Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerened with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austente-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transormation product, thesoft-impingement effect should be taken into account in the modeling. In that case, the Gilmouret al., analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/λ interface remains the same as the overal carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct inR ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied. C. CAPDEVILA, Research Associate, formerly with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), 28040 Madrid, Spain     

Austenite formation during intercritical annealing

A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.     

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s⁻¹) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

     

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M^σ_s temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M^σ_S temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.     

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density. In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels with different active particle types and densities. The morphology of the obtained intragranular microstructures has been found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.     

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30?min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C?s^?1. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ?→?α with a cooling rate of 5°C?s^?1. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.     

The effect of cooling rate on the microstructures formed during solidification of ferritic steel

This article describes in detail the effect of cooling rate on the microstructure of a low-carbon Fe-12 pct Cr alloy. The alloy was prepared using a relatively simple technique, i.e., rapid cooling of the melt in a copper wedge mold. The dependence of microstructure on the cooling rate (∼40 to 10⁵ K/s) has been determined by X-ray diffraction (XRD), microhardness measurement, optical microscopy (OM), and transmission electron microscopy (TEM). It has been found that the matrix structure over a large cooling rate range is composed of columnar ferrite grains, the size of which decreases with increasing cooling rate. Precipitation of second phases has been observed at either the ferrite grain boundaries or within the ferrite grains. The former takes place along the entire wedge sample, whereas the latter characterizes a region 12 mm away from the tip of the wedge sample. The essential structure of the grain boundary precipitates was identified as martensite, which is a transformation product of austenite precipitated at high temperatures. Retained austenite was identified at the tip region as isolated particles (<4 μm). The precipitates within the ferrite grains appeared as planar colonies consisting of two sets of needles. The density of these precipitates increases with increasing the cooling rate while their size decreases. Characteristic precipitate-free zones (PFZs) at the ferrite grain boundaries were observed and are discussed.     

<Emphasis Type="Italic">In-situ</Emphasis> observations of lattice parameter fluctuations in austenite and transformation to bainite

The isothermal transformation of high-carbon austenite-to-bainitic ferrite has been investigated with the in-situ technique of time-resolved X-ray diffraction using synchrotron radiation. The measurements indicate that prior to transformation, the austenite divided into regions with significantly different lattice parameters. It is possible that this is due to the development of carbon-rich and carbon-poor regions in the austenite, as a precursor to transformations including the bainite reaction. The lattice parameter became uniform as transformation progressed and the fraction of carbon-poor austenite decreased. The ferrite itself exhibited a large range of lattice parameters during the early stages of transformation, due to the trapping of carbon.     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450°C, following austenitization at 1250°C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

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.     

Theory of modelling the isothermal austenite grain growth in a Si‐Mn TRIP steel

The development of a model to predict the isothermal austenite grain growth during soaking of a low carbon Si‐Mn TRIP steel is described. After reviewing the existing models for isothermal grain growth, a general model dⁿ=d₀ⁿ + K₁t exp(K₂/T) was selected and a procedure was delineated to calculate the values of the different constants of the equation starting with the real three‐dimensional austenite grain size. This paper also deals with an improved etching technique to reveal the austenite grain boundaries.     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

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.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

Effect of Niobium on Transformations From Austenite to Ferrite in Low Carbon Steels

Niobium has an important effect on the transformation behaviour,grain size refinement and precipitation strengthening during hot rolling and subsequent cooling in low carbon steels,with even a low content of niobium having a strong effect on the transformation rate from austenite to ferrite.However,the effects of niobium on transformation behaviour have not been fully characterised and understood to date.This paper examines in detail austenite grain growth as a function of austenitisation time in high strength low alloy (HSLA) steels with three different niobium contents,together with the effect of niobium on the isothermal transformation kinetics from austenite to ferrite as a function of temperature.It is shown that austenite has the slowest grain growth rate in the steel with the highest niobium content.When austenite grain sizes are consistent,the steel with the highest niobium content was found to have the slowest transformation rate from austenite to ferrite.