Abstract
The tensile behaviours of four sheet steels reveal that their strength, ductility, and anisotropy vary with direction in the plane of the sheet. The flow curves in each of three carbon steels (DP600, DP450, and GA220) are represented with Hollomon's law, in which slight differences in strength coefficient and hardening exponent reflect a direction dependence. The ductilities of the carbon steels (20–30 per cent elongation) contrast with the greater amount (50 per cent) found for 18–8 stainless steel, the flow curve of which more closely followed Ludwik's law. The contrasting formabilities of the four steels are revealed within n and r values found parallel and transverse to the roll. Contrary to expectation, the r values for each of these forming steels do not increase much above unity. Indeed, the majority of r values found were slightly less than unity, when based upon natural plastic strains. A common source of error in the r value is also examined. The correct r measurement should be based upon plastic strain and not total strain. This requires a subtraction of the elastic component of strain from the total strain, either by using the elastic modulus or, as was used here, by a repeated load-unload test procedure. The failure to remove elastic strain in a low region of total strain, where elastic and plastic components are of comparable magnitudes, can mislead, as when r values are found from a non-linear, total strain plot. Although the error from this source is diminished in a higher region of total strain, where its plastic component is dominant, the non-linear total strain plot cannot determine the r value precisely. Plastic strain plots for the three carbon steels are found to be sensibly linear throughout their uniform strain range. This allows an association to be made between a single, true r value and a given in-plane direction. A graphical compensation for correcting the error arising in r value determination, when not omitting elastic strain, is given for each material.
