Influence of silicon,aluminium,phosphorus and boron on hot ductility of TRansformation Induced Plasticity assisted steels

Abstract

The hot ductility of steels having high aluminium or phosphorus contents, which are currently being considered as possible replacements for the conventional high silicon TRansformation Induced Plasticity (TRIP) steel, has been examined. Tensile specimens were cast in situ and tested in the temperature range 750 - 1000 ° C at a strain rate of 3 × 10^-3 s^-1. The ductility trough for the conventional high silicon TRIP steel was controlled by the austenite - ferrite transformation, intergranular failure occurring when a thin band of the softer ferrite phase formed around the austenite grains. Void formation at the sulphides situated in the soft ferrite at the boundaries then occurred, and the strain concentrated locally there. The thin bands of ferrite were deformation induced and, as such, formed at temperatures above Ar ₃ and could form at as high a temperature as Ae ₃. Adding ferrite formers such as silicon, phosphorus and aluminium increased the Ae ₃ temperature and thus widened the trough. The high aluminium (2%) TRIP steel exhibited good ductility throughout the temperature range examined, since large amounts of ferrite were always present, preventing strain concentration, and the AlN particles were too coarse to influence the hot ductility. In contrast, the 1%Al containing steel gave poor ductility below 850 ° C, the band of strain induced ferrite being extremely thin. The ductility trough in the titanium containing high phosphorus steel was poor, owing to fine precipitation of TiN. Adding boron to the steel and reducing the manganese content from 1.4 to 1% resulted in better ductility. Generally, the TRIP type steels had superior ductility to the conventional niobium containing high strength low alloy steel.