Multi-scale Microstructure Refinement And Mechanical Properties Of1200MPa Low Alloy Multi-phase Steels

An economical Fe-Mn-Si-Cr low carbon low alloy steels have been selected as the testingmaterials in this paper. We used two microstructure control techniques TMCP (TMCP)-Tempering and Quenching-Partitioning (Q-P) to get bainite/austenite (B/A) andmartensite/austenite (M/A) samples, respectively, which are multi-scale refined, metastable,multi-phase high strength steels. The components of different phases in these samples wereanalyzed by X-ray diffraction (XRD). The characteristics of microstructure and the multi-scalerefinement behavior were investigated by Atomic Force Measurement (AFM) and ElectronBackscattered Scattering Detection (EBSD). The nanoindentation mechanical behavior,dynamic impact toughness and toughening mechanism were compared between these samples.The contents are as followed:The fractions of austenite in B/A and M/A samples are nearly equivalent, which are9.1%and8.3%, respectively. Film-like austenite, which of sizes is refined to submicron, isdiscontinuously distributed between B laths and M laths. AFM analysis shows that t width ofaustenite in B/A is100～200nm and that of austenite in M/A is slightly higher than100nm.The width of substructures in B lath is refined to nanometer. AFM analysis shows thatwidth of sub-laths in B lathes is less than100nm.The width of B laths and M laths are both refined to submicron. AFM analysis shows thatthe width of B laths is200～300nm and that of M laths is100～200nm.The effective grain size of blocks is refined to micron. EBSD analysis shows that packetsboundaries in B/A and M/A samples are all high angle boundaries, respectively. Effective grainsize of bainite packets is1.20μm and that of martensite packets is2.25μm.Strength index of M/A is higher than that of B/A. Nanoindentation tests show that thehardness of bainite and bainite-austenite structure in B/A is respectively6.93GP and6.71GPawhile that of martensite and matrensite-austenite structure in M/A is respectively9.37GPa and8.24GPa. The elasticity modulus of bainite and bainite-austenite structure in B/A is respectively6.93GP and6.71GPa while that of martensite and matrensite-austenite structure in M/A isrespectively6.93GPa and6.71GPa. Plasticity index of B/A is higher than that of M/A. Nanoindentation tests show that theindentation strain of bainite and bainite-austenite structure in B/A is respectively0.163nm/nmand0.222nm/nm while that of martensite and matrensite-austenite structure in M/A isrespectively0.136nm/nm and0.166nm/nm. The investigations of working-hardening curve ofmulti-phase structures clearly demonstrate the generation of transformation induced plasticity(TRIP) effect.B/A samples have better toughness than M/A samples. Instrumented impact test revels thatB/A samples shows semi brittle fracture while M/A samples shows brittle fracture. The charpyimpact energy (AKV), crack propagation energy and dynamic fracture toughness (KId) of B/Asamples are both higher than that of M/A samples. The multi-scale refinement ofmicrostructure, substructure and the submicron size retained austenite distributed between lathsenhance the resistance of crack initiation and propagation, which improve the toughness.