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

Abstract

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...

Carbide-Free Bainite: Compromise between Rate of Transformation and Properties

“…The first consists of steels whose properties are achieved by continuous cooling transformation. [14][15][16][17][18][19][20][21] The second is where the hardenability is controlled so that the transformation temperature is dramatically suppressed, giving slender plates of bainitic ferrite, under isothermal conditions. [22][23][24][25][26][27][28][29][30][31] The transformation time in the latter case, however, can be greater than 10 days at temperatures as low as 125°C, making the process suitable for large components; strength values in excess of 2000 MPa can routinely be achieved.…”

Microstructure evolution of advanced high-strength TRIP-aided bainitic steel

“…The knowledge concerning the relationships between the manufacturing conditions, the microstructure and the mechanical properties of coldrolled and annealed products is sufficiently documented in literature studies. 5,6,10,14 However, there are only few reports on the microstructure-property relationships in hot-rolled medium-C, carbide-free bainitic steels. 4,7 It is known that thermomechanical processing requires a detailed time-temperature regime during a sheet's cooling.…”

“…Thus, increased carbon contents are used or microalloying is employed to improve the strength levels with grain refinement and precipitation strengthening, in a similar way as in the HSLA steels. [9][10][11][12] Recently, medium-Mn bainitic-austenitic steels have also been developed. 8,13 Medium-C or medium-alloyed sheet steels are used for the products capable of mechanical binding and chemical adhesion 14 or they can be also used for forgings and wires.…”

“…However, the most important is the mechanical stability of retained austenite, dependent on its carbon content, size, dislocation density, morphology (blocky, interlath), etc. [3][4][5]10 An efficient monitoring of the evolution of retained austenite and strain-induced martensite as a function of strain is crucial for understanding microstructure-property relationships. This approach can be used with typical TRIP-aided ferrite-based steels.…”

Exploring Carbide-Free Bainitic Structures for Hot Dip Galvanizing Products

“…[3][4][5][6][7][8] Design methodologies based on diffusionless bainite transformation theory were applied to develop steels with a carbide-free bainitic (CFB) microstructure consisting of a mixture of bainitic ferrite, retained austenite, and some martensite. 9,10) Using thermodynamics and kinetics models, CFB steels with a 0.2 and 0.3 wt.% carbon content were designed and manufactured following a conventional hot rolling practice.…”