An investigation on the effect of carbon and silicon on flow behavior of steel
Introduction
Phase transformations during hot deformation of metals and alloys play a significant role in the design of metal forming programs and controlling the product microstructure [1]. There has been considerable debate regarding the effect of different alloy elements including carbon and silicon on the hot deformation behavior of the steel [2], [3], [4], [5], [6], [7], [8], [9], [10]. Some researchers have studied the role of carbon and silicon on strength of steel utilizing experimental models [2], [3], [4], [5], [6], while others have employed mathematical modeling and simulation to distinguish the role of alloy elements on the dynamic and static transformations in austenite. However, the influence of carbon and silicon has not been extensively studied and further investigation is required to clarify the effects of these elements on the hot deformation behavior of steel.
The purpose of the present paper is the evaluation of the role of carbon and silicon on dynamic recovery and recrystallization as well as on the flow stress of steel regarding initial austenite grain size. To do this, three grades of steels were chosen. Single hit hot compression tests at different temperature, stain rates, and holding times together with Begrstrom approach [11] and Avrami equation [12] are utilized to derive the kinetics of dynamic transformation and the flow stress of the steels. Based on the kinetic equations the role of carbon and silicon on the flow behavior of the steels is discussed.
Section snippets
Transformation model
Diverse models have been proposed to describe the hot deformation of metals regarding dynamic recovery [11], [13]. Most of these approaches are based on the balance between work hardening and work softening due to dislocation multiplication and annihilation during hot deformation. Bergstrom [11] has derived the variation of dislocation density according to work hardening and softening as below:where U represents the work hardening and Ω describes the amount of the dynamic recovery
Materials and experimental procedures
The materials used in this research consisted of three grades of steels with the chemical compositions shown in Table 1. To remove the casting structure, the steels were heated to 1250 °C and rolled in this temperature. Hot compression samples were machined out of the as received hot rolled bars with the deformation axis parallel to the hot rolling direction. A height to diameter ratio of 1.5 was selected for the samples to ensure homogeneous deformation. The hot deformation experiments were
Results and discussion
Typical stress–strain curves and initial austenite grain size for 10-min holding time are presented in Fig. 2, Fig. 3, Fig. 4, Fig. 5, respectively. As seen from Fig. 2, Fig. 3, Fig. 4 the shape of all curves are similar to the classical stress–strain curves consisting of initial work hardening at lower strains followed by work softening and a peak in the curves at higher strains. Also, it is observed for all kind of steels increasing the strain rate leads to a larger peak in strain and stress.
Conclusion
In the present research, utilizing the hot compression single hit experiments, the Bergstrom approach and the Avrami equation, the flow stress as well as the kinetics of dynamic recrystallization and recovery of three grades of steels were evaluated. The results show that:
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Increasing the carbon content from 0.08 to 0.7 wt.% leads to reduction of activation energy from 338 to 316 kJ/mol. Also, the rate of dynamic recrystallization is increased. On the other hand, increasing the carbon content
Acknowledgements
The authors acknowledge with gratitude the Department of Mining and Metallurgy at McGill University for the hot compression tests. Also, they would like to thank the Sharif University of Technology for a period of sabbatical leave during which this work was carried out.
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