Crystallographic and morphological aspects of AlN precipitation in high Al, TRIP steels

Abstract

The dendritic AlN associated with poor ductility on straightening during continuous casting is believed to always crystallise out in the hcp structure. The present paper shows that AlN precipitation can also be present as an fcc phase. Because AlN precipitation is sluggish and needs suitable nucleants like MnS to aid its precipitation, the crystal structure of AlN is dependent on the order in which the austenite and MnS precipitate out during solidification. When austenite forms first, as with the 1%Al transformation induced plasticity (TRIP) steel, MnS nucleants are not available and AlN is forced to precipitate at the austenite grain boundaries as fcc so encouraging intergranular failure. With the 1.5%Al TRIP steel, on solidification, MnS is present in abundance, giving rise to the conventional hcp AlN.

Keywords:

• TRIP assisted steel
• Casting
• AlN
• MnS
• Precipitation crystallography
• TEM