New forming method of manufacturing cylindrical parts with nano/ultrafine grained structures by power spinning based on small plastic strains

A new spinning method to manufacture the cylindrical parts with nano/ultrafine grained structures is proposed, which consists of quenching, power spinning and recrystallization annealing. The microstructural evolution during the different process stages and macroforming quality of the spun parts made of ASTM 1020 steel are investigated. The results show that the microstructures of the ferrites and pearlites in the ASTM 1020 steel are transformed to the lath martensites after quenching. The martensite laths obtained by quenching are refined to 87 nm and a small amount of nanoscale deformation twins with an average thickness of 20 nm is generated after performing a 3-pass stagger spinning with 55% thinning ratio of wall thickness, where the equivalent strain required is only 0.92. The equiaxial ferritic grains with an average size of 160 nm and nano-carbides are generated by subsequent recrystallization annealing at 480°C for 30 min. The spun parts with high dimensional precision and low surface roughness are obtained by the forming method developed in this work, combining quenching with 3-pass stagger spinning and recrystallization annealing

Grain refinement in dual-phase steels

Deformation-induced ferrite transformation (DIFT) was applied in laboratory tests to produce fine-grained dual-phase (DP) steels. Four different chemistries were investigated, starting from a conventional DP 600 chemistry of 0.06 wt pct C-1.9 wt pct Mn-0.16 wt pct Mo and subsequently varying Nb and Mo additions. For all investigated steels, ultrafine ferrite (UFF) with a grain size of 1 to 2 μm can be obtained when a sufficient amount of deformation (e.g., a true strain of 0.6 or above in axisymmetric compression) is applied to an austenite microstructure with a grain size in the range of 10 to 20 μm at 25 °C to 50 °C above the austenite-to-ferrite transformation start temperature (Ar 3) characteristic for the given cooling condition. Rapid post-deformation cooling at rates of approximately 100 °C/s yields the desired UFF-martensite microstructure. Electron backscattered diffraction (EBSD) mapping reveals a high percentage (approximately 40 pct) of low-angle boundaries in these microstructures, except for the steel that is just microalloyed with Nb. The steel with the plain-carbon-base chemistry was subjected to hot torsion simulations of a hot strip rolling processing schedules that incorporate a DIFT pass after a conventional seven-stand finish mill schedule. Executing the DIFT pass at 650 °C to 675 °C produced an UFF microstructure, illustrating the potential for the design of novel thermomechanical processing paths to produce hot-rolled ultrafine DP steels