Ferrite Recrystallization Research Articles

Deformation Induced Pearlite Transformation and Spheroidization: Effect of Alloying Additions

The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar1 temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.

Metallographic Analysis of Non-Orientation Silicon Steel W20 in Different Shearing Deformation Area

In order to further understand the relationship between technology and metallographic microstructure for the large-scale production of non-orientation silicon steel, hot assembles rolling mill samples at different shearing deformation strip were selected and analyzed. The results show that the surface microstructure in finishing section at the first meeting to the fourth-pass is band-shape deformed ferrite plus equiaxed recrystallization ferrite, while the center is deformed ferrite. With the increase of shearing deformation, number of rheological ferrite increase, and become more slender.

Warm Deformation Microstructure of a Plain Carbon Steel

Grain refinement in a plain carbon steel under intercritical warm deformation was studied by torsion testing. Based on the experimental results. the warm flow behaviour and microstructural evolution of ferrite were researched with particular emphasis on the effect of the strain rate in controlling the grain refinement mechanism of ferrite. The deformed microstructures were investigated at various strain rates using optical microscopy and electron back-scattered diffraction (EBSD). The EBSD observations indicate that an increase in the strain rate leads to the development of new fine ferrite grains with high angle boundaries. Furthermore, it shows that the annihilation of dislocations occurs more readily at lower strain rate. The elongated ferrite grains continuously dynamically recrystallize to form the equiaxed fine ferrite grains. Thereby, the aspect ratio of elongated grains decreases with increasing the strain rate. Furthermore, the peak stress and steady state stress of ferrite both increase with increasing strain rate. Based on the study, the effect of strain rate on the development of fine ferrite grains during continuous dynamic re-crystallization of ferrite was analyzed in detail.

3D Cellular Automata Modelling of Solid–state Transformations Relevant in Low–alloy Steel Production

A three-dimensional cellular automata (CA) model is developed for the kinetic and microstructural modelling of the relevant metallurgical mechanisms occurring in the annealing stage of low–alloy steels: recrystallisation, pearlite–to–austenite transformation and ferrite–to–austenite transformation on heating and austenite–to–ferrite transformation on cooling. In this model the austenite–to–ferrite transformation is described by a mixed–mode approach, which implies that the transformation kinetics is controlled by both the interface mobility and the diffusivity of the partitioning elements. This approach also allows incorporation of the ferrite nucleation occurring on structural defects. The developed CA algorithm, in which the transformation rules for the grain boundary and interface cells are controlled by the growth kinetics of the forming phase, allows three-dimensional systems to be treated within relatively short simulation times. The simulated microstructure reproduces quite well the microstructure observed in experimental samples. A good agreement is obtained between the experimental and simulated ferrite recrystallisation and ferrite and austenite transformation kinetics. The present approach also models the development of the carbon concentration profile in the austenite, which is, for instance, essential for subsequent martensite formation.

Recrystallization of Ferrite in Spheroidite of Fe-0.67%C Steel

Recrystallization of Ferrite in Spheroidite of Fe-0.67%C Steel

Microstructure Evolution and Mechanical Properties of 780 MPa Hot Dip Galvanized Dual-Phase Steel

The microstructure evolution of 780 MPa hot dip galvanized dual-phase (DP) steel at heating stages of the annealing process was analyzed using a Gleeble−3500 thermal/mechanical simulator. A multifunction continuous annealing simulator was employed to investigate the effect of annealing process on microstructure and mechanical properties of hot dip galvanized DP steel. The experimental results show that ferrite recovery and recrystallization, pearlite dissolution and austenite nucleation and growth take place in the annealing process of hot dip galvanized DP steel. The hardenability can be significantly improved by trace addition of vanadium. When the soaking temperature reaches 780 °C, the tensile strength and total elongation of DP steel can reach 785MPa and 15%, respectively. The microstructure of DP steel mainly consists of a mixture of ferrite and martensite.

A microstructure model for recrystallisation and phase transformation during the dual-phase steel annealing cycle

A three-dimensional cellular automata model is developed for the description of the relevant metallurgical mechanisms occurring in the annealing stage of dual-phase steels: ferrite recrystallisation, pearlite-to-austenite and ferrite-to-austenite transformation on heating and austenite-to-ferrite transformation on cooling. Based on the local grain-boundary and interface velocity, the latter controlled by both interface mobility and carbon diffusivity, the model is able to simulate the microstructure development throughout the annealing stage. The model also provides information on the carbon gradient in austenite at the end of the cycle, which is relevant for the prediction of martensite formation during the subsequent quenching. The simulated structure thus provides a realistic representation of many microstructural aspects.

Study on the ferrite grain refinement during intercritical deformation of a microalloyed steel

In the present research, the ferrite grain refinement during intercritical deformation of a low carbon microalloyed steel within the two phase (α + γ) region was investigated using hot torsion testing. The physical processes that occurred during intercritical deformation were discussed by observing the optical microstructure and analyzing the flow curve responses. The shape of the flow curve suggests that the certain dynamic softening mechanisms take place during deformation. Dynamic softening mechanisms compensate for the hardening effect of deformation and gradually keep balance with it. This flow softening is the result of deformation-induced ferrite transformation and continuous dynamic recrystallization of ferrite. Strain increasing promotes both of the softening mechanisms. Consequently, ultrafine ferrite grains continuously nucleate not only at grain boundaries but also inside austenite and pre-eutectoid ferrite. As a result, ultrafine ferrite with average grain size of ∼1.8 μm achieved. It is concluded that with strain increasing, in addition to deformation-induced ferrite transformation, continuous dynamic recrystallization of ferrite contributes to the further ferrite grain refinement.

Hot deformation behavior of a Cr-containing low carbon steel in the ferrite range

A low carbon steel with Cr addition of 0.46wt% combined with trace elements of Mn and Ti was studied. The apparent activation energy of deformation and the hot deformation equation of the steel in the ferritic range were determined by means of single hot compression tests. The hot-rolled strip of 3 mm in thickness rolled in the ferritic range was obtained using a laboratory hot rolling mill. The mechanical properties show that the values of yield strength and ultimate tensile strength are 230 and 330 MPa, respectively, and the elongation is 33%. The average r-value is 1.1. Large polygonal ferrite recrystallization grains with about 40 μ m in size and the strong {111} recrystallization texture can be obtained in the hot-rolled strip.

Effect of alloying elements and coiling temperature on the recrystallization behavior and the bainitic transformation in TRIP steels

The effects of alloying elements and coiling temperature on recrystallization behavior and bainitic transformation were investigated based on 0.07C-Mn-Cr-Nb steel with a low carbon equivalent. Based on the ferrite recrystallization behavior, the proper intercritical annealing temperature of all studied steels was suggested to produce TRIP steel with good strength and elongation balance. All steels coiled at 550 °C showed much faster ferrite recrystallization behavior than steels coiled at 700 °C. In addition to the coiling temperature, the effect of increasing carbon content on the ferrite recrystallization was minor at a coiling temperature of 550 °C, but much more prominent at a coiling temperature of 700 °C. The highest Mo added steel showed the best strength and elongation balance, and the highest carbon and Mo added steel showed the highest tensile strength at a coiling temperature of 550 °C. The steel containing a higher amount of elemental Al (0.7 wt.% Al) exhibited much better elongation than the lower Al added steel (0.04 wt.% Al) in TS 780 MPa grade, about 24 % and 19 %, respectively.

Development of ultrafine bainite + martensite duplex microstructure in SAE 52100 bearing steel by prior cold deformation

We explore microstructural refinement through complete/partial recrystallization of prior cold-deformed ferrite during austenitizing or austempering of SAE 52100 steel to obtain ultrafine bainite and martensite. Optical and scanning/transmission electron microscopy coupled with compositional microanalysis were employed to determine the volume fraction and dimension (thickness/length) of ferritic sheaves and carbides. Characterization of mechanical properties indicates that 5–15% cold deformation significantly enhances the impact strength (∼72 J) with marginal improvement in hardness and tensile strength over that in the undeformed condition.

冷間加工した高Ｃｒ型肌焼鋼の浸炭時オーステナイト粒形成挙動の解明

It is known that austenite grain coarsening during carburizing is likely to occur in cold-forged steel parts. We estimate that carbide morphology of spheroidizing annealed microstructure prior to cold forging have an influence on the austenite grain coarsening behavior strongly through the ferrite recrystallization during heating to carburizing temperature. In this study, the formation of austenite grain during heating for carburizing of the cold-upset carburizing steel containing higher Cr than conventional JIS SCR420 was investigated. The carburizing steel with 2.1 mass% Cr showed relatively uniform distribution of spheroidized carbides after spheroidizing prior to cold working. It contained much less lamellar carbides than JIS SCR420 with almost 1 mass% Cr. This spheroidizing annealed microstructure is likely to deform more uniformly during cold working than lamellar carbide bearing microstructure. Consequently, both recrystallized ferrite grain and subsequent austenite grain were formed relatively coarse and uniform during heating for carburizing. Because of its higher austenite grain coarsening temperature, the carburinzing steel with 2.1 mass%Cr is suitable for the combined processes of cold forging and carburizing. In this steel, the formation of spherical carbide at ferrite and austenite interface occurs preferentially during spheroidizing and as a result, few lamellar carbides are observed.

Numerical investigation of deformation-induced dynamic transformation in Fe-C alloy by using a Q-state potts Monte Carlo model

The deformation induced dynamic transformation （DIDT） of a Fe-C alloy above austenite transformation equilibrium temperature is simulated by using a Q-state potts Monte Carlo （MC） model. The austenite-to-ferrite transformation, dynamic recrystallization （DRX） of austenite and ferrite, and the ferrite-to-austenite reverse transformation can be simulated simultaneously in the same MC model by building suitable MC transition rules. Meanwhile, an affine transformation model based on vector operation is also coupled with the MC model for the first time for tracking the changes in grain shape during dynamic transformation. The formation of ferrite during DIDT and the influence of austenite DRX on DIDT are simulated based on this MC model. The simulation results show that the competition between the DRX of austenite and the austenite-to-ferrite transformation causes the oscillation behaviour of ferrite kinetics.

Dynamic Recrystallization of Ferrite in Low Carbon Steels

Dynamic recrystallization(DRX) of ferrite in low carbon steels was investigated by hot compression tests at temperatures of 550 to 700oC at strain rates of 0.001 to 10s-1. The results indicate that DRX of ferrite can occur in low carbon steels and lead to grain refinement. With increasing Zener-Hollomon parameter Z, its mechanism changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization, the turning point is approximately at Z=1×1016s-1 for a low carbon steel with 0.171wt% C. The results also indicate that changing the minor constituents of the low carbon steel from pearlite colonies to fine cementite particles has an effect on promoting DRX of ferrite, and the increase of Mn content and the presence of tiny Nb precipitates have opposite effects respectively. However, all these changes are of benefit to the refinement of recrystallized grains.

Recrystallisation at Intercritical Annealing in Low Carbon Steels

Subgrain growth in deformed ferrite and incomplete recrystallisation during intercritical annealing in low carbon (LC) steels was investigated by EBSD and FEGTEM/EDS. It was confirmed that fine dual phase (α+γ) microstructures could be obtained even without the addition of microalloying elements such as Nb and Ti, if the steels were heated above Ac1 temperature before the completion of primary recrystallisation and then intercritically annealed. The fine microstructure was found to be mainly due to the inhibition of primary recrystallisation, and also due to the inhibition of subgrain growth in deformed matrix by finely dispersed γ phase formed during heating. Mn segregation at α/γ interfaces seems to indicate that the kinetics of boundary migration in the existence of γ is controlled by the volume diffusion of substitutional alloying elements across the α/γ interfaces.

Effect of Warm Deformation on Ferrite Microstructure Evolution in a Ti-Microalloyed Steel

Warm deformation is one of the promising hot rolling strategies for producing thin hot rolled steel strips. A better understanding of the microstructure evolution during warm deformation is important for a successful introduction of such processing into the industrial production. In the present research, the effect of deformation strain on the ferrite microstructure development in a low carbon Ti-microalloyed steel was investigated through warm torsion testing. Microstructural analysis with optical microscope and electron back-scattering diffraction was carried out on the warm deformed ferrite microstructures. The results show that at the early stage of deformation an unstable subboundaries network forms and low angle boundaries are introduced in the original grains. Then, with further straining, low angle boundaries transform into high angle boundaries and stable fine equiaxed ferrite grains form. It was considered that dynamic softening and dynamically formation of new fine ferrite grains, with high angle boundaries, were caused by continuous dynamic recrystallization of ferrite.

Interactions between Recrystallisation and Phase Transformations during Annealing of Cold Rolled Nb-Added TRIP-Aided Steels

Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

Influence of Deformation Parameters on Deformation Induced Ferrite Transformation (DIFT) in Plain Low Carbon Steel

The behaviors of deformation induced ferrite transformation (DIFT) and dynamic recrystallization of ferrite (DRX) in the plain carbon steel under various deformation parameters were investigated. The occurrence of phenomena of DIFT and DRX depends on different deformation parameters. Pivotal role for the occurrence of DIFT phenomena is strain rate and temperature. The strain or reduction only changes process of DIFT occurrence. The change of strain rate and deformation temperature will be induced the occurrence of different phenomena such as DRX or DIFT.

Effect of Nb on Ferrite Recrystallization and Austenite Decomposition in Microalloyed Steels

Effect of Nb on Ferrite Recrystallization and Austenite Decomposition in Microalloyed Steels

Dynamic recrystallization of ferrite in a low-carbon steel

Plane strain compression tests were performed on a low-carbon steel from 550 °C to 700 °C (ferritephase range) at strain rates of 10 to 5 × 10−4 s−1, and the deformation microstructure evolution was investigated by means of scanning electron microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results indicate that under the present deformation conditions, dynamic recrystallization of ferrite can occur in the low-carbon steel and lead to grain refinement. With increasing Zener-Hollomon parameter Z, the mechanism of this process changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization; the turning point is approximately at Z=1 × 1016 s−1. The increase of parameter Z leads to the decrease of recrystallized grain size of ferrite under steady state of deformation, and can lead to the formation of ultrafine microstructures with average grain size of about 2 µm.