The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

Abstract

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 lm 3 . The presence of aluminium and phosphorus further stabilizes the austenite grains.

Formation of Microphases and Constituents from Remaining Austenite Decomposition in API X80 Steel Under Different Processing Conditions

“…The cooling rate affects the product from the remaining austenite transformation and its volumetric fraction [7][8][9][10][11] . However, the stability of austenite phase stems from significant differences in grain volume and carbon content from grain to grain 12 . This inhomogeneity of the carbon content in remaining austenite will form a complex microstructure after the final cooling stage 13 .…”

Microstructural Control of the Austenite Stability in Low-Alloyed TRIP Steels

“…Average grain volume ( V γ < > ) and carbon content ( c x γ < > ) of austenite before and after cooling down to 100 K for both compositions, taken from ref. [5]. The width of the distribution (standard deviation) is indicated in parenthesis.…”

“…Moreover, a fraction of the initial metastable austenite grains still remain untransformed at the lowest temperature of 100 K in all samples. The average austenite carbon content ( c x γ < > ) and grain volume ( V γ < > ) have been compared before and after cooling down to 100 K using high-energy X-ray diffraction [5], see Table 1. These data indicate that only small grains with a high carbon content remain untransformed at 100 K. Finally, in Fig.…”

“…This transformation seems to constitute the key process to obtain a high work hardening rate and a large uniform elongation in these materials [2,4]. We have recently derived a new equation that links the martensitic transformation temperature not only to the carbon content of the austenite grains but also to the grain size [5]. These two microstructural parameters are controlled by the heat treatment and the chemical composition of the material.…”

“…To achieve this goal, we have varied the heat treatment parameters systematically in two aluminum-based TRIP steel grades. We have subsequently studied the thermal stability of the retained austenite by using two in-situ bulk techniques: magnetization measurements using a SQUID magnetometer [6] and high-energy X-ray diffraction at a synchrotron source [5,7].…”