Micro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel

Abstract

Micro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel are investigated through experimental simulation and microstructural characterization. The results show that the deformation strengthening effect is fully exerted by thermo‐mechanical control process in steel containing 0.015 % lanthanum. Finally a kind of multi‐layer bainite ferrite microstructure featured by 3.5 μm blocks, 0.41 μm plates and 108 nm sub‐plates with ultrafine sub‐subunits and 55 nm θ‐M3C inside is achieved, which enhances the strength and toughness synergistically. And the nanoscale refinement mechanism of bainite ferrite plates lies in the formation of massive ultrafine sub‐subunits with the average size of 20 nm ×32 nm. Besides, a large number of twinning martensite with the size of 2 nm to 20 nm and high‐density entangled dislocations can be found on the boundaries of ultrafine sub‐subunits. Further, the density of dislocation is increased by 2.92×1014 cm−2 and its contribution to the strength is calculated to be 18 MPa. Moreover, micro‐alloying effects of lanthanum in thermo‐mechanical control process are explored to be that lanthanum enhances the interaction between bainite ferrite plates and dislocations, strengthens the entanglement of carbon and θ‐M3C with dislocations, and promotes the pinning effect of θ‐M3C on ultrafine subunits.