This paper reviews the various ferritic microstructures produced by austenite decomposition in continuously cooled low-carbon and ultralow-carbon steels and irons. Various terminologies used to identify the different ferritic microstructures and presented and related to several classification systems which have been developed for bainitic and ferritic microstructures in steels. Microstructural aspects are emphasized and discussed relative to transformation kinetics and mechanisms of austenite transformation to various morphologies of ferrite.

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It has been known for some time that steel microstructures are almost always composed of mixtures of phases and microconstituents. It has also been recognized that the minor phase can often exert a major influence on the final mechanical properties of structural steels. Over the last two decades, several important advances have been made in terms of the properties which have become attainable in these steels. These advances have occurred largely through a better understanding of the control of multi-phase microstructures. The paper reviews the benefits that have accompanied a better understanding of these complex microstructures.

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The features of the γ–α transformation under isothermal conditions and during continuous cooling for a wide range of rates (0.1–500000 K/s) were studied on Fe–C alloys, on chromium steels containing up to 0.4%C and 4.3; 6.6 and 9.4% Cr and on some low–carbon alloyed steels. It is established that there are discrete, staged kinetics of the γ–α transformation in Fe–Cr alloys and alloyed steels. The transition from a diffusionless transformation to a bainite transformationin low–carbon chromium steels is examined.
All the results on the kinetics of the γ–α transformation are obtained for iron alloys which show that, there are four stages of the transformation in steels and suggest a connection between the formation of upper and lower bainite and the transformation at stages II and III observed in pure Fe, respectively.
A theory is developed which explains the two–stage dependence of the starting point of the martensitic transformation in structural steels on the cooling rate by the change in the interaction of the moving γ/α boundary with carbon atoms. The upper temperature stage of the transformation is realized in conditions where growth is blocked by carbon atoms, the lower temperature stage–when the interphase boundary separates from the impurity atmosphere.

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The models so far proposed in order to explain the mechanism of bainitic transformations have been reviewed and the problems arising from them have been discussed. The morphology and the crystallography of bainitic structures are quite similar to those of martensite which forms via diffusionless processes. The kinetic properties of bainite, however, exhibit typical of diffusional (reconstructive) natures such as C-curve formations in TTT diagrams. If bainitic transformations occurred in a displacive mechanism where at least the substitutional atoms are assumed to displace diffusionlessly, bainitic reaction would disappear in the interstitial-free steels, the Bs-temperatures, being equal to the Ms-temperatures. It has, however, been confirmed that bainitic reactions really occur in such extremely low carbon steels with their own C-curves. In order to get a comprehensive understanding for these conflicting results, some experiments using extremely low carbon steels have been carried out. It has been demonstrated that the results can be comprehensively explained by the model where the lattice change and the relaxation of elastic stress due to it are assumed to occur via individual atomic jumps and a lattice invariant shear, respectively.

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Widmanstätten austenite laths forming from δ-phase in a δ/γ duplex phase stainless steel have been investigated metallographically. The austenite laths form in the temperature range between 1100 and 750°C with a C-curve in a T-T-T diagram.
The formation of Widmanstätten austenite laths accompanies a sharp surface relief similar in appearance to that of martensite. Widmanstätten laths formed at 1000°C where relatively large alloy partition occurs do not change the morphology by the further isothermal holding. Those formed at lower temperatures, 900 and 800°C, where the partition is relatively small, however, decompose into the rows of small austenite fragments, and the alloying elements are partitioned quite largely between the γ fragments and the δ-matrix. The driving force for this interphase boundary migration producing the γ-fragments is thought to arise from the partial supersaturation of alloying elements in the initially formed laths. The interphase boundary migration with alloy partition does not produce additional surface reliefs, suggesting that a pure diffusional transformation does not induce surface reliefs. Such an austenite lath formation can be explained consistently in terms of a shear-assisted diffusional transformation model where the lattice change occurs via a diffusional individual atomic jumps and the resulting elastic strain is relaxed by lattice invariant shear.

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The experimental growth kinetics of bainitic plates in a series of Fe-6 mass%Ni-0.290.71mass%C alloys were compared with the calculation from Zener-Hillert, Trivedi and Atkinson's edgewise growth theories. All theories can account well for the reported lengthening rates in the temperature range between 230 and 500°C except in the 0.29 mass%C alloy. In this alloy, it may become necessary to consider the carbon-partitionless growth (massive transformation) and the interfacial reaction etc., although the carbon content exceeds the solubility limit in ferrite. The reason why morphological difference between upper and lower bainite does not affect the edgewise growth kinetics is considered.

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Fe-0.002%C, Fe-0.01%C, Fe-1.5%Mn-0.001%C and Fe-1.5%Mn-0.01%C steels were vacuum-melted and examined. Fe-0.05% and Fe-1.5%Mn-0.05%C steels were also melted and used for comparison. The effects of cooling rate and chemical composition on microstructures were examined by optical and electron microscopy. Autoradiography of boron was also performed in order to know relation between γ and α grain boundaries and effects of microstuctures on discontinuous yielding were also examined. The fraction of quasi-polygonal ferrite (α_q) increases with the cooling rate and the contents of C and Mn. Dislocation density in α_q is higher than that in polygonal ferrite (α_p) and dislocations tend to form cell structures in α_q matrix, whereas dislocations which do not form cell structures were also observed. Normal cooling rate dependence of ferrite grain size was not observed in Fe-0.002%C and Fe-1.5%Mn-0.001%C steels. By autoradiography of boron it was known that not a small fraction of grain boundaries of ferrite might be coherent. Even in the ultra-low carbon steels cooled at 360°C/s, cementite was observed on ferrite grain boundaries. Discontinuous yielding was suppressed by bainite transformed during continuous cooling, while Mn enhanced discontinuous yielding. Bainite and martensite could be observed in he specimens of Fe-0.002%C and Fe-1.5%Mn-0.001%C steels cooled at the rate higher than 360°C/s. In such specimens, the continuous change from α_q to bainite was observed. Martensite can be distinguished from bainite by its higher microhardness and thinner lath width.

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Scanning tunneling microscopy (STM), together with light optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), of ultra fine structure of bainitic ferrite (α) shows that subunits inside a bainitic plate consist of sub-subunits, and STM of the surgace relief effect accompanying bainitic structure further confirms the existence of sub-subunits. The multi-layer structural constitution of bainite exhibits the typical self-embedded characteristics, and the fractal multi-dimension has been determined to be 2.74. Moreover, STM shows that an individual relief corresponding to a bainitic plate is composed of a sheaf of miniature reliefs which seem to arise from the formation of bainitic platelets, such as subplates, subunits and/or sub-subunits. The discovery of bainitic sub-subunits is helpful to give insight into transformation mechanism and interpret the complex and variable morphology of bainite. Based on the findings regarding the multi-layer structure of bainite, it is proposed that bainite forms via ledgewise growth which is essentially accompanied by sympathetic nucleation, i.e., sympathetic-nucleation-ledgewise-growth (SNLG). In this paper the SNLG mechanism will be theoretically discussed and practically applied to explain the multi-layer structure of bainite.

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The formation mechanism of intermediate transformation products and the effects of B as well as carbide forming elements such as Cr, Nb and Ti on the isothermal transformation characteristics of low carbon 3 wt% Mn steels have been investigated and the following results have been obtained.
(1) Widmanstatten ferrite and bainite exibit both the diffusional and the displacive aspects in their formation processes.
(2) The addition of the above described elements markedly retards the formation of primary ferrite, Widmanstatten ferrite and bainite in the specimens directly quenched to isothermal transformation temperatures from 1200°C, while the precipitation of coarse carbide particles in austenite accelerates the fololowing transformations.
(3) Ferrite nucleation on coarse carbide particles can be explained in terms of the decrease in the activation energy for nucleation by the lattice correspondence at the interface.
(4) The coarse carbide particles precipitated at austenite grain boundaries are related to the ferrite enclosing these particles with Kurdjumov-Sachs relationship.

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Effects of deformation in unrecrystallized austentie region on bainite microstructure were investigated in low carbon Nb-B bearing steels. Particular emphases were placed on the variation of the morphology of bainitic ferrite in isothermally transformed specimen.
SEM observation shows that, in the case of non-deformation, a number of straightly elongated bainitic ferrite laths form parallel from austenite grain boundaries, showing an aspect of typical upper bainite structure. In the case of 30%-deformation, although bainitic ferrite laths are shaped like curves of bows, the deformation has a llittle influence on their length. In the case of 50%-deformation, on the contrary, the length is significantly decreased in the growth direction of the lath. Furthermore, the number of bainitic ferrite lath in a "bainite packet" (an aggregate of bainitic ferrite laths with same crystallographic orientation) is decreased by the enhancement of the nucleation within austenite grains. The decrease of both the length and the number of bainitic ferrite lath leads to complicate the appearance of a microstructure.
TEM observation, however, confirmed that those bainitic ferrite laths were also surrounded by two sets of parallel planes close to {451}_α in the case of 50%-deformation, as previously reported for typical bainitic ferrite laths in the case of non-deformation. Therefore it suggests that bainitic ferrite laths transformed from heavily deformed austenite has also the same crystallographic characteristics with typical one.

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An experimental ultra-low carbon bainitic steel was prepared to investigate the effect of a prior compressive deformation on the morphology of the transformation procuct during continuous cooling. It is found that at the higher cooling rate the deformed austenite tends to form the non-parallel plates of acicular ferrite, and that at the lower cooling rate the deformed austenite tends to form the parallel plates of bainitic ferrite. The orientation relationships between adjacent grains of acicular ferrite have been studied using the analysis of axis-angle pair. The result shows that the adjacent variants have nearly the same orientation in space, which is analogous to the case in alloy-steel weld metals. Furthermore, based on a thermodynamic analysis, it is indicated that acicular ferrite transformation (from deformed austenite) also exhibits the phenomenon of incomplete reaction, where the reaction ceases well before the residual austenite achieves its equilibrium carbon concentration.

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The effects of austenite grain size and deformation in the unrecrystallized austenite region on bainite transformation behavior and microstructural characteristics were investigated under continuous cooling condition and compared with those in ferrite-pearlite transformation and martensite transformation. Austenite grain size does not affect the transformation temperature and resulting microstructure of bainitic ferrite except that nucleation site of bainitic ferrite is increased with the decrease in equiaxed austenite grain size. But the volume fraction of the transformed products depends on the temperature during continous cooling and does not change with the prior austenite grain size. These characteristics are quite different from those in ferrite-pearlite transformation. The deformation in the unrecrystallized austenite region raises the transformation temperature and decreases the hardness of bainitic ferrite. Kurdjumov-Sachs relationship holds between austenite and bainitic ferrite irrespective of prior austenite microstructure. No contribution of austenite grain boundary for the growth of bainitic ferrite seems to indicate the intensive involvement of the displacive mechanism in this transformation.

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The preferential conditions of intragranular acicular ferrite (IAF) in heat-affected zone were studied by adding sulfur and calcium into a Ti-killed steel. Sulfur and calcium addition refined the austenite grain due to the increase in the number density of inclusions. An intermediate austenite grain favored the formation of IAF due to the prevention of Widmanstatten ferrite and polygonal ferrite. The inclusions changed from Ti-oxides dominant to Ti-oxysulfides dominant and Ti-Ca-oxysulfides dominant due to small addition of sulfur and calcium. The nucleation potential of inclusions was found to increase in the sequence of Ti-oxides, Ti-oxysulfides, and Ti-Ca-oxysulfides. This phenomenon could be attributed to the increase of the lattice disregistry between inclusions and ferrite matix or the decrease of strain field around inclusions. Both effects prevented the transformation of austenite to ferrite at elevated temperatures, thereby promoting the formation of IAF. The highest volume fraction of IAF was, therefore, obtained at an intermediate sulfur content with small amount of calcium due to the achievement of optimum nucleation potential and grain size.

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Microstructures and age hardening characteristics of 0.04C-3.5Ni-0.6Cr-1.8Cu-0.4Mo-0.03Nb Cu bearing HSLA steel manufactured by reheat quenching (RQ) and direct quenching (DQ) processes, were investigated. Especially, the effect of DQ treatment on quenched microstructures and niobium carbide and ε-Cu precipitation were examined. A finer quenched microstructure was obtained in DQ processed steel compared with RQ processed steel, due to the heavily deformed austenite introduced by DQ. In DQ processed steel, lath and block sizes of martensite decreased and internal dislocation density increased with the increase of austenite deformation in the non-recystallization region. Hence, the strength of quenched DQ steel increased with the amount of deformation in non-recrystallization region. ε-Cu precipitation characteristics in the aging stage changed very sensitively to the quenched microstructures. The size and growth rate of ε-Cu precipitate decreased with the amount of deformation in the non-recrystallization region, because heavily dislocated structure supplied more preferred nucleation sites of ε-Cu precipitates and retarded the precipitate growth. Also, DQ process affected the niobium carbide precipitation in this steel. In RQ steel, most of niobium precipitated as a coarse carbide in the reheating stage for austenitization. Whereas, in DQ steel, most of niobium precipitated very finely in the aging stage and thus highly contributed to the increase of the strength and tempering resistance. Therefore, it was councluded that DQ processed steel had higher strength and tempering resistance than RQ processed steel due to the finer ε-Cu and niobium carbide precipitates.

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