Influences of Manganese Content and Heat Treatment on Mechanical Properties of Precipitation-Strengthened Steels

In order to reduce the cost and improve the ductility of maraging steels without sacrificing strength, the high content of costly Ni element is partially substituted by inexpensive Mn to achieve metastable austenite. Thermodynamic simulation, composition screening and critical experiments are integrated to design ultrastrong dual-phase steels with the compositions of Fe-10Ni-3Al-4Mo-xMn (x = 0-10, wt%) by tailoring austenite fraction and heat treatment. The results show that for ageing following solid solution and quenching (soft ageing for commercial maraging steels, SA), the phase fraction of fcc should be less than that of bcc to achieve B2 precipitation in the bcc matrix. For the 9Mn alloy with a volume fraction of fcc more than 50%, ageing following cold rolling (hard ageing, HA) recovers the B2 precipitation that is hindered by fcc reversion during SA, resulting in an ultrahigh ultimate strength (similar to 1600 MPa) with an improved tensile ductility (similar to 7%) compared with the SA alloys (similar to 1450 MPa and similar to 5% at best). The improved strength-ductility synergy at ultrahigh strength is attributed to transformation-induced plasticity (TRIP) and ultrafine grain size, both of which turn the fracture mode from transgranular cleavages to ductile dimples. This work demonstrates a promising strategy by addition of austenite-stabilized element and ageing without solid solution for achieving low cost and high strength-ductility synergy, which paves the way for the development of precipitation-strengthened dual-phase steels.