Effect of Heating Rate on the Austenite Formation in Low-Carbon High-Strength Steels Annealed in the Intercritical Region

Abstract

Austenite formation during intercritical annealing was studied in a cold-rolled dual-phase (DP) steel based on a low-carbon DP780 composition processed in the mill. Two heating rates, 10 and 50 K/s, and a range of annealing temperatures from 1053 K to 1133 K (780°C to 860°C) were applied to study their effects on the progress of austenitization. The effect of these process parameters on the final microstructures and mechanical properties was also investigated using a fixed cooling rate of 10 K/s after corresponding annealing treatments. It was found that the heating rate affects the austenite formation not only during continuous heating, but also during isothermal holding, and the effect is more pronounced at lower annealing temperatures. Faster heating delays the recrystallization kinetics of the investigated steel. The rate of austenite formation and its distribution are strongly influenced by the extent of overlapping of the processes of recrystallization and austenitization. It appeared that the heating rate and temperature of intercritical annealing have a stronger effect on the final tensile strength (TS) of the DP steel than holding time. Both higher annealing temperatures and long holding times minimize the strength difference caused by a difference in heating rate.

EFEITO DOS PARÂMETROS DE RECOZIMENTO CONTÍNUO NA MICROESTRUTURA E PROPRIEDADES MECÂNICAS DE UM AÇO REVESTIDO POR IMERSÃO A QUENTE DA CLASSE DE RESISTÊNCIA DE 800 Mpa

“…Nesse estudo se constatou que até aproximadamente a temperatura de 748°C (Ac  ) a austenita se desenvolveu dentro da própria perlita e/ou regiões ricas em carbono, sendo a cinética desse processo controlada pela difusividade do carbono. Acima desta temperatura, tendo-se dissolvidos todos os carbonetos, a nucleação da austenita se estendeu aos contornos de grãos da ferrita, o que pode ser comprovado pela quantidade de segundo constituinte disperso na matriz ferrítica [6,7]. Outro fato que corrobora com essa hipótese é a presença do constituinte MA (martensita-austenita) nas microestruturas simuladas com encharque a 780°C através do HDPS, conforme mostrado na figura 4.…”

Effect of higher heating rate during continuous annealing on microstructure and mechanical properties of cold-rolled 590 MPa dual-phase steel

“…In the unrecrystallized sample, carbide particles distributed along deformed ferrite grain boundary which provides an increased nucleation density compared with the fully recrystallized samples [11]. Subsequently, the austenite grows rapidly until they cover almost all the deformed ferrite grain boundaries and at the same time the ferrite recrystallization occurs.…”

Electric-Thermo-Mechanical Analysis of Joule Heating in Dilatometric Specimens

“…Many of these studies use commercial dilatometry equipment to determine transformation kinetics through dilatometric specimens of different shapes and sizes, for example, solid cylindrical shape, hollow, or flat in small sizes, ensuring that the changes in volume obtained are due solely to the phase transformation or the internal changes in the material and not to distortion caused by the thermal shock in the heating and quenching stages. Likewise, the dilatometry devices can use different heating media such as magnetic induction, thermal radiation, and direct current or Joule heating [3], [7] and [8]. This latter medium is characterized by providing thermal homogeneity in the material and good control of the heating rate (HR), which can remain constant for the whole heating stage.…”