Cold-rolling and inter-critical annealing of low-carbon steel: Effect of initial microstructure and heating-rate

Abstract

The effects of starting microstructure and heating rate on the inter-critical annealing treatment have been investigated by 80% cold-rolling of different initial microstructures namely ferrite–pearlite and ferrite–martensite with blocky and fibrous martensitic morphologies, followed by inter-critical annealing treatment using two different heating rates (∼0.5 °C/s and ∼300 °C/s). Sub-critical annealing has also been carried out to understand the effect of starting structures on cold-rolled and completely recrystallized microstructures. Due to the fine-scale lamellar structure comprised of alternate layers of ferrite and martensite, ferrite-fibrous martensite starting structure showed the finest ferrite grain sizes (3–6 μm) and more uniform distribution of martenisitc islands compared to ferrite–pearlite and ferrite-blocky martensite after the inter-critical annealing. Slow heating resulted in a coarser ferrite grain size with more uniform distribution of martensite, compared to rapid heating. Recrystallization–transformation interaction and the avoidance of ferrite grain growth contributed to the finer grain sizes after rapid annealing. Nature and distribution of θ-particles and the austenite islands played an important role in controlling the ferrite recrystallization and grain growth. Finer microstructural constituents offered superior combination of strength, ductility and strain-hardening ability to ferrite-fibrous martensite starting structures after rapid annealing, compared to other structures. Rapidly heated samples showed higher strength than the slowly heated samples.

Introduction

Conventional, sub-critical annealing of the cold-rolled steel is either carried out in batch-annealing furnace or in continuous-annealing furnace. This annealing schedule comprised of slow heating (heating rate <10 °C/s) to the annealing temperature below A_c1 (∼550–700 °C), prolonged isothermal holding (>1 h) followed by slow-cooling [1]. In general sub-critical (recrystallization) annealing offers yield strength (YS) ∼200–300 MPa [1], whilst, inter-critical annealing above A_c1 and water-cooling (cooling rate, R_C≥50 °C/s) can raise YS to ∼600 MPa in cold-rolled, dual-phase steels [2], [3], [4]. Development of ultra-fine grained steels (average grain size <3 μm) by applying severe plastic deformation (SPD) processes can increase the strength even higher (YS>800 MPa), but affect the strain-hardening ability and uniform elongation (e_u≤5%) [5]. SPD techniques are also difficult to apply in the industrial scale.

Considering the above limitations, different starting microstructures have been subjected to cold-rolling and sub-critical annealing treatment, in place of conventional ferrite–pearlite structure to develop ultrafine ferrite grains [6], [7], [8], [9], [10], [11], [12], [13], [14]. Tsuji and co-workers [6], [7], [8] produced ultra-fine ferrite (UFF) grained steel with grain size less than 1 μm by 50% cold-rolling of martensite and annealing at 500 °C. Grange [9] performed short and rapid austenitizing of undeformed martensite, which has been recently repeated by Azizi-Alizamini et al. [10] and Nakada et al. [11] on both undeformed and cold-rolled martensite. However, the deformation of martensite is difficult as it requires high-flow stress [8] and suffers from transverse cracking problem at the edge of the specimen [12]. Considering the problems associated with cold-deformation of martensite, rolling of mixed phase structures can be preferred for the development of ultra-fine ferrite grain structures [8], [9], [10], [11], [12], [13], [14], [15].

Ultra-fine dual-phase (UFDP) steels show better strength and uniform elongation than the UFF, carbide steels [5], [10], [16], which encouraged the development of rapid-transformation annealing (RTA) or ultra-rapid annealing (URA) treatment [17], [18], [19], [20]. RTA is characterized by rapid-heating (R_H∼200–300 °C/s) of cold-rolled steels above A_c1, followed by short isothermal holding (∼5–60 s) and rapid cooling (R_C≥100 °C/s) [15], [16], [17], [18], [19]. Higher heating rate in RTA, compared to the conventional inter-critical annealing (R_H∼20–50 °C/s) prevents the growth of recrystallized ferrite grains and helps in obtaining fine-grain size [17], [18], [19], [20]. Heating-rate also plays an important role in inter-critical annealing as it can vary over a wide range starting from slow-heating (say, ∼0.5 °C/s) to RTA (∼300 °C/s). Compared to the studies on sub-critical annealing lesser attention has been paid to understand the effect of initial microstructures on inter-critical annealing.

Therefore, in the present study ferrite–pearlite and ferrite–martensite structures have been cold-rolled and inter-critically annealed after slow- and rapid-heating