Mechanical twinning in E2412 electrical steel

Resumo

The deformation of fine-grain E2412 steel containing 3.63% Si, with different grain size is considered. Twinning predominates in its deformation. The samples are extended on an Instron-5565 machine at strain rates \(\dot \varepsilon \) ≈ 0.002–0.660 s^–1 at 183–393 K. Samples of two types are investigated: with around 80% of the grains in the ranges 1.5–9.0 mm and 0.025–0.225 mm. (The mean grain sizes are d_me1 = 3.55 mm and d_me2 = 0.12 mm, respectively.) The relation between the number of twins and the types of steps formed on the strain and strain-rate diagrams in loading is established for the grains with d_me1 = 3.55 mm. At small loading rates, on account of the high growth rate of the twins, step formation on the strain diagrams is accompanied by marked decrease in the load. With increase in the loading rate, the spread of the load is reduced; Δσ changes sign at a strain rate \(\dot \varepsilon \) ≈ 0.04 s^–1. In fine-grain steel (d_me2 = 0.12 mm), there are no visible jumps in the load with the appearance of twins. In the fine-grain steel, the growth time of twins in the grain is small. On account of their high rate of development, the number of twins is also small. The distribution of twinned grains is plotted as a function of the grain size, at different temperatures and loading rates. The peak in the size distribution of the twinned grains is shifted to larger grains in comparison with the overall grain-size distribution of the polycrystal. There is some optimal grain size that facilitates twinning. As a rule, it is higher than the mean grain size determined from the initial grain-size distribution. The number of twins in a particular grain depends on the test temperature and the strain rate. There is a deformation temperature at which the number of twins in the grain is constant at the strain rates employed.