Recrystallization of 304SS during and after high strain rate deformation

During the hot working of austenitic stainless steels the shape of the flow curve is strongly influenced by the strain rate. Low strain rate deformation results in flow curves typical of dynamic recrystallization (DRX) but as the strain rate increases the shape changes to a ‘flat-top’ curve. This has traditionally been thought to indicate no DRX is taking place and that dynamic recovery (DRV) is the only operating softening mechanism. Examining the work-hardening behaviour and corresponding deformation microstructures showed this is not the case for austenitic stainless steel, as clear evidence of dynamic recrystallization process can be seen. The post-deformation recrystallization kinetics can be modelled using a standard Avrami equation with an Avrami exponent, n, of 1.15. With an increasing value of the Zener-Hollomon parameter it was found that the kinetics of recrystallization become less strain rate sensitive until at the highest values (highest strain rates/lowest temperatures) the recrystallization kinetics become strain rate insensitive.

Recrystallization kinetic behavior of copper-bearing strip cast steel

In the present study, copper-bearing low carbon steels were produced by direct strip casting (DSC) method on a pilot scale. The effects of copper on mechanical, microstructural, and recrystallization behavior were investigated. As-cast microstructure mainly consists of polygonal ferrite and Widmanstatten ferrite. The increase in Cu increases the amount of Widmanstatten ferrite and induces the formation of bainite in the as-cast condition. It was found that copper increases strength and hardness by solid solution strengthening, grain refinement, and precipitation hardening and the increment is significant above 1% Cu in as-cast condition. Six different compositions were selected for recrystallization study. All the samples were cold rolled to 70% reduction and annealed at three different temperatures, 600, 650, and 700°C for various times. Recrystallization responses were strongly dependent on initial microstructure and Cu content and the effect is dramatic between 1 and 2% Cu. Recrystallization time and temperature were found to be increased with increase in copper content.

Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

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 1000 s 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 10 s resulted in the formation of fine, largely coherent NbC particles with a mean diameter of ∼5 nm, 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 1000 s led to the formation of semi-coherent, gradually coarser and more widely spaced particles with a mean diameter of 8 nm 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. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of alloying and extrusion temperature on the microstructure and mechanical properties of Mg-Zn and Mg-Zn-RE alloys

Extruded Mg-Zn-RE alloys have been shown to exhibit excellent combinations of yield strength and ductility, but it is not completely clear how adding rare earth metals to Mg-Zn alters the microstructure and affects the mechanical properties. Microstructural changes and the resulting mechanical properties from changes in composition and extrusion temperature have been investigated for Mg-. x Zn-. y RE (. x=2.5 and 5. wt.%, y=0 and 1. wt. %, and RE=Gd and Y) alloys. Adding RE to Mg-Zn increased the strength and reduced the ductility, while increasing the zinc concentration in the Mg-Zn-RE alloys had the reverse effect.

A Novel Mo and Nb Microalloyed Medium Mn TRIP Steel with Maximal Ultimate Strength and Moderate Ductility

The multi-phase, metastable, and multi-scale (M3) constitution of a novel transformation-induced plasticity (TRIP) steel (Fe-0.17C-6.5Mn-1.1Al-0.22Mo-0.05Nb, wt pct) was designed through thermodynamic calculations combined with experimental analysis. In this study, Mo and Nb microalloying was used to control the fraction of retained austenite and its mechanical stability during tensile deformation and to improve the yield strength. Thermodynamic calculations were developed to determine the critical annealing temperature, at which a large fraction of retained austenite (~38 pct) would be obtained through the effects of solute enrichment. The experimental observation was in good agreement with the predicted results. According to the critical annealing temperature, such an ultrafine (<200 nm) M3, microstructure with optimum mechanical stability was successfully achieved. The results of this work demonstrated the superior performance with improved yield strength of 1020 to 1140 MPa and excellent ductility (>30 pct), as compared with other TRIP steels. Both angle-selective backscatter and electron backscatter diffraction techniques were employed to interpret the transformation from the deformed martensitic laths to the ultrafine austenite and ferrite duplex structure.

Numerical investigation of influence of the Martensite Volume Fraction on dp steels fracture behavior on the basis of digital material representation model

Development of the methodology for creating reliable digital material representation (DMR) models of dual-phase steels and investigation of influence of the martensite volume fraction on fracture behavior under tensile load are the main goals of the paper. First, an approach based on image processing algorithms for creating a DMR is described. Then, obtained digital microstructures are used as input for the numerical model of deformation, which takes into account mechanisms of ductile fracture. Ferrite and martensite material model parameters are evaluated on the basis of micropillar compression tests. Finally, the model is used to investigate the impact of the martensite volume fraction on the DP steel behavior under plastic deformation. Results of calculations are presented and discussed in the paper.

A comprehensive method to identify the kinetics of static recrystallization using the hot torsion test results with an inverse solution

Many difficulties exist in directly following the static recrystallization of metals, particularly during hotworking. Indirect measurement of static recrystallization has been extensively performed in the literature where, for example, the recrystallization behavior of austenite in steels has commonly been measured indirectly using the fractional softening method. This method relies on the yield stress changes during recrystallization which are physically simulated by hot torsion or compression tests. However, the inherent heterogeneity of deformation during a mechanical test leads to a non-uniform static recrystallization distribution in the test sample. This, in turn, poses a serious question concerning the reliability of the measurement since the stress calculation techniques during recrystallization are not adequately developed in the existing literature. This paper develops a computer-based method to account for heterogeneous deformation during fractional softening measurements based on the hot torsion test data. The importance of the fractional softening gradient in determining the kinetics is emphasized and deficiencies in our understanding of the basic mechanisms are highlighted. A computer-based method is introduced to generate the experimental and computational components in a cost function. The cost function is then utilized by an inverse solution to calibrate the design parameters in a static recrystallization model.

Warm deformation and annealing behaviour of iron-silicon-(carbon) steel sheets

Single pass warm rolling and compression experiments were carried out from ambient to 800°C for ultra-low carbon (ULC) steel with ∼100 ppm carbon and interstitial free (IF) steels, both with two levels of silicon. Subsequently, annealing was done in order to recrystallize the deformed specimens. The main purpose of this study was to understand the effects of rolling temperature and silicon on stress responses and textures. This study comprises two main themes: flow stress and strain rate sensitivity during compression and shear banding and textures in warm rolled specimens. The effects of deformation temperature on in-grain shear bands were different between ULC-Si and IF-Si steels. As in previous work with more conventional steels, in-grain shear bands in the IF grade had low sensitivity to rolling temperature, while those in the ULC grade depended significantly on the deformation temperature. However, the temperature profile of shear banding in the ULC grade was approximately 150°C higher than in previous work. Deformation and recrystallisation textures for both IF and ULC grades depended on their rolling temperatures. The variation of both grain size and texture after annealing can be explained by the rise and fall of in-grain shear banding activity which is related to the strain rate sensitivity.

The need to revise the current methods to measure and assess static recrystallization behavior

Heterogeneous deformation developed during "static recrystallization (SRX) tests" poses serious questions about the validity of the conventional methods to measure softening fraction. The challenges to measure SRX and verify a proposed kinetic model of SRX are discussed and a least square technique is utilized to quantify the error in a proposed SRX kinetic model. This technique relies on an existing computational-experimental multi-layer formulation to account for the heterogeneity during the post interruption hot torsion deformation. The kinetics of static recrystallization for a type 304 austenitic stainless steel deformed at 900 °C and strain rate of 0.01s^-1 is characterized implementing the formulation. Minimizing the error between the measured and calculated torque-twist data, the parameters of the kinetic model and the flow behavior during the second hit are evaluated and compared with those obtained based on a conventional technique. Typical static recrystallization distributions in the test sample will be presented. It has been found that the major differences between the conventional and the presented technique results are due to the heterogeneous recrystallization in the cylindrical core of the specimen where the material is still partially recrystallized at the onset of the second hit deformation. For the investigated experimental conditions, the core is confined in the first two-thirds of the gauge radius, when the holding time is shorter than 50 s and the maximum pre-strain is about 0.5.

Computational inverse analysis of static recrystallization kinetics

© 2015 Published by Elsevier Ltd. All rights reserved. Accurate static recrystallization (SRX) models are necessary to improve the properties of austenitic steels by thermo-mechanical operations. This relies heavily on a careful and accurate analysis of "the interrupted test data" and conversion of the heterogeneous deformation data to the flow stress. A "computational-experimental inverse method" was presented and implemented here to analyze the SRX test data, which takes into account the heterogeneous softening of the post-interruption test sample. Conventional and "inverse" methods were used to identify the SRX kinetics for a model austenitic steel deformed at 1273 K (with a strain rate of 1 s-1) using the hot torsion test assess the merits of each method. Typical "static recrystallization distribution maps" in the test sample indicated that, at the onset of the second pass deformation with less than a critical holding time and a given pre-strain, a "partially-recrystallized zone" existed in the cylindrical core of the specimen near its center line. For the investigated scenario, the core was confined in the first half of the gauge radius when the holding time and the maximum pre strain were below 29 s and 0.5, respectively. For maximum pre strains smaller than 0.2, the specimen did not fully recrystallize, even at the gauge surface after holding for 50 s. Under such conditions, the conventional methods produced significant error.

Effect of twinning on microstructural evolution during dynamic recrystallisation of hot deformed as-cast austenitic stainless steel

An as-cast austenitic stainless steel was hot deformed at 1173 K, 1223 K, and 1373 K (900 °C, 950 °C, and 1100 °C) to a strain of 1 with a strain rate of 0.5 or 5 s⁻¹. The recrystallised fraction is observed to be dependent on dynamic recrystallisation (DRX). DRX grains nucleated at the initial stages of recrystallization have similar orientation to that of the deformed grains. With increasing deformation, Cube texture dominates, mainly due to multiple twinning and grain rotation during deformation.