Microstructural evolution and precipitation behavior of the warm-rolled medium Mn steels containing Nb or Nb-Mo during intercritical annealing

Pan, Haijun; Ding, Hua; Cai, Minghui

doi:10.1016/j.msea.2018.08.055

Cited by 49 publications

(19 citation statements)

References 35 publications

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“…The effects of C, Mn, Si and Al content on the microstructure and properties of medium Mn steels have been reviewed in more detail in [304][305][306]. In order to enhance the strength of medium Mn steels, nano-sized carbide or coherent B2 ordered NiAl nanoparticles can be introduced via micro-alloying elements addition, such as Mo, Ti, Nb and V [296,305,[307][308][309][310][311][312], or Ni and Al addition [313][314][315][316][317], respectively. In addition, the addition of micro-alloying elements is also beneficial to the refinement of prior austenite grain size [318].…”

Section: Desired Microstructures and Required Chemical Compositionmentioning

confidence: 99%

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Dai

Chen

Ding

et al. 2021

Materials Science and Engineering: R: Reports

145

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Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating.This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

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Section: Desired Microstructures and Required Chemical Compositionmentioning

confidence: 99%

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Dai

Chen

Ding

et al. 2021

Materials Science and Engineering: R: Reports

145

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“…In order to prevent carbide precipitation, the concept with a higher aluminum content (~1.5%) and Mo addition was applied [29]. Mo was added for solid solution strengthening and to enhance producing bainite-like microstructures [31][32][33].…”

Section: Chemical Compositionmentioning

confidence: 99%

“…Cai et al [30] reported that the addition of 0.22 wt.% Mo and 0.05 wt.% Nb to a 6.5 wt.% Mn steel improved the strength-ductility combination due to the increased volume fraction and mechanical stability of retained austenite. Mo together with Ti and Nb microadditions affected a grain size due to the tendency to form complex (Ti, Nb, Mo)C precipitates, which cause the grain refinement [31,32]. Mo is also the only element which limits the cementite precipitation even at 450 • C [20].…”

Section: Effect Of Thermomechanical Processingmentioning

confidence: 99%

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Grajcar

Kilarski²,

Kozłowska

2018

Metals

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Detailed studies on microstructure-property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel-17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase. case, the phase transformation kinetics depends significantly on the austenite deformation degree [5,8,9]. In case of the first generation AHSSs, the main advantage of the TMP is the ability to refine the ferrite grain size by controlling the austenite pancaking [10,11]. Moreover, the thermomechanical treatment can increase the amount of retained austenite to obtain the optimal TRIP (TRansformation Induced Plasticity) effect [12,13]. In case of medium manganese steels (third generation AHSS), the TMP is also a good alternative for the cold-rolling process [14][15][16]. However, most publications on medium-Mn steels concern the cold-rolling process and subsequent inter-critical heat treatment.Cold-rolled medium-Mn TRIP steels are susceptible to plastic instabilities associated with dynamic strain aging (DSA) and serrated flow (PLC-Portevin-Le Chatelier) effects due to the heat treatment required after cold-rolling. From a technological point of view, the PLC and DSA effects must be avoided. The DSA phenomenon gives rise to non-homogeneous plastic flow during the sheet-forming processes and may lead to surface defects on formed parts [17][18][19]. Our previous reports on thermomechanically processed medium-Mn sheet steels indicate that the problem can be avoided [7].Newly developed fine-grained ferrite-austenite or bainitie-austenite steels contain manganese in a range of 3-12%, while carbon content is ca. 0.1-0.2%. These steels contain also aluminum and silicon (1-3%) additions which delay the carbides formation during the bainitic transformation. The increased Mn amount leads to obtain the high fraction of retained austenite (~10-30%). The Mn addition also increases the carbon solubility and lowers the cementite precipitation temperature [20][21][22]. Al is added to partially replace silicon due to the problems related to galvanizing, hot-rolling and welding [23][24][25]. However, Gir...

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“…As to the coarsening rate of precipitates, it can be described by the following equation [ 32 ]:

where

is the coarsening rate of the precipitates,

is the interface energy between ferrite and precipitates,

is the molar volume of the precipitates,

is the diffusion coefficient of the controlling elements,

is the solute atomic concentration of the controlling elements,

is the molar volume of the solute element, and

is the solute atomic equilibrium concentration of the controlling elements. According to Equation (2), the

value decreased as the

value decreased.…”

Section: Resultsmentioning

confidence: 99%

“…It can be inferred that the decrease of interface energy caused by the partial replacement of Nb by Mo can refine the size of precipitates. Additionally, Pan et al had pointed out that the size of (Nb, Mo)(C, N) was smaller than that of Nb(C, N) [ 32 ]. Therefore, due to the addition of Mo, the average size of precipitates was reduced significantly, and the number density of precipitates was increased substantially.…”

Section: Resultsmentioning

confidence: 99%

Effect of Molybdenum on the Impact Toughness of Heat-Affected Zone in High-Strength Low-Alloy Steel

Ping

et al. 2021

Materials

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The microstructure, precipitates, and austenite grain in high-strength low-alloy steel were characterized by optical microscope, transmission electron microscope, and laser scanning confocal microscopy to investigate the effect of Mo on the toughness of steel. The microstructure was refined and the toughness was enhanced after the addition of 0.07% Mo in steel. The addition of Mo can suppress the Widmanstätten ferrite (WF) formation and promote the transformation of acicular ferrite (AF), leading to the fine transformed products in the heat-affected zone (HAZ). The chemical composition of precipitates changed from Nb(C, N) to (Nb, Mo)(C, N) because of the addition of Mo. The calculated lattice misfit between Nb(C, N) and ferrite was approximately 11.39%, while it was reduced to 5.40% for (Nb, Mo)(C, N), which significantly affected the size and number density of precipitates. A detailed analysis of the precipitates focusing on the chemical composition, size, and number density has been undertaken to understand the contribution of Mo on the improvement of steel toughness.

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Microstructural evolution and precipitation behavior of the warm-rolled medium Mn steels containing Nb or Nb-Mo during intercritical annealing

Cited by 49 publications

References 35 publications

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Effect of Molybdenum on the Impact Toughness of Heat-Affected Zone in High-Strength Low-Alloy Steel

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