Conditions for the occurrence of acicular ferrite transformation in HSLA steels

Abstract

For the class of steels collectively known as high strength low alloy (HSLA), an acicular ferrite (AF) microstructure produces an excellent combination of strength and toughness. The conditions for the occurrence of the AF transformation are, however, still unclear, especially the effects of austenite deformation and continuous cooling. In this research, a commercial HSLA steel was used and subjected to deformation via plane strain compression with strains ranging from 0 to 0.5 and continuous cooling at rates between 5 and 50°C s -1 . Based on the results obtained from optical microscopy, scanning electron microscopy and electron backscattering diffraction mapping, the introduction of intragranular nucleation sites and the suppression of bainitic ferrite (BF) laths lengthening were identified as the two key requirements for the occurrence of AF transformation. Austenite deformation is critical to meet these two conditions as it introduces a high density of dislocations that act as intragranular nucleation sites and deformation substructures, which suppress the lengthening of BF laths through the mechanism of mechanical stabilisation of austenite. However, the suppression effect of austenite deformation is only observed under relatively slow cooling rates or high transformation temperatures, i.e., conditions where the driving force for advancing the transformation interface is not sufficient to overcome the austenite deformation substructures.

Formation of Non-Metallic Inclusion and Acicular Ferrite in Ti–Zr Deoxidized Steel

“…From the characteristics of inclusions, the number of inclusions in 2#Steel was less than that reported in other literatures, 10,11,13) but the proportion of fairly chaotic arrangement ferrite plates was little difference. Therefore, in the present study the AF nucleation may be not only induced by inclusions, but also induced by primary ferrite, dislocation, 28,29) and other intragranular defects.…”

Influence of cooling rate on the grain-refining effect of austenite deformation in a HSLA steel

“…The reason for the transition from AF dominant to BF dominant was explored in our previous study [8]. A brief explanation is provided here.…”

“…However, the nucleation sites for AF and BF are different, austenite deformation substructures for AF and austenite grain boundaries for BF. [8]. Consequently, AF microstructure can only be formed at relatively slow cooling rates, and the transformed microstructure gradually transitions from AF dominant to BF dominant with the rise of cooling rates.…”

“…There have been many investigations concerning the effect of processing parameters on the effective grain sizes of steel microstructures [6][7][8][9][10][11]. However, some of the results are still controversial, especially for the influence of austenite deformation below the austenite nil-recrystallisation temperature (Tnr).…”

In-situ observation on the influences of Ti on phase transformation of weld metal processed by high-heat input welding

“…Under this background, the vertical electro-gas welding (EGW), electroslag welding and other high-heat input welding methods have received great attention. [1][2][3][4][5] On the condition of high-heat input welding, the mechanical properties of the welding line and weld metal are readily deteriorated due to the high peak temperature of the weld metal, prolonged retention at high temperature and coarse microstructures. The acicular ferrite (AF) is one of the medium-temperature (500-700°C) transformation products.…”