Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

Abstract

Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV$$_{10}$$ 10 ) of 320 J cm$$^{-2}$$ - 2 compared to 66 J cm$$^{-2}$$ - 2 in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution.

Effect of Manganese on the Strength–Toughness Relationship of Low-Carbon Copper and Nickel-Containing Hull Steel

“…Huang [26] subjected 0.12C-3.0Mn steel to a three-step heat treatment method of complete quenching + two-phase quenching + critical zone quenching to achieve an impact work of 200 J at −80 • C. Liu [27] achieved good low-temperature impact toughness (>90 J at 77 K) by designing metastable austenite with a core-shell structure and specific volume fraction in maraging steel. Kwok [28] were designed two medium Mn steels with 0.2 and 0.5 wt pct C. The fracture surfaces were investigated and the Transformation Induced Plasticity (TRIP) effect was found to occur more readily in the Low C Charpy specimen. The low C steel had a corrected Charpy impact energy (KV10) of 320 J cm (−2) compared to 66 J cm (−2) in the high C steel, despite both having a ductility of over 35 pct.…”