Investigation Of Hierarchical Microstructure On Mechanical Property Of Lath Martensite Steels

In this thesis,the low-carbon steel 20CrMnTi and medium-carbon steel30CrMnSi were carried out from different heat treatment processes with the aims of obtaining lath martensite hierarchical microstructure,including prior austenite grain?d_r?,packet?d_p?,block?d_b?and lath?d_l?.The size and orientation of hierarchical micro-structures were characterized quantitatively by means of OM,SEM,EBSD,TEM et al.As well,the macroscopic properties?with tensile and impact?and fracture toughness(K_IC)were measured for different conditions.The relation between properties and microstructure were determined from Hall-Petch relationship and obtained the effective control unit and criterion and revealed the important role of micro-structures to the mechanic properties.The microstructure was lath martensite found in 30CrMnSimedium-carbon steel after quenching and tempered.With the prior austenite grain coursing,the packet and block size increased.The martensite lath was related to the quenching temperature and the higher the quenching temperature,the samller the lath.In addition,with the prior austenite grain coursing,the yield strength,tensile strength,reduction of area and elongation were all reduced,the fracture toughness test is improved,and the impact toughness is basically unchanged.The strength index was in accordance with Hall-Petch.Relations,high-angle grain boundaries play a crucial role in strength,of which the martensite block size contributes most to the strength.20CrMnTi steel is low carbon low alloy steel.Due to high N content in the smelting process,a large amount of TiN inclusion particles precipitated in the matrix.SEM,EBSD and EDS quantitative characterization analysis results show that:with the increase of quenching temperature,the prior austenite grain,packet and block size increases;TiN inclusion was the high temperature stable phase.The shape is mostly square and the distribution is diffuse.With the increasing of the prior austenite grain size,the tensile specimens wereall plastically fracture,meanwhile the fracture toughness and impact toughness were cleavage brittle fractures.TiN inclusions impacted the plasticity of coarse-grained specimens slightly lower,the large-angle interface was still an important factor affecting the intensity.With the increasing of the prior austenite grain size,the impact toughness and fracture toughness of20CrMnTi steel decreased;combined with the Hall-Petch relationship,it was found that the prior austenite grain,packet and block all had the effect of preventing the crack propagation.Among them,the martensitic packet size contributes the most.TiN inclusion is the main cause of cleavage cracking of 20CrMnTi steel.Under high stress concentration,TiN inclusion particles can cause two cleavage crack initiation mechanisms,resulting in cleavage fracture of the sample.