Grain Refinement by Rapid Transformation Annealing of Cold Rolled Low Carbon Steels

Abstract

A novel thermal treatment, rapid transformation annealing (RTA), has been applied to six different cold rolled low-carbon (LC) steel sheets with the aim of refining their microstructure. The process involves rapid heating to just above the austenite (g) to ferrite (a) transformation temperature and subsequent rapid cooling to room temperature. Grain sizes around 2 µm in two different Nb-Ti HSLA steels, 5 µm in a Ti-LC steel and 6 µm in a plain LC (0.037%C) steel have been produced using fast cooling rates (200°C/s). Non-equiaxed structures are obtained in a Nb-Ti HSIF steel and in a plain LC (0.135%C) (CM) steel due to their higher Mn content. However, very fine equiaxed grains (2 µm) are obtained by rapid intercritical annealing (RIA) in the CM steel. Irrespective of the microalloying concept, the grain growth of recrystallized a grains before their transformation was inhibited in CM and in both HSLA steels. This inhibition is connected with the overlapping of a recrystallization and a-g transformation processes which is essential in order to achieve extreme grain refinement either by RTA or RIA.

Identification of the parameters controlling the grain refinement of ultra-rapidly annealed low carbon Al-killed steels

“…2a). As was already observed by other authors [1,2,4], these temperatures increase when the heating rate is increased. Moreover, it has to be noted that the temperatures determined in the present work are in rather good agreement with those measured by [1,2] on a low carbon steel with 50 × 10 −3 %C.…”

“…According to [3,4], the degree of grain refinement of low carbon steels resulting from the use of RTA cycles is generally higher for austenitic annealing and is greatly influenced by the interaction that may occur in the intercritical domain between the recrystallisation and the ferrite-to-austenite phase transformation. In particular, the grain diameter of low carbon steels has been shown to decrease when this interaction becomes stronger.…”

“…During the last 10 years, novel process techniques (frequently involving severe plastic deformation) have been developed to refine the microstructure of steels. In recent publications [1][2][3][4], it has been suggested that rapid transformation annealing (RTA) cycles may be used to obtain substantial grain refinement of low carbon steels. These cycles that can be performed either in the intercritical domain (between A C1 and A C3 ) or in the austenitic domain combine rapid heating to the annealing temperature, a short holding time and rapid cooling.…”

Microstructure and mechanical property of cold rolled low carbon steel after prolonged annealing treatment

“…(i) heavy cold reduction (>50%) of martensite followed by recrystallisation during conventional annealing 2-5 (ii) rapid transformation annealing of cold rolled ferrite-pearlite structures. [6][7][8] Cold rolling of martensite requires high rolling load, and the risk of cracking exists during rolling. Rapid transformation annealing technique is also difficult to implement in an industrial set-up.…”