| The research and development of medium–Mn steels with the product of ultimate tensile strength(UTS)and total elongation(TEL)(UTS×TEL)over ~30 GPa·% is an effective approach to realize the goals of weight reduction and crashworthiness improvement simultaneously as well as to guarantee economical efficiency and manufacturing feasibility.Although the mechanical properties of medium-Mn steels,such as UTS×TEL,have been significantly enhanced in recent years,many issues related to industrial production and application,such as hydrogen related problems,should be explored further to make them commercially viable to manufacture automobile components.For this purpose,the hydrogen embrittlement(HE)behavior of industrial trial-produced 0.1C-5Mn medium-Mn steel subjected to hot-rolled(HR),cold-rolled(CR)and warm-rolled(WR)plus intercritical annealing(IA)was systematically studied by using electrochemical hydrogen charging,thermal hydrogen analysis,slow strain rate tensile test,scanning electron microscopy and transmission electron microscopy.The hydrogen-induced cracking and propagation mechanism of the obtained ultrafine duplex microstructure was clarified,and emphasis was paid to the effects of retained austenite(RA)stability and carbides on the HE susceptibility of medium–Mn steel.Furthermore,the technical way to improve the HE resistance of the tested medium-Mn steel through tempering treatment was discussed.Moreover,the behavior of the tested medium-Mn steel subjected to warm-and hot-stamping under different heating temperatures was investigated.The main conclusions are summarized as follows:The susceptibility to HE evaluated by the relative TEL loss is significant for the HR+IA medium-Mn steel primarily due to remaining of the boundaries of prior austenite grains and subsequent intergranular cracking.Therefore,the effect of tempering treatment on the hydrogen embrittlement(HE)behavior of as-annealed specimen was investigated.The results show that when the tempering temperature was lower than 400 ?,the UTS×TEL as high as 31 GPa·% remained nearly unchanged,however,the HE index(HEI),which is defined as the relative loss of TEL,decreased notably from 62% for the as-annealed specimen to 16% for the specimen tempered at 400 ?.The UTS×TEL decreased by about 16% while the HEI was only 4% when the tempering temperature was increased up to 500 ?.The HEI increased to 45% when the tempering temperature was increased up to 600 ?.The enhanced resistance to HE of the tempered steel is primarily attributed to the microstructural evolution of RA and carbides during the tempering process.It is thus proposed that the susceptibility to HE of medium-Mn steel could be significantly reduced through suitable tempering treatment after IA on the premise of no notable loss of its UTS×TEL.The HE behavior of the CR medium-Mn steel intercritically annealed at 650 °C for different times as well as tempering treatment was investigated.The experimental results revealed that the CR+IA specimen exhibits a duplex microstructure consisting of globular shaped RA and ferrite.An excellent combination of UTS×TEL could be obtained when the specimen was annealed at 650 °C for 10 min.The susceptibility to HE increases with increasing the IA time,from 12% for 5 min to 62% for 60 min.SEM analysis of the fracture surfaces of fracture specimens revealed that the hydrogen-charged annealed specimens were fractured to leave both dimples filled with grains and empty dimples.The dimples filled with grains were basically a brittle intergranular cracking occurring along the boundaries of RA and fresh martensite(formerly RA)grains by the hydrogen-assisted cracking mechanism.It is thus concluded that the HE behavior of intercritically annealed CR medium-Mn steel is primarily controlled by both the amount and mechanical stability of RA.Besides,the effect of low temperature tempering treatment on both the HE behavior and UTS×TEL of intercritically annealed CR medium-Mn steel is relatively small,while tempering at a higher temperature of 500 ? could significantly alleviate the HE susceptibility for the specimen annealed at 650 ? for 10 min,however,this beneficial effect is not so significant for the specimen annealed at 650 ? for 360 min.The effect of warm-rolling and subsequent intercritical annealing on the HE susceptibility of the tested medium-Mn steel was investigated.It was found that a duplex microstructure consisting of both equiaxed and lamellar morphologies of RA and ferrite was obtained after IA of the WR steel sheet,which thus exhibits more excellent ductility and UTS×TEL compared with that of the CR+IA steel sheet.With an increase in the IA time of the WR steel sheet,the RA stability decreases and thus the HE susceptibility increases.Unlike that of the HR+IA specimen,the WR steel sheet underwent recovery and recrystallization during subsequent IA and thus the prior austenite grain boundaries disappeared,so the HE susceptibility of the warm-rolled steel sheet was significantly alleviated.In addition,the delamination fracture of the hydrogen-charged WR+IA specimen is also helpful to improve the HE resistance because it contains a part of fine lathy austenite with high stability.The investigation of the HE susceptibility of warm-and hot-stamped medium-Mn steel under different heating temperatures revealed that the HEI increased at first,and then it tended to decrease with increasing heating temperature.This increase-decrease pattern of HE susceptibility is ascribed primarily to the microstructural evolution and strength variation with heating temperature.An optimum value of UTS×TEL was obtained when the tested medium-Mn steel was heated at 850 °C,however,its HEI is as high as 74%.The HE fracture surfaces of the warm-and hot-stamped medium-Mn steel were intergranular fracture.The HE susceptibility of the warm-stamped 0.1C-5Mn steel heated at 850 ? is higher than that of hot-stamped 22MnB5 steel heated at 950 ?,which is ascribed mainly to the presence of plenty of carbides because of self-tempering of the latter which could act as effective hydrogen trap. |
PDF Full Text Request