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To ensure the high strength of 55Si2MnMoV steel and greatly improve the ductility, three samples with various initial microstructures (annealing, quenching, and austempering) were subjected to intercritical annealing (820 °C) and austempering (325 °C) for 25 min, respectively. The effects of the initial microstructure on the final microstructure and mechanical properties were systematically studied. XRD revealed that the volume of retained austenite (RA) of the three decreased significantly with the location close to the tensile fracture surface. While the conversion proportion of RA in the quenched sample peaked at 16.5%, indicating that RA transforms into martensite to a greater extent during tensile, thus delaying the occurrence of necking. The ferrite grains in the quenched sample were significantly refined and uniformly distributed, and a few were polygonal. Additionally, the grains of the austempered sample were mostly lath because of the inheritance of the initial microstructure. The ultimate tensile strength (UTS) of the quenched sample reaches 1093.61 MPa, which is slightly lower than that of the annealed sample, but the plasticity is greatly improved. The corresponding product of strength and elongation (PSE) reaches the peak value of 67.05 GPa%, which is 26.22% higher than that of the annealed sample, this is mainly attributed to the transformation of RA in a broad range of strain and the uniform equiaxed microstructure. Also, the strength ratio of the three samples fluctuates between 0.67 and 0.71, and their failure modes are all ductile fractures.

The effect of retained austenite stability on plastic deformation was investigated by stepwise tensile test of medium carbon steel material at two different bainite isothermal temperatures below or above the martensite start (Ms) point. The results show that higher retained austenite transformation leads to higher elongation. Compared with the above-Ms bainitic steel (320 °C-12 h), the below-Ms bainitic steel of retained austenite (240 °C-48 h) has higher mechanical stability. This is because the transformation induced plasticity effect (i.e., the TRIP effect) of retained austenite plays a significant role in the process of tensile strain. The block retained austenite distributed between the bainite sheaf deforms rapidly in the initial stage, and the film retained austenite between the bainitic-ferrite lath transformation is slow in the later stage of tensile deformation. This is attributed to the low stability of block retained austenite, which easily transforms into hard and brittle martensite under external stress, while the film retained austenite has high stability and cannot easily undergo phase transformation under the action of external force. Meanwhile, the work hardening ability of the below-Ms bainitic steel is higher than that of the above-Ms bainitic steel at the initial stage of tension, but the opposite occurs in the later stage of tension.

A new concept for the alloy design of advanced structural steels was figured out with the experimental elaboration of deformation behaviour in two novel medium carbon bainitic steels proposed sparse mixed regression model. Fe-0.53C-1.55Si-0.48Mn-0.59Cr-0.05Ni (wt%, Steel-A) and Fe-0.64C-1.74Si-2.11Mn-0.2Cr-0.15Ni (Steel-B) were studied. These steels were austempered to get bainite structure with retained austenite of different fraction and morphology. The chemical composition and heat treatment conditions were designed by sparse mixed regression modelling. In the Steel-A, the retained austenite fraction is 0.07 and it is in film form. In the Steel-B, the retained austenite fraction is 0.37 and most of them are in blocky form. Both the samples showed high tensile strength (>1.6 GPa) and good elongation (>12%). The Steel-A showed poor uniform elongation, but large post-uniform elongation. In the Steel-A, the retained austenite is stable during deformation, so inherent ductility of bainitic ferrite matrix largely contributed to higher post-uniform elongation. On the other hand, in Steel-B, the occurrence of deformation-induced martensitic transformation leads to large uniform elongation, through increasing the work hardening rate and delaying the onset of plastic instability.

A heterogeneous microstructure composed of multiscale ferrite (spanning ultrafine, fine, and coarse grains) and granular or short rod cementite was obtained by warm rolling pseudoeutectoid microstructure. Combined with the short-time annealing process, the deformation tendency of ferrite grains and the dissolution and precipitation of cementite were adjusted. The mechanical properties of medium carbon steel are close to that of low alloy high strength steel. It was found that the GNDs are distributed in a gradient at the interface of the soft domain, and the closer to the boundary the higher the density, which contributes to the occurrence of the hetero-deformation induced (HDI) hardening mechanism and provides additional strength and plasticity. The increase in elongation of the annealed samples is due to the recrystallization of ferrite and the redistribution of cementite, which realizes the combination of HDI mechanism, Orowan strengthening and annealing softening mechanism. Additional strengthening mechanisms are calculated and a physical model of cementite ripening during annealing is proposed. The tensile fracture shows that the cracks mainly propagate along the grain boundaries of the layered microstructure, and the high density of dimples can make the propagation path of the cracks tortuous and increase the elongation.