Transformation In Low Carbon Steel Research Articles

EBSD investigation on microstructure transformation in low carbon steel during continuous cooling

In this study the microstructure in low carbon steel during phase transformation was systematically investigated using dilatometry, optical microscopy as well as EBSD. The specimens after annealing at 900°C for 3 min were subsequently cooled at 0·3–100°C s−1 for dilatometry, in order to determine the continuous cooling transformation (CCT) diagram. Then the microstructures were analysed by optical microscopy (OM) and electron backscattering diffraction (EBSD). Dilatometry, optical microscopy as well as image quality technique in EBSD were combined together to determine the continuous cooling transformation diagram of low carbon steel. As increasing in the cooling rate from 1 to 30°C s−1, the fraction of ferrite is almost 90% and the phase transformation occurs from pearite to bainite at the cooling rate between 10 to 20°C s−1.Dans cette étude, on a examiné systématiquement la microstructure de l’acier à faible teneur en carbone lors de la transformation de phase en utilisant la dilatométrie, la microscopie optique ainsi que la DERD. Après un recuit à 900°C pendant 3 min, on a ensuite refroidi les échantillons entre 0·3 et 100°C s−1 pour l’étude de dilatométrie, afin de déterminer le diagramme de transformation en refroidissement continu (CCT). On a ensuite analysé la microstructure par microscopie optique (MO) et par diffraction des électrons rétrodiffusés (DERD). On a combiné la dilatométrie, la microscopie optique ainsi que la technique de haute qualité de l’image de DERD pour déterminer le diagramme de transformation en refroidissement continu de l’acier à faible teneur en carbone. Avec l’augmentation de la vitesse de refroidissement de 1 à 30°C s−1, la fraction de ferrite atteint presque 90% et la transformation de phase de perlite à bainite a lieu à une vitesse de refroidissement entre 10 et 20°C s−1.

Study of the interaction of solutes with Σ5 (013) tilt grain boundaries in iron using density-functional theory

Substitutional alloying elements significantly affect the recrystallization and austenite-ferrite phase transformation rates in steels. The atomistic mechanisms of their interaction with the interfaces are still largely unexplored. Using density functional theory, we determine the segregation energies between commonly used alloying elements and the Σ5 (013) tilt grain boundary in bcc iron. We find a strong solute-grain boundary interaction for Nb, Mo, and Ti that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundary, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk.

Bainite Transformation of Low-carbon and Boron-containing Steel under Continuous Cooling

The effects of deformation temperature and strain on bainitc transformation of low carbon steel and boron-containing steel were investigated under continuous cooling conditions by means of dilatometrie measurement and microstructurc observation. The results show that with decreasing the deformation temperature from 1000 to 800 °C and increasing strains, bainitc start temperature for boron-containing low carbon steel increases, whereas it decreases for low carbon steel under the same condition. The bainitc microstructurcs arc easily obtained for boron-containing steel deformed at different temperatures and different strains compared with plain low carbon steel. With increasing the continuous cooling rate, the bainitc start temperature under deformed condition is about 20 °C higher than that under undcformed condition for boron-containing steel, but it changes slightly when the cooling rate is 20 °C/s or more.

Phase-field modeling of Widmanstätten ferrite formation during isothermal transformation in low carbon steels

The simulation of Widmanstätten ferrite formation, developing from grain-boundary ferrite allotriomorphs during the isothermal austenite to ferrite transformation in binary Fe–C alloys, has been realized using Loginova and KKS phase-field models. It was found that the KKS model can quantitatively predict the transformation kinetics at a larger interfacial thickness comparing with the Loginova model. The plate spacing predicted by the KKS model is more comparable with the experimental result. Both models show that lengthening rates of plates increase linearly with increasing interfacial thickness. The simulation results demonstrate that the formation of Widmanstätten ferrite plates with realistic morphologies requires high anisotropy of interface energy. The increased strength of anisotropy causes a higher lengthening rate of Widmanstätten ferrite plate. A large interfacial energy leads to a high growth velocity of broad sides of Widmanstätten ferrite plate. The disappearance of some plates and the coalescence of two adjacent plates coarsen plate spacing, which is in good agreement with the experimental observation. The lengthening rate increases and the plate spacing decreases with increasing degree of undercooling. As the supersaturation is increased the plate lengthening rate is obviously improved and the spacing of ferrite group is refined.

Effect of austenite grain size on kinetics of dynamic ferrite transformation in low carbon steel

The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae3 in a 6Ni–0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula.

Occurrence of dynamic ferrite transformation in low-carbon steel above Ae3

Dynamic ferrite transformation behavior was investigated over a wide temperature range using a 6Ni–0.1C steel. Softening in flow stress due to dynamic transformation was observed at temperatures above Ae3, the ortho-equilibrium austenite-to-ferrite transformation temperature. Microstructural observation revealed that ferrite grains formed at temperatures above Ae3 showed deformation microstructures, and those grains were reversely transformed to austenite during subsequent holding at the same temperature. Therefore, we concluded that dynamic ferrite transformation could certainly occur even above Ae3.

Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 ‐ C and 850 ‐ C leads to flne ferrite grains. The microstructure characterization and mechanism of the flne ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered difiraction (EBSD) technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries (HAGBs) are dominant in both of the deformation induced microstructures occurring below and above Ae3, with only a few low angle grain boundaries (LAGBs) existing in the grain interiors. It implies that the dynamic strain-induced transformation (DSIT) happens above and below Ae3 temperature and has the same phase transition mechanisms. The reflnement of ferrite is the cooperative efiect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon difiusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with reflned ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the reflned structure is partly responsible for the ductility loss.

Analysis of Transformation Plasticity in Steel Using a Finite Element Method Coupled with a Phase Field Model

An implicit finite element model was developed to analyze the deformation behavior of low carbon steel during phase transformation. The finite element model was coupled hierarchically with a phase field model that could simulate the kinetics and micro-structural evolution during the austenite-to-ferrite transformation of low carbon steel. Thermo-elastic-plastic constitutive equations for each phase were adopted to confirm the transformation plasticity due to the weaker phase yielding that was proposed by Greenwood and Johnson. From the simulations under various possible plastic properties of each phase, a more quantitative understanding of the origin of transformation plasticity was attempted by a comparison with the experimental observation.

Variant selection of lath martensite and bainite transformation in low carbon steel by ausforming

Abstract Variant selection of lath martensite and bainite transformed in an ausformed low-alloy low-carbon steel is analyzed quantitatively by applying reconstruction method of austenite orientation. It was found that transformation texture having {1 1 1} α and {0 1 1} α along compression axis is developed by ausforming both in martensite and bainite transformation. Development of such texture can be explained qualitatively from deformation texture in austenite without taking variant selection into account. However, their intensities cannot be reproduced quantitatively, indicating that specific variants are favorably selected by ausforming. Quantitative analyses of variant fractions revealed that martensite variants whose habit planes are nearly parallel to the primary and secondary slip planes in austenite are formed preferentially. Similar variant selection rule is observed in ausformed bainite while it is not as strong as in ausformed martensite. A preferential nucleation model is proposed for variant selection by ausforming in which variants with habit planes nearly parallel to the primary or secondary slip planes in austenite are nucleated on the microband boundaries in austenite introduced by compression.

Effect of B and B + Nb on the bainitic transformation in low carbon steels

Development of new, advanced high and ultra-high strength bainitic steels requires the selection of the optimum balance of bainite promoting elements allowing the production of the desired bainitic microstructure over a wide range of cooling rates. The addition of boron or a combined addition of boron and niobium is well known to retard strongly the polygonal ferrite formation but very little knowledge has been acquired on the bainitic transformation. Therefore, the purpose of this study is to investigate the influence of boron and boron plus niobium on the bainite transformation kinetics, microstructural evolution and mechanical properties in a low carbon steel (Fe–0.05C–1.49Mn–0.30Si). Isothermal and continuous cooling transformation diagrams were determined and followed by a detailed quantitative characterisation of the bainite microstructure and morphology using complementary advanced metallographic techniques (FEG-SEM-EBSD, SIMS and TEM). The relationship between microstructure and hardness has been evaluated. Finally, results of SIMS and TEM analyses coupled with microstructural investigations enable to propose a mechanism to explain the effect of the synergy between boron and niobium on the bainitic transformation and the resultant microstructure.

SIMULATION OF LEDGEWISE GROWTH KINETICS OF PROEUTECTIOD FERRITE UNDER INTERFACIAL REACTION–DIFFUSION MIXED CONTROL MODEL

A mixed control model considering both carbon diffusion and interfacial reaction is es- tablished for ledgewise growth kinetics of proeutectiod ferrite during → � isothermal transformation in low carbon steels. In this model, the growth rate of ledgewise ferrite and the carbon concentration at the ledge riser are determined by both carbon diffusion at ledge riser in austenite and interfacial reaction rate of → � transformation, which is different from the traditional local equilibrium(LE) model. Simulation is done by utlizing this model to analyze the ledgewise growth kinetics of proeu- tectiod ferrite. And based on analysis of the characteristics of growth kinetics of proeutectiod ferrite obtained by the simulation, a factor S, which includes the effects of carbon diffusion, interface migra- tion, temperature and composition, is proposed to characterize the control mode of growth kinetics. By judging this factor S, we can predict that if the carbon diffusion or interfacial reaction dominates the growth kinetics of proeutectiod ferrite. In the case that the value S is relatively small, the growth kinetics is mainly controlled by interfacial reaction. In the case that the value of S is quite large, the growth kinetics is controlled by carbon diffusion, which returns to the traditional local equilibrium model. The mixed control simulation results for ledgewise growth of proeutectoid ferrite at 720 in Fe-1%C (atomic fraction) alloy show a good agreement with the experimental results previously reported.

Austenite Grain Ultrarefinement and the Formation of Nanocrystallized Structure through Transformation of Low Carbon Steels

The effects of chemical composition, original microstructure, and heating rate on austenite grain refinement were investigated, and the results indicated that ultrafine austenite with grain size 1 µm can be obtained from the original microstructure of ferrite/pearlite after being warm rolled and cold deformed, and the ultrafine austenite could also be obtained through combined addition of Nb–V–Ti and by properly increasing heating rate. The transformation of the ultrafine austenite was also investigated, and the results indicated that when the heavy deformation was applied below Ar3 during cooling, the uniform and equiaxed ferrites with grain size of 0.1 ∼ 0.3 µm, close to nanosize, can be obtained. The main mechanism of deformation-induced transformation of ultrafine austenite to ferrite is that the nucleation of ferrite along austenite boundary is enhanced by the boundary slipping of austenite grains.

Two-dimensional time-resolved x-ray diffraction study of dual phase rapid solidification in steels

The high intensity heat source used for fusion welding creates steep thermal gradients of 100 °C/s from 1800 °C. Further, the influence of preferred orientation is important for the observation of a directional solidification that follows the dendrite growth along the ⟨100⟩ direction toward the moving heat source. In the present study, we observed the rapid solidification of weld metal at a time resolution of 0.01–0.1 s by a two-dimensional time-resolved x-ray diffraction (2DTRXRD) system for real welding. The diffraction rings were dynamically observed by 2DTRXRD with synchrotron energy of 18 keV while the arc passes over the irradiation area of the x-rays. The arc power output was 10 V-150 A, and the scan speed of the arc was 1.0 mm/s. The temperature rise in instruments was suppressed by a water-cooled copper plate under the specimen. Further, the temperature distribution of the weld metal was measured by a thermocouple and correlated with the diffraction patterns. Consequently, solidification and solid phase transformation of low carbon steels and stainless steels were observed during rapid cooling by 2DTRXRD. In the low carbon steel, the microstructure is formed in a two step process, (i) formation of crystallites and (ii) increase of crystallinity. In stainless steel, the irregular interface layer of δ/γ in the quenched metal after solidification is expected to show the easy movement of dendrites at a lower temperature. In carbide precipitation stainless steel, it is easy for NbC to grow on δ phase with a little undercooling. Further, a mistlike pattern, which differs from the halo pattern, in the fusion zone gave some indication of the possibilities to observe the nucleation and the early solidification by 2DTRXRD.

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.

Transformation Textures in Steels: A Simulation Based on Micromechanical Modeling Data

The phase transformation of low-carbon steels is evaluated by means of computer simulations performed with a viscoplastic self-consistent (VPSC) code. The simulations allow calculating micromechanical data at high-temperature deformation that are currently inaccessible through experiments. The micromechanical data are later on used in phase transformation computer codes to evaluate the pre-eminence of certain transformation variants following different criteria. The texture results are compared with experimental results available in the literature. The main fibers and components develop and behave differently depending on the selection of hardening laws, fragmentation criteria, active slip systems, etc. The results are compatible with high-temperature textures characteristic of a high stacking fault energy (HSFE)–like alloy and selection of variants based on the activity of the high-temperature slip systems.

低炭素鋼の連続冷却過程におけるフェライトプレート形成その場観察

In-situ observation of austenite-to-ferrite transformation in low carbon steel was conducted with a laser scanning confocal microscope. Development and kinetics of ferrite plates during continuous cooling process were studied. It was in-situ observed that the ferrite plates nucleated at either austenite grain boundary or allotriomorphic ferrite, and grew with keeping the Kurdjumov–Sachs orientation relationship with prior austenite. Surface relief was induced by the formation of ferrite plate. When the ferrite plate nucleated at pre-existing grain boundary ferrite, gradual change in growth direction by the sympathetic nucleation was confirmed both by the observation and EBSP analysis. Temperature dependency of the observed growth rates of the plates was in a good agreement with the Zener–Hillert model, which indicates the growth is controlled by the diffusion of carbon.

Growth modes of individual ferrite grains in the austenite to ferrite transformation of low carbon steels

The mesoscale deterministic cellular automaton (CA) method and probabilistic Q-state Potts-based Monte Carlo (MC) model have been adopted to investigate independently the individual growth behavior of ferrite grain during the austenite ( γ)–ferrite ( α) transformation. In these models, the γ– α phase transformation and ferrite grain coarsening induced by α/ α grain boundary migration could be simulated simultaneously. The simulations demonstrated that both the hard impingement (ferrite grain coarsening) and the soft impingement (overlapping carbon concentration field) have a great influence on the individual ferrite growth behavior. Generally, ferrite grains displayed six modes of growth behavior: parabolic growth, delayed nucleation and growth, temporary shrinkage, partial shrinkage, complete shrinkage and accelerated growth in the transformation. Some modes have been observed before by the synchrotron X-ray diffraction experiment. The mesoscopic simulation provides an alternative tool for investigating both the individual grain growth behavior and the overall transformation behavior simultaneously during transformation.

The formation of ultrafine ferrite through static transformation in low carbon steels

A novel approach was used to produce an ultrafine grain structure in low carbon steels with a wide range of hardenability. This included warm deformation of supercooled austenite followed by reheating in the austenite region and cooling (RHA). The ultrafine ferrite structure was independent of steel composition. However, the mechanism of ferrite refinement changed with the steel quench hardenability. In a relatively low hardenable steel, the ultrafine structure was produced through dynamic strain-induced transformation, whereas the ferrite refinement was formed by static transformation in steels with high quench hardenability. The use of a model Ni–30Fe austenitic alloy revealed that the deformation temperature has a strong effect on the nature of the intragranular defects. There was a transition temperature below which the cell dislocation structure changed to laminar microbands. It appears that the extreme refinement of ferrite is due to the formation of extensive high angle intragranular defects at these low deformation temperatures that then act as sites for static transformation.

Prediction of Flow Curves and Micro-Structural Evolution in Strain Induced Dynamic Transformation of Low Carbon Steel

The relationships between flow stress curve and microstructure evolution in strain induced dynamic phase transformation (SIDT) of low carbon steel (0.22wt.%) were quantitatively investigated. The deformation was carried out at just above Ar3 temperature (710°C) as a function of strain rate (0.01-5/sec). The softening process of SIDT was well agreed with calculated result derived from Avrami’s and constitutive equation at higher strain rate than 0.5/sec. However, the calculated results differed from the experimental curve at strain rate of less than 0.2/sec. This is due to fact that the dynamic transformation from austenite to ferrite can not be completed owing to less stored energy during hot deformation.

Deformation induced ferrite transformation in low carbon steels

Deformation induced ferrite transformation (DIFT) is a kind of solid state transformation induced through deformation, which can be applied to be as the effective method to produce fine or ultrafine ferrite grains. This paper reviews the research progress in the theory and application of DIFT from five aspects: evidence and study methods, thermodynamics and kinetics, transformation mechanisms, factors influencing DIFT, application of DIFT in production of fine grained C–Mn steel and ultrafine-grained microalloyed steel.