Austenite phase formation in rapidly solidified Fe-Cr-Mn-C steels

Steels having compositions (wt%) 0.05-0.5C, 12.5-20Cr, 8-25Mn and 0-0.51N have been chill-block melt-spun to ribbons in order to investigate systematically, by X-ray diffractometry and electron microscopy, the effects of rapid solidification and of solute concentrations on the formation of the austenite phase. The austenite is most easily formed at (wt%) 16Cr-8Mn for 0.3C ribbons while alpha'-martensite or epsilon-martensite was observed at lower concentrations of Cr or Mn and alpha-ferrite appeared for Cr > 18 wt%. The volume fraction of austenite in the steel ribbons studied was found, by multiple regression analysis, to obey the equation gamma(%) = 94 + 26.8C + Mn x (8.4 - 0.08Mn - 0.44Cr) - (Cr - 17.7)(2). Thus, the effect of Mn on gamma formation followed a non-linear function, containing an interaction term including the Cr and Mn contents, and first- and second-order terms involving the Mn concentration. This indicates the ranges over which Mn is a gamma-former or an alpha-former. Iso-austenitic lines, constructed on the basis of this new equation, are nearly orthogonal to those in the Schaeffler diagram for Cr-Mn steels so that use of the latter for prediction of the austenite content in the present case would be inappropriate. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.