Bessemer Memorial Lecture: The dimensions of steel

Bhadeshia, H. K. D. H.

doi:10.1179/174328107x174753

Cited by 17 publications

(14 citation statements)

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“…Results from reference [57] showed a total elongation proportional to the initial volume fraction of austenite in nanostructured bainite. In this case, fracture presumably takes place when the austenite content decreases down to a critical value, measured at fracture, close to 10% [64]. This value is thought to be the percolation limit, above which austenite maintains a continuous path through the test sample.…”

Section: Ductility and Retained Austenite Contentmentioning

confidence: 97%

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Ductility of Nanostructured Bainite

Morales-Rivas

García‐Mateo

Sourmail³

et al. 2016

Metals

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Nanostructured bainite is a novel ultra-high-strength steel-concept under intensive current research, in which the optimization of its mechanical properties can only come from a clear understanding of the parameters that control its ductility. This work reviews first the nature of this composite-like material as a product of heat treatment conditions. Subsequently, the premises of ductility behavior are presented, taking as a reference related microstructures: conventional bainitic steels, and TRIP-aided steels. The ductility of nanostructured bainite is then discussed in terms of work-hardening and fracture mechanisms, leading to an analysis of the three-fold correlation between ductility, mechanically-induced martensitic transformation, and mechanical partitioning between the phases. Results suggest that a highly stable/hard retained austenite, with mechanical properties close to the matrix of bainitic ferrite, is advantageous in order to enhance ductility.Keywords: nanostructured bainite; ductility; microstructure; mechanical properties Nanostructured Bainite: Heat-Treatments and MicrostructureThere is an increasing interest in nanocrystalline steels because of their unique mechanical properties, achieving ultra-high strength while maintaining good values of ductility [1,2]. However, the manufacture of nanocrystalline steels is frequently very cost-consuming, involving the addition of expensive alloying elements, severe plastic deformation or complex thermomechanical routes in order to obtain the desired refinement of the microstructure during the material processing. A new generation of steels, nanostructured bainitic steels, is one promising solution because of the simplicity in terms of alloy design and processing. On the one hand, they are low-alloyed steels, with a typical chemical composition range being: (0.6-1)C-(≥1.5)Si-(0.7-2)Mn-(0.4-1.7)Cr-(0-0.2)Mo wt % [3]. On the other hand, the heat treatment consists of complete austenitization followed by isothermal holding at temperature T as low as T/T m < 0.25, where T m is the absolute melting temperature. The transformation itself leads to the nanocrystalline structure without further technological efforts, and has received much attention in recent years [4][5][6][7]. These novel microstructures have achieved the highest strength-toughness combinations ever recorded in bainitic steels (2.2 GPa-30 MPa·m 1/2 ) [8][9][10][11].The microstructure of nanostructured bainite consists basically of two phases: a hard matrix of bainitic ferrite and a carbon-enriched retained austenite, the second dispersed phase. It is characterized by the lack of coarse precipitates of cementite, due to the addition of Si with resulting content of near

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Section: Ductility and Retained Austenite Contentmentioning

confidence: 97%

“…This value is thought to be the percolation limit, above which austenite maintains a continuous path through the test sample. It suggested that the formation of hard martensite cannot be tolerated if the austenite volume fraction decreases below the percolation limit, 10%, leading to fracture [64,65].…”

Section: Ductility and Retained Austenite Contentmentioning

confidence: 99%

Ductility of Nanostructured Bainite

Morales-Rivas

García‐Mateo

Sourmail³

et al. 2016

Metals

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“…Retained austenite is a ductile phase compared to the bainitic ferrite and would be expected to enhance ductility. The resultant retained austenite volume fraction in the majority of the samples is about 10%, minimum amount of austenite required to reach an acceptable ductility [19]. Bhadeshia [19] demonstrated that the formation of strain-induced martensite can only be tolerated if the austenite maintains a uniform and percolated structure through the material.…”

Section: Role Of Microstructural Features On Mechanical Propertiesmentioning

confidence: 98%

“…3). This morphology of austenite is expected to influence favorably on the ductility [19,20]. The tensile test results of 0.2BAIN 1 and 0.2BAIN 3 steels are reported in Table 5 and represented in Fig.…”

Section: Tablementioning

confidence: 99%

Theoretical design and advanced microstructure in super high strength steels

et al. 2009

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“…Results are in accordance with Xray analysis that revealed a retained austenite content of 9 % out of the necked region in a fractured tensile sample of CENIM 6-CT 500°C. Bhadeshia 15) attributed this behaviour to the geometrical isolation of austenite. It seems that the formation of strain-induced martensite, vital to reach acceptable ductility properties, can only be tolerated if the austenite maintains a uniform and percolated structure through the material.…”

Section: Micrographs Inmentioning

confidence: 99%

Effects of Morphology and Stability of Retained Austenite on the Ductility of TRIP-aided Bainitic Steels

et al. 2008

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In order to improve the ductility of carbide free bainitic microstructures, consisting of a bainitic ferrite matrix and a mixture of austenite and martensite, the TRIP effect i.e. the strain induced transformation of retained austenite to martensite, should be controlled. In this sense, the effect of the chemical composition on the mechanical stability of the retained austenite and the morphology, size, and distribution of this phase has been studied to determine the role that plays on the ductility behaviour of advanced bainitic steels. Results suggest that apart of the retained austenite, the morphology of the bainitic matrix is an important factor controlling ductility. Bainitic microstructures formed by coiling with coarse and blocky bainite morphology have shown higher uniform deformation values than those obtained by air cooling with the typical thin bainite platelets.KEY WORDS: ductility; TRIP effect; bainite; steels. mechanical stability to martensitic transformation.Further improvement can be achieved by TRIP effect i.e. strain induced transformation of retained austenite to martensite. In order to take full advantage of this effect, the mechanical stability of austenite, i.e. its capability to transform to martensite under strain, must be controlled. In this sense, the role of retained austenite on the mechanical properties of ultra high strength bainitic steels has been analysed in this work. The effect of the retained austenite chemical composition, morphology, size, and distribution on its mechanical stability of this phase has been studied. Moreover, the influence of the amount and size of martensite, and the morphology of bainitic ferrite matrix on the ductility behaviour of advanced bainitic sheet steels has been also examined. Materials and Experimental ProcedureChemical composition of these novel bainitic steels was designed to have the same bainitic transformation region in the TTT diagram and the same T o curve 6) as those of Ni 2 bainitic steel designed in previous work 9) and taken here as reference ( Table 1). The reference steel proved to have impressive mechanical properties when fully transformed to bainite. 10) Designed alloys were manufactured by ARCELOR RESEARCH (France) as 180ϫ80ϫ12 mm 3 plates. Their actual composition is shown in Table 1. All laboratory heats were elaborated in a 60 kg vacuum induction furnace under inert atmosphere (Ar, N 2 ). The generator power was 80 kW. Pure (Ͼ99.9 %) electrolytic iron and addition of the alloying elements one after each other were used. Carbon deoxidation was performed and an analysis of C, S, N, O was made on line during elaboration for the final adjustment of composition. During elaboration, the temperature was controlled by a thermocouple.Hot rolling simulations were performed on ARCELOR pilot plant according to the general scheme presented on Fig. 1. The desired bainitic microstructure was obtained in all the steels performing air cooling after accelerating cooling, and in CENIM 6 and 7 performing coiling after accelerating cooling. Spec...

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Bessemer Memorial Lecture: The dimensions of steel

Cited by 17 publications

References 50 publications

Ductility of Nanostructured Bainite

Ductility of Nanostructured Bainite

Theoretical design and advanced microstructure in super high strength steels

Effects of Morphology and Stability of Retained Austenite on the Ductility of TRIP-aided Bainitic Steels

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