Interpretation of a dilatometric anomaly previous to the ferrite-to-austenite transformation in a low carbon steel

Abstract

During continuous heating of a cold rolled low carbon (LC) steel a dilatometric anomaly has been detected previous to the well known ferrite-to-austenite transformation. This anomaly is due to the texture change and/or the dislocation annihilation that accompany the recrystallization process.

Austenite Formation in Plain Low-Carbon Steels

“…Gridnev et al [33] attributed this observation to annihilation of point and line defects as well as microcracks. De Cock et al [10,32] also showed that the reduction in dislocation density during recrystallization can account for the amount of dilation (%0.05 pct) that is observed during heating of 80 pct CR low and ultralow carbon steels before austenitization at 10 K/s. Thus, it is concluded that the deviation from linear thermal expansion is related to recrystallization of the sample during heating and is consistent with the volume change caused by a reduction in the average dislocation density.…”

“…Microstructural observations at 873 K (600°C) and 1123 K (850°C) showed that the microstructure changed from the CR to a completely recrystallized one during the first heating stage. Thus, a reduction in dislocation density and texture changes from an alpha fiber to a gamma dominant texture [10,32] that occur during ferrite recrystallization remain as possible explanation for the deviation. A reduction in specific volume of 0.37 pct for 80 pct cold-drawn steel wires during annealing and before austenite formation was reported by Gridnev et al [33] With an assumption of isotropic change in length during dilation, one can estimate DL/ L o through DL/L o % DV/3V o for a small relative volume change in which DL is the length change, L o is the initial length, DV is the volume change, and V o is the initial volume of the sample.…”

“…They observed that recrystallization of deformed ferrite and spheroidization of pearlite lamellae can take place prior to austenite formation in the CR sample. Using dilatometry data, de Cock et al [10] showed that ferrite recrystallization prior to phase transformation results in dimension changes prior to austenite formation in low and ultra-low carbon steels. The sequence of these microstructural changes is dependent strongly on the employed heating rate in both HR and CR structures.…”

In situ study of austenite formation by dilatometry in a low carbon microalloyed steel

“…Some of the most usual experimental techniques for the in-situ study of phase transformations in steels are differential scanning calorimetry [7] and magnetization measurements [8]; however, dilatometry is probably the most widely used technique [9][10][11][12][13][14][15][16]. There exist semi-empirical models to quantify the progress of ferrite and/or pearlite on cooling [13,14,17].…”

Global recrystallisation model of low carbon sheet steels with different cementite contents

“…The pole figures (110), (200) and (211) were measured and a series expansion technique was employed to calculate the orientation distribution function (ODF), along with ghost correction [15]. The typical cold rolling and recrystallisation textures of LC steels may be described by two texture fibres: an incomplete α-fibre characterised by <110> parallel to RD; and a complete γ-fibre which comprises all crystals with {111} plane parallel to ND [16].…”