Toughness of Ultrafine Grained Ferritic Steels Fabricated by ARB and Annealing Process

Abstract

A plain IF steel and a P-added IF steel having various ultrafine grain sizes from 0.24 to 11 mm were fabricated by the accumulative roll bonding (ARB) process followed by annealing. Dynamic fracture toughness of the ultrafine grained IF steels was investigated as a function of grain size by miniaturized Charpy impact test. The static strength of the IF steels significantly increased with decreasing the grain size, while the uniform elongation was limited in the ultrafine grained samples. A number of delamination appeared in the impact-tested specimens, especially in the ultrafine grained materials at low temperatures. It was concluded that the frequent delamination is not owing to insufficient roll-bonding in the ARB specimens but it is rather a characteristic feature of the ultrafine grained materials fabricated through heavy deformation. Because of the delamination, the absorbed energy in the impact test continuously decreased with decreasing the test temperature. On the other hand, an obvious change from the ductile fracture surface characterized by dimples into the brittle fracture surface mainly due to intergranular fracture was recognized at a certain low temperature. The ductile-brittle transition temperature determined from the microscopic fracture surfaces greatly decreased with decreasing the grain size, and finally no brittle fracture happened even at À190 C when the grain size was smaller than 5 mm or 2 mm in the plain IF steel or the P-added IF steel, respectively. It was concluded that the ultra-grain refinement is quite effective to improve the low-temperature toughness of ferritic steels and that it is possible to make phosphorus substantially harmless by grain refinement.

Brittle-Ductile Transition in Low Carbon Steel Deformed by the Accumulative Roll Bonding Process

“…This implies that grain refining should rather decrease the fracture toughness at low temper- atures, i.e., increase the BDT temperature, which contradicts the fact that grain refining improves the fracture toughness at low temperatures, as observed in the Charpy impact tests 11) and the present study. Suppose (i À 1) dislocations had been emitted from the crack tip.…”

“…One of the methods is grain refining. [7][8][9][10] Tsuji et al 11) deformed interstitial free (IF) steel by an accumulative roll-bonding (ARB) process and measured the absorbed energy at various temperatures by conducting Charpy impact tests. They demonstrated that the IF steel gained both strength as well as ductility at low temperatures.…”

A Review on Nano-Scale Precipitation in Steels

“…This suggests different heat treatment schedules can be explored to mitigate temper embrittlement. Other than this, Tsuji et al [81] also reported that thin lamellae with ultrafine grain size of (<2 µm) could improve impact toughness even in the presence of harmful phosphorus through the initiation of delamination during fracture. Grain boundary engineering, for example, by changing the structure of grain boundaries, is also another alternative to solve temper embrittlement in steels [82].…”

Effect of Extrinsic Grain Boundary Dislocations on Mechanical Properties of Ultrafine-Grained Metals by Molecular Dynamics Simulations

“…Ultrafine-grained (UFG) metals produced by SPD processes show high strength consistent with the Hall-Petch relationship, and they also exhibit unusual mechanical properties compared to coarse-grained metals, for instance, a sudden decrease in uniform elongation, 5,6) strain-rate sensitivity of flow stress, 7,8) and improved low-temperature toughness. 9) However, the detailed mechanisms of these unusual mechanical properties have not been fully understood yet.…”