Effect of Bimodal Grain Size Distribution on Scatter in Toughness

Abstract

Blunt-notch tests were performed at À160°C to investigate the effect of a bimodal ferrite grain size distribution in steel on cleavage fracture toughness, by comparing local fracture stress values for heat-treated microstructures with uniformly fine, uniformly coarse, and bimodal grain structures. An analysis of fracture stress values indicates that bimodality can have a significant effect on toughness by generating high scatter in the fracture test results. Local cleavage fracture values were related to grain size distributions and it was shown that the largest grains in the microstructure, with an area percent greater than approximately 4 pct, gave rise to cleavage initiation. In the case of the bimodal grain size distribution, the large grains from both the ''fine grain'' and ''coarse grain'' population initiate cleavage; this spread in grain size values resulted in higher scatter in the fracture stress than in the unimodal distributions. The notchbend test results have been used to explain the difference in scatter in the Charpy energies for the unimodal and bimodal ferrite grain size distributions of thermomechanically controlled rolled (TMCR) steel, in which the bimodal distribution showed higher scatter in the Charpy impact transition (IT) region.

Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

“…[11,12] It is also possible to have inappropriate austenite refinement before strain begins accumulating, in some cases, as a consequence of a low soaking temperature [13] or in others due to microalloying segregation. [14] Also, in thin slab direct rolling technologies, the refinement and conditioning of austenite prior to transformation may be limited. The initial as-cast coarse grain sizes are present at the entry of the first stand.…”

“…In a recent work, it was observed that a higher fraction of coarse grain sizes in the microstructure increases the chance of finding those grains at the cleavage origin, leading to a wider scatter in fracture stress in comparison to more homogeneous grain size distributions. [14] Various terms were proposed to name the transformation microstructures. Those terms mostly used are by Bramfitt and Speer [16] and the ISIJ Bainite Committee.…”

Development of Bimodal Grain Structures in Nb-Containing High-Strength Low-Alloy Steels during Slab Reheating

“…[17,18] It has been shown that interdendritic segregation during solidification results in an inhomogeneous distribution of Nb-precipitates in continuously-cast microalloyed steel slabs (0.1 wt pct C, 0.020 to 0.045 wt pct Nb), which can cause a bimodal grain structure to develop during reheating at certain temperatures. [19][20][21] This article extends the previous analysis by considering the full precipitate distributions in the segregated regions and their thermodynamic stability, in order to predict abnormal grain growth leading to bimodal grain structures.…”

Effect of Heat Input on Microstructure and Toughness of Coarse Grained Heat Affected Zone of Q890 Steel

“…However, some researchers suggest that effective grain size (the minimum size of microstructural unit) over which the cleavage crack propagates in an uninterrupted path can improve toughness. 16,17) But both the evolution of the density of high angle boundary and effective grain size with different thermal cycles has not been shown transparently and discussed in detail. Furthermore, the combined evolution of all factors controlling (M-A constituents and high angle boundary) and their influence on CGHAZ toughness need to be conducted to the heat input variation.…”