Deformation-induced martensitic transformation behavior of type 304 stainless steel sheet in draw-bending process

Deformation-induced martensitic transformation and workhardening behavior in draw bending process was investigated, on a Type 304 stainless steel sheet, in comparison with that in uniaxial tension experiments. The Vickers hardness of the draw-bent sheet at the surface is much larger than that at the mid-plane, and it becomes remarkably larger with increasing blank holder force. The significant increase of hardness in the deformed sheet is due to alpha'-martensitic transformation. The volume fraction of alpha'-martensite in the draw-bent sheet is smaller than that in the uniaxially pulled sheet with the same plastic strain. In uniaxial tension the sheet is plastically deformed in one direction monotonically, but in contrast, in draw-bending tension-to-compression (i.e., bending-to-unbending) deformation takes place when the sheet is drawn over the die-corner. The difference in the evolution of the martensite between draw-bending and uniaxial tension is explained from such a difference in deformation mode. Under cyclic deformation, in the reverse deformation, the martensitic transformation stagnates in a certain extent of plastic strain because of the Bauschinger effect. Including such a case of stress reversal, the evolution of the martensitic transformation is given as a unique function of the effective stress, rather than the effective plastic strain. Thus the behavior of the martensitic transformation of the material during plastic deformation would be understood from the stress-induced phase transformation mechanism. (C) 2015 Elsevier B.V. All rights reserved.