Multi-pass Hot Rolling Research Articles

Development of dislocation-based unified material model for simulating microstructure evolution in multipass hot rolling

Dynamic recrystallization and recovery are two competing processes. Both may continue after hot deformation, such as during passes in multipass hot rolling processes, reducing dislocation density of materials and allowing larger plastic deformation to be achieved. The main objective of this research is to develop a set of mechanism-based unified viscoplastic constitutive equations which model the evolution of dislocation density, recrystallization and grain size during and after hot plastic deformation. This set of constitutive equations are determined for a C-Mn steel using an evolutionary programming (EP) optimization technique and implemented into the commercial finite element (FE) solver ABAQUS for process simulations. Numerical procedures to simulate multipass rolling are developed. FE analysis is carried out to simulate the evolution of grain size, dynamic/static recrystallization and recovery, and to rationalize their effects on the viscoplastic flow of the material in a two-pass hot rolling process.

Modelling of microstructural evolution in multipass hot rolling

Mechanism-based unified viscoplastic constitutive equations are developed to model the evolution of grain size, recovery, dynamic and static recrystallisation and their effects on viscoplastic flow of materials. This set of constitutive equations are determined for a C–Mn steel using a GA-based optimisation technique and implemented into the commercial FE solver ABAQUS for process simulations. Numerical procedures to simulate multipass rolling are developed. FE analysis is carried out to rationalise the evolution of grain size, dynamic/static recrystallisation and recovery, viscoplastic flow and stress relaxation in a multipass hot rolling process.

A set of unified constitutive equations for modelling microstructure evolution in hot deformation

Recrystallisation and recovery are two competing processes. Metals showing high degrees of dynamic recovery are unlikely to recrystallise dynamically, since it is difficult to accumulate the dislocation density to a certain level to initiate recrystallisation. Recrystallisation and recovery may continue after hot deformation, such as during passes in multipass hot rolling processes. Both processes reduce dislocation density of materials and allow large plastic deformation to be achieved. The main aim of this research is to develop a set of mechanism-based unified viscoplastic constitutive equations, which models the evolution of dislocation density, recrystallisation and grain size during and after hot plastic deformation. Deformation mechanisms of metals at high temperatures and techniques of modelling the physical effects are analysed. The effects of dislocation density and recrystallisation on viscoplastic flow and grain size evolution of a micro-alloyed steel are rationalised. Optimisation techniques and procedures are developed to determine the unified viscoplastic constitutive equations from experimental data.

A new approach to predicting partial recrystallization in the multi-pass hot rolling process

An exploratory approach to handling partial recrystallization in multi-pass hot rolling where the heterogeneity of steel microstructures is inherent is presented. The proposed model is based on a modification of the conventional model in which the microstructure of deformed austenite at each pass is simply taken as homogeneous during the multi-pass rolling. The usefulness of the modified model is demonstrated by applying it to a four-pass oval-round (or round-oval) rod rolling sequence. The pass-by-pass recrystallized fraction and austenite grain size (AGS) computed from the modified model are compared with those from the conventional model. The result showed that in multi-pass rolling at higher rolling speed, the recrystallization behavior and evolution of the austenite grain size at a given pass was strongly influenced by the modeling method of the partial recrystallization attributed to microstructural heterogeneity.

Multipass rolling of aluminium alloys: finite element simulations and microstructural evolution

Use of numerical predictive methods such as finite element analysis is becoming progressively more common for modelling industrial hot metal working and forming processes. These tools are used not only to predict the thermomechanical behaviour of metals but increasingly to predict microstructural changes by linking them to physical models of recrystallisation and textural evolution. This paper describes the development and application of a fully integrated model for the prediction of thermomechanical and microstructural behaviour during multipass hot rolling of aluminium alloy AA 3104. Finite element code ABAQUS/standard has been used in the work and the process is modelled assuming plane strain conditions. It is shown that for this alloy the static recrystallisation which occurs during interpass cooling does not significantly influence the thermomechanical response during subsequent rolling passes.

Flow stress equation for multipass hot-rolling of aluminum alloys

A series of simple axisymmetric compression tests were carried out on the computer servo-controlled Gleeble 1 500 machine when strain rates ranged between 0.05–25 s−1 and deformation temperature 300–500 °C. The results show that flow stress is related to the Zener-Hollonom parameter Z and strain, as well as the static recrystallization fraction between passes during multipass hot deformation of 5182 aluminum alloy. Hence, a modified exponential flow stress equation was presented by considering the values of InA and β as functions of strain, and by using the uniform softening method and incorporating the static recrystallization fraction between passes to consider the effects of residual strain during multipass hot-rolling of 5182 aluminum alloy. The validity of the equation was examined by a typical non-isothermal multipass deformation test.

Effect of hot working variables on microstructure and properties of Fe-25AI

The evolution of microstructures of extruded ingot Fe-25AI material during multipass hot rolling at 800°C (B2 ordered) and 1000°C (disordered) has been studied. Grain refinement was found to be difficult at 1000°C because, in the processing times, recrystallisation and grain growth were too rapid. At 800° C, however, recrystallisation and grain growth were significantly slower, which makes grain refinement and microstructural control feasible. Good agreements were found between the loads measured in rolling and the flow stress values obtainedfrom plane strain compression testing. Roll chilling was found to cause a significant increase inflow stress because the surface of the Fe-25AI material is cooled below Tc in this case. After a number of consecutive rolling passes, a dramatic increase in the flow stress was observed, which can be attributed to the additional effect of work hardening, combined with the effects of roll chilling and ordering. The properties of the material processed by different hot rolling schedules were examined by tensile testing and, by manipulation of hot rolling and heat treatment parameters, it was possible to produce higher proof stress and lower elongation, or vice versa.

Spray deposition of an iron aluminide

High velocity oxyfuel (HVOF) spray forming of an iron aluminide [Fe–12.5 Al–2.93 Ni–0.02 B (wt%), containing 300 p.p.m. oxygen], followed by heat treatment for 24 h at 500°C, 18 h at 600°C and 20 min at 800°C, and multipass hot rolling at 800°C has been studied. Three different thicknesses (0.43, 0.93 and 1.33 mm) of sprayed deposit were produced by spraying for different times (approximately 10, 20 and 30 min). The spray-deposited layers exhibited some oxide and some porosity. This porosity was reduced by heat treatment. The as deposited layer had a high degree of B2 order, and a B2 antiphase domain size of 4.5 nm. On hot rolling this material to a reduction of 38%, it was found to be more susceptible to edge cracking than similar material processed by an ingot–extrusion–hot rolling route. In heat treatment, the aluminide-sprayed layer formed a non-protective Fe2O3 oxide, rather than the usual Al2O3 that forms on the binary alloy. This is attributable to the Ni content of the iron aluminide powder employed. © 1998 Kluwer Academic Publishers

Hot rolling and superplastic forming response of net shape processed Ti-6AI-4V produced by centrifugal spray deposition

The hot rolling and superplastic forming response of Ti-6Al-4 V deposits, produced using a unique low pressure centrifugal spray deposition (LPCSD) facility, have been examined. Fundamental information has been generated relating to the parameters which govern microstructural development (porosity distribution, grain size, and α/β phase volume fraction) both during the deposition process and following post-spray thermomechanical processing. As sprayed deposits were characterised by relatively coarse transformed β microstructures, with α lath and prior β grain sizes varying from about 0.6 to 1.1 μm and from 70 to 400 μm respectively, depending on the spray forming environment (Ar or vacuum) and the position within the preform. Despite these coarse microstructural features, and relatively high levels of porosity, single and multipass hot rolling reductions of up to 80% were successfully achieved, yielding microstructures and strain rate sensitivity factors (m values) compatible with superplasticforming grade sheet specifications.

Hot rolling and superplastic forming response of net shape processed Ti-6AI-4V produced by centrifugal spray deposition

The hot rolling and superplastic forming response of Ti-6Al-4 V deposits, produced using a unique low pressure centrifugal spray deposition (LPCSD) facility, have been examined. Fundamental information has been generated relating to the parameters which govern microstructural development (porosity distribution, grain size, and α/β phase volume fraction) both during the deposition process and following post-spray thermomechanical processing. As sprayed deposits were characterised by relatively coarse transformed β microstructures, with α lath and prior β grain sizes varying from about 0.6 to 1.1 μm and from 70 to 400 μm respectively, depending on the spray forming environment (Ar or vacuum) and the position within the preform. Despite these coarse microstructural features, and relatively high levels of porosity, single and multipass hot rolling reductions of up to 80% were successfully achieved, yielding microstructures and strain rate sensitivity factors (m values) compatible with superplasticforming grade sheet specifications.

Modeling recrystallization kinetics, grain sizes, and textures during multipass hot rolling

A physically based model for the evolution of recrystallization microstructures and textures during hot rolling of aluminum is presented. The approach taken differs from similar models developed for steels. The present model is based on recent experimental investigations directed toward identifying the nature of the nucleation sites for recrystallized grains of different crystallographic orientations. Particle stimulated nucleation (PSN) and nucleation from cube bands and grain boundary regions have been incorporated in the model. The multipass aspect complicates the modeling due to partial recrystallization between the rolling passes. Two different approaches have been suggested to handle this. The model has been applied to predictions of recrystallization kinetics, recrystallized grain sizes, and recrystallization textures during multipass hot rolling of aluminum. The predictions are reasonable compared to experimental results.

A model for recrystallisation kinetics, texture and grain size applied to multipass hot rolling of an AlMgMn aluminium alloy

A physically based model for primary recrystallisation, including recrystallisation kinetics, grain size and texture evolution, during hot rolling of aluminium is presented. The model is based on extensive experimental investigations that have been directed towards identifying the nature of the nucleation sites for recrystallised grains of different crystallographic orientations. Particle stimulated nucleation and nucleation from cube bands and grain boundary regions have been incorporated in the model. The model has been applied to the hot rolling of an AlMgMn alloy processed in 13 passes. Reasonable model predictions have been obtained for cases of reversing rolling and a 3-stand tandem mill, including variations in the exit gauge temperature.

Origin of cube texture during hot rolling of commercial AlMnMg alloys

Investigation of samples taken at different gauges during multi-pass hot rolling has been made with emphasis on characterisation of the through-thickness variation in microstructure and texture. The evolution of the cube texture ((001),(100)), which is a major cause of anisotropy after hot rolling is of particular interest. In the present study a detailed investigation of the origin of the cube texture component during hot rolling has been performed. Information on the characteristics of this special crystallographic orientation during the hot rolling operation has been obtained by applying the electron back scattering pattern (EBSP) technique in SEM. The results show that the cube orientation is metastable during hot deformation in the sense that large fractions of the cube are able to survive through heavy reductions without rotating towards the stable texture components. Cube grains, which during a few hot rolling passes get flattened to a critical thickness, subsequently serve as sites for the nucleation of second generation cube grains.

Some new results in thermomechanical processing of microalloyed steels

In recent times, efforts have been directed towards a better understanding of the mechanisms associated with deformation and restoration of austenite during continuous multipass hot rolling of microalloyed steels. The correspondence between the condition of austenite before transformation and the resultant microstructure upon cooling holds the key to the attainment of interesting properties.In the present paper, some results obtained on the deformation of austenite in a microalloyed steel, using a hot compression machine, are presented. Here, the idea was to simulate actual plate rolling or hot strip rolling conditions and study the evolution of microstructure at different stages of the hot deformation process i.e. after precise reductions at given strain rate and temperature of deformation.The paper further discusses recent results obtained by us on the influence of hot deformation parameters (strain, finish rolling temperature, temperature of deformation) and cooling rates (air cooling, spray water cooling) on the microstructure of microalloyed steel. The precise conditions leading to the evolution of acicular ferrite and bainitic microstructures have been identified. New information on the influence of short tempering treatments (15 min at 550, 600 and 650°C) on the microstructure and properties of a microalloyed steel are also outlined.

Modelling Microstructure and Its Effects during Multipass Hot Rolling.

By collaborative work the Sheffield Leicester Integrated Model for Microstructural Evolution in Rolling (SLIMMER) has been developed for hot rolling of flat products. The background physical metallurgy is presented together with the expressions used to describe microstructure evolution for a range of ferrous and non-ferrous metals. The finite difference thermal model at the heart of SLIMMER computes heat loss to air, descalers, rolls and water cooling while allowing for oxidation and deformation heating. The use of temperature compensated time enables isothermally determined equations for microstructure evolution to be applied to practical non-isothermal conditions. Rolling loads and torques are calculated using Sims theory with an accurate prediction of mean flow stress. Examples of rolling niobium microalloyed steel plate and the effect of initial grain size illustrate the capabilities of SLIMMER and show some of the validation of the predictions.

A Model for Recovery and Recrystallization of Hot Deformed Austenite Considering Structural Heterogeneity.

A mathematical model for the recovery and recrystallization of austenite during a multi-pass hot rolling was developed. Contrasting to the conventional model, in which the condition of austenite before each deformation is expressed only by the average grain structure with homogeneous dislocation density, in the present model, the austenite is expressed taking into account the microstructure heterogeneity. Thereby, the distribution of grain size and dislocation density can be predicted. In modelling the microstructural change in the multi-pass hot deformation process, austenite was divided into a large number of small elements having, in each, different grain sizes as well as different dislocation densities. The distribution of grain size and dislocation density can be expressed by summing up the conditions of all the elements. The optimum number of elements were determined to be 100 considering the accuracy of prediction and the calculation time. The comparison of recrystallized fraction between the present and conventional models indicated that the present model gave much-improved results, especially when partially recrystallized austenite was hot rolled as often experienced in the latter stage of hot strip mill rolling and in the entire process of plate mill rolling.

The thermomechanical processing of AlSiC particulate composites

The development of microstructure during the deformation and annealing of discontinuously reinforced metal matrix composites is discussed, with particular reference to composites having a pure aluminium matrix. The deformation inhomogeneities induced by the ceramic particles and the role of the particles on the nucleation and growth of recrystallized grains during subsequent heat treatment are considered. The effect of the ceramic particles on the microstructures developed during multipass hot rolling is shown to be complex, depending on particle size and volume fraction. The effect of deformation processing on the ceramic is discussed and, in particular, the fracture of ceramic particles, platelets and whiskers is considered.

工業用アルミニウムの多パス熱間圧延時の組織変化

In order to investigate subgrain formation and precipitation in a 1100 aluminum alloy during hot-rolling process, DC cast ingots were subjected to four conditions of homogenization and then multi-pass hot-rolling. The size of subgrains formed during hot-rolling changed depending on the condition of rolling pass through Z-parameter, but was not affected by both the distribution of precipitates and the amout of solid solution determined by the homogenizing condition. Precipitation took place during hot-rolling. Precipitates were considered to be preferentially formed during rolling, but not formed during holding period between passes. In the present range of temperatures, the amount of precipitates increased with decreasing rolling temperature. The precipitates formed in the matix during homogenization resulted in the enhanced precipitation during rolling.

Prediction of temperature distribution, flow stress and microstructure during the multipass hot rolling of steel plate and strip.

The hot rolling of steel was simulated using data from laboratory experiments and mill trials. Particular attention was paid to prediction of the temperature distribution through the thickness of the rolled plate or strip. The effects taken into account are radiation and convection from the surface when the material is between stands, and conduction to the rolls and the temperature increase due to mechanical work when the material is in the roll gap. An explicit finite difference method is used to calculate the temperature distribution through the thickness of the workpiece during processing.On the basis of the present temperature model and of the constitutive and recrystallization kinetics equations developed earlier for the steels under investigation, a computer model was developed for the prediction of rolling force and microstructural evolution. The predictions of these models are in good agreement with measurements on both experimental and commercial steels. They can accordingly be used for off-line applications, such as mill construction and rolling schedule design and optimization. They also constitute a step towards the on-line control of plate and hot strip mills.

Microstructural development during thermomechanical processing of particulate metal-matrix composites

AbstractThe development of microstructure during the deformation and annealing of discontinuously reinforced metal-matrix composites is discussed with particular reference to composites having a pure aluminium matrix. The deformation inhomogeneities induced by the ceramic particles are examined, and the role of the particles in the nucleation and growth of recrystallised grains during subsequent heat treatment is considered. The effect of the ceramic particles on the microstructures developed during multipass hot rolling is shown to be complex, depending on particle size and volume fraction. Superplasticity is shown to occur by several mechanisms in particulate composites. The effect of deformation processing on the ceramic is discussed, with emphasis on the fracture and realignment of ceramic particles, platelets, and whiskers.MST/1299