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Abstract
The steels C-Mn-Al and C-Mn-Nb-Al have been chosen so that their compositions would give a wide range of Ae3 and Ar3 (undeformed) temperatures. The steels were heated to 1330°C and cooled at 60 K min-1 to test temperatures in the range 1050 to 600°C and strained to failure at a strain rate of 3 × 10-3 S-l. In all cases troughs were obtained in the curve of reduction of area versus test temperature, the width varying markedly with composition. Although the Nb containing steel with the lowest transformation temperature had the widest trough, the depths of the troughs were similar. This relative insensitivity of the depth of the trough to the presence of Nb is believed to be owing to the high P levels in these steels reducing the amount of Nb(CN) precipitated in the region of the grain boundaries. Below the Ae3 temperature, the troughs were owing to the presence of a thin film of deformation induced ferrite allowing strain concentration to occur around MnS inclusions. Above the Ae3 temperature continuation of the trough was caused by grain boundary sliding in the austenite. Recovery of ductility at the high temperature end of the trough corresponded to the onset of dynamic recrystallisation and this was delayed in the Nb containing steels so that the trough was extended to higher temperatures. Recovery of ductility at the low temperature end always corresponded to the presence of a large amount of ferrite (∼50%) in the structure. In some cases this required having the ferrite present 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 that ductility recovered just below Ae3. For these lower Mn and C steels, this ability for the thin films of ferrite to progress into the matrix giving enhanced ductility may be associated with the presence of a high Si level (0.5%). The commercial implications of the work are discussed.
