Thermal stability of retained austenite in TRIP steels studied by synchrotron X-ray diffraction during cooling

A study of microstructure, transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel

Relation between microstructure and mechanical properties of a low-alloyed TRIP steel

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M
 s temperature induced by carbon enrichment.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

Recent advances in the development of high performance steels presenting improved properties of strength and ductility rely on the TRIP effect, i.e. on the mechanically-induced martensitic transformation of the retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilisation and retention of austenite at room temperature have become of primary importance, leading to specifically designed steel grades and thermal or thermomechanical treatments. Particularly, carbon enrichment of the austenite during intercritical annealing and bainite transformation was found to be very effective in retaining austenite. This metastable austenite then progressively transforms during straining, bringing about a large increase of the work hardening rate. This increase results from the stress and strain partitioning continuously evolving with the appearance of the hard martensite. (c) 2004 Elsevier Ltd. All rights reserved.

Transformation-induced plasticity for high strength formable steels

Dilatometry is a useful technique to obtain experimental data concerning transformation kinetics in ferrous alloys. This technique is commonly used in cooling experiments to study the austenite decomposition of hypo-eutectoid steel grades. In the standard analysis of the dilatation signal there are two factors that are normally neglected. During the pro-eutectoid ferrite formation the austenite enriches in carbon, resulting in a non-linear temperature dependence of the specific austenitic volume. Furthermore, the specific volume of the formed ferrite is considerably different from that of the formed pearlite. In total not taking into account these two effects can lead to an error in the determined fraction ferrite of up to 25%. A method is presented that takes into account the two above-mentioned factors. In order to determine both the fraction ferrite and the fraction pearlite, in the analysis the temperature range of the transformation is divided into a ferrite-formation range and a pearlite-formation range. Two possible criteria for this division are discussed, and it is shown that the choice does not have an essential influence on the results.

Dilatometric analysis of phase transformations in hypo-eutectoid steels

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. (C) 1999 Elsevier Science S.A. All rights reserved.

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

The massive transformation from austenite to ferrite (γ→α) in binary substitutional Fe–X alloys, where X represents successively about 1 or 2 at.% of Co, Cu, Mn, Cr or Al, is experimentally investigated by means of dilatometry. The resulting transformation curves have been modelled by an interface-controlled growth model, taking the characteristics of the austenitic microstructure into account. The assumption of an Arrhenian temperature dependence of the interface mobility, defined as the ratio between the interface velocity and the driving force, is found to give a consistent picture of the observations with an activation energy for atoms crossing the interface of 140 kJ mol−1. The spurious presence of interstitial nitrogen is shown to have a significant effect on the mobility, as large as a factor 8 at concentration levels on the order of 10−4.

A study on the austenite-to-ferrite phase transformation in binary substitutional iron alloys

Dilatation measurements were carried out on cylindrical specimens taken from the longitudinal and the normal directions of hot rolled C-Mn steel strip, giving rather different results for the heat treatments that led to a banded ferrite-pearlite microstructure. Under those conditions, the standard methods of deriving the fractions transformed from the uniaxial dilatation signal give rise to systematic errors. A model describing the anisotropic dilatation behaviour for such an evolving banded structure is presented. The model reproduces the experimentally observed differences semiquantitatively as a function of the band orientation in the specimen. Calculations indicate that the most adequate description of the anisotropic dilatation behaviour during formation of an evolving banded structure should be based on a time dependent model, which accounts for the phase transformation history. The consequences of the anisotropic behaviour for determination of the fractions transformed are discussed.

Anisotropic dilatation behaviour during transformation of hot rolled steels showing banded structure

The kinetics of the phase transformation between the high temperature FCC-phase austenite and the low temperature BCC-phase ferrite as it occurs during controlled cooling of hot rolled low carbon steels is described using a physical model that considers austenite grain size, nucleus density, composition effects, and the austenite/ferrite interface mobility. The model is verified against experimental dilatometry data for three lean carbon-manganese steel grades. The model yields adequate reproductions of the transformation kinetics. Ferrite grain coarsening during the transformation appears to have a significant effect on the final microstructure.

T.A. Kop

Papers

Dilatometric analysis of phase transformations in hypo-eutectoid steels

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

A study on the austenite-to-ferrite phase transformation in binary substitutional iron alloys

Anisotropic dilatation behaviour during transformation of hot rolled steels showing banded structure

Modelling the Austenite to Ferrite Phase Transformation in Low Carbon Steels in Terms of the Interface Mobility