Etienne Girault

Metallographic methods for revealing the multiphase microstructure of TRIP-assisted steels

Classical etching techniques used for the investigation of steel microstructures allow the simultaneous observation of only a restricted number of phases. So far, this limitation has not been too detrimental, because most low-carbon steel grades possess a quite simple microstructure. The recent interest in the so-called TRIP-assisted multiphase steels characterized by complex microstructures requires new developments in metallographic methods. This paper proposes an extension of already known techniques to allow the study of four kinds of TRIP-aided steels. The actual restrictions justifying the development of an improved method are emphasized. In spite of its simplicity, the procedure has the advantage of allowing the simultaneous observation of the four phases that generally compose the microstructure of TRIP-assisted steels; that is, ferrite, bainite, austenite, and martensite. Light and electron microscopy as well as diffraction techniques are used to demonstrate the interest of the method.

Comparison of the effects of silicon and aluminium on the tensile behaviour of multiphase TRIP-assisted steels

Ghent University,Laboratory for Iron and Steelmaking, Technologiepark 9, B-9052 Ghent, Belgium(Received July 12, 2000)(Accepted in revised form November 21, 2000)Keywords: TRIP-steels; Microstructure; Phase transformations; Mechanical propertiesIntroductionMultiphase TRIP-assisted steels are a new generation of low alloy high strength steels that exhibitexceptional formability [1]. The remarkable strength to ductility balance results from the occurrenceduring testing of the Transformation Induced Plasticity (TRIP) phenomenon [2], which involves thestrain-induced transformation of austenite to martensite. The presence of austenite in the initialmicrostructure appears to be critical to the achievement of the desired properties. The retention ofaustenite is usually obtained by the combined effect of an appropriate chemistry and a typicalheat-treatment. In this respect, it is known that silicon and aluminium may both retard the kinetics ofcarbide formation and thus favour the austenite stabilisation by a bainitic holding stage [3]. Despite thisqualitative knowledge, very little literature can be found that rigorously compares the effect of siliconand aluminium on the austenite retention, on the extent of the TRIP effect, and on the resulting tensilebehaviour, all other chemical constituents have been kept constant [4]. The objective of this paper is toquantitatively assess the influence of aluminium and silicon contents, in view of the development ofmultiphase TRIP-assisted steels.Materials and Experimental ProcedureThe chemical compositions of the steels studied in this work are given in Table 1. Specific care wastaken to keep the same carbon content for each alloy. The slabs were initially hot and cold-rolled tothicknesses between 0.8mm and 1.0mm, following classical processing routes.The desired multiphase microstructure was obtained as displayed in Figure 1. The cold-rolledmaterial was first annealed for 4 minutes in the (a1g) region at a temperature 25°C above its Ac1temperature. It was then rapidly cooled and held at an intermediate temperature (i.e. between 375°C and450°C), where bainite formation takes place and contributes to the stabilisation of the austenite. Theheat-treatment was eventually interrupted by quenching the samples to room temperature. After the

Bainite transformation of low carbon Mn–Si TRIP-assisted multiphase steels: influence of silicon content on cementite precipitation and austenite retention

Studies dealing with TRIP-assisted multiphase steels have emphasized the crucial role of the bainite transformation of silicon-rich intercritical austenite in the achievement of a good combination of strength and ductility. The present work deals with the bainite transformation in two steels differing in their silicon content. It is shown that both carbon enrichment of residual austenite and cementite precipitation influences the kinetics of the bainite transformation. A minimum silicon content is found to be necessary in order to prevent cementite precipitation from austenite during the formation of bainitic ferrite in such a way as to allow stabilisation of austenite by carbon enrichment

Austenite texture and bainite/austenite orientation relationships in TRIP steel

Abstract A bainitic medium carbon steel with 10–15 vol.% of retained austenite has been used to measure the texture of recrystallised and deformed austenite and of the corresponding bainite. The local orientations of bainite and retained austenite, and their orientation relationships, have been measured with an automated EBSD device mounted on a FEG-SEM.

Study of the temperature dependence of the bainitic transformation rate in a multiphase TRIP-assisted steel

A prerequisite to the development of multiphase TRIP-assisted steels is a good understanding of the bainitic transformation that takes place during the related thermo-mechanical processing. In this framework, the present paper proposes to investigate the formation of bainite when originating from intercritical austenite in a Si bearing steel. The experimental results suggest the contribution of a martensitic type mechanism to the transformation process. Yet, the overall bainitic reaction rates are found to strongly depend on the holding temperature. This original kinetics is correlated with the typical microstructure the steel exhibits after the intercritical annealing stage. To this extent, the crucial role of the adjacent development of bainitic ferrite for the observed temperature dependence is discussed