No abstract

The hydrogen embrittlement behavior of an ultra-high strength (1180 MPa grade) dual phase steel sheet composed of ferrite and tempered martensite, as compared with that of a single phase steel sheet composed of tempered martensite, has been investigated by a sustained tensile-loading test. No fracture of the dual phase steel occurs under the low hydrogen-charging current density of 5 A/m 2 except under high applied stress substantially larger than the yield stress. With the high current density of 50 A/m 2 , the time to fracture of the dual phase steel varies widely, but is almost the same as that of the single phase steel. The critical applied stress for fracture of the dual phase steel is higher than that of the single phase steel. Under the high applied stress, however, the time to fracture of the dual phase steel is shorter than that of the single phase steel, and a unique intergranular-like morphology is observed at the crack initiation area on the fracture surface. Upon plastic deformation before the sustained tensile-loading test under the high applied stress, the time to fracture of the dual phase steel increases and the initiation area on the fracture surface exhibits typical quasi-cleavage features. The results of the present study indicate that the hydrogen embrittlement of the dual phase steel displays some anomalous behavior.KEY WORDS: hydrogen embrittlement; delayed fracture; high strength steel; dual phase steel; martensite; ferrite.

No abstract

The aim of this research is to investigate generation of micro voids affecting stretch flange formability in cold-rolled Dual-Phase (DP) steel sheets having a low volume fraction of martensite. The volume fraction of martensite was the dominant factor of the hole expanding ratio instead of the difference in hardness between ferrite and martensite when the difference in hardness between ferrite and martensite was large. On the other hand, the difference in hardness became the dominant factor of the hole expanding ratio in DP steel sheets having a low difference in hardness. Micro voids around the punched hole and the fracture edge of tensile deformation were observed in order to understand these different results. The void density is able to be associated with the hole expanding ratio under the same strain condition and the void density depends on the martensite spacing.

as the initiator; here it has been discussed kinetically from the point of view of cyclopolymerization. The rate of polymerization, Rp, was not proportional to the square root or the first power of the initiator concentration, (I), but Rp/(I)1/2 and (I)1/2 bore a linear relationship. The rate of polymerization, the residual unsaturation, and the degree of polymerization decreased with a decrease in the monomer concentration. The relation between the rate of polymerization over the degree of polymerization, Rp/Pn,o and the monomer concentration, (M), was also linear, as in Rp/(I)1/2 and (I)1/2. The ratio of the rate constant of the unimolecular cyclization reaction to that of the bimolecular propagation reaction of the uncyclized radical, Kc, was estimated to be 4.0 mol/l from the dependence of the residual unsaturation on the monomer concentration.