MODELLING FOR PREDICTION OF CONTINUOUS COOLING TRANSFORMATION KINETICS OF HOT-DEFORMED AUSTENITE STEEL

1.IntroductionAsaneffectivethermomechanicalcontrolprocess,controlledrollingandcontrolledcoolingprocesshasbeensuccessfullyusedinproductionofsteelsinordertoimprovestrengthandductility.WOrk-hardenedaustenite(7)-- ferrite(or),pearlite(P)andbainite(B)transformationsarethemaintransformationsinhotrolledstripsandplatesduringcoolingafterrolling.Precisepredictionofcolltinuouscoolingtransformationkineticsbehaviourplaysanimportantroleinselectingreasonablethermomechanicalcontrolprocess.Onthebasisoftransf…     

Non-isothermal thermomechanical metallurgical model and its application to welding simulations

Abstract

This paper presents a thermomechanical metallurgical macroscopic model for steels. The model is based on an existing model that is extended for non-isothermal behaviour in combination with phase transformations. The model and its numerical implementation in ABAQUS are described using vector notation for stress and strain tensors. Model parameters are presented for the dual phase steel DP600 and the structural steel S355. For DP600, thermomechanical model parameters, i.e. hardening and strain rate dependency, have been obtained by fitting temperature and strain rate dependent tensile tests. A metallurgical model was implemented using data obtained from phase field models for the austenite growth and continuous cooling transition diagrams for phase transformations from austenite to low temperature phases. The model is applied to welding simulations of DP600 overlap joints and S355 T joints. The final distortion is compared to experiments and it is shown that the model presented is able to reproduce the experimental results very well.     

Recrystallization and the possibility of the realization of the α-γ diffusionless transformation during the ultrarapid laser heating of steels

Analysis is given of phase and structural transformations occurring upon ultrarapid laser heating in steels with different initial structures, namely, after annealing, after preliminary quenching, quenching and tempering, and after quenching with subsequent deformation and tempering. It is shown that a significant suppression of diffusion processes occurs during laser heating; this circumstance substantially affects the nature of the phase and structural transformations proceeding during laser processing. Special attention is given to studying the process of recrystallization and to the phenomenon of structural heredity during laser heating. The process of recrystallization during laser heating is considered as consisting of two stages, namely, an ordered lattice rearrangement (α-γ transformation) and the recrystallization of austenite that suffered phase-transformation-induced hardening (“phase naklep”). The effect of tempering and plastic deformation on the recrystallization of a preliminarily quenched steel consists in the intensification of the second stage, i.e., of the recrystallization of the transformation-hardened austenite. It is shown that the α-γ transformation during the laser heating of steels with the initial structure of lath martensite occurs by the “mechanism of recovery,” i.e., via the formation and growth of austenite nuclei. In steels with the initial structure of pearlite, the nucleation of austenite during laser heating can occur by a shear martensite-like diffusionless mechanism with the observance of characteristic orientation relationships between the initial ferrite and the newly formed austenite.     

Phase Transformations of an Fe-0.85 C-17.9 Mn-7.1 Al Austenitic Steel After Quenching and Annealing

Low-density Mn-Al steels could potentially be substitutes for commercial Ni-Cr stainless steels. However, the development of the Mn-Al stainless steels requires knowledge of the phase transformations that occur during the steel making processes. Phase transformations of an Fe-0.85 C-17.9 Mn-7.1 Al (wt.%) austenitic steel, which include spinodal decomposition, precipitation transformations, and cellular transformations, have been studied after quenching and annealing. The results show that spinodal decomposition occurs prior to the precipitation transformation in the steel after quenching and annealing at temperatures below 1023 K and that coherent fine particles of L1₂-type carbide precipitate homogeneously in the austenite. The cellular transformation occurs during the transformation of high-temperature austenite into lamellae of austenite, ferrite, and kappa carbide at temperatures below 1048 K. During annealing at temperatures below 923 K, the austenite decomposes into lamellar austenite, ferrite, κ-carbide, and M₂₃C₆ carbide grains for another cellular transformation. Last, when annealing at temperatures below 873 K, lamellae of ferrite and κ-carbide appear in the austenite.     

Thermodynamic modeling of the Cr-Pd and Mo-Pd systems

To assist the quantitative design of ultra-high strength (UHS) quantum steels containing Pd, a set of self-consistent thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Pd and Mo-Pd systems. These thermodynamic parameters are of importance to control the kinetics of solid-solid phase transformations, such as martensitic transformation, and precipitation of M₂C carbide and austenite, relevant to the design of UHS steels in the Fe-C-Co-Cr-Mo-Ni-Pd system. The present thermodynamic modeling complements our previous report on the thermodynamic modeling of the Co-Pd, Fe-Pd, and Ni-Pd systems to facilitate quantitative design of UHS quantum steels.     

Thermodynamic modeling of the Cr-Pd and Mo-Pd systems

To assist the quantitative design of ultra-high strength (UHS) quantum steels containing Pd, a set of self-consistent thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Pd and Mo-Pd systems. These thermodynamic parameters are of importance to control the kinetics of solid-solid phase transformations, such as martensitic transformation, and precipitation of M₂C carbide and austenite, relevant to the design of UHS steels in the Fe-C-Co-Cr-Mo-Ni-Pd system. The present thermodynamic modeling complements our previous report on the thermodynamic modeling of the Co-Pd, Fe-Pd, and Ni-Pd systems to facilitate quantitative design of UHS quantum steels.     

Multiscale mechanics of TRIP-assisted multiphase steels: II. Micromechanical modelling

The stress and strain partitioning between the different phases of transformation-induced plasticity (TRIP)-aided multiphase steels is evaluated using a mean field homogenization approach. The change of the austenite volume fraction under straining is predicted using a micromechanics-based criterion for the martensitic transformation adapted to the case of small, isolated, transforming austenite grains. The parameters of the model are identified from the mechanical response and transformation kinetics measured under uniaxial tension for two steels differing essentially by the austenite stability. The model is validated by comparing the predictions with tests performed under different loading conditions: pure shear, intermediate biaxial and equibiaxial. An analysis of the effect of the austenite stability on strength and ductility provides guidelines for optimizing properties according to the stress state.     

Kinetics of deformation processes in high-alloyed cast transformation-induced plasticity/twinning-induced plasticity steels determined by acoustic emission and scanning electron microscopy: Influence of austenite stability on deformation mechanisms

The austenite stability and the stacking fault energy of high-alloyed metastable transformation-induced plasticity/twinning-induced plasticity (TRIP/TWIP) steels, both depending on the chemical composition, have a strong influence on the deformation processes and stress/strain-induced martensitic phase transformation. Aiming at a better understanding of the kinetics of TRIP/TWIP-assisted plastic deformation, acoustic emission (AE) measurements were performed during room temperature tensile deformation of high-alloyed cast model steels with different austenite stability. The real-time AE investigations were complemented by detailed scanning electron microscopy investigations of deformed microstructures using electron backscattered diffraction to determine the martensitic phase transformation and electron channelling contrast to visualize dislocations and their arrangements. The quantitative AE analysis revealed different AE patterns at different plastic strains, which were correlated with underlying deformation mechanisms and microstructural transformations.     

Structural and phase transformations,mechanical properties,and shape-memory effects in quasibinary Ni<Subscript>50</Subscript>Ti<Subscript>38</Subscript>Hf<Subscript>12</Subscript> alloy obtained by quenching from the melt

Methods of transmission and scanning electron microscopy and chemical microanalysis, electron diffraction, and X-ray diffraction were used to systematically study the structure and the chemical and phase composition of the Ni₅₀Ti₃₈Hf₁₂ alloy synthesized by rapid quenching from the melt and subjected to various heat treatments. The critical temperatures of the devitrification of the initially amorphous rapidly quenched alloy and the B2 ? B19′ thermoelastic martensitic transformations have been determined. The lattice parameters of the B2 austenite and thermoelastic B19′ martensite have been measured. The main features of the formation of an ultrafine-grained structure in the alloy and the subsequent phase transformations (martensitic transformation and the decomposition with the formation of an intermetallic phase of the (Ti,Hf)₂Ni type) have been studied depending on the regimes of heat treatment. Based on the results of measurements of mechanical properties upon tension (σ_M, σ_u, and δ) and the shape-memory effects (degree of shape recovery depending on the deformation by bending; and magnitude of the reversible strain ε_rev), regimes for obtaining high-strength and plastic states of the alloy with a shape-memory effect have been established.     

In situ observation of austenite–ferrite interface migration in a lean Mn steel during cyclic partial phase transformations

High-temperature laser scanning confocal microscopy (HT LSCM) has been applied to investigate the austenite–ferrite interface migration during cyclic phase transformations in situ in a Fe–Mn–C alloy. It has been found that during the cyclic phase transformations the transformation proceeds via the migration of existing austenite–ferrite interfaces. The interfaces migrate in a retraceable way. For the first time, the so–Called stagnant stage has been observed directly. The new in situ observations show that the interface migration rates for interfaces in different grains are comparable with each other prior to soft impingement, while the equilibrium migration distances for different interfaces can be quite different, depending on the local grain size. The average interface velocities as measured by HT LSCM are in very good agreement with the velocities derived from dilatometric data, and those are predicted by a local equilibrium transformation model.     

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 采用非等温差示扫描量热法（DSC）测试了不同冷却速率下低合金钢中奥氏体→珠光体转变温度和转变时间,并计算了奥氏体→珠光体转变激活能。结果表明,随着冷却速率的提高,相变温度降低,相变时间缩短,相变激活能随着相变体积分数的增加而逐渐减小。     

Study of Decomposition of Supercooled Austenite of Powder Steels with the Help of a Novel Digital Magnetometer

A magnetometric complex for studying the decomposition of supercooled austenite is described. The kinetics of phase transformations is studied by the developed method. The kinetic parameters are determined and kinetic diagrams of phase transformations for conditions of stage hardening are plotted. A mathematical model of decomposition of supercooled austenite under conditions of isothermal and stage hardening is developed on the basis of these kinetic parameters and diagrams. An experimental check of adequacy of the models obtained is performed for the whole temperature and time range of the transformation.__________Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 24 – 29, April, 2005.     

Modeling of structure of double-phase low-carbon chromium steels

A physical model for determining the relative amount of phase components and the size of ferrite grains after decomposition of austenite in the process of cooling of double-phase steels is suggested. The main products of austenite transformation, i.e., polygonal ferrite, pearlite, bainite, and martensite, are considered. The driving forces of the transformation and the concentration of carbon on the phase surface are determined with the use of methods of computational thermodynamics. The model is based on equations of the classical theory of nucleation and growth. It allows for the structural features of the occurrence of γ → α transformation and contain some empirical parameters. The latter are determined using data of dilatometric measurements of the kinetics of austenite transformation and metallographic measurements of the size of ferrite grains. The model is used for predicting the kinetics of the transformation under the complex cooling conditions implemented by the VOEST-ALPINE STAHL LINZ GmbH rolling mill within the computer system for control of mechanical properties of hot-rolled strip.     

Influence of Steel Grade on Surface Cooling Rates and Heat Flux during Quenching

Immersion quenching is one of the most widely used processes for achieving martensitic and bainitic steels. The efficiency and quality of quenching are generally tested using standard quench probes for obtaining the cooling curves. A host of parameters like quenchant type, steel grade, bath agitation, section thickness, etc., affect the cooling curves. Cooling curve analyses covered under ASTM standards cannot be used to assess the performance of a quenchant for different grades of steel, as they use a common material for the probe. This article reports the development of equipment, which, in conjunction with mathematical models, can be used for obtaining cooling curves for a specific steel/quenchant combination. The mathematical models couple nonlinear transient inverse heat transfer with phase transformation, resulting in cooling curves specific to the steel grade-quenchant combination. The austenite decomposition models were based on an approach consistent with both the TTT diagram of the steel and Fe-C equilibrium phase diagrams. The TTT diagrams for the specific chemistry of the specimens and the thermophysical properties of the individual phases as functions of temperature were obtained using JMatPro software. Experiments were conducted in the laboratory for computing surface temperature and heat flux at the mid-section of a 25-mm diameter by 100-mm-long cylindrical specimen of two types of steels in two different quenchants. A low alloy steel (EN19) and a plain carbon steel (C45) were used for bringing out the influence of austenite transformation on surface cooling rates and heat flux. Two types of industrial quenchants (i) a mineral oil, and (ii) an aqueous solution of polymer were used. The results showed that the cooling curves, cooling rate curves, and the surface heat flux depended on the steel grade with the quenchant remaining the same.     

Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading

An ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9 at.% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading. Both macroscopic stress–strain measurements and volume-averaged lattice strains are reported and discussed. The loading behavior is described in terms of elasto-plastic deformation of austenite, emergence of R phase, stress-induced martensitic transformation, and elasto-plastic deformation, grain reorientation and detwinning of martensite. The unloading behavior is described in terms of stress relaxation and reverse plasticity of martensite, reverse transformation of martensite to austenite due to stress relaxation, and stress relaxation of austenite. Microscopically, lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19′ phases are examined during loading and unloading. It is shown that the phase transformation occurs in a localized manner along the gage length at the plateau stress. Phase volume fractions and lattice strains in various crystallographic reflections in the austenite and martensite phases are examined over two transition regions between austenite and martensite, which have a width on the order of the wire diameter. Anisotropic effects observed in various crystallographic reflections of the austenitic phase are also discussed. The results contribute to a better understanding of the tensile loading behavior, both macroscopically and microscopically, of NiTi shape-memory alloys.     

Effect of deformation on hydrogen trapping and effusion in TRIP-assisted steel

The trapping of hydrogen at a variety of sites in multiphase transformation-induced plasticity (TRIP) steels has been characterized using thermal desorption spectroscopy and the results have been modelled using diffusion theory. It is discovered that austenite serves as a reversible trapping site which is more potent than grain boundaries or dislocations in ferrite. Plastic deformation which leads to the partial martensitic transformation of the austenite results in an alteration in the trapping condition of the inherited hydrogen. It is demonstrated that these phenomena can be incorporated into a mathematical model which permits the desorption of hydrogen to be predicted quantitatively as a function of, for example, the heating rate, phase fractions and phase transformation. An interesting outcome is that the mechanical degradation of the steel by hydrogen is more pronounced in TRIP steel containing austenite which is relatively less stable to martensitic transformation during deformation. This is because the phase transformation causes a reduction in the trap binding energy, thus enhancing the apparent mobility of the hydrogen.     

Influences of cyclic loading on martensite transformation of TRIP steels

While austenite transformation into martensite induces increasing of the crack initiation life and restraining of the growth of fatigue cracks in cyclic-loading processes, TRIP-assisted steels have a better fatigue life than the AHSS (Advance High Strength Steels). As two key parameters in the cyclic loading process, strain amplitude and cyclic frequency are used in a kinetic transformation model to reasonably evaluate the phase transformation from austenite into martensite with the shear-band intersections theory, in which strain amplitude and cyclic frequency are related to the rate of shear-band intersection formation and the driving force of phase transformation. The results revealed that the martensite volume fraction increased and the rate of phase transformation decrease while the number of cycles increased, and the martensite volume fraction was almost constant after the number of cycles was more than 2000 times. Higher strain amplitude promotes martensite transformation and higher cyclic frequency impedes phase transformation, which are interpreted by temperature increment, the driving force of phase transformation and the rate of shearband intersection formation.     

Austenite to bainite phase transformation in the heat-affected zone of a high strength low alloy steel

The austenite to bainite phase transformation was investigated in a low alloy structural steel after simulated welding heat treatment, by means of light microscopy, electron backscatter diffraction and transmission electron microscopy. Upper bainite packets result from the growth of groups of laths having close crystallographic orientations but highly misoriented habit planes. Self-accommodation of the transformation eigenstrain was evaluated for various bainite configurations using a micromechanical model. The observed pairs of variants seem to help limiting plastic strain in the austenite phase, thus enhancing growth of the bainite phase during cooling.     

钢的连续冷却转变图的神经网络计算模型及预测软件设计

建立了基于人工神经网络技术预测钢的过冷奥氏体连续冷却转变(CCT)图的计算模型,并在此基础上设计了一种预测CCT图的软件—Eagleye2003。在计算模型中综合考虑了CCT图的物理意义和几何特征,同时为了方便计算也忽略了一些次要因素。通过该软件可以根据钢的化学成分和热处理工艺预测其连续冷却转变图,这对钢的合金设计及组织、性能预测都有很大帮助。实例证明该软件具有较高的预测精度。     

In-situ neutron diffraction analysis on deformation behavior of duplex high Mn steel containing austenite and ?-martensite

The deformation behavior of Fe-17Mn-0.02C steel containing ?-martensite within austenite matrix has been investigated via in-situ neutron diffraction study at 298 K and 77 K. Based on the analyses of changes in phase fraction and lattice strain, it has been shown that the steel shows the deformation-induced phase transformation of austenite ?? ?-martensite ?? ????-martensite and the direct transformation of austenite ?? ????-martensite at both temperatures. However, the kinetics of such transformations vary with temperature, resulting in a higher and more persistent work hardening at 77 K than at 298 K.