The precipitation behavior of nanometer-sized carbides in ferrite in Nb–V-bearing low-carbon steel was studied by electron microscopy and nanoindentation hardness measurements. The results indicated that interphase precipitation and random precipitation could occur simultaneously for the specimen isothermally treated at 700 °C for 60 min, while in other specimens, only random precipitation was observed. This phenomenon might be explained by mass balance criterion during the diffusional phase transformation. Nanohardness result indicated that the average hardness of the specimens isothermally held at 600 °C for 20 min was 3.87 GPa. For the specimen isothermally holding at 650 °C for 20 min, the average hardness was 4.10 GPa and the distribution of the nanohardness was in a narrower range compared with that of the specimen isothermal holding at 600 °C for 20 min. These implied that the carbides in the specimens isothermal treated at 650 °C were more uniformly dispersed, and the number density of the carbides was greater than that treated at 600 °C. Using Ashby–Orowan model, the contribution of precipitation strengthening to yield strength was estimated to be ~110 MPa for the specimen isothermally treated at the temperature of 650 °C for 20 min.

•A good combination of strength and toughness can be obtained controlled by NG-TMCP.•Nanoscale cementite was observed in Ti-microalloyed steel except for TiC.•The contribution of precipitates to yield strength can be greater than ∼300 MPa.We elucidate here the strengthening mechanisms in titanium microalloyed low-carbon steels, which were rolled into plates of 12 mm thickness using a combination of thermomechanical controlled processing (TMCP) and ultrafast cooling (UFC). The ultrafast cooling combined with thermomechanical controlled processing is referred by us as new generation (NG)-TMCP. Chemical phase analysis, small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (TEM) were used to study the characteristics of nanoscale cementite precipitates and microalloyed precipitates. Besides nanoscale TiC, cementite precipitates of size less than ∼35 nm with high volume fraction were observed in Ti-microalloyed steel. Cementite with high volume fraction had a stronger precipitation strengthening effect than nanometer-sized TiC. The precipitation strengthening contribution of nanoscale precipitates of different types and size was estimated together with solid solution strengthening and grain refinement strengthening contribution. The estimated yield strength was consistent with the experimental value.TEM micrograph of (a) interphase TiC, (b) random TiC, (c) Fe3C, (d) HRTEM image, (e) corresponding FFT diffractogram, and (f) IFFT lattice image interphase TiC.

•Nano/ultrafine-grained austenitic steel with combination of high strength and good ductility is obtained.•Effect of grain size (until ∼200 nm) on mechanical properties and Hall-relationship is studied.•Strain hardening behavior along with deformation mechanism of nano/ultrafine grained austenitic steel is studied.Nano/ultrafine-grained (Nano/UFG) structure was obtained in Fe-17Cr-6Ni austenitic steel using a combination of severe cold deformation and reverse-transformation annealing. The microstructural evolution during severe cold reduction and annealing was studied to elucidate the effect of grain size on mechanical properties and strain hardening behavior. Austenitic steel with the smallest average grain size of ∼220 nm was obtained and exhibited a good combination of high strength and high ductility when the cold reduction was ∼75% and annealing was carried out at 700 °C for 20 s. The relationship between grain size and yield strength was in good agreement with Hall-Petch relationship until ∼200 nm grain size. For coarse-grained steel, the strain hardening rate (SHR) plots comprised of 4 stages and the increase of SHR was attributed to deformation-induced martensite transformation (DIMT). While for Nano/UFG ASS, the SHR plots contained only 3 stages and the increase of SHR was attributed to the comprehensive effect of DIMT and twining.

•PPT curve of carbides in ferrite is typical “C” shape, with nose locating at ~650 °C.•V was incorporated into NbC during precipitation and increased during coarsening.•V/Nb ratio in carbide and lattice parameter was dependent on holding time.The chemical composition and lattice parameter of the nanometer-sized carbides in ferrite of Nb-V bearing low-carbon steel were studied by high resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The results indicate that the average size of the carbides increased with increase in isothermal holding time. V was found to be gradually incorporated into NbC lattice to form a complex (NbxV1−x)C particle during precipitation and the atomic ratio of V/Nb gradually increased during coarsening, which leads to a decrease in the lattice parameter of carbide and reduces the misfit strain between the carbide and ferrite matrix, resulting in acceleration of precipitation kinetics.The HRTEM image and corresponding IFFT diffractograms of nanometer-sized carbides obtained from the specimen isothermally treated at 650 °C for (a), (b) 10 min, (c), (d) 20 min, (e), (f) 60 min and the atomic ratio of vanadium to niobium in carbides as a function of isothermal holding time (g).