Retained austenite (RA) transformation and its role in the mechanical properties of three low-carbon micro-alloyed quenching and partitioning (Q&P) steels was investigated utilizing in situ tensile tests and electron microscopy. Meanwhile, RA’s strain-induced martensite transformation (SIMT) was analyzed and discussed in terms of the strengthening mechanism. The results show that the ductility of the Q&P steels relies on the size and morphology of RA. In addition, both affect RA’s mechanical or thermostability. Dislocation density and carbon trapping should be considered in estimating the yield strength in the two-step Q&P process. V and Nb-Ti elements promote the formation of blocky RA. Ti accelerates the formation of film-like RA. For experimental Q&P steels with different processes and compositions, the true stress always keeps a linear relationship with the amount of transformed martensite, i.e., 30.38~46.37 MPa per vol. 1% transformed martensite, during the in situ tensile deformation.

Impact fracture behavior at low temperatures was investigated in medium manganese steel with bcc-fcc duplex microstructures. The impact energy was above 150 J (−80~20 °C) and the fractography showed dimples for inter-critical annealing at 630 °C (QHA) because of the high retained austenite stability and low martensite dislocation density. The impact energy was from 180 J (20 °C) to 60 J (−80 °C) and the fractography was intergranular for inter-critical annealing at 610 °C (QLA) because of the low stability of RA and carbides precipitated at the prior austenite grain boundaries. The impact energy was below 60 J (−80~20 °C) and the fractography showed cleavage for direct quenching (DQ) because of the high dislocation density of martensite.

Currently, the fish-scaling resistance of most hot-rolled enamel steels is improved by adding Ti to form fine TiC carbides as hydrogen traps. Given that the hydrogen capture capacity of NbC is higher than that of TiC, the manufacture of hot-rolled enamel steels via Ti-Nb microalloying has a promising future. In the present study, a Ti-Nb microalloyed hot-rolled enamel steel was developed, and its microstructure, mechanical properties, and fish-scaling resistance were studied by optical microscopy, transmission electron microscopy, tensile test, and hydrogen permeation test. The results show that the microstructure of hot-rolled experimental steel is composed of ferrite and fine carbides, with a large number of fine precipitates uniformly distributed in the ferrite grains. After the first and second enamel firings, the average sizes of ferrite grain and precipitates gradually increase, the yield strength decreases from 711 ± 9 MPa to 471 ± 17 MPa and 409 ± 8 MPa, the tensile strength decreases from 761 ± 7 MPa to 524 ± 15 MPa and 490 ± 12 MPa, and the elongation increases from 21.0 ± 2.8% to 27.8 ± 1.8% and 33.9 ± 1.1%. The hydrogen permeation value (TH value) decreases from 35.9 min/mm² to 6.8 min/mm² and 3.9 min/mm² after the first and second enamel firings. That is, the fish-scaling resistance of hot-rolled experimental steel is significantly reduced after enamel firing, which is caused by the coarsening of precipitates, resulting in a significant reduction in the density of irreversible hydrogen traps (from 1.21 × 10²⁵ cm⁻³ to 6.50 × 10²³ cm⁻³ and 4.27 × 10²³ cm⁻³). A large amount of semi-coherent precipitates is the key to obtaining the good fish-scaling resistance of hot-rolled enamel steel.