One of the principal requirements for accurate thermomechanical process modelling is reliable constitutive data for the materials being processed. Such data are normally acquired in laboratory tests at constant strain rate and temperature but with the advent of finite element (FE) modelling techniques it is now possible to adopt more realistic assumptions about the complex loading paths occurring in most metal forming processes. These complex-loading paths may involve non-uniform strain rate, non-uniform temperature, strain reversal and the initiation of defects. The proposed project should be seen in a wider context as contributing to the development of a through-process modelling capability, which will enable optimization of process variables to provide the required microstructural evolution and final product properties.
The objectives of the proposed programme are:
To investigate the constitutive behaviour of steels by means of laboratory test simulation of complex loading paths such as strain reversal, variation of strain rate and variation of temperature during deformation.
To identify the mechanisms responsible for the constitutive behaviour observed during the laboratory tests by quantifying the sub-structure obtained in terms of dislocation density, cell size, sub-grain boundary misorientation and recrystallisation behaviour using electron optical techniques such as TEM and EBSD where appropriate.
To carry out laboratory scale hot rolling trials to investigate the effects on constitutive behaviour of complex strain paths such as those in forward and reverse rolling, horizontal-vertical rolling sequences and very high strain rate processes such as rod rolling.
To formulate and further develop alternative constitutive models for predicting flow stress, microstructural evolution, damage initiation and evolution. Also to compare alternative constitutive modelling approaches and identify the advantages and disadvantages of each approach for a particular application.
To develop appropriate methodologies and incorporate the constitutive models into finite element models of rolling and to validate the models by comparison with measurements taken in full scale hot rolling mills.
Each sub-section below describes a task which is referred to in the Programme Bar Chart.
It is important to establish a common basis of knowledge between all partners, taking account of different approaches and identifying deficiencies in modelling capability againstPage 436future requirements. It is also necessary to identify areas where the different modelling approaches may be compared so that any advantages or limitations may be determined.
It is also proposed to carry out a benchmark simulation exercise in which a mutually agreed metal forming process, e.g. hot rolling of a specified steel grade, is modelled and the results compared to illustrate the relative importance of flow stress measurement, constitutive model and FE model on the accuracy of final predictions of quantities such as rolling load and strain distribution.
Axisymmetric compression tests will be carried out in a Gleeble Thermomechanical Simulator and in which the strain rate is changed abruptly or continuously or the deformation temperature is changed rapidly. The transient behaviour of the flow stress following the strain rate or temperature changes will be examined and the subsequent static or post dynamic recrystallisation kinetics will be determined. Both the stress relaxation and double compression testing techniques will be employed. Ti-Nb HSLA and medium carbon steels will be investigated.
The influence of strain reversal on flow stress and subsequent recrystallisation behaviour will be investigated by means of axisymmetric, alternate tension/compression tests in the Gleeble Thermomechanical Simulators at Corus UK Limited and Oulu University.
The various tensile/compressive sequences will be followed by stress relaxation in order to observe any differences in recrystallisation behaviour. The specimen microstructures will be examined and characterised as described in Section 2.3 below in order to identify the mechanisms responsible for differences in constitutive behaviour. Oulu will investigate Ti- Nb HSLA and medium carbon steels. Corus UK Limited will investigate CMn, CMn-Nb and 316 stainless steels.
Torsion tests will be used to provide information about constitutive behaviour following strain reversal. The tests will be carried out for a large set of pre-strain values after which the full flow stress curve for the material can be obtained and the effect of reversal on static and dynamic recrystallisation determined. Hot torsion tests in single pass, multipass and multipass plus strain reversal will be carried out. The materials investigated will be CMn, CMn-Nb and stainless steels.
A novel method of imparting bi-directional strain to a compression sample in the Gleeble Thermomechanical Simulator (strains at 90° to each other) will be investigated as a means of simulating deformation in horizontal-vertical rolling. Deformation sequences will be carried out using uni-directional and bi-directional strains interrupted by quenching to study the microstructure changes during testing. Recrystallisation behaviour will also be determined following the two modes of straining. The specimen microstructures will bePage 437characterised as described in Section 2.3 below in order to rationalise any differences in recrystallisation behaviour. The material investigated will be 316 stainless steel.
In addition to more conventional testing at around 1000-1100°C, a series of tests (both strain reversal and changing strain rate) will be conducted at lower temperatures where the microstructure of the steel is ferritic. The behaviour of ferrite has not been examined earlier in such tests but it is relevant in relation to ferritic rolling for example. The observed phenomena in the ferritic phase will be compared with those in the austenite phase. The materials investigated will be CMn-Nb and medium carbon steels.
Laboratory rolling trials will be carried out to investigate the differences in recrystallisation behaviour between horizontal-horizontal and horizontal-vertical rolling sequences to the same equivalent strains. In the former pass sequence the strains in successive passes are of the same sign whereas in the latter sequence the strains are reversed in successive passes. The material investigated will be 316 stainless steel.
The effect of strain reversal will be investigated by forward and reverse rolling. The development of microstructure and mechanical properties will be compared with those obtained by forward rolling only. The effect of forward and reverse rolling on the development of thermal distributions, which may affect the microstructural evolution, will be investigated for selected materials.
The experimental rod mill at TU Freiberg provides the opportunity to derive information about the constitutive behaviour of steels at very high strain rates, i.e. of the order of 1000 s-1. Such strain rates are not achievable with conventional laboratory test equipment. However, it is possible, that information about the constitutive behaviour of steels at such high strain rates may be inferred from experimental rolling data by use of a suitable theoretical process model. This approach is to be investigated.
In the laboratory tests described above it is important that a detailed understanding of the fundamental mechanisms responsible for the material constitutive behaviour is developed. This is necessary in order to formulate physically meaningful constitutive models. Therefore, characterization of the microstructures will be accomplished using electron optical methods (TEM/EBSD) where appropriate, i.e. for stainless steels or deformed ferritic structures, to quantify the sub-structure in terms of dislocation density, cell size and sub-grain boundary misorientation.
All partners will be involved with the formulation of constitutive laws using a number of alternative approaches.
For example MEFOS will develop further the dislocation density based model for predicting flow stress under varying strain rate conditions and taking account of dynamic recrystallisation. Corus UK Limited will investigate the physical-phenomenological modelling approach. CSM will develop constitutive equations for multiaxial loading, including damage evolution, for use in the three dimensional case of long product rolling. CEIT will develop a physical/semi-empirically based constitutive model to describe the effect of strain reversal on the flow stress curves and on the static and dynamic recrystallisation. TU Freiberg will make further developments to existing constitutive models, e.g. Spittel-ansatz and dislocation density models.
The constitutive models formulated in Section 2.4 above will be incorporated into FE models of rolling.
Emphasis will be placed on developing the methodologies for incorporating complex constitutive models into FE models. This is usually accomplished by means of a user defined material subroutine (UMAT).
TU Freiberg, MEFOS and Corus UK Limited have excellent laboratory/pilot scale rolling facilities which are fully instrumented and will provide initial data for validation of the models. TU Freiberg will utilise optical visioplasticity techniques and the analysis of grey scale images will be further developed for measurements of local surface deformations in the roll gap. These measurements will be compared with FE model predictions. Corus UK Limited and CSM have access to full scale industrial hot rolling mills and will provide data for final validation of the models over a wide range of steel compositions and rolling conditions.
In order to explain the constitutive behaviour observed when the materials are subjected to complex loading paths and in order to facilitate the development of constitutive models, electron optical metallographic techniques such as TEM and EBSD will be employed to characterise the sub-structures obtained in terms of dislocation density, cell size and grain boundary misorientation.
Particular attention will be given to the development of methodologies for incorporating constitutive models into FE models. It is important to ensure that the increased complexity of the models does not make their use impracticable because of the high computing demand.
The exchange of test data and models between partners will enable alternative approaches and procedures to be compared and assessed.
Corus UK Limited will be responsible for the overall management of the project and will appoint a Project Co-ordinator. Each partner will appoint a Technical Manager to liaise with the Project Co-ordinator.
The research described above will be placed in the area covered by the Executive Committee (or the Expert Group): D3
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