Modeling tensile deformation of dual-phase steel
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I.
INTRODUCTION
D u A L - p h a s e steels derive their characteristic properties from the presence of martensite and austenite islands dispersed in a ferrite matrix. These steels are produced by annealing plain carbon or low alloy steels in the ferriteaustenite (a + y) region and cooling below the martensite start temperature at a suitable rate. [11 In spite of the large amount of literature published, many critical questions remain unanswered; for example, the optimum amount of martensite/austenite constituent, the role of retained austenite, and the role of residual stress in each constituent. The structure-property relationships developed so far to describe the tensile deformation behavior of dual-phase steel have had limited success due to adoption of overly simplistic assumptions and/or analytical methods. In these approaches, the dual-phase steel was treated as a twophase system (ferrite and martensite), whereas in reality there is a significant quantity of retained austenite in addition to martensite. A variety of analytical approaches have been used; the more important examRles are a simple rule of mixtures for stress and/or strain, [2 '3] a continuum mechanics and finite element method, used jointly to study inhomogeneity of plastic deformation within a two-phase microstructure, 14] a continuum mechanics approach in which the internal stress produced by inhomogeneous distribution of plastic strain is simultaneously taken into account, tSl an analytical expression for the influence of phase contiguity on yielding added to a continuum mechanics model, t61 and a semi-mechanistic model which accounts for the load transfer between two phases by using an intermediate law of mixtures. [7'8] Several important factors which influence deformation behavior of dual-phase steel have been studied analytically and experimentally, even if they were not employed in a structure-property equation. Tension/compression tests were performed to determine the Bauschinger effect, 19] i.e., the development of long range internal stress field due to inhomogeneous plastic deformation. The importance of the mechanical stability of retained austenite in enhancing the ductility of dual-phase steel was analytically eluci-
CHONGMIN KIM is Staff Research Engineer, Metallurgy Department, General Motors Research Laboratories, Mound and 12 Mile Roads, Warren, MI 48090-9055. Manuscript submitted May 14, 1987. METALLURGICAL TRANSACTIONS A
dated, tS] as was the significance of extrinsic transformation accommodation hardening. II~ Dislocation density and its contribution to deformation hardening in dual-phase steels were estimated, till The analytical model that incorporates continuum mechanics and the internal stress contribution [5] appears to be more useful than other models because of its capability to describe the general tensile deformation behavior of dualphase steels. Also, upon further modification of the model, it is possible to explain the most important aspect of all, i.e., the influence of the amounts of secondary microstructura
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