Ultra-fine grained bulk steel produced by accumulative roll-bonding (ARB) process

Much attention has been directed recently to ultra-grain refining of metallic materials, where the grain size is reduced to less than 1 mu m. It is expected that submicrometer grained structure would result in high strength and toughness at ambient temperature [1] as well as high strain rate or low temperature superplasticity at elevated temperatures [2,3]. It has been reported recently that ultra-fine grains can be obtained by intense plastic straining [4]. Several special intense straining processes, such as Torsion Straining (TS) [5-10], Equal-Channel Angular Pressing (ECAP) [8-12] and Mechanical Milling (MM) of powder metals [13-15], have succeeded in producing ultra-fine grains. On the other hand, the disadvantage of these processes is that they are not applicable to large bulk materials. Not small functional materials but large structural materials do require high strength and toughness. From this critical viewpoint, we have recently developed a novel intense straining process for bulk materials, named Accumulative Roll-Bonding (ARB) [16]. We firstly tried to apply ARE to the aluminum alloys, and the bulk sheets with ultra-fine grains whose grain sizes are several hundred nano-meters were successfully produced [17,18]. The ARBed aluminum alloys with ultra-fine grains showed large strength at ambient temperature, which is up to 3.7 times larger than that of the starting materials [17,18], Further, it has been also clarified that the ARBed Al-Mg alloy sheet with submicrometer grains shows low-temperature superplasticity at 473 K which is half the melting temperature of the material [19]. The purpose of the present study is to clarify whether or not it is possible to produce the bulk steel sheets with ultra-fine grains by ARE process. Because steel is the most useful structural material, the ultra-grain refining of steel is greatly desired. The ultra-grain refining and resulted strengthening of steels could largely reduce the weight of any constructions, and the strengthening without alloying elements would be preferable for recycling. However, no investigation concerning the intense straining of bulk steels has been carried out by now possibly due to the difficulty in processing, although limited results about small materials, such as grain refining by powder metallurgical process (MM) [13-15] or TS of thin discs [7], have been reported.