Dynamic Ferrite Transformation Research Articles

A hot deformation constitutive model applicable for complete austenite and dynamic ferrite transformation interval

Dynamic ferrite transformation is a vital microstructure evolution mechanism for carbon alloy steels and an essential technology for manufacturing high-performance steel materials. In practice, carbon steel hot working processes are usually carried out at various temperatures, involving the evolutionary behavior of the single-phase austenite and the ferrite dynamic transformation process. Few models can simultaneously predict the microstructural state and deformation resistance in the single-phase austenitic interval and the dynamic ferrite transformation interval. The deformation mechanisms involve work-hardening, dynamic recovery, and dynamic recrystallization of austenite and ferrite phases, as well as the dynamic transformation of austenite to ferrite and coordinated deformation between the two phases. Therefore, this paper extends the application range of the model based on the viscoplastic unified constitutive model by considering the above deformation mechanisms. The constitutive model proposed in this paper applies to both the single-phase austenite interval and the dynamic ferrite transformation interval. Moreover, it can accurately predict carbon alloy steel's microstructure state and deformation resistance in different intervals. This paper can help to construct a hot deformation constitutive model in a wide range of deformation temperature intervals of steel materials.

Improved Mn–segregation bands and its influence on deformation behavior in a severely warm–rolled 10Mn lightweight steel

The hot–rolled microstructures of medium Mn lightweight steels led to the formation of Mn–segregation bands due to a relatively high Mn content of 9–12 wt%, ultimately deteriorating their overall mechanical properties. In this study, a multi–pass warm rolling of 95% reduction was proposed to minimize the Mn–segregation bands in the 10 wt% Mn lightweight steel. This was followed by the detailed microstructural examinations and ex–situ tensile testing to evaluate the mechanical behavior response and deformation mechanism operating upon straining. Severe warm rolling accelerated the dynamic ferrite transformation and recrystallization in the Mn–rich austenitic bands, thus improving the microstructural homogeneity. The austenite stability was further improved so as to enable continuous strain hardening by transformation/twinning–induced plasticity (TRIP/TWIP) effects. This, thereby, led to obtain high tensile strength of 985–1201 ± 15 MPa together with large elongation of 34.3–76.0 ± 1.1%. In addition, the strain compatibility between ferrite and austenite also proves to give rise to an alleviation of strain localization at the ferrite/austenite interface, which also postponed the plastic instability and contributed to the large ductility by the coordinated TRIP/TWIP effects.

Role of the crystallographic texture in anisotropic mechanical properties of a newly-developed hot-rolled TRIP steel

In general, hot-rolled steels have higher strength but lower ductility in the transverse direction. In this work, the effects of the crystallographic texture on the anisotropic tensile properties of a new hot-rolled TRIP steel, named DF-TRIP steel, were investigated by electron backscatter diffraction. The tensile tests showed that the hot-rolled steel exhibited nearly the same yield strength but different TRIP effects when loaded along the rolling direction and transverse direction. The low anisotropy in yield strength was explained by the negligible difference in rolling texture in the ferrite due to strain-induced dynamic ferrite transformation. However, a lower Taylor factor of the austenite was found along the rolling direction, leading to early onset of slip and thus providing nucleation sites for strain-induced martensitic transformation. This new hot-rolled steel shows higher ultimate strength and better total uniform elongation in the rolling direction.

Nature of dynamic ferrite transformation revealed by in-situ neutron diffraction analysis during thermomechanical processing

Nowadays, a new concept of process utilizing dynamic ferrite transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1 μm, has been proposed. This paper reports transformation mode of dynamic ferrite transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of in-situ neutron diffraction analysis during thermomechanical processing. Dynamic ferrite transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed ferrite is the main mechanism for the formation of ultrafine-grained structure.

Deformation-Induced Dynamic Ferrite Transformation During Hot-Rolling in Oxide Dispersion-Strengthened Ferritic Steel with 9 Wt Pct Cr Content

9CrODS steel, a candidate fission and fusion structural material, was subjected to hot-rolling with varying parameters of surface temperature and cooling rate just after hot-rolling. The deformation-induced dynamic ferrite transformation was confirmed at the rolling temperature 805 °C above Ar3 (780 °C). This transformation exhibits three characteristic features: transformation for extremely short duration (0.044 second), retaining carbon content equal to the original without long-distance carbon diffusion, and elongated coarse ferrite grains (10 μm). The massive transformation was proposed for the dynamic ferrite transformation from the hot-rolled austenite. The driving force for massive transformation was quantitatively estimated considering dislocations accumulated by hot-rolling. It was also shown that the oxide particles in 9CrODS steel play a critical role for dynamic ferrite transformation by suppressing the dynamic recrystallization at hot-rolling.

Coopetitive micro-mechanisms between recrystallization and transformation during/after dynamic strain-induced transformation in aluminum-containing low-carbon steel

In this work, the coopetitive relationships among dynamic ferrite transformation, reverse ferrite transformation, and austenite recrystallization were investigated by using dynamic dilatometry, optical metallography, and electron backscattering diffraction in an aluminum-containing low-carbon steel subjected to hot compressions in the two-phase region. The microscopic mechanism of concurrent dynamic softening was studied based on the analysis of transformation crystallography. Moreover, this analysis method was used to discover the occurrence of reverse-transformation-induced recrystallization. In this microscopic mechanism, new austenite grains formed by reverse transformation can act as seeds for recrystallization. These paths for microstructural control in steels are not common, but they can be enabled by critical thermo-mechanical treatments combined with proper alloy design.

Effect of heavy warm rolling on microstructures and mechanical properties of AISI 4140 steel

In this study, ultrafine structured AISI 4140 steels with ultrahigh strength, sufficient ductility and low yield ratio were obtained through warm rolling and subsequent quenching without tempering. The effect of warm rolling reduction ratio ranging from 60% to 90% on microstructures and mechanical properties of steels was investigated. It was clarified that the microstructures of steels were remarkably refined after heavy warm rolling. Ultrafine grained ferrite was formed because of dynamic ferrite transformation and dynamic recrystallization. Great enhancements in YS, UTS and microhardness after warm rolling were attributed to grain refinement and subsequent formation of martensite. The increasing ferrite and decreasing martensite contents contributed to the decrement of strength and increment of ductility in steels when the warm rolling reduction ratio increased. The WR (60%) steel exhibited the highest strength with YS and UTS of 1570MPa and 2370MPa respectively. And the WR (85%) steel exhibited an optimal balance of strength and ductility with YS, UTS and elongation of 1080MPa, 2022MPa and 7.9% respectively.

Multi-Phase-Field Analysis of Stress–Strain Curve and Ferrite Grain Formation during Dynamic Strain-Induced Ferrite Transformation

A numerical model of dynamic strain-induced ferrite transformation (DSFT) was developed by combining the multi-phase field model with the Kocks–Mecking model. Using the developed model, a three-dimensional simulation of the DSFT in a Fe-C alloy was performed to study the correlation between the variation in flow stress and the microstructure evolution during the DSFT. The simulation results indicated that the developed model successfully simulated the characteristic DSFT behavior, i.e., both the stress–strain curve with a single peak and the formation of an ultrafine-grained ferrite microstructure. The variation in the flow stress during the DSFT was characterized by the volume fraction of the ferrite phase.

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae3 temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.

On-line spheroidization process of medium-carbon low-alloyed cold heading steel

Conventionally manufactured 35CrMo cold heading steel must undergo spheroidization annealing before the cold heading process. In this paper, different types of deformation processes with various controlled cooling periods were operated to achieve on-line spheroidal cementite using the Gleeble-3500 simulation technique. According to the measured dynamic ferrite transformation temperature (Ad3), the deformation could be divided into two types: low temperature deformation at 810 and 780°C; “deformation-induced ferrite transformation” (DIFT) deformation at 750 and 720°C. Compared with the low temperature deformation, the DIFT deformation followed by accelerated cooling to 680°C is beneficial for the formation of spheroidal cementite. Samples subjected to both the low-temperature deformation and DIFT deformation can obtain granular bainite by accelerated cooling to 640°C; the latter may contribute to the formation of a fine dispersion of secondary constituents. Granular bainite can transform into globular pearlite rapidly during subcritical annealing, and the more the disperse phase, the more homogeneously distributed globular cementite can be obtained.

Dynamic Softening of Flow Stress during Dynamic Ferrite Transformation

This study using a 6Ni-0.1C steel confirmed the relationship between the change in a fraction of dynamically transformed ferrite and the dynamic softening in stress-strain curve during dynamic transformation above ortho-equilibrium austenite-to-ferrite transformation temperature. Dynamic softening in stress-strain curve was well-fitted with a form of Avrami equation as a function of strain, and it corresponded with the change in fraction of ferrite. The slope of work-hardening rate was increased due to the additional softening phenomenon, i.e. dynamic transformation, to dynamic recovery of austenite. Dynamic softening of austenite, which has been considered as a typical evidence of dynamic recrystallization, could be interpreted as a response of dynamic transformation to ferrite.

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.

Flow stress analysis for determining the critical condition of dynamic ferrite transformation in 6Ni–0.1C steel

In order to clarify the occurrence of dynamic ferrite transformation in a 6Ni–0.1C steel, the stress–strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic transformation to ferrite. Analysis of the maximum stress in the stress–strain curves indicated that dynamic ferrite transformation occurred above a certain value of the Zener–Hollomon parameter (Z). The critical deformation condition (ZC) for the occurrence of dynamic transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of ferrite, and the maximum flow stress came close to the flow stress of ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the ferrite grain, inhomogeneity in ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of ferrite transformation during deformation, i.e. dynamic ferrite transformation.

Ferrite Transformation during Deformation of Super-cooled Austenite

The research results achieved by the Korean national project, HIPERS-21, on the ferrite transformation during the deformation of super-cooled low carbon austenite were summarized. Fine ferrite grains formed during the deformation of austenite, i.e. dynamically. The rate of ferrite nucleation was estimated to be accelerated several hundred times by the deformation of austenite. However, the grain refinement could not be explained by the accumulated strain alone. The application of stress during the ferrite transformation is known to effectively weaken the orientation relationship of the ferrite grains with austenite, making the coalescence of the grain difficult during the growth of ferrite and effectively increasing the nominal ferrite nucleation rate. The application of the dynamic ferrite transformation to industrial hot rolling or plate milling was also summarized. The multi-pass rolling technique was introduced to produce a fine ferrite grain structure and controlled cooling was adopted to produce a multi-phase structure in order to improve the work hardening rate of fine grained steels.

Combined Macro–Micro Modeling for Rolling Force and Microstructure Evolution to Produce Fine Grain Hot Strip in Tandem Hot Strip Rolling

T.M.C.P. (Thermo Mechanical Control Process) has been widely used in the steel industry. We have produced fine grain hot strips industrially through high reduction and low temperature asymmetric rolling which decreases rolling force by generating a cross shear zone.We needed combined macro–micro model to predict rolling force in the finishing train and microstructure evolution of fine grain hot strips. First, deformation analysis model, an asymmetric rolling theory based on numerical analysis using Orowan's theory was proposed. Second, to obtain the material data for the kinetics of microstructure change in the low temperature austenite region, the compression tests were performed and we measured recrystallization ratio by EBSD (Electron back scattering diffraction). Third, microstructure evolution model to formulate the ferrite transformation using the intragranular nucleation was introduced.Accuracy of the new combined macro–micro model for rolling force and microstructure evolution was very excellent and it enabled us to confirm dynamic ferrite transformation occurrence during high reduction low temperature rolling at the actual tandem hot strip mill.

Effect of Thermo-Mechanical Variables on the Dynamic Ferrite Transformation Induced by Hot Deformation in 0.22C-Mn Steel

Hot torsion of a C (0.22 wt%)-Mn steel was used to investigate the influence of thermomechanical arameters on the strain induced dynamic transformation (SIDT) of ferrite. The pecimens were strained as a function of strain rate (0.05/sec - 5/sec) and strain (- 5.0) at right bove Ar3 temperature. The critical strain to initiate dynamically transformed ferrite nuclei during deformation increased as increasing the strain rate. On the other hand the completion of SIDT was hifted to larger strain by decreasing strain rate. This is due to the fact that the dynamic ransformation of ferrite was processed in the interior of austenite grain as well as at grain boundary y large stored energy and many nucleation sites for high strain rate. The dynamic transformed micro-structure of ferrite was developed to higher angle and the grain size could be refined to ~3 ㎛ at strain of 3.0 and 5/sec.