Crucial microstructural feature to determine the impact toughness of intercritically annealed medium-Mn steel with triplex-phase microstructure

Kim, Min Tae; Park, Tak Min; Baik, Kyeong Ho; Choi, Won Seok; Choi, Pilsoon; Han, Jeongho

doi:10.1016/j.actamat.2018.10.043

Cited by 51 publications

(28 citation statements)

References 32 publications

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“…A notable difference between the impact testing results and the tensile testing results is that alloy 51, despite its lower prior γ grain size of 50 µm, underperforms compared to the impact testing results of alloy 52. The presence of δ-ferrite grains (Figure 8a) is well reported in the literature to have an adverse effect on the impact properties of steel alloys [44,45], giving an argument for the lower impact properties of 51 compared to 52. Conventionally, adverse effects on the ductility of nB alloys are caused by a large amount of unstable blocky austenite grains, which prematurely transform to martensite upon the application of low loads.…”

Section: Mechanical Behaviorsupporting

confidence: 55%

Nano-Bainitic Steels: Acceleration of Transformation by High Aluminum Addition and Its Effect on Their Mechanical Properties

Akram

Soliman

Palkowski

2021

Metals

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Additions of 3 and 5 wt.% Al have been investigated as a low-cost method for transformation acceleration in nano-bainitic steels. For both Al contents, two groups of steels with C-content in the range ~0.7 to ~0.95 wt.% were studied. Thermodynamic and physical simulations were used in alloy and heat treatment design. Characterization was performed via dilatometry, scanning and transmission electron microscopy, Synchrotron X-ray diffraction, and tensile and impact testing. Fast bainitic-transformation time-intervals ranging from 750–4600 s were recorded and tensile strengths up to 2000 MPa at a ductility of ~10 elongation percent were attainable for the 3 wt.% Al group at an austempering temperature of 265 °C. Higher Al additions were found to perform better than their lower Al counterparts as the austempering temperature is dropped. However, Al lowered the austenite stability, increased the martensite start temperature, austenitization temperatures and, consequently, the prior austenite grain size, as well as limiting the austempering temperatures to higher ones. Additionally, the lowered austenite stability coupled with higher additions of hardenability elements (here carbon) to maintain the martensite start at around 300 °C, causing the 5 wt.% Al group to have a large amount of low stability retained austenite (and consequently brittle martensite) in their microstructure, leading to a low elongation of around 5%.

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Section: Mechanical Behaviorsupporting

confidence: 55%

Nano-Bainitic Steels: Acceleration of Transformation by High Aluminum Addition and Its Effect on Their Mechanical Properties

Akram

Soliman

Palkowski

2021

Metals

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“…The macroscopic fractography of transverse impact specimen of IA‐30 was shown in Figure . The splitting or secondary cracking phenomenon could be observed, which was determined by the micro‐laminated microstructure . The deep secondary cracking showed complete cleavage fracture.…”

Section: Resultsmentioning

confidence: 99%

“…Hence, the behavior of toughness is related to the change in the content of retained austenite, and a maximum value of 53 J was obtained for annealing time of 30 min. The retained austenite is considered to weaken the stress concentration at the crack tip through plastic deformation, and toughness can be increased …”

Section: Resultsmentioning

confidence: 99%

Structure‐Property Relationship in a Micro‐laminated Low‐Density Steel for Offshore Structure

Zhang

et al. 2019

steel research int.

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A low‐density steel (7.5 g cm−3) steel produced by direct‐quenching and intercritical annealing (IA) process is designed for offshore structure. The microstructure, mechanical properties, and corrosion resistance of steels are studied. It is found that after IA process, the microstructure is composed of micro‐laminated delta‐ferrite, tempered martensite, and reverted austenite. Two stage partitioning processes, the reheating process prior to rolling, and IA process lead to enrichment of C, Mn, and Ni elements in the reverted austenite. The enrichment of alloying elements stabilized austenite, and the retained austenite content is up to 18%. The growth of reverted austenite at lath‐lath interface is simulated by DICTRA software, and the results show that the growth of reverted austenite are controlled by diffusion of alloying elements. The ductility and toughness has a strong relationship with content of retained austenite. The mechanical properties are excellent with high yield strength (>620 MPa) and high total elongation (>40%). The experimental steel has excellent resistance to marine atmospheric corrosion, because of enrichment of Al and Ni in the rust layer, that leads to a compact layer. In addition, the partitioning of Al and Ni into different phase, promotes the corrosion resistance of rust layer.

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“…[19] Mn segregation at the d-ferrite interface has also been shown to reduce impact properties in medium Mn steels. [20] It is, therefore, important that alloy development efforts consider the size, morphology, and distribution of d-ferrite in medium Mn steels and how these factors may change during a transition from lab-scale to full-scale production material.…”

Section: Introductionmentioning

confidence: 99%

A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

Kwok

Slater

et al. 2021

Metall Mater Trans A

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Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 $$^{\circ} $$ ∘ C for various times to study the degree of chemical homogeneity and $$\delta $$ δ -ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some $$\delta $$ δ -ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed $$\delta $$ δ -ferrite in the 5 kg ingot turned into coarse $$\delta $$ δ -ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed $$>900$$ > 900 MPa yield strength, $$>1100$$ > 1100 MPa tensile strength, and $$>40$$ > 40 pct elongation with $$<10$$ < 10 pct difference in strength and no change in ductility when compared to a fully homogenized sample.

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Crucial microstructural feature to determine the impact toughness of intercritically annealed medium-Mn steel with triplex-phase microstructure

Cited by 51 publications

References 32 publications

Nano-Bainitic Steels: Acceleration of Transformation by High Aluminum Addition and Its Effect on Their Mechanical Properties

Nano-Bainitic Steels: Acceleration of Transformation by High Aluminum Addition and Its Effect on Their Mechanical Properties

Structure‐Property Relationship in a Micro‐laminated Low‐Density Steel for Offshore Structure

A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

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