Polygonal Ferrite Transformation Research Articles

The austenite to polygonal ferrite transformation in low-alloy steel: multi-phase-field simulation

The austenite to ferrite phase transformation is a critical structural transformation in steel production, where the morphology and grain size of ferrite substantially influence the mechanical properties of steel materials. In this work, the influences of cooling rate, prior austenite grain size (PAGS), and Mn content on the microstructure evolution and component distribution of austenite-to-polygonal ferrite phase transformation are investigated by a multi-phase-field model. It is found that higher cooling rates intensify the driving force for austenite to polygonal ferrite phase transformations and delay the phase transformation process. As PAGS decrease, the increased proportion of austenite grain boundary offers more nucleation sites for polygonal ferrite and thus refines the polygonal ferrite grain. Additionally, increased Mn content results in significant grain refinement due to a reduction in the transformation temperature of austenite to polygonal ferrite. This work provides valuable insights into adjusting and designing desired microstructures of polygonal ferrite for enhancing the mechanical performance of steel.

Effect of Deformation Temperature on the Microstructure and Mechanical Properties of High-Strength Low-Alloy Steel During Hot Compression

The microstructure and mechanical properties of high-strength low-alloy steel were investigated at deformation temperatures of 800-1100 °C and strain rates of 0.1-10 s−1 using an MMS-200 thermal mechanical simulator. The results indicated that the increased deformation processes observed between the starting and finishing temperatures during hot compression testing caused a polygonal ferrite transformation in the material. The polygonal ferrite grain sizes increased with increasing transformation temperatures and gradually grew larger at higher deformation temperatures. Widmanstatten ferrite and acicular ferrite were also formed at high temperatures from 1000-1100 °C, which accordingly led to an increase in Vickers microhardness. In addition, the flow stress in the material increased with an increase in the strain and a decrease in the deformation temperature.

Effect of Dissolution and Precipitation of Nb on Phase Transformation, Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.

Isochronal phase transformation of Nb–V–Ti microalloyed ultra-high strength steel upon cooling

The transformation behavior of Nb–V–Ti microalloyed ultra-high strength steel during continuous cooling was investigated by Gleeble-1500 thermal mechanical simulator. Thermal dilation curves were measured at different cooling rates, based on which continuous cooling transformation (CCT) curve was established. The microstructure at slow cooling rate, primarily consists of polygonal ferrite, pearlite and lower bainite, while at intermediate cooling rate besides dominated lower bainite, small amount of polygonal ferrite can also be observed. At higher cooling rate, martensite lath is obtained. The nano-sized precipitates at slow cooling rate include M23C6, M2C, M3C, rich-Nb MC and rich-Ti MC and at intermediate cooling rate and higher cooling rate, the type of precipitates were similar, M3C and two types of MC. At cooling rates ranging from 0.2 to 1°C/s, Vickers hardness of the steel decreases significantly. While at intermediate and higher cooling rates, the Vickers hardness increases gradually with increasing cooling rate. In addition, the polygonal ferrite transformation start temperatures (Ar3) decreases with increasing cooling rate, which is related to the carbide precipitation.

Austenite to polygonal-ferrite transformation and carbide precipitation in high strength low alloy steel

AbstractBy differential scanning calorimetry measurement and microstructural examination, the effects of cooling rates on the kinetics and microstructure of polygonal ferrite transformation have been investigated in a high strength low alloy steel. The peak temperature of ferrite transformation exhibits an evident dependency on cooling rate. With the cooling rate decreasing, the ferrite fraction corresponding to the maximum transformation rate is decreased, as well as the size of polygonal ferrite grains. More ferrite is nucleated at lower cooling rate. On the basis of the effects of austenite grain size and carbide precipitation on ferrite formation, it is suggested that carbide precipitation plays a more dominant role in the kinetics of ferrite transformation. During austenite–ferrite transformation, the ferrite nucleation still proceeds, and the nucleation would not be saturated by pre-existing nuclei.

Microstructure Evolution of HSLA Pipeline Steels after Hot Uniaxial Compression.

The mechanical properties of the high-strength low-alloy pipeline steels were mainly controlled by the subsequent phase transformations after rolling. The influence of hot uniaxial compression on the phase transformation of acicular ferrite was explored by viewing of the deformation degree, the deformation temperature, and the strain rate. The results show that the increase of deformation amounts raises the transformation starting and finishing temperature during the subsequent cooling and also promotes the polygonal ferrite transformation, which leads to the decrease of Vickers hardness accordingly. With the increasing of the deformation temperature, the achieved microstructure becomes coarsened and thus decreases the hardness. As the strain rate increases, the microstructure is refined and thus the hardness increases gradually; increasing the strain rate appropriately is beneficial to the refinement of the microstructure.

Effect of hot deformation and Nb precipitation on continuous cooling transformation of a high-Nb steel

In this work, the effects of hot deformation on continuous cooling transformation of a high-Nb steel were investigated on a Gleeble 3500 thermal simulator. The amounts of dissolved Nb were determined by inductively coupled plasma-atomic emission spectrometry. Furthermore, the effects of hot deformation and Nb precipitation on phase transformation were discussed. Results showed that high-Nb steel is suitable for acicular ferrite pipeline steels because the acicular ferrite microstructure can be obtained in a wide cooling rate range. Hot deformation strongly accelerates the polygonal ferrite transformation and increases the critical cooling rate to obtain a full acicular ferrite microstructure. Moreover, hot deformation markedly refines the final microstructure and improves the mechanical properties of acicular ferrite obtained at a high cooling rate. However, hot deformation can also promote Nb precipitation during holding and even cooling at low rates after hot deformation. Nb precipitation dramatically promotes the polygonal ferrite, weakens the effect of Nb in solution on phase transformation and strengthening, and decreases the microhardness.

The Effect of Simulated Thermomechanical Processing on the Transformation Behavior and Microstructure of a Low-Carbon Mo-Nb Linepipe Steel

The present work investigates the transformation behavior of a low-carbon Mo-Nb linepipe steel and the corresponding transformation product microstructures using deformation dilatometry. The continuous cooling transformation (CCT) diagrams have been constructed for both the fully recrystallized austenite and that deformed in uniaxial compression at 1148 K (875 °C) to a strain of 0.5 for cooling rates ranging from 0.1 to about 100 K/s. The obtained microstructures have been studied in detail using electron backscattered diffraction complemented by transmission electron microscopy. Heavy deformation of the parent austenite has caused a significant expansion of the polygonal ferrite transformation field in the CCT diagram, as well as a shift in the non-equilibrium ferrite transformation fields toward higher cooling rates. Furthermore, the austenite deformation has resulted in a pronounced refinement in both the effective grain (sheaf/packet) size and substructure unit size of the non-equilibrium ferrite microstructures. The optimum microstructure expected to display an excellent balance between strength and toughness is a mix of quasi-polygonal ferrite and granular bainite (often termed “acicular ferrite”) produced from the heavily deformed austenite within a processing window covering the cooling rates from about 10 to about 100 K/s.

Influence of Deformation Temperature on γ-α Phase Transformation in Nb–Ti Microalloyed Steel during Continuous Cooling

The dynamic continuous-cooling-transformation (CCT) diagrams of Nb–Ti microalloyed steel for four different deformation temperatures of 850, 900, 950 and 975°C were plotted by means of a combined method of dilatometry and metallography. The influence of deformation temperature on continuous cooling transformation behavior and transformation microstructure was elucidated. The reason that the polygonal ferrite transformation field is shifted to the right and moved down as the deformation temperature is increased and the progress of transformation for lowest continuous cooling rate of 0.5°C/s was obviously retarded at the lowest deformation temperature of 850°C was discussed in details based on transformation kinetics and strain-induced precipitation kinetics, respectively. Moreover, the polygonal ferrite grain size for lowest continuous cooling rate of 0.5°C/s can be significantly refined from approx. 15.4 μm to 9.4 μm as the deformation temperature is reduced from 975°C to 850°C. In addition, the empirical equation to calculate γ-α phase transformation start temperature was established, and the calculated Ar3 temperatures are in good agreement with measured ones.

Comparative Study on Austenite Decomposition and Cu Precipitation During Continuous Cooling Transformation

The nature of Cu precipitation during continuous cooling of austenite is an essential issue in thermomechanical processing of Cu-bearing high-strength low-alloyed steel. The Cu precipitation behaviors and microstructural evolution are comparatively studied at two representative cooling rates of 0.1 and 10 K/s using dilatometry, optical microscopy, transmission electron microscope, and atom probe tomography. The microstructure developed at the cooling rate of 0.1 K/s contains a variety of microconstituents of polygonal ferrite, acicular ferrite, bainite, martensite, and retained austenite, which is attributed to the varying concentrations of untransformed austenite and consequently changing austenite decomposition kinetics. The Cu precipitation occurs only in association with polygonal ferrite transformation by interphase precipitation mechanism. No Cu precipitates are detected in acicular ferrite, retained austenite, and martensite. At the cooling rate of 10 K/s, the microstructure is dominated by acicular ferrite dispersed with the C-Ni precipitates that are assumed to be formed on the retained austenite. The Cu precipitation generally occurs from aging because of the high temperature and supersaturated austenite in association with acicular ferrite transformation. The nature of Cu precipitation in different cooling rates greatly depends on the initial composition or the extent of solute enrichment in the prior untransformed austenite and the associated phase transformation mechanisms, resulting in the differences in sizes, morphology, and compositions of Cu precipitation.

Effiect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation (TTT) behavior of the low carbon steels with two Si contents (0.50 wt pct and 1.35 wt pct) was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effiects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more homogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calculation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effiect is weakened by the decrease of the isothermal temperatures.

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si-Mn TRIP steel was investigated by means of microstructural observation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.

Effect of silicon addition on recrystallization and phase transformation behavior of high-strength hot-rolled trip steel

Because Si is a noncarbide forming element, a multiphase microstructure consisting of ferrite, bainite, and retained austenite, at room temperature, can be formed by controlling the thermomechanical process strictly. The cooling schedules must be restricted by the formation of pearlite and cementite. In the present article, a new integrated mathematical model for prediction of microstructure evolution during controlled rolling and controlled cooling is developed for a typical kind of low carbon Si-Mn TRIP steel, which consists of temperature simulation, recrystallization, and transformation models. The influence of Si contents has been thoroughly investigated. The calculated results indicate that Si retards recrystallization, restrains austenite grain growth as well as accelerates polygonal ferrite transformation.

Effects of deformation and Mo, Nb, V, Ti on continuous cooling transformation in Cu–P–Cr–Ni–Mo weathering steels

The continuous cooling transformation (CCT) diagrams of the weathering steels Cu–P–Cr–Ni–Mo were investigated by means of a combined method of dilatometry and metallography. For five of the steels (B, C, D, E and F), since there is a metastable austenite gap completely open or half-open between the ferrite and the bainite transformation regions, it is possible to obtain dual-phase microstructure directly by hot-rolling and appropriate cooling. Hot deformation increased the transformation start temperature Ar3 significantly, enlarged the polygonal ferrite transformation temperature range, and shortened the gestation period of pearlite and polygonal ferrite transformation. Mo effectively retards the bainite and the pearlite transformations, elevates the pearlite-finish temperature, and induces the metastable austenite gap formed.

Effects of Thermo-Mechanical Process on the Microstructure and Mechanical Properties of Low Carbon HSLA Steels

Effects of deformation at austenite region and cooling rate on the microstructure and mechanical properties of low carbon (0.06 wt. % C) high strength low alloy steels have been investigated. Average grain size decreased and polygonal ferrite transformation promoted with increasing deformation amount at austenite region due to increase of ferrite nucleation site. Microstructure was also influenced by cooling rate resulting in the development of a mixture of fine polygonal ferrite and acicular ferrite at 10°C/s cooling rate. Discontinuous yielding occurred in highly deformed specimen due to the formation of polygonal ferrite. However, small grain size of highly deformed specimen caused lower ductile-to-brittle transition temperature than slightly deformed specimen.

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstatten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstatten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.