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Mechanical Behaviour and Microestructural evolution of strained steels at warm temperatures under different strain modes

The leveling or straightening operations are applied after hot working to ensure that the material has the required surface quality for further practical applications. During these processes, each differential volume element of the profile or sheet at both sides of the neutral axis undergoes a low cyclic strain reversal path due to the applied loading sequence. Traditionally, these operations have been carried out at low temperatures; however new sheet metal production lines can pre-flattened the material at intermediate temperatures, just after hot rolling.

Industrial performance has shown that uncontrolled leveling operations may affect negatively the mechanical properties of the material. The mechanical and microestructural evolution during leveling processes at warm temperatures is an issue that has not been studied in detail so far. In this line, this thesis has been focused on the study of the effect of cyclic strain reversals at intermediate temperatures on the mechanical behavior and microstructural evolution of two low carbon steels. This thesis is a continuation of the research line carried out in Ceit- ik4 and Tecnun in the Materials department and Thermomechanical Treatments Area. The work has been divided into the following points:

  • The effect of the strain reversal on the austenite to ferrite phase transformation. Multipass torsion tests at decreasing temperatures and static conditions, and at constant temperature and dynamic conditions have been carried out.
  • The effect of process variables (austenite conditioning, cooling rate, test temperature, etc..) on the mechanical and microstructural behavior of the material during leveling has been studied deeply. A large programme of multipass torsion tests with cyclic strain reversal, at constant temperature in the range 800ºC-500ºC, have been performed in order to distinguish the effect of each variable.
  • A mixed isotropic-kinematic plasticity model has been developed to describe the stress-strain curves produced during the cyclic strain reversals. The effective equivalent strain concept has been introduced that allows, as a first approach, to explain the effects produced by the strain reversal on the mechanical and microstructural evolution.
  • A bending- unbending reverse bending test has been designed to reproduce more realistically rectification processes. This test, based on the three-point bending test, can be simulate complex thermomechanical cycles. In addition, a finite element model of the test has been implemented so as to analyze the effect of boundary conditions on the mechanical response during bending.
  • This work has been developed within a European project funded by the Research Fund for Coal and Steel entitled "Optimization of Finishing Processes for Eliminating Rectification of Plate and Section Products", running from July 2007 to December 2010. The aim of this project has been the optimization of the hot/warm rectification processes to reduce, and eventually eliminate, the need for cold rectification. Other industrial and technological partners as Corus (now Tata) Steel UK Limited (United Kingdom), Riva Acciaio SpA (Italy) and Akademia Gornicza-Hutnicza (Poland) together with CEIT have been involved in this project.

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