Complex carbide precipitates in a quenched and tempered low alloy Cr–Mo–V steel after long-term aging at 650 °C for 13,000 h and 30,000 h were investigated in this study. The mass fraction and sizes of precipitates were quantified by electrolytical extraction technique. The types of precipitate were further studied by combined X-ray diffraction and transmission electron microscopy with selected area electron diffraction and energy dispersive spectrometry. A series of carbide precipitates, namely MC, M₇C₃, M₆C, and M₂C, were found existing in the near-equilibrium state. The precipitate sequence of these carbides was identified as MC + M₇C₃ + M₂C → MC + M₂C + M₇C₃ + M₆C → MC + M₇C₃ + M₆C. It was clarified that the stable phases for the investigated steel aged at 650 °C were composed of MC, M₇C₃, and M₆C. For the first time, the in-situ transformations of M₂C to M₆C and M₇C₃ to M₆C were directly observed. It was also observed that the nucleation site of the M₆C was located at the interface of M₇C₃ carbides and the matrix. The orientation relationships between the secondary phases of the in-situ transforming carbides aged for 13,000 h and 30,000 h at 650 °C were established. The coherent interfaces between these secondary phases became incoherent with prolonged aging treatment due to the exerted strain field of the growing carbides.

Four types of ultra-low carbon bainite (ULCB) steels were obtained using unified production methods to investigate solely the effect of niobium content on the performance of ULCB steels. Tensile testing, low-temperature impact toughness testing, corrosion weight-loss method, polarization curves, electrochemical impedance spectroscopy (EIS), and the corresponding organizational observations were realized. The results indicate that the microstructure of the four steels comprise granular bainite and quite a few martensite/austenite (M/A) elements. The niobium content affects bainite morphology and the size, quantity, and distribution of M/A elements. The elongation, yield strength, and tensile strength of the four types of ULCB steels are above 20%, 500 MPa, and 650 MPa, respectively. The impact toughness of the four types of ULCB steels at −40 °C is lower than 10 J. Steel with Nb content of 0.0692% has better comprehensive property, and maximum charge transfer resistance in 3.5 wt.% NaCl solution at the initial corrosion stage. The corrosion products on the surface of steel with higher niobium content are much smoother and denser than those steel with lower niobium content after 240 h of corrosion. The degree of corrosion decreases gradually with the increase of niobium content at the later stage of corrosion.

The uniaxial tensile behaviors of Fe-19Cr-16Mn-2Mo-0.49N and Fe-18Cr-16Mn-2Mo-0.85N high-nitrogen and nickel-free austenitic stainless steels at two strain rates of 10⁻² s⁻¹ and 10⁻⁴ s⁻¹ were comparatively investigated. The related deformation microstructure was characterized and fracture mechanism was analyzed. The results show that the nitrogen content and strain rate both have significant effects on the tensile properties of the tested steels. As the strain rate is the same, the tested steel containing a higher nitrogen content has higher R_m and R_p0.2. However, R_m is higher at a lower strain rate and R_p0.2 is higher at a higher strain rate in the case of the same nitrogen content. The tested steel with a lower nitrogen content (0.49 wt.%N) tensioned at a lower strain rate of 10⁻⁴ s⁻¹ obtains the highest elongation, while the tested steel with a higher nitrogen content (0.85 wt.%N) tensioned at a higher strain rate of 10⁻² s⁻¹ has the lowest elongation. The tensile plastic deformation is mainly governed by slip and twinning, affected jointly by stacking fault energy and short-range order. Dislocation slip featured by planar slip bands and twin-like bands is the main deformation structure in the tested steel containing a higher nitrogen content (0.85 wt.%N) tensioned at a lower strain rate of 10⁻⁴ s⁻¹, whereas twinning deformation becomes more prominent with decreasing nitrogen content and increasing strain rate.

In order to provide important guidance for controlling and obtaining the optimal microstructures and mechanical properties of a welded joint, the continuous cooling transformation diagram of a new low-carbon Nb-microalloyed bainite E550 steel in a simulated coarse-grain heat-affected zone (CGHAZ) has been constructed by thermal dilatation method in this paper. The welding thermal simulation experiments were conducted on a Gleeble-3800 thermo-mechanical simulator. The corresponding microstructure was observed by a LEICA DM2700M. The Vickers hardness (HV) and the impact toughness at −40 °C were measured according to the ASTM E384 standard and the ASTM E2298 standard, respectively. The experimental results may indicate that the intermediate temperature phase transformation of the whole bainite can occur in a wide range of cooling rates of 2–20 °C/s. In the scope of cooling rates 2–20 °C/s, the microstructure of the heat-affected zone (HAZ) mainly consists of lath bainite and granular bainite. Moreover, the proportion of lath bainite increased and granular bainite decreased as the cooling rate increasing. There is a spot of lath martensite in the microstructure of HAZ when the cooling rate is above 20 °C/s. The Vickers hardness increases gradually with the increasing of the cooling rate, and the maximum hardness is 323 HV₁₀. When the cooling time from 800 °C to 500 °C (t_8/5) is 5–15 s, it presents excellent −40 °C impact toughness (273–286 J) of the CGHAZ beyond the base material (163 J).