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Low‐alloy cast steel with ultrahigh strength is produced by a combination of normalizing and reheating, followed by water quenching and high‐temperature tempering. The cast steel quenched at 1000 °C for 30 min and tempered at 550 °C for 120 min exhibits superior tensile ductility. More precisely, the tensile strength is 1683.5 MPa, yield strength is 1635.3 MPa, elongation is 12.0%, and microhardness is 500.3 HV. The microstructure is composed of martensite lath + ferrite + M23C6 carbide. The strengthening mechanisms of the steel are mainly precipitation, grain refinement, and dislocation strengthening. When the quenching temperature is increased from 950 to 1000 °C, the M3C carbides precipitated between the martensite lath are transformed from short rod‐like to spherical shapes, which improves the plasticity of the steel. When the tempering temperature is increased from 550 to 630 °C, the number and size of the carbides also increase. The M23C6 carbides interact with the dislocations. M23C6 carbides play a crucial role in the precipitation strengthening and efficiently improve the strength of the steel. The fracture is changed from a mixed ductile and brittle fracture to a ductile fracture.

In the present research, the microstructure and mechanical properties of low-density Fe-0.72Mn-3.7Al-0.53C steel were investigated after solution treatment at 900 °C, 1000 °C, 1110 °C and 1200 °C for 1 h. The density of steel is about 7.0 g·cm−3 due to the addition of a higher content of aluminum elements. The microstructure was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the mechanical behavior was analyzed by room temperature tensile testing. The results show that the microstructure of the steel is ferrite and martensite after solution treatment, and that martensite can be divided into dislocation martensite and twinned martensite according to different substructures. Part of the martensite grows in a mirror-symmetrical manner in order to adjust the strain energy that increases with the system undercooling to form twinned martensite. After solution treatment at different temperatures, the tensile strength and elongation of the steel increased and then decreased with the increase of the solution treatment temperature, and the tensile strength could reach 928.92 MPa, while maintaining excellent toughness and elongation at 5.89%.