| A new class of austenitic steel for automotive applications exhibiting the TWIP (twinning- induced plasticity) effect, which provides an excellent combination of strength and ductility, is reviewed. Special attention is given to the microstructural design of TWIP steels, their alloying concept, thermomechanical processing, physical mechanisms responsible for superior ductility at room temperature (≥ 100%) and high strength, which ensures a value of σB × δ ≥ 5 × 104 MPa × %, being highest among all steels. The correlation between microstructure and mechanical properties is discussed in detail. TWIP steel microstructural design is aimed to provide a stacking fault energy (SFE) between 20 and 40 mJ/m2. In the steels with SFE values below 20 mJ/m2, the ε (hcp) or α' (bcc) strain-induced martensite evolves during cold deformation. These steels exhibit effect of transformation-induced plasticity (TRIP), which leads to σB× δ=3÷4 104 MPa × %. At SFE values ≥ 40 ÷ 50 mJ/m2, the mechanical twinning is suppressed, and the value of σB× δ does not exceed 2 × 104 MPa ×%. The role of twinning and dislocation slip in achieving ultra-high strength and ductility is considered. The specific mechanisms of solid-solution strengthening associated with the formation of a C-Mn dipole or octahedral clusters CMn6 are discussed. The influence of thermomechanical treatment on the microstructure, crystallographic texture and mechanical properties of TWIP steel is also discussed. It was shown that extensive grain refinement provides a high increase in yield stress, and intense plastic straining can be applied to produce high-strength TWIP steels with ductility ≥ 20%. |
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