Factors influencing ferrite/pearlite banding and origin of large pearlite nodules in a hypoeutectoid plate steel

Abstract

The microstructure and distribution of alloying elements in a hot rolled, low alloy plate steel containing (wt-%) 0·15%C, 0·26%Si, l·49%Mn, and 0·03%Al were examined using light microscopy and electron probe microanalysis. Microstructural banding was caused by microchemical banding of manganese, where alternate bands of proeutectoid ferrite and pearlite were located in solute lean and solute rich regions, respectively. Bands were well defined for a cooling rate of 0·1 K s−1, but banding was much less intense after cooling at 1 K s−1. At a cooling rate of 0·1 K s−1 and for austenite grains smaller than the microchemical band spacing, austenite decomposition occurred via the formation of ‘slabs’ of proeutectoid ferrite in manganese lean regions resulting in the growth of ferrite grains across austenite grain boundaries. Abnormally large austenite grains result in the formation of large, irregularly etching pearlite nodules which traversed several bands. In specimens cooled at 1 K s−1, ferrite/pearlite banding did not exist in regions where austenite grains were two or more times larger than the microchemical band spacing.MST/1397