Initiation Of Dynamic Recrystallization Research Articles

Determining the No-Recrystallization Conditions for Industrial Hot Strip Rolling of Steels Using Laboratory Simulations

Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products. The present work focuses on computation of incubation time tinc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting tinc equal to interpass time. Simulations allow for prediction of the onset of austenite static recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Modeling the Flow Curve of Hot Deformed Austenite

A 0.02%C plain carbon and a 0.22%C TRIP steel were tested in compression in the temperature range 900°C to 1150°C and strain rate range 0.05s-1 to 1s-1. Thirty-two experimental flow curves were obtained in this way. The critical conditions for the initiation of dynamic recrystallization were determined by the double differentiation method. Using a dislocation density model to describe the austenite flow stress, the work hardening parameters r and h were derived and are used to model the flow curve in the absence of dynamic recrystallization. The latter was employed to calculate the fractional softening attributable to dynamic recrystallization. The kinetics of dynamic recrystallization are then described using Avrami kinetics. Finally, the dependences of the Avrami and work hardening parameters on Z, the Zener-Hollomon parameter, are used to model compression flow curves at strain rates an order of magnitude greater than the ones employed in the tests.

Determining the No-Recrystallization Conditions for Industrial Hot Strip Rolling of Steels Using Laboratory Simulations

Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products, as well as for the products undergoing cold rolling and annealing or coating. Recrystallization can only be avoided if its incubation time is longer that the time intervals characteristic for a particular hot rolling process. The present work focuses on computation of incubation time tinc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting tinc equal to interpass time. Simulations allow for prediction of the onset of austenite static or metadynamic recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Dynamic and Static Recrystallization Behavior of Low Carbon High Niobium Microalloyed Steel

The recrystallization behavior of a low carbon high niobium microalloyed steel was investigated using continuous and interrupted hot compression tests. The results showed that the initiation of dynamic recrystallization (DRX) could be detected from inflection in the plot of the strain hardening rate against stress regardless of the stress peak appearance. According to the Zener-Hollomon parameter equation, the activity energy of DRX ( Q def) was obtained and a new modified expression calculating Q def was proposed in consideration of the chemical composition of experimental steel. Applying the 2% offset method, the static softening fraction was determined. The graphic representation of the softening fraction vs interruption time gave the information about the non-static recrystallization temperature (about 1000 °C) and the relationship of precipitation-time-temperature. Static recrystallization kinetics followed the Avrami's law at high deformation temperature, and different values of the exponent m were given to illustrate the effect of niobium element on static recrystallization at different deformation temperatures.

Tensile properties and microstructure of AZ31B magnesium alloy sheet processed by repeated unidirectional bending

The mechanical properties of the AZ31B magnesium alloy sheet processed by repeated unidirectional bending (RUB) with a basal pole tilted in the rolling direction were systematically investigated at different temperatures. Compared with the as-received sheet exhibiting approximately the same average grain size, the fracture elongation increased while the proof stress decreased at room temperature. The initial texture had a significant influence on the mechanical properties at room temperature, while it showed a much weaker effect on the mechanical properties because of the activation of non-basal slips and initiation of dynamic recrystallization during deformation at elevated temperature.

Dynamic Recrystallization of 3104 Aluminum Alloy during Isothermal Compression Deformation at Elevated Temperatures

The dynamic recrystallization characteristic of 3104 alloy has been investigated by isothermal compression deformation at temperatures from 573K to 773K under constant strain rates ranges from 10-3s-1 to 1s-1. It was shown that the form of flow stress curves were very sensitive to temperature and strain rate. At high temperatures and low strain rates, dynamic recrystallization of 3104 alloy deformation occured. The critical value for dynamic recrystallization start was determined from an inflection point that was observed in the plot of work hardening rate vs. flow stress and verified metallograhically based on quenching samples interrupted during deformation. In the experimental domain studied, the ratio of the critical strain for the initiation of dynamic recrystallization to that of the peak strain falled in the range 0.4-0.6. Furthermore, the dependence of εc on Zener-Hollomon parameter( Z) has been developed.

The strain dependence of postdynamic recrystallization in 304 H stainless steel

Using double-hit hot compression tests, the softening behavior of 304 H stainless steel was studied during unloading. The prestrains used were associated with the initiation of dynamic recrystallization (DRX) (e c), the peak strain (e p), 1/2 (e c+e p), the strain at maximum softening rate (e i), and the onset of steady state flow (e s). The following conditions of deformation were used: T=1000 °C, 1050 °C, and 1100 °C, $$\dot \varepsilon $$ =0.01 and 0.1 s−1, and delay times of 0.3 to 1000 seconds. To define the above important strains, single-hit hot compression tests were performed over a wider range of deformation conditions than the double-hit ones—i.e., 900 °C to 1100 °C and $$\dot \varepsilon $$ =0.01 to 1 s−1. The results show that a transition strain (e*) separates the strain-dependent range of postdynamic softening from the strain-independent range. At strains between e c and e*, both metadynamic and static recrystallization contribute to interhit softening. The value of e* obtained in this work was e*=4/3 e p. It was also found that the strain hardening rate was identical at all the critical strains (e*) and took the value −22 MPa.

The Mechanical Behaviour and Microstructures of Interstitial Free Steel Strained in Monotonic or Cyclic Torsion at 1223 K

The work hardening behavior of metals under cold cyclic or multiaxial deformation involves lower hardening rates than those caused by monotonic strain to the same total magnitude. The hardening is lower as the strain per cycle decreases. Similar studies under hot working conditions, for a limited number of cycles, also lead to lower hardening, as well as to changes in the shape of the flow curves and to delays in the initiation of Dynamic Recrystallization (DRX) and in the post-processing static softening kinetics. These results point to a decrease in stored energy caused by cyclic straining. A higher number of hot deformation cycles leads to lower steady state stresses (σssr) than those corresponding to monotonic deformation (σss). Besides, DRX can be suppressed and replaced by Dynamic Recovery (DRV), and the successive cycles have been associated with the repeated build-up and disintegration of dislocation structures. The present research discusses the hot deformation of an IF steel in the austenitic temperature range, for monotonic torsion or a large number of reversed torsions with various strain amplitudes (Δe). It was confirmed that σssr depends on the relative amplitude of the strain cycles (Δe/ep, where ep is the strain at the peak stress in the monotonic torsion of the material), and that lower values of Δe/ep eliminate DRX, which is replaced by DRV. The ferritic grain size after deformation into the cyclic steady state stress depends on the value of σssr, similarly to results in the literature for monotonic torsion of IF steels.

Influence of Boron on Dynamic Recrystallization and Continuous Cooling Transformation of High Strength Interstitial Free Steels

The paper examines the effect of boron (B) on the dynamic recrystallization and continuous cooling transformation (CCT) behavior in Nb-Ti microalloyed high strength interstitial free (IF) steels. For this purpose, two Nb-Ti microalloyed IF steels containing 0.003wt.% and 0.0005wt.% B, respectively, and one IF steel without B were chosen. The dynamic recrystallization behavior was investigated using hot compression testing. The character of the austenite to ferrite transformation during continuous cooling was studied by dilatometry test and CCT diagrams for the IF steels have been constructed. It was found that the initiation of dynamic recrystallization is delayed as the amount of boron increases. Addition of B retards the austenite to ferrite transformation as well. Under cooling rates of 0.5 and 1oC s-1, which correspond to slow cooling rates in the hot strip mill, the addition of B leads to the development of acicular ferrite and bainite phases. On the other hand, at similar cooling conditions the B free IF steel was observed to have a polygonal ferrite microstructure.

Dynamic Twin Nucleation at Triple Junctions in Fe-32%Ni Stainless Steel

The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Critical condition for dynamic recrystallisation of high carbon steels

A softening mechanism for dynamic recrystallisation has been determined through analysis of the continuous compression flow curves of both hypoeutectoid and hypereutectoid steels, 0·7 and 0·9% carbon respectively, in the range 900 – 1050°C for strain rates 0·01 to 1 s−1. The critical stress and strain for the initiation of dynamic recrystallisation were determined from the inflection points in plots of strain hardening rate against flow stress.

Critical Strain for the Initiation of Dynamic Recrystallization in a Ni-Fe Base Alloy

In order to quantitatively analyze the critical strain for the initiation of dynamic recrystallization in Ni-Fe-based Alloy 718, a series of uniaxial compression tests was conducted in the temperature range 927°C - 1066°C and the strain rate range 5 x 10-4 s-1 - 5 s-1 with varying initial grain size. The critical strains were graphically determined based on one parameter approach and microscopically confirmed. The effect of γ'' (matrix-hardening phase) and δ (grain boundary phase)on the critical strain was simply discussed. The constitutive model for the critical strain of Alloy 718 was constructed using the experimental data obtained from the higher strain rate and the temperature range between 940°C and 1040°C.

Initiation of Dynamic Recrystallization in Constant Strain Rate Hot Deformation

In constant strain rate tests, the occurrence of dynamic recrystallization (DRX) is traditionally identified from the presence of stress peaks in flow curves. However, not all materials display well-defined peaks when tested under these conditions. Using plain carbon, Nb-bearing and 321 austenitic stainless steels, it is shown that the onset of DRX can also be detected from inflections in plots of the strain hardening rate θ against stress a or, equivalently, from inflections in In θ-In a and In θ-e plots regardless the presence of stress peaks in the flow curves. These observations are verified by means of metallography. A unified description of the flow curve is introduced based on normalization of the stress and strain by the respective peak or steady state values. This approach reveals that, in a given material, the ratio of DRX critical stress to the peak or steady state stress is constant, as is that of the critical strain to the corresponding strain values. Furthermore, it is shown that the present technique can be used to establish the occurrence of DRX when this cannot be determined unambiguously from the shape of the flow curve.

Influence of Reversible Hydrogen Alloying on Formation of SMC Structure and Superplasticity of Titanium Alloys

A method of production of a submicrocrystalline (SMC) structure (d<1μm) in large workpieces of the commercial pure titanium and the two-phase titanium alloys based on initiation of dynamic recrystallization (DRX) during hot working and reversible hydrogen alloying has been developed. The method involves continuous grain refinement due to DRX at decreasing temperature. Hydrogen alloying decreases deformation stresses and increases ductility at lower temperatures than in hydrogen-free alloy and promotes the formation of the finest grain structure. The grain sizes of 0.1-0.2 μm and 0.04 μm have been produced in the hydrogenated commercial pure Ti and Ti-11.4Al-1.7Mo-0.88Zr (at.%) based alloys, respectively. An oxidized layer retards the hydrogen removal in these alloys at temperatures lower than 500°C. Therefore the SMC structure during degassing can be retained only in complex alloyed two-phase titanium alloys. After degassing the grain size as low as 0.04 μm has been obtained. The hydrogen alloying of commercial pure Ti prevents observing of low temperature superplasticity (SP), but sharply increases superplastic properties at higher temperatures. The low temperature SP at 550°C occurs in the two-phase titanium alloy with the grain size 0.04 μm that has been obtained by using reversible hydrogen alloying.

Formation of a submicrocrystalline structure in TiAl and Ti 3Al intermetallics by hot working

A method of producing a submicrocrystalline structure (grain size d<1 μm) in large pieces of hard-to-deform materials like titanium aluminides has been developed. The method is based on initiation of dynamic recrystallization (DRX) during hot working, utilizing the decrease in grain size during DRX which occurs when the hot working temperature decreases or the strain rate increases, both leading to an increase in flow stress. In practice, the method involves continuous grain refinement due to DRX as the working temperature decreases. A grain size of 0.1–0.4 μm was produced with no porosity in TiAl- and Ti 3Al-based alloys. Partial disordering was detected in a Ti 3Al alloy with a submicrocrystalline (SMC) structure. The grain refinement hardens the intermetallic alloys at room temperature. In a fully ordered Ti 3Al alloy, room temperature ductility increases when the grain size decreases, while ductility of partially disordered Ti 3Al and TiAl alloys is close to zero at room temperature. Materials with SMC structures show superplastic properties at temperatures in the range of 600–900°C, i.e. 200–400°C below the temperature range for superplasticity in the materials with micron-sized grains.

Initiation of dynamic recrystallization under inhomogeneous stress states in pure copper

The initiation of dynamic recrystallization (DRX) in 99.9% pure copper has been investigated for the case of homogeneous and inhomogeneous deformation. Tests were conducted at 400°C on conventional cylindrical test specimens and on specially designed truncated cone specimens in which strain, strain rate, and stress vary spatially. A range of interrupted tests was carried out in order to identify the strain to initiation of DRX for the case of homogeneous deformation, and to track the motion of the “recrystallization front” for the case of inhomogeneous deformation. A material model implemented into finite element software, and employing the initiation criterion of Poliak and Jonas is presented. The model is calibrated using the homogeneous test data, and is used to predict deformation, initiation of DRX, and microstructural change in the truncated cone specimens. The computed results are compared with experiments, and the model is shown to be capable of tracking the motion of “recrystallization fronts” through the deforming material under conditions of inhomogeneous stress. It is also demonstrated that one truncated cone specimen test can replace several tests using conventional cylindrical specimens to study the relationship between dynamic recrystallized grain size and saturated stress.

A model for the initiation of dynamic recrystallization in two-phase materials

Abstract A material model is presented for the prediction of the initiation of dynamic recrystallization in two-phase materials in which the second phase does not deform. Initiation is based on the criterion of Poliak and Jonas in which both energy and kinetic conditions are considered. The criterion is generalized for multiaxial stress states, and coupled with elastic-viscoplastic constitutive equations for high-temperature rate-sensitive materials undergoing strain hardening, dynamic recovery and strain softening due to dynamic recrystallization. The model is embodied within a finite-deformation finiteelement code. Micromechanical finite-element cells are then developed to model two-phase materials, and the dependence of the initiation of dynamic recrystallization on the second-phase volume fraction, shape and distribution is investigated. Stress-state dependence is also examined. Increasing volume fraction of the second phase is found to lead to considerably enhanced hardening at low strains but. at hi...

A one-parameter approach to determining the critical conditions for the initiation of dynamic recrystallization

The onset of dynamic recrystallization is treated in terms of a model based on the principles of irreversible thermodynamics. In the analysis, the initiation of dynamic recrystallization is governed by both energetic and kinetic critical conditions. The former requires that the energy stored during a given deformation schedule attain its maximum, while the latter requires the dissipative processes associated with deformation to decelerate to a critical level. The kinetic critical condition leads to a minimum value of ∥−∂θ∂σ∥ (θ is the conventional strain hardening rate) when the critical state is attained and to the appearance of an inflection point in the θ-σ curve. In a range where the strain rate sensitivity is constant, the onset of dynamic recrystallization corresponds to a constant value of Γ = ∂ ln σ∂ϵ. The initiation of dynamic recrystallization has much in common with the beginning of flow localization. These conclusions of the model are verified using experimental data obtained during the high temperature compression of nickel and of a type 305 austenitic stainless steel.

Recrystallization behaviour of a CMn steel, a titanium-interstitial-free steel and a niobium-microalloyed steel during strip rolling

In order to characterize the Lake Erie strip mill of Stelco, Canada, three grades of steel (CMn, interstitial free (IF) and niobium microalloyed) were subjected to torsional simulation. The flow stresses and microstructural behaviours have been compared. As anticipated, for long interpass times (typical of plate-rolling schedules) the temperature T nr of no recrystallization can be clearly established only for the niobium steel, the other grades undergoing nearly full recrystallization at all temperatures. Short interpass times, typical of strip-rolling schedules, lead to little pass-to-pass strain accumulation in the three steels. In the temperature range 1000-900 °C, static recrystallization is largely responsible for the high degree of interpass softening in the CMn steel. In the case of the IF steel, during the latter stages of finishing, there is some strain accumulation, and both static and dynamic recrystallization occur to a significant degree. For the niobium steel, because of solute effects, the short interpass times (about 1 s) do not permit much static recrystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(C, N) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the niobium steel followed by the initiation of dynamic recrystallization would lead to lower rolling loads in the strip mill for this grade than in the plate mill at the same temperature.

Modeling of flow stress and rolling load of a hot strip mill by torsion testing.

In order to characterize the Lake Erie Works hot strip mill, Stelco Steel, two grades of steel (C-Mn and Nb) were subject to torsional simulation. The flow stresses and rolling loads are considered in this report. The microstructural behaviours of the two grades are compared in a separate publication. As anticipated, for long interpass times, the Tnr temperature (temperature of no-recrystallization) can be clearly established only for the Nb steel, the other grade under-going nearly full softening, i.e. recrystallization, at all temperatures. By contrast, for short interpass times of the order of 2 s, typical of the strip mill, there is little pass-to-pass strain accumulation in either steel. In the temperature range 1 020 to 920°C, static recrystallization is largely responsible for the hight degree of interpass softening in the C-Mn steel. In the case of the Nb steel, because of solute effects, the short interpass times do not permit much conventional recystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(CN) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the Nb steel, followed by the initiation of dynamic recrystallization, leads to lower rolling loads in the strip mill for this grade than in the plate mill at the same rolling temperatures.