Dynamic Recrystallization Of Ferrite Research Articles

Increased hot-formability and grain-refinement by dynamic recrystallization of ferrite in an in situ TiB2 reinforced steel matrix composite

It is a challenge to hot-roll TiB2 reinforced steel matrix composite because of strain concentration at the TiB2/matrix interface. In the present study, dynamic recrystallization in ferrite was promoted for the first time to increase the hot-formability and refine the grain size of TiB2 reinforced steel-matrix composite.

Investigation on Hot Compression Precipitation and Recrystallization Behavior of 23.7Cr–2.2Ni–1.3Mo–14Mn–0.26N High‐Manganese Low‐Nickel Duplex Stainless Steel

Isothermal compression deformation studies for 23.7Cr–2.2Ni–1.3Mo–14Mn–0.26 N low‐nickel duplex stainless steel (DSS) are investigated under deformation conditions of 800–1150 °C/0.01–10 s−1. The flow curve and deformation microstructure analysis show that deformation softening changes from ferrite dynamic recrystallization (DRX) to austenite DRX, with increasing temperature from 800 and 900 °C to 1050 °C deformed at 0.01–1 s−1. The sigma (σ) precipitate formation is mainly affected by the interaction of deformation temperature and strain rate. In addition to deformation time controlled by strain rate, the refining microstructure of ferrite DRX accelerates σ precipitate formation due to the increase in diffusion channels deformed at 900 °C/0.01 s−1 and 900 °C/1 s−1. The hot deformation activation energy is calculated as 367.03 kJ mol−1, and the relationship between the critical conditions for DRX initiating and Zener–Hollomon parameters is established. The decreasing Z value contributes to deformation softening at low strain, and the DRX softening effect is enhanced deformed at 800–900 °C/0.01–0.1 s−1. Meanwhile, the modified dynamic crystallization kinetics model of tested DSS at a strain rate of 1 s−1 is established.

Hot Deformation Behavior of Fe-6.8Al Low-Density and AISI 1040 Steels

Abstract Steels are the traditional choice for automobile bodies due to their low cost and good combination of strength, ductility, and formability. Aluminum-containing low-density steels have recently been materials of immense attention due to the quest for a weight reduction of automobiles. For these applications, steel parts are produced by casting, followed by forming operations at an elevated temperature. Furthermore, the application of low-density steels is highly envisaged in automobile applications. Therefore, in the present investigation, the hot deformation behavior of single phase Fe-6.8Al low-density steel, and American Iron and Steel Institute (AISI) 1040 dual phase steel has been evaluated by developing strain rate sensitivity maps. Hot compression tests were carried out at temperatures ranging between 500 and 900°C and 650 and 900°C for the Fe-6.8Al and AISI 1040 steels, respectively, at strain rates between 0.001 and 10 s−1. Microstructural features have been evaluated for both the steels and correlated to the deformation behavior. For the Fe-6.8Al steel, strain localization in the form of shear bands is detected during deformation below 600°C in the strain rate range between 1 and 10 s−1. The dynamic recrystallization of ferrite in Fe-6.8Al steel is also noted during compression at 800 and 900°C for strain rates 0.01 and 0.001 s−1. For the AISI 1040 steel, based on the microstructural features, dynamic recovery and dynamic recrystallization is reported to occur at 700 and 850°C, respectively. The optimized deformation conditions (temperature and strain rate) for the Fe-6.8Al and AISI 1040 steels are determined to be 900°C, 0.001 s−1 and 850°C, 0.001 s−1, respectively.

Mechanism of ferrite grain refinement in the (γ + α) region of weathering steel Cu–P–Cr–Ni–Mo

Abstract

Microstructure Evolution of a New Dual-Phase Steel via Multi-Pass Compression Deformation in the (α + γ) Region

Grain refinement provides an effective way to improve the strength of low-carbon steel without loss of room temperature ductility. The microstructure evolution of Cu–P–Cr–Ni–Mo weathering steel during multi-pass deformation was investigated via hot compression simulation. The results indicated that ultrafine ferrite grains of 1.8 μm can be obtained in the (α + γ) region at multi-pass deformation conditions. The second phase of bainite and martensite was distributed as band form on the ferrite matrix and became thinner with increasing number of passes. The microstructural analysis results showed that at the initial pass deformation, strain-enhanced ferrite transformation is the main mechanism for structure refinement and at the later pass deformation, continuously dynamic recrystallization of ferrite is the main mechanism for ferrite grain refinement.

20#,60Si2Mn,9Cr18MoV Steel Ferrite Area Dynamic Recovery and Recrystallization Research

By using the Gleeble - 1500 hot simulation test machine we studied 20#, 60Si2Mn, 9Cr18MoV the three kinds of low, medium and high carbon steels to observe the dynamic recovery and recrystallization of ferrite while in the process of thermoplastic deformation. We calculated the hot deformation activation energy of each kind of steel by combining the stress-strain curve we got in the experiment and the theoretical model of Z parameter. It turns out Q9Cr18MoV<Q20#<Q60Si2Mn. In the meanwhile, microstructure observation through transmission electron microscope shows that the dynamic recrystallization of ferrite is more likely to happen in 9Cr18MoV steel than in 20# steel and in 60Si2Mn steel. These results indicate that the dynamic recrystallization of ferrite is not only determined by stacking fault energy but also closely related with the strain storage energy release degree.

Dynamic Recrystallization of Ferrite with Particle-Stimulated Nucleation in a Low-Carbon Steel

Microstructure evolution of a low-carbon steel with the initial microstructure of ferrite matrix plus cementite particles during hot compression deformation was investigated at the strain rates of 0.001 s−1, 0.01 s−1, and 1 s−1 at 973 K (700 °C) by means of field-emission scanning electron microscope, electron backscattered diffraction, and transmission electron microscopy. The results indicated that dynamic recrystallization (DRX) of ferrite took place at all of three strain rates, which can be classified as discontinuous DRX at 0.001 s−1, 0.01 s−1, and as continuous DRX at 1 s−1. The formation of the nuclei of DRX of ferrite was mainly ascribed to the occurrence of particle-stimulated nucleation (PSN), accompanied with the lattice rotation and the formation of new high-angle boundaries. The occurrence of PSN was dependent on the development of a subgrain in the regions with high density of dislocations around cementite particles, without the need for the formation of the deformation zone.

Dynamic Recrystallization of Ferrite in a Low Carbon Steel with the (α+θ) Duplex Microstructures

Dynamic recrystallization (DRX) of ferrite in a low carbon steel with the (α+θ) duplex microstructures was investigated using hot compression tests in combination with SEM, TEM and EBSD, and the effect of the size of cementite particles was analyzed. The results indicated that during hot deformation of the low carbon steel DRX of ferrite took place and the increase in the average size of cementite particles was of benefit to DRX. The formation of DRX grains was attributed to particle stimulated nucleation (PSN), by the well development of the subgrain near cementite particles. At the beginning of hot deformation, DRX grains were mainly formed near cementite particles with the size of about 1μm or above. With the increase in the strain, such grains were also formed around pairs or groups of particles with the size of 0.5μm to 1μm.

Numerical simulation of dynamic strain-induced austenite–ferrite transformation in a low carbon steel

A modified cellular automaton modeling has been performed to investigate the dynamic strain-induced transformation (DSIT) from austenite ( γ) to ferrite ( α) in a low carbon steel. In this modeling, the γ– α transformation, ferrite dynamic recrystallization and the hot deformation were simulated simultaneously. The simulation provides an insight into the mechanism of the ferrite refinement during the DSIT. It is found that the refinement of ferrite grains derived from DSIT was the result of the increasing ferrite nuclei density by the “unsaturated” nucleation, the limited ferrite growth and the ferrite dynamic recrystallization. The effects of prior austenite grain size and strain rate on the microstructural evolution of the DSIT ferrite and the characteristics of the resultant microstructure are also discussed.

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.

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.

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.

Dynamic Recrystallization of Ferrite in a Low Carbon Nb-Microalloyed Steel

The hot deformation behavior of a low carbon Nb-microalloyed steel is investigated by hot compression test in the ferrite phase region compared with a low carbon steel with similar compositions, and the effect of Nb on dynamic recrystallization of ferrite is analyzed. Results indicate that during hot deformation in the ferrite phase region, the effect of Nb solely depends on the size of NbC precipitates. Tiny particles which average size is about 7.5nm have a retarding effect on dynamic recrystallization process of ferrite, on the contrary, coarser particles which average size is about 30.6nm have a promoting effect and are of benefit to the refinement of recrystallized grains.

Deformation enhanced transformation and dynamic recrystallization of ferrite in a low carbon steel during multipass hot deformation

Multipass hot deformation with an industry-acceptable pass reduction has been investigated by hot compression. A fine ferrite grain size can be achieved by carefully controlling the interaction of the deformation-enhanced transformation and dynamic recrystallization of ferrite. By the development of a ferrite grain size of 3–5 μm, the yield strength of Q235 grade plain carbon steel can be doubled, without significant loss of ductility, compared with material produced by conventional processing.

Important factors to obtain homogeneous and ultrafine ferrite–pearlite microstructure in low carbon steel

The microstructural evolutions during compressions at different temperatures and strains were investigated using a Gleeble-1500 test machine for a low carbon steel, with and without the formation of pre-eutectoid ferrite. The physical processes that occurred during heavy deformation were discussed by observing the microstructure and analyzing the true stress–true strain responses. The important factors to obtain the equiaxed, homogeneous and ultrafine ferrite, include the initial microstructure before deformation, the deformation temperature and the true strain imposed. The refinement mechanisms for attaining the ultrafine-grained ferrite were discussed in terms of the strain-assisted transformation (SAT) of austenite to ferrite and the dynamic recrystallization of ferrite. The adequate ferritic microstructure has been obtained by deforming the undercooling austenite with a sufficient strain in a reasonable temperature range. The SAT should preferentially be considered in obtaining the ultrafine ferrite because it occurs more easily than the dynamic recrystallization of ferrite. Decreasing the recovery rate of ferrite and increasing the defect concentration in austenite is beneficial to the formation of ultrafine ferrite.

Dynamic recrystallization of ferrite in interstitial free steel

The present study using IF steel confirmed that dynamic recrystallization can occur also in ferrite where it has been generally considered that recovery is an only restoration process during hot deformation. Although the occurrence of DRX has been clarified by microstructural observations and crystallographic determinations, stress-strain curves do not show obvious drop of stress which has been typically reported in the case of DRX of austenite. This result indicates that it is quite difficult to distinguish whether DRX occurs in ferrite only by stress-strain behavior. The noticeable feature of DRX of ferrite is inhomogeneity of recrystallization, i.e., some of the initial grains are hard to recrystallize. This is presumably due to orientation dependence of recrystallization, which is the essential feature of ferrite.

Dynamic recrystallization of ferrite in a low carbon steel during hot rolling in the (F+A) two-phase range

The dynamic restoration mechanisms of metals during hot working have been one of the main topics in the past two decades. In most cases, it has been found that dynamic recovery is the sole dynamic restoration mechanism in ferrite ([alpha]-iron) due to its high stacking fault energy. Dynamic recrystallization is found to occur only in high purity ferrite and in ferritic interstitial free steels. In the latter case, the nucleation of dynamic recrystallization takes place via the bulging of grain boundaries. However, all of the above work has been done on single-phase ferrite, and the dynamic restoration behavior of ferrite in coarse two-phase structures is still unclear. In a previous study on the hot deformation of low carbon steel in the ferrite-austenite (F+A) two-phase range, the authors found that the nucleation mechanism of dynamic recrystallization in the austenite phase was greatly influenced by the presence of F/A phase boundaries. In this paper, the dynamic restoration of ferrite in (F+A) two-phase structures and the role of F/A phase boundaries were investigated.

Recrystallization of ferrite in steel R6M5

1. Recrystallization of deformed ferrite in steel R6M5 commences at about 780°C and it continues to the maximum recrystallization temperatures for existence of α-solid solution. The temperature ranges for recrystallization and superplasticity coincide. 2. Dynamic recrystallization of ferrite in steel R6M5 with rolling for one pass is incomplete with all degrees of deformation. 3. Static recrystallization occurs entirely with e≥50%. The recrystallized ferrite grain size is finer than the original by a factor of four to five. 4. Polygonization of deformed ferrite may markedly stabilize the structure and there-by make recrystallization difficult.

Dynamic restoration of 24% chromium ferritic steels

Kinetics of ferrite dynamic recrystallization with 24% chromium steels was studied by means of laboratory tests. Recrystallization was studied in a temperature range of 1173 to 1573 K at deformation rates\(\dot \varepsilon = 1 \times 10^{ - 2} \) to 5×101 s−1. The kinetics of ferrite dynamic recrystallization was evaluated by means of a parametric equation dependent on the Zener-Hollomon parameter. The results proved that in using the recrystallization activation energyQRD= = 273·9 k J mol−1 the constants of the parametric equation, determined earlier for austenite, are valid for ferrite as well. In addition, the dependence of the grarn size on deformation stress and on carbonitride stabilization was assessed.