Effect of two-step bainite treatment on the morphology and texture of retained austenite and mechanical properties of austenitizing pretreated transformation-induced plasticity steel

In this paper, the experimental steel was treated with an austenitizing pretreatment followed by a two-step bainite treatment. The volume fraction of the retained austenite was significantly increased at room temperature, and moreover, film-like retained austenite accounts for more than 85% of the total retained austenite. The grain size of the blocky retained austenite was controlled to be less than 2 μm. The texture of the retained austenite has changed significantly: The retained austenite exhibited strong {hkl} textures, in which the retained austenite had a lower Schmidt factor when subjected to uniaxial stretching parallel to the rolling direction; its stability was greatly improved. The one-step bainite treatment based on an austenitizing pretreatment can effectively improve the mechanical properties of TRIP steel by an increase of more than 20% compared with the conventional TRIP heat treatment process. However, the two-step bainite treatment based on an austenitizing pretreatment process applied herein can further significantly improve the comprehensive mechanical properties of the experimental steel compared with the one-step bainite treatment based on an austenitizing pretreatment process, which has increased by 23.8% on average. When the two-step bainite treatment process is 500 °C × 20 s + 420 °C × 220 s, the experimental steel exhibits optimal comprehensive mechanical properties with an ultimate tensile strength of 885 MPa, an elongation of 32.2%, and a product of the strength and elongation of 28,947 MPa%.