Effect of Hot Deformation on Bainite Structure in Low Carbon Steels.

Fujiwara, Kazuki; Okaguchi, Shuji; Ohtani, Hiroo

doi:10.2355/isijinternational.35.1006

Cited by 34 publications

(10 citation statements)

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“…It is reasonable to assume that densely stacked dislocation arrays introduced intragranularly by austenite deformation can act as nucleation sites for AF. Actually, the nucleation of AF has been found at various deformation substructures, like deformation bands [9,13,56,57] and dislocation cell walls [58]. Therefore, despite a wide range of cooling rates used (5-50°C s -1 ) in this research, for samples undeformed (strain2 = 0) or slightly deformed (strain2 = 0.1), laths nucleated primarily on PAGBs where densely stacked dislocation arrays can be accommodated, and these laths finally developed into BF packets with a parallel morphology as shown in Figs.…”

Section: Transition From Bf To Afmentioning

confidence: 99%

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

2017

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For the class of steels collectively known as high strength low alloy (HSLA), an acicular ferrite (AF) microstructure produces an excellent combination of strength and toughness. The conditions for the occurrence of the AF transformation are, however, still unclear, especially the effects of austenite deformation and continuous cooling. In this research, a commercial HSLA steel was used and subjected to deformation via plane strain compression with strains ranging from 0 to 0.5 and continuous cooling at rates between 5 and 50°C s -1 . Based on the results obtained from optical microscopy, scanning electron microscopy and electron backscattering diffraction mapping, the introduction of intragranular nucleation sites and the suppression of bainitic ferrite (BF) laths lengthening were identified as the two key requirements for the occurrence of AF transformation. Austenite deformation is critical to meet these two conditions as it introduces a high density of dislocations that act as intragranular nucleation sites and deformation substructures, which suppress the lengthening of BF laths through the mechanism of mechanical stabilisation of austenite. However, the suppression effect of austenite deformation is only observed under relatively slow cooling rates or high transformation temperatures, i.e., conditions where the driving force for advancing the transformation interface is not sufficient to overcome the austenite deformation substructures.

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Section: Transition From Bf To Afmentioning

confidence: 99%

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

2017

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“…Actually, the nucleation of AF has been observed on many deformation defects, such as deformation bands [18,[44][45][46] and dislocation cell walls. [47] C.…”

Section: B Transition From Bf To Afmentioning

confidence: 99%

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Zhao

Palmiere

2017

Metall and Mat Trans A

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Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 lm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures.

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“…Yamamoto et al [11] and Yang et al [10] suggested that AF nucleates on deformation bands, and Yakubtsov and Boyd [14] indicated that dislocations are the intragranular nucleation sites. The preceding articles [9][10][11]14] report observations for conditions where deformed austenite is a prerequisite for AF formation. By contrast, Lee et al [15] have studied the effects of austenite deformation on AF formation, under conditions where AF is the predominant transformation product in undeformed austenite.…”

Section: Introductionmentioning

confidence: 94%

“…Fujiwara et al [9] showed that AF forms in deformed austenite during isothermal transformation and that the transformation rate is higher for AF than for CB. It was also shown that the AF lath length decreases with increasing austenite deformation strain below T NR , and it was suggested that AF nucleates on dislocation cell (DC) walls.…”

Section: Introductionmentioning

confidence: 98%

“…The current systems for classifying microstructures resulting from bainite transformations in undeformed austenite are summarized in Reference 7. The main feature that emerges from studies of bainite formation in deformed austenite of low-C microalloyed steels [8][9][10][11][12][13][14][15] is the distinction between bainite that nucleates on austenite grain boundaries and that which nucleates in intragranular sites. In this article, we refer to the former as conventional bainite (CB) and the latter as acicular ferrite (AF).…”

Section: Introductionmentioning

confidence: 99%

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