Study On High Strain-rate Tensile Properties And Fatigue Performance Of Thin Dual-phase Steel Sheets

Being a widely used automotive steel,dual phase(DP)steel has received much more attention in collision safety and fatigue performance during its service.In this study,the micro-deformation distribution,void formation and fracture mechanism of DP steels with different strength levels at different strain rates were illustrated by high-speed tensile tests.Besides,based on the comparison between the loading of cantilever beam bending fatigue and of symmetrical tension-compression fatigue of DP steels,an evaluation method on fatigue lives of automobile thin steel sheets in the large strain range was explored.The fatigue damage,deformation and fracture mechanism of thin DP steel foils under cantilever beam bending fatigue loading were analyzed and discussed,and factors affecting the fatigue lives of DP steels were summarized.The high-speed tensile tests of DP590 and DP780 steels show that their yield strength and tensile strength increase with the increase of strain rate,and their uniform elongations increase at first,then decrease and increase again.In the low strain rate range,the dislocation tangles is the main dislocation morphology in both DP steels;as the strain rate increases,there is an increase of dislocation tangle density inside ferrite grains;when the strain rate increases to 9×10² s^-1,dislocation tangles tend to transit to dislocation bands.The micro-deformation concentration zones are at martensite/ferrite interfaces in both DP steels in different strain rate ranges.The micro-deformation inside ferrite grains in DP590 steel declined gradually as the strain rate increases,while the distribution of micro-deformation inside ferrite grains in DP780 steel changes little.Martensitic/ferrite interface cracking and martensite cracking are both common in DP590 steel at lower strain rates,while martensite/ferrite interface cracking is dominant at higher strain rates.The cracking zones in DP780 steel do not change significantly with the variation of strain rates.Namely,martensite/ferrite interface cracking is dominant at all strain rates.From the comparision of total strain amplitude-fatigue life relations of cantilever beam bending fatigued specimens of cool rolled DP590 steel with thickness of 95 ?m and symmetrical tension-compression fatigued ones with thickness of 2 mm,it can be found that the low-cycle fatigue lives are identical,while there is a large difference between the high-cycle fatigue lives of the two sorts of specimens.This is mainly due to the difference of fatigue damage mechanism and crack propagation behavior between the two types of fatigue loading.Thus,using ultra-thin specimens for cantilever beam bending fatigue loading controlled by constant deflection,the low-cycle fatigue lives of automotive thin steel sheets with thickness of 2 mm or even thinner can be quickly evaluated when the symmetrical tensile-compression fatigue loading is difficult to perform at large strain range.The cantilever beam bending fatigue experiments of DP500,DP590 and DP780 steel specimens show the low-cycle fatigue lives of three DP steels are very close and their high-cycle fatigue lives are different;no matter in the low-cycle fatigue range or high-cycle fatigue range,the percentage of martensite/ferrite interface cracking of all three DP steels increases with the increase of martensite volume fraction,and the relative ratio of the martensite/ferrite interface cracking percentage is close to that of the corresponding martensite volume fraction.The percentage of cracking site at the intrusion/extrusion inside ferrite grains in DP500 steel is the highest,while it is the lowest in the high-cycle fatigue regime;the percentage of cracking sites at the intrusion/extrusion inside ferrite grains and ferrite grain boundaries in DP590 and DP780 steels does not vary too much.By summarizing the plot of total strain amplitude-fatigue lives of DP steels,it can be found that the cyclic deformation mechanism in the low-cycle fatigue regime of DP steels are very close,which is mainly related to the plastic deformation behavior of ferrite matrix.Besides,ferrite grain size has a more obvious effect on the low-cycle fatigue lives of DP steels than martensite volume fraction does.