The intermediate transformation of Mn-Mo-Nb steel during continuous cooling

Lee, Jye-Long; Hon, Min-Hsiling; Cheng, Gwo-Hwa

doi:10.1007/bf01086469

Cited by 24 publications

(9 citation statements)

References 13 publications

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“…However, some aspects about the AF transformation are still unclear, especially as related to its formation conditions, including the effects of austenite deformation and subsequent continuous cooling. For example, increasing austenite deformation [4,9,12,13] has been shown to increase the AF fraction, while 'AF-like' microstructures are produced when continuously cooled without austenite deformation [14][15][16][17]. These observations are contradicted from research [18][19][20] which indicates that even after significant amounts of austenite deformation, the transformation product still consists of parallel BF laths and the typical AF microstructure is absent.…”

Section: Introductionmentioning

confidence: 99%

“…For example, AF was found in the final microstructure with cooling rates below 10°C s -1 [14], but BF has been shown to be dominant at a slower cooling rate (5°C s -1 ) [19], and as the cooling rate rises to 35°C s -1 , the fraction of intragranularly nucleated AF laths is increased, due to the decreased transformation onset temperature and thus the increased thermodynamic driving force, which is supposed to promote the intragranular nucleation as expected by nucleation theory [21].…”

Section: Introductionmentioning

confidence: 99%

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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: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

2017

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“…Alternatively, micro/alloying elements were added as compensations to the strength decreasing. In such a case, many alloying elements (Cr, Mo, Ni, B, etc) are used to promote the transformation from austenite to bainite at the intermediate temperature, [1] which lies between the polygonal ferrite transformation temperature and martensite transformation temperature. Therefore, those with the intermediate microstructures are identified as low carbon bainitic steels.…”

Section: Introductionmentioning

confidence: 99%

Microstructure and Mechanical Properties of a Low Carbon Bainitic Steel

Lan¹,

Du²,

Liu³

2012

steel research int.

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A low carbon bainitic steel with microstructure of granular bainite (GB), acicular ferrite (AF), and bainitic ferrite (BF) is obtained under different deformation and cooling rate. The effect of deformation and cooling rate on microstructural characteristics such as the type of the matrix, the size, and area fraction of the martensite-austenite (M-A) constituents is investigated. In addition, the nanohardness of these three kinds of matrix as well as that of the M-A constituents in them is characterized. Further, the effect of matrix and M-A constituents on strength-toughness balance is studied. Results indicate that deformation expands the transformation region. The size as well as the area fraction of the M-A constituent decreases with the increasing of the cooling rate. After deformation, the area fraction of the M-A constituents increases. Nanohardness of GB, AF, and BF increases orderly, but that of the M-A constituents in them decreases accordingly. The nanohardness of the M-A constituent is significantly affected by its carbon concentration. AF is the optimum microstructure having superior strengthtoughness balance.

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“…This is because both AF and BF are displacive transformation products, following a near Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationship with parent austenite grains. [3,33] For the K-S or N-W orientation relationships, the disorientation angles of boundaries between different laths transformed in the same parent austenite grain do not fall into range 3 between 21 and 47 deg. [34] The boundaries with disorientation angles within range 3 shown in Figure 6 are the boundaries between laths transformed from different austenite grains at PAGBs, but not vice versa.…”

Section: Ebsd Mappingmentioning

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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The intermediate transformation of Mn-Mo-Nb steel during continuous cooling

Cited by 24 publications

References 13 publications

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

Conditions for the occurrence of acicular ferrite transformation in HSLA steels

Microstructure and Mechanical Properties of a Low Carbon Bainitic Steel

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

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