Effect of intercritical deformation on tensile performance of a low-carbon Si-Mn steel processed by quenching and bainitic partitioning

The effect of intercritical deformation on retained austenite and tensile performance of a low-carbon Si-Mn steel in modified quenching and bainitic partitioning processes was evaluated.The results showed that the intercritical deformation can play a positive role in stabilizing and refining the retained austenite,and possessed promising potential in balancing tensile strength and ductility of multiphase high-strength steels.The experimental low-carbon Si-Mn steel exhibited multiphase configuration comprising polygonal ferrite,granular bainite and granular structure after two different modified quenching and bainitic partitioning processes,and the bainitic ferrite laths got refined by intercritical deformation.The volume fraction of retained austenite in film-like and blocky morphology was increased from 11.5%to 13.9%due to applied intercritical deformation,and the larger amount of retained austenite provided the sufficient transformation-induced plasticity effect and resulted in enhanced work hardening degree;in response,enhanced ultimate tensile strength 1260 MPa and fracture elongation 22.1%were obtained,leading to increased product of strength and elongation in value of 27.7 GPa% compared to 20.8 GPa% of undeformed structure.

Ultra-fine-Grained Ferrite Prepared from Dynamic Reversal Austenite During Warm Deformation

The ultra-fine-grained ferrite(UFGF) with the size of less than 1 μm is often difficult to be obtained for low-alloyed steel in practical production processing.In this study,considering the rod and wire production process,a new method for preparing the UFGF with submicron scale is proposed by warm deformation of six passes with total strain of 2.6,followed by the cooling process in Gleeble-3500 thermo-mechanical simulator.The results show that the UFGF with an average size of 0.64 μm could be obtained via the phase transformation from austenite grains with an average size of 3.4 μm,which are achieved by the deformation-induced reversal austenization during the high strain rate warm deformation.The main driving force for the reversal transformation is the stress.And the interval between the passes also plays an important role in the reversal austenization.

Effect of time interval on microstructure evolution of medium carbon steel during warm deformation

The thermal mechanical simulator system (Gleeble-3500) was used to carry out the double passes compression tests. The microstructure evolution of medium carbon steel with different time intervals during warm deformation was investigated by scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction. The results show that the effects of time interval on microstructure evolution are very important. The low-angle boundaries transform into high-angle boundaries (HABs) during interval. The fraction of HABs and homogeneity of particle carbides increase with increasing time interval. In addition, the homogeneity of carbides also increases with the strain. However, the effects of time interval on the microstructure evolution are negligible when it exceeds 100 s.

Mechanical and fatigue properties of SA508-Ⅳ steel used for nuclear reactor pressure vessels

The mechanical and fatigue properties of SA508-Ⅳ steel with martensite and granular bainite, respectively, were studied. The mechanical tests results showed that the ultimate tensile strength and impact toughness of the specimen with martensite were 830 MPa and 158 J, respectively, and those of the specimen with granular bainite were 811 MPa and 115 J, respectively. The former had higher tensile strength and impact toughness than the latter. The impact tests results showed that the former belonged to typical dimple fracture, while the latter belonged to brittle fracture. The fatigue tests results showed that the fatigue life of the specimen with martensite was 2717 cycles, and that of the specimen with granular bainite was 1545 cycles under the strain amplitude of ± 0.45%. The specimen with martensite had fewer crack initiation points, narrower fatigue striations separation, and larger volume fraction of high-angle grain boundaries than the latter. The fewer crack initiation points meant fewer fatigue cracks, the narrower fatigue striations separation meant slower crack propagation rate, and the larger volume fraction of high-angle grain boundaries could more effectively hinder fatigue crack propagation. Based on these facts, the fatigue life of the specimen with martensite was higher than that of the specimen with granular bainite.