Abstract. Grain refinement in metals is well-known as one of the most effective methods to enhance their strength without addition of other elements. In this study, repetitive heat treatment combined with subsequent cold-rolling and recrystallization were investigated to obtain ultrafine-grained ferrite. Ultrafine-grained (UFG) ferritic structure having a mean grain size smaller than 1 m was fabricated by repetitive heat treatment at 810 °C for 180 s and cold rolling by 90 % plus a recrystallization heat treatment at 600 °C. Starting from this UFG ferrite, finegrained dual phase (DP) steel composed of ferrite and martensite phases with grain sizes smaller than 3 m could be also obtained by intercritical heat treatment at 740 °C for 30 minutes followed by water-quenching. The mechanical properties of the ultrafine-grained ferritic and DP structured specimens were evaluated by tensile test. Results of the tensile test showed that finegrained ferritic and DP structures had higher yield and tensile strength than the coarse-grained ferritic and DP structures of the same steel. IntroductionDual phase (DP) steels composed of ferrite and martensite phases are widely used for sheet materials in the automotive industry due to their good mechanical properties, such as high strength, adequate ductility and good formability. It is generally known that several factors, such as grain size of each phase, volume fraction and distribution of martensite, change the mechanical properties of DP steels. Among them, it is expected that grain refinement is an effective way to further improve the mechanical properties of DP steels. Recently, several studies reported that fine-grained DP steels showed excellent mechanical properties compared with DP steels with conventionally coarse grain sizes [1][2][3][4][5]. In order to fabricate bulky metals with ultrafine-grained structures, severe plastic deformation (SPD) processes, such as Equal Channel Angular Pressing, Accumulative Roll-Bonding and High Pressure Torsion, have been widely used [6][7][8][9]. The ultrafine-grained DP steels have also been obtained successfully using SPD processes [10,11]. However, it seems difficult to apply SPD processes to mass production of steels because of the limited dimensions of the SPD processed materials. Several alternative processes for refining grain size other than SPD have been also proposed, for example, repetitive heat treatment [12,13] and recrystallization of cold-rolled DP steel [14]. In the present study, grain refinement of ferrite and DP structures through repetitive heat treatment and subsequent cold-rolling and recrystallization was investigated.

Low carbon dual-phase (DP) steels composed of soft ferrite and hard martensite have been widely used in the automotive industry due to their good strength-ductility balance and large strain hardening ability. DP steels have a wide variation in mechanical properties depending on several microstructural features such as grain size, phase fraction and distribution. Among them, the grain refinement of DP steels is known to be an effective option for enhancing mechanical performance in strength and ductility (especially post-uniform elongation). However, the exact reason for the significant improvement of post-uniform elongation by grain refinement has not been fully understood. It is considered that the characterization of local deformation behavior and micro-void formation/growth behavior in connection with microstructures is an essential approach for understanding the enhanced post-uniform elongation realized in the fine-grained DP specimen. In the present study, we prepared two kinds of DP specimens with mean ferrite grain sizes of 14.9 μm (coarse-grained DP) and 7.1 μm (fine-grained DP), and carefully investigated local strain distribution of tensile specimen and micro-void formation/growth behavior using digital image correlation (DIC) analysis and SEM observations. The fine-grained DP specimen exhibited a gradual strain localization after necking and had sufficient strain capacity that could endure against fracture. The fine-grained DP structure had a great number of micro-voids in the necked region, but almost all the micro-voids maintained a very small size, which was contrasted with the case of coarse-grained DP specimen containing very large-sized micro-voids. Such a significant difference in micro-void size/number characters between two kinds of DP specimens would be one possible reason for exhibiting greatly different post-uniform elongation behavior.