Influence of Alloying Elements on the Transformation Behavior of Medium-Manganese Steels

Medium-manganese steels are promising candidates for use as air-hardening forging steels, thus eliminating the need for subsequent heat treatment steps. This publication deals with the influence of the main alloying elements, namely, carbon, silicon, manganese, and chromium, on the transformation behavior during continuous cooling of medium-manganese steels. Therefore, dilatometer-tests, microstructure characterization by SEM and EBSD and in addition, hardness and retained austenite measurements were performed to derive CCT diagrams. The substitution of 1% manganese with 1% chromium in the steel 0.2C1Si4Mn results in a similar transformation behavior except for a higher Ms temperature (335°C instead of 303°C) and a slightly lower hardness. In general, all 4% manganese steels and the 3% manganese + 1% chromium steel show relatively high hardness values after slow cooling, which makes them suitable for use as air-hardening steels. All steels exhibit increasing retained austenite fractions with decreasing cooling rates, which can be attributed to autopartitioning. It could be shown that ferrite formation starts significantly earlier than the 1% transformation line in the classical CCT-diagram indicates, which is suspected to affect toughness.