Fatigue Properties Of Fe-20Mn-0.6C TWIP Steel With Different Grain Sizes

In order to be competitive in automobile markets,high-strength alloys are more and more frequently innovated and developed by the automobile manufacturers,both to reduce vehicles weight and to improve passenger safety.Twinning induced plasticity(TWIP)steel has high strength and plasticity as well as good comprehensive mechanical properties.It has become the research highlight in the field of the international automobile structural steel,and has been widely used in the cockpit,front side beam,front and rear anti-collision beam,door anti-collision beam and so on,showing strong competitiveness.However,due to cyclic loading under the working condition,it is easy to suddenly crack or overladen crack after a period of fatigue loading,resulting in huge economic losses and personal casualties.Therefore,it is of great significance to explore the fatigue damage and behaviors under repeated loading conditions.The purpose of this study is to explore the effect of fatigue damage caused by plastic deformation on fatigue life of Fe-20Mn-0.6C TWIP steel.By evaluating the effect of fatigue load on hardness,the varaiation of dislocation density and deformation twin density is verified.The fatigue properties of Fe-20Mn-0.6C TWIP steels with different grain sizes were analyzed according to the fracture morphology,metallographic structure and dislocations configuration.The experimental results as follows:Fe-20Mn-0.6C TWIP steel with different grain sizes were prepared by changing rolling process,which mainly consist of austenite and annealing twin through the whole grain.The average grain size of investigated steel by hot rolling is 50 ?m with regular shape,uniform size and obsolete rolling direction.The average grain size of investigated steel by two-stage controlled rolling is 20?m the geometric shape of the grain is irregular,and the large grain is surrounded by small grain with irregular shape.Experienced steel with grain size of 50 ?m and 20 ?m as research objects were applied to tensile and compressive fatigue tests with constant strain amplitude of 0.6%,1.0%and 1.6%.Cyclic softening,stress saturation and cyclic hardening were observed from the fatigued samples.As the grain size is 50 ?m,the fatigue life of Fe-20Mn-0.6C TWIP steel at different strain ranges is 6037 cycles,1103 cycles,247 cycles,respectively.As the grain size is 20 ?m,the fatigue life of Fe-20Mn-0.6C TWIP steel at different strain ranges is 2843s cycle,1600 cycles,375 cycles respectively.At constant grain size,the plastic deformation increases with increasing strain amplitude.The cyclic hardening is obvious,which leads to the increase of peak stress and fatigue damage.At constant grain size,the fatigue life is inversely proportional to grain size at the strain amplitude of 1.0%and 1.6%.However,secondary cyclic hardening occurs at the strain amplitude of 0.6%,which results in abnormal maximum peak stress.The cyclic hardening rate(d=20 ?m)is larger than that of cyclic hardening rate(d=50 ?m).So fatigue life is directly proportional to grain size at the strain amplitude of 0.6%.The fracture morphology of different grain size steel was observed by SEM.The main features include crack initiation zones,crack propagation zones and transient fracture zones.The fluvial cleavage steps,the beachlike cleavage surface and the shellfish lines arched in the direction of crack propagation can be seen in the macroscopic fracture.In the crack initiation zone,fatigue stripes can be found.In addition,the distance between fatigue stripes becomes wide with increasing applied load along with the distance from the cross section edge of sample.At the crack growth stage,the fracture morphology of the experimental steel with the grain size of 50 ?m exhibits as fatigue striation at the cleavage step and the dimple appears at the strain range of 1.6%.However,the fracture morphology of the steel with the grain size of 20 ?m mainly includes smooth cleavage plane.Dimples with the same morphology as tensile fracture appeared in the transient fracture region.The microstructure of experimental steels with different grain size at different strain amplitude was observed by OM,including deformation twin,annealing twin and austenite.When the strain amplitude increases from 0.6%to 1.6%,the deformation twin density of Fe-20Mn-0.6C TWIP steel with the grain size of 50 ?m increases with increasing strain amplitude,meanwhile,the hardness increased from 300 HV to 386 HV.At the strain amplitude 0.6%,the secondary cyclic hardening phenomenon resulted in a raise peak stress of experimental steel with grain size of 20 ?m.Therefore,there is a small difference in content of deformation twin between the strain amplitude 0.6%and 1.0%.The dislocation density of fatigue deformation samples under different strain amplitude was analyzed by XRD.When the grain size is 50 ?m,the dislocation density of Fe-20Mn-0.6C TWIP steel at strain amplitude 0.6%,1.0%and 1.6%is 1.0×1018 m-2,3.1×1018 m-2 and 10.4×1018 m-2,respectively.With the increasing strain amplitude,the lattice distortion caused by both twin and stacking faults increases,leading to the increasing dislocation density.When the grain size is 20 ?m,the total dislocation density of Fe-20Mn-0.6C TWIP steel increases with increasing strain amplitude.However,the deformation twin content increases due to secondary cyclic hardening at the strain amplitude of 0.6%,which results in the dislocation density at the strain amplitude of 0.6%is larger than that at the strain amplitude of 1.0%.The dislocation density of Fe-20Mn-0.6C TWIP steel at strain amplitude 0.6%,1.0%and 1.6%is 3.2×1018 m-2,2.1×1018 m-2 and 8.5×1018 m-2,respectively.TEM observations show that the microstructures of fatigue samples with 50 ?m grain size include twin,stacking faults,persistent slip band(PSB),various dislocation configurations.Due to the relatively large grain size obtained after hot rolling and solid solution treatment,the critical stress value is small,which is not sufficient for climbing and cross slip for the Fe-20Mn-0.6C TWIP steel with grain size of 50 ?m.With increasing strain amplitude,dislocation density increases and aggregates in PSB.On the other hand,the structure of dislocations is closer to a vein shape in the the Fe-20Mn-0.6C TWIP steel with grain size of 20 ?m.The leaf vein appears between PSBs,which leads to the secondary cycle hardening in the late fatigue stage.At strain amplitude of 1.0%and 1.6%,the deformed regions are separated into two independent regions including twin and reticular dislocation structures by the dislocation wall.