Rapid Transformation Annealing: a Novel Method for Grain Refinement of Cold-Rolled Low-Carbon Steels

Abstract

A novel heat treatment, rapid transformation annealing (RTA) is introduced, which uses the refining effect of the phase transformation during recrystallization annealing. The RTA is comprised of rapid heating including the full or partial phase transformation of cold-rolled, nonrecrystallized, or partly recrystallized a microstructure to c microstructure combined with subsequent rapid cooling to ambient temperature. In this way, a substantial grain refinement compared to conventional industrial annealing of cold-rolled strip can be realized. Minimum homogeneous grain sizes of 2.3 lm are produced in microalloyed high-strength low-alloy (HSLA) steels. A plain low-carbon steel shows despite any microalloying grain sizes of about 2.5 lm. Grain refinement results in a substantial increase in strength and in a pronounced strain hardening in the H IV stage. Simultaneously, the uniform and total elongation slightly decrease, which is a physical consequence of Conside`reÕs instability criterion. By a refinement from 4.2 to 2.3 lm, the yield and tensile strength of a HSLA grade can be increased by about 130 MPa to 535 and 650 MPa, while the total elongation is reduced by about four percentage points to 21 pct, i.e., a still moderate formability is provided. In general, the ductility-strength relationship remains similar to the conventional behavior. For all investigated steel grades, the HallPetch (HP) relationship is fulfilled. Furthermore, it is proved that RTA cycles can be realized in pilot scale as far as induction heating devices are applied.

Formation of Ultrafine Grained Dual Phase Steels through Rapid Heating

“…However, this approach would be applicable to hot-rolled products rather than coldrolled and coated sheets that are of primary interest to the current work. Rapid heating and cooling experiments have also been used to develop very fine structures in steels using rapid transformation annealing (RTA) 15) and flash processing. 16) Andrade-Carozzo and Jacques 17) showed that rapid heating of an initial cold-rolled ferrite-pearlite structure into the intercritical annealing region results in grain refinement in the final dual phase structures of a Nb-bearing low carbon steel.…”

Grain-refining ability of ultra-rapid annealing for low-carbon steel: severe plastic deformation

“…As the interaction occurs, the kinetics of recrystallization and transformation are increased. 3,6,13,16,21 The austenite phase nucleates first at the deformed ferrite grain boundaries. 22 On the other hand, austenite provides a preferred recrystal- lization nucleation site for the non-recrystallized ferrite.…”

“…[6][7][8][9] This condition provides the ability to control the microstructure by means of the simultaneous occurrence of recrystallization and phase transformation. URA shifts the start temperature of recrystallization to higher values near and above Ac 1 .…”

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

“…In the case of rapid heating during continuous annealing, ferrite recrystallization is strongly suppressed during the heating stage, while the austenite formation and its distribution are strongly influenced by overlapping of the processes of recrystallization and austenitization and thus it could refine the grain structure, to improve both strength and ductility at MATEC Web of Conferences the same time [8]. The purpose of this study was to explore the influence of higher heating rate on the microstructure and tensile behavior of DP590 steel.…”