Research On Mechanical Properties And Hydrogen Embrittlement Performance In Medium Mn Steels By Microstructural Control

The rapid development of the automotive industry has put forward higher requirements for automotive steels.Medium Mn transformation-induced plasticity(TRIP)steels,which are denoted as 3rd advanced high-strength steels(AHSS)of automobiles,is becoming increasingly attractive because of its advantages in mechanical properties,low material costs,easy material processing and lightweight,etc.Improving the comprehensive mechanical properties and the service performance of medium Mn steels by adjusting the microstructure,heat treatment and rolling process are the basic researches for industrialization.On the basis of Fe-6Mn-0.2C-3Al medium Mn steel,the composition and the microstructure are adjusted by adding a little Si(0.6 wt.%).These two specimens are denoted as 0Si and 0.6Si,respectively.The influence of the annealing time,strain rate,heat treatment and rolling process on mechanical properties and hydrogen embrittlement(HE)of the medium Mn steels are systematically investigated.The main conclusions are as follows:(1)The processes of inter-critical annealing were carried out at 740? for 3?120 min and then air cooled to RT.The microstructure,mechanical properties and fracture behavior after different annealing times are as follows:Compared to 0Si,in addition to the ultra-fine-grained austenite and ?-ferrite,0.6Si consists of abundant coarse-grained ?-ferrite.The hardness of ?-ferrite reduces sharply during annealing.The mechanical properties are slightly lower in 0.6Si than in 0Si for a short annealing time.For example,the product of strength and elongation(PSE)of 0Si and 0.6Si are 13.8?37.9 GPa·%and 17.1-25.3 GPa·%,respectively,when the annealing time is 3?7 min.However,the mechanical properties are much higher in 0.6Si than in 0Si for a long annealing time.For example,the PSE of 0Si and 0.6Si are 38.6-31.8 GPa·%and 58.2-55.6 GPa·%,respectively,when the annealing time is 30?60 min.Microcracks of 0Si initiate at the ?(?')/? interface,and those of 0.6Si mainly start at the interface of ?(?')/? and(?(?')+?)/?.When the hardness in ?-ferrite is higher than in austenite and ?-ferrite,the propagation of microcracks along the(?(?')+?)/? interface in 0.6Si contributes to the microcracks,parallel to the tensile directions,which lead to the delaminations on fractograph.When the hardness in ?-ferrite is lower than in austenite and ?-ferrite,microcracks which are vertical to the tensile directions preferentially pass through the ?(?')/? microstructure until microcracks arrest occur,extending to ?-ferrite.(2)Two specimens with the best mechanical properties have been chosen(0Si annealed for 10 min and 0.6Si annealed for 30 min).The mechanical properties and fracture behavior after different strain rates(1×10-4/s?1×10-1/s)are as follows:As the strain rate increases,the ultimate tensile strength(UTS)and the total elongation(TEL)in both 0Si and 0.6Si decreases continuously which results from the restriction of the TRIP effect,and the TEL decreases faster in 0.6Si than in 0Si.For example,the TEL of 0Si decreases from 44%to 33%,and that of 0.6Si decreases from 55%to 35%,when the strain rate increases from 1×10-4/s to 1×10-1/s.As the strain rate increases,the area reduction of 0Si is almost the same(about 70%),nevertheless,that of 0.6Si increases continuously(from 51%to 72%).The strain rates have nothing to dot with the fracture behavior for both 0Si and 0.6Si.However,as the strain rate decreases,the initiation sites number of microcracks increases,whereas the propagation rate decreases;the secondary cracks on fractographs increases,nevertheless,the dimple size decreases.(3)Four specimens have been chosen(both 0Si and 0.6Si annealed for 3 min and 30 min).The influence of the composition and the annealing time on HE performance and hydrogen induced fracture behavior have been systematically investigated.The main results are as follows:Regarding annealing time:as the annealing time increases,HE susceptibility(TEL loss and UTS loss)of both 0Si and 0.6Si increases continuously.For example,TEL loss and UTS loss in both 0Si/0.6Si are 13.5%/46.7%and 0.0%/1.7%,respectively,when the annealing time is 3 min;TEL loss and UTS loss in both 0Si/0.6Si are 79.2%/76.5%and 26.8%/6.3%,respectively,when the annealing time is 30 min.Regarding the composition:the HE susceptibility in 0Si is lower than in 0.6Si when annealing time is for 3 min,nevertheless,the HE susceptibility in 0.6Si is lower than in 0Si when annealing time is for 30 min.Compared to the fracture behavior in air,hydrogen promotes the initiation and propagation of microcracks,thereby resulting premature fracture.Regarding 0Si,hydrogen has nothing to do with the microcrack's initiation,nevertheless,the propagation of hydrogen induced microcracks(HICs)of 0Si reveals intergranular and transgranular.Regarding 0.6Si,the hydrogen induced fracture behavior is not affected by hydrogen,whereas the dimples on fractographs transform into quasi-cleavage.(4)Quenching and partitioning(QP),intercritical annealing reverted transformation(IA)and warm rolling(WR)are summarized.QP160,IA810 and WR760 are selected and the order of the mechanical properties from high to low is as follows:WR760>IA810>QP160.The HE susceptibilities(TEL loss)of WR760,QP160 and IA810 are 3.1%,24.2%and 54.2%,respectively,when the hydrogen charging time is 2 h.It reveals that WR760 reveals the highest HE resistance and IA810 shows the lowest HE resistance.In summary,the appropriate WR process may be one of the effective methods to improve the mechanical properties and the HE resistance of the medium Mn steels.(5)Without deteriorating the mechanical properties of commercial martensitic steels(S0),the second tempering can significantly improve HE resistance after different second tempering time(30 min,60 min,120 min).These samples were labeled as S30,S60 and S120.For example,TEL loss and UTS loss in both S0/S60 are 100.0%/79.3%and 35.9%/1.7%,respectively.The main reasons for the higher HE resistance of S60 are as follows:1:The increase in MoyCx precipitates;2:The growth of the cementite and matrix interface,and the strained interface acts as hydrogen traps with high binding energy;3:The decrease in dislocation density.