Low strain processing of LC and ULC steels

Temper rolling and tension levelling are commonly used to manufacture flat rolled steel. Both processes lengthen the steel at strains up to 3% by applying a load and stretching the strip. By latering the balance between the load and the tension the formability of the low carbon and ultra low carbon steel may be optimised.

Local laser heat treatment in dual-phase steels

This research deals with processes leading to local strengthening effects in hot-rolled dual-phase (DP) steels. For this purpose, a method was investigated to achieve local strengthening, namely, local laser heat treatment (LHT). DP sheet steels were globally and homogenously deformed with different degrees of prestrains by cold rolling and subsequently locally heat treated by laser. Following this treatment with selected parameters, the microstructure of the surface and cross section of the heat-treated area as well as the mechanical properties were evaluated by light optical microscopy (LOM), scanning electron microscopy (SEM), as well as transmission electron microscopy (TEM), hardness measurement, and tensile testing. It can be stated that with partial heat treatment, local high strengthening can be produced. At lower heat treating temperatures, this effect could be attributed to bake hardening (BH). Increasing the prestrain as well as temperature results in improving the local properties. With increased heat treating temperature, the initial microstructure near the surface is affected. Partial strengthening of DP steels by laser can open up new fields of application for locally using the strengthening effect to only influence relevant areas of interest, thus providing the potential for saving energy and designed the component's behavior.

Finite element analysis of an axi-symmetric forward spiral extrusion of Mg-1.75Mn alloy

A potential severe plastic deformation process known as axi-symmetrical forward spiral extrusion (AFSE) has been studied numerically and experimentally. The process is based on the extrusion of cylindrical samples through a die with engraved spiral grooves in a near zero shape change manner. The process was simulated using a three dimensional finite element (FE) model that has been developed using commercial software, ABAQUS. In order to verify the finite element results, hot rolled and annealed samples of the alloy were experimentally processed by AFSE. The required extrusion forces during the process were estimated using the FE model and compared with the experimental values. The reasonable agreement between the FE results and experimental data verified the accuracy of the FE model. The numerical results indicate the linear strain distribution in the AFSE sample is only valid for a core concentric while the strain distribution in the vicinity of the grooves is non axi-symmetric. The FE simulation results from this research allows a better understanding of AFSE kinematics especially near the grooves, the required extrusion force and the resultant induced strain distribution in the sample. To compare the mechanical properties of the Mg-1.75Mn alloy before and after the process, a micro shear punch test was used. The tests were performed on samples undergoing one and four passes of AFSE. After four passes of AFSE, it was observed that the average shear strength of the alloy has improved by about 21%. The developedfinite element model enables tool design and material flow simulation during the process.

Microstructures, mechanical properties and in vitro corrosion behaviour of biodegradable Mg-Zr-Ca alloys

The microstructures, mechanical properties, corrosion behaviour and biocompatibility of the Mg-Zr-Ca alloys have been investigated for potential use in orthopaedic applications. The microstructures of the alloys were examined using X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The mechanical properties of Mg-Zr-Ca alloys were determined from compressive tests. The corrosion behaviour has been investigated using an immersion test and electrochemical measurement. The biocompatibility was evaluated by cell growth factor using osteoblast-like SaOS2 cell. The experimental results indicate that the hot-rolled Mg-Zr-Ca alloys exhibit much finer microstructures than the as-cast Mg-Zr-Ca alloys which show coarse microstructures. The compressive strength of the hot-rolled alloys is much higher than that of the as-cast alloys and the human bone, which would offer appropriate mechanical properties for orthopaedic applications. The corrosion resistance of the alloys can be enhanced significantly by hot-rolling process. Hot-rolled Mg-0.5Zr-1Ca alloy (wt %) exhibits the lowest corrosion rate among all alloys studied in this paper. The hot-rolled Mg-0.5Zr-1Ca and Mg-1Zr-1Ca alloys exhibit better biocompatibility than other studied alloys and possess advanced mechanical properties, corrosion resistance and biocompatibility, suggesting that they have a great potential to be good candidates for orthopaedic applications. © 2012 Springer Science+Business Media New York.

Orientation dependence of the deformation microstructure in a Fe-30Ni-Nb model austenitic steel subjected to hot uniaxial compression

The present work was aimed at a detailed investigation of the orientation dependence of the microstructure characteristics in a Fe-30Ni-Nb austenitic model steel subjected to hot uniaxial compression at 1198 K (925 °C) at a strain rate of 1 s−1 to several strain levels up to 1.0. The quantification of the substructure evolution as a function of strain was performed for the stable 〈011〉 oriented grains. Other grain orientations were also investigated in detail at a strain of 0.2. The 〈110〉 oriented grains contained self-screening arrays of “microbands” (MBs) aligned with high Schmid factor {111} slip planes. The MB crystallographic alignment was largely maintained up to a strain of 1.0, which suggests that the corresponding boundaries kept continuously rearranging themselves during straining and did not follow the sample shape change. The mean MB spacing decreased and misorientation angle increased with strain towards saturation, indicating the operation of the “repolygonization” dynamic recovery mechanism. The non-〈011〉 oriented grains displayed a strong tendency to split during deformation into deformation bands having alternating orientations and being mutually rotated by large angles. The bands were separated by transition regions comprising arrays of closely spaced, extended sub-boundaries collectively accommodating large misorientations across very small distances.

Ultrafine grained ferrite formed by interrupted hot torsion deformation of plain carbon steel

A plain carbon steel was deformed using a hot torsion deformation simulator. A schedule known to produce strain-induced ferrite was used with the strain interrupted for increasing intervals of time to determine the effect of an isothermal hold on the final microstructure. Microscopy and electron back-scattered diffraction (EBSD) were used to analyse the changes that occurred in the partially transformed microstructure during the hold and the subsequent applied strain. The strain-induced ferrite coarsened during the hold and this coarsened ferrite was refined during the second deformation. These results were compared to those obtained for a different plain carbon steel deformed in single pass rolling close to the Ar3 temperature.

Fragmentation of orientation within grains of a cold-rolled interstitial-free steel

The formation of a favourable recrystallization texture in interstitial-free (IF) steels depends on the availability and activation of particular nucleation sites in the deformed microstructure. This paper presents a description of the deformed microstructure of a commercially cold-rolled IF steel, with particular emphasis on the microstructural inhomogeneities and short-range orientational variation that provide suitable nucleation sites during recrystallization. RD-fibre regions deform relatively homogeneously and exhibit little short-range orientational variation. ND-fibre regions are heavily banded and exhibit considerable short-range orientational variation associated with the bands. While the overall orientational spread of ND-fibre grains frequently is about the ND-axis, the short-range orientational variation often involves rotation about axes in the TD-ND plane that are nearer to the TD than the ND.

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

In the present study, wedge-shape samples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.

Understanding the deformed microstructure of cold-rolled IF and LC steel

This paper presents an overview of a series of investigations of the microstructure and texture of cold-rolled IF and LC steel. The investigations made extensive use of orientation mapping using electron backscattered diffraction (EBSD) in a field emission gun scanning electron microscope (FEG-SEM). The effect of grain boundaries on the deformed microstructure was examined by comparing the textures of regions near grain boundaries and in the interiors of grains.  A general weakening of the texture, but a strengthening of the {OOI}<110> component, occurs in the vicinity of grain boundaries. Misorientation angle and axis distributions were used to characterise the fragmentation of grains belonging to different orientation classes. The influence of carbon on the deformed microstructure and nucleation during recrystallization was clarified by examining the microstructures of LC and IF steels during rolling and annealing. The
results of the investigations emphasize the important role of shear banding in determining the fragmentation behaviour of ND-fibre grains and the orientations of viable recrystallization nuclei within the deformed microstructure.

Predicting the rolling force in hot steel rolling mill using an ensemble model

Accurate prediction of the roll separating force is critical to assuring the quality of the final product in steel manufacturing. This paper presents an ensemble model that addresses these concerns. A stacked generalisation approach to ensemble modeling is used with two sets of the ensemble model members, the first set being learnt from the current input-output data of the hot rolling finishing mill, while another uses the available information on the previous coil in addition to the current information. Both sets of ensemble members include linear regression, multilayer perceptron, and k-nearest neighbor algorithms. A competitive selection model (multilayer perceptron) is then used to select the output from one of the ensemble members to be the final output of the ensemble model. The ensemble model created by such a stacked generalization is able to achieve extremely high accuracy in predicting the roll separation force with the average relative accuracy being within 1% of the actual measured roll force.

Analysis of work hardening and recrystallization during the hot working of steel using a statistically based internal variable model

A mathematical model has been developed which describes the hot deformation and recrystallization behavior of austenite using a single internal variable: dislocation density. The dislocation density is incorporated into equations describing the rate of recovery and recrystallization. In each case no distinction is made between static and dynamic events, and the model is able to simulate multideformation processes. The model is statistically based and tracks individual populations of the dislocation density during the work-hardening and softening phases. After tuning using available data the model gave an accurate prediction of the stress–strain behavior and the static recrystallization kinetics for C–Mn steels. The model correctly predicted the sensitivity of the post deformation recrystallization behavior to process variables such as strain, strain rate and temperature, even though data for this were not explicitly incorporated in the tuning data set. In particular, the post dynamic recrystallization (generally termed metadynamic recrystallization) was shown to be largely independent of strain and temperature, but a strong function of strain rate, as observed in published experimental work.

Microstructural evolution during hot deformation of duplex stainless steel

The microstructure evolution during hot deformation of a 23Cr-5Ni-3Mo duplex stainless steel was investigated in torsion. The presence of a soft δ ferrite phase in the vicinity of austenite caused strain partitioning, with accommodation of more strain in the δ ferrite. Furthermore, owing to the limited number of austenite/austenite grain boundaries, the kinetics of dynamic recrystallisation (DRX) in austenite was very slow. The first DRX grains in the austenite phase formed at a strain beyond the peak and proceeded to <15% of the microstructure at the rupture strain of the sample. On the other hand, the microstructure evolution in δ ferrite started by formation of low angle grain boundaries at low strains and the density of these boundaries increased with increasing strain. There was clear evidence of continuous dynamic recrystallisation in this phase at strains beyond the peak. However, in the δ ferrite phase at high strains, most grains consisted of δ/δ and δ/γ boundaries.

The production of ultrafine ferrite during hot torsion testing of a 0.11 Wt Pct C steel

Ultrafine ferrite grain sizes were produced in a 0.11C-1.6Mn-0.2Si steel by torsion testing isothermally at 675 °C after air cooling from 1250 °C. The ferrite was observed to form intragranularly beyond a von Mises equivalent tensile strain of approximately 0.7 to 0.8 and the number fraction of intragranular ferrite grains continued to increase as the strain level increased. Ferrite nucleated to form parallel and closely spaced linear arrays or “rafts” of many discrete ultrafine ferrite grains. It is shown that ferrite nucleates during deformation on defects developed within the austenite parallel to the macroscopic shear direction (i.e., dynamic strain-induced transformation). A model austenitic Ni-30Fe alloy was used to study the substructure developed in the austenite under similar test conditions as that used to induce intragranular ferrite in the steel. It is shown that the most prevalent features developed during testing are microbands. It is proposed that high-energy jogged regions surrounding intersecting microbands provide potential sites for ferrite nucleation at lower strains, while at higher strains, the walls of the microbands may also act as nucleation sites.

Local Austenite grain size distribution in hot bar rolling of AISI 4135 Steel

In this paper, the local distribution of austenite grain size (AGS) was experimentally determined by conducting single round-oval and square-diamond pass hot bar rolling experiments of AISI4135 steel. The rolling experiments were carried out using the laboratory mill. The local distribution of AGS was also determined numerically. In order to predict AGS distribution, the AGS evolution model was combined with three dimensional non-isothermal finite element analyses by adopting a modified additivity rule. AGS evolution model was experimentally determined from hot torsion test according to Hodgson's model. The predicted results were in a reasonably good agreement with experimental results.