Study On The Construction Of Gradient Structure And Optimization Of Mechanical Properties Of Steel

Metal materials are widely used in aerospace,automobile manufacturing,transportation and other field because of their excellent strength and toughness.With the progress of science and technology,higher requirements are put forward for the mechanical properties of metal materials,that is,the synergy of high strength and high ductility/toughness.However,the relationship between strength and plasticity/toughness of traditional metal materials is inverted,which greatly limits the development of metal materials.In recent years,it has been found that the inversion relationship of traditional metal materials can be overcome by constructing gradient structures.The excellent mechanical properties obtained by gradient structures point out a promising way for the manufacture of advanced materials with high performance.However,the effectiveness of different gradient structures on performance improvement and other influencing factors remain to be further studied.In addition,due to the limited thickness of the gradient structure prepared by predecessors,the impact performance of the lack of research.In this thesis,commercial low-alloy steel is selected as the research object.Through the construction of gradient grain structure（GGS）and gradient dislocation structure（GDS）,the effects of microstructure,grain size distribution and crystal texture on the mechanical properties of low-alloy steel with gradient structure are explored,and the strengthening and toughening mechanism is deeply studied.（1）The grain size distribution of the GGS and dislocation density distribution of GDS can be effectively manipulated by tuning the pre-torsion or annealing parameters and multiple strength-ductility combinations are obtained,which are described as an inverse linear rule,in stark contrast to the banana-shaped strength-ductility trade-off dilemma in monolithic materials.In low strength（σ_y～400-550 MPa）range,superior strength-ductility-toughness synergies are achieved by both the GDS and GGS,while in high strength（σ_y～550-750 MPa）range,the GGS presents much better strength-ductility-toughness combinations than the GDS.（2）Compared with coarse grains（σ_y～360 MPa,U_r～114.8 MJ/m³）,the yield strength（σ_y～736 MPa）of GGS with grain size distribution from～3.4±1.6μm on the surface to～8.0±3.5μm on the core is doubled.The static toughness（U_r～125.1 MJ/m³）is also improved.In addition,we found that the strong<110>texture formed in the tensile direction is beneficial to enhance the high strength-plasticity combination of the gradient grain structure.（3）Investigated the tensile and impact properties and strengthening-toughening mechanisms of gradient-structured low-alloying steel,in which both the grain size and retained geometrical necessary dislocations（GNDs）density change continuously,produced by pre-torsion and annealing treatments.The enhanced tensile properties were attributed to the continuous work-hardening from the fine-grain region at the surface to the coarse-grain region at the core,caused by the sustaining accumulation of GNDs.（4）A simultaneous enhancement of yield strength（σ_y～397 MPa）and impact toughness(A_kv～129.69 J/cm²)was obtained at a specific gradient-structure with the grain size distribution from～8.3±4.8μm at the surface to～22.4±10.2μm at the core.The characterization of the fracture morphology of the sample with high strength and toughness showed the prominent ductile fracture by microvoid coalescence,which was contrast to the predominant brittle cleavage fracture of the sample with high strength but low toughness.This study is of significance to develop advanced structural materials with the exceptional strength-toughness combination by architecturing proper gradient structure.