ALTERNATIVE PHASE-FORMATION IN THIN STRIP CASTING OF STAINLESS-STEELS

Factors controlling the solidification structure produced by twin roll thin strip easting of 304 stainless steels have been studied. A trans ition of the primary solidification mode from ferrite to metastable au stenite was observed. This transition can be ascribed to supercooling of the melt by relatively high cooling rates, of the order of 10(3) K s(-1) near the chilling rolls. The degree of primary austenite solidif ication depends also on possible alloy composition fluctuations betwee n different melt charges, and can vary from some localised austenitic regions adjacent to the strip surface to fully austenitic solidified s trips. An unequally directed segregation of Cr and Ni was found within the as solidified regions of primary ferrite, whereas in regions soli dified as primary austenite an equally directed segregation of both el ements occurred. Magnetic measurements revealed that the volume fracti on of ferrite changed from 1.3 to 1.9 vol.-% in the primary ferritic s olidified thin strips, and from 0.1 and 0.2 vol.-% in samples solidifi ed as primary austenite.