Effect Of Strain Rate On Mechanical Properties And Microstructures Of High Manganese TRIP Steel

High-Mn transformation-induced plasticity（TRIP）steel has attracted a great deal of attention owing to its significant strain hardening capacity,impressive impact resistance and combination of high strength and ductility.This kind of steel is used to manufacture automotive safety crash structure and energy-absorbing buffer parts.Those structural parts are usually subjected to deformation loads at different strain rates in sudden accidents.Therefore,it is of great significance to study the mechanical properties and deformation mechanism of high-Mn TRIP steel under different strain rates to understand the microstructure-property relationship for the high Mn TRIP steel.In this thesis,the influence of strain rate on microstructure relating to mechanical properties of Fe-20Mn-3Al-3Si steel without interstitial atom and Fe-20Mn-0.3C TRIP steel with interstitial atoms were studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,backscattered electron diffraction（EBSD）,transmission electron microscopy（TEM）and mechanical tests,i.e.quasi-static compression,tensile tests and Hopkinson dynamic compression.The results show that the yield strength of Fe-20Mn-3Al-3Si steel during dynamic defromation is higher than that under quasi-static compression.But the strain hardening rate of the dynamically impact sample is lower,which leads to lower flow stress in the dynamic deformation sample when the strain exceeds 0.11 compared with the quasi-static compression sample.The low strain hardening capacity is attributed to the suppression of martensite transformation and promotion of dynamic recovery of dislocations.For the quasi-static deformation,the strain induced phase transformation product are mainly composed ofε_hcp-martensite andα’_bcc-martensite,and the orientation relationship（OR）between theα’_bcc-martensite andγ_fcc-austenite keeps K-S OR.Besides the formation ofε_hcp-martensite andα’_bcc-martensite,a large number ofε_hcp-twins including{10 1^—1}ε-twins and{10 1^—2}ε-twins are formed at the intersection of twoε_hcp-martensite laths in the dynamic deformed sample.At this time,the OR betweenα’_bcc-martensite formed alone andγ_fcc-matrix is K-S OR,and the OR betweenα’_bcc-martensite adjacent to the{10 1^—1}ε-twins andγ_fcc-matrix is an approximate Pitsch OR.The tensile tests of austenitic Fe-20Mn-0.3C steel at different strain rates(10^-3s^-1～10⁰ s^-1)show that the yield strength and ultimate tensile strength of austenitic Fe-20Mn-0.3C steel decrease with the increase of strain rate,which exhibits a negative strain rate sensitivity.While the total elongation firstly increases and then decreases with the increase of strain rate.There is higher volume fraction ofε_hcp-martensite and dislocation density in the lower strain rate deformed samples.The deformation products are mainly composed ofε_hcp-martensite and deformation twins in 10^-3 s^-1 deformed sample.For 10^-1 s^-1deformed sample,there is almost noε_hcp-martensite and the deformation products are mainly composed of deformation twins ofγ_fcc-austenite and dislocation.Therefore,the dominant deformation mechanism at lower strain rate(10^-3 s^-1)deformed sample includes strain-inducedε_hcp-martensitic transformation（TRIP effect）,deformation twinning（TWIP effect）.The deformation mechanism of higher strain rate(10^-1 s^-1)deformed sample is mainly deformation twinning（TWIP effect）and dislocation slip.The tensile tests of duplex-phase Fe-20Mn-0.3C steel at different strain rates(10^-3 s^-1～10⁰ s^-1)show that the strain rate has no obvious effect on the yield strength of the samples.But the ultimate tensile strength and total elongation firstly increases and then decreases with the increase of strain rate.The effect of strain rate on the volume fraction of strain-inducedε_hcp-martensite is not obvious.For lower strain rate(10^-3 s^-1)deformed sample,the dislocation density is lower and the shear band appear in the sample.The fracture way of lower strain rate deformed samples is ductile fracture and quasi-cleavage fracture.The higher strain rate(10^-1 s^-1)deformed sample have higher dislocation density and exhibit ductile fracture.