Delamination Effect on Impact Properties of Ultrafine-Grained Low-Carbon Steel Processed by Warm Caliber Rolling

Abstract

Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500°C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong a-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 lm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100°C to À40°C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

Effect of initial notch orientation on fracture toughness in fail-safe steel

“…This fracture surface characterized by a quasi-cleavage shown in Fig. 3e was very similar to the delaminating structure observed in the TF(0°) sample [3,14,15]. Eventually, the peak-bending load exhibited the lowest value and the J decreased remarkably because a barrier lying normal to crack propagation is nonexistent.…”

“…Figure 4 shows the image-quality map obtained by the EBSD analysis on the cross section of the plane along the RD for the TF sample and the inverse pole figures for the RD and TD2. Here, the {100} h110i texture and {110} h110i texture that cause delamination by crack branching in the TF(0°) sample [6,10,15] are highlighted in EBSD map, and the LD for each sample is illustrated. The insets in Fig.…”

Microstructure and microtexture assessment of delamination phenomena in charpy impact tested specimens

“…Furthermore, crystallographic plane materials, with (100) orientation aligned in the rolling direction, show a strong tendency to occur cleavage fracture during impact test 3,4,[6][7][8][9]14 . To identify whether the separation of the (100) planes occurred in this case, the microtexture on the edge of the parallel section's (Figure 3b)) fractured surface was measured, where the delamination was generated.…”

“…In addition, this allows a better understanding of the mechanisms responsible for the nucleation and propagation of delamination cracks, which are a major problem in HSLA steels 4,5 . Furthermore many works have investigated the influence of the delamination phenomena on impact toughness of microalloyed HSLA steels [5][6][7][8][9] . Some of these works reported that the presence of delamination can decrease the uppershelf higher energy in an impact test.…”

“…Others have reported that the formation of delamination during the fracture process can increase it. Recently, Inoue et al 7 proposed a new model in which the absorbed energy in the impact test is enhanced by the presence itself. This mechanism is shown in Figure 1, where the formation of two types of delamination can be seen: the crack-arrester type; and the crack-divider type.…”