Nowadays, modern casting theories and technologies have got marked progress in reducing steel casting’s defects, such as shrinkages, cracks, porosities, and segregations, which make it possible to manufacture industrial parts with casting instead of forging billet. Compared with the traditional technology, the new method will have many obvious advantages in reducing heating times and discharge, saving materials and energy, and improving productivity. In order to produce parts with sound mechanical properties by employing the new technology, it is important to probe the flow behavior of as-cast carbon steel under hot deformation for premium controlling processing parameters, reasonable planning procedures and a reliable constitutive equation for precise simulation. In this paper, high temperature flow behavior of as-cast 1026 carbon steel is investigated by conducting hot compression experiments on Gleeble-3500 simulator in the temperature range from 1 173 K to 1 473 K at an interval of 100 K and the stain rate range from 0.1 s−1 to 2.0 s−1. The relationships of deformation parameters (temperature, strain rate) with material’s flow behavior are found. The deformation activation energy and the stress index are worked out and the mathematical model of the flow stress under hot deformation is established by means of the liner regression analysis of true stress-strain data. Meanwhile, the effect of initial grain sizes on flow behavior of as-cast 1026 steel is also studied by compressing samples cooled to 1 173 K from 1 273 K, 1 373 K and 1 473 K respectively. The experimental results reveal that strain hardening and flow softening mainly characterize the flow behavior. It is also found that with the increase of deformation, the flow stress first increases rapidly, then reaches the peak slowly, after that it begins to decrease and finally comes to a steady value. At the temperature of 1 173 K, material’s softening is not apparent even if the strain rate is increased, while at the strain rate of 2 s−1, it is also not apparent even when the deformation temperature is raised to 1 473 K, so the final forging temperature is supposed to be about 1 173 K and the maximum stain rates should be below 2 s−1. In addition, at the same deformation temperature and strain rate, the more refined initial grain, the easier material dynamically recrystallizes and the lower the steady stress is. Therefore, the heating process of material is expected to be tightly controlled. The maximum error of flow stress between the model predictions and actual results is only 5.90%. The good agreement signifies the applicability of this method as a general constitutive equation in hot deformation studies.

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