TEM observation and strengthening mechanism of cementite nanoparticles of heterogeneous structure 1045 steel

A heterogeneous structural (HS) 1045 steel prepared by aluminothermic reaction (AR) casting method was fabricated by combining severe deformation cold rolled with subsequently heating. Its detailed microstructure evolution, mechanical properties, strengthening and toughening mechanisms were investigated. The heterogeneous structure (HS) is characterized with trimodal grain size distribution of ferrite (with microcrystalline grain sizes of >1000 nm, ultrafine grain sizes of similar to 1000 nm, nanocrystalline grain sizes of similar to 100 nm grains, respectively) and cementite nanoparticles (with particle sizes of 11 similar to 120 nm). The HS 1045 steel shows an outstanding strength-ductility synergy. When the cold rolled steel was heated at 400 degrees C with 1 h, comparing with casting steel, the yield strength and tensile strength increased by 87.4% and 35%, to 1420 and 1602 MPa, respectively, and surprisingly, the ductility was only with a small sacrifice, the elongation rate is maintained at the level of 10.6%. The improved strength is mainly attributed to multiple strengthening mechanisms, fine-grain strengthening, cementite nanoparticles strengthening, dislocation strengthening, and solution strengthening. The reasonable ductility can be attributed to the heterogeneous structure, as it can offer an extra work hardening ability introduced by generation of geometrically necessary dislocations (GNDs) near heterointerfaces.