Effects of ALPS on Mechanical Property in Bainitic Steels

Aggregate of bainite laths with almost parallel slip systems between neighboring bainite laths, hereafter referred to as ALPS, has great effect on the improvement of mechanical properties in steels. Elongation increases remarkably with increasing the number of bainite laths within an ALPS. When a bainite lath begins to deform, the neighboring bainite lath also easily deforms to relax the deformation strain, because of good parallelism between their slip systems. Under the cooperative deformation of bainite laths, the area of interface between neighboring bainite laths increases during deformation. The increase in the area of interface between neighboring bainite laths suppresses localization of dislocations at the interface, that is, dislocation density per unit area of the interface between neighboring bainite laths hardly increases, resulting in the suppression of nucleation of cracks at the interface between neighboring bainite laths. Ductile fracture would occur along the boundary between ALPSs. It could be suggested that larger ALPS and/or ALPS consisting of large number of bainite laths induce larger elongation in steels.On the other hand, it has been reported that tensile strength increases in proportion to inverse of square root ofd, thedbeing the average size of bainite laths [1]. In order to form fine bainite lath, dislocation network instead of inclusion in austenite was utilized as nucleation site for bainite lath. Great barriers to be overcome exist for the improvement of both strength and toughness. An idea for the improvement of both strength and toughness is shown in this study.