Bainite Transformation in a Silicon Steel.

Abstract

The processes of upper bainite formation, the crystallographic aspects and the internal structure in a 1.83% silicon steel have been investigated. The T-T-T diagram is separated into two C-curves as in the case of steels containing strong carbide forming elements. In the early stage of upper bainitic transformation, very fine needlelike ferrite subunits with the parallelogram cross sections elongated in a γ // α direction form on a{111} γ sheet in a side-by-side fashion. In the later stage of transformation, these subunits are coalesced on a{111} γ plane and produces upper bainite laths. This bainite is always related to the parent austenite with the Kurdjumov-Sachs relationship and accompanies the surface reliefs. e carbide particles precipitate mainly after the ferrite subunits formation or after the coalescence of them. Therefore, the carbide precipitation is not a fundamental characteristic of bainite transformation but is a secondary effect. e carbide needles precipitating within a bainite lath by long time holding are aligned almost parallel to the α with almost constant intervals and are related to the ferrite by the Jack relationship. These aspects are exactly similar to those in tempered martensite. Such an e carbide configuration is thought to arise from the precipitation on the dislocations introduced during transformation.