Superplastic characteristic of Mn–Si–Cr alloyed ultrahigh carbon steel realized through a novel process

A novel process based on simple heat treatment was developed in order to explore the superplastic characteristic of Mn–Si–Cr alloyed ultrahigh carbon steel. After austenitizing at A1 − Acm and slow cooling, a microstructure with superplastic “potential” was obtained. The microstructure with superplastic “potential”, mainly composed of martensite and spherical carbides, could transform to a fine (austenite + ferrite + spherical carbides) microstructure beneficial for superplasticity during subsequent warm deformation at just below A1. The superplastic characteristic during warm deformation is as follows: flow stress stays at 30–50 MPa and the m value reaches 0.4–0.5 at a strain rate of 10–4–2 × 10−4 s−1. The novel process has two advantages: ultrahigh strength (HRC52) and excellent ductility (the reduction of area ∼45%) are ensured after superplastic forming without the need of supplementary heat treatment; internal stress in the microstructure obtained after superplastic forming can be avoided to a great extent.