INFLUENCE OF AR-3 AND AE(3) TEMPERATURES ON HOT DUCTILITY OF STEELS

The steels C-Mn-Al and C-Mn-Nb-Al have been chosen so that their compo sitions would give a wide range of Ae(3), and Ar-3 (undeformed) temper atures. The steels were heated to 1330 degrees C and cooled at 60 K mi n(-1) to test temperatures in the range 1050 to 600 degrees C and stra ined to failure at a strain rate of 3 x 10(-3) s(-1). In all cases tro ughs were obtained in the curve of reduction of area versus test tempe rature, the width varying markedly with composition. Although the Nb c ontaining steel with the longest transformation temperature had the wi dest trough, the depths of the troughs were similar. This relative ins ensitivity of the depth of the trough to the presence of Nb is believe d 60 be owing to the high P levels in these steels reducing the amount of Nb(CN) precipitated in the legion of the grain boundaries. Below t he Aes temperature, the troughs were owing to the presence of a thin f ilm of deformation induced ferrite allowing strain concentration to oc cur around MnS inclusions. Above the Ae(3) temperature continuation of the trough was caused by grain boundary sliding in the austenite. Rec overy of ductility at the high temperature end of the trough correspon ded to the onset of dynamic recrystallisation and this was delayed in the Nb containing steels so that the trough was extended to higher tem peratures. Recovery of ductility at the low temperature end always cor responded to the presence of a large amount offer rite (similar to 50% ) in the structure. In some cases this required having the ferrite pre sent before deformation, as in the lower transformation steels having 1.4%Mn and 0.085%C, but for the higher transformation steels having 0. 6%Mn and 0.15%C, deformation induced ferrite formed very readily so th at ductility recovered just below Ae(3). For these lower Mn and C stee ls, this ability for the thin films of fer rite to progress into the m atrix giving enhanced ductility may be associated with the presence of a high Si level (0.5%). The commercial implications of the work are d iscussed. MST/3930.