To explore the effect of inclusions in the coarse-grain heat-affected zone (CGHAZ), four groups were designed using the ‘Al/Ti deoxidised + Nb/V microalloyed’ technical route. The toughness of the CGHAZ of Al–Ti–V–N steel was greatly promoted by the presence of polygonal ferrite (PF) after high heat input rather than the traditionally used acicular ferrite (AF). Furthermore, micron-sized inclusions in AF or PF were analysed, and the results indicated that the TiN and V (CN) particles attached on the Ti/Al oxide inclusions induced the nucleation of AF and PF. That can be concluded that the nucleation mechanism of PF is similar to that of AF. In addition, smaller micron- and nanoscale-size inclusions observed by transmission electron microscopy could not induce the nucleation of AF and PF because of the low heterogeneous nucleation energy. Nevertheless, these inclusions can have a drag effect on grain boundary migration.

Because of the features of high strength and high plasticity, TRIP duplex stainless steels can achieve the purpose of lightweight and reduce fuel consumption and emissions. In order to further develop and apply the lightweight metastable duplex stainless steel, the martensite transformation and the microscopic mechanism of Mn-N alloyed TRIP duplex stainless steel under the cyclic loading were studied. The in-situ measurement platform of martensite transformation under the cyclic loading was built by the INSTRON 8801 series servohydraulic testing machine and FMP30 ferrite measuring instrument. The volume fraction of martensite transformation under symmetrical cyclic loading of different cycles with strain amplitudes of 0.5%, 0.7%, 0.9%, 1.1% and 1.3% were measured. The transformation law of martensite under the cyclic loading was analyzed, and the kinetics model of martensite transformation under the cyclic loading was established; The martensite transformation law, the effect of austenite grain orientation on martensite transformation and the microscopic mechanism of martensite transformation under the cyclic loading were analyzed by the electron backscattering diffraction(EBSD) and the transmission electron microscope(TEM).

The cyclic plastic characteristics of metal materials are different from the deformation characteristics under monotonic loading, which has an important effect on the safety of structures in service under cyclic loading. However, GS-20Mn5, which is commonly used in large hydraulic machine beams, offshore platforms and large Bridges, is still lacking the studies of mechanical response characteristics under cyclic loading. In this study, the cyclic softening/hardening characteristics of GS-20Mn5 are studied by a series of cyclic loading tests under uniaxial strain control. Combined with transmission electron microscope (TEM) analysis of cyclic loading tests under typical strain levels, the microscopic mechanism of cyclic softening/hardening is discussed. The results show that the cyclic softening/hardening properties of GS-20Mn5 cast steel are sensitive to amplitudes and cycles. At smaller strain amplitudes (0.16%,0.2% and 0.3%), the cyclic hardening properties of GS-20Mn5 cast steel are rapid at the beginning of the cycle, followed by cyclic softening and then slow secondary cyclic hardening at the end. However, under larger strain amplitudes (0.4% and 0.5%), the cyclic hardening continues during the cyclic loading, and the hardening rate is bigger at the beginning of the cyclic loading and smaller at the later cyclic stage. The cyclic softening/hardening characteristics of GS-20Mn5 cast steel are related to the dislocation structure of ferrite and pearlite. Taking the strain amplitude of 0.2% as an example, the initial cyclic hardening is mainly caused by the proliferation and interaction of dislocations in ferrite. Dislocation spots and cell walls in ferrite grains are mainly caused cyclic softening at the initial stage, the secondary cyclic hardening is directly related to dislocation proliferation and entanglement in pearlite.