Fractional Softening Research Articles

Evolution of strain-induced precipitates in a model austenitic Fe–30Ni–Nb steel and their effect on the flow behaviour

The present work investigated the evolution of strain-induced NbC precipitates in a model austenitic Fe–30Ni–Nb steel deformed at 925°C to a strain of 0.2 during post-deformation holding between 3 and 1000s and their effect on the reloading flow stress. The precipitate particles preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls. Holding for 10s resulted in the formation of fine, largely coherent NbC particles with a mean diameter of ∼5nm, which displayed a cube-on-cube orientation relationship with austenite and caused the maximum increase in the reloading steady-state flow stress. A further increase in the holding time from 30 to 1000s led to the formation of semi-coherent, gradually coarser and more widely spaced particles with a mean diameter of 8nm and above, which led to a gradual decrease in the reloading steady-state flow stress. The holding time increase resulted in progressive disintegration of the dislocation substructure and dislocation annihilation through static recovery processes, which was also reflected by the measured softening fractions. The precipitate particle shape changed during post-deformation annealing from elliptical to faceted octahedral and subsequently to tetra-kai-decahedral.

Analysis of Titanium Microalloying in As-Received and Oxidized Crofer® 22 APU

The addition of titanium to ferritic stainless steel alloys has been shown to benefit the high temperature characteristics of these alloys for use in solid oxide fuel cell (SOFC) interconnects. TiN precipitates can reduce the fractional softening of interconnects at high temperatures by preventing recrystallization and recovery of the cold-worked alloy [1]. TiN has a lower solubility in austenite than ferrite [2]. During solidification of molten steel, TiN precipitates in the austenitic region and is then annealed in the ferritic region [2]. During long term SOFC operation, titanium oxide forms and provides a keying effect at the interface of the alloy and chromia scale [3]. Titanium oxide also forms at the surface of the chromia scale to reduce chromium volatilization [4]. Analysis of an interconnect alloy, Crofer ® 22 APU, before and after oxidation was performed to show the initial development of titanium precipitate morphology, structure, and diffusion.

Evolution of microstructure and mechanical properties of cold-rolled SUS430 stainless steel during a continuous annealing process

This study investigates the effect of the annealing temperature on the microstructure, strength, elongation, and anisotropy properties of cold-rolled SUS430 stainless steel by a continuous annealing experiment. The SUS430 stainless steels of cold-rolled reductions of 84% and 60% were used in this study. The evolution of the microstructure and mechanical properties of SUS430 stainless steel during the annealing process was obtained. As continuous annealing proceeds, the softening behavior of the cold-rolled SUS430 stainless steel strip presented a two-stage trend characterized as recovery and recrystallization. Recovery mainly occurred at a temperature ranging from 600°C to 700°C. The hardness of the strip presented a certain decrease and unobvious change in microstructure was observed in this stage. Recrystallization of the deformed grains initiated at the temperature higher than 700°C, and the hardness of the strip decreased significantly. The mean recrystallized grain size of the sample cold rolled to 84% and 60% reductions are 13.5µm and 20µm, respectively. The tensile and yield strength of the specimens decrease as annealing temperature increases, and the elongation displays a sigmoidal increase with annealing temperature. Annealing temperature has little influence on the mean Lankford value (rm); however, an increase in cold-rolled reduction from 60% to 84% increases the value of rm from 0.9 to 1.3.The yield strength Re, the elongation δ, and the quantitative relationship between the yield strength Re and the softening fraction X were determined for cold-rolled SUS430 stainless steel in the range of εCR=1.0–2.2. The results of our study provide a theoretical basis for the prediction of the mechanical properties of cold-rolled SUS430 stainless steel as well as for the development of new annealing processes.

Metadynamic Recrystallization Kinetics of Twin Roll Cast AZ31 Alloy during Hot Deformation

Metadynamic recrystallization takes place during isothermal annealing of materials in which an incomplete dynamic recrystallization occurred. In magnesium alloys this question has been investigated less until now. The metadynamic recrystallization behavior of twin-roll-cast and annealed AZ31 strip after hot deformation was investigated under conditions corresponding to hot strip rolling process of twin-roll-cast Mg coils. Plane strain compression tests were performed using double hit compression tests at temperatures of 250 - 400 oC, strain rates of 0.1 - 5 s-1 and interpass time of 0.5 – 10 s. Based on the experimental results, the kinetic equations and grain size model were established. Results show the effects of deformation parameters, including forming temperature and strain rate, on metadynamic recrystallization softening fractions and grain size in the two-pass hot deformed AZ31 alloy. Results also reveal that the pre-strain (beyond the peak strain) has little influence on the metadynamic recrystallization behavior in AZ31 alloy. Comparisons between the experimental and the predicted results were carried out. A good agreement between the experimental and the predicted results was obtained.

The study on kinetics of static recrystallization in the two-stage isothermal compression of 300M steel

The static recrystallization behavior in the two-stage isothermal compression of 300M steel is investigated at the deformation temperatures ranging from 1173K to 1373K, strain rates ranging from 0.1s−1 to 5.0s−1, height reductions ranging from 5% to 15%, initial grain size ranging from 36.77μm to 175.68μm, and the inter-pass times ranging from 1s to 30s after the first stage isothermal compression. The experimental results show that the static softening fraction rapidly increases with the increasing of deformation temperature, strain rate and height reduction, while it decreases with the increasing of initial grain size. According to the present experimental results, the kinetics of static recrystallization for 300M steel is proposed. The calculated results are in a good agreement with the experimental results, and the present approach is effective to predict the static recrystallization behavior in the two-stage isothermal compression of 300M steel.

Static Recrystallization and Precipitation Behavior of a Weathering Steel Microalloyed with Vanadium

The static recrystallization (SRX) and precipitation behavior of a weathering steel microalloyed with vanadium were investigated through double-pass compression tests under controlled conditions using the MMS-300 thermal-mechanical simulator. The deformation temperatures ranged from 800 °C to 1000 °C, and the inter-pass time from 1 s to 500 s. The simulation results showed that SRX occurred after 5–10 s at the first compression deformation. The softening fraction of SRX was found to increase with increasing the deformation temperature and the pre-strain. However, the softening fraction scarcely changed during the process of strain-induced precipitation. In addition, the kinetics of SRX was described by the Avrami equation, and the Avrami exponent appeared to be closely associated with the deformation temperature. The microstructure evolution was investigated at the initiation and completion of recrystallization. The amount and distribution of the precipitates were analyzed. The relationship between the driving force of SRX and the pinning force of precipitation was discussed. Besides, the recrystallization inhibition was detected at the early stage of precipitation, and the pinning forces were found to be of a magnitude comparable to the driving force. Moreover, the pinning forces were found to increase with the degree of precipitation and reach a peak at the intermediate stage of precipitation, and finally reduce as the particles coarsened.

Microstructure evolution at the onset of discontinuous dynamic recrystallization: A physics-based model of subgrain critical size

• Evolution of subgrains to recrystallization nuclei is possible at very low strains. • Dynamic recrystallization nuclei increases on a sigmoidal curve with strain. • Nucleation rate increases up to a maximum and decreases to zero at peak strain. • Growth of recrystallized nuclei diminishes to zero at or over the peak. • Site saturation is the mechanism of dynamic recrystallization up to the peak. A physical model based on the evolution of subgrains size is proposed to describe the nucleation and growth processes during discontinuous dynamic recrystallization. The evolution of subgrains to viable recrystallization nuclei was found possible at very low strains. Afterwards, the number of stable nuclei considerably increased on a sigmoidal trend with strain and reached a saturated state at about 0.6 times the peak strain. The dependence of nucleation rate on strain was modeled using an Avrami-type equation and the driving force for the growth of recrystallized nuclei was similarly modeled in terms of strain. It is also shown that “site saturation” is the governing mechanism for the initiation of the discontinues dynamic recrystallization at the grain boundaries. The flow stress of the material was calculated using the law of mixture of recrystallized and unrecrystallized regions with fractional softening as the stress-partitioning factor. Satisfactory agreement between predicted and experimental results was obtained, thereby confirming the validity of the proposed model.

Mathematical Modelling of Hot Rolling: A Practical Tool to Improve Rolling Schedules and Steel Properties

Most of the commercial metallic materials undergo at least one hot deformation stage during fabrication. Hot deformation processing leads to the production of plates, strips, rods, pipes and other shapes at lower overall cost when compared to the cold deformation/annealing route. Comprehensive study of the metallurgical phenomena during hot deformation has enormous potential application in the control of industrial rolling processes. Understanding of the microstructural and mean flow stress evolution lead to sound steel developments and innovative rolling schedules. The models predict parameters such as grain size, fractional softening (static and dynamic) and strain induced precipitation which are useful to improve rolling schedules. Effects such as incomplete softening and strain accumulation can be easily detected as well as their consequences on the final grain size and mechanical properties. In this regard, special attention must be given to steels, the most important metallic material in terms of history, present and future. In this paper, three hot rolling routes will be analyzed in order to produce high strength linepipe steels. Examples were selected on how the use of modelling during development stage can help to meet mechanical properties, mainly toughness and drop weight tear test. Firstly, it is presented a brief overview on mathematical models applied to hot rolling. Thin slab casting/direct rolling, hot strip mill and plate mill are exemplified in the present work. The development of new steel grades can greatly accelerated with the aid of modelling, which is an useful, low-cost technique.

Study of Recrystallization in High Manganese Steels by Means of the EBSD Technique

The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.

Research on Austenite Static Recrystallization Kinetics of Ultra-High Strength Dual Phase Steel

In this paper, double deformation test was conducted in a MMS-300 thermomechanical simulator. Austenite static recrystallization kinetics of ultra-high strength dual phase steel was investigated. The results have shown that the softening fractions increase with holding time as a whole. Under deformation temperatures of 950°C, 1000°C and 1050°C, the softening fractions increase with the increase of the deformation temperature. And the softening curve appears relatively flat under deformation temperatures of 950°C. The austenite no-recrystallization temperature (Tnr) of ultra-high strength dual phase steel was between 900°C and 950°C. Double deformation test conditions which promoted Nb(CN) precipitation resulted in a decrease of a austenite recrystallization. Strain induced precipitation of Nb (CN) was the main reason why the softening curve appeared a plateau.

The metadynamic recrystallization in the two-stage isothermal compression of 300M steel

The two-stage isothermal compression are carried out at the deformation temperatures of 1173–1373K, strain rates of 0.1–5.0s−1, and a strain of 0.69 and the inter-pass times of 0.2–5.0s after the first stage isothermal compression in order to investigate the metadynamic recrystallization of 300M steel. The experimental results show that the metadynamic softening fraction rapidly increases with the increasing of deformation temperature and strain rate. According to the present experimental results, the kinetic equation for the metadynamic recrystallization of 300M steel is proposed. The volume fraction in the metadynamic recrystallization Xm is presented as follows: Xm=1−exp[−0.693(t/t0.5)0.69], and the time for the metadynamic softening fraction of 50%, t0.5=9.7×10−7ε̇−0.27 exp[14103/(RT)]. The calculated results are in a good agreement with the experimental, which indicates that the present kinetic equation can be used to characterize the metadynamic recrystallization behavior in the isothermal compression of 300M steel.

Multi-stage thermomechanical behavior of AISI 410 martensitic steel

The flow softening behavior of AISI 410 martensitic stainless steel was investigated by double hit compression tests in the temperature range of 950–1100°C and the strain rates of 0.01, 0.1, and 0.5s−1. The tests were conducted with delay times varying between 1 and 50s after achieving a strain of 0.1, 0.16 and 0.2 in the first stage. The fractional softening has been predicted by the offset stress method and the kinetic equations have been proposed to predict the static recrystallization behavior of hot deformed AISI 410 martensitic stainless steel. Experimental results indicate that the flow softening is a function of the inter-pass time for different strain rates and deformation temperatures. Using analytical approach, the static recrystallization activation energy and kinetics model was obtained and built. A good agreement between experimental results and predicted values of the model was achieved.

Modelling the Static Recrystallisation Texture of FCC Metals Using a Phase Field Method

A three dimensional phase field model has been developed to simulate the texture formed during the static recrystallisation of FCC metals with medium or high stacking fault energy, such as aluminium, copper and nickel. Before recrystallisation the deformation texture as well as the stored energy was simulated using a three dimensional crystal plasticity finite element model. This output calculated on the distorted finite element mesh was first mapped onto the regular grid of the phase field model using a linear interpolation method and then used as initial condition for the subsequent recrystallisation texture modelling. This model has successfully predicted the typical recrystallisation texture components: cube {001}<100>, R {124}<211> in the aluminium alloy. In addition, the softening fraction and three dimensional microstructure produced during static recrystallisation have also been simulated by this model.

Recrystallization of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation. Part Ш: Metadynamic recrystallization

► The kinetic equations of metadynamic recrystallization were developed. ► The grain size model for metadynamic recrystallization was proposed. ► The effects of forming parameters on the microstructural evolution were studied. The metadynamic recrystallization (MDRX) behavior of 30Cr2Ni4MoV ultra-super-critical (USC) rotor steel during hot deformation was investigated based on the first part of this study, in which the evolution of the dynamically recrystallized structure was studied in detail. Compression tests were performed using double hit schedules at temperatures of 970–1250 °C, strain rates of 0.001–0.1 s −1 and inter-pass time of 1–100 s. Based on the experimental results, the kinetic equations and grain size model were established. Results show that the effects of deformation parameters, including forming temperature and strain rate, on MDRX softening fractions and austenite grain size in the two-pass hot deformed 30Cr2Ni4MoV steel are significant. Results also reveal that the pre-strain (beyond the peak strain) has little influence on the MDRX behaviors in 30Cr2Ni4MoV steel. Comparisons between the experimental and the predicted results were carried out. A good agreement between the experimental and the predicted results was obtained, which verified the developed models.

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.

Hot deformation behavior and constitutive modeling of VCN200 low alloy steel

The hot working behavior of VCN200 medium carbon low alloy steel was analyzed by performing hot compression tests in temperature range of 850–1150°C and at strain rates of 0.001–1s−1. Flow curves were typical of dynamic recrystallization during hot working over temperature range of 900–1150°C and strain rates of 0.001–1s−1. However, at lower temperatures no indication of flow softening was observed. The constitutive analysis using the hyperbolic sine function was performed and the value of apparent activation energy for the hot deformation determined to be about 435kJ/mol. The flow curves up to the peak were successfully modeled using a dynamic recovery model. All the factors in this model were defined in terms of the Zener–Hollomon parameter. A modified form of Avrami equation was used to estimate the fractional softening due to the dynamic recrystallization for any given strain in flow curve. Using this model, the flow curves were successfully predicted and generalized to different deformation conditions.

Modeling the initiation of dynamic recrystallization using a dynamic recovery model

Dynamic recrystallization flow curve was studied in AISI 410 martensitic stainless steel by performing hot compression tests in a temperature range of 900–1150 °C and at strain rates of 0.001–1 s −1. The Estrin and Mecking's equation for dynamic recovery was used to model the work hardening region of the flow curves. The critical strain and stress for the initiation of dynamic recrystallization were determined using the method developed by Poliak and Jonas. The critical dislocation density for starting dynamic recrystallization was estimated using the Estrin and Mecking's dynamic recovery model. A modified Arrhenius-type equation was used to relate the critical dislocation density to strain rate and temperature. The proposed model was also verified by the model proposed by Roberts and Ahlblom and developed to describe the variation of dislocation density and fractional softening due to dynamic recrystallization up to the peak of flow curve.

A new method to predict the metadynamic recrystallization behavior in 2124 aluminum alloy

The metadynamic recrystallization behaviors in deformed 2124 aluminum alloy were investigated by isothermal interrupted hot compressive tests, which were carried out at the deformation temperatures of (653–743) K, strain rates of (0.01–10) s−1 and inter-stage delay time of (30–180) s. A new approach, “peak stress method”, is proposed to calculate the softening fractions induced by the rapid metadynamic recrystallization. The kinetic equations were developed to predict the metadynamic recrystallization behaviors in hot compressed 2124 aluminum alloy. Both the experimental and predicted results show that the effects of deformation parameters, including strain rate, deformation degree and temperature, on the softening behaviors in the two-pass hot compressed 2124 aluminum alloy are significant. A good consistency between the experimental and predicted results indicates that the proposed kinetic equations can precisely estimate the softening behaviors and metadynamic recrystallization kinetics of the hot compressed 2124 aluminum alloy.

Static Recrystallization Behavior of Twin Roll Cast Low-Carbon Steel Strip

The static recrystallization kinetics of low-carbon steel cast strip was investigated by means of interrupted hot tensile tests. As-cast strip was reheated and soaked and its austenite grain size was similar to the width level of the as-cast columnar structure. The tests were carried out on Gleeble-3500 thermomechanical simulator. The deformation temperature is in the range of 800 to 1200 °C with strain rate of 0.01 to 1 s −1. The prestrain is fixed at 0.04 to 0.12, and the inter-hit delay time varies from 1 to 3000 s. Effect of deformation conditions and initial micro-structure on static recrystallization behavior was investigated. The activation energy (Q six) and Avrami exponent ( n) of static recrystallization were determined to have 241 kJ/mol and 0.54 respectively by linear regression of the experimental results. A kinetics model was proposed to describe the static recrystallization kinetics in low-carbon steel cast strip. The predicted softening fractions are in good agreement with the experimental results, indicating that the proposed equations can give an accurate estimate of the softening behaviors for the low-carbon steel cast strip.

Recrystallization behavior of a Nb-microalloyed steel during hot compression

Using a Thermecmastor-Z hot simulator, dynamic recrystallization (DRX) and static recrystallization (SRX) behavior of a Nb-microalloyed steel was investigated by single-hit compression tests and double-hit compression tests, respectively. The experimental results show that DRX will more easily occur at higher deformation temperature and lower strain rate. The deformation activation energy and stress exponent for the Nb-microalloyed steel are calculated to be 379.29 ± 23.56 kJ/mol and 5.76 in temperature range of 950 °C to 1100 °C by regression analysis, respectively. Furthermore, a semi-empirical model is developed to identify the peak stress and strain for DRX. It is found that SRX kinetics follows Avrami’s law, and the softening fraction predicted by the model agrees well with experimental results.