Effect Of Pre-strain On Microstructure,mechanical Properties And Hydrogen Embrittlement Of A Novel Al-containing Medium Mn Steels

In recent years,with increasingly demands for automotive industry energy saving,vehicle emission reduction and consumer safety concerns,the research and development of medium Mn steels with superior mechanical properties to balance vehicle lightweight,collision safety and economic costs have attracted more and more attentions.Although the mechanical properties of medium Mn steel have been significantly enhanced through compostion optimization and microstructure adjustment,the problems of unavoidable plastic deformation in the process of cold forming and in service and correspondingly hydrogen embrittlement(HE)risk are still the limiting factors for the applications of this type of steel.Therefore,the influneces of pre-strain up to 25% on the microstructure,mechanical properties and HE behavior of a novel Al-containing medium Mn steel(0.25C-8.67Mn-0.54Si-2.69 Al,wt%)subjected to hot-and cold-rolling and intercritical annealling were studied systematically by means of electrochemical hydrogen charging,thermal hydrogen analysis,hydrogen permeation test,slow strain rate tensile(SSRT)test,scanning electron microscope(SEM),electron backscattered diffraction(EBSD)and transmission electron microscope(TEM).The conclusions are drawn as follows:The microstructure of the hot-rolled and intercritically annealed tested steel is primarily lathy multiphases including intercritical ferrite(IF)and retained austenite(RA),in which the RA fraction is ?60 vol% and the product of ultimate tensile strength(UTS)and total elongation(TEL)(UTS×TEL)as high as 69.1 GPa·% was obtained.With an increase in the pre-strain,the dislocation density in the IF was significantly increased,and part of the large size unstable RA preferentially transformed into martensite causing a decrease of the remaining RA fraction and an enhanced mechanical stability of the remaining RA.As a result,both the UTS and yield strength(YS)increased while the TEL decreased with increasing pre-strain.The as-hot-rolled and annealed tested steel exhibited comparatively high susceptibility to HE,and the HE susceptibility index(HEI),which was defined as the relative TEL loss,increases from 75.9% for the non-pre-strained steel to 83.2% for the 15% pre-strained steel.The fractured SSRT specimen showed a primarily brittle quasi-cleavage fracture with many secondary cracks in the crack initiation region.The cold-rolled steel sheet processed by intercritical annealing at 725 ? for 60 min has an ultrafine grained morphology primarily consisting of equiaxed IF and RA with rather low dislocation densities.The pre-strained specimens exhibited formations of both mechanical twins and ?-and ?'-martensite,indicating the occurrence of TWIP(twinning-induced plasticity)effect besides TRIP(transformation-induced plasticity)effect during tensile deformation and the occurrence of ?-martensite as a transition phase.Like that of the hot-rolled and annealed steel,the mechanical properties of the cold-rolled and annealed tested steel were notaly affected by pre-strain.The tensile behavior,which originally exhibited notable Lüders plus Portevin-Le Chatelier(PLC)banding phenomenon,was significantly changed by pre-straining.A pre-strain equalling to the Lüders strain,i.e.,3%,could cause an elimination of the Lüders bands while it had almost no influence on the PLC bands.The increase of the dislocation density,the enhanced stability of the RA and the strain-induced martensite and mechanical twins caused the reduction and elimination of the Lüders bands with increasing pre-strain.As for the cold-rolled and annealed tested steel,the reversible hydrogen content of the pre-strained and hydrogen-charged specimen increased significantly while the hydrogen diffusion coefficient decreased with increasing pre-strain.Like that of the hot-rolled and annealed steel,the cold-rolled and annealed tested steel exhibited relatively high susceptibility to HE.The HE susceptibility increased with increasing pre-strain.SEM fractographs reveal that fracture was dimple ductile for the uncharged specimens with different degrees of pre-strain,while the fracture appearance changed gradually from mixed intergranular and quasi-cleavage brittle fracture near the surface to dimple ductile fracture interior for hydrogen-charged pre-strained specimens.