Ultra grain refinement and hardening of IF-steel during accumulative roll-bonding

A strip of commercial Ti-IF-steel was processed by accumulative roll-bonding (ARB) up to three cycles. The mean grain size and hardness distribution profiles along the thickness were obtained using atomic force microscopy and instrumented indentation tests. The grain refinement occurred mainly at the subsurface regions where the shear strain is high. The stacking of the strips before the subsequent rolling passes places the ultra-fine grains in the center region, leading to a complete ultra grain refinement along the thickness after only three ARB cycles. A parabolic grain size distribution profile results along the thickness that is a direct consequence of the shear strain during ARB so that the number of superimposed parabolas is 2n−1, where n is the number of ARB cycles. For all samples the hardness distribution along the thickness bears a direct correspondence to the local grain size following the well-known Hall-Petch relation. It is concluded that there is a direct relationship among shear strain, grain refinement and hardening of ARB processed materials with the shear strain being the main instrument for grain refinement in the present process.