-additivity rule. The thermomechanical part of the modeling process was carried out using the finite-element method. Experimental results at different temperatures, strain rates, and strain levels were obtained using a Gleeble thermomechanical simulator. A comparison of results of the model with experiments shows good agreement.     

A Conversional Model for Austenite Formation in Hypereutectoid Steels

An on-heating conversional model has been proposed to calculate the volume fraction of austenite in hypereutectoid steels from dilation strain. The transformation strain equation for austenite formation from ferrite and cementite was developed with lattice parameters modified to include alloying element effects. The conversional model was verified by comparing calculations with experimentally measured transformation temperatures and constituent volume fractions in the transformation temperature range for various heating rates. A continuous heating transformation (CHT) diagram was produced from model results.     

A kinetic model of the γ → α + Gr eutectoid transformation in spheroidal graphite cast irons

The eutectoid transformation of austenite in spheroidal graphite cast iron can follow one of two paths: (a) transformation to a mixture of ferrite and graphite or (b) transformation to pearlite. The extents to which the two reactions occur determine the relative amounts of ferrite and pearlite in the microstructure and, hence, the properties of the iron. In this paper, the kinetics of the γ → α+ Gr reaction is studied, and a model is developed to predict the isothermal transformation rates. The transformation occurs at a rate determined by the rate of carbon diffusion. The diffusion of carbon through ferrite, as well as through austenite, has been considered. The model predicts that the volume fraction of austenite transformed isothermally increases with increasing number density of graphite spheroids. Predictions of the model are compared with data available in literature.     

1.6 GPa级中碳高强贝氏体钢残余奥氏体调控机理

 为了研究中碳高强贝氏体钢中的残余奥氏体体积分数在不同等温情况下的变化规律,通过X射线衍射试验、热模拟试验和扫描电子显微镜观察等,分析了等温淬火条件对中碳高强贝氏体钢中残余奥氏体体积分数和组织的影响。结果表明,最终残余奥氏体的体积分数受贝氏体相变和马氏体相变的共同影响。贝氏体相变量决定了未转变奥氏体的体积分数及其化学稳定性,从而影响随后的马氏体相变量及最终残余奥氏体体积分数。此外,随着相变温度的升高,开始由于贝氏体相变量逐渐减少,残余奥氏体体积分数先增加(300~350 ℃),随后由于马氏体相变量增加,残余奥氏体体积分数减少(350~400 ℃)。     

Influence of Carbon Equivalent Content on Phase Transformation During Inter-critical Heating of Dual Phase Steels Using Discrete Micro-scale Cellular Automata Model

In this paper, a cellular automata method based model is proposed for simulating phase transformation kinetics of inter-critical heating of dual phase (DP) steel. This developed model deals with the kinetics of pearlite dissolution, ferrite transformation and austenite grain growth based on carbon diffusion process. Diffusion equation is discretized and solved by finite difference method whereas austenite grain growth is controlled by transition rules applied in cellular automata algorithm. The model is operated in the temperature range of 730–890 °C for four different specimens of DP steel. This model predicts appropriately the microstructure and volume fraction of formed austenite during inter-critical heating of DP steel. In addition, this study shows that the presence of carbon and alloying elements enhances carbon equivalent of DP steel, helps in austenite formation.     

稀土管线钢轧制工艺的模型研究

为了研究稀土对变形奥氏体晶粒的影响,建立了稀土管线钢的轧制模型,实验验证表明,模型可以精确预测稀土管线钢轧制过程中奥氏体晶粒的长大行为.研究发现,稀土改变了X80管线钢的奥氏体变形行为,原有的TMCP工艺不适合稀土管线钢.对于稀土管线钢而言,通过适当的调整轧制工艺,稀土的微合金作用可以得到充分发挥,相变之前的奥氏体组织明显细化.     

形变奥氏体在控冷过程中相变行为的研究

以超组元算法和LFG模型为热力学基础,以Cahn的相变动力学和Scheil叠加法为动力学基础,建立了连续冷却过程中奥氏体向铁素体、珠光体和贝氏体转变的相变行为的预测模型,研究了变形、冷却速率和终轧温度对相变的影响.     

微合金钢连铸坯表面裂纹敏感性预测模型

为了从凝固及相变特性角度解决微合金钢连铸坯表面裂纹问题,建立了与合金化相关联的初始凝固包晶反应度模型、奥氏体晶粒长大模型、铁素体转变量模型以及碳氮化物的析出模型。结合铸坯实际冷却条件,进一步建立了包晶反应度预测、初生奥氏体晶粒长大、铁素体转变、析出相析出等对铸坯表面裂纹敏感性的预测模型。针对某J55钢连铸板坯,奥氏体晶粒尺寸超过1 mm、铁素体析出量为10%、二次相析出量增加时,横裂纹敏感性最大。表面裂纹敏感性预测模型有助于实现基于成分微调和组织调控的微合金钢连铸、热装等生产过程表面裂纹控制技术。     

Nb-V-Ti微合金化高强度钢08MnCr连续冷却转变曲线和组织

利用ThermecMaster-Z热模拟实验机测定了一种Nb-V-Ti微合金化高强度钢08MnCr(S2)在910～1 200℃不变形(静态)和变形(动态)奥氏体0.05～30℃/s冷速下连续冷却转变(CCT)曲线,并分析和观察了对应的相变及组织。实验结果表明,提高轧后的冷却速度使Ar₃降低,导致钢的晶粒进一步细化;冷却速度大于10℃/s开始出现贝氏体转变。提高加热温度时相变温度降低,变形奥氏体相变温度较不变形奥氏体相变温度高。冷速较低时,铁素体晶粒呈多边形;冷速高时,铁素体晶粒多呈尖角形。     

Influence of alloying elements on the kinetics of strain-induced martensitic nucleation in low-alloy, multiphase high-strength steels

Low-alloy multiphase transformation-induced-plasticity (TRIP) steels offer excellent mechanical properties in terms of elongation and strength. This results from the complex synergy between the different phases, i.e., ferrite, bainite, and retained austenite. The precise knowledge of the austenite-to-martensite transformation kinetics is required to understand the behavior of TRIP steels in a wide array of applications. The parameters determining the stability of the metastable austenite were reviewed and investigated experimentally, with special attention paid to the effect of the chemical composition, the temperature, and the size of the austenite particles. The results show that the stability and rate of transformation of the austenite particles in TRIP steels have a pronounced composition dependence: austenite particles transform at a faster rate in CMnSi TRIP steel than in TRIP steels in which Si is fully or partially replaced by Al and P. The results clearly support the view that (1) both a high C content and a submicron size are required for the room-temperature stability of the austenite particles and (2) the effect of the chemical composition on the transformation is due to its influence on the intrinsic stacking-fault energy. In addition, the composition dependence of the Md ₃₀ temperature was derived by regression analysis of experimental data. The influence of the size of the retained austenite particles on their Ms ^σ temperature was studied by means of a thermodynamic model. Both the analysis of the transformation-kinetics data and the microstructural analysis by transmission electron microscopy revealed the very limited role of autocatalysis in the transformation.