Effects of Mn on Isothermal Transformation Microstructure and Tensile Properties in Medium- and High-carbon Steels

The effects of Mn addition on the microstructure formed through an isothermal transformation at 873 K and its tensile properties were investigated over a wide range of concentrations in medium- and high-carbon steels with 0.4-1.0 mass% C. The Mn addition changed the pearlite transformation mode from relatively slow non-partitioning pearlite with a small amount of proeutectoid ferrite to an extremely slow partitioning pearlite without any proeutectoid ferrite in the hypoeutectoid steel. The transformation rate of the partitioning pearlite associated with the Mn partitioning between ferrite and cementite in Fe-C-M alloy was more than three orders of magnitude lower than the pearlite transformation in Fe-C binary alloy at a given interlamellar spacing. The decrease in the transformation rate in the non-partitioning and partitioning pearlite transformation was caused by the decrease in the carbon flux controlling the pearlite transformation, which can be explained by the theory of local equilibrium at the austenite/ferrite and austenite/cementite interphases. The Mn addition increased the thermal stability of the lamellar cementite. Corresponding to the change in the transformation microstructure, the Mn addition improved the tensile properties in the pearlite steel, particularly the strength and local deformability balance, regardless of the difference in the transformation mode between the non-partitioning and the partitioning transformation, unless the proeutectoid cementite precipitated at prior austenite grain boundaries. The strength increase of the pearlite after the Mn addition was caused by the refinement of the interlamellar spacing and/or the increase of the lattice strain in the pearlitic ferrite.