Fracture Toughness And Damage Mechanisms Of The Ductile Ferritic Steel And High-manganese Steel Sheets

Air pollution and greenhouse effect caused by automobile exhaust have brought serious social and economic problems.In order to meet the increasingly stringent exhaust emission regulations,vehicle lightweight is one of the important solutions.The use of advanced high strength steel(AHSS)is an important way to realize automobile lightweight.Typical representatives include ferrite martensite dual phase steel and high manganese steel.However,the engineering application of advanced high strength steel needs accurate evaluation and indepth analysis based on fracture performance,in order to provide key basis for body design and material selection.As the main carrier of plastic strain,ferrite phase in advanced high strength steel interacts with various strengthening phases to make the material show good tensile strength plastic matching.However,the local stress concentration at the soft / hard interface will promote the formation of damage;After damage nucleation,it will grow and connect in ferrite phase,and eventually lead to unstable fracture.In order to deeply understand the intrinsic fracture properties of ferrite phase in advanced high strength steel,this thesis takes interstitial free ferrite(IF)steel as the research object,and systematically studies its microstructure,uniaxial tensile properties,fracture toughness and damage mechanism.In addition,high manganese steel has the best tensile plasticity among advanced high-strength steels,but there are significant fracture problems in the process of forming and service.The high elongation of high manganese steel under uniaxial tension is mainly provided by uniform plasticity.It often breaks rapidly after necking or fails before necking.This fracture problem is contained in ε-martensitic phase is particularly prominent in high manganese steel.In this thesis,two high manganese steel model materials Fe26 Mn and Fe30 Mn with different austenite stability are taken as the research objects,and the fracture toughness and damage mechanism are systematically studied.The research results of two material systems in this thesis reveal the key intrinsic and extrinsic factors of fracture toughness of high plastic plates.The results of IF steel and Fe30 Mn show the importance of the extrinsic factor of local necking work at the crack tip,which means that it is of great significance to improve the work hardening ability and fracture strain at the same time to improve the apparent fracture toughness.The comparison of DP steel and IF steel and the comparison of fracture toughness of Fe26 Mn and Fe30 Mn alloy show that micro heterogeneity is the controlling factor of intrinsic fracture toughness.It is urgent to improve the damage resistance of heterogeneous structure through tissue regulation to delay the occurrence of damage.The main conclusions of this study are as follows:(1)The yield strength and tensile strength of IF steel are much lower than that of ferrite martensite dual phase steel DP600 and DP780,while the uniform elongation and total elongation are much higher than that of DP600 and DP780.However,according to the measurement results of basic fracture work method,the fracture toughness of IF steel sheet is not higher than that of dual phase steel.At a thickness of about 1 mm,the basic fracture work of IF steel is higher than DP780 but lower than DP600.The fracture toughness of interstitial free ferritic steel decreases significantly with the decrease of thickness,which is due to the fact that the energy dissipation in the fracture process is mainly determined by the local necking work at the crack tip.(2)The damage analysis of tensile fracture specimens of IF steel shows that the grains at the fracture are obviously elongated and refined;There are high-density dislocations in the structure,but no significant hole formation is observed.The damage analysis of DP600 shows a typical ferrite / martensite interface crack,which grows along the ferrite grain boundary after nucleation;In the damage analysis of DP780,it is also found that there is obvious damage accumulation in the microstructure below the fracture.The damage mechanism is also ferrite /martensite interface cracking,and the microcracks grow along the ferrite grain boundary parallel to the tensile direction.(3)Fe26Mn has two-phase structure(face centered cubic phase and close packed hexagonal phase)at room temperature,and Fe30 Mn has single-phase austenite structure.The yield strength and tensile strength of Fe26 Mn are higher than that of Fe30 Mn,and the uniform elongation is lower than that of Fe30 Mn.Fe26Mn has only a small amount of deformation after necking,while Fe30 Mn has a very significant necking deformation.However,there is little difference in the basic fracture work between Fe26 Mn and Fe30 Mn sheet,indicating that the fracture toughness of these two high manganese steel sheets with different austenite stability is similar.The fracture strain of Fe30 Mn is higher than that of Fe26 Mn,and more energy is dissipated in the local necking process at the crack tip.Fe26 Mn is that the energy dissipated in the damage process in the fracture process area is higher than that of Fe30 Mn.(4)Damage such as holes and microcracks are densely distributed near the fracture of Fe26 Mn.The damage is related to the formation of dense hexagonal phase: cracking at the phase interface or cracking inside the dense hexagonal phase.There are significant local uneven deformation around microcracks and holes.Fe30 Mn damage is mainly caused by the formation of microcracks in the dense hexagonal phase produced by deformation,and high local orientation difference is also accompanied around the microcracks.