| In order to meet the energy saving and environmental protection requirements,automotive steels need to develop in the direction of lightweight,which enables them to exhibit good mechanical properties,excellent impact resistance and formability.Among them,TWIP steel can meet all the needs of automotive steels well as a potential candidate.However,TWIP steel generally shows a high sensitivity to the delayed fracture,and so far very limited work has been carried out on the delayed fracture of austenitic TWIP steels.The work in this thesis is combined with the project titled 'Fracture and Hydrogen Embrittlement of TWIP Steel' of RWTH Aachen,Germany Steel Forschungsgemeinschaft(DFG)within the Collaborative Research Center(SFB)761 C6"steel-ab initio.Therefore,the present work mainly focuses on the dependence of delayed fracture on the grain size and hydrogen content in the well-known Fe-0.6C-(17-22)Mn-xAl alloy system by quasi-static tensile test,slow strain rate tensile test and deep drawing test,etc.Hydrogen was electrochemically charged using potentio-statica at room temperature.Hydrogen contents were quantified by hot extraction and Thermal Desorption Analysis(TDA).In-depth investigation of mechanical properties at different hydrogen levels elucidated the influence of hydrogen content,chemical composition and lattice defects on the delayed fracture behavior.In addition,the dislocation and twinning structures were characterized by Transmission Electron Microscopy(TEM)to reveal the influence of grain size on the dominant deformation mechanisms.The fracture surfaces were observed by Scanning Electron Microscope(SEM)to reveal the combined effects of grain size and hydrogen content on the fracture morphology.The main results of this dissertation are summarized as follows:(1)The adsorption,diffusion and dissolution of hydrogen on metal surfaces were analyzed,and the influence of the solution composition and the current density electrochemical hydrogen charging process was investigated.By means of experimental analysis,the solution 0.05mol/L H2SO4 + 1.4g/l(NH2)2CS was applied for the charging of the current TWIP steel.According to the first and second law of Fick,the hydrogen distribution in the sample after hydrogen charging was simulated,and the experimental results showed that the hydrogen content of the sample decreased significantly after exposing in atmosphere for 24h.(2)The influence of charging time on the amount of total hydrogen content was studied by hot extraction.Hydrogen contents in all the materials increased significantly with extending the charging period.The 5 ppm hydrogen was obtained in the samples after 72h hydrogen pre-charging,while the hydrogen content doubled after 166h hydrogen pre-charging.Thermal Desorption Analysis(TDA)reveals that the first desorption peak at 200 ? corresponds to the part of diffusive hydrogen;whereas the second desorption peak at?600 ? is closely associated with the non-diffusive hydrogen.(3)The Fe-22Mn-0.6C TWIP steels with average grain sizes of 4?m,7?m,20?m and 45?m were electrochemical hydrogen charged to 5ppm and lOpmm,and the combined effects of grain size and hydrogen content on the drawability of the experimental material were investigated in detail.The results showed that the 4?m and 7?m-sized samples failed to form,while the 20?m and 45?m-sized samples without hydrogen charging were successful for deep drawing.The 20?m and 45?m-sized samples reveal the delayed fracture with increasing the hydrogen content.The material was more sensitive to the delayed cracking with the decrease in grain size or the increase in the hydrogen content.(4)Delayed fracture surfaces of the materials under different experimental conditions were revealed using SEM,which can be divided into three zones:the initiation zone,intermediate and arrested zone.The material A fracture surface without hydrogen charging was characterized by cleavage fracture,quasi-cleavage fracture and brittle fracture,respectively.The fracture surfaces of material C and D with 5 ppm hydrogen show ductile fracture,quasi-cleavage fracture and brittle fracture,respectively.As the hydrogen content increased to 10ppm,the fracture surface of material C and D exhibited brittle intergranular cracking.(5)Based on the slow strain rate tensile testing and microstructural observation,the effects of grain size and hydrogen content on the hydrogen embrittlement of Fe-22Mn-0.6C TWIP steel were studied.The results showed that the fracture stress,fracture strain and fracture time of materials are closely related to the hydrogen content after hydrogen charging.Stress or strain loss ratio increased with increasing the hydrogen content,and the strain loss ratio is higher than the stress loss ratio,which means that the material strain is more sensitive to the hydrogen.Stress and strain loss drops as the grain size decreases with the same hydrogen content,which confirms that the grain refinement improved the resistance to hydrogen embrittlement of materials.Fracture surface after slow strain rate testing shows ductile fracture,and dimple size increases as the grain size increases without hydrogen.After hydrogen charging,the brittle fracture edge region shows intergranular fracture,and the central region exhibits ductile fracture.With the increase of hydrogen content,the depth of intergranular crack region increases.The depth of intergranular fracture region increases as the grain size increases with the same hydrogen content.(6)The combined effects of grain size and hydrogen content on the mechanical properties of Fe-17Mn-lAl-0.6C TWIP steel were studied,and the results showed that yield and ultimate tensile strengths,and work hardening rate of the steel decreased,while the elongation increased as the grain size increased.The tensile strength and elongation decreased with pre-charged hydrogen.As the hydrogen content increased from 5ppm to 10ppm,the strain loss increased from 6.7%to 11%in materials B,while the strain loss of material C increased from 9.3%to 17%.(7)Delayed fracture behavior of TWIP steels with different Al content were studied,and the results showed that the delayed fracture did not occur in the Fe-17Mn-lAl-0.6C TWIP steel containing the 10 ppm hydrogen content,while this phenomenon occured in the 10 ppm hydrogen-added Fe-22Mn-0.6C steel after deep drawing and exposing in the air for several hours.It is demonstrated that Al increases the stacking fault energy of the material,leading to the more homogeneous distribution of deformation twins and the smaller residual stress,and thereby improving the resistance to delayed fracture of TWIP steels. |
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