Alternative phase formation in thin strip casting of stainless steels

Abstract

Factors controlling the solidification structure produced by twin roll thin strip casting of 304 stainless steels have been studied. A transition of the primary solidification modefromferrite to metastable austenite was observed. This transition can be ascribed to supercooling of the melt by relatively high cooling rates, of the order of 10³ K S ⁻¹ near the chilling rolls. The degree of primary austenite solidification depends also on possible alloy composition fluctuations between different melt charges, and can vary from some localised austenitic regions adjacent to the strip surface to fully austenitic solidified strips. An unequally directed segregation of Cr and Ni was found within the as solidified regions of primary ferrite, whereas in regions solidified as primary austenite an equally directed segregation of both elements occurred. Magnetic measurements revealed that the volume fraction of ferrite changed from 1·3 to 1·9 vol.-% in the primary ferritic solidified thin strips, and from 0·1 and 0·2 vol.-% in samples solidified as primary austenite.

MST /1997